-
In yet another approach to Paleozoic platforms, this study takes
a restricted time slice and compares the cycle patterns in two
different platforms: the Middle Appala- chians of Eastern North
America, and the Great Basin on the Western side.
Shoal-water emergence cycles and ramp cycles are dif-
ferentiated, and time-subsidence plots provide a way of comparing
the histories. Forward computer-modelling can be designed to
produce somewhat similar sequences. The facies patterns and cycle
patterns of these platforms differ markedly from those of the Early
Carboniferous and those of the Triassic.
The periodicities have not been established owing to the great
uncertainties of stage durations. In addition, frequency ratios, so
helpful in the Mesozoic, are not readi- ly applicable to these
early Paleozoic times inasmuch as the precessional and obliquity
frequencies were probably considerably higher than those prevailing
now, whereas the eccentricity frequencies have probably not
changed. Such differences might in the future allow a better ap-
proach to the problem of deceleration of the earth's spin rate, but
this will require cleaner cyclicity data than are now
available.
RELATION OF EUSTASY TO STACKING PATTERNS OF METER-SCALE
CARBONATE CYCLES, LATE CAMBRIAN, U.S.A.
DAVID OSLEGER Department of Earth Sciences
University of Cahfornia Riverside, California 92521
AND
J. FRED READ Department of Geological Sciences
Virginia Polytechnic Institute and State University Blacksburg,
Virginia 24061
A~rRncr : An interbasinal study of Late Cambrian cyclic
carbonate successions in the Appalachian and Cordilleran passive
margins suggests that superimposed orders ofeustasy controlled the
development of large-scale depositional sequences and the component
peritidal to subtidal meter-scale cycles that comprise them. The
focus of this paper is on the small-scale cyclicity, its probable
control by Milankovitch-forced sea-level oscillations, and how
stacking patterns of meter-scale cycles can be used to define
internal com- ponents of larger-scale sequences and estimate
variations in relative sea level.
Fining-upward peritidal cycles showing evidence of episodic
emergence grade seaward into coarsening-upward subtidal cycles
which lack evidence of emergence and form a continuum across the
Cambrian carbonate platforms. Eustasy appears to exert the dominant
control on the simultaneous development of peritidal and subtidal
cycles on Late Cambrian carbonate platforms. Evidence for
Milankovitch forcing of glacio-eustatic sea-level oscillations is
provided by a 4:1 bundling of fifth-order meter-scale cycles (~ 96
ky) within fourth-order cycles spanning tens of meters (~ 440 ky)
within the Big Horse Member of the Orr Formation in the House Range
of Utah. The 4:1 bundling may manifest the short eccentricity to
long eccentricity ratio ofthe Milankovitch astronomical
rhythms.
Systematic changes in the stacking patterns of meter-scale
cycles can be used in conjunction with Fischer plots to define
long- term sea-level cycles. On Fischer plots ofperitidal cyclic
successions, long-term relative sea-level rises are characterized
by thick, subtidal-dominated cycles with thin laminite caps.
Long-term relative sea-level falls are defined by stacks of thin,
laminite-dominated cycles that show brecciated cycle caps and
quartz sands toward the relative sea level lowstand. On Fischer
plots of dominantly subtidal cyclic successions, long-term
sea-level rise is characterized by storm-dominated, open marine
carbonate cycles or thick, deep ramp, shale-based cycles. Falling
segments of the Fischer plot are characterized by thin, shallow
subtidal cycles composed of restricted lithofacies. Cycle stacking
patterns (parasequence sets) provide the crucial link between the
meter-scale cycles (parase- quences) and the larger scale sequences
and their component systems tracts.
One- and two-dimensional models of pedtidal and subtidal cycle
development indicate that, whereas peritidal cycle thickness is
primarily controlled by accommodation space, deeper subtidal cycle
thickness is primarily controlled by sedimentation rate. Lithofa-
ties within peritidal cycles reflect the sedimentologic response to
fluctuations in sea level; lithofacies within subtidal cycles may
be related to fluctuations in the zones of fairweather and
storm-wave reworking that oscillated in harmony with sea-level
fluctuations and may have acted as a subtidal limit to upward
aggradation. The 2-D modelling illustrates how stacked peritidal to
deep subtidal carbonate cycles with thicknesses, compositions and
stacking patterns similar to the Late Cambrian of North America can
be generated by Milankovitch-driven composite eustasy.
INTRODUCTION
Hierarchies of stratigraphic cyclicity have long been recognized
throughout the geologic record (e.g., Barrell 1917; Fischer 1964;
Koersehner and Read 1989; Gold-
hammer et al. 1990; Borer and Harris 1991) and appear to be
related to the combined effects of several orders of relative
sea-level oscillations. Shallowing-upward, meter- scale carbonate
cycles (parasequences) tend to be system- atically arranged within
larger-scale successions (parase-
JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 6 !, NO. 7, Dec., 199 l,
P. 1225-1252 Copyright 1991, SEPM (Society for Sedimentary Geology)
0022-4472/91/0061-1225/$03.00
-
1226 DA V1D OSLEGER AND J. FRED READ
H(
LATE CAMBRIAN SEDIMENTARY FACIES
[~] Cratonal siliciclaslics
Shallow marine carbonates
Basinal sAiciclastics
Fio. ! .--Location map of sections measured in the study. Late
Cam- brian base map modified from Palmer (1974) to show the inner
and outer detrital belts and the middle carbonate belt.
quence sets). Stacking patterns of the meter-scale cycles
(stratigraphic trends in cycle thickness and composition) can be
used to identify large-scale sequences, their com- ponent systems
tracts, and long-term relative sea-level changes. An interbasinal
study of Late Cambrian peri- cratonic cyclic carbonates was
conducted by logging me- ter-scale cycles of time-equivalent cyclic
successions on separate platforms to evaluate various types of
cycles, their stacking patterns, and potential mechanisms that may
have controlled their origin.
The objectives of this this paper are to: 1) describe Late
Cambrian peritidal to deep subtidal cycles and interpret the
environmental conditions under which upward-shal- lowing occurred;
2) evaluate the controlling mechanisms of meter-scale cycle
formation (specifically the connection with Milankovitch orbital
variations); 3) illustrate char- acteristic stacking patterns of
cycles that define rising and falling portions of sea-level curves
using Fischer plots; and 4) use quantitative I -D and 2-D modelling
to con- strain the probable conditions under which coeval peri-
tidal and subtidal cycles were deposited.
STRATIGRAPHIC AND TECTONIC SETTINGS
Complete sections of Late Cambrian strata were mea- sured and
logged bed-for-bed in the House Range of west central Utah and in
the Appalachian Mountains in Ten- nessee, Virginia and eastern
Pennsylvania (Fig. 1). Bio- stratigraphic control of the formations
for each of the localities (Fig. 2) was obtained from published
work
(Palmer 1965, 1971a, 1971b; Derby 1965; Rassetti 1965; Hintze
1974; Hintze and Palmer 1976; Hintze et al. 1980; Eby 1981; Taylor
and Miller 1981; Miller et al. 1982; Orndorff 1988; Sundberg 1990).
Primary estimates of subsidence rate and cycle duration were made
using the DNAG time scale values for the duration of the Late
Cambrian (Palmer 1983). However, considerable contro- versy exists
regarding the total duration of Cambrian time (Cowie and Harland
1989), with new age dates (Benus 1988) supporting a much shorter
time span. Therefore, a conservative 50% margin of error is
incorporated into all calculations involving total Late Cambrian
time.
Field locations were chosen on the basis of: 1) quality of
exposure and absence of structural complications, 2) availability
of biostratigraphic data (especially biomere boundaries), and 3)
platform location along the transition between shallow-water
carbonates and deeper-water fine- grained siliciclastics where
intertongueing relations best define excursions in sea level. A
total of 2200 m of section was logged and numerous other sections
previously de- scribed in the Appalachians (Zadnik 1960; Markello
1979; Koerschner 1983; Demicco 1981) and in Utah-Nevada (Palmer 197
la; Kepper 1972; Lohmann 1976; Rees 1986) were field-checked. Hand
samples of individual lithofa- cies were slabbed and thin sectioned
to provide additional detail for paleoenvironmental
interpretations. Details re- garding the exact locations of
sections and logs of strati- graphic intervals can be found in
Osleger (1990).
The Appalachian and Cordilleran passive margins orig- inated in
response to breakup of a Late Proterozoic su- percontinent around
625 to 555 Ma (Bond et al. 1984). Both passive margins developed
wedge-shaped prisms of post-rift subtidal to peritidal carbonates
and interlayered siliciclastics during Late Cambrian time (Fig. 3).
The Ap- palachian passive margin contains up to 1.6 km of Middle to
Late Cambrian shallow water carbonates and intrashelf basin shale
and siltstone (Read 1989). The Cordilleran passive margin of the
western United States accumulated approximately 2.0 km of post-tiff
Middle to Late Cam- brian carbonates and fine siliciclastics
(Stewart and Poole 1974; Levy and Christie-Blick 1989).
SHALLOWING-UPWARD METER-SCALE CYCLES
A spectrum of meter-scale peritidal to deep subtidal carbonate
cycles (1-15 m) can be recognized in Late Cam- brian strata of the
two passive margins (Figs. 4, 7). Suc- cessions of fining-upward
peritidal cycles (Wilson 1952; Chow and James 1987; Demicco 1985;
Koerschner and Read 1989) extend entirely across the broad Late
Cam- brian passive margin of the Appalachians but are re- stricted
to a narrow zone near the Wasatch hinge line of the coeval
Cordilleran passive margin (Palmer 1971a; Kepper 1972). Peritidal
cycles in the Cordillera of Utah grade seaward into shallow to deep
subtidal cycles show- ing an upward increase in grain size, bed
thickness, and other indices of higher energy. Subtidal cycles are
not capped by intertidal lithofacies, nor do the subtidal cycles
exhibit exposure features such as microkarsting or vadose
dissolution/cementation. The cycles form a continuum
-
EUS7"AS Y AND CYCLE STACKING PATTERNS OF LATE CAMBRIAN
CARBONATES 1227
~ I '~0 ~ ~ i BIOMERE w
1
Z E P~CHA.~OtD
m ~
UJ CEPHAUlD
m "523 AM - X W
E MARJUMIID
TRILOBITE ZONE
MISSISSIOUOIA
SAUKIA
SARA TOGIA
TAENICEPHAL US
EL VJNIA
OUNDERBERGIA
APHEL ASPIS
CREPICEPHAL US
CEDARfA
BOLASPIDELLA
HOUSE RANGE, UTAH
LAVA DAM
'RED TOPS z u
HELLN- MARIA Z
SNEAK- OVER
CORSET SPRING ~
JOHNS WASH
CANDLAND
BIG HORSE
WEEKS FM.
SW VIRGINIA NE TENN.
COPPER RIDGE I
CONOCO- CHEAGUE
( MAYNARD.
VILLE
8
ELBROOK
EASTERN PENN.
ALLENTOWN DOLOMITE
FIG. 2.--Biostraligraphic chart of Late Cambrian strata in the
Cordilleran and Appalachian passive margins.
across the carbonate platforms and are genetically linked to one
another by shared lithofacies (Fig. 4) (Osleger 1991). These
asymmetric, meter-scale cycles are the parase- quences of sequence
stratigraphic terminology in that they are "relatively conformable
successions of genetically re- lated beds bounded by marine
flooding surfaces" (Van Wagoner et al. 1987).
The vast majority of meter-scale cycles recognized on both
passive margins are asymmetric with relatively thin basal
lithofacies recording abrupt drowning and relatively thick upper
lithofacies recording gradual shoaling. To- ward the outer platform
of both passive margins, some cycles exhibit subequal amounts of
deepening and shal- lowing lithofacies. These symmetric cycles are
relatively rare, however, and are restricted to deeper water
positions on each platform. No deepening-upward cycles were rec-
ognized.
Estimations of average cycle duration are complicated by errors
in the absolute time scale, the effects of com- paction, and
assumptions of constant sedimentation rates. Acknowledging these
potential sources of error, average cycle durations for
non-decompacted Late Cambrian cy- cles range from roughly 40 to
almost 150 ky. Taking a conservative 50% margin of error into
account, this range of durations may extend from about 20 to 225
ky, the normal range expected for meter-scale cycles (Algeo and
Wilkinson 1988).
Vertical and Lateral Consistency of Cycles
Late Cambrian meter-scale cycles of the Appalachians are
extremely rhythmic vertically in outcrop with only minor variations
in the arrangement of component litho- facies (Demicco 198 l;
Koersehner and Read 1989). Peri-
tidal cycles comprise successions of hundreds of stacked cycles
but are difficult to correlate laterally in the Ap- palachians
because of the distance between outcrops, the lack of marker beds
and the lack of precise biostratigraph- ic control. However, groups
of time-equivalent Late Cambrian cycles with distinct stacking
patterns (fourth- and third-order scale) can be correlated along
the Ap-
LATE CAMBRIAN PLATFORM MORPHOLOGIES
APPALACHIAN REEF - RIMMED PLATFORM NW SE
UTAH - NEVADA DISTALLY - STEEPENED RAMP W E
FIG. 3.--Late Cambrian platform morphologies of the Appalachian
and Cordilleran passive margins. Formation and group names super-
imposed on lithologic symbols.
-
1228 DA VID OSLEGER AND J. FRED READ
GRADATION OF CYCLE "TYPES ACROSS A LATE CAMBRIAN SHALLOW TO DEEP
RAMP
LAMINITE-CAPPED PERmDAL CYCLE
THROMBOLJTE BIOHERM, SHALLOW SUBTIDAL CYCLE
SL m
1 o -~.~- o o o o oOnOoOoO/.
--~- l
~ID GRAI~T~E, SHALLOW SUBTIDAL CYCLE
~ RYPTALGAL LAMINITE
~ ] I ~ SKELETAL'PELLETAL THICK I~:-~-~-' PACKSTONE WITH
LAMINITE I ' I STORM BEDS
/ , , ' t~- c r ^- .
SKELETAL PACKSTONE, MID-RAMP CYCLE
FWWB
t ~ = t "
SPICUUT1C WACKESTONE, DEEP RAMP CYCLE
~ RIBBON ~ BURROWED ROCK WACKESTONE
[~] THROMBOUTIC ~ PELOIDAL WACKESTONE BOUNDSTONE PACK, STONE
700IDJNTRACLAST ~ ARGILLACEOUS GRAIN, STONE NODULAR WACKESTONE
FIG. 4.--Arrangement of peritidal to deep subtidal cycle types
across a hypothetical Late Cambrian platform. Note the location of
the zones
of fairweather and storm-wave reworking and their relation to
cycle types.
palachians from Virginia to Pennsylvania using Fischer plots
(Read 1989; Osleger and Read, unpublished data).
Subtidal cycles of the Utah Cordillera are repetitive over 15 to
40 successive cycles before gradually changing to a different cycle
type. In the House Range, cycles can be tracked as subparallel
bands for many kilometers along the mountain flank and meter-scale
deep subtidal cycles can be correlated between outcrops greater
than 45 km apart (Fig. 5), indicating that the subtidal cycles are
not local facies mosaics. No up-dip peritidal cycles exist that can
be directly correlated with down-dip subtidai cycles in the House
Range.
Lithofacies and Depositional Environments
Peritidai Cycles.--Laminite-capped cycles (0.4-7.0 m) of the
Appalachian Late Cambrian are composed of a basal ooid-intraclast
grainstone lag deposit overlain by either ribbon carbonates or
thrombolite boundstones (Ta- ble 1; Fig. 4). The cycles are capped
by mudcracked thick laminites and/or cryptalgal laminites; quartz
arenites or carbonate clast breccias may cap some cycles,
particularly during long-term relative sea-level fall. The cycles
exhibit abrupt upper and lower boundaries but have gradational
internal boundaries between iithofacies. Peritidal cycles extend
over much of the Appalachian reef-rimmed shelf (Zadnik 1960;
Reinhardt 1977; Demicco 1981; Read
1985) and are recognized within the Elbrook, Copper Ridge,
Conococheague and Allentown Formations.
The basal ooid-intraclast sandy lag deposit migrated onto the
underlying tidal fiat cap from shallow offshore wave-agitated
shoals during initial rapid transgression. Hardgrounds developed on
the lag deposit as the trans- gressive rise of sea level outpaced
sediment production. As the rate of relative sea-level rise
decreased, throm- bolites locally established themselves on
marine-ce- mented grainstone lags and grew to sea level. Ribbon
rocks accumulated adjacent to bioherms in shallow sub- tidal to
lower intertidal conditions. The rippled peloidal silts/fine sands
were laid down during storms with drapes of lime mud settling out
during the waning stages (De- micco 1983).
Progressive shallowing and progradation is reflected in the
upward transition into increasingly mudcracked rib- bon rocks, SH
and LLH stromatolites and thick laminites. Centimeter-scale thick
laminites are mechanically-de- posited couplets of fine pelodial
silts and mud drapes laid down on the intertidal fiats by storm and
tidal currents (Hardie and Ginsburg 1977). This lithofacies often
caps cycles or grades up into cryptalgal laminites. Lack of bur-
rowing, abundant mudcracks, silicified evaporite nodules and
windblown quartz sand within laminite lithofacies indicate
hypersaline and semiarid conditions. Cratoni- cally-derived quartz
sands (Wilson 1952; Koerschner and Read 1989) were probably brought
in during long-term
-
E USTAS Y ,tND CYCLE STACKING PA TTERNS OF LATE CAMBRIAN
CARBONATES 1229
falls in relative sea level and were incorporated into cycle
caps during short-term regression or reworked by the suc- ceeding
marine transgression into the basal lag deposit of the succeeding
cycle.
Relatively few characteristics of supratidal conditions have
been observed in peritidal cycles of the Appalachi- ans (Hardie and
Shinn 1986; Koerschner and Read 1989). The absence of bedded
evaporites (other than isolated silicified nodules), the relative
scarcity of dissolution breccias and the lack of erosional relief
along the sharp top of laminite caps may indicate erosional removal
dur- ing the formation of a deflation surface down to paleo-
watertables. Supratidal evaporites may have been dis- solved during
the occasional rains that occurred in the semi-arid climate. Lack
of land plants would have not favored caliche development, and the
prevailing desic- cating conditions might have inhibited
cementation, making erosional removal of supratidal sediment by eo-
lian action a tenable mechanism for forming the planar surface at
the tops of most laminite caps. However, some evidence of exposure,
non-deposition and erosion is ex- hibited. Relatively scarce,
irregular veneers of carbonate clast breccia fill solution-enhanced
lows at the top of some caps and are probably thin regoliths that
developed on the non-vegetated Late Cambrian exposed flats. Fossil
molds and geopetally-filled leach voids in subtidal lime- stones
indicate flushing by undersaturated meteoric wa- ters (Koerschner
and Read 1989) during relative sea level falls.
Shallow Subtidal Cycles.--Shallow subtidal cycles are defined by
the interpreted paleowater depth of the cycle cap and include
cycles capped by thrombolite bioherms, ooid grainstones and
skeletal packstones. Paleowater depths are based upon modem analogs
of the lithofacies and their sedimentary structures.
Cycles capped by thrombolite bioherms (1.5-12.0 m) consist of a
basal dark gray peloidal packstone overlain by stacked
thrombolite-stromatolte bioherms and later- ally equivalent light
gray cross-bedded peloidal-oncolitic grainstone (Table 2; Fig. 4).
These cycles record progra- dation of shallow subtidal bioherms and
associated high energy grainstones over slightly deeper subtidal
peloidal packstones of a restricted shelf. More than thirty of
these cycles are recognized within the upper Hellnmaria Mem- ber of
the Notch Peak Formation throughout the House Range of west central
Utah.
These cycles were initiated by onlap of peloidal pack-
stones/wackestones onto thrombolite-stromatolite bio- herrns.
Horizontal to low angle cross-lamination, lack of recognizable
skeletal material, and dark gray bioturbated textures suggest
restricted, quiet water (but not necessarily deep) deposition. With
slowing of the rate of short-term relative sea-level rise,
thrombolitic bioherm complexes were able to establish themselves on
hardgrounds or other stable substrates. The bioherms are laterally
discontin- uous, suggesting development as isolated, shallow sub-
tidal "patch reefs" on top of the basal peloidal veneer. Continued
slow rates of relative sea-level rise are indi- cated by the
stacking of individual bioherms up to 12 m thick without
intervening bedded lithofacies. Many of the
STEAMBOAT PASS, S. HOUSE RANGE
ORR RIDGE, N. HOUSE RANGE
I " I I
I I
i
"1 J ,
! 1 ~ _ _
i i ' .,;I wP
PTEROCEPHAL I ID
. B IOMERE - - ~-
- " , - -I i!~! ~ i:ii!ii!ii ii~ii,, / 1 " ! " | ' l
," ,J I I i V l "
/ I " - I |
FIG. 5.--Correlation of deep subtidal cycles within the
Sneakover Pass Member, Orr Formation, across 45 km within the House
Range. Correlation based upon the Orr/Notch Peak and Steamboat
Spring/ Sneakover formational boundaries.
thrombolitic bioherms have stromatolitic laminae out- lining the
outer surface of the mound, and some stro- matolitic biohermal
layers are interbedded within the dominantly thrombolitic complex
(Fig. 4). This implies either episodic shallowing to intertidal
depths or perhaps variations in salinity (and associated grazing
and boring epifauna) related to periodically restricted conditions
on the platform (Aitken 1967; Kennard and James 1986).
Shallow subtidal conditions for the thrombolites are supported
by the laterally equivalent light gray, crossbed- ded
peloidal-oncolitic-oolitic-intraclastic grainstones
-
1230 DA V1D OSLEGER AND J. FRED READ
TABLE 1.--Peritidal lithofacies
Intraclast Breccia (5-20 cm) Bedding Characteristics: Laterally
discontinuous veneers of angular to
elongate intraclasts in a calcrete matrix within mudcracked
laminite lithofacies; often abundant quartz sand; intraclasts often
poorly sorted with no preferred orientation or grading; irregular
upper surface usu- ally overlain by oolitic grainstone.
Internal Composition/Texture: Angular elasts composed of LLH
stro- matolites and cryptalgal laminites and have corrodcd edges
and doio- mitic rinds; quartz sand grains are moderately sorted,
subrounded and frosted; vuggy voids common.
Cryptaigal Laminite (0.2-3.0 m) Bedding Characteristics:
Dolomite; mm-scale planar and crinkly lam-
inations; mudcracks, deep prism cracks, tepees and silicified
evaporite nodules common; thin flat pebble conglomerates and mud
chip in- traclast layers; occasional 1-3 grain thick quartz sand
stringers; grades upward from ribbon rock or more commonly thick
laminite; occa- sionally capped by irregular cherty breccias but
more typically over- lain by intraclastic transgressive lag of
overlying cycle; some cycles are reversing with the cryptalgal
laminite coarsening upward into less mudcracked thick laminite.
Internal Composition/Texture: Laminar couplets composed of basal
silt- size peloidal packstones grading up into mudstone laminae;
some low-angle cross-lamination and micro-scoured bases in peloidal
silts; some laminoid fenestrae.
Thick Laminite (0.2-3.0 m) Bedding Characteristics: Dolomite;
cm-scale laminations of silt and
mud couplets; laminations planar to wavy to discontinuous;
common cross-laminated scour fills and current ripples; some short
mudcracks and silicified evaporite nodules; commonly overlie ribbon
rocks; usu- ally grade up into cryptalgal laminite but sometimes
will cap incom- plete cycles.
Internal Composition/Texture: Couplets consist of peloid-quartz
silt packstone that grades up into dolomitic mudstone; some
well-round- ed quartz sand laminae 1 or 2 grains thick; some
laminoid fenestrae.
Ribbon Carbonate (0.5-4.0 m) Bedding Characteristics:
Alternating irregular layers of peloidal lime-
stone and dolomitic mud; discrete burrowing; some gutter scours
with cross-laminated peloidal fill; shallow mudcracks become more
abun- dant upward; common flat pebble conglomerate beds with
internal scours, hardgrounds, and mud drapes; often flank and
overlie throm- bolite bioherms; fine upward into thick
laminites.
Internal Composition/Texture: Peloidal packstones grade upward
into argillaceous dolomite caps; peloidal laminae contain minor
quartz silt and skeletal debris and show occasional scoured bases
and low- angle cross-lamination; flat pebble beds composed of
imbricate dis- coidal clasts of laminated peloidal packstone or
doiomitic mudstone; matrix between clasts consists of sand-size
intmclasts, trilobite and echinoderm debris, pellets, and minor
quartz silt.
Stromatolite Bonndstone (0.5-2.5 m) Bedding Characteristics: SH
and LLH stromatolites typically encrust
tops of thrombolite bioherms or basal grainstone lags; fingers
often coalesce into fan-like forms or crenulated sheets; club
shapes common toward the tops of bioherm complexes; commonly
surrounded lat- erally by intraclast-peloidal packstones or ribbon
rocks which onlap and smother the stromatolites.
Internal Composition/Texture: Alternating mm-scale laminae of
dolo- mitic peloidal silts and muds; some irregular and laminoid
fenestrae; some thin quartz silt laminae and coarser laminae of
intraclasts.
Thrombolite Boundstone (0.5-2.5 m) Bedding Characteristics:
Globose to upward-widening flat-topped bio-
herms to coalescent biostromes; individual bioherrns often
stacked on top of one another (up to 12 m thick) without
intervening contin- uous bedded lithologies; digitate fingers (1--6
cm high x 1-2 cm wide) have erosional edges, are grouped in
clusters and often overlie massive cores of thrombolites; lime sand
interhead fill flank and locally onlap and blanket bioherms;
bi-directional and high-angle cross-bedding, scours and multiple
hardgrounds common in interhead fill; lime sands fine upward into
ribbon carbonates; thrombolites typically nucleate on underlying
intraclastic grainstone lag.
Internal Composition/Texture: Clotted micritic textures;
commonly grain-rich reflecting the composition of the interhead
fill; small in- traclasts, skeletal debris, ooids and pellets are
dominant components; fingers show traces of Girvanella, thin
stringers of micfite and mi- crospar cements; Renalcis recognized
toward the base of many of the bioherrns; common irregular
fenestrae with geopetal fillings.
which resemble modem, high energy, non-skeletal grain- stones
enveloping growing stromatolitic bioherms in ti- dal channels in
the Bahamas (Dill et al. 1986). Irregularly laminated oncolites,
coated peloids, and high-angle tabular erossbedded gra ins tones
that a l te rnate w i th hor i zonta l ly bedded packstones reflect
variable energy conditions. The lack of open marine fauna within
the inter-bioherm grain- stones may reflect either elevated
salinities on the re- stricted platform or intermittent high wave
or tidal en- ergies on mobile sandy substrates that precluded the
establishment of grazing organisms. Only the robust mol- lusks
Mathevia and Matherella are found associated with the bioherms,
supporting the case for high energy con- ditions.
Cycles capped by ooid grainstone (0.5-4.2 m) consist of burrowed
wackestone/packstone grading up into on- colite-skeletal
packstone/grainstone capped by oolitic grainstone (Table 2; Fig.
4). The succession of lithofacies record progradation of oolitic
shoals over deeper ramp lithofacies and occur in the Big Horse
Member, Orr For- mation of the House Range of Utah (Lohmann
1976).
Burrowed wackestone/packstones are subtidal facies deposited
below fairweather wave base under normal ma- rine conditions.
Pervasive bioturbation (ichnofabric in- dex 3-5; Droser and Bottjer
1986), bioclastic debris, and clusters of pellets suggest an active
infauna. Laterally dis- continuous skeletal packstone lenses with
erosional bases and burrowed tops are rapidly deposited storm beds
that escaped homogenization by burrowers. The abundant quartz silt
may have been transported from the craton across the inner detrital
belt (Palmer 197 la) and onto the carbonate platform through a
west-trending subtidal channel that debouched near the House Range
(Lohmann 1977).
With shoaling, skeletal sand sheets migrated across the burrowed
wackestones and were reworked by storm and wave currents.
Megarippled units suggest emplacement as sand waves and shallow
bars. The upward transition from open marine skeletal packstones to
oncolitic-peloi- dal grainstones indicates increasingly shallow,
restricted conditions (Enos 1983), perhaps peripheral to active
ooid shoals (Hine 1977).
-
I:'US7;1SY AND ('YCLE ST.4C'KING P.4TTERNS OF LATE CAMBRIAN
CARBONATES 1231
Ooid Grainstone (0.1 -1.5 m) Skeletal-Oncolitic Grainstone
(0.5-2.5 m) Bedding Characteristics: Thin to medium bedded, common
high angle
crossbedding; random stacked hardgrounds; gradationally overlie
skeletal-oncolitic grainstones; abruptly overlain by burrowed
(ii3-ii5) wackestone/packstone lithofacies.
Internal Composition/Texture: 90-95% well-sorted ooids; common
oo- litic intraclasts and random subangular quartz sand grains;
ooids are concentrically laminated and have
echinoderm-trilobite-quartz silt nuclei; isopachous marine cement
rims and fine to medium equant spar cements; some dolomitized
ooids.
Peloidal Grainstone (1.0-4.0 m) Bedding Characteristics: Thin to
medium-bedded dolomite; light to
medium gray cross-bedded grainstones; laterally equivalent to
(and gradationally onlap) thrombolitic-stromatolitic biohenns;
abruptly overlain by either thin-bedded, evenly-laminated dark gray
peloidal dolomites or thrombolitic bioherms.
Internal CompositioniTexture: Fine to medium crystalline
dolomite; dominantly peloidal grainstones with variable numbers of
large on- colites, ooids and intraclasts (all seen as ghosts); flat
pebble conglom- erates in lenses and scours; robust Mathevia and
Matherella mollusk shells; some internal mm-scale gentle
cross-lamination of peloids.
Bedding characteristics: Thin to medium bedded; megarippled,
high angle crossbeds; commonly coarsen upward from skeletal
packstone up to interbedded oncolitic and skeletal grainstones;
random in- terbedded lenses of burrowed (ii3-ii5) wackestone low in
lithofacies and occasional interbedded short digitate stromatolite
fingers high in lithofacies; gradationally overlies burrowed
wackestone lithofacies and underlies oolitic grainstone
lithofacies.
Internal CompositioniTexture: Cm-size, well-rounded oncolites
with large intraclast cores; some multigenerational oncolites;
moderate sorting; random peloids and muddy intraclasts; micritic
and abundant turbid marine cements. Skeletal packstone/grainstones
composed of abundant trilobite and echinoderm debris and common
pellets and rounded elongate muddy intraclasts; some alignment and
imbrication of allochems; some crystal silt-filled geopetal voids
and shelter po- rosity.
The crossbedded oolitic grainstone cap resulted from
progradation of ooid shoal complexes as migrating spill- over lobes
that formed in response to storm or tidal cur- rents (Hine 1977;
Harris 1979). Rounded oolitic intra- clasts indicate early marine
cementation and the lack of leached ooids or other vadose features
suggests continual submergence.
Cycles capped by skeletal packstone (1.0-7.5 m) are composed of
basal nodular argillaceous wackestone over- lain by burrowed,
storm-deposited wackestone/pack- stone coarsening upward into a
skeletal packstone cap. (Table 2; Fig. 4). They occur in the lower
Big Horse Mem- ber (Orr Formation) of the House Range. These cycles
developed on the mid-ramp at intermediate water depths above the
zone of storm wave reworking seaward of ooid grainstone shoals. The
succession of lithofacies record gradually increasing storm
influence as the platform shal- lowed to skeletal shoal depths.
The basal nodular, argillaceous wackestone is a distal storm
facies deposited on the middle ramp between bur- rowed wackestones
and packstones and deeper water sil- iciclastic muds (Aigner 1985).
The low angle cross-lam- inated rnicropeloidal and quartz silty
layers within the modular limestones were probably transported
seaward from the shallow ramp during periodic storms. Nodules may
have formed early by submarine lithification under weak bottom
currents (Mullins et al. 1980) or may be the result of late
pressure solution and compaction. Argilla- ceous muds fell out of
suspension during waning storm activity. The two sediment types
were mixed by the bur- rowing infauna.
As the depositional surface shallowed with aggradation, the
argillaceous content of the sediment decreased, grain size and
skeletal content generally increased, and the abundance of storm
beds with hummocky cross-stratifi- cation increased.
Storm-deposited skeletal packstones (5- 20 cm) are laterally
discontinuous, fine upward and, from
base to top, consist of: 1) sharp, scoured bases, 2) skeletal
debris with peloids and mud perched above shells with pendant
bladed marine cements extending down into now- occluded shelter
pores, 3) hummocky cross-stratified pe- loidal packstones and 4)
bioturbated calcisiltite caps. This allochthonous debris was
transported by storm-generated currents and then reworked by
oscillatory shear currents (Aigner 1985). Upward within individual
cycles, skeletal material becomes more abundant and storm deposits
ap- pear amalgamated with numerous wavy beds of subtly graded
skeletal debris. Common platy ginanellid crusts are imbricated and
suggest that the packstone cycle cap may have formed within the
photic zone (Pfeil and Read 1980).
Deep Ramp/Intrashelf Basin Cycles.-Deep subtidal cycles are
characterized by sedimentary structures within the cycle cap
indicative of deposition below the zone of storm wave reworking.
These cycles are commonly shaly and may be capped by spiculitic
wackestone, skeletal storm beds or flat-pebble conglomerates.
Cycles capped by spiculitic wackestone (0.7-3.1 m) are composed
of basal nodular argillaceous mudstone over- lain by burrowed
spiculitic wackestone with upward-in- creasing skeletal packstone
lenses (Table 3; Figs. 4-6). These cycles occur in the Sneakover
Member (Orr For- mation) and in the Hellnmaria and Lava Dam Members
(Notch Peak Formation) of the House Range. Very sim- ilar deep ramp
cycles have been described in the Upper Muschelkalk of the
South-German Basin (Aigner 1985) and the Catalan Basin of Spain
(Calvet and Tucker 1988). Spiculitic wackestone-capped cycles of
the House Range developed on the deep ramp very near the base of
storm wave reworking seaward of the skeletal-packstone capped
cycles. The abundant bioturbation (ii3-ii5) and trilobite and
echinoderm debris within storm beds attest to well- oxygenated,
normal marine conditions.
Shaly cycles capped by skeletal storm beds (2.5-1 5.0 m)
-
1232 DA VID OSLEGER AND J. FRED READ
TABLE 3.--Deep subtidal lithofacies
Thin-bedded Peioidal Packstones (1.0-4.0 m) Bedding
Characteristics: Dolomitized, dark gray, thin to medium-bed-
ded peloidal packstones; random cm-scale horizontally laminated
horizons; abruptly overlie light gray cross-bedded peloidal facies
and thrombolite bioherms; occasionally differentially compacted
beneath biohermal mounds.
Internal CompositionFFexture: Consist internally of dolomitized
dark gray peloids and patchy micrite; evenly laminated; probable
burrow- ing traces; lack any of the sedimentary features and grain
types ex- hibited in the distinct cross-bedded peloidal grainstone
lithofacies.
Nodular Argillaceous Waekestone (1.0-8.0 m) Bedding
Characteristics: Discontinuous, nodular, wavy and thin-bed-
ded; recessive weathering; argillaceous seams with
concentrations of ferroan dolomite separate platy
mudstone/wackestoae; quartz silt lenses common; random very
low-angle cross-laminated lenses and nodules; some ram-scale
horizontal laminations; common horizontal burrows on tops of
bedding planes.
Internal Comoosifion/Texture: Variable intermixed textures from
mud- stone through packstone with wackestone dominant; micropeioids
and quartz silt dominant along with finely comminuted skeletal
debris including sponge spicules; patchy lime mud; occasional
floating elon- gate trilobite fragments; peloids and quartz silt
alternate in low angle mm-scale cross-laminations with divergent
dip angles (hummocky cross-stratification?)
Burrowed Wackestone/Packstone (0.5-4.0 m) Bedding
Characteristics: Nodular to wavy thin beds ofdoiomitic, mot-
tled limestone; thin quartz siltstone lenses abundant;
occasional skel- etal packstone lenses; pervasively bioturbated
(ii4-iiS); gradational contact with overlying oncolitic-skeletal
packstones; abrupt lower contact with ooid grainstones of
underlying cycle.
Internal Com0osition/Textare: Silt-size finely comminuted grains
dom- inant with floating, randomly oriented echinoderm-trilobite
debris common; clusters of peloids; abundant subangular quartz silt
to fine sand; discrete burrows commonly dolomitized (ferroan);
medium equant dolomite/calcite void-filling cements.
Spiculitic Wackcstone (0.7-2.5 m) Bedding Characteristics:
Ledge-forming, medium to thick-bedded; mot-
tled medium to dark gray; black chert in discontinuous lenses
and nodules; thoroughly bioturbated (ii5-ii6) with ferroan
dolomitized horizontal burrows; skeletal packstone lenses become
more common toward top of lithofacies; gradual lower contact with
underlying nod- ular mudstones and abrupt upper contact with
overlying nodular mudstones.
Internal Composition/Textare: Alternating cm-scale
mudstones/wacke- stones and subordinate lenses and wavy beds of
skeletal packstones; wackestones are strongly burrow-homogenized
(ii5-ii6); sponge spic- ules dominate with common trilobite and
echinoderm debris; pellets associated with burrows; some admixed
quartz silt; packstone lenses have erosive scoured bases and are
composed of unbroken elongate skeletal debris and muddy intraclasts
with occluded shelter porosity and perched peloidal muds; some
grading is evident with lenses with mud drapes at the top; some
gentle micro-cross-laminations ofpeloids with bidirectional
orientations.
consist of a thick basal shale abruptly overlain by upward-
coarsening skeletal wackestone/packstones (Table 4; Fig. 7). These
cycles occur in the Candland Shale, Corset Spring Shale and
Steamboat Pass Members of the Orr Formation of Utah. These cycles
commonly characterize long-term rises in relative sea level that
cause onlap of deep outer ramp siliciclastic facies onto shallow
ramp carbonate fa- cies.
The thick basal shale formed below the zone of storm wave
reworking. Siliciclastic clays accumulated in a dys- aerobic
environment, as indicated by the olive green to dark gray color,
mildly bioturbated laminae, and sparse trilobite and phosphatic
brachiopod fauna. The clays were probably derived from the craton
and were transported across the carbonate belt (perhaps through the
House Range Embayment trough) and onto the deep ramp as dilute
clouds or bottom-hugging nepheloid layers (Board- man and Neumann
1984).
The uppermost carbonate beds of these shale-domi- nated cycles
reflect rapid shallowing from shale to bio- turbated wackestones up
into skeletal packstones. A few of these cycles shallow up to large
(1.5 x 1.5 m) throm- bolitic bioherms that nucleated on flat-pebble
conglom- erate storm beds. The abrupt transition in paleowater-
depths between the deep, quiet water shales (perhaps water depths
of > 40 to 60 m) and the shallow, clear water carbonates
(perhaps water depths between 5 to 20 m) suggests that these cycles
probably did not form by simple aggradation, which would provide a
maximum of only 15 m of shallowing, but rather experienced a
relative sea level rise (shales) followed by relative sea level
fall (car-
bonates) (Osleger 1991). No evidence of subaerial ex- posure of
the skeletal carbonates or the bioherms is rec- ognized, indicating
that sea level never fell below the platform. With renewed relative
short-term sea level rise, carbonate sedimentation ceased and the
skeletal sands or bioherms were abruptly covered with clays
deposited be- low the zone of storm wave reworking.
Shaly cycles capped by flat-pebble conglomerates (0.8- 5.5 m)
consist of a basal calcareous green-brown shale grading upward into
cross-laminated peloidal grainstones and quartz siltstones. The
cycles are capped by amalga- mated flat-pebble conglomerate beds
(Table 4; Fig. 7). They occur in the Nolichucky Formation, Virginia
and Tennessee (Markello and Read 1982), and in Late Cam- brian
strata of central Texas (Osleger 1990), Montana (Sepkoski 1982),
and the southern Canadian Rockies (Aitken 1978).
The Nolichucky cycles record deposition above and below a
fluctuating zone of storm wave reworking in a shallow intrashelf
basin. The Conasauga basin was ad- jacent to the craton and derived
its siliciclastic sediment from distant deltas (Hasson and Haase
1988; Read 1989). The base of storm wave reworking may have been
shallow due to the barrier effect of the peritidal Elbrook platform
to seaward (Markello and Read 1982). Progressive shal- lowing
within individual cycles is indicated by an increase in grain size
and in storm-generated sedimentary struc- tures. The peloidal
grainstone/quartz siltstone lithofacies was deposited under the
influence of oscillatory shear currents as indicated by parallel
lamination and micro- hummocky cross-stratification.
-
EUSTASY AND CYCLE STACKING PA TTERNS OF LATE CAMBRIAN CARBONATES
1233
FIo. 6.-- Deep ramp cycles with spiculitic wackestone caps,
Sneakover Member, Orr Formation, House Range. Basal lithofacies is
composed of argillaceous nodular wackestones and exhibits a
recessive weathering pattern. Overlying ledge-forming cap consists
of spiculitic wackestone with upward-increasing storm-deposited
packstone lenses composed of open marine skeletal debris.
The flat-pebble conglomerate caps of the cycles were deposited
during severe storms that eroded the underlying semi-l ithified
peloidal grainstone and redeposited the rounded, elongate clasts
within tabular to lenticular beds (Sepkoski 1982).
Multi-generational clasts and thin mud drapes that separate
conglomerate beds within amalga- mated units were formed by mult
iple storm events. Di- verse skeletal debris within the matrix
between clasts re- flects normal marine conditions.
MECHANISMS CONTROLLING METER-SCALE CYCLE DEVELOPMENT
Mechanisms proposed to explain the genesis of meter- scale
carbonate cycles have focused on perit idal cycles common
throughout the rock record. The recognition of shallow to deep
subtidal cycles that formed simultaneous- ly with perit idal cycles
requires some modif ication of the mechanisms proposed for the
perit idal cycles. Three models have been suggested to explain the
origin of shal- lowing-upward, meter-scale cycles: 1)
autocyclicity, 2) ep- isodic subsidence, and 3) high-frequency
oscil lations in eustatic sea level. Each of the proposed
mechanisms must explain the upward shallowing of individual cycles,
the repetit ive stacking of similar cycles throughout a vertical
sequence, and the simultaneous development of tidal flat- capped
cycles and subtidal cycles across a carbonate plat- form.
A utocycl icity
The autocyclic model (Ginsburg 1971; Wilkinson 1982; Hardie et
al. 1991) depends upon the periodic progra- dation of tidal flats
over the subtidal carbonate factory to restrict the size of the
carbonate source area, effectively
SHALE-BASED CYCLES INTRASHELF BASIN
i 3WB/
FIAT-PEBBLE CONGLOMERATE SHALY CYCLE
DEEP SHALY RAMP
KEY TO LITI'-IOLOGIES '~ -~ ,SKELETN.
PACK.STONE
~ FLAT PEBBLE CONGLOMERATE
l ~ PELOIDAL PACK/GRNNSTONE
BURROWED WACKESTONE
GREEN-BROWN SHALE
SL B
iiii!iiiiiiSWB !i!i!ii
SKELETAL PACKSTONE SHALY CYCLE
FIG. 7. - - Late Cambrian shaly cycles of the Conasauga
intrashelfbasin of the Appalachians and of the Cordilleran deep
ramp of Utah. Siliciclastic shales are abruptly overlain by
"clear-water carbonates" with storm-deposited caps. Note the
possible shallower position of storm-wave base in the protected
intrashelf basin.
-
1234 DA bTD OSLEGER AND J. FRED READ
TABLE 4.--Shaly deep ramp/intrashe~'basin lithofacies
Flat Pebble Conglomerate (0.1-0.6 m) Bedding Characteristics:
Amalgamated irregular thin beds and lenses
cap coarsening-upward cycles; scoured bases common into
underlying peloidal grainstones and quartz siltstones; elongate
clasts imbricated to edgewise to random orientations; mud drapes
separate individual beds within amalgamated units; matrix includes
skeletal debris and glauconite; abruptly overlain by shale
(Noliehueky) or peloidal silt- stone (Point Peak) lithofacies.
Internal Composition/Texture: Elongate rounded clasts typically
com- posed of laminated peloidal grainstone and quartz siltstone of
un- derlying lithofacies; clasts less commonly composed of skeletal
pack- stone (Nolichucky) or micritic-spiculitic (Point Peak); many
clasts have iron-stained rinds and are often bored; matrix between
clasts consists of peloids and skeletal debris (abundant
brachiopod-trilobite- echinoderm in Point Peak); quartz silt
common; some occluded shel- ter porosity and perched peloidal
muds.
Peloid Grainstone/Quartz Siltstone (0.4-2.0 m) Bedding
Characteristics: Interlaminated calcareous siltstones and silty
peloidal grainstones in thin irregular beds; internal parallel
and hum- mocky cross-lamination; typically grades from quartz silty
toward the base to dominantly peloidal grainstones at the top of
the lithofacies; lower contact with shale lithofacies (Nolichucky
Fm only) begins with very thin siltstone beds intercalated within
the shale eventually be- coming pure calcareous peloidal siltstone;
this facies forms the base of the cycles in the Point Peak where
they are platy bedded and. less well-cemented but otherwise
identical to the Nolichucky laminated peloidal quartz siltstones;
Cruziana trace fossils; coarsens upward into amalgamated flat
pebble conglomerates or, less commonly, skeletal- ooid
packstones.
Internal Composition/Texture: Peloids and quartz silt are
dominant grain types with subordinate finely-comminuted skeletal
debris; some laminae show very fine normal grading; fine skeletal
debris--domi- nantly echinoderms and trilobites.
Calcareous Shale (0.5-10.0 m) Bedding Characteristics: Olive
green to dark gray fissile shale; breaks into small chips upon
separation suggesting bioturbation; random, very
thin lime mudstone beds increase in frequency upward in the
lithofacies; overlain by calcareous quartz siltstone lithofacies
(Nolichucky) or thin nodular wackestone (Candland and Corset
Springs); abruptly overlie flat pebble eonglomerate/skeletal-ooid
grainstone lithofacies or thrombolitic bioherms.
Internal Composition/Texture: Composed of clay-sized micas and
associated clay minerals as well as calcareous micropeloids;
occasionally fine trilobite and phosphatic brachiopod
fragments.
shutting down carbonate production until tectonic sub- sidence
recreates broad shoal-water areas. Implicit in the model are the
assumptions of static sea level over tens to hundreds of thousands
of years and complete shoaling to tidal levels. Weaknesses in this
model are the inordi- nately long lag times (> 20 ky) necessary
for the creation of water depth sufficient to resume carbonate
production and the assumption of complete non-deposition over tens
of thousands of years (Grotzinger 1986b; Koerschner and Read 1989;
Read et al. 1991). Perhaps the biggest draw- back to autocyclic
control is the simultaneous develop- ment of purely subtidal cycles
that, by definition, have no progradational tidal fiat cap that
could influence the shrinking of the carbonate factory (Grotzinger
1986b). The inability of the autocyclic model to explain incom-
plete shallowing of subtidal cycles that develop seaward of
peritidal cycles precludes it as a potential controlling process on
Late Cambrian cycle development.
Other autocyclic models invoke 1) the lateral migration of tidal
channels to produce shallowing-upward peritidal cycles (Cloyd et
al. 1990) and 2) autocyclic responses to "sediment production,
tidal variations, and wave and storm activity" to explain the lack
of lateral correlatability of Cambro-Ordovician pefitidal cycles in
eastern Ten- nessee (Kozar et al. 1990). As with the progradational
model (Ginsburg 1971), variations in sediment accu- mulation and
redistribution cannot explain the origin of regional subtidal
cycles, but may contribute to variability in the internal
composition of individual cycles (Osleger 1991). Autocyclic
mechanisms may only be viable as an explanation of stratigraphic
"noise" within individual cy- cles but probably do not control the
development of re- petitive stacks of cycles or the synchronous
development of pefitidal and subtidal cycles on Late Cambrian plat-
forms.
Episodic Subsidence
Repeated pulses of downfaulting have been proposed (Hardie et
al. 1986; Cisne 1986) to generate abruptly the accommodation
potential for asymmetric cycle devel- opment. If the stress limits
between faulting episodes were rhythmic based on some threshold
value, then this model could conceivably explain the coexistence
ofpefitidal and subtidal cycles. However, the lateral extent of
such events would be limited and could not explain the widespread
nature of carbonate cycles across entire platforms (e.g., Demicco
1985; Grotzinger 1986a; Hardie and Shinn 1986). Additionally,
modern examples of tectonic pulsing (Yeats 1978; Bull and Cooper
1986; Atwater 1987) are restricted to tectonically active settings,
poor analogs for ancient mature passive margins such as existed
during Late Cambrian time. Other tectonic mechanisms such as
intraplate stress (Cloetingh 1986; Karner 1986) are too slow
(0.01-0.1 m/ky) and non-periodic to produce high- frequency
meter-scale cycles. It seems hard to conceive of repeated tectonic
pulses (each 20 to 200 ky duration) over millions of years to
produce repetitive cycles (all within a fairly narrow range of
thicknesses) on mature passive margins.
Eustatic Oscillations
High-frequency oscillations in sea level, probably con- trolled
by fluctuations in glacial ice volume, provide the simplest
explanation for the origin of meter-scale peritidal and subtidal
cycles (Fischer 1964; Matthews 1984; Good- win and Anderson 1985;
Goldhammer et al. 1987; Koerschner and Read 1989; numerous others).
Consid- ering the evidence for eustatic control on third-order se-
quence development (Vail et al. 1977; Haq et al. 1987;
-
EUSTAS Y AND CYCLE $724 CKING PATTERNS OF LATE CAMBRIAN
CARBONATES 1235
Ross and Ross 1988; Osleger and Read, unpublished data), it
seems likely that higher frequency sea-level fluctuations were
superimposed on the longer-term sea level events and, by
association, were also eustatic in origin. Super- imposed orders of
eustatic sea-level oscillations (com- posite eustasy, Goldhammer et
al. 1990) provide the best explanation for the upward shallowing of
individual cy- cles, stacking patterns with cyclic successions, and
the simultaneous development of peritidal cycles and subti- dal
cycles across carbonate platforms.
Although it seems clear that sea level fluctuated eustati- cally
to generate individual meter-scale cycles as well as stacked cyclic
successions, the forcing mechanism behind high-frequency sea level
oscillations is far from certain. It has been proven that
Plio-Pleistocene sea levels fluc- tuated in response to variations
in global ice volume as a function of changes in solar insolation
forced by Mil- ankovitch astronomical rhythms (e.g., Hays et al.
1976; Berger 1977). Temporal association with continental gla-
ciations has made glacio-eustasy and Milankovitch or- bital forcing
probable as a cause of the Permo-Carbon- iferous cyclothems
(Wanless and Shepard 1936; Heckel 1986). It has been more difficult
to make a case for Milan- kovitch control on stratigraphic
cyclicity in ancient rock sequences deposited during times of more
equable cli- mates and no known major glaciations. Van Houten
(1964), Olsen (1986) and Anderson (1986) used varve- calibrated
sedimentation rates to show Milankovitch pe- riodicities for rocks
of Triassic and Permian age. Schwarz- acher and Fischer (1982),
Schwarzacher and Haas (1986), and Goldhammer et al. (1987) used a
5: ! recurrence ratio of meter-scale cycles within megacycles,
representing the precession signal modulated by the short
eccentricity sig- nal, as evidence for Milankovitch control of
Mesozoic cycles. Borer and Harris ( 1991) recognized a 4:1 ratio
for Permian cycles and suggested that the bundling mani- fested 100
ky short eccentricity cycles superimposed with- in the 400 ky long
eccentricity cycle.
Other attempts at showing a Milankovitch influence on ancient
cyclic sequences have depended upon the av- erage periodicities of
cycles that roughly coincide with the range of Milankovitch periods
of 19-23 ky, 41 ky, 95-123 ky, or 413 ky. It has been recognized
that the periods of precession and obliquity signals have changed
through geologic time due to changing earth-moon rela- tionships,
whereas the long and short eccentricity cycles probably have
remained constant through time since they are based on
interplanetary gravitational forces (Walker and Zahnle 1986; Berger
et al. 1989). The ranges of vari- ance (the 21 ky precession signal
may have approached 17 ky and the 41 ky obliquity signal may have
approached 28 ky during the Early Paleozoic) are insignificant when
compared to the large errors associated with absolute age dates for
Early Paleozoic rocks. As cautioned by Hardie and Shinn (1986) and
Algeo and Wilkinson (1988), cal- culations of average cycle period
within the Milankovitch band are to be expected for meter-scale
cycles and are insufficient evidence for orbital control on cycle
forma- tion.
An objective way of determining cycle periods is by
spectral analysis of cyclic successions where dominant
periodicities can be extracted and ratios between the pe- riods can
be used to establish Milankovitch control (e.g., Schwarzacher and
Fischer 1982; Herbert and Fischer 1986; Kominz and Bond 1990).
However, spectral anal- ysis is particularly difficult for shallow
platform carbon- ates of Early Paleozoic age for the following
reasons. 1) Peaks on the power spectra are difficult to calibrate
since long-term accumulation rates used to convert thickness per
cycle to time per cycle are dependent upon the ra- diometric time
scale with its large uncertainties (Fig. 2). 2) The assumption of
constant sediment accumulation throughout the duration of the
cyclic succession is un- likely due to differential sedimentation
rates for different lithofacies (Kominz and Bond 1990) and the
effects of long-term changes in sea level. 3) "'Missed beats" are a
common phenomenon of shallow platform carbonates (Hardie and Shinn
1986; Koerschner and Read 1989; Goldhammer et al. 1990), resulting
in a noisy spectrum. 4) Peritidal cyclic successions are a poor
proxy for time, because much of the cycle period is taken up by
non- deposition (Read et al. 1986; Read et al. 1991). Spectral
analysis of Early Paleozoic shallow platform carbonates may only be
viable in conjunction with techniques for deriving better time
series such as gamma analysis (Ko- minz and Bond 1990), a method
that deserves further testing.
Late Cambrian Eustasy and Milankovitch Rhythms
Evidence from the Cordilleran passive margin suggests that
Milankovitch orbital variations may have controlled low-amplitude
glacio-eustatic fluctuations during the Late Cambrian. In the Big
Horse Member of the Orr For- mation of the House Range, meter-scale
fifth-order cycles are stacked into shallowing-upward successions
at the fourth-order scale as well as at the third-order scale (Fig.
8). The Big Horse Member comprises the upper portion of one
long-term third-order shallowing-upward sequence (220 m thick;
approximately 4.8 m.y. duration). The long- term sequence is
composed of stacked deep ramp cycles in the lower portion gradually
shoaling up to stacked shallow ramp cycles with large thrombolite
bioherrns marking the top of the sequence. This third-order se-
quence has superimposed within it 11 fourth-order de- positional
cycles (l 5-45 m thick; average of ~ 440 ky) that are typically
composed of three to four fifth-order cycles (0.5-8.0 m thick;
average of ~ 96 ky). The low- ermost fifth-order cycle within each
fourth-order bundle is typically the thickest and is dominated by
deeper water lithofacies. The fifth-order cycles gradually thin
upward and shallow upward within each fourth-order bundle.
The 4:1 bundling is illustrated in a mirror plot of de- viations
from average cycle thickness within the Big Horse Member (Fig. 9).
The mirror plot is simply a tracing of a Fischer plot of the Big
Horse Member (discussed below) and was created to accentuate the
bundled nature of the cycles. Assuming that the estimate of
long-term accu- mulation rate (0.046 m/ky) derived from the DNAG
time scale is reasonable, the 4:1 bundling may manifest the
-
1236 DA VID OSLEGER AND J. FRED READ
250
On"
I ,~'] I o . j ~o ,
BIG HORSE MEMBER Big Horse Member ORR FORMATION House Range,
Utah
180 o OoO . ~ - -
,o~oo- ~ j 4
170-, '~" [ , ~1 ! A! - _' 2. " M, ~o*O=O
- - B
- _~ ? ~ "~ 3O
Fie. 9.--Mirror plot of deviations from average cycle thickness
in the Fio. 8.--Hierarchy of cycles within the Big Horse Member,
Orr For-
mation, House Range, Utah. Column on the left shows long-term
third- order shallowing evident from the storm-influenced deep ramp
cycles with open marine faunas in the lower Big Horse progressively
giving way to shallow subtidal cycles characterized by restricted
lithofacies upward in the Big Horse Member. Dashes to the right of
the left column denote generalized fourth-order cycles that are
shown in derail in the columns on the right. Composition of the
fourth-order cycles suggests rapid deepening in the basal cycle
followed by progressively shallower conditions toward the upper
cycles. Note the 4:1 bundling of fifth-order cycles within
fourth-order sets.
short eccentrically (95-123 ky) to long eccentricity (413 ky)
ratio. The bundles exhibiting a 3:1 ratio may simply have missed a
cycle beat, perhaps as a low ampl i tude sea- level event oscil
lated above the platform with no apparent sedimentologic
response.
No evidence can be recognized within the fifth-order
eccentricity cycles for superimposed cyclicity that may represent
the precession or obl iquity signals. Because the Cordil leran
passive margin extended essentially E -W at about 10 to 15*N during
the Late Cambrian (Scotese and McKerrow 1990), the lack of a 41 ky
obliquity cycle is to be expected because the effect of changing
axial tilt is
Big Horse Member, Orr Formation, House Range. The plot is simply
a tracing of the Fischer plot shown in Figure 13 and oriented
vertically. Fourth-order bundles are marked by the abrupt
appearance of thick basal cycles over thinner cycles below.
minimal toward low latitudes (Berger 1978). The appar- ent lack
of a precession signal may be due to a number of interrelated
factors. 1) The relative ampl itudes of the precession-generated
sea level signals may have been low compared to those of the
dominant eccentricity cycle. 2) Perhaps the deposit ional setting o
f the Cordi l leran shal- low to deep ramp was not a sensitive
enough recorder of each individual sea level event. Only the
higher-ampli- tude ~ 100 ky sea-level event may have caused a sedi-
mentologic response on the carbonate platform in the vicinity of
the House Range. 3) Phase relations of the interacting Mi
lankovitch frequencies may have sup- pressed the precession signal
due to destructive interfer- ence. It seems likely that at certain
t imes in the geologic past, constructive interference has acted to
enhance the individual Mi lankovitch frequencies and, conversely,
de- structive interference has acted to mask the Mi lankovitch
frequencies. Phase relations may provide a partial expla- nation of
cyclic successions with no evidence of bundling
-
EUSTASY AND CYCLE ST.4CKING PATTERNS OF LATE CAMBRIAN CARBONATES
1237
or for weakly cyclic intervals within an overall strongly cyclic
section.
Glacio-Eustasy During the "Non-Glacial" Late Cambrian
The connection between Milankovitch orbital varia- tions, the
shrinkage and growth of continental ice sheets and eustasy has been
well-documented (e.g., Berger et al. 1984). However, a direct link
between changes in solar insolation related to Milankovitch
astronomical rhythms and changes in sea level and sedimentation
during glob- ally warm periods of Earth history has yet to be found
(Barron et al. 1985). To account for the low to moderate amplitude
(perhaps 15-25 m based on 2-D modelling) sea-level oscillations
proposed to simultaneously generate Late Cambrian peritidal and
subtidal cycles, a sink for the storage and release of moderate
volumes of seawater needs to be identified.
Paleogeographic reconstructions for the Late Cambrian place most
continental land masses between 60N and S latitudes (Scotese and
McKerrow 1990). Only Baltica and the southern margin of Gondwana
extend into higher southern latitudes where climates may have been
signif- icantly cooler than the generally warm global climate.
Ziegler et al. (1981) have suggested that the paleogeo- graphic
configuration of the continents during the Cam- brian facilitated a
latitudinal zonation of prevailing winds and ocean currents within
the high latitudes that may have reduced the absorbtion of solar
radiation, enhancing the possibility of cooler Cambrian climates
than previ- ously believed. Additionally, climate modelling of pre-
sumably warm periods of Earth history suggest that the interiors of
mid- to high latitude continents may have had subfreezing
temperatures and that no global climate is truly "equable" (Sloan
and Barton 1990). Even though no major large-scale continental
glaciers existed during the Late Cambrian, diamictites and striated
cobbles have been reported in lower Tremadocian strata of Argentina
and Bolivia (Erdtmann and Miller 1981) which were lo- cated in a
part of Gondwana believed to have experienced cool climates during
the Late Cambrian-Early Ordovician (Scotese and McKerrow 1990).
Alpine glaciers may have been present in ancestral mountain belts
of continental interiors of major land masses and provide a
possible sink for small portions of the 20 (_+5)-meter sea-level
oscillations estimated for the Late Cambrian meter-scale cycles.
However, the apparent absence of a reservoir large enough for the
rapid storage and release of moderate vol- umes of seawater remains
a major weakness in the con- nection between Milankovitch orbital
variations and Late Cambrian meter-scale cyclicity.
STACKING PATTERNS OF METER-SCALE CYCLES
Characteristic meter-scale fifth-order cycles system- atically
change upward within third- and fourth-order sequences and define
distinct stacking patterns. Cycle stacking patterns provide the
crucial link between the
20
1
og lo ._>
Average Cycle Duration Path of Relative
I , . ~ C, hange in Sea
Subsidence
100 200 300 ~0 T~e (ky)
FiG. 10.--Explanatory diagram of the Fischer plot technique. The
horizontal scale of the plot represents time and the vertical scale
is the cumulative cycle thickness in meters. For each cycle the
amount of accommodation space provided by linear subsidence is
plotted over the duration of the average cycle period. Cycle
thickness is plotted vertically. The net difference can be
interpreted to define the change in accom- modation space through
time.
meter-scale cycles and the larger scale sequences and their
component systems tracts. Stacks of genetically related cycles are
the parasequence sets of sequence stratigraphic terminology.
Fischer plots (Fig. 10) illustrate deviations from av- erage
cycle thickness throughout a stratigraphic interval. They can be
interpreted as graphic displays of relative changes in
accommodation space through time (Fischer 1964; Goldhammer et al.
1987, 1990; Read and Gold- hammer 1988; Read 1989). Each
fifth-order cycle is as- signed an average cycle duration by
dividing the total estimated duration of the cyclic succession by
the number of meter-scale cycles. This average cycle duration is
mere- ly a device for assigning time per cycle and does not imply
that each cycle was actually deposited over the same du- ration.
The horizontal axis could just as easily be divided into equivalent
units that equal "cycle number". If the plot was constructed so
that cycle thickness equaled time, the resulting plot would define
a horizontal line. Thus it is necessary to assign a constant time
of deposition to each cycle to generate relative rises and falls on
the plot.
Interpretation of individual Fischer plots should only be made
in unison with temporally equivalent Fischer plots that show
similar patterns of rises and falls (Koerschner and Read 1989; Read
et al. 1991). Fischer plots of Late Cambrian cyclic strata have
been correlated between the Cordilleran and Appalachian sections
and provide excellent evidence for eustatic control on third- order
sequence development (Osleger 1990; Osleger and Read, unpublished
data). The correlated plots suggest that stacks of thick cycles
plot as positive slopes and are pre- sumed to have formed under
conditions of increased ac- commodation space provided by relative
sea-level rise. Stacks of thin cycles plot as negative slopes and
are pre- sumed to reflect reduced accommodation space during
-
1238 DA I,'ID OSLEGER AND J. FRED READ
m
15
14.1 > W - J
Ut W >
5
CONOCOCHEAGUE FORMATION KEY TO LITHOFACIES ~- - - - - CRYPTALGAL
LAMINITE
WYT.EWt.LE. VIRG,NIA M,N,T I I ~ 'X I"" ~ ~7 RIBBON ROCK ,~I ~ ~
l~7.~"~,~q THROMBOLITE BOUNDSTONE
O-
10
5 t '~ , ,O ,1
-:2 ~ _ Om
m
L_J
- - -W'--w--
- -7 - - - v
,X~, :o , ' , . oJQ=
I ~ ~s~' l - -
0 m -~.~. .~. .~_~j __ 0m
A) SUBTIDAL-DOMINATED B) LAMINITE-DOMINATED C) PERITIDAL CYCLES
PERITIDAL CYCLES PERITIDAL CYCLES WITH QUARTZ SAND
F]G. 11 .--Fischer plot of the Conococheague Formation
constructed from the Wytheville, Virginia section (from data in
Koerschner and Read 1989). Cycles containing quartz sand are black.
Stacking patterns of representative cycles are shown pulled out
from their position on the Fischer plot. Note the difference in
scales between the three columns of cycles and how the
subtidal-dominated cycles are considerably thicker than the
peritidal-dominated cycles.
relative sea level fall. The method seems to be best suited for
peritidal cycles or subtidal cycles that shallow to near sea
level.
Fischer Plots and Peritidal Successions
The relationship between cyclic peritidal carbonates of the
Conococheague Formation of southwestern Virginia and long-term
relative sea-level events defined by its Fi- scher plot (Koerschner
and Read 1989) is shown in Figure 11. The Conococheague Formation
is composed of hun- dreds of stacked peritidal cycles that record
periodic, high- frequency fluctuations in relative sea level
(Demicco 1985; Koerschner and Read 1989). The Fischer plot defines
a major relative sea-level rise and fall within this portion of the
Conococheague Formation. Stacking of thick, sub- tidal-dominated
cycles with thin laminite caps occurs during the rising portions of
the plot (Fig. 11, column A).
Stacking of thin, laminite-dominated cycles occur during falling
portions (Fig. 1 1, column B). Brecciated cycle caps and quartz
sands become common toward the troughs on the plot (Fig. l 1,
column C).
Similar cycle stacking patterns are exhibited in the peri- tidal
Allentown Formation of eastern Pennsylvania (Fig. 12). During
long-term rises on the plot, cycles are thick with oolitic bases
and thin stromatolitic caps (Fig. 12, column B). Ooids have dropped
cores and cycle caps are brecciated indicating meteoric diagenesis
during episodic short-term emergence. During long-term falls on the
plot, cycles show thin oolitic transgressive lags overlain by
thrombolites that grade up into LLH stromatolites and cryptalgal
laminites (Fig. 12, columns A and C). The caps o f many cycles are
marked by regolithic breccias devel- oped on the emergent tidal
flat. Toward the troughs on the plot (Fig. 12, column C),
erosionally-capped cycles contain quartz sand.
-
E I~S'TAS Y AND ( 'YCLE STACKING PA TTERNS OF LATE CAMBRL4N
CARBONATES 1239
>
<
<
KEY TO LITHOFAGIES ALLENTOWN FORMATION ~ R~.~X~ e^P .
I" "- ' " ; " "I OOMOG~SrONE /~I
" N ""s
' _ . . .
B) SUBTIDAL-DOMINATED PERmDAL CYCLES
o-
lo-
20-
A) TIDAL FLAT-DOMiNATED PERmDAL CYCLES
\
3- T:~ " "~~.~ -
. . . . " " ' ' ' ' " O
". :4 : . I Om C) PERmDAL CYCLES
WITH QUARTZ SAND
FIG. 12.--Fischer plot of the lower Allentown Formation
constructed from the Easton, Pennsylvania section with cycle
slacking patterns expanded from their position on the plot. Small
dots below individual cycles on Fischer plot denote regolithic
cycle caps. Note the variation in scales between the three
intervals and the relative thicknesses of the component cycles.
Oolitic grainstone bases of cycles on the rising portions of the
Fischer plot are considerably thicker than those on the falling
portions. Tidal flat caps are considerably thinner on cycles that
formed during the relative sea-level rise but dominate in the
cycles that formed on the relative sea level fall.
The Conococheague and Allentown Fischer plots il- lustrate the
significant difference in cycle thickness and lithofacies
composition between stacks of cycles gener- ated during rising and
falling relative sea level. Peritidal cycle thickness is controlled
by the total amount of ac- commodation space provided by subsidence
and eustasy. For these peritidal cycles, stacks of thicker cycles
were formed during relative sea level rise that generated ac-
commodation space beyond that provided by subsidence. Stacks of
thinner cycles were formed during relative sea level fall that
reduced accommodation space provided by subsidence. Quartz sands
were brought in and brecciated laminite caps were developed during
relative sea-level lowstands that exposed craton interiors and the
inner platform. Assuming relatively constant tectonic subsi- dence,
the control on the long-term changes in relative
sea level is believed to be eustasy, on the basis of cor-
relation of the Fischer plots above with equivalent sec- tions in
the Appalachians and Utah (Osleger and Read, unpublished data).
Fischer Plots and Subtidal Successions
Stacking patterns of dominantly subtidal cyclic succes- sions
and their relationship to Fischer plots are shown on Figure 13. The
Fischer plot of the upper Big Horse and Candland Shale Members of
the Late Cambrian Orr Formation of the House Range shows stacks of
deeper subtidal cycles on the rising segments of the plot. These
stacks of genetically-related cycles are characterized by
storm-dominated carbonate cycles (Column A) or thick, deep ramp,
shale-based cycles (Column C). Falling por-
-
1240 DAVID OSLEGER AND J. FRED READ
ORR FORMATION, HOUSE RANGE, UTAH
20-
. J uJ > lO-
w W 0-
~ -10 W
-20
10 ~ ~ I O ' - -
Om ,
A) STORM-DOMINATED DEEP RAMP CYCLES
(BIG HORSE)
KEY TO LffHOFACIES * * ~ ' ,~)OOID-ONCOLITE GRST
I ~ ~/ SKELETALPKST
[ . ~ BURROWED WKST Lil~mlml/ A RGI LLACEOUS WKST ~ O L I V E
GREEN SHALE
= T IME >
tO - e .~. . __ . .~ . . " J
E m
o o o~
/ 0m
B) SHALLOW SUBTIDAL CYCLES WITH OOID GRAINSTONE CAPS (BIG
HORSE)
C) SHALY CYCLES WITH SKELETAL STORM BED CAPS (CANDLAND
SHALE)
Fxo. 13.--Fischer plot of the upper Big Horse and Candland Shale
Members of the Orr Formation, House Range, Utah. The scale is the
same for all three stacks of cycles; note the thin shallow subtidal
restricted cycles versus the substantially thicker, deeper
subtidal, open marine carbonate and shaly cycles.
tions of the Fischer plot are characterized by thin, oolite
grainstone-capped cycles (Column B). Common lithofa- cies within
these shallow ramp cycles are oncolitic pack- stones-grainstones,
ooid grainstones, thrombolite bio- herms and SH and LLH
stromatolites, all indicative of shallow, restricted
conditions.
Fourth-order cycles on the Fischer plot of the Big Horse Member
occur as bundles of one thick cycle followed by two to three
thinner cycles (Figs. 8, 9 and 13). The mirror plot of Figure 9 was
created from the Fischer plot of Figure 13 and better illustrates
the 4:1 bundling of meter-scale cycles within the Big Horse Member.
Thicknesses for these fourth-order cycles range from 15 to 45 m and
their average duration is ~ 440 ky. Cycles on the fourth-order
rises are consistently composed of thick cycles dominated by deep
subtidal lithofacies whereas fourth-order falls are consistently
composed of thin cycles dominated by shal- low subtidal lithofacies
(Fig. 8). This is essentially the same stacking pattern recognized
within the third-order sequence only repeated over shorter time
increments.
The systematic arrangement of similar subtidal cycles on rising
and falling limbs of Fischer plots suggests that,
like the peritidal cycles, they record changes in accom-
modation space generated by third-order relative sea-lev- el
fluctuations. This suggests that time-equivalent suc- cessions of
meter-scale peritidal or shallow subtidal cycles may be correlated
using Fischer plots and combined to define third-order, and perhaps
fourth-order, sea-level events. I fa good degree ofcorrelatibility
can be attained between geographically distinct sections, then the
long- term fluctuations may be considered to have been eustatic in
origin (Osleger and Read, unpublished data).
Effect o f Sedimentation Rate on the Form o f Fischer Plots
Some deeper subtidal successions of cyclic carbonates show
Fischer plots whose trend is opposite to that ex- pected from the
above examples. Within the Notch Peak Formation of Utah, a thick
succession of shallow subtidal cycles capped by thrombolite
bioherms shallows to tidal depths before grading up into a series
of deeper water cycles capped by spiculitic wackestone (Fig. 14).
Using stacking patterns established from other similar cycle
-
EUSTASY AND CYCLE STACKING PA TTERNS OF LA TE CAMBRIAN
CARBONATES 1241
,< W Q. ~
O z ~ -
i - -
~ - ~ C ~ _
A A A ~
: t - - DEEP RAMP, m SPICULITIC -- WACKESTONE -- CYCLES
PERITIDAL _ CYCLES 7
- 7 r
- \ \ \ \
\ \
THROMBOLITE- ! _ STROMATOLITE
BOUNDSTONE / CYCLES "3
WJ
m
DEEP TIDAL SUBTIDAL FLAT \
\
/ /
c \ 45 30 15 0'm RELATIVE "~
WATER / CUMULATIVE CYCLE THICKNESS DEPTH
FIG. 14.--Vertically-oriented Fischer plot of the upper Notch
Peak Formation, House Range, Utah. Dashes to right of strat column
indicate cycle tops. Groups of like cycles are noted with arrows.
Key horizons are connected to the Fischer plots by the dashed
lines; they are not perfectly horizontal because the stratigraphic
column is in thickness and the Fischer plot is in time. Interpreted
paleowater depth curve to the fight is shown for comparison.
-
1242 DAVID OSLEGER AND J. FRED READ
A) MODEL SEA LEVEL CURVE \ ^
'r
m 0 100 200 Q
I- EMERGENT .~ WATER SUPRATIDAL SURFACE
DEPTHS / AGGRADING I EO,MENT ,.SEA EVE
~ ' ~ , ~ \ \ ' ~ " ~SHALLOW SUBTIDAL " ~ ' ~ . . . ~ " 'TIDAL
FLAT I:ACiES . . . . FACIES
~" -SUBSIDENCE I I
1 O0 200 TIME (KY)
FIG. 15A.--Explanatory diagram of the 1-D modeling. The sea
level curve is composed of in-phase symmetrical 20 and 40 ky
periods and asymmetrical 100 ky periods superimposed on a long-term
sea-level rise/fall. Any combination of amplitudes of sea level
cycles can be input and define the vertical axis. Sloping lines to
lower right represent linear subsidence of deposited sediments
through time. The lines sloping to the upper right represent the
aggrading sedment surface and changes in slope reflect differing
sedimentation rates of water depth-dependent lithofacies. The
period of non-deposition following drowning is a pre- determined
lag time.
types, one would expect the Fischer plot of the restricted
thrombolitic cycles to be associated with a long-term fall in sea
level, whereas the plot of the deep water cycles would be expected
to form a long-term rise in sea level. However, the thick
thrombolitic cycles plot as a positive slope on the Fischer plot,
whereas the thin spiculitic wackestone cycles plot as a negative
slope.
One reason for this counterintuitive result may be that the
relative thicknesses of subfidal cycles are controlled by
sedimentation rate rather than by sea-level-determined
accommodation space. Thick thrombolitic cycles may have accumulated
rapidly within the zone of optimal car- bonate productivity,
rapidly filling to near sea level. In contrast, the thinner,
argillaceous, deeper water cycles may simply have accumulated
slowly. The resulting trend on the Fischer plot is an apparent
long-term rise and fall in sea level generated by
water-depth-dependent sedi- mentation rates of the cycles rather
than by sea-level- controlled accommodation space.
A second possible interpretation is that the thickness of
subtidal cycles may indeed be controlled by accom- modation space
but that the upper limit to vertical ag- gradation may be the base
of normal fairweather or storm- wave reworking rather than tidal
level (Osleger 1991). Subtidal cycle thickness could be limited by
reworking and redistribution of sediment when the depositional sur-
face intersects an energy barrier associated with a zone of active
wave or storm-current winnowing. The deep ramp cycles of the upper
Notch Peak are capped by thin lenses of skeletal packstone storm
beds, which suggests
that the base of storm reworking may have precluded any further
vertical aggradation. Consequently, the resulting cycles are thin
and define an anomalous negative slope on the Fischer plot of
Figure 14.
This example illustrates that Fischer plots of mixed cycle types
must not be interpreted alone but should be used in conjunction
with time-equivalent plots to deter- mine their value. Caution must
be exercised when inter- preting the form of individual Fischer
plots of subtidal cycles without looking at the internal
composition of the cyclic succession as well as at other plots of
coeval inter- vals.
MODELLING OF CYCLE STACKING PATTERNS
One- and two-dimensional computer modelling are valuable
techniques for assessing the effects of controlling variables on
the generation of cyclic sequences and for testing the feasibility
of models related to the origin of meter-scale cycles.
One-dimensional models (Read el al. 1986) graphically track the
simultaneous interaction of eustatic sea level, the sediment
surface, and long-term subsidence to generate synthetic
stratigraphic columns. Two-dimensional models (Koerschner and Read
1989; Read et al. 1992) integrate a more sophisticated set of
parameters to produce synthetic geologic cross-sections that
simulate the vertical and lateral facies distribution of cycles and
the internal geometry of longer-term de- positional sequences. The
model types can be used to complement one another by showing the
simpler concepts with the one-dimensional modelling and then
reproduc- ing a more detailed, more realistic simulation of actual
stratigraphic data using the two-dimensional modelling.
The one-dimensional models (Read et al. 1986) incor- porate
linear long-term subsidence, simplified sea-level curves composed
of high-frequency in-phase oscillations superimposed on a
longer-term rise/fall, water-depth-de- pendent sedimentation rates
of lithofacies, and lag time after flooding to produce synthetic
stratigraphic columns (Fig. 15). The two-dimensional models (Read
et al. 1992) (Fig. 16) incorporate many of the same variables as
the one-dimensional models but with significant refinements.
Antecedent topography and platform slope is digitized before the
program runs and is constrained by modern analogs of carbonate
platform morphologies. The model divides the platform into 200
localities whose increment width varies with the pre-determined
length of the plat- form. Tectonic subsidence is separated into
regional and rotational components, and isostatic subsidence is
cal- culated for each time slice to account for sediment and water
loading. The synthetic cross-sections produced are truly
two-dimensional because the isostatic response to sediment and
water loading at single localities affects ad- jacent localities
along the elastic beam 200 km on either side (Read et al.
1991).
The form of the eustatic sea-level curve can be gen- erated by
any combination of high-frequency, asymmet- ric or symmetric sine
waves superimposed on a long-term sine wave or digitized curve. The
input values for the sea-level curve allow for any combination of
cycle periods
-
EUSTASY AND CYCLE STACKING PA TTERNS OF L4 TE CAMBRIAN C4RBONA
TES 1243
PERITIDAL CYCLES ~. 40- ~! E J 30- UJ > LU 20- .2
m 10" '~" . . . . . . . . . . ' - 03 I . . . . . . . . . . .
~---
-
1244
0rn A
DA V1D OSLEGER AND d. FRED READ
PERITIDAL PLATFORM B
_ _ I
SU BTIDAL PLATFORM
20 t V.E.=5000
40 Okra 160 260
C
D I
E 30 v ._1 I..U > 20- uJ
< 10- uJ
0 200 400 600 800
TIME (k.y.)
KEY TO LITHOFACIES I~1 TIDAL FLAT I I SHALLOW SUBTIDAL
DEEP SUBTIDAL DEEPEST SUBTIDAL
FIo. 16A.--2-D model ofa peritidal to subtidal transition across
a hypothetical platform. Water depths and sedimentation rates of
facies are the same as in the i-D models of Figure 15. Amplitudes
of the sea level oscillations are also the same as in the I-D
models but the periods have been input as 19, 23, 41 and 100 ky and
allowed to interfere to produce the complex sea level curve in the
inset. Initial slopes on the peritidal platform are < 0.01 m/kin
and are ~ 0.04 m/km on the subtidal platform, comparable to modem
carbonate platforms. The apparent abrupt break in slope around 550
km is an artifact of the vertical exaggeration (5000) and
translates to 0.2 m/kin or a fraction of a degree. Rotational
subsidence at the outer edge of the platform is 0.015 m/ky.
Duration of the run is 800 ky and time lines are denoted every 200
ky.
posed upon a third-order "driver" (Goldhammer et al. 1990). The
lithologic composition of the synthetic cycles was highly irregular
and totally unlike Late Cambrian cycles observed in the field.
Narrowing the range of ran- dom periods alleviated the problem to
an extent, but stratigraphic trends of thickening and thinning
cycles were absent. The experiments suggest that periods of Late
Cambrian sea-level oscillations were constrained within relatively
narrow ranges.
Previous discussion of the origin of meter-scale cycles has
shown composite eustasy to be the most likely mech- anism
controlling cycle development. The 4:1 bundling of Late Cambrian
cycles in the Big Horse Member sug- gests sea-level control by
Milankovitch orbital variations. On the assumption of
Milankovitch-forced glacio-eu-
stasy, sea-level fluctuations with cycle periods of 19, 23, 41
and 100 ky were used in the modelling.
Amplitudes of Relative Sea Level Oscillations. -- Am- plitudes
of relative sea-level fluctuations that generated the Late Cambrian
meter-scale cycles must account for the simultaneous development of
peritidal cycles as well as deep ramp shaly cycles that formed on
different parts of the Late Cambrian platforms. For peritidal
cycles where the uppermost datum is known to be tide level, the av-
erage cycle thickness may be a minimum approximation of the total
amount of accommodation space created by the combined effects of
subsidence and sea level (Grot- zinger 1986a; Goldhammer et al.
1987; Koerschner and Read 1989).
Stratigraphic thickness of subtidal cycles cannot be used
FiG. 16B.--Columns of stacked synthetic cycles generated in the
2-D model of Figure 16A. Column A is from the inner peritidal
platform; column B is from the outer peritidal platform; column C
is from the inner subtidal platform; column D is from the outer
subtidal platform. Actual stacked cycles of Late Cambrian cyclic
successions are aligned below the synthetic cycles for comparison.
Key to lithofacies is the same as in Figures 15 and 16A.
-
F.U,S'TASY AND CYCLE STAUK1NG PATTERNS OF LATE CAMBRIAN
CARBONATES 1245
A) INNER B) OUTER C) INNER D) OUTER PERITIDAL PERITIDAL SUBTIDAL
SUBTIDAL PLATFORM PLATFORM PLATFORM PLATFORM
48-
25-
0m- ALLENTOWN FORMATION
PERITIDAL CYCLES
SNEAKOVER MBR. ORR FORMATION SUBTIDAL CYCLES
52 ~ ' -
II II
]
D
m
25
0m CANDLAND SHALE MBR.
ORR FORMATION DEEP RAMP SHALY CYCLES
-
1246 DA V1D OSLEGER AND J. FRED READ
to approximate the amplitudes of relative sea level os-
cillations. The only way to estimate the amplitudes of the
high-frequency relative sea-level oscillations that gener- ated
subtidal cycles is to use the difference in water depths between
estimated storm-deposited and fairweather-re- worked lithofacies.
The base of storm wave reworking may be defined geologically by the
first appearance of hummocky cross-stratified carbonate packstones
and grainstones with interbedded wackestones above suspen-
sion-settled carbonate mudstones and siliciclastic shales. The base
of fairweather wave reworking may be defined geologically by the
transition from storm-influenced sandy muds upward into winnowed
carbonate sands. Geological estimates of fairweather- and
storm-wave base are spec- ulative and must be based on
oceanographically-defined modern analogs.
Fairweather wave base has been estimated at 10 to 20 m on the
Yucatan shelf(Logan et al. 1969) and 8 to 20 m in the Persian
Gulf(Purser and Evans 1973). The Cor- dilleran passive margin may
have fronted a semi-enclosed ocean basin (Stewart and Suczek 1977)
where storm wave base may have been approximately 60 m minimum,
based upon the semi-enclosed Yucatan platform (Logan et al.
1969).
Potential amplitudes of short-term sea-level oscilla- tions can
be estimated using the ranges between fair- weather and storm wave
base. Estimating the maximum range to be about 50 m (60 m storm
wave base minus 10 m fairweather wave base) and the minimum range
to be about 10 m (30 m storm wave base minus 20 m fair- weather
wave base), a reasonable mid-range might be about 30 m. Several
reasons exist for even this mid-range value of about 30 m to be too
high. One constraint on the total amplitude of the high-frequency
sea level oscil- lations is provided by the composition of
Appalachian peritidal cycles. The extent of exposure ofperitidal
cycles is dependent on the amplitude, and therefore the rate of
fall, of the sea-level fluctuation. Rapid sea-level falls would
preclude the development of thick tidal flat caps (Koer- schner and
Read 1989) and result in dominantly subtidal, disconformity-capped
cycles similar to the Plio-Pleisto- cene of the Bahama platform
(Beach and Ginsburg 1980) and the Quaternary of south Florida
(Perkins 1977). Sea- level fall rates had to have been reasonably
slow to allow for the accumulation of tidal fiat caps that average
be- tween 1 to 2 m in thickness. Koerschner and Read (198