Facies Architecture and Sequence Stratigraphy of Fine ... · Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene

ABSTRACT

This study documents the effects of changinglake level (limnostasy), tectonics, sediment yield,and basin physiography on the facies architecture,sequence stratigraphy, and reservoir quality ofthree fine-grained deltas deposited along the east-ern margin of late Pleistocene Lake Bonneville.Analysis of facies architecture indicates that theWeber and Spanish Fork deltas were strongly wave-modified because they were situated along openlyexposed portions of the shoreline. The Bear Riverdelta, nestled in a relatively isolated northeast armof the lake, records both fluvial and wave process-es. These fine-grained deltas were fed by low-gradi-ent rivers that drained large regions that weresparsely glaciated, whereas other contemporane-ous, coarse-grained “Gilbert” deltas were fed bysteep-gradient rivers that drained local source areasthat were strongly influenced by glaciers.

Limnostasy, tectonics, and sediment yield weresimilar for all three fine-grained deltas, implyingthat the most inf luential forcing parameter onsequence stratigraphy is basin physiography. Basinphysiography (specifically ramp length and accom-modation) most strongly controlled the externaland internal geometry of the lowstand systemstract of each delta.

Potential hydrocarbon reservoir quality (e.g.,grain size and sorting) is largely a product of

635AAPG Bulletin, V. 83, No. 4 (April 1999), P. 635–665.

©Copyright 1999. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received October 28, 1996; revised manuscript receivedDecember 11, 1997; final acceptance July 27, 1998.

2Exxon Exploration Company, P.O. Box 4478, Houston, Texas 77210-4478; e-mail: [email protected]

3Department of Geology and Geophysics, University of Utah, Salt LakeCity, Utah 84112.

This project was funded by NSF grant EAR 9303519. A research grantfrom the Geological Society of America supported radiocarbon dating. Wethank Mark Milligan, Kerry Barker, and Matt Rees for field assistance.Constructive criticisms by W. B. Harris, K. Kelts, R. G. Stanley, and KevinBohacs much improved the original manuscript.

Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene Lake Bonneville,Northern Utah and Southern Idaho1

David R. Lemons2 and Marjorie A. Chan3

drainage basin size and stream gradient. Relativedifferences in ramp lengths seemed to determinethickness and lateral continuity of delta frontdeposits. Relative differences in accommodationappeared to determine the internal geometry ofthe delta front deposits, especially in the low-stand systems tract. These deltas can serve asanalogs for lacustrine exploration and productionwhere many forcing parameters typically areunknown.

INTRODUCTION

Lake Bonneville (Figure 1) was the largest of the94 Pleistocene lakes that occupied the Basin andRange physiographic province of the westernUnited States (Williams and Bedinger, 1984;Grayson, 1993). The deposits left by this lake offerthe opportunity to examine the dynamics of rapidlake level change, tectonics, physiography, and sed-iment flux as expressed in the stratigraphy of theWeber, Bear, and Spanish Fork deltas. The purposeof this paper is to evaluate the stratigraphy of thesefine-grained deltas in the context of facies architec-ture and nonmarine sequence stratigraphy. Ourdiscussion focuses on description of lithofacies,architectural elements, and depositional styles,especially as they relate to hydrocarbon reservoirsize, shape, and quality.

The cornerstone of previous work on LakeBonneville is G. K. Gilbert’s (1890) classic work,which includes his description of coarse-grained or“Gilbert” deltas, as well as his earlier associatedpaper (Gilbert, 1885) on topographic features oflake shores. Following these works was a 50 yrperiod of research on Lake Bonneville related land-form evolution and glacial geomorphology (e.g.,Atwood, 1909; Blackwelder, 1931; Pack, 1939).Subsequent works on Lake Bonneville focused ondetailed geologic mapping and stratigraphic studies(e.g., Hunt et al., 1953; Williams, 1962; Morrison,

1965a, b, c). More recent studies documented LakeBonneville stratigraphy, chronology, climate histo-ry, paleolimnology, paleohydrology, and geomor-phology (e.g., McCoy, 1981; Scott et al., 1983;Currey and Oviatt, 1985). Previous studies of thefine-grained deltas (Feth, 1955; Bissell, 1963;Bright, 1963; Feth et al., 1966) were made prior tothe advent of process-response sedimentary mod-els, facies architecture, and sequence stratigraphy;therefore, these studies did not place thesedeposits into a comprehensive depositional frame-work in the context of lake level. The well-con-strained age, limnostatic (lake level change) con-trol, basin physiography, tectonics, climate, andsediment yield of these Bonneville deposits providea model for hydrocarbon reservoir properties oflacustrine subsurface deposits where many of thesecontrolling parameters are typically unknown orpoorly understood.

GEOLOGIC SETTING

Lake Bonneville

Late Pleistocene Lake Bonneville occupied atopographically closed basin at the eastern marginof the Great Basin in the Basin and Range physio-graphic province. Both fine- and coarse-graineddeltas developed along the lake’s margin (Milliganand Lemons, 1998) (Figures 1, 2).

Lake Bonneville deposits are packaged into theBonneville alloformation. The Fielding geosol

(∼40–25 ka), where present, separates Bonnevilledeposits from the underlying lacustral and intrala-custral sediments of the Cutler Dam alloformation(Oviatt et al., 1987). The Weber and Spanish Forkdeltas were deposited on previous lake depositsand the Bear River delta was deposited, at leastpartially, on the Tertiary Salt Lake Group. Depth tobedrock is up to 1500 m deep in the Salt LakeValley (Radkins, 1990). Unconsolidated sedimentsare up to 2100 m thick near Preston, Idaho(Stanley, 1972).

Wasatch and East Cache Fault Zones

The Weber and Spanish Fork deltas are situat-ed along the Wasatch fault zone, which formsthe western structural margin of the WasatchMountains (Figure 2). The Wasatch fault zone isa long (∼340 km), active, normal fault compris-ing ten seismical ly independent segments(Machette et al., 1991). Uplift along the Wasatchfault zone began between 17 and 12 Ma andlocally has produced at least 3–11 km of verticaloffset (Zoback, 1983; Parry and Bruhn, 1987).The displacement rate averaged between 0.4 and0.67 mm/yr for this time period (Naeser et al.,1983; Parry and Bruhn, 1986); however, morerecent slip rates along the Wasatch fault zone arehigher (M. Machette, 1996, personal communi-cation). The Bear River delta lies along the west-ern margin of the Bear River Range, which isbounded by high-angle normal faulting along theEast Cache fault zone (Figure 2). This fault zonehas been inactive since the late Pleistocene(McCalpin, 1994).

Tectonically caused topographic relief betweenthe Bonneville basin and the Wasatch and BearRiver ranges, as well the Uinta Mountains (wherethe Weber and Bear rivers originate), controllederosional relief and river gradient. These factorsplayed a significant role in determining sedimentgrain size for deltas deposited along the easternmargin of Lake Bonneville. The lower gradients ofthe Weber, Bear, and Spanish Fork rivers resulted infine-grained deltas (Figure 3; Table 1). This con-trasts with locally sourced rivers, such as theAmerican Fork, Big Cottonwood, and Box ElderCreek (Figure 2), which have higher gradients andsupplied the coarse-grained material formingGilbert deltas (Milligan, 1995).

Glaciation and Paleoclimate

During the existence of Lake Bonneville, boththe Weber and Bear River drainages underwentglaciation of their uppermost reaches in the Uinta

636 Lake Bonneville Deltas

Figure 1—Location of late Pleistocene Lake Bonneville atits highest (Bonneville) shoreline (Oviatt et al., 1992)and study area containing the three deltas.

0 300

km

N

IDAHO

WYOMING

UTAH

COLORADO

NEVADA

Lake Bonneville

Great SaltLake

ARIZONA

2

1

3

1. Weber River delta2. Bear River delta3. Spanish Fork delta

Mountains during Pinedale glaciation (Atwood,1909; Richmond, 1965). In addition, the BearRiver drainage underwent glaciation in the upperportions of the Bear River Range in northern Utahand southern Idaho (DeGraff, 1976), as well asminor glaciation in southeastern Wyoming.Areally, less than 5% of either drainage basinunderwent glaciation during Pinedale deposition.Limited study on glaciation in Spanish Fork Canyonsuggests little glaciation occurred during Pinedaledeposition (Atwood, 1909; Rawson, 1957).Compared with other drainages that sourcedcoarse-grained Gilbert deltas (e.g., American Forkand Big Cottonwood deltas, whose drainage areaswere glaciated up to approximately 25%), a rela-tively minor amount of coarse-grained glacial debrisexisted in the Weber, Bear, and Spanish Forkdrainages (Milligan, 1995). The relative lack ofglaciation and subsequent coarse-grained glacialdebris is partially responsible for the fine-grainednature of the Weber, Bear, and Spanish Fork deltas.

Climate, especially precipitation, may have anoverriding effect on sediment yield rates in moun-tainous terrain (Hicks et al., 1990). Late Pleistoceneprecipitation rates for the approximate highstandperiod of Lake Bonneville were estimated based onsediment yields for the fine-grained Weber Riverdelta and the coarse-grained American Fork delta (aGilbert delta located about 40 km south of Salt LakeCity). Both deltas’ sediment yields indicate maxi-mum paleoprecipitation rates as much as 33% high-er than present-day rates in the Wasatch and Uintamountains (Lemons et al., 1996). Basin-floor meanannual paleotemperatures for the same time periodare estimated at about 13°C cooler than present(McCoy, 1981; Mears, 1981; Lemons et al., 1996).

FACIES ARCHITECTURE

Facies architecture is based on a hierarchy ofinternal bounding surfaces separating depositional

Lemons and Chan 637

Figure 2—Index mapshowing study localities of fine-grained and coarse-grained deltasexposed along the WasatchMountains and Bear RiverRange (modified fromMachette, 1988).

IDAHO

UTAHLAKE

AREAOF

FIGURE

GREATSALTLAKE

Salt LakeCity

Ogden

0 10 20 30 40 50

km

N

BEARLAKE

IDAHO

UTAH

WYOMING

AREA OFFIGURE

WYOMING

UTAH

2

UINTAMTNS.

WASATCHMOUNTAINS

1

mountains

water

normal fault (ball ondownthrown side)

3

41o

40o

42o

112o

BEARRIVER

RANGE

UTAHLAKE

112O

WASATCHMOUNTAINS

111O

Provo

1

Wasatch Fault Zone

Bear River

Ogden River

Weber River

JordanRiver Provo

River

Spanish ForkRiver

1. Weber River delta

3. Spanish Fork delta

2. Bear River delta

AmericanFork

Provo

Big CottonwoodCreek

Box ElderCreek

PRESTON

Wasatch faultzone East Cache

fault zone

modern river drainages

coarse-grained deltas

units of different temporal and spatial scales (e.g.,small-scale ripples deposited in minutes or hours toentire depositional systems deposited in hundredsor thousands of years or more). These boundingunits separate lithofacies and assemblages of litho-facies (architectural elements) that represent a par-ticular process or suite of processes occurringwithin a depositional system (Campbell, 1967;Miall, 1988a, b, 1990, 1991, 1994; Walker, 1990).

We used facies architecture concepts to system-atically describe and interpret the lithostratigraphyof the Weber, Bear, and Spanish Fork deltas. In thisstudy, a modified version of the bounding surfacehierarchy proposed by Miall (1988a, b) is used(Figure 4; Table 2). This hierarchy comprises sixscales of bounding surfaces, with each progressivebounding surface (in descending order) encom-passing a spatially and temporally larger deposit.We determined the facies architecture through (1)measured sections, (2) photomosaic overlays, and(3) interpreting these data to form a systematicseries of lithofacies and architectural elements usedto describe the entire delta.

Architectural Element Description andInterpretation

Thirteen, seven, and eight lithofacies are identi-fied in the Weber, Bear, and Spanish Fork deltas,

respectively, and five, three, and four architecturalelements are similarly recognized (Tables 3–8;Figures 5–7). The architectural elements consist ofassemblages of the various lithofacies. Horizontaland vertical scales of the architectural elementsrange from meters to kilometers. The horizontaland vertical extents of individual lithofacies withinthese elements occur on a smaller scale of centime-ters to meters.

Delta-front sheet sand, lacustrine clay, and beachgravel architectural elements are volumetricallygreatest in all three deltas. In addition, delta-marginand f luvial channel elements are present in theWeber River delta, and fluvial gravel and loess ele-ments exist in the Spanish Fork delta.

SEQUENCE STRATIGRAPHY

The concepts and principles of sequence stratig-raphy developed in the marine realm (e.g., Vail,1987; Posamentier and Vail, 1988; Van Wagoner etal., 1988, 1990; Posamentier et al., 1992) can beapplied in a straightforward manner to the lacus-trine delta exposures of the Weber, Bear andSpanish Fork deltas. The use of sequence stratigra-phy rather than lithostratigraphy to correlate genet-ic packages of sediments may be more appropriatein lacustrine basins, where similar lithologies maybe produced by several lake cycles (Oviatt et al.,1994). Interpretations of sequence stratigraphy inthis study are enhanced by the well-establishedhydrograph (Figure 8) with which to relate the sed-imentary packages.

All three deltas exhibit systems tracts ofmarkedly different character (especially the low-stand), as well as unique assemblages of lithofa-cies and architectural elements; however, thereare features common to all three deltas. (1) Strike


Table 1. Drainage Basin Characteristics of LakeBonneville Deltas

Size atBonneville

Delta ShorelineRiver System Type Gradient (km2)

Weber River Fine grained 0.01* 3328**Bear River Fine grained 0.003–0.004* 11,225**Spanish Fork Fine grained 0.02* 886**American Fork Coarse grained 0.06† 160†

Big Cottonwood Coarse grained 0.06† 135†

Box Elder Creek Coarse grained 0.09† 98†

(Brigham City)

*Computed from U.S. Geological Survey 30 × 60° quadrangle maps(scale: 1:100,000).

**Linearly interpolated between gauging stations (ReMillard et al., 1993).†From Milligan (1995).

10

100

1000

104

105

fine-grained deltas

coarse-grained deltas

Gradient

0.001 0.01 0.1

2

1

3

456

Dra

inag

e B

asin

Siz

e (k

m2 )

Figure 3—Cross-plot of river gradient vs. drainage basinsize for streams that deposited deltas along the easternmargin of late Pleistocene Lake Bonneville. Fine-graineddeltas were sourced by low-gradient rivers with largedrainage basins that underwent only minor glaciation.Coarse-grained deltas (“Gilbert” deltas) were sourced byhigh-gradient rivers with small drainage basins thatwere strongly affected by glaciation (numbers refer todeltas listed in Table 1).

vs. dip variability in stratal stacking patterns (e.g.,Martinsen and Helland-Hansen, 1995) in the trans-gressive and highstand systems tracts is low, indi-cating minimal effects of local subsidence trendsand strike-variable sediment supply. (2) Healing-phase deposits (relatively sand-poor, early trans-gressive deposits primarily eroded from preexistingdelta plain/coastal plain sediments) of Posamentierand Allen (1993) are not recognized. (3) Fluvialincision into the transgressive and highstand sys-tems tracts does occur, unlike the development ofthe lowstand systems tract in most ramp physio-graphic settings (Posamentier and Allen, 1993). (4)No turbidites are recognized in the lowstand sys-tems tract. (5) As a consequence of lowstand ero-sion, the lowstand systems tract is not physicallyattached (e.g., Ainsworth and Pattison, 1994) to thehighstand systems tract.

Forcing Parameters

Previous work has established important bound-ary conditions for interpreting forcing parameterson deposition: (1) lake level change vs. time forLake Bonneville, (2) tectonic uplift rates along theWasatch Mountains and Bear River Range, (3)resulting physiography due to uplift and lake levelchange, and (4) sediment yield rates along the east-ern margin of Lake Bonneville. The relative impactof the four forcing parameters (Posamentier andJames, 1993) is assessed for each delta (Table 9).

LimnostasyThe term “limnostasy” is suggested for basin-

wide changes in lake level. This definition contrastswith that for “eustasy,” which refers to global

Lemons and Chan 639

Figure 4—Hierarchy ofbounding surfaces (numbered 1–6 in ascending order) used in developing the faciesarchitecture for theWeber, Bear, and SpanishFork river deltas (modifiedfrom Miall, 1988a) (Table2). TST = transgressive systems tract, HST = highstand systems tract,LST = lowstand systemstract.

clay

clay

4

4

3

2intraclasts

10s m

1—10

s m

geosol

6

5 4

10s km

100s

mWest East

TST/HST

LSTWasatch/BearRiver range

reactivationsurface

1

changes in sea level (e.g., Dott, 1992). In dealingwith nonmarine strata, this distinction seemsappropriate. Unlike most marine settings, LakeBonneville has a preexisting well-established hydro-graph of base level (lake level) change vs. time(Figure 8); therefore, packages of sedimentsequences can be definitively tied to the hydro-graph on a smaller scale than in most marinesequence stratigraphic studies. The Weber, Bear,and Spanish Fork deltas were deposited on the timescale (∼20 k.y.) of a fifth-order high-frequency cycle(Mitchum and Van Wagoner, 1991).

TectonicsEstimated ranges of subsidence values (dip sepa-

ration along the Wasatch and East Cache faultzones) for all three deltas are shown in Table 9.Maximum subsidence values for the exposed por-tions of all three deltas range from 0 to 25 m.Similarly, minimum subsidence values range from 0to 12 m. Contemporaneous lake level fluctuations

were on the order of 300 m; therefore, accommo-dation (space made available for potential sedimentaccumulation) (Posamentier et al., 1988) was large-ly controlled by lake level. Slip rates along theWasatch fault over the last 15 k.y. may be muchhigher than longer term slip rates (M. Machette,1996, personal communication); consequently,subsidence values during delta formation (∼28–10ka) may be less than suggested.

Sediment Yield RatesThe sediment yield rate for the Weber River delta

is estimated to be 705 m3/km2/yr based on deltavolume, drainage size, and time involved in deltadeposition (Lemons et al., 1996). This value isabout two times higher than modern basins of simi-lar size (Gregory and Walling, 1973), although it isdifficult to find direct analogs in terms of climate,relief, and bedrock geology. Paleoprecipitation esti-mates based on this sediment yield rate suggest thatmaximum annual paleoprecipitation rates during


Table 2. Bounding Surface Hierarchy for the Weber, Bear, and Spanish Fork Deltas

Bounding Surfaces and Attributes

1. First-Order Surfaces (same as Miall, 1988a)A. Cross-bed set bounding surfacesB. Little or no erosionC. Represent virtually continuous sedimentation of a train of similar bed forms; therefore, lithofacies and

sedimentary structures directly overlying surface are the same (can include reactivation surfaces)D. Bounds microform deposits

2. Second-Order Surfaces (same as Miall, 1988a)A. Coset bounding surfacesB. Change in lithofacies due to change in flow conditions or a change in flow directionsC. Lithofacies and bed forms above and below surface are differentD. No significant time breakE. Bounds mesoform deposits

3. Third-Order Surfaces (modified from Miall, 1988a)A. Erosional features within macroforms (not necessarily crosscutting erosion)B. Intraclast breccia commonly overlies surface

4. Fourth-Order Surfaces (same as Miall, 1988a)A. Upper bounding surfaces of macroformsB. Change in lithosome due to change in flow conditionC. Bounds architectural elements

5. Fifth-Order Surfaces (modified from Miall, 1988a)A. Bounds deposits of major rapid lake level fluctuations (transgression/regression), e.g., surface between

Bonneville and Provo level deltas (which separates the highstand systems tract and lowstand systems tract)B. Also includes surfaces resulting from smaller oscillations of lake level (e.g., Keg Mountain oscillation)

6. Sixth-Order Surfaces (modified from Miall, 1988a)A. Bounds mappable sediment packages of a single lake cycle (e.g., Fielding geosol at base of Bonneville

alloformation)B. Alloformation boundaries

Lemons and Chan 641

Table 3. Weber River Delta Lithofacies Classification

Facies Lithofacies Sedimentary Structures Interpretation

Gravel, Gravel, clast- Typically upward-fining, Beach or shoreline massive to supported, sandy matrix; crude horizontal bedding deposits sourcedcrudely sometimes matrix supported from local canyonsbedded gravelly sand

Gravel, Gravel or granules, Horizontal to low-angle Associated with wave-subhorizontal usually occurs as bedding (<10°) influenced deposits, usually

discontinuous lenses; found in delta-frontvery fine-grained to medium- sands depositedgrained, sandy matrix near mountain front

Gravel, Gravel or granules in a Angular to concave Current ripplescross- fine to coarse-grained foresets with dips up formed in fluvial channelbedded sandy matrix to 30°; commonly have deposits; associated

erosive basis with Sxb† lithofacies

Sand, Very fine to medium Wave-ripples with ripple, Wave influencedwave grained sand, may be index of 5–10 and in shallow waterrippled silty; contains higher ripple symmetry index of ~1; (~0.5–1 m), usually

silt and clay content may be intricately woven, delta-front andwhen developed in some soft-sediment delta-margin depositsdelta margin deposits, deformation and sometimes interstratified intraclast brecciawith subhorizontal sand

Sand, Very fine to fine grained Thin clay laminations Restricted to deltaflaser silty sand, smaller draped over wave margin depositsbedding amounts of clay ripples

Sand, Very fine to medium Angular to concave Current ripples cross- grained sand, may foresets with dips up formed in fluvialbedded contain granules to 30°; commonly have channel deposits or

and small pebbles erosive basis beach gravels

Sand, Very fine to medium Horizontal to low-angle Usually found insubhorizontal grained sand, may be bedding (<10°), some delta-front and

silty; contains higher soft-sediment deformation, delta-margin depositspercentage of silt and clay some intraclast brecciawhen developed indelta-margin environments;can contain granules andsmall pebbles whendeveloped in fluvial channeldeposits; sometimesinterstratified with wave-rippledsand

Sand, Very fine grained sand, Most hummocks have Associated withhummocky silt, and some clay amplitudes of about 6–8 distal portionscross- cm, whereas internal of delta-margin deposits;stratification laminations have rarely found in delta-front

amplitudes of about 2–3 depositscm; hummocks havelateral spacings from 1–3 m

(Continued on next page)

the approximate highstand of Lake Bonneville mayhave been up to 33% higher than present rates(Lemons et al., 1996). Decreasing precipitation asLake Bonneville waned apparently resulted inlower sediment yields. Sediment yield data are cur-rently not available for the Bear and Spanish Forkrivers; however, the close proximity of all threedrainage basins and the apparent importance ofprecipitation suggest that sediment yield rates aresimilar.

PhysiographyThe physiography of the surface on which each

delta was deposited resembles a ramp. The ramplength and accommodation for each delta areshown in Table 9. There is some uncertaintyregarding when the Bear River headwaters werediverted into the Bonneville basin from the SnakeRiver basin; however, the Bear River was flowinginto the Bonneville basin prior to the initiation ofLake Bonneville (Bright, 1963; McCoy, 1987;Hochberg, 1995).

The Weber and Spanish Fork deltas have shortramp lengths (tens of kilometers), whereas the

Bear River delta has a long ramp length (>100km). The Spanish Fork delta has approximately90 m less potential accommodation (∼180 m)than either the Weber or Bear River delta (∼270m). The Weber and Spanish Fork deltas are situ-ated along portions of former Lake Bonnevillewhere they were openly exposed to waves andcurrents. In contrast, the Bear River delta is situ-ated in a relatively isolated arm in the northeast-ern portion of former Lake Bonneville over 100km from the present-day Great Salt Lake; there-fore, the Bear River delta has a depositional sur-face with a lower gradient than the other twodeltas. Even though all three deltas have ramp-like physiography with low depositional gradi-ents, variations in accommodation and downdipramp length lead to markedly different sequenceexpressions, particularly in the lowstand sys-tems tract.

An important distinction needs to be madebetween ramp length and ramp gradient withregards to marine vs. nonmarine depositional set-tings. In marine sequence stratigraphy, the shore-line is assumed to be more or less linear. Thus, in amarine ramp setting, ramp gradient can vary, but


Table 3. Continued


Sand, Fine to medium Current ripples with RSI* Only recognized incurrent grained sand, may up to 4.5 and RI** up to 14 delta-margin deposits;rippled be silty associated with local

canyons contributingunusually large amounts ofsediment; rarely found

Sand Very fine to medium Trough cross-beds up to Restricted to fluvialtrough grained sand, may contain 0.3 m across channel environmentscross- granules or small pebbles along more distalbedded portions of delta

adjacent to mountains;rarely found

Fines, Silt and clay, smaller Horizontal to low-angle Usually found insubhorizontal amounts of very fine bedding (<10°), some delta-front and delta-

to fine grained sand soft-sediment deformation margin deposits

Fines, Clay Massive to Lacustrine depositsclay laminated

Marl, Friable marl containing None recognized Fauna suggestswhite abundant ostracods, deposition in a

gastropods, bivalves wetland environmentepiphytic diatoms, fish marginal to thebones, and plant remains delta (R. Forester, 1996,

personal communication)

*RSI = ripple symmetry index = length of horizontal projection of stoss side/length of horizontal projection of lee side (Reineck and Singh, 1980).**RI = ripple index = ripple length/ripple height (Reineck and Singh, 1980).†Sxb = sand, cross-bedded.

Lemons and Chan 643

Tab

le 4

. W

eber

Riv

er D

elta

Arc

hit

ectu

ral

Ele

men

ts

Pri

nci

pal

Lith

ofa

cies

Geo

met

ry a

nd

Elem

ent

Ass

emb

lage

*R

elat

ion

sIn

terp

reta

tio

n

Del

ta-fr

on

t sh

eet

san

dSw

r, S

sh, F

sh,

Shee

t, b

lan

ket

(up

to

~10

0M

ost

ab

un

dan

t el

emen

t; w

ave

rip

ple

s, d

elta

elo

nga

tio

n t

o s

ou

th, a

nd

Gsh

, Sh

csm

th

ick

×10

00s

m w

ide

and

lack

of

pre

serv

ed d

elta

-pla

in d

epo

sits

ind

icat

e h

igh

wav

e ac

tivi

tylo

ng)

; in

terf

inge

rs b

asin

war

d(W

righ

t, 1

977;

Ort

on

an

d R

ead

ing,

199

3) g

ener

ally

dir

ecte

d f

rom

th

ew

ith

lacu

stri

ne

clay

an

dn

ort

hw

est

(Will

iam

s, 1

994)

; em

pir

ical

eq

uat

ion

s d

eriv

ed b

y T

ann

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war

d w

ith

all

oth

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971)

an

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(199

4) in

dic

ate

dep

osi

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n in

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pal

eow

ater

dep

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f 0.

5–1

m

Del

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Sfb

, Sh

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Wed

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10s

m t

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k ×

Rec

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hu

mm

ock

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sh, S

wr,

100s

to

100

0s m

wid

e);

cro

ss-s

trat

ific

atio

n a

lon

g th

e so

uth

ern

mar

gin

s o

f th

e d

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to

wh

ite

Mw

dep

osi

ted

alo

ng

mar

gin

sm

arl d

epo

site

d in

wet

lan

d e

nvi

ron

men

ts (

R. F

ore

ster

, 199

6, p

erso

nal

of

del

ta a

nd

sh

ore

war

dco

mm

un

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) to

cu

rren

t-ri

pp

led

san

ds

alo

ng

loca

l can

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so

f d

elta

-fro

nt

shee

t sa

nd

s;co

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tin

g u

nu

sual

ly h

igh

am

ou

nts

of

fin

e-gr

ain

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and

to

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nt

wit

h in

crea

sin

gd

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siti

on

in w

ave-

infl

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ced

mu

dfl

at a

nd

san

dfl

at e

nvi

ron

men

tsd

ista

nce

alo

ng

stri

ke f

rom

(Alle

n a

nd

Co

llin

son

, 198

6); a

lon

g so

uth

ern

po

rtio

ns

of

del

ta,

rive

r m

ou

th; l

arge

-sca

leth

is e

lem

ent

has

un

der

gon

e m

ajo

r p

ost

-Bo

nn

evill

e d

epo

siti

on

slu

mp

ing

alo

ng

the

sou

ther

nla

tera

l sp

read

ing

(Van

Ho

rn, 1

975)

mar

gin

of

the

del

ta

Flu

vial

ch

ann

el d

epo

sits

Sxb

, Gx

b, S

tx,

Len

ticu

lar

[met

ers

thic

k ×

Rel

ativ

ely

rare

; gen

eral

ly c

oar

se g

rain

ed (

smal

l gra

vel,

gran

ule

s, a

nd

Ssh

, Gsh

10s(

?) m

lon

g]; t

ypic

ally

san

d)

and

cro

ss-b

edd

ed, a

nd

can

ex

hib

it e

rosi

ve b

ases

; so

urc

ed f

rom

pre

serv

ed n

ear

mo

un

tain

loca

l sm

alle

r ca

nyo

ns;

rep

rese

nt

eph

emer

al, s

edim

ent-

char

ged

fro

nt,

ass

oci

ated

wit

hst

ream

s p

rovi

din

g lo

caliz

ed c

last

ic in

flu

x t

o t

he

sho

relin

ed

elta

-mar

gin

dep

osi

ts

Bea

ch g

rave

lsG

mc,

Gsh

, Ssh

,R

ecta

ngu

lar

wed

ge (

10s

mLo

caliz

ed a

dja

cen

t to

th

e W

asat

ch M

ou

nta

ins

and

fo

rm a

lin

ear

Swr

thic

k ×

1000

s m

wid

e);

stra

nd

of

bea

ch o

r sh

ore

line

dep

osi

ts (

Nel

son

an

d P

erso

niu

s, 1

993)

;ca

ps

del

ta a

t B

on

nev

ille

rep

rese

nt

coar

se-g

rain

ed s

edim

ent

sou

rced

by

loca

l sm

alle

rsh

ore

line;

pre

sum

ably

can

yon

s w

ith

ste

ep g

rad

ien

tsp

rese

nt

in lo

wer

par

ts o

fd

elta

alo

ng

mo

un

tain

fro

nt,

bu

t n

ot

exp

ose

d; s

ou

rced

fro

mlo

cal c

anyo

ns

Lacu

stri

ne

clay

FcSh

eet,

bla

nke

t (u

p t

o 1

0sP

rese

nt

bas

inw

ard

of

del

ta-fr

on

t sh

eet

san

ds;

th

inly

lam

inat

ed w

ith

m t

hic

k ×

1000

s m

wid

e an

dlig

ht

tan

an

d r

edd

ish

bro

wn

alt

ern

atin

g b

and

s ~

0.5–

1.0

cm t

hic

k,lo

ng)

; typ

ical

ly in

terf

inge

rssu

gges

tin

g an

ox

ic b

ott

om

wat

ers

and

so

me

typ

e o

f se

aso

nal

ity

insh

ore

war

d w

ith

del

ta-fr

on

tse

dim

ent

infl

ux

(A

nd

erso

n a

nd

Dea

n, 1

988;

Sm

oo

t, 1

993)

; tw

osh

eet

san

ds;

per

sist

sla

rge-

scal

e in

terv

als

(~10

–20

m)

of

lacu

stri

ne

clay

dep

osi

tio

nd

ow

nd

ip in

to d

eep

erp

arts

of

the

bas

in

Fsh

= f

ines

, su

bhor

izon

tal,

Gm

c =

gra

vel,

mas

sive

to

crud

ely

bedd

ed,

Gsb

= g

rave

l, cr

oss=

bedd

ed,

Gsh

= g

rave

l, su

bhor

izon

tal,

Mw

= m

arl,

whi

te,

Sfb

= s

and,

fla

ser

bedd

ed,

Shc

s =

san

d, h

umm

ocky

cros

s-st

ratif

ied,

Ssh

= s

and,

sub

horiz

onta

l, S

tx =

san

d tr

ough

, cro

ss-b

edde

d, S

wr

= s

and,

wav

e rip

pled

, Sxb

= s

and,

cro

ss-b

edde

d.

ramp length measured from the shoreline general-ly does not. In the Bonneville basin, all three deltashave a low depositional gradient; however, due tobasin and range physiography, the ramp length forthese deltas varies from tens of kilometers to over100 km. Both ramp length and gradient contributeto accommodation; however, unlike marine set-tings, where ramp gradient is a more importantcontributor to accommodation, ramp length is moreimportant in the Bonneville basin.

Weber River Delta

The sequence stratigraphy of the Weber Riverdelta closely resembles that of a passive continentalmargin (Vail, 1987; Van Wagoner et al., 1988, 1990;Steckler et al., 1993), even though it is situated inan intracratonic setting. The transgressive systemstract consists of a retrogradational parasequence

set with a minimum thickness of about 130 m. Thisis a minimum value because the base of theBonneville alloformation is not exposed. The trans-gressive systems tract contains two parasequences(labeled 1 and 2 on Figure 10) composed primarilyof fine-grained delta-front sheet sands overlain bylacustrine f looding surfaces or their shorewardequivalent (Figures 9–11). This setting suggeststhat lake level rise was punctuated, allowing fortwo cycles of progradation of delta-front sedi-ments (i .e. , sedimentation rates equal to orgreater than accommodation created by risinglake level). These parasequences were subse-quently transgressed during rapid phases of lakelevel rise where sedimentation rates were exceed-ed by rising lake level.

A detailed look at the pinchout of the lacustrineclays into delta-front sheet sands suggests some typeof seasonality or cyclicity. This is evidenced by arepetitive pattern of lacustrine clay, subhorizontal


Table 5. Bear River Delta Lithofacies Classification


Gravel, Gravel, clast- Typically upward-coarsening, Beach or shorelinemassive to supported, usually crude horizontal bedding deposits, sourced fromcrudely sandy at base of local drainages,bedded upward-coarsening recognized in outcrop at

cycle Bonneville shoreline andProvo and post-Provodeposition sediments(Bright, 1963)

Sand, Very fine to Wave ripples may be Wave-influencedwave medium grained intricately woven, delta-front depositsrippled sand, may be silty some soft-sediment

deformation

Sand, Very fine to Horizontal to low-angle Delta-front depositssubhorizontal medium grained bedding (<10°), some

sand, may be silty soft-sediment deformation

Sand, Very fine to Current (unidirectional) Fluvially influencedcurrent medium grained ripples, some soft- delta-front depositsrippled sand, may be silty sediment deformation

Sand, Very fine to Climbing current ripples, Fluvially influencedclimbing medium grained usually preserved as delta-front depositsrippled sand with lee side ripple laminae-in-drift

preserved, stossside sometimespreserved

Fines, Silt and clay, Horizontal to low-angle Delta-front depositssubhorizontal smaller amounts bedding (<10°), some

of very fine to fine soft-sediment deformationgrained sand

Fines, clay Clay Massive to laminated Lacustrine deposits

Lemons and Chan 645

Tab

le 6

. B

ear

Riv

er D

elta

Arc

hit

ectu

ral

Ele

men

ts

Pri

nci

pal

Lith

ofa

cies

Elem

ent

Ass

emb

lage

*G

eom

etry

an

d R

elat

ion

sIn

terp

reta

tio

n

Del

ta-fr

on

t sh

eet

san

ds

Scr,

Sw

r, S

sh,

Shee

t, b

lan

ket

(up

to

~10

0 m

Mo

st a

bu

nd

ant

elem

ent;

wav

e ri

pp

les

and

cu

rren

tSc

l, Fs

hth

ick

×10

00s

m(?

) w

ide

and

and

clim

bin

g cu

rren

t ri

pp

les

ind

icat

e co

mb

inat

ion

of

lon

g); l

ater

ally

co

nti

nu

ou

s in

w

ave

and

flu

vial

infl

uen

ce; p

hys

iogr

aph

ical

ly lo

cate

do

utc

rop

ove

r at

leas

t 2

km in

dip

in is

ola

ted

no

rth

east

arm

of

lake

[C

ach

e V

alle

y b

ayd

irec

tio

n, i

nte

rbed

s b

asin

war

d

of

Gilb

ert

(189

0)];

alt

ho

ugh

del

ta e

xp

erie

nce

d le

ssw

ith

lacu

stri

ne

clay

wav

e ac

tivi

ty t

han

th

e W

eber

Riv

er d

elta

, ab

sen

ce o

fd

elta

-pla

in d

epo

sits

(B

righ

t, 1

963)

ind

icat

es t

her

e w

asst

ill a

hig

h d

egre

e o

f re

wo

rkin

g

Bea

ch g

rave

lsG

mc

Len

s-sh

ape

or

po

ssib

lyG

rave

l pro

ven

ance

ap

pea

rs t

o b

e p

rin

cip

ally

fro

m t

he

late

rally

co

nti

nu

ou

s o

ver

sho

rtM

ioce

ne–

Plio

cen

e Sa

lt L

ake

Gro

up

an

d n

earb

yd

ista

nce

s [m

eter

s th

ick

×10

s–10

0sP

aleo

zoic

fo

rmat

ion

sm

(?)

wid

e]; c

aps

del

ta a

tB

on

nev

ille

sho

relin

e; a

pp

aren

tly

sou

rced

by

loca

l, st

eep

er g

rad

ien

td

rain

ages

an

d c

anyo

ns

Lacu

stri

ne

clay

FcSh

eet,

bla

nke

t [1

s–10

s o

f m

P

rese

nt

bas

inw

ard

of

del

ta-fr

on

t sh

eet

san

ds

inth

ick

×10

00s

m(?

) w

ide

and

an

inte

rfin

geri

ng

rela

tio

nsh

ip; m

ost

of

the

clay

lon

g]; l

ater

ally

co

nti

nu

ou

s in

is

th

inly

lam

inat

ed w

ith

ligh

t ta

n a

nd

red

dis

ho

utc

rop

ove

r at

leas

t 2

km in

dip

bro

wn

alt

ern

atin

g b

and

s su

gges

tin

g an

ox

icd

irec

tio

n; i

nte

rfin

gers

sh

ore

war

db

ott

om

wat

ers

and

so

me

typ

e o

f se

aso

nal

ity

wit

h d

elta

-fro

nt

shee

tin

sed

imen

t in

flu

x; s

ever

al s

mal

l alt

ern

atin

g cy

cles

san

ds;

pre

sum

ably

per

sist

s o

f d

elta

-fro

nt

shee

t p

rogr

adat

ion

an

d t

ran

sgre

ssiv

ed

ow

nd

ip b

asin

war

dla

cust

rin

e cl

ay d

epo

siti

on

wit

hin

on

ela

rge-

scal

e p

acka

ge o

f la

cust

rin

e cl

ay (

corr

elat

ive

wit

h u

pp

erm

ost

lacu

stri

ne

clay

in t

he

Web

er R

iver

del

ta);

low

er c

lay

may

be

pre

sen

t, b

ut

was

no

t re

cogn

ized

in o

utc

rop

or

pu

blic

ly a

vaila

ble

dri

llers

’ lo

gs

*Fc

= fi

nes,

cla

y, F

sh =

fine

s, s

ubho

rizon

tal,

Gm

c =

gra

vel,

mas

sive

to c

rude

ly b

edde

d, S

cl =

san

d, c

limbi

ng r

ippl

ed, S

cr =

san

d, c

urre

nt r

ippl

ed, S

sh =

san

d, s

ubho

rizon

tal,

Sw

r =

san

d, w

ave

rippl

ed.


Table 7. Spanish Fork Delta Lithofacies Classification


Gravel, Gravel, clast Vague upward-fining and Can occur as beach gravelmassive to supported; pebbly to upward-coarsening cycles, deposits preserved in narrow crudely sandy matrix; can contain crude horizontal bedding, corridor along Wasatchbedded subhorizontal fine- to dips usually <10° Mountains or as fluvial gravel

coarse-grained sand deposits that are gradedstringers and lenses; to the Provo shorelinerarely has small-scale and cap delta front depositschanneling

Gravel, Gravel, clast supported, Beds that dip >20°; Fluvial gravels deposits that arecross- very fine to fine grained can have erosive base when graded to Provo shorelinebedded matrix, may contain sand associated with channeling and cap delta-front deposits

stringers with dips >20°; into underlying delta-frontassociated with massive depositsto crudely bedded gravels

Sand, Very fine to medium Wave-ripples with RI* Wave influenced in shallowwave grained sand, may be of 5–12 and RSI** water (~1–2 m); onlyrippled silty; commonly of ~1; may be intricately found in delta front deposits

interbedded on 1s–10s woven; paleocurrentcm scale with subhorizontal measurements indicatefines predominant wave

orientation from thenorth–northwest

Sand, Very fine to medium Horizontal to low-angle Usually found in delta-frontsubhorizontal grained sand, may be bedding (<10°), may deposits; less

silty; commonly contain intraclast breccia; frequently seen atinterbedded on centimeter some soft-sediment transition fromscale with subhorizontal fines deformation; rarely has fluvial gravels to loessindicating some type of erosive base whenseasonality; rarely stratigraphically overlyingcontains thin gravel fluvial gravels (massive tostringers crudely bedded gravels or

cross-bedded gravel)

Sand, Medium to coarse Angular to tangential Finer grained fluvialcross-bedded grained sand, can contain foresets with dips of deposit that occurs

granules or small pebbles 10–15°; can have erosive at transition from delta-frontbase; sometimes to fluvial gravels elementshas lower angle dips and at fluvial gravels-to-loess(<10°); can have soft- element transition;sediment deformation that rarely seen in thin sandaffects underlying deposits stringers in massive to crudely

bedded gravels

Sand, Very fine to coarse Angular to concave foresets Rarely found in bothplanar grained sand, may be dipping from 5–20°; delta-front deposits and incross-bedded silty, commonly relatively rare; sometimes sand stringers in massive to

interbedded on cm grades laterally into crudely bedded gravelsscale with subhorizontal fines wave-rippled sand

Sand, Very fine to coarse Trough cross-beds up Recognized only intrough grained sand; may to 1 m across delta-front deposits of thecross-bedded contain granules transgressive systems tract

Lemons and Chan 647

Table 7. Continued


Fines, Silt with smaller Horizontal to low-angle Found in delta-front elementsubhorizontal amounts of very fine bedding (<10°), some or as loess capping

grained sand and clay; soft-sediment deformation delta stratigraphicallycalcareous when occurs and slumping; sometimes above fluvial gravelsas loess forms a drape over

underlying, unconsolidatedwave-rippled sand; sometimescontains intercalated wave-rippledsand lenses

*RI = ripple index = ripple length/ripple height (Reineck and Singh, 1980).**RSI = ripple symmetry index = length of horizontal projection of stoss side/length of horizontal projection of lee side (Reineck and Singh, 1980).

sand, and wave-rippled sand that repeats on a 10–30cm scale (Figure 5F). Landward of delta-front sheetsand deposition, contemporaneous delta-margin, flu-vial channel, and beach gravels were deposited.

The lacustrine flooding surfaces could correspondto periods of rapid subsidence (faulting) that loweredthe depositional surface and produced “instanta-neous” lacustrine flooding (Plint and Browne, 1994);however, the vertical displacements associated withmovement on the Wasatch fault along the WeberRiver delta are too low (usually in the range of 1.5 to2.5 m), making this subsidence scenario unlikely(Machette et al., 1991).

The subsequent highstand systems tractdeposits are exposed directly adjacent to theWasatch Mountains (Figures 10, 11). A paucity oflacustrine clay indicates that coarse clastic sedi-mentation kept up with lake level rise, at leastnear the Wasatch Mountains, during highstanddeposition. The maximum thickness of the high-stand systems tract is approximately 85 m. Part ofthe highstand systems tract was eroded when thewater level in Lake Bonneville fell catastrophical-ly to the Provo level. The bulk of the preservedhighstand systems tract deposits consist of delta-margin, f luvial channel, and beach gravel sedi-ments deposited landward of delta-front sheetsands. The limited exposures of the highstandsystems tract (Figure 11) show the deposits to besubhorizontal. Thus, it is impossible to recognizethe maximum f looding surface by downlap.Presumably, the highstand systems tract resultedin aggradational to progradational parasequencesets (Van Wagoner et al., 1988, 1990).

Lowstand systems tract sediments consist of alter-nating delta-front sheet sands and lacustrine clays(Figure 5) recording a punctuated post-Provo regres-sion of the lake. Minimum thickness of the lowstandsystems tract is on the order of tens of meters. Fluvialincision resulted in steep, unstable clay slopes alongthe margins of the river, and a series of landslides has

occurred along these slopes (Blackett, 1979). Theoverlying lowstand systems tract consists partly ofcannibalized highstand and transgressive systemstract sediments that were deposited at the toe of theWeber River delta during a forced regression or rela-tive lake level lowering (Posamentier et al., 1992;Posamentier and Morris, 1996) of Lake Bonnevillethat formed the subaerially exposed sequenceboundary (Posamentier and Vail, 1988).

Bear River Delta

The sequence stratigraphy of the Bear Riverdelta was heavily inf luenced by its long, low-gradient ramp geometry. Outcrop exposure of thetransgressive systems tract is incomplete, themaximum exposed thickness being about 50 m.Transgressive systems tract deposits formed dur-ing the early stages of Lake Bonneville as it trans-gressed northeastward are not exposed. Theexposed portion of the transgressive systemstract (situated at the mouth of the OneidaNarrows; Figure 12) consists of very fine grainedprogradational delta-front sheet sands capped bya lacustrine f looding clay unit that forms oneparasequence (Figures 6E, 13). Because the BearRiver delta was situated in a relatively isolatednortheastern arm of Lake Bonneville (Figure 1)and because the prevailing winds were from thenorthwest there was limited wave activity; conse-quently, the delta-front sheet sands display morefluvial inf luence than those of the Weber Riverdelta. The relatively rapid rise of Lake Bonneville,however, resulted in wave-reworking of any delta-plain sediments (e.g., distributary channel depositsand organics) into delta-front deposits. Landwardof delta-front sheet sand sedimentation, beach grav-els were contemporaneously deposited.

As a result of the Bear River delta’s low gradi-ent ramp, there is more large-scale interfingering

648 Lake Bonneville DeltasT

able

8. Sp

anis

h F

ork

Del

ta A

rch

itec

tura

l E

lem

ents

Pri

nci

pal

Lith

ofa

cies

Geo

met

ry a

nd

Elem

ent

Ass

emb

lage

*R

elat

ion

sIn

terp

reta

tio

n

Del

ta-fr

on

t sh

eet

san

dSw

r, S

sh, S

px

b,

Shee

t o

r b

lan

ket

(at

leas

tR

ipp

le s

pac

ing

and

gra

in s

ize

ind

icat

e d

epo

siti

on

Stx

, Fsh

seve

ral 1

0s m

th

ick

×in

wat

er d

epth

s o

f 1–

2 m

(T

ann

er, 1

971;

Asp

ler

et10

00s

m w

ide

and

lon

gal

., 19

94);

ab

sen

ce o

f d

elta

-pla

in d

epo

sits

, ab

un

dan

cein

low

stan

d s

yste

ms

trac

t);

of

wav

e ri

pp

les,

an

d r

ipp

le o

rien

tati

on

s su

gges

t h

igh

geo

met

ry in

tra

nsg

ress

ive

wav

e ac

tivi

ty d

irec

ted

fro

m t

he

no

rth

wes

t; la

ke le

vel

syst

ems

trac

t an

d h

igh

stan

do

scill

atio

ns

reco

gniz

ed in

fo

rese

ts a

s an

gula

r re

lati

on

ssy

stem

s tr

act

no

t kn

ow

n;

bet

wee

n d

ipp

ing

fore

sets

an

d s

ub

ho

rizo

nta

l bed

s;fo

rms

bas

inw

ard

-dip

pin

gsy

nd

epo

siti

on

al s

lum

pin

g m

ay r

epre

sen

tfo

rese

ts w

ith

dip

s o

f 25

–p

aleo

seis

mic

ity

alo

ng

the

Was

atch

fau

lt (

D.

35°

wh

en p

art

of

low

stan

dC

urr

ey, 1

996,

per

son

al c

om

mu

nic

atio

n);

sea

son

alit

y in

syst

ems

trac

t (f

allin

g la

ked

epo

siti

on

is s

ugg

este

d b

y re

pea

ted

cyc

les

of

fin

e- t

ole

vel)

; su

bh

ori

zon

tal b

eds

med

ium

-gra

ined

san

d a

nd

th

inn

er b

eds

of

clay

ey s

iltw

hen

par

t o

f tr

ansg

ress

ive

syst

ems

trac

t o

r h

igh

stan

dsy

stem

s tr

act

(ris

ing

lake

leve

l)

Loes

sFs

hSh

eet

or

bla

nke

t (m

eter

sLo

ess

dep

osi

tio

n p

rob

ably

sta

rted

ab

ou

t 12

.5 k

a, b

ut

thic

k ×

1000

s m

wid

ese

ems

to h

ave

bee

n g

reat

est

du

rin

g th

e ea

rly

and

lon

g); o

verl

ies

grav

els

and

mid

dle

Ho

loce

ne

(Mac

het

te a

nd

Lu

nd

, 198

7)(m

assi

ve t

o c

rud

ely

bed

ded

grav

el o

r cr

oss

-bed

ded

gra

vel)

or

san

ds

(su

bh

ori

zon

tal s

and

s)o

f fl

uvi

al g

rave

ls in

th

e ce

ntr

alp

ort

ion

of

the

del

ta; s

ilty

Bea

ch g

rave

lsG

mc

Rec

tan

gula

r ri

bb

on

(10

s m

Co

arse

-gra

ined

sed

imen

t so

urc

ed b

y lo

cal,

stee

per

thic

k ×

1000

s m

wid

e)gr

adie

nt

stre

ams

and

rew

ork

ed b

y w

aves

pre

serv

ed in

a n

arro

wco

rrid

or

alo

ng

the

Was

atch

Mo

un

tain

s ab

ove

th

e P

rovo

sho

relin

e; p

resu

mab

lyp

rese

nt

in lo

wer

po

rtio

ns

of

del

ta a

lon

g th

e m

ou

nta

infr

on

t, b

ut

no

t ex

po

sed

;so

urc

ed f

rom

loca

l can

yon

s;er

osi

on

of

this

ele

men

taf

ter

the

cata

stro

ph

ic f

all

of

Lake

Bo

nn

evill

e to

th

eP

rovo

sh

ore

line

pro

vid

edso

urc

e fo

r fl

uvi

al g

rave

ls

Lemons and Chan 649

Tab

le 8

. C

on

tin

ued

Pri

nci

pal

Lith

ofa

cies

Geo

met

ry a

nd

Elem

ent

Ass

emb

lage

*R

elat

ion

sIn

terp

reta

tio

n

Flu

vial

gra

vels

Gm

c, G

xb

, Ssh

Elo

nga

te w

edge

th

at t

hin

sFo

rms

a to

pse

t u

nit

th

at d

irec

tly

ove

rlie

s b

asin

war

d-

Sxb

bas

inw

ard

an

d is

gra

ded

dip

pin

g d

elta

-fro

nt

shee

t sa

nd

s; c

on

tact

wit

hto

th

e P

rovo

sh

ore

line

un

der

lyin

g d

elta

-fro

nt

shee

t sa

nd

s is

so

met

imes

ero

sive

(~10

m t

hic

k ×

1000

s(e

.g.,

loca

l sco

urs

on

th

e o

rder

of

a fe

w m

eter

s); f

luvi

alm

wid

e an

d lo

ng)

;gr

avel

s re

pre

sen

t re

wo

rked

bea

ch g

rave

ls o

rigi

nal

lyo

verl

ies

del

ta-fr

on

t el

emen

t;d

epo

site

d n

ear

the

Was

atch

Mo

un

tain

s (M

ach

ette

,ca

n h

ave

ero

sio

nal

bas

e19

92);

co

arse

-gra

ined

nat

ure

an

d c

ross

-bed

ded

bar

fo

rms

in s

hal

low

-wat

er g

eom

etri

es in

dic

ate

dep

osi

tio

n b

y b

raid

ed s

trea

ms

as L

ake

Bo

nn

evill

ere

gres

sed

fro

m t

he

Pro

vo s

ho

relin

e

*Fsh

= f

ines

, su

bhor

izon

tal,

Gm

c =

gra

vel,

mas

sive

to

crud

ely

bedd

ed,

Gxb

= g

rave

l, cr

oss-

bedd

ed,

Spx

b =

san

d, p

lana

r cr

oss=

bedd

ed,

Ssh

= s

and,

sub

horiz

onta

l,Stx

= s

and

trou

gh,

cros

s-be

dded

,S

wr

= s

and,

wav

e rip

pled

, Sxb

= s

and,

cro

ss-b

edde

d.

Figure 5—Architectural elements for the WeberRiver delta. Major boundingsurfaces labeled using thebounding surface hierarchy are shown inTable 2. Black-and-whitestaff divisions are in 15 cmintervals. (A) Lacustrineclay (LC) draped over delta-front (DF)wave-rippled sand. Two zones of intraclastbreccia form third-orderbounding surfaces. (B) Two sequences of delta-margin (DM) flaser-bedded sand overlain by fluvial channeldeposits (FC). The fluvialchannel exhibits an erosive base with cross-beds prograding to the left (west), and apebbly lag that gradesupward into fine to medium grained sand. (C) Beach gravels consisting of massive tocrudely bedded gravel andsubhorizontal sand. (D) Delta-front (DF)deposits overlain by lacustrine clay (LC). At thebase of some lacustrineclay sequences, thinly bedded alternatingsequences of delta-frontand lacustrine sediments(commonly draped overwave ripples) suggestepisodic transition to deeper water lacustrinedeposition or episodicstorm (and other)sand deposition. (E) Thin-bedded lacustrineclay. (F) Lacustrine clay(LC) interfingering in acyclic manner with updipdelta-front (DF) sands. A cycle typically consists ofdelta-front subhorizontalsand overlain by delta-front wave-rippledsand subsequently overlainby darker colored lacustrine clay. (G) Abruptchange from lacustrineclay (LC) into delta-front(DF) deposition indicatingrapid delta-front progradation and the for-mation of a fourth-orderbounding surface. Note the gravels (Gsh) containedin the delta-front deposits(arrow).

DM

FC

DM

4

4

(E)

(B)

(F)

0

1/2mLC

DF

0

1m

(C)

(G)

DF

LC

4

Gsh

(D)

LC

4DF

3

3

4

LCDF

(A)

(more than 20–30 m vertically) between thedelta-front and lacustrine clay deposits in com-parison to the Weber River delta. Some workershave subdivided this interfingering into a greaternumber of parasequences (Anderson and Link,1998); however, we interpreted these to be localto the Bear River delta (not present basinwide in theother deltas) and thus grouped these deposits into asingle parasequence. The exposed lacustrine clayappears to be correlative to the uppermost lacustrineclay of the Weber River transgressive systems tract;however, the updip termination of the clay is approx-imately 25–30 m lower in elevation in the Bear River

delta than in the Weber River delta. This discrepancyprobably results from isostatic rebound differencesbetween the Weber and Bear River deltas. The iso-statically adjusted Bonneville shoreline is 1552 m(Currey and Oviatt, 1985). The current Bonnevilleshoreline ranges from 1555 ±2 m south of Preston,Idaho, to 1584 ±1 m near Salt Lake City (Crittenden,1963; Currey, 1982; Bills et al., 1994). Thus, isostaticrebound resulting from the desiccation of LakeBonneville has resulted in approximately 25–30 m ofrebound in the Weber River delta, with only a fewmeters of rebound in the Bear River delta; therefore,intrabasinal correlation of f looding surfaces and

Lemons and Chan 651

Figure 6—Architectural elements for the Bear Riverdelta. Major bounding surfaces are labeled usingthe bounding surface hierarchy shown in Table 2. (A) Delta-frontsheet sands deposited during regression of Lake Bonneville. Arrowpoints to the base of a wave-rippled, low-angle clinoform set. (B) Similar to(A), but note high-angleclimbing-ripples (Scl) with transport direction to left (west). From the base of the outcrop, thiserroneously appears to beangular eastward-dippingcross-beds suggesting transport direction to the right. (C) Crudely bedded beach gravels. (D) Lacustrine clay (view to the north).

(C)

(D)

0

2m

3

(A)

(B)

Scl

3

0

2m

0

2m

other sequence components based solely on eleva-tion, even in relatively undisturbed Quaternarydeposits, may be misleading.

Highstand systems tract deposits are poorlyexposed. Exposures south of the Oneida Narrows arecomposed of delta-front sheet sands and beach grav-els. Small exposures north of the mouth of the OneidaNarrows show a gentle gradient for the upper deltasurface (less than 0.1°) from the delta origin over adownstream distance of over 16 km (Bright, 1963).The highstand systems tract deposits are subhorizon-tal and show no evidence of downlap onto the maxi-mum flooding surface. Maximum thickness of thehighstand systems tract is on the order of 85 m.

Lowstand systems tract deposits consist of poorlyexposed, basinally isolated (patchy deposits in a bas-inward position relative to previous shoreline),downstepping shoreline deposits reworked fromhighstand systems tract and transgressive systems

tract deposits. These are associated with the forcedregression of Lake Bonneville, which produced thesubaerially exposed sequence boundary as the BearRiver entrenched into the soft sediments of its owndelta when the water level of Lake Bonneville fellcatastrophically from the Bonneville to the Provoshoreline. Fluvial incision created steep, unstableslopes along the margin of the river, resulting in theBear River landslide complex (Mahoney et al., 1987).Due to lower wave energy in the Bear River delta,there was not much Provo-level erosion of highstandsystems tract deposits. The downstream distancefrom the Bear River delta to the present-day GreatSalt Lake is much larger than the distance from theWeber River delta to the lake (approximately 125 vs.20 km), which means that the lowstand systems tractis spread out over a much larger area. Some lowstandsystems tract deposits are preserved on the sides ofthe valley incised by the Bear River. These deposits


(A)

(C)

(B)

5

(D)

(E)

L 4

FG

0

2m

0

2m

Figure 7—Architectural elements for the SpanishFork delta. Black-and-whitestaff divisions are in 15 cmintervals. Major boundingsurfaces labeled using thebounding surface hierarchy shown in Table 2. (A) Transgressivedelta-front sheet sands. (B) Regressive, wave-rippled, delta-frontsheet sands consisting of basinward-dipping (to theright) foresets. Angularcontact with overlying subhorizontal beds indicates a major lake level oscillation and formsa fifth-order bounding surface. (C) Loess (L) capping fluvial gravels(FG). (D) Beach gravels. (E) Fluvial gravels containing cross-beddedbar forms (dipping to the left or south) deposited by braidedstreams.

include low-angle, wave-rippled clinoforms (Figure6A, B). Maximum thickness of the lowstand systemstract is on the order of tens of meters.

Spanish Fork Delta

The sequence stratigraphy of the Spanish Forkdelta was strongly affected by lack of accommo-dation and the smaller drainage-basin size of theSpanish Fork River. Outcrop exposure of thetransgressive systems tract is poor and consists ofa few exposures of subhorizontal delta-front

sheet sands preserved adjacent to the WasatchMountains below the Provo shoreline (Figures 7,14–16). Lacustrine f looding clays that typicallyserve as parasequence boundaries in the Weberand Bear River deltas may exist, but are notexposed. Shoreward of delta-front sheet sand sedi-mentation, beach gravels were contemporaneouslydeposited. Maximum thickness of the transgressivesystems tract is unknown.

Highstand systems tract deposits are also poorlyexposed. Part of the highstand systems tract was can-nibalized when the water level in Lake Bonneville fellcatastrophically to the Provo level. The Spanish Fork

Lemons and Chan 653

(Madsen and Currey, 1979)

Lake level, altitudes adjustedfor net isostatic rebound(Currey and Oviatt, 1985)

Provo

Stansburyoscillation

oscillation

Bonneville

shoreline

Gilbert

Bonnevilleflood

AGE (103 yr)

1015202530

4100 1250

4200

4300

4400

4500

4600

1300

1350

1400

4700

4800

4900

5000

5100

1450

1500

1550

ALTITUDE

Glacial maxima, Pinedale deposition

shoreline

shoreline

ft m

Keg Mountain

Figure 8—Reconstructed Lake Bonneville lake levels (Currey and Oviatt, 1985) with glacial maxima during Pinedaledeposition shown in shaded area. Altitudes are adjusted for isostatic rebound and faulting. At its maximum heightof 1552 m above sea level, the highstand of Lake Bonneville was slightly later than the glacial maxima duringPinedale deposition (Madsen and Currey, 1979). Lake Bonneville began to rise about 28 ka and continued totransgress until the Stansbury oscillation (22–20 ka), when it formed the Stansbury shoreline (1372 m). The lakethen continued to rise, with minor fluctuations, to its highest level (1552 m), where it formed the Bonneville shore-line sometime shortly after 15.3 ka (Oviatt et al., 1992). At the Bonneville shoreline, the lake overflowed intermit-tently near Red Rock Pass into the Snake River drainage of southeastern Idaho (Currey et al., 1984). The overflowwaters eventually caused hydraulic failure of relatively unconsolidated sediments forming the basin rim near RedRock Pass, scoured a channel down to well-indurated materials, and released the catastrophic Bonneville flood atapproximately 14.5 ka (Jarrett and Malde, 1987). This flood lowered lake level approximately 100 m to the Provoshoreline, where the lake overflowed intermittently, until the post-Provo regression (14–12 ka) ended the Bon-neville deep-lake cycle (Oviatt et al., 1992). The Great Salt Lake remains today as a much-evolved descendant of theformer Lake Bonneville.

654 Lake Bonneville DeltasT

able

9. Su

mm

ary o

f Fo

rcin

g P

aram

eter

s

Ram

p C

har

acte

rist

ics*

*Le

ngt

hA

cco

mm

od

atio

nSe

dim

ent

Del

taT

ecto

nic

s*P

hys

iogr

aph

y(k

m)

(m)

Yie

ld†

Lim

no

stas

y††

Web

erLa

te P

leis

toce

ne

and

Ho

loce

ne

slip

rat

es ~

0.9–

Sho

rt r

amp

~20

~27

0~

705

m3 /

km2 /

yrSa

me

for

all

Riv

er1.

9 m

m/y

r fo

r la

st 1

5 k.

y. (

Nel

son

an

dth

ree

del

tas

del

taP

erso

niu

s, 1

993)

; tim

e re

pre

sen

ted

in e

xp

ose

dd

elta

is ~

12–1

3 k.

y.; t

her

efo

re, m

axim

um

sub

sid

ence

wo

uld

hav

e b

een

~12

–25

m

Bea

rP

re-la

te Q

uat

ern

ary

slip

rat

es a

lon

g Ea

st C

ach

eLo

ng

ram

p~

100

~27

0C

om

par

able

to

Sam

e fo

r al

lR

iver

fau

lt z

on

e n

ear

del

ta a

re ~

0.05

–0.1

0 m

m/y

r;W

eber

Riv

er d

elta

thre

e d

elta

sd

elta

ho

wev

er, n

o a

pp

aren

t m

ove

men

t in

late

Qu

ater

nar

y (M

cCal

pin

, 198

8, 1

994;

McC

alp

inan

d F

orm

an, 1

991)

; acc

om

mo

dat

ion

sp

ace

likel

y a

fun

ctio

n o

f la

kele

vel o

nly

Span

ish

Slip

rat

es a

lon

g th

e W

asat

ch f

ault

zo

ne

nea

rSh

ort

ram

p~

17~

180

Co

mp

arab

le t

oSa

me

for

all

Fork

Span

ish

Fo

rk a

re m

ore

dif

ficu

lt t

o d

eter

min

eW

eber

Riv

er d

elta

thre

e d

elta

sd

elta

du

e to

co

mp

lex

ity

of

the

fau

lt a

s it

fo

rms

a m

ajo

rco

nca

ve-t

o-t

he-

wes

t b

end

(M

ach

ette

, 199

2);

ho

wev

er, l

ate

Ple

isto

cen

e sl

ip r

ates

of

0.5–

3.0

mm

/yr

seem

rea

son

able

(M

. Mac

het

te, 1

996,

per

son

al c

om

mu

nic

atio

n);

tim

e re

pre

sen

ted

inex

po

sed

del

ta is

~7

k.y.

; th

eref

ore

, max

imu

msu

bsi

den

ce d

uri

ng

that

tim

e w

ou

ld h

ave

bee

n

~3.

5–21

m

*Sub

side

nce

alon

g W

asat

ch o

r E

ast C

ache

faul

t zon

e.**

Ram

p le

ngth

is m

easu

red

from

mou

th o

f riv

er (

i.e.,

at th

e W

asat

ch M

ount

ains

or

One

ida

Nar

row

s) to

sho

relin

e of

pre

sent

-day

Gre

at S

alt L

ake.

Acc

omm

odat

ion

is m

easu

red

from

Bon

nevi

lle s

hore

line

(i.e.

, hig

hest

lake

leve

l) to

the

pres

ent-

day

valle

y flo

or im

med

iate

ly b

asin

war

d of

eac

h de

lta.

† Fro

m L

emon

s et

al.

(199

6).

††La

ke B

onne

ville

hyd

rogr

aph

from

Cur

rey

and

Ovi

att (

1985

).

Lemons and Chan 655

I-84

Weber River

Ogden RiverMiddle ForkWeber River

Bonnevilleshoreline

Wasatchfault

Great Salt Lake

North

89

A’

B

I-15

WasatchMountains

North ForkWeber River

South ForkWeber River

21

1 20km

km outcrop localitydriller’s log

normal fault(ball on downthrown side)

core description

Figure 9—Base map of the WeberRiver delta.

TST

LST

truncated section

TST

HST

Great Salt Lake

WasatchMountains

~ 300 m

East

~ 20 km

West

delta-front sheet sands

lacustrine clay

delta-margin, fluvial channel,and beach gravel deposits

delta-front sheet sands with interbedded clayMFS

sequenceboundary

correlativeconformity

Wasatch fault

Fielding geosol

2

1

Provoshoreline

Bonnevilleshoreline

Pre-Bonneville sediments

Figure 10—Simplified two-dimensional model of the sequence stratigraphy of the Weber River delta. TST = trans-gressive systems tract, HST = highstand systems tract, LST = lowstand systems tract, MFS = maximum flooding sur-face; 1 and 2 are parasequences.

River has a smaller drainage-basin size than the Weberand Bear rivers, resulting in a small delta. Wave erosionassociated with the Provo level of Lake Bonnevilleremoved a large portion of the highstand systemstract. Preserved highstand systems tract deposits exist

in a narrow corridor directly adjacent to the WasatchMountains and consist of delta-front sheet sands andbeach gravels. Because the boundary between thetransgressive and highstand systems tracts is notexposed, maximum thickness is unknown.


NS

E

W

Bonneville shoreline

Provo shoreline

TST

HST

LST

NS

E

W

NS

E

W

lacustrine clay delta-front sheet sand

fluvial channel, delta-margin,and beach gravel deposits

delta-front sheet sandand clay

WasatchMountains

WasatchMountains

WasatchMountains

Bonnevilleshoreline

100s

m10

0s m

100s

m

10s km10s km

10s km10s km

10s km10s km

(A)

(B)

(C)

Figure 11—Block diagrams illustrating the progressive development of the sequencestratigraphy of the Weber Riverdelta. The development of theBear River delta is similar to thatof the Weber River delta, exceptthat the lowstand systems tract is spread out over a much largerarea and exists as a series of basinally isolated shorelinedeposits.

The lowstand systems tract of the Spanish Forkdelta is quite different than that of the Weber andBear River deltas. This systems tract mainly consistsof basinward-dipping foresets composed of fine-to medium-grained delta-front sheet sands cappedby f luvial gravels and loess (Figure 7C, E). Theforced regression of Lake Bonneville formed a sub-aerially exposed sequence boundary and led towidespread erosion of the transgressive and high-stand systems tract. Accommodation for theSpanish Fork delta is approximately 90 m less thanthat for the Weber and Bear River deltas; thisreduced accommodation caused reworked trans-gressive and highstand systems tract finer grainedsediments to be deposited as strongly progradation-al, wave-rippled delta-front clinoforms. Acceleratormass spectrometer radiocarbon dates of two gastro-pod shells found in these deposits indicate thereworked (18,700 ±70 yr ago, deposited during laketransgression; Beta-94278; Figure 8) and regressive(14,740 ±60 yr ago, deposited after fall of lake toProvo level; Beta-94279; Figure 8) nature of these

deposits. The oblique (vs. sigmoidal) nature of the cli-noforms indicates rapid progradation with slow aggra-dation (Helland-Hansen, 1993). Wave-ripple spacingsand grain size suggest deposition in water depths of1–2 m (Tanner, 1971; Aspler et al., 1994). Angularcontacts between basinward-dipping foresets andoverlying subhorizontal beds record at least twomajor oscillations of the post-Provo regression of LakeBonneville (Figure 7B). Reworked coarse-grainedtransgressive and highstand systems tract sediments(e.g., beach gravels) were later deposited as a veneerof fluvial gravels that thins basinward, graded to theProvo shoreline. Following deposition of the fluvialgravels, a thin veneer of loess was deposited along thecentral portions of the delta.

DISCUSSION

The forcing parameters (in the sense ofPosamentier and Allen, 1993) governing the devel-opment of depositional sequences are limnostasy,

Lemons and Chan 657

TreasuretonOneidaNarrows

Preston

Banida

Riverdale

BearRiver

34 36

0 1km 1/2

1/2km

1 N

outcrop locality driller’s log

normal fault(ball on downthrown side)

91

34

EastCachefault zone

Bonnevilleshoreline

91

8991

34

91

km

km

0 5 10

510 N

I-15

Logan

BrighamCity

Great Salt Lake

Idaho

Utah

Bonnevilleshoreline

BearRiver

36

Bea

r R

iver

Ran

ge

WasatchMountains

Preston

Bonnevilleshoreline

Bonnevilleshoreline

Whitney

Figure 12—Base map of the Bear River delta.


0

1

2

21

outcrop localitydriller’s lognormal fault(ball on downthrown side)

km

km

Payson Salem

SpanishFork

Springville

I-15

50691

Bonnevilleshoreline

Bonnevilleshoreline

Bonnevilleshoreline

Wasatchfault

8950

650

SpanishForkRiver

ProvoBayUtah Lake

North

Figure 13—Simplified two-dimensional model of the sequence stratigraphy of the Bear River delta.

Figure 14—Base map of the Spanish Fork delta.

Pre-Bonneville sediments

Fielding geosol

LST

~300 m

Bear River Range

portion of delta exposednear Preston, Idaho

truncated section

Great Salt Lake

?

?

~125 km

delta-front sheet sand(with interbedded clay)

delta-frontsheet sand

lacustrine clay beach gravels

HST

TST

East Cachefault zone

sequence boundary

MFS

tectonics, sediment yield rates, and physiography(Table 9). The change in the water level of LakeBonneville was similar for all three deltas. Tectonicsubsidence ranged from maximum of 25 m to mini-mum of 0 m; however, compared to changing lakelevel, which was on the order of 300 m, subsidencewas relatively insignificant (Oviatt et al., 1994).Sediment yield rates have been determined only forthe Weber River (Lemons et al., 1996), but areassumed to be similar for all three rivers. The oneunique parameter for each delta is physiography;therefore, the variability in the stratal architectureof each delta, especially within the lowstand sys-tems tract, appears to be chief ly the result ofdifferences in ramp length and accommodation(Figure 17). This finding is consistent with the sug-gestion of Posamentier and Allen (1993) that sedi-ment flux and physiography determine the stratalarchitecture between bounding surfaces, whereaseustasy and tectonics determine the timing of the

sequence boundary surface. As the result of thecatastrophic fall of Lake Bonneville, all three deltashave synchronous sequence boundaries.

As an alternative to the traditional sequencestratigraphic model (e.g., Posamentier et al., 1988;Van Wagoner et al., 1988), the lowstand systemstract identified in these three deltas could be classi-fied as the forced regressive wedge systems tract(e.g., Hunt and Tucker, 1992; Helland-Hansen andGjelberg, 1994; Mellere and Steele, 1995). Theforced regressive wedge systems tract forms duringfalling base level and is bounded above by thesequence boundary, representing the lowestpoint in base level. After sequence boundary for-mation, the lowstand prograding wedge systemstract develops as relative base level begins to rise.Using this model, the present-day Great Salt Lakewould represent lowest base level, the sequenceboundary would be currently forming, and thepreviously defined lowstand systems tracts would

Lemons and Chan 659

?

?

Dimple Dell geosolWasatch fault

WasatchMountains

truncatedsection

sequenceboundary

~ 200 m

~ 17 km

LST

TST & HST

delta-front sheetsand (TST and HST)

fluvial andbeach gravels

delta-frontforesets (LST)

loess

UtahLake

West East

lacustrine clay ?

Figure 15—Simplified two-dimensional model of the sequence stratigraphy of the Spanish Fork delta.

be packaged into the forced regressive wedge sys-tems tract.

These lacustrine deltas can serve as analogs forhydrocarbon exploration and production in similarbasins. For all three deltas, potential hydrocarbonreservoir quality (e.g., grain size and sorting) islargely a product of drainage-basin size and streamgradient, whereas size and shape of the delta-frontdeposits is more a product of accommodation andramp length of the preexisting depositional surface(Tables 4, 6, 8). Compaction, diagenesis, and othersimilar processes also would be important withtime and burial. The relatively small drainage-basinsize and higher stream gradient of the Weber andSpanish Fork rivers provided coarser grainedsands (fine to medium grained sand) and less siltand clay than the Bear River (very fine grained

sand). As a consequence, the delta-front sheetsands of the Weber River delta (and presumablythe Spanish Fork delta) have higher permeabili-ties than similar deposits of the Bear River delta(Lemons, 1997).

Relative differences in ramp lengths influencedthickness and lateral continuity of delta-frontdeposits. The relatively short ramp length of theWeber River and Spanish Fork deltas resulted inthicker, more laterally continuous delta-frontdeposits. The relatively long ramp length of theBear River delta resulted in thin, patchy delta-front deposits (spread out over a much largerarea), especially in the lowstand systems tract.Relative accommodation differences influencedthe internal geometry of the delta-front deposits,especially in the lowstand systems tract. Lake


NS

E

W

Bonneville shoreline

Provo shoreline

(B) LST

WasatchMountains

NS

E

W

WasatchMountains

(A) TST and HST

delta-front foresetsfluvial and beachgravel depositsdelta-front sheet sand

?

??

regressingshoreline

??

?

100s

m10

0s m

Dimple Dellgeosol

Dimple Dellgeosol

10s km10s km

10s km10s km

Figure 16—Block diagrams illustrating the progressive development of thesequence stratigraphy of the Spanish Fork delta.

level rise was so rapid that accommodation wasnot a significant factor until Lake Bonnevillecatastrophically fell to the Provo level. Relativelylow accommodation in the Spanish Fork deltayielded a series of strongly progradational clino-forms in the lowstand systems tract. Relativelyhigher accommodation in the Weber River deltaresulted in subhorizontal lowstand systems tractdeposits. The lowstand systems tract of the BearRiver delta contains both subhorizontal depositsand low-angle clinoforms.

CONCLUSIONS

(1) Three fine-grained deltas on the eastern marginof Lake Bonneville exhibit distinctive facies architec-ture and sequence stratigraphic packages. Significantglaciation and steep river gradients contributed to

the formation of coarse-grained Gilbert deltas alonglocally sourced rivers, whereas minor glaciation andlower river gradients contributed to contemporane-ous fine-grained deltas fed by larger rivers.

(2) The Weber River delta is a wave-influenced deltacomposed of five architectural elements: delta-frontsheet sands, delta-margin deposits, fluvial channeldeposits, beach gravels, and lacustrine clays. Thesequence stratigraphy of the Weber River delta closelyresembles that of a passive margin. The transgressivesystems tract consists of two parasequences forming aretrogradational parasequence set. The parasequencesare composed of lacustrine flooding clays overlain byprograding delta-front sheet sands. The highstand sys-tems tract was partially removed by wave erosion. Thelowstand systems tract consists partly of cannibalizedtransgressive and highstand systems tract sedimentsdeposited in the delta-front environment at the toe ofthe delta during a forced regression of the lake.

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Figure 17—Effects of accommodation and ramp lengthon the sequence stratigraphy of the Weber, Bear, and Spanish Forkdeltas (WRD, BRD, and SFD, respectively). Physiography (asexpressed in accommodation andramp length) is mainly responsiblefor the variability in stratal architecture of each delta, especially in the lowstand systemstract. The high accommodationand short ramp length in theWeber River delta resulted in asequence stratigraphy similar tothat of a passive margin. The highaccommodation and long ramplength of the Bear River delta led to a lowstand systems tract consisting of poorly exposed, basinally isolated patches of downstepping shoreline deposits.The low accommodation andshort ramp length of the SpanishFork delta yielded a lowstand systems tract consisting of basinward-dipping clinoforms.

accommodation highlow

long

shor

tra

mp

len

gth

BRD

WRDSFDLST

LST

LST

delta-front sheetsand (TST and HST)

fluvial andbeach gravels

delta-frontforesets (LST)

loess

delta-front sheet sandand interbedded clay

delta-front sheet sand

lacustrine clay

delta-margin, fluvial channel,and beach gravel deposits

delta-front sheet sand

delta-front sheet sand(and interbedded clay)

lacustrine clay

beach gravels

(3) The Bear River delta was influenced by bothwave and fluvial processes. This delta is composedof three architectural elements: delta-front sheetsands, beach gravels, and lacustrine clays. Thesequence stratigraphy of the Bear River delta washeavily influenced by its long, low-gradient rampgeometry. Outcrop exposure of the transgressivesystems tract shows one parasequence composedof lacustrine flooding clays overlain by progradingdelta-front sheet sands. The highstand systems tracthas similar poor exposures with basinally isolateddownstepping shoreline deposits formed during aforced regression of the lake.

(4) The Spanish Fork delta is a wave-influenceddelta composed of four architectural elements:delta-front sheet sands, beach gravels, fluvial grav-els, and loess. The sequence stratigraphy of theSpanish Fork delta was strongly affected by lack ofaccommodation and the smaller size of the delta.The transgressive systems tract is poorly exposedand partially consists of subhorizontal delta-frontsheet sands. The highstand systems tract was most-ly removed by wave erosion. The lowstand systemstract consists mainly of strongly progradational,basinward-dipping foresets formed in the delta-front environment.

(5) Limnostasy, tectonics, and sediment yieldwere similar for all three fine-grained deltas; there-fore, by process of elimination, the forcing parame-ter that exerted the greatest inf luence on thesequence stratigraphy of all three deltas appears tobe physiography. This physiographic control ismost pronounced in the lowstand systems tract ofeach delta.

(6) Potential hydrocarbon reservoir quality (e.g.,grain size and sorting) is largely a product ofdrainage-basin size and stream gradient. Relativedifferences in ramp lengths affected thickness andlateral continuity of delta-front deposits. Relativedifferences in accommodation determined theinternal geometry of the delta-front deposits, espe-cially in the lowstand systems tract.

(7) The well-documented age, limnostasy, basinphysiography, tectonics, climate, and sedimentyield of these Bonneville deposits make an excel-lent analog for understanding lacustrine subsurfacehydrocarbon reservoirs where stratal surfaces maybe preserved but forcing parameters typically areunknown.

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Lemons and Chan 665

David R. Lemons

David R. Lemons is a geologistwith Exxon Exploration Company.He received his Ph.D. in geologyfrom the University of Utah in 1997.His work focused on the stratigra-phy, permeability distribution, andpaleoclimatic implications of fine-grained lacustrine deltas in theBonneville basin. He completed hisB.S. and M.S. degrees in geology atBaylor University in 1984 and 1987,respectively. After obtaining his M.S. degree, he workedfor Oryx Energy as a production and exploration geologist.

Marjorie Chan

Marjorie Chan received her Ph.D.from the University of Wisconsin–Madison in 1982. She has been atthe University of Utah for 15 yearsand currently is a full professor. Sheand her students have worked on awide range of clastic sedimentologyand stratigraphy problems in thewestern United States, ranging fromthe Precambrian up through thePleistocene.

ABOUT THE AUTHORS

Facies Architecture and Sequence Stratigraphy of Fine ... · Facies Architecture and Sequence Stratigraphy of Fine-Grained Lacustrine Deltas Along the Eastern Margin of Late Pleistocene

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