U-Pb detrital zircon geochronology and provenance of the Tertiary Kootznahoo Formation, southeastern Alaska: A sedimentary record of Coast Mountains exhumation Nathan Evenson Senior Integrative Exercise March 10, 2010 Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota.
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U-Pb detrital zircon geochronology and provenance of the Tertiary Kootznahoo Formation, southeastern Alaska: A sedimentary record of Coast Mountains
exhumation
Nathan Evenson Senior Integrative Exercise
March 10, 2010
Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota.
TABLE OF CONTENTS
IntroductionGeologic SettingThe Kootznahoo FormationField Sampling MethodsSedimentary Petrography Methods ResultsU/Pb Detrital Geochronology Methods Results U/Th Ratios Statistical ComparisonsDiscussion Source Region Classification Source Regions of Detrital Zircons in the Kootznahoo Formation Major Kootznahoo Formation DZ populations Kootznahoo Formation DZ populations older than 200 Ma Implications of Detrital Zircon and Sedimentary Petrography Data Links between Kootznahoo Formation deposition and exhumation of the CMB Timing and continuity of deposition in the Kootznahoo basin Kootznahoo Formation deposition and the northward transport of outboad terranesConclusionsAcknowledgementsReferencesAppendix 1: U/Pb Analytical MethodsAppendix 2: Isotopic Data
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U-Pb detrital zircon geochronology and provenance of the Tertiary Kootznahoo Formation, southeastern Alaska: A sedimentary record of Coast Mountains
exhumation
Nathan S. Evenson Senior Integrative Exercise
March 10, 2010 Carleton College
Advisors: Cameron Davidson, Carleton College
Karl Wirth, Macalester College Tim White, Pennsylvania State University
ABSTRACT Quartzo-feldspathic arenites of the Kootznahoo Formation in Southeast Alaska contain detrital zircon U-Pb age distributions dominated by plutonic zircon populations at 190- 160, 93-85, and 65-50 Ma. These populations suggest derivation from the adjacent Coast Mountains Batholith complex. Consideration of maximum depositional ages for the Kootznahoo Formation and exhumation rates of the Coast Mountains Batholith suggest that most of the Kootznahoo Formation in the study area was deposited between 60-25 Ma. An abrupt influx of Paleogene-aged zircons suggests that the initial un-roofing of the Coast Plutonic Complex, east of the Coast shear zone, is captured within the Kootznahoo stratigraphy. We use the Kolgomorov-Smirnov statistical test to demonstrate a depositional affinity between the Kootznahoo Formation exposed in lower Little Pybus Bay on Admiralty Island and the lower portion of the Kootznahoo stratigraphy in the Keku Straits region to the south on Kupreanof Island. Our data suggests a depositional history of episodic rapid subsidence and sedimentation alternating with slow subsidence and erosion of previously deposited strata. An episode of slow sedimentation at approximately 54 Ma may be a result of uplift that occurred following the subduction of an oceanic ridge. Observed similarities between the detrital U-Pb zircon ages of the Kootznahoo Formation and the Kulthieth and Poul Creek Formations, deposited on the Yakutat block presently located ~500 km to the north, suggest that these formations were adjacent and shared a sediment source between 45-35 Ma. Keywords: Detrital zircon, Kootznahoo Formation, Coast Mountains Batholith, U/Pb geochronology, provenance, exhumation
INTRODUCTION
Detrital zircon (DZ) studies are a robust method of analyzing the depositional history
and provenance of clastic sedimentary successions and the tectonic and magmatic evolution
of the rocks from which these sediments were derived (Gehrels et al., 2006; Gehrels et al.,
2000). Previous DZ studies have established probable source regions for sedimentary and
metasedimentary strata along the western margin of North America (Gehrels et al., 1995;
Gehrels and Kapp, 1998; Kapp and Gehrels, 1998). Recent advances in analytical techniques
permit greater sampling densities, resulting in the generation of high-resolution age spectra.
Such spectra have been employed to elucidate the details of the tectonic chronology of the
Cordilleran margin (e.g. DeGraaff-Surpless et al., 2003; Haeussler et al., 2004) and other
areas (e.g. Weislogel et al., 2006). DZ data are often complemented by optical point count
analyses. Together, these data provide detailed information on the source terrane lithology
and sediment maturity (Dickinson and Suczek, 1979; Dickinson, 1985). In this study, I
examine high-resolution detrital zircon and sedimentary provenance data from the Tertiary
(Paleocene-Miocene) Kootznahoo Formation of southeast Alaska.
The Mesozoic-Cenozoic geologic history of western North American is defined by
frequent, major changes in the regional tectonic regime, including shifts between convergent
and transform motion (Crawford et al., 2005; Haeussler et al., 2003), accretion and transport
of allocthonous terranes along the margin (Monger et al., 1982; Gehrels and Berg, 1994), and
magmatic events that accompanied these processes (Gehrels et al., 2009; Crawford et al.,
2005). Changes in tectonic setting during the Cretaceous to middle Tertiary are closely tied
to the emplacement, uplift, and exhumation of the plutonic rocks of the Coast Mountains
batholith (CMB), located in present-day British Columbia and southeast Alaska (Crawford et
1
al., 1987; Gehrels et al., 2009). Gehrels et al. (2009) provide a comprehensive review of U-
Pb geochronology of the CMB and the nearby Coast Shear Zone (CSZ). Zircon age data
suggest the occurrence of three periods of relatively high magmatic flux, at 160-140 Ma,
120-78 Ma, and 55-48 Ma. Spatial distribution of U-Pb ages suggests eastward migration of
magmatic activity after 120 Ma. In this study, I compare DZ populations and sedimentary
provenance data from the Kootznahoo Formation to the results of Gehrels et al. (2009) and
geochronological studies of other Cordilleran terranes. The primary objective of these
comparisons is to determine probable sediment source regions for the Kootznahoo
Formation, and to use observed shifts in provenance to make inferences about the timing of
uplift, exhumation, and possible margin-parallel transport history of these source regions.
Within the study area, the Kootznahoo Formation is present in a number of exposures
separated by large covered areas (Fig. 1). Cover material may obstruct the expression of
transform faults or other structures that have modified depositional relationships between
exposures. White et al. (in prep) have created a tentative stratigraphic sequence (Fig. 2) that
links these scattered exposures together. An additional objective of this study is to apply
internal comparisons and tests of similarity to our DZ and sedimentary framework analyses
with respect to the stratigraphy of White et al. (in prep) to determine whether Kootznahoo
DZ data support correlations that these workers have made.
GEOLOGIC SETTING
The Kootznahoo Formation lies unconformably on variably deformed rocks of the
Alexander terrane and the Gravina belt (Loney, 1964; Muffler, 1967; this study). The
Alexander terrane, a sequence of Paleozoic to middle Jurassic sedimentary and igneous
2
PortageBay
Hamil tonBay
Big JohnBay
DavidsonBay
KadakeBay
KuiuIsland
KuiuIsland
KupreanofIsland
Kake Airport
Dak
anee
k Bay
Ro
ck
y P
as s
Po
rt Ca
md
en
Keku Stra i t
Kupreanof Island
133°40
56°55
56°50
56°45
133°45133°50133°55134°00
0 3 km
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
Tmgb
MDc
MDc
MDc
Pp
Pp
Ksm
Tk
Tk
Ksm
Ksm
Pp
Pp
Qs
Tk
Tk
Tk
TkQTf
QTf
QTf
QTf
QTf
QTf
QTb
QTb
QTb
QTb
QTb
QTb
QTc
QTc
QTc
QTc
vRT
cRT
hRT
hRT
kRT
vRT
vRT
vRT
vRT
vRT
QTc
Qs
QTf
Geologic Map of the Keku Straight Area and Little Pybus Bay (Inset), Southeast Alaska.Modified from Lathram et al. (1965), Muffler (1967), and Brew et al. (1984)
Surficial deposits (Quaternary): Alluvium, glaciofluvial deposits, and tidal mud flats.Felsic volcanics (Quaternary and Tertiary): Grey to buff altered volcanic flows, tuff, or shallow intrusives with myrolitic cavities.Basalt (Quaternary and Tertiary): Dark grey to black aphanitic basalt.
Hound Island volcanics (Triassic): Dark green to black basaltic pillow breccia and pillow lava with some interbedded limestone.
Hamilton limestone (Triassic): Dark grey thinly bedded limestone.
Qs
QTf
QTb
QTc
Tk
Ta
Ksm
Pp
MDc
vRT
Seymour Canal Formation (Late Jurasic and Early Cretaceous): Dark gray to black slate, greywacke, and conglomerate.
ALASKA
MAP LOCATION
hRT
cRT
kRT
Volcaniclastic deposits (Quaternary and Tertiary): Grey to buff conglomerate and lithic sandstone. Gradational contact with Tk.
Admirality Island volcanics (Eocene and Oligocene): Dark grey basalt and andesite flows .
Tmgb Gabbro (Oligocene): Grey to black phaneritic gabbro with olivine and clinopyroxene.
Kootznahoo Formation (Paleocene-Miocene): Grey to buff arkosic sandstone, conglomerate, and black shale. Coal, fossil leaves and wood present.Turbidites (Early Cretaceous?): Rhythmically layered grey to black sandstone, siltstone, and mudstone with carbonate concretions.
Cornwallis limestone (Triassic): Grey medium to thick bedded oolitic limestone.Keku volcanics (Triassic): Altered felsic flows and breccia, basalt, volcanoclastics and limestone.Pybus Formation (Permian): White to light grey limestone, dolostone and chert.
Cannery Formation (Mississipian and Devonian): Dark grey to bluish green thinly bedded volcanic argillite and greywacke with chert.
DZ and FrameworkSample location
Strike and dip of bedding
09TH01
09TH1015
10
15
135
12
16
10
10
10
5
25
15
25 15
15
1510
20
1515
09LA05
09LA08
09TH08
09LA1009NE18
09NE14
09NE12
09NE06
09NE0814
12
20
20
15
15
KJs
57° 19’134° 11’ 134° 05’
57° 15 ’
Ad
mir
al i
ty I
sla
nd
09LA01
09LA14
Lit tle
Pyb
us
Ba
y
Tk
KJs
KJs
MDc
Ta
Ta
Tk
1 km
CamdenPoint Tk
DakaneekPoint
09NE20
09NE1509NE07
09NE29
Framework onlySample location
1 Location of measuredstratigraphic section(see Fig. 4)
1
2
PointHamil ton
3
4
5Porcelaini teBeach
Figure 1. Geologic map of the Keku Strait region and Little Pybus Bay. Sample locations and section numbers correspond to Fig. 2. Note that the Little Pybus Bay outcrops (inset) are separated from the Keku Strait outcrops by Frederick Sound, a distance of approximately 50 km. Map modified from Davidson (pers. comm.)
3
MDc
Ta
09LA1459(55) Ma
BJBS
BJB
Dav. B.Dak. B.
Dak. Pt.
Pt. Ham.
53.5 ± 0.6 Ma
~65 mcoveredinterval
?
?
2
4
s fs cs cglKsm
Cmd. Pt.
Low. PB
Upp. PB
26.5 ± 0.3 Ma
09NE1429(28) Ma
09NE1225(24) Ma
? 09LA1057(31) Ma
5
Ham. B.
Hound Is.Volcanics
3
vRT
?
20 mVertical Scale
LPB
1
MDc Tk
s = shale/mudstonefs = fine sandstonecs = coarse sandstonecgl = conglomerate
EXPLANTION
= sill/intrusion
Stratigraphic Units and Lithologies
= correlation uncertain
= volcanic flow/tuff= covered interval
= unconformity
Horizontal scale
= column number (from Fig. 1)
?
1
Ta
Ksm Tmgb
vRT
26.5 ± 0.3 Ma Ar-Ar cooling age(White et al, in prep.)
=
Units from geologic map (Fig. 1)
Other lithology symbols
09LA0159(52) Ma
09NE0658(54) Ma
09NE0859(53) Ma
09TH0858(53) Ma09LA0858(54) Ma09LA0559(54) Ma
09TH1060(53) Ma
09TH0187(80) Ma
09NE1225(24) Ma
=
s fs cs cgl
s fs cs cgl
Figure 2. Kootznahoo Formation stratigraphic sections (modified from White et al., in prep.). Column numbers correspond to locality from which the column was measured (shown on Fig. 1) Horizontal scale indicates grain size. Marked levels of uncertain correlation indicate areas where stratigraphic relationships have been inferred across large (10+ kilometers) locations. Sample heights are listed with corresponding maximum depositional ages (discussed in the Results section).
DZ sample with maximumdepositional age listed:yngst. pop. (yngst. grain)
4
rocks and their metamorphic equivalents, is joined with the Wrangellia and Peninsular
terranes to form the Insular superterrane, or Wrangellia Composite Terrane (WCT) (Monger
et al., 1982; Gehrels and Berg, 1994). The Gravina belt is a band of upper Jurassic to middle
Cretaceous clastic sediments interbedded with basaltic/andesitic flows and volcaniclastics
(Gehrels and Berg, 1994; Cohen and Lundberg, 1993). Jurassic-age Gravina strata were
deposited unconformably on rocks of the Alexander and Wrangellia terranes to the west,
while the eastern margin of the belt has been obscured by and incorporated into
metamorphism and thrust faulting associated with the accretion and subduction of the WCT
(Kapp and Gehrels, 1998). Sedimentary rocks of the Gravina belt are derived from
subduction arc volcanics and basement strata of the Alexander terrane to the west (Cohen and
Lundberg, 1993; Cohen et al., 1995) as well as northern Cordilleran terranes to the east
(Kapp and Gehrels, 1998). Cessation of sedimentation in the Gravina basin occurred at 95-
85 Ma, when the changes in plate motion caused a return to a compressive regime, resulting
in the collapse of the basin and the deformation and uplift of Gravina strata (Engebretson et
al., 1985; McClelland and Mattinson, 2000; Gehrels et al., 2009).
Convergence during the late Cretaceous (up to ~65 Ma) caused regional deformation,
crustal thickening, and deep emplacement of plutons at the continental margin (McClelland
and Mattinson, 2000; Crawford et al., 1999). The CSZ developed as a structure to
accommodate crustal adjustments and west-side up exhumation that occurred in response to
this thickening (McClelland and Mattinson, 2000). Evidence from metamorphic and plutonic
rocks in and adjacent to the CSZ suggests that during the Paleocene, regional tectonics
shifted from a primarily convergent regime to one that included a substantial transform
5
component (Engebretson et al., 1985; Crawford et al., 1999; Klepeis et al., 1998; McClelland
and Mattinson, 2000; Hollister and Andronicos, 2000; Gehrels et al., 2009).
After 65 Ma, ductile extensional processes facilitated rapid exhumation of the roots the
CMB (Hollister, 1982; Crawford et al., 1987; Rusmore et al., 2005). The presence of melt
along low-angle shear zones facilitated extension (Crawford et al., 2009). Tonalitic-
granodioritic plutons were emplaced syntectonically during east-side up motion along the
CSZ (Ingram and Hutton, 1994) and other transform and normal shear zones at depths of 15-
20 km (Metcalf and Davidson, 1997; Crawford et al., 1999; Andronicos et al., 2003). These
younger plutons were exhumed to shallow crustal levels by ductile extension that persisted
until ~50 Ma (Harrison et al., 1979, Rusmore et al., 2005). Some workers attribute complex
changes in tectonic regime, rapid exhumation and Paleocene magmatism to the subduction of
a spreading ridge and adjacent young, buoyant oceanic crust (Cloos, 1993; Thorkelson, 1996;
Hauessler et al., 2003; Bradley et al., 2003, Madsen et al., 2006). The Kootznahoo basin
system likely developed as an upper crustal response to these lower crustal extensional
processes (as described in Rohr and Currie, 1997).
After 50 Ma, exhumation of the roots of the batholith continued via brittle deformation
of the upper crust. Plate reconstructions suggest that oblique convergence that occurred
during the Paleocene and Eocene shifted to dextral transform motion with a small
transtensional component by ~45-40 Ma (Engebretson et al., 1985; Stock and Molnar, 1988).
Studies of the sediments and structural geology of the Queen Charlotte Basin (located
outboard of the southern reaches of the CMB between Vancouver Island and Alaska’s
southern border) suggest that this margin-normal extensional component initiated rifting in
the middle Eocene, facilitating subsidence along N-S striking normal faults (Rohr and
6
Dietrich, 1992; Hyndman and Hamilton, 1993; Irving et al., 2000). Near Prince Rupert,
similarly oriented normal faults in the CSZ have been dated to 30 Ma (Davidson et al., 2003).
The distribution of brittle extensional structures of middle Tertiary age suggest that the basin
system in which Kootznahoo sediments accumulated was expanded and/or otherwise
modified by brittle extensional processes similar to those described in studies cited above.
The shift to dextral transform motion provided a mechanism for margin-parallel
northward terrane transport. The Chugach terrane is a block of subduction-related deformed
submarine volcanics interbedded with pelagic sediments (Plafker, 1987). Currently located
along the south-central Alaskan coast (~58°N), some paleomagnetic and geologic evidence
suggest that the CPW terrane was located south of 50°N in the early Tertiary, and was
transported to its present position by the middle Miocene along dextral strike-slip faults
(Cowan, 1982; Nilsen and Zuffa, 1982; Cowan, 2003). The Yakutat block consists of
basement of Chugach-Wrangellia affinity, with Eocene-Pleistocene age sedimentary cover
(Plafker, 1987). Detrital zircon evidence suggests that the Yakutat terrane originated off the
coast of British Columbia, and experienced moderate northward transport along the Queen
Charlotte Fault after its detachment from the margin at ~45 Ma (Hyndman and Hamilton,
1993; Perry et al., 2009). It is possible that one or both of these terranes were adjacent to the
Kootznahoo during active deposition, and therefore shared a sediment source, or even
contributed sediment to the Kootznahoo basin.
THE KOOTZNAHOO FORMATION
The Kootznahoo Formation is a composed primarily of feldspathic arenite of Paleocene
to Miocene age, which was deposited in a fluvio-deltaic to marginal marine environment
7
(Brew et al., 1984; White et al., in prep). The Kootznahoo Formation was deposited between
56°50’N and 57°50’N, in a basin oriented parallel to and approximately 50 km west of the
Coast Shear Zone (CSZ) (Dickinson and Pierson, 1988). The Kootznahoo Formation is well-
exposed in the Keku Strait area of Kupreanof Island, in the Kootznahoo Inlet and Little
Pybus Bay on Admiralty Island, and on the southwest side of Zarembo Island (Lathram et al.
1965; Brew et al. 1984; Dickinson and Pierson, 1988). The focus of this study is restricted to
exposures in the Keku Strait and Little Pybus Bay.
The base of the Kootznahoo Formation is in all places an unconformity. In and near
Little Pybus Bay, it lies unconformably on the Late Triassic Hyd Formation of the Alexander
terrane and the deformed Early Jurassic Seymour Canal Formation of the Gravina Belt
(Loney, 1964; Cohen and Lundberg, 1993). In the Keku Strait area, Muffler (1967) observed
the Kootznahoo Formation deposited unconformably on the Triassic Hound Island Volcanics
in the Hamilton Bay locality. I observed an unconformity between the Kootznahoo
Formation sandstones and unnamed Cretaceous Gravina mudstones (unit Ksm on Fig. 1) on
the western side of Point Hamilton (09TH10; Figs. 1 and 3d).
In the Keku Strait area, the Kootznahoo Formation is exposed at several short (20-75 m
of stratigraphic thickness) exposures (see Fig. 1). As shown in Figure 2, the Kootznahoo
Formation in the Keku Strait (right and center columns) consists primarily of coarse-grained
to granular sandstone with poor sorting and highly angular grains. Prominent, mutually
truncating trough cross-stratification is a feature of most of the sandstone beds (Fig. 3e).
Interbedded with the sandstone are beds of shale, some of which bear coal seams or plant
fossils. Preserved stumps, wood fragments, and paleosols are present in sandstone beds
throughout the section. Conglomerate beds are distributed throughout the section as thick
8
A.
E.
D.
C.
Ksm
Ksm
Ksm
Tk
Tk
Arkosic Tk
AIV conglomerate
Ta?
A.B.
Figure 3. Field photos from Keku Strait and Little Pybus Bay outcrops. Photo series progresses clockwise from upper left. A.) Sub-rounded andesitic blocks in matrix-supported congolmerate, from uppermost portions of Kootznahoo stratigraphy in Little Pybus Bay. Clasts likely derived from the Admiralty Island Volcanics (Ta). Gradations on field book are 2 cm. B.) Rounded plutonic and metamorphic clasts in cobble-pebble conglomerate from upper Kootznahoo Formation in Little Pybus Bay. Arrow denotes large garnet crystal in metamorphic clast. C.) Abrupt transition between arkosic sandstone/conglomerate and volcaniclastic AIV conglomerate in upper Kootznahoo Formation, Little Pybus Bay. Black line denotes apparent erosional surface. Sample 09LA01 taken nearby, from 3-4 m below erosional surface. Note honeycomb-type weathering on the outcrop face, indicated by black arrow. Person for scale. D.) Basal contact between Kootznahoo Formation sandstone (Tk) and Cretaceous mudstone (Ksm), near 09TH10 sample location on Point Hamilton. Angular nature of Ksm clasts and apparent rip-up tongue of Ksm that extends above the contact (denoted by white arrow) suggest high-energy deposition of Tk sandstone on partially lithified Ksm mudstone. Hammer is 30 cm long. Photo credit: K. Wirth. E.) Prominent channel forms in at Kadake Bay outcrop, typical of most outcrops of Kootznahoo Formation sandstone. Hammer is 25 cm long.
9
throughout the section. Conglomerate beds are distributed throughout the section as thick
beds and lenses within sandstone, but are concentrated primarily in the Hamilton Bay and
Porcelainite Beach sections. Clast sizes vary, most are 5-25 cm in diameter. In lower
portions of the section, conglomerate clasts are primarily plutonic, gneissic, low- to medium-
grade metamorphic, and vein quartz in composition – volcanic clasts are conspicuously
absent. Clasts of the thick conglomerate beds in the upper portions of the Keku Strait section
(Porcelainite Beach) are entirely volcanic. The section ends in volcaniclastic sandstone and
conglomerate interbedded with Tertiary basalt flows.
The Kootznahoo Formation of the Little Pybus Bay (LPB) locality is lithologically
similar to the lower portions of the Keku Strait section. Notable differences between the two
localities include an increase in the ratio of conglomerate to other lithofacies in the section
and a decrease in measured section thickness (a difference of ~300 meters) (Dickinson and
Pierson, 1988; White et al., in prep). Conglomerate clasts in this locality are similar in
composition to those of the conglomerate beds of the Keku Strait section – consisting
primarily of plutonic and metamorphic (gneissic) rocks, as well as populations of argillite
and chert (Figure 3b; Loney, 1964; Lathram et al., 1965). In the volcaniclastic conglomerate
beds (herein referred to as the AIV conglomerate) that are deposited on the arkosic strata of
the upper Little Pybus Bay section, clasts are almost entirely of volcanic composition, and
are more angular than those found in lower beds (Figure 3a-c). Lying conformably on the
AIV conglomerate are Eocene basaltic-andesitic flows of the Admiralty Island Volcanics
(Loney, 1964).
Correlation between the Little Pybus Bay and Keku Strait sections is based largely on
the work of Lathram et al. (1965). Based on plant fossils taken from shale beds in both
10
localities, Wolfe correlates the Little Pybus Bay section with the Hamilton Bay portion of the
Keku Strait section, assigning these strata a Paleocene age. Other geologic indicators of
depositional age (the positions of which are shown on Fig. 2) in the two sections include a
biotite Ar-Ar date of 53.5 ± 0.6 Ma for a tuff layer in the Big John Bay South section, and a
whole rock Ar-Ar date of 26.5 ± 0.5 Ma for a basaltic flow in the Lower Porcelainite Beach
portion of the Keku Strait section (White et al., in prep.).
The continuity of drainages that fed the Kootznahoo basin, as well as the continuity of
the basin itself, is uncertain (Buddington and Chapin, 1929; Muffler, 1967; Brew et al.,
1984). The Kootznahoo basin formed during deformation that occurred in the late
Cretaceous and early Paleogene (Loney, 1964; Muffler, 1967). Some authors propose that
the basin existed as a continuous drainage (Buddington and Chapin, 1929). Other authors
suggest that the basin may not have been continuous during deposition of some or all
portions of the Kootznahoo stratigraphy, and differences in provenance and lithology
between localities reflect smaller drainages of local source areas (Brew et al., 1984;
Dickinson and Vuletich, 1990).
FIELD SAMPLING METHODS
The goal of the sampling strategy employed in this study was to obtain a set of
samples that were spread evenly across the known Kootznahoo stratigraphy at each of the
two field localities. Where possible, locations of samples relative to a known stratigraphic
datum – geodetic markers, volcanic tuff layers, or other geologically distinct features – were
measured using a Jacob’s staff and compass. Locations of all samples were marked with at
least one portable GPS unit.
11
Each detrital zircon sample consisted of approximately 3-5 kilograms of rock. Most
samples were sandstones with relatively few clasts of larger sizes; a few others were drawn
from the sandstone matrix of a conglomerate. Each sample was taken from a limited
stratigraphic interval; no more than a meter of stratigraphic height separated the rock pieces
that compose a given sample. When possible, samples were taken from a single laterally
contiguous bed. The stratigraphic positions of all samples are shown on Fig. 2.
SEDIMENTARY PETROGRAPHY
Methods
Eighteen thin sections were made from samples from the upper half of the
Kootznahoo sections in the two field localities. All thin sections were stained with sodium
cobaltnitrite for potassium feldspars, and all were examined to determine their detrital
assemblages, alteration states, and other characteristics. Examples of common assemblages
and alteration textures can be seen in Fig. 4.
Ten of these thin sections were subjected to optical point-count analysis. Counting
categories are listed in Table 1. Only thin sections that were free of pervasive diagenetic
alteration and/or apparent secondary matrix development, both of which can obscure the
depositional framework composition of the rock, were subjected to this analysis. For each
modal analysis, at least 350 framework grains were counted. Fewer than 10% of the total
grains counted were classified as unidentifiable or matrix/cement. Point-count analysis was
12
A. B.
C. D.
Figure 4. Common detrital assemblages and textures of arkosic Kootznahoo Formation sandstones. A.) Fine-grained metamorphic mica+quartz lithic fragment (Lm). Partially replaced amphibole (Hbl) fragment is visible above the scale bar. Other components include Plg, Qm (XPL, 40x, 09NE06). B.) Warped biotite grain, partially replaced by chlorite (Chl, purple birefringence) (XPL, 40x, 09NE08). C.) and D.) PPL and XPL views of an identical field, which contains several large Kfs grains, variably altered to calcite (Cal). Note the prominent polysynthetic twinning preserved in the Plg grain in the upper left portion of the field (40x, 09NE24). E.) Foliated, biotite bearing metamor-phic fragments (Lm) in arkosic matrix, note both compaction alteration and warping of biotite grains (PPL, 40x, 09NE20). F.) Volcanic lithic (Lv)-dominated sample from the uppermost portions of the Keku Strait stratigraphy. Note the ubiquity of plagioclase phenocrysts in Lv grains (PPL, 40x, 09NE12).
performed after the Gazzi-Dickinson (G-D) method1 (Dickinson, 1970; Dickinson and
Suczek, 1979).
Results
Results of Kootznahoo sandstone modal analyses are listed in Table 2. Kootznahoo
Formation sandstones are texturally submature; most samples contain populations of poorly
sorted, angular to sub-rounded grains. The scarcity of silt/clay matrix and the relative
abundance of feldspar over quartz lead to the classification of Kootznahoo sandstones as
feldspathic to quartzo-feldspathic arenites (average composition (excluding 09NE12) =
Q36F57L07). Detrital assemblages are relatively continuous at the locality scale, but vary
widely throughout the whole of the Kootznahoo section. Plagioclase feldspar,
monocrystalline quartz, and potassium feldspar are the primary detrital components. Micas
(Fig. 4b,e) (primarily biotite, some allogenic chlorite) and polycrystalline quartz are an
accessory component of most samples, while amphibole (Fig. 4a) and chert are found in
accessory amounts in only a few samples. Lithic fragments, particularly volcanic lithic
fragments, display the greatest variability of abundance among analyzed samples (Fig. 4f).
Alteration of detrital grains is observed in feldspars, micas, and some lithic
fragments. Plagioclase grains are typically partially to fully replaced by a number of
minerals, most commonly calcite or albite. Relict polysynthetic twinning is present in many
fully replaced plagioclase grains. Potassium feldspar is also often partially altered to calcite
and/or albite (Fig. 4c-d). Complete alteration of potassium feldspar grains is rare. This
1Two important differences between the G-D method and the “traditional” methods exist. First, in the G-D method, any sand-size mineral fragment that can be identified is counted as a mineral, regardless of any apparent affinity with any type of lithic fragment. Second, polycrystalline quartz (Qp in Table X) is counted as such only if the grain appears to be pure, plutonic/metamorphic quartz. The traditional method allows 10% (total grain area) of imperfections – such grains are counted as lithic fragments in the G-D method (Ingersoll et al., 1984).1
15
Table 2. K
ootznahoo Formation sandstone m
odal analysis results. Porc. B. = Porcellainite B
each, Pt. Cam
den = Point Cam
den, BJB
S= Big John B
ay South, B
JB = B
ig John Bay, LPB
= Little Pybus Bay.
Table 3. L
ocality group average ternary compositions. K
eku Straits locality group includes BJB
, BJB
S, and Pt. Cam
den samples from
Table 1. Locality G
roup Q
F
L Q
m
F Lt
Qp
Lv Lsm
Q
m
P K
Q
m/Q
t P/F
Porc. B
each 0.01
0.05 0.94
0.01 0.05
0.94 0.00
0.92 0.08
0.13 0.78
0.09 1.00
0.90 Little P
ybus Bay
0.36 0.44
0.20 0.24
0.44 0.32
0.37 0.13
0.50 0.35
0.41 0.24
0.75 0.63
Keku S
trait 0.33
0.62 0.05
0.31 0.62
0.08 0.33
0.00 0.67
0.33 0.37
0.30 0.93
0.56 Locality
Sample
No.
Qm
Q
p C
hert P
K
Lv Lm
Ls
Hv
Mica
Cem
ent U
nident. Total
Frmw
ork P
orc. B.
09NE
12 3
0 0
18 2
326 7
23 0
4 53
14 450
379 P
orc. B.
09NE
14 84
28 14
96 57
15 26
30 0
19 36
20 425
350 P
t. Cam
den 09N
E18
110 8
1 124
98 0
11 7
1 45
31 16
452 359
BJB
S
09NE
20 100
18 0
133 37
7 76
2 0
31 39
7 450
373 B
JBS
09N
E15
93 8
0 197
100 0
0 0
0 27
17 8
450 398
BJB
S
09NE
07 93
23 0
122 69
6 1
3 1
17 62
9 406
317 B
JBS
09N
E06
86 7
0 167
70 0
17 1
0 25
17 7
397 348
BJB
09N
E08
92 8
0 125
60 0
1 3
17 27
36 26
395 289
LPB
09LA
01 168
26 2
185 0
0 8
2 0
16 26
18 451
391 LP
B
09NE
29 137
29 0
173 3
1 12
2 0
47 25
21 450
357
Locality Sam
ple N
o. Q
F
L Q
m
F Lt
Qp
Lv Lsm
Q
m
P K
Q
m/Q
t P/F
Porc. B
. 09N
E12
0.01 0.05
0.94 0.01
0.05 0.94
0.00 0.92
0.08 0.13
0.78 0.09
1.00 0.90
Porc. B
. 09N
E14
0.36 0.44
0.20 0.24
0.44 0.32
0.37 0.13
0.50 0.35
0.41 0.24
0.75 0.63
Pt. C
amden
09NE
18 0.33
0.62 0.05
0.31 0.62
0.08 0.33
0.00 0.67
0.33 0.37
0.30 0.93
0.56 B
JBS
09N
E20
0.32 0.46
0.23 0.27
0.46 0.28
0.17 0.07
0.76 0.37
0.49 0.14
0.85 0.78
BJB
S
09NE
15 0.25
0.75 0.00
0.23 0.75
0.02 1.00
0.00 0.00
0.24 0.51
0.26 0.92
0.66 B
JBS
09N
E07
0.37 0.60
0.03 0.29
0.60 0.10
0.70 0.18
0.12 0.33
0.43 0.24
0.80 0.64
BJB
S
09NE
06 0.27
0.68 0.05
0.25 0.68
0.07 0.28
0.00 0.72
0.27 0.52
0.22 0.92
0.70 B
JB
09NE
08 0.35
0.64 0.01
0.32 0.64
0.04 0.67
0.00 0.33
0.33 0.45
0.22 0.92
0.68 LP
B
09LA01
0.50 0.47
0.03 0.43
0.47 0.10
0.74 0.00
0.26 0.48
0.52 0.00
0.87 1.00
LPB
09N
E29
0.46 0.49
0.04 0.38
0.49 0.12
0.66 0.02
0.32 0.44
0.55 0.01
0.83 0.98
16
suggests that misidentification of potassium feldspar grains is unlikely. Books of plutonic
mica crystals frequently take a warped appearance from chemical alteration, and also
observed bent around other grains, a result of the compaction process. Examples of most of
these alteration processes can be seen in Figure 4.
Lithic fragments of several lithologies are present in Kootznahoo sandstones.
Metamorphic lithic fragments are the most widespread (Fig. 4a,e) – these include fine-
grained phyllitic/schistose fragments and medium-fine grained gneissic fragments that show
variation in grain size, composition, and foliation expression across the grain. Volcanic rock
fragments are typically very rare, with the exception of sample 09NE12 (Q01F05L94), taken
from the upper portions of the Porcelainite Beach locality (Fig. 4f). Most volcanic fragments
contain euhedral plagioclase phenocrysts, some appear to composed almost entirely of
plagioclase. Sedimentary lithic fragments are rare in most samples; where present, they take
the form of chert and siltstone fragments
Figure 5 shows results of Kootznahoo Formation sandstone modal analyses plotted on
ternary diagrams from Dickinson (1985) (ternary end-members are defined in Table 1). On
the QFL and QmFLt plots (Fig. 5a-b), most Kootznahoo sands plot in the basement uplift and
dissected arc fields. Five samples from the Big John Bay and Big John Bay South localities
form a cluster, and the two sandstones from Little Pybus Bay are also grouped. The QPK
plot (Fig. 5c), which emphasizes variation in feldspar composition, shows that that most
Kootznahoo Formation sands share a similar Qm/P ratio (~2:3), but they vary in K-feldspar
composition along the line corresponding to that ratio. Samples from Little Pybus Bay are
devoid of K-feldspar, all other samples contain appreciable amounts.
17
P55Qm45
P60Qm40
Qm
Lt F
A.
B.
C.
Locality Symbols
Keku Straits
Porcellainite Beach
Little Pybus Bay
BasementUplift
BasementUplift
DissectedArc
DissectedArc
TransitionalArc
TransitionalArc
UndissectedArc
UndissectedArc
Qm
K P
Provenance Categories
Continental Block
Recycled Orogen
Magmatic Arc
Q
L F
Figure 5. Ternary plots of modal analysis results. See Table 1 for definitions of each ternary end member used. A.) and B.) QFL and QmFLt plots, respectively, of modal analysis data. Polygon with triangle at center represents the region enclosed by one standard deviation of average components of analyses from the Keku Straits locality group (Table 3). Note that most samples plot in the basement uplift field. The two analyses that plot in the dissected arc field are from the upper portions of the Keku Strait section. These analyses may represent a lithologic transition to sample 09NE12 (plotted near the lithic end member), which was taken from the primarily volcaniclas-tic upper portions of Porcellainite Beach. C.) QPK diagram of modal analysis data. Note that analyses from Little Pybus Bay plot on the Qm-P axis, between Qm40 and Qm45. Most Keku Straits analyses retain a similar Qm/P ratio (shown by the dotted lines), varying only in the proportion of Kfs present.
18
U/PB DETRITAL ZIRCON GEOCHRONOLOGY
Methods
Detrital zircon samples were processed at Union College and Macalester College.
Both institutions used standard procedures to avoid introduction of sample bias and sample
contamination. Zircon separates were transported to the LaserChron Lab at the University of
Arizona-Tuscon, where they were prepared and mounted for analysis per standard
LaserChron procedures.
U-Th-Pb isotopic data were collected using a multicollector inductively coupled
plasma-mass spectrometer coupled to a laser ablation system. Detailed information about the
specifics of the instrumentation and standard analytical procedures can be found in Appendix
1 and Gehrels et al. (2008). Analyses were conducted using a beam diameter of ~30 um,
generating a pit depth of ~12 um. Fragments of a Sri Lankan zircon standard (564 Ma,
Gehrels et al., 2008) mounted with the unknown zircons were analyzed regularly to monitor
and correct for element fractionation.
Isotopic data were processed using Isoplot 3.0 (Ludwig, 2003). Interpreted ages are
based on 206Pb/238U for <960 Ma grains and on 206Pb/ 207Pb for >960 Ma grains. This division
at 960 Ma results from the increasing uncertainty of 206Pb/ 238U ages and the decreasing
uncertainty of 206Pb/ 207Pb ages as a function of increasing age.
Results
U/Pb Detrital Ages
1169 U-Pb isotopic ages of zircon grains from 12 samples of the Kootznahoo
Formation were collected for this study (isotopic data listed in Appendix 2). All <500 Ma
19
analyses are concordant to slightly discordant. 1158 (99%) of all analyzed grains are
Paleozoic age or younger, and 1108 (95%) are younger than 200 Ma. These data are plotted
on relative probability plots in Figure 6. Pronounced peaks in the cumulative dataset (Fig. 7)
include a very narrow peak at 30-24 Ma, moderately narrow peaks at 65-50 Ma and 90-80
Ma, and a broad peak at 185-160 Ma.
Though populations of zircons older than 200 Ma are present, none of these
populations are comparable in magnitude to those younger than 200 Ma. Grains older than
200 Ma are distributed relatively evenly over the 400-200 Ma interval (Fig. 7). Those grains
that are of Proterozoic age or older (n=12) fall in age between 1.3 and 2.9 Ga, with a cluster
of five grains between 1.65 and 1.95 Ga; however, these grains are slightly to highly
discordant.
Maximum depositional ages (MDAs) of samples, given for each sample in Figure 2
and Figure 6, are given either by the youngest zircon grain or the youngest population of
zircon ages (populations determined with the Age Pick program; see Vermeesch et al., 2004;
Haeussler et al., 2004 for a discussion of MDA determination). Samples from the Keku
Strait locality have MDAs 87-25 Ma. Sample 09TH01, collected from the lowermost portion
of the section in Hamilton Bay yields the oldest MDA, 87 Ma. Above this stratigraphic
level, MDAs range 59-56 Ma, with the exception of 09NE14 and 09NE12 (Porcelainite
Beach), which have distinct peaks between 30-25 Ma. Note that grains with ages <45 Ma
(n=2) are present in 09LA10, but are not classified as a population.
All samples from localities in the Keku Strait (with the exception of 09TH01) have
age distributions such that 60-80% of their total populations are younger than 75 Ma, with
most ages concentrated between 60-55 Ma (Fig. 8). All of these samples have numerous
20
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
Num
ber
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350 400
Relative probability
09LA08Dakaneek Bay
n=96
+ 1879 ± 442 Ma + 1953 ± 664 Ma
+ 1961 ± 1333 Ma+ 2290 ± 263 Ma
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350 400
Num
ber
Age (Ma)
09TH08Davidson Bay
n=97
MDA53.4 ± 0.6 Ma58 Ma (n=35)
0
10
20
30
40
50
0 50 100 150 200 250 300 350 400
Relative probability
Age (Ma)
09NE08Big John Bay
n=98
Figure 6. Individual sample probability plots for all <400 Ma detrital zircons from the Kootznahoo Formation. Rectangles represent 10-Ma bins of analyses, each set of which is traced by a probability curve. Note that the vertical scale for age bins is not consistent between all plots. Numbers of analyses for each sample and analyses of >400 Ma grains from the sample are listed below locality information. Two indicators of maximum depositional age (MDA) are given for each sample: age/uncertainty of the youngest grain, as well as the youngest population of grains determined by the Age Pick program (Gehrels, 2009) with the number of grains contributing to that population.
Figure 7. Relative probability plots for all Kootznahoo detrital zircon ages <400 Ma. A.) shows ages <400 Ma. Note the relative scarcity of zircons older than 200 Ma. B.) shows ages <200 Ma. Note the sharp peaks at 30-25 Ma, 65-55 Ma, 90-80 Ma, and 190-160 Ma. Zircons that compose the 30-25 Ma peak are from two samples only, the rest are spread relatively evenly across most samples.
0
50
100
150
200
250
300
0 25 50 75 100 125 150 175 200
Relative probability
Num
ber
A.
B.
23
peaks in the 75-50 Ma age range, including peaks at 60-57 Ma and 71-65 Ma. All samples
except for those from the Big John Bay (09NE08) and Big John Bay South (09NE06) have
95-85 Ma peak. Most samples have few or no grains present from the age range of 150-100
Ma (sample 09NE14 is an exception). All samples except for the lowermost (09TH01) and
the uppermost samples (09NE12) have one to several moderately pronounced peaks between
190-160 Ma.
The youngest population in both samples from Little Pybus Bay is 59 Ma. Both
samples have large 65-59 Ma peaks. Both samples have more pronounced peaks between
90-75 Ma than do most upper Keku Strait samples, though stratigraphically lower Keku
Strait samples do contain grains of this age range (Fig. 8). Sample 09LA14, taken from the
base of the LPB section, has a pronounced, broad peak at 185-160 Ma, a feature shared by
several Keku Strait samples but not 09LA01, the sample from the top of the LPB section.
U/Th Ratios
Detrital zircons from most Kootznahoo Formation samples display low U/Th ratios
(Appendix 2). Zircon grains that display high U/Th ratios (10<U/Th) likely crystallized in an
environment where metamorphic fluids were present (Williams, 2001; Gehrels et al., 2009).
Only in samples 09TH01 and 09TH10 do large portions of the entire population display U/Th
ratios greater than10. In these samples, approximately half of all zircons aged 90-80 Ma
display U/Th>10. Almost all zircons younger than 80 Ma display U/Th ratios below the
metamorphic cutoff; though trace (n<10) populations of metamorphic zircon that fall within
this age range are present in all samples.
24
Peak ofEastern
MagmaticBelt activity
160-140 Ma WMB
magmaticflare-up
120-78 Ma magmaticflare-up, 100-80 Ma metamorphic event
67-52 MaQuottoon, other
CSZ related magmas
55-48 Ma magmatic flare-up
Admiralty Island Volcanics, other Oligocene-age
volcanics
Rel
ativ
e Pr
obab
ility
0 25 50 75 100 125 150 175 200 Age (Ma)
09NE12
09NE14
09LA10
09NE08
09NE06
09TH08
09LA08
09LA05
09TH10
09TH01
09LA01
09LA14
Littl
e Py
bus
Bay
Kek
u St
rait
Figure 8. Stacked relative probability plots for all zircon ages <200 Ma. With the exception of the Eastern Magmatic Belt, peaks of CMB-sourced detrital zircons to the left of the heavy black vertical line at 80 Ma are originally sourced from plutons emplaced east of the CSZ, those to the right are from west of the CSZ. Colored strips represent likely sources of plutonic and metamorphic zircons. Green belts represent high flux magmatic events, while the yellow belt represents the peak flux of the long-lived Eastern Magmatic Belt (Gehrels et al., 2009). The blue belt, which overlaps with the youngest high-flux magmatic event, represents the emplacement of the Quottoon and related plutons adjacent to the east side of the CSZ (Crawford et al., 1999). Zircons in the red strip are likely sourced from Oligocene extrusive rocks (Haeussler et al., 1992; Ford et al., 1996).
25
Statistical Comparisons
The Overlap-Similarity (O-S) test produces a measure of the amount which a given
relative probability spectrum’s age peaks overlap with those of another (overlap), and an
independent measure of whether the proportions of overlapping age peaks of the two spectra
are similar (similarity) (Gehrels, 2000). The K-S test is a non-parametric test that compares
cumulative age probability curves of similarly-sized samples to determine the likelihood
(expressed as a p-value) that any two age curves can be achieved by random sampling of a
single parent population (Guynn, 2006). If p<0.05, one can be 95% confident that the
differences between a population are not due to random sampling error, but rather, derivation
from different parent populations. Other workers have used the K-S test in the comparison of
detrital zircon age spectra (e.g. Berry et al., 2001; DeGraaff-Surpless et al. 2003; Amidon et
al., 2005). Though the K-S test is regarded as a more rigorous test of similarity (Fedo et al.,
2003), results of both tests are provided in Table 4 (K-S) and Table 5 (O-S) for the purpose
of comparison2.
Samples 09TH01 and 09NE12 were excluded from statistical comparisons on the
basis of their unique, clearly unimodal age spectra (Fig. 6). Both tests suggest that the
populations of most samples are highly statistically similar – 09LA14 (lower LPB) is the
only sample included in the comparisons that produces p-values that suggest derivation from
a different source region than a majority of the other samples. In K-S comparisons, 09NE14
was not significantly different from any other sample (Table 4). In the results of both tests,
mutually similar samples 09TH10 and 09LA05 have notably lower similarity values in
comparison to samples from higher stratigraphic levels in the Keku Strait section, as
2Excel program versions both of these tests, as well as the Age Pick program, are available on the LaserChron website (www.geo.arizona.edu/alc).
26
suggested by the lack of uncolored cells in the rows of Table 4 that correspond to these
samples. It should be noted that these two samples share a pronounced 80-90 Ma peak with
the sample that lies directly below in the Kootznahoo stratigraphy, 09TH01.
A comparison of samples from the basal and top portions of the Little Pybus Bay
section (09LA14 and 09LA01, respectively) suggests that these sediments were drawn from
different source areas. K-S and O-S tests strongly suggest that the Keku Strait sample that
09LA14 is most similar to be 09TH10. Comparisons involving 09LA01 are ambiguous:
results suggest that 09LA01 is nearly equally similar to Keku Strait samples from
Porcelainite Beach (09NE14) and a cluster of samples in the Dakaneek Bay and Dakaneek
point localities (09LA05, 09LA05, and 09TH08). As seen on Fig. 2, these two localities are
separated by hundreds of meters of Kootznahoo sediments.
DISCUSSION
Source Region Classification
Quartz, feldspar, and lithic fragments dominate the framework compositions of
Kootznahoo Formation sandstones. Modal analyses of most Kootznahoo sandstones plot in
the “basement uplift” and lithic-poor “dissected arc” fields of ternary classification diagrams
(Fig. 5) (Dickinson, 1970). This suggests that these rocks are derived from feldspar-rich
continental basement or the plutonic roots of a magmatic arc (Dickinson and Suczek, 1979,
Dickinson, 1985). The immature nature of these sediments, evidenced by angular grain
shapes (Fig. 4) and presence of coarse-grained plutonic and metamorphic fragments in most
samples, suggests that these sands experienced minor mechanical breakdown, and were
therefore subjected to relatively short transport (Pettijohn et al., 1987).
27
09NE
1409LA
1009N
E06
09NE
0809TH
0809LA
0809LA
0509TH
1009LA
0109LA
1409N
E14
0.1530.091
0.3380.449
0.0660.153
0.7000.514
0.05109LA
100.153
0.1990.267
0.5320.408
0.0010.007
0.0900.000
09NE
060.091
0.1990.353
0.4590.167
0.0350.199
0.0170.006
09NE
080.338
0.2670.353
0.5850.303
0.0020.029
0.1790.000
09TH08
0.4490.532
0.4590.585
0.9860.088
0.0510.400
0.00009LA
080.066
0.4080.167
0.3030.986
0.0210.010
0.4840.000
09LA05
0.1530.001
0.0350.002
0.0880.021
0.5360.421
0.03309TH
100.700
0.0070.199
0.0290.051
0.0100.536
0.0420.245
09LA01
0.5140.090
0.0170.179
0.4000.484
0.4210.042
0.00109LA
140.051
0.0000.006
0.0000.000
0.0000.033
0.2450.001
0.00-0.050.05-0.150.15-0.350.35-0.500.55-1.0
09LA01
09LA01
09NE14
0.60109N
E1409LA
100.506
0.56809LA
1009N
E060.590
0.5000.597
09NE06
09NE08
0.6710.586
0.5110.648
09NE08
09TH08
0.6060.520
0.4740.572
0.56509TH
0809LA
080.657
0.5910.515
0.6270.641
0.60609LA
0809LA
050.820
0.6930.520
0.6210.683
0.6070.722
09LA05
09TH10
0.5870.603
0.4640.487
0.4780.641
0.5440.600
09TH10
09LA14
0.3130.355
0.4310.435
0.2950.509
0.2980.334
0.579
Table 4. K-S Test of sim
iliarity results. Results are in the form
of p-values, with bin color coding schem
e displayed to the right of the table. Sam
ples are arranged in stratigraphic order. C
omparisons of populations that produce a p-value of less than 0.05 (uncolored cells) suggest that these sam
ples were
drawn from
different parent populations. Note that m
ost p<0.05 results in the Keku S
trait samples are found in com
parisons involving 09LA05 and 09LA
10.
LPB Keku Strait
LPBK
eku Strait
Table 5. Overlap-Sim
ilarity (O-S) test results. R
esults shown in table represent the product of the overlap and sim
ilarity values (O x S
) from each
comparison.
28
Sample 09NE12 falls in the “undissected arc” region of the QtFL and QmFLt
diagrams (Fig. 6). Composed almost entirely of angular volcanic lithic fragments (Fig. 4f),
this sample is evidence of a major shift toward more volcanic provenance in the upper
portions of the Keku Strait section. This shift in provenance is visible on the outcrop scale in
the volcaniclastic upper Porcelainite Beach portion of the Keku Strait section, and is mirrored
in the AIV conglomerate at the top of the exposed Little Pybus Bay section (Fig. 2).
The Paleocene-early Eocene Arkose Ridge Formation in the Matanuska Valley of
southern Alaska (Trop and Ridgway, 2000) is analogous to the Kootznahoo Formation. Its
framework compositions are very similar to those of Kootznahoo arkoses (Arkose Ridge
average = Q23F67L10, Kootznahoo arkoses average = Q36F57L07). In addition, as observed in
the Kootznahoo stratigraphy, an increase in the proportion of lithic fragments is observed in
upper portions of the Arkose Ridge section. Trop and Ridgway (2000) and Trop et al. (2003)
concluded that the Arkose Ridge Formation was sourced from local Mesozoic/Cenozoic
plutonic rocks undergoing active uplift, as well as volcanic sediments from eruptive activity
that was contemporary with its deposition. Based on field observations and petrographic
data, I arrive at a similar conclusion for Kootznahoo sandstones.
Source Regions of Detrital Zircons in the Kootznahoo Formation
Kootznahoo Formation DZ data, when considered as a whole, display four major
populations (Fig. 7). Nearly 50% of all analyzed grains are assigned ages between 65-50 Ma.
Another large population is present at 93-85 Ma, and a moderate, broad population is
observed at 190-160 Ma. Zircons aged 30-24 Ma dominate one sample (09NE12), but are
absent or present only in trace amounts in all other samples.
29
Gehrels et al. (2009) provide a review of the U-Pb zircon and titanite geochronology
of the CMB, approximately 400 km south of our field area (Gehrels and Berg, 1994). Their
analyses suggest three periods of high magmatic flux in the past 200 years (at 160-140 Ma,
~120-80 Ma, and 55-48 Ma), and regional metamorphic events that facilitated zircon growth
at 88-76 Ma and 62-52 Ma. Gehrels et al. (2009) separate magmatic activity into three
spatiotemporal groups. The eastern magmatic belt was stationary during the emplacement of
its constituent plutons 225-100 Ma, while after 110 Ma, the western magmatic belt (located
west of the CSZ), and the <100 Ma magmatic belt define a trend of eastward magmatic
migration that crossed the CSZ at about 80 Ma (Fig. 9).
Major Kootznahoo Formation detrital zircon populations
The cumulative detrital zircon (DZ) age probability curve from this study shares
peaks with the probability curves of two of Gehrels et al. (2009)’s magmatic belts (Fig. 9).
The 190-160 Ma Kootznahoo Formation DZ peak, present in most samples from this study
(Fig. 8), falls near the probability peak of the eastern magmatic belt. Subduction-generated
plutons of this magmatic belt (225-110 Ma, Gehrels et al., 2009) intruded Paleozoic-
Mesozoic continental margin strata of the Yukon-Tanana terrane and accreted arc rocks of
the Stikine and Taku terranes. These host rocks are located exclusively to the east of the
CSZ (Gehrels and Berg, 1994; Currie and Parrish, 1997; Gehrels and Kapp, 1998; Mahoney
et al., 2009). The pronounced 93-85 Ma and 65-50 Ma peaks correspond closely to peaks in
magmatic flux in the <100 Ma magamatic belt (Fig. 8, Gehrels et al., 2009). The 93-85 Ma
population is likely sourced from plutons concentrated 10-30 km west of the CSZ that were
emplaced during an episode of high flux magmatism (Fig. 8, Gehrels et al., 2009). This
30
episode was initiated by a shift to a margin-normal compressive tectonic regime ~100 Ma
(Engebretson et al., 1985). U/Th ratios greater than 10/1 suggest that a proportion of this
zircon population is also drawn from metamorphic rocks generated during regional
metamorphism that occurred during this time period (Williams, 2001; Gehrels et al., 2009).
This shift to a compressive regime juxtaposed the WCT and the Stikine/Yukon-
Tanana terranes, caused the subduction and imbrication of Gravina strata, and initiated an
episode of crustal thickening that persisted until ~65 Ma. Plutons from melt generated in the
lower portions of this overthickened crustal welt were emplaced at deep to mid-crustal levels
in a regime of dextral transpression to transtension between 67 and 52 Ma (Klepeis et al.,
1998; Crawford et al., 1999). These plutons, which are today exposed in a narrow band just
to the east of the CSZ, almost certainly provided a local, plutonic source for the major 65-50
Ma Kootznahoo DZ population. The low (<5/1) U/Th ratios for most Kootznahoo Formation
DZ analyses for this age range are consistent with this conclusion.
The uppermost sample from the Keku Strait locality (09NE12) contains a 30-24 Ma
population that makes up the bulk of the sample. Trace amounts of zircons in this age range
were also encountered in the sample directly below 09NE12 in the section (09NE14).
Sedimentary provenance results for 09NE12 suggest a proximal, undissected volcanic source.
Likely source formations include the Admiralty Island Volcanics (Haeussler et al., 1992,
Ford et al., 1996), presently exposed ~40 km to the northwest, or a volcanic component of
the Kuiu-Etolin magmatic suite (Crawford et al., 2009; Gehrels and Berg, 1992), exposed
Western magmatic beltEastern magmatic belt<100 Ma magmatic beltKootznahoo DZ
Figure 9. Relative probability plot comparing the Kootznahoo Formation detrital zircon population with U/Pb age curves of the major spatiotemporal magmatic belts defined by Gehrels and others (2009). Note the match between the 190-160 Ma detrital peak and the peak of the Eastern magmatic belt curve, as well as the similarities between the distribution of younger, major detrital peaks and the <100 Ma magmatic belt curve.
32
Kootznahoo Formation detrital zircon populations older than 200 Ma
Accessory zircon populations (1-10 grains) are present in most samples detrital zircon
samples. These populations have ages concentrated at 280-200 Ma, 370-310 Ma and 2.9-1.4
Ga. Zircon grains of these ages were likely derived from metamorphosed plutonic and
sedimentary rocks within and adjacent to the CMB. These host rocks were uplifted, eroded,
and deposited along with younger plutonic rocks that contributed <200 Ma zircons that make
up the majority of the Kootznahoo Formation detrital zircon population.
Zircons of 280-200 Ma age (relatively abundant in 09TH01) are present in
metamorphosed plutons within the Alexander terrane basement, which is present west of the
CSZ (Gehrels and Saleeby, 1987). Grains of 370-300 Ma age (relatively abundant in
09TH01, 09TH10, and 09LA14) are likely derived from plutons that intruded the Yukon-
outboard portion of the Tanana terrane, and are present as a detrital component of the strata
of the Taku terrane and the Gravina assemblage (Gehrels and Kapp, 1998; Kapp and Gehrels,
1998; Gehrels, 2001). Gravina sediments and their metamorphic equivalents are present
from a few kilometers northeast of to 10-20 km southwest of the CSZ, while Yukon-Tanana
and Taku strata are present in and adjacent to the Coast Mountains, northeast of the CSZ
(Gehrels and Berg, 1992). Yukon-Tanana strata are also the probable source of recycled
Proterozoic and Archean grains, the ages of which are concentrated in the 2.0-1.6 Ga range
(relatively abundant in 09LA14 and 09LA08) (Gehrels et al., 1995).
Implications of Detrital Zircon and Sedimentary Petrography Data
In this section, I focus my analysis on the implications provenance data from this
study have for the timing and continuity of Kootznahoo Formation deposition, the links
33
between the evolution of the detrital age distribution and the exhumation of the Coast
Mountains batholith (CMB), and the consideration of similarities between the Kootznahoo
Formation detrital signature and that of other sedimentary rocks along the margin.
Links between Kootznahoo Formation deposition and exhumation of the CMB
Determination of depositional age in clastic sedimentary strata, especially those of
non-marine to marginal marine depositional environments, can be difficult, as this rock type
often lacks fossils, continuous layers of volcanic material, and other typical sedimentary rock
age indicators. In such case, where detrital zircon data are available, MDAs determined from
these data are often used as indicators of age. MDAs typically provide only a crude estimate
of true depositional age (Fedo et al., 2003). This lack of accuracy is a result of the time that
exists between crystallization of zircon and its deposition in sedimentary strata – in other
words, exhumation, erosion, and transport are the key processes in determining the difference
between the MDA and the true depositional age of a given sample.
Several studies of plutonic rocks associated with Jurassic to middle Cretaceous
convergent tectonics and late Cretaceous-early Eocene development of the Coast Shear Zone
(CSZ) suggest that exhumation rates were moderate to relatively rapid, and varied spatially
relative to the CSZ. Late Cretaceous plutons emplaced to the west of the CSZ (the proposed
source of the 90-80 Ma Kootznahoo DZ populations) display moderate to rapid rates of uplift
(0.5-1.0 mm/yr, Stowell and Crawford, 2000; Butler et al., 2002; Himmelberg et al., 2004;
Crawford et al., 2009). In the CMB, most plutons aged 65-50 Ma are part of the Quottoon
Pluton, which was emplaced along the eastern margin of the CSZ in several sill-like pulses
between 72-55 Ma (Ingram and Hutton, 1994; Butler et al., 2001). The Quottoon pluton
34
records rapid exhumation (~1.0 mm/yr) between 59 and 52 Ma (Wood et al., 1991; Crawford
et al., 1999; Butler et al., 2001; Rusmore et al., 2005). These exhumation rates suggest that
the time period between crystallization and deposition of zircon in the Kootznahoo for rocks
of CMB origin was ~10-20 Ma for <100 Ma plutons west of the CSZ, and ~3-15 years for
plutons east of the CSZ, with grains of ages <60 Ma displaying the most rapid cooling
histories. Petrographic evidence for short post-erosion transport of Kootznahoo sediments
and a Late Cretaceous-Eocene paleoclimate ideal for an aggressive, brief weathering process
(e.g. Tarduno et al., 1998) suggest that the time difference between crystallization age and
maximum depositional age in this study area is best approximated by the duration of source
rock exhumation.
Limited geologic and fossil evidence (Lathram et al., 1965; Muffler et al., 1967;
White et al., in prep) suggest a Paleocene age for the lower portions of the Kootznahoo
stratigraphy. A biotite K-Ar date of 53.5 ± 0.6 Ma for a tuff layer in the Big John Bay South
section, and a whole rock Ar-Ar date of 26.5 ± 0.5 Ma for a basaltic flow in the Lower
Porcelainite Beach portion of the Keku Strait section give additional constraints on the
depositional age of portions of the Kootznahoo stratigraphy (Fig. 2, White et al., in prep.).
Below, I present a chronology that uses these geologic age markers and MDAs adjusted to
reflect exhumation to link the depositional chronology of the Kootznahoo to the history of
CMB exhumation, with an emphasis on changes in uplift processes across the CSZ.
Sandstone at the base of the Hamilton Bay locality in the Keku Strait (sample
09TH01) contains zircons that suggest derivation from rocks west of the CSZ (Fig. 6). The
major 90-80 Ma and accessory 280-200 Ma DZ populations in this sample are likely drawn
from the western magmatic belt of Gehrels et al. (2009) and Alexander terrane basement
35
rock, respectively. Potential source rocks for the 370-310 Ma accessory population (Gravina,
Yukon-Tanana) are present both west and east of the CSZ. These source rocks were uplifted
85-65 Ma, when the CSZ formed to accommodate west side-up motion (McClelland and
Mattinson, 2000). This uplift event was driven by the partial subduction of Alexander
terrane crust. The buoyancy of this crustal material may have caused it to delaminate from
the lithosphere and rebound (McClelland and Mattinson, 2000; Mahoney et al., 2009).
Resulting uplift of plutons from mid-crustal levels, carried along with Alexander and Gravina
host strata, occurred at moderate rates. Source plutons for 90-80 Ma populations likely
reached the surface at ~70-60 Ma and were deposited by 65-55 Ma, an age consistent with
other proposed ages of lowermost Kootznahoo strata (White et al., in prep).
At approximately 65 Ma, a shift to east side-up motion on the CSZ drove an increase
in sediment sourced from rocks east of the CSZ. This shift occurred in response to high heat
levels and pluton emplacement in the overthickened crust, and is linked with a switch to a
dextral transpressional/transtensional style of deformation (Klepeis et al., 1999; Crawford et
al., 2000; Gehrels et al., 2009). Most samples in the Kootznahoo Formation have MDAs of
60-55 Ma, and large DZ populations from 75-52 Ma. Plutons that shed zircons of this age
range were emplaced primarily east of the CSZ, and were exhumed at moderate to rapid rates
from 70-52 Ma. During the period when exhumation rates were at their highest (59-52 Ma),
the time between crystallization and exhumation may have been as low as 2-5 Ma (Rusmore
et al., 2005). Northeast-directed movement of upper crust on detachment-scale normal faults
accelerated denudation and cooling of 75-52 Ma plutons (Andronicos et al., 2003).
Exhumation patterns suggest that these plutons began to reach the surface 60-55 Ma, with
36
increasing exhumation rates until 52 Ma causing continuous release of sediment. These
volumes of sediment were deposited in the Kootznahoo from ~60 Ma to 35-30 Ma.
Detrital zircon populations suggest that the majority of the Kootznahoo stratigraphy
was deposited during this stage of CMB exhumation (Fig. 8). Samples 09TH10 and 09LA05
(lower Keku Strait, column 2 on Fig. 2), as well as the 09LA01 (lower Little Pybus Bay)
have large populations of both 90-80 Ma and 65-50 Ma grains. Moderate to rapid
exhumation rates recorded in these plutons suggests that these strata were deposited 60-50
Ma. The portion of the section above this stratigraphic level and below the Porcelainite
Beach locality has a relatively homogenous DZ signature that can be attributed to zircon
sources aged 65-50 Ma, but not to those aged 90-80 Ma. The absence of this second
population implies a complete shift to a sediment source east of the CSZ. The presence of a
53.5 Ma tuff date in Big John Bay South suggests that the approximately 300 meters of
sediment between this layer and the base of the Point Hamilton section (Fig. 2; sample
09TH10, MDA=60-53 Ma) accumulated in the Keku Strait portion of the basin over a period
of 6 Ma or less. This rapid accumulation of sediment likely required high levels of sediment
flux and a period of active basin subsidence to create space for this accumulation.
At 45-40 Ma, shifts in the regional tectonic regime initiated dextral transtension
(Engebretson et al., 1985; Stock and Molnar, 1988). This shift is the inferred driver of
normal faulting and subsidence in the Queen Charlotte Basin (QCB) (Rohr and Dietrich,
1992; Hyndman and Hamilton, 1993; Irving et al., 2000), and it may have had similar effects
in the Kootznahoo basin. Comparison to the QCB is problematic, as basin extension began
there in the late Eocene to Early Oligocene (Hyndman and Hamilton, 1993), whereas the
presence of the 53.5 Ma tuff date in the Big John Bay South locality (White et al., in prep)
37
suggests substantial accumulation of Kootznahoo sediments by early Eocene time. I propose
that the transtensional shift that caused extension in the QCB contributed to subsidence and
sediment accumulation in later portions of Kootznahoo deposition, providing accommodation
space for strata at and above the level of Camden Point (Fig. 2).
After the transition to a transtensional regime, ductile extension and exhumation
processes all but ceased. Shallow plutonism and volcanism accompanied brittle extension
(Davidson et al., 2003). This younger magmatic activity was the source of sharp 34-24 Ma
peaks in the samples in the uppermost Keku Strait stratigraphy (09NE14 and 09NE12). The
most probable source of these zircons is the Admiralty Island Volcanic suite, which is
located 40 km north of the field area (Haeussler et al., 1992, Ford et al., 1996).
Timing and continuity of deposition in the Kootznahoo Basin
Internal comparison of detrital zircon and sedimentary petrography data can be used
to examine continuity of deposition within the Kootznahoo basin. Examination of Figure 8
reveals clear changes in the relative magnitudes of major DZ populations through the Keku
Strait stratigraphy. The primary changes that occur moving up through this inferred section
include the decline of the 90-80 Ma population in lowermost three samples (09TH01,
09TH10, and 09LA05, herein referred to as the “basal group”), and the increase of the 30-24
Ma population in Camden Point portion of the section.
Results of the K-S test (Table 4) suggest that there is a relatively high level of
statistical similarity between most Kootznahoo DZ samples and their inferred stratigraphic
neighbors. Only two comparisons between samples adjacent to one another in the
stratigraphy of White et al. (in prep.) yield a result suggesting derivation from different
38
source populations (p<0.05). This result suggests that the outcrop correlations made by
White et al. (in prep.) are permissive.
K-S test results suggest that a shift in DZ age distribution occurs between the basal
group and the rest of the Keku Strait samples. This suggestion is based on two observations:
first, the age distributions of basal group samples are significantly different from most other
Keku Strait samples, and second, the only neighboring pair in the inferred Keku Strait section
to return a comparison result suggesting different source areas is 09LA05 (the uppermost
sample of the basal group) and the superposed sample, 09LA08.
Both the K-S and O-S tests also suggest that the lowermost sample in the Little Pybus
Bay section (09LA14) is most similar to the samples of the Keku Strait basal group,
particularly 09TH10. The upper sample from Little Pybus Bay, 09LA01, displays similar
affinities to both the basal group and to 09NE14, the sample from Lower Porcelainite Beach.
The statistical similarity between 09LA01 and 09NE14, and the relatively dissimilarity
between these two samples and 09LA14 (Table 4) suggest that the upper portion of Little
Pybus Bay should be correlated to the upper portions of the Keku Strait stratigraphy.
However, petrographic data show that upper Little Pybus Bay samples (09LA01 and
09NE29) are devoid of K-feldspar, a trait that distances them from all other samples
subjected to petrographic analysis (Fig. 6c). This difference in lithology likely reflects a
difference in source rock composition. The incomplete nature of the petrographic dataset
obscures my ability to make further relevant comparisons. Therefore, my results with respect
to the correlation of the upper portion of Little Pybus Bay are ambiguous.
The apparent discontinuity in DZ age distribution, present in both Little Pybus Bay
and the Keku Strait (Fig. 2), likely developed during a hiatus in deposition. The base of the
39
Little Pybus Bay section (09LA14) correlates to one of the basal samples (09TH10) of the
Keku Strait section, suggesting that these lower portions of both sections were fed by similar
sediment sources. During the sedimentation process, I suggest that the basin filled to
capacity, causing a temporary cessation of sediment accumulation. During this hiatus,
erosion and reworking of portions of deposited strata likely occurred, which may explain the
low similarity between 09LA01 and 09LA14 (p=0.001). It is also possible that no erosional
surface developed, and the difference in detrital age populations may instead be a result of
some other change that caused a shift in the source regions that contributed sediment to the
basin during active sedimentation.
The Kootznahoo basin was a fluvial-deltaic to marginal marine basin (Fig. 10,
Muffler, 1967; Brew et al., 1982; White et al., in prep) similar to the forearc basin system of
the Matanuska Valley in southern Alaska (Trop et al., 2003; Trop, 2008). In such basins,
especially those undergoing active subsidence that creates a steep scarp along one or both of
the basin margins, sedimentation is fed by fans of coarse-grained sand and conglomerate that
radiate from drainages along the basin margin. In areas of the basin that are more distant
from these fan depocenters, finer-grained sedimentation occurs in low energy environments.
As the basin fills, coarse-grained deposits migrate towards the basin axis, whereas in periods
of subsidence, zones of conglomeratic deposition retreat (Leeder, 1999). In Figure 10, jumps
in the distribution of deposits of different grain sizes represent changes in the base level of
sedimentation, caused by episodes of active subsidence.
Limited accumulation of sediment in the Kootznahoo basin began as early as 65-60
Ma with the deposition of material at the base of Hamilton Bay, which lacks grains younger
than 80 Ma. MDAs of 60-53 Ma for other basal samples in the Kootznahoo stratigraphy
40
KsmTrTr
Tr
Point Hamiltoncomposite
Little Pybus Baycomposite
~225 m
Hamilton Bay
Major source region discontinuityDisconformity surface?Incipient
normal fault
~75 m
Big John BayBig John Bay South
~65-58 Ma: Basin-wide
subsidence drives continuous, rapid sedimentation in both the Keku Strait and Little
Pybus Bay
58-54 Ma:Uneven
subsidence style develops, more rapid in Keku
Strait. Deposition of fine-grained
BJB/BJBS sediments. Possible
hiatus/erosion in LPB
Port Camdencomposite
Little Pybus Baycomposite
Little Pybus Baycomposite
54-35 Ma:Low net
sedimentation, unconformity
development in KS and LPB. Interpreted as
basin-wide slowing or
cessation of subsidence.
~150 m
54 Ma tuff layer in upper portions
ExplanationShale, mudstoneSandstoneGranular sand to conglomerateVolcaniclastic sedimentsVolcanic flows
Unconformitydevelopmentduring slowsubsidence
Slow sedimentation below this level
35-25 Ma:Deposition of
fine to coarse-grained
sediment in both LPB and KS.
Coarse-grained volcaniclastics and volcanic
flows dominate stratigraphy by
27 Ma Figure 10. Kootznahoo Basin development model. Time series of vertically exaggerated, schematically scaled cross sections oriented approximately parallel to the inferred NW-SE elongation of the basin (Dickinson and Vuletich, 1990). Lithology patterns represent expansion of coarse-grained fan deposits (see text for description). Rapid subsidence occurs in the basin between ~57-54 Ma (B.) and 35-25 Ma (C.). During episode of slow subsidence/deposition, unconformity development likely occurred in both the Little Pybus Bay and Port Camden Composite sections. Dotted vertical lines represent approximate placements of measured sections from Fig. 2. Thin, curved horizontal lines are schematic chronostrati-graphic lines.
A.)
B.)
C.)
Ksm Tr
Tr
Ksm Tr
Tr
41
suggest that sedimentation had begun throughout the basin by 60 Ma (Fig. 10a). Statistical
similarity between DZ populations at the bases of Little Pybus Bay and Keku Strait
(09TH10) point to continuous sedimentation between these localities. During this short
period of relatively rapid deposition, the development of a major unconformity is unlikely.
Therefore, I tentatively attribute the apparent discontinuity in age distributions to a shift in
the source regions.
Shortly after 60 Ma, active subsidence in the Keku Strait portion of the Kootznahoo
basin commenced. This first episode of subsidence, which was limited to the Keku Strait
area, allowed the accumulation of ~250-300 meters of sediment of relatively homogenous
provenance below the stratigraphic level of Big John Bay South by 54 Ma. The rapid
sedimentation that occurred between 60-54 Ma was followed by deposition of volcaniclastic
material and volcanic flows that are present at the upper ends of both sections. The MDAs of
samples that contain the earliest components of volcanic populations in the Keku Strait,
beginning with Point Camden, are 33-25 Ma. This implies that 1) there is section missing at
this stratigraphic level, or, 2) the short portion of the measured section present between the
54 Ma tuff layer and the 33 Ma MDA of Point Camden represent a relatively long period of
slow sedimentation. During this period, I suggest that sediment reworking, mixing and
erosion occurred in both sections, resulting in the deposition of relatively heterogeneous age
distributions observed in 09LA01 and 09NE14 at 40-30 Ma. By 30-25 Ma, deposition of
volcaniclastic conglomerates and their source flows had begun. The deposition of these
volcanic strata was accommodated by by basin-wide subsidence that occurred due to a shift
to transtensional tectonics at approximately 45 Ma (Hyndman and Hamilton, 1993).
42
The middle Eocene shift to transtensional tectonics provides a mechanism for
subsidence in the Kootznahoo basin after 45 Ma, but this change does not account for the
earlier subsidence or discontinuity development. The pattern of discontinuity development
followed by further sediment accumulation that we observe in this episode is similar to a
pattern observed by Trop (2008) and Trop et al. (2003) in sediments of the Matanuska Valley
of southern Alaska. Trop (2008) attributes these events to the subduction of a young,
buoyant oceanic ridge, such as the Resurrection-Kula ridge (Haeussler et al. 2003; Bradley et
al., 2003, Taylor et al., 2005). To account for such changes in the Kootznahoo, ridge
subduction must occur at ~58-54 Ma. Haeussler et al. (2003) do not place a ridge near the
site of Kootznahoo deposition during this time period. A modification of their model,
presented by Madsen et al. (2006) suggests that ridge subduction may have begun in this area
at ~53 Ma.
Kootznahoo Formation deposition and northward transport of outboard terranes
The extent to which margin-parallel transport of terranes is a part of the tectonic history
of the Cordillera is a subject of major debate (e.g. Cowan, 2003). Though it is unlikely that
the Kootznahoo experienced syn- or post-depositional transport, similarities between the
provenance of the Kootznahoo and that of sedimentary formations of the Yakutat terrane
suggests that these sedimentary provinces may have shared a source region in Paleocene-
Eocene time.
43
Num
ber
0
50
100
150
200
250
300
350
400
450
0 50 100 150 200 250
Age (Ma)
Figure 11. Comparison of detrital zircon age populations between the Kootznahoo Formation and the Kulthieth-Poul Creek formations, which are deposited on the Yakutat terrane. Note that the Kulthieth-Poul Creek age distribution is similar to that of the Kootznahoo, with the exception of the young volcanic peak at 27-29 Ma in the Kootznahoo Formation. Modified from Perry et al. (2009)
Kootznahoo Formation DZ
Kulthieth/Poul Creek DZ
44
The middle Eocene-Oligocene Kulthieth and Poul Creek Formations (Plafker, 1987)
deposited on the Yakutat block, currently located southeast of Prince William Sound, display
a similar DZ provenance as that of the Kootznahoo Formation (Fig. 11). Kulthieth/Poul
Creek strata share a large 65-50 Ma DZ population with the Kootznahoo, and have a similar
petrographic character that suggests a relatively immature plutonic source (Perry et al.,
2009). Sources of the 65-50 Ma DZ population were first exhumed to the east of the CSZ by
60-55 Ma, and that rapid exhumation processes persisted until 52-50 Ma (Crawford et al.,
1999; Rusmore et al., 2005). DZ fission track populations from Kulthieth/Poul Creek strata
with cooling ages of 50-40 Ma imply a somewhat later exhumation history, but not so much
as to negate the similarities in U/Pb ages (Perry et al., 2009). We interpret these similarities
to suggest that Yakutat terrane, which most workers believe was transported northward from
45-0 Ma, was at the paleolatitude of Kootznahoo Formation deposition at 45-35 Ma. At this
point in time, according to our conceptual model of Kootznahoo basin development (Fig. 10),
the Kootznahoo basin was experiencing little or no subsidence, which may have resulted in
the filling of the basin and the bypass of sediments onto the Yakutat terrane.
Pronounced 30-24 Ma age peaks in samples from the Porcellainite Beach outcrops
represent zircon populations that we presume, based on sample lithology and age correlation
of these populations, to be volcanic in origin. Fission-track and U-Pb data indicate that such
a population is not present in the Kulthieth and Poul Creek Formations, suggesting that
northward transport of the Yakutat had begun by this time (Perry et al., 2009).
45
CONCLUSIONS
Detrital zircon and petrographic data suggest that the majority of the Paleocene-
Miocene Kootznahoo Formation of the Keku Strait and Little Pybus Bay was derived
from local, quartzo-feldspathic, primarily plutonic sources that experienced major zircon
crystallization events at 190-160, 95-80, and 65-50 Ma. I identify the Jurassic-early
Tertiary plutons of the Coast Mountains Batholith as the most probable sediment source
for the bulk of the Kootznahoo Formation. Comparison to the plutonic/metamorphic age
distribution of Gehrels et al. (2009) suggests that changes in the relative abundance of
major and accessory detrital zircon populations correspond to changes in the uplift and
exhumation of the CMB across the Coast Shear Zone. Correlations between plutonic
sources and detrital zircon ages suggest depositional ages of 65-54 Ma for lower
Kootznahoo Formation strata. In the upper portions of the Kootznahoo stratigraphy, a
shift to a detrital zircon age distribution dominated by middle-late Tertiary ages and a
volcaniclastic petrographic signature suggests a change to local volcanic sediment
sources.
Statistical comparisons of detrital zircon samples from the Kootznahoo Formation
support the stratigraphy presented by White et al. (in prep). Comparisons of Little Pybus
Bay and Keku Strait samples link lower Little Pybus Bay with the lowermost portions of
the Keku Strait section. Synthesis of detrital zircon ages, statistics, and the Kootznahoo
stratigraphy suggests a depositional history filled with uneven and episodic subsidence
and sedimentation, punctuated by periods of erosion and unconformity development.
Major subsidence events occurred at 60-54 Ma and 45-35 Ma. These periods of
subsidence may have been punctuated by the subduction of an oceanic ridge.
46
Similarities between the detrital zircon age distribution, exhumation chronology,
and sedimentary petrography of the Kootznahoo Formation and the Eocene-Oligocene
Kulthieth and Poul Creek Formations of the Yakutat terrane suggest that these areas may
have shared a sediment source 45-35 Ma. I interpret this similarity to indicate that the
Yakutat block was adjacent to the Kootznahoo Formation during this time period, before
beginning its northward transport prior to 30 Ma.
ACKNOWLEDGEMENTS
I thank the Keck Geology Consortium and Carleton College for providing funding
for this research. I express my profound gratitude to Cameron Davidson for his unlimited
availability, wise advice, and the countless discussions that benefitted this paper a great
deal. I thank Peter Haeussler, and Sue Karl, and especially Tim White for sharing their
experience and previous work on the Kootznahoo Formation, therein making my work
possible. In the field, Captain Karl Wirth embodied the perfect combination of academic
advisor, maritime navigator, and field photographer – I thank him for sharing his
expertise. Lenny Ancuta shared the burden of zircon sample processing, and he, along
with Tiffany Henderson, provided good company in the field and good ideas back at
home. Thanks to Louis Baggetto and Jeff Thole at Macalester College for instruction and
support in zircon sample processing procedures. Finally, thanks to all family and friends,
especially my fellow geology majors, for their support through this often difficult, but
ultimately satisfying process.
47
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APPENDIX 1. U/PB ANALYTICAL METHODS (after Gehrels et al., 2009)
U-Pb geochronology of zircons was conducted by laser ablation multicollector
inductively coupled plasma mass spectrometry (LA-MC-ICPMS). The analyses involve
ablation of zircon with a New Wave/Lambda Physik DUV193 Excimer laser (operating
at a wavelength of 193 nm) using a spot diameter of 35 microns. The ablated material is
carried in helium into the plasma source of a GVI Isoprobe, which is equipped with a
flight tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously.
All measurements are made in static mode, using 10e11 ohm Faraday detectors for 238U,
232Th, 208Pb, and 206Pb, a 10e12 ohm faraday collector for 207Pb, and an ion-counting
channel for 204Pb. Ion yields are ~1.0 mv per ppm. Each analysis consists of one 12-
second integration on peaks with the laser off (for backgrounds), 12 one-second
integrations with the laser firing, and a 30 second delay to purge the previous sample and
prepare for the next analysis. The ablation pit is ~12 microns in depth.
For each analysis, the errors in determining 206Pb/ 238U and 206Pb/ 204Pb result in a
measurement error of ~1-2% (at 2-sigma level) in the 206Pb/ 238U age. The errors in
measurement of 206Pb/ 207Pb and 206Pb/ 204Pb also result in ~1-2% (at 2-sigma level)
uncertainty in age for grains that are >1.0 Ga, but are substantially larger for younger
grains due to low intensity of the 207Pb signal. For most analyses, the cross-over in
precision of 206Pb/ 238U and 206Pb/ 207Pb ages occurs at 0.8-1.0 Ga.
Common Pb correction is accomplished by using the measured 204Pb and
assuming an initial Pb composition from Stacey and Kramers (1975) (with uncertainties
of 1.0 for 206Pb/ 204Pb and 0.3 for 207Pb/ 204Pb). Measurement of 204Pb is unaffected by the
56
presence of 204Hg because backgrounds are measured on peaks (thereby subtracting any
background 204Hg and 204Pb), and because very little Hg is present in the argon gas.
Inter-element fractionation of Pb/U is generally ~20%, whereas apparent
fractionation of Pb isotopes is generally ~2%. In-run analysis of fragments of a large
zircon crystal (generally every fifth measurement) with known age of 564 ± 4 Ma (2-
sigma error) is used to correct for this fractionation. The uncertainty resulting from the
calibration correction is generally 1-2% (2-sigma) for both 206Pb/ 207Pb and 206Pb/ 238U
ages.
57
Sample 09LA01, Upper Little Pybus Bay Isotope ratios Apparent ages (Ma)Coordinates (WGS 84): N57.22054, W134.15404