Field-Trip Guide to Volcanic and

Volcaniclastic Deposits of the Lower

Jurassic Talkeetna Formation, Sheep

Mountain, South-Central Alaska

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2006-1124
Field-Trip Guide to Volcanic and

Volcaniclastic Deposits of the Lower

Jurassic Talkeetna Formation, Sheep

Mountain, South-Central Alaska

Amy E. Draut

U.S. Geological Survey, Pacific Science Center, Santa Cruz, CA 95060

Peter D. Clift

School of Geosciences, University of Aberdeen, AB24 3UE, U.K.

Robert B. Blodgett

U.S. Geological Survey–Contractor, Anchorage, AK 99508

U.S. GEOLOGICAL SURVEY

Open-File Report 2006-1124

2006
U.S. Department of the Interior

P. Lynn Scarlett, Acting Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia 2006

Revised and reprinted: 2006

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LIST OF FIGURES

FIGURE 1. Regional map of the field-trip area.

FIGURE 2. Geologic cross section through Sheep Mountain.

FIGURE 3. Stratigraphic sections on the south side of Sheep Mountain.

FIGURE 4. Stratigraphic sections on the north side of Sheep Mountain.

FIGURE 5. Photographs of Talkeetna Formation facies on Sheep Mountain.

FIGURE 6. Depositional environments within the Talkeetna Formation.

ADDITIONAL DIGITAL INFORMATION

For an online PDF version of this report, please see:

http://pubs.usgs.gov/of/2006/1124/

For more information on the U.S. Geological Survey’s Alaska Paleontological

Database, please see: http://www.alaskafossil.org

DIRECT CONTACT INFORMATION

Regarding this Report:

Dr. Amy E. Draut (Research Geologist): adraut@usgs.gov

REPORT REFERENCE

Draut, A.E., Clift, P.D., and Blodgett, R.B., 2006, Field-trip guide to volcanic and

volcaniclastic deposits of the Lower Jurassic Talkeetna Formation, Sheep

Mountain, South-Central Alaska, U.S. Geological Survey Open-File Report 2006-

1124, 21 p.
Abstract

This guide provides information for a one-day field trip in the vicinity of

Sheep Mountain, just north of the Glenn Highway in south-central Alaska. The

Lower Jurassic Talkeetna Formation (Imlay, 1981), consisting of extrusive

volcanic and volcaniclastic sedimentary rocks of the Talkeetna arc complex

(Kelemen and others, 2003; Clift and others, 2005a), is exposed on and near

Sheep Mountain. Field-trip stops within short walking distance of the Glenn

Highway (approximately two hours’ drive from Anchorage) are described, which

will be visited during the Geological Society of America Penrose meeting entitled

Crustal Genesis and Evolution: Focus on Arc Lower Crust and Shallow Mantle,

held in Valdez, Alaska, in July 2006. Several additional exposures of the

Talkeetna Formation on other parts of Sheep Mountain that would need to be

accessed with longer and more strenuous walking or by helicopter are also

mentioned.

Regional Geology of the Talkeetna Arc

The Jurassic Talkeetna crustal section of south-central Alaska (Fig. 1;

Trop and others, 2002; Kelemen and others, 2003; Clift and others, 2005a) is the

most complete volcanic-arc section exposed in any accreted arc to have been

studied worldwide thus far. It includes ~7 km of exposed section, similar to that

observed seismically in the Tonga and Mariana Arcs (Suyehiro and others, 1996;

Crawford and others, 2003). The Talkeetna Arc (Clift and others, 2005a) forms

part of the Peninsular Terrane described by Plafker and Berg (1994), one of

many terranes accreted to the active margin of North America during the

Mesozoic and Cenozoic (for example, Gehrels and Berg 1994; Nokleberg and

others, 1994). The Peninsular Terrane had amalgamated with two other oceanic

terranes, the Alexander and Wrangellia Terranes, at least by Late Jurassic time

(~160 Ma; Gardner and others, 1988; Trop and others, 2002), forming the

composite Wrangellia Terrane that subsequently accreted onto the North

American continent during the Late Jurassic and Early Cretaceous (for example,

Csejtey and others, 1982; Plafker and others, 1989; Nokleberg and others, 1994;

Trop and others, 2002). The exact sequence of docking with Wrangellia and

North America is not yet resolved, although it is clear that the Talkeetna Arc had

collided with a terrane to its north by ~160 Ma when the coarse clastic Naknek

Formation was deposited (Trop and others, 2005).

The lack of any significant continental sedimentary contribution to arc

petrogenesis was interpreted by Clift and others (2005a) to indicate that the

Talkeetna Arc could not have faced north while colliding with a North American

passive margin. Moreover, the tectonically truncated arc section is juxtaposed

against the Cretaceous accretionary complex to the south in the Chugach

Mountains (Clift and others, 2005a, b), which suggests the presence of erosive

5
plate boundary during the Early Jurassic, consistent with north-dipping

subduction. The age and degree of deformation decreases progressively to the

south within the accretionary prism, further implying protracted north-dipping

subduction (Plafker and others, 1994).

The best radiometric age control on the Talkeetna Arc comes from the

ages of plutons exposed along the southern boundary of the arc complex near

the Border Ranges Fault (201–181 Ma), as well as from plutons in the Talkeetna

Mountains to the NW (177–171 Ma; Rioux and others, 2003, submitted). Volcanic

and volcaniclastic rocks of the Talkeetna Formation (Martin, 1926) were

described by Winkler (1992), apparently based on misinterpretation of Kelley

(1980), as “Upper Triassic (?) to Lower Jurassic” and this is the classification still

used for this formation in the USGS database of stratigraphic nomenclature.

However, no studies have confirmed a Late Triassic age for this unit and so we

refer to it hereafter as Lower Jurassic. The lowermost volcaniclastic materials of

the Talkeetna Formation have been dated as Hettangian (Early Jurassic,

200–196 Ma) based on ammonite fossils from exposures of the Talkeetna

Formation on the Alaska Peninsula (Imlay, 1981; Poulton and others, 1992, Pálfy

and others, 1999). Other volcaniclastic and quartzitic rocks of the Talkeetna

Formation on the northeast side of Puale Bay on the Alaska Peninsula (Imlay

1981; Pálfy and others, 1999) have been dated as Hettangian to Lower

Sinemurian (Lower Jurassic, ~200–193 Ma). The precise stratigraphic

relationship of these rocks at Puale Bay with those of the Talkeetna Formation to

the north would benefit from further evaluation. Volcaniclastic rocks of the

Talkeetna Formation in the Talkeetna Mountains (its type area) have been dated

(Imlay 1981; Poulton and others, 1992) as Late Sinemurian to Toarcian (Lower

Jurassic, ~190–180 Ma).

The top of the Talkeetna Formation is, in most places, overlain

unconformably by sandstones of the Middle Jurassic Tuxedni Group, which has

been dated as Early Bajocian (172–168 Ma) on the basis of molluscan

assemblages (Grantz 1960). There are few distinctive marker horizons within the

Talkeetna Formation that would allow biostratigraphic age correlation between

different exposed sections. However, the different facies found along and across

strike of the exposure allow the paleogeography to be reconstructed, even if it is

not possible to directly compare synchronous units in different regions. Within

individual massifs there is no clear, coherent up-section facies evolution (Clift

and others, 2005a).

The Talkeetna Formation and coeval mid-crustal gabbros were all

generated in an oceanic setting, far from the influence of a continental magmatic

source (Kelemen and others, 2003; Clift and others, 2005a; Greene and others,

in press), similar to the western Pacific Tonga and Mariana Arcs. Although the

polarity of Talkeetna subduction and the timing of its accretion to North America

have been debated (Reed and others, 1983; Plafker and others, 1989; Wallace

and others, 1989), there is little doubt that the arc was fully oceanic; Clift and

others (2005b) have argued that it was likely also in a state of tectonic erosion.

The juxtaposition of arc volcanic and plutonic rocks against younger accretionary

material in the Chugach Mountains across the Border Ranges Fault is interpreted

6
to reflect the loss of crust due to tectonic erosion before a switch to active

subduction accretion during the Cretaceous (Plafker and others, 1994). Strikeslip faulting along the Border Ranges, Castle Mountain, and other associated

faults in this area could have truncated the arc section in a horizontal direction

but is thought not to have affected the vertical thickness of the formation; ~5 km

of Talkeetna Formation volcanic and volcaniclastic material is preserved in the

Sheep Mountain massif alone, and there is no evidence for major thrusting or

extension within Talkeetna Formation exposures at either Sheep Mountain, south

of the Matanuska River, or farther north in the Boulder Creek, Horn Mountains, or

Little Oshetna Valley areas (the sedimentology and geochemistry of which have

been discussed in detail by Clift and others, 2005a).

As well as allowing study of arc volcaniclastic sedimentary rocks at greater

depth than is possible in modern arcs, exposures of the Talkeetna Formation

extend across and along the strike of the arc, allowing facies variability to be

explored. In addition, the geochemical composition of the northernmost volcanic

exposures in the Little Oshetna Valley are consistent with that of backarc

deposits, and distinct from the main exposures of the arc farther south (Clift and

others, 2005a). Clift and others, (2005a), following the observation of Burns

(1985), argued that the oldest volcanic rocks in the Talkeetna Formation are

exposed south of the Matanuska River, that the central part of the stratigraphy is

exposed on Sheep Mountain, and that the upper part crops out in the East

Boulder Creek area and in the Horn Mountains. The middle of the Talkeetna

Formation exposed on Sheep Mountain (Fig. 2) is separated from the volcanic

exposures south of the Matanuska River by an inferred strike-slip fault that

follows the trend of the Matanuska River (also inferred by Grantz 1961). Although

the presence of this fault could imply that some unknown volcanic thickness is

missing from the section, the 7 km thickness reconstructed by Clift and others,

(2005a) is consistent with paleobarometry constraints from the upper part of the

gabbro section (B. Hacker, personal communication 2004), suggesting that the

section is largely intact.

The Sheep Mountain massif consists almost entirely of Talkeetna

Formation volcanic and volcaniclastic materials. Sheep Mountain itself is

essentially a fault-bounded block, with various faults separating the Talkeetna

Formation from Cretaceous sediments of the Matanuska Formation on the east

and north (showing up-to-the-north stratigraphic displacement), an ENE-trending

fault running through the Talkeetna Formation on the west side of Sheep

Mountain, and the strike-slip fault inferred within Talkeetna Formation exposures

along its south side.

About 13 km north of Sheep Mountain is the east-west trending Caribou

Syncline, part of which (Syncline Mountain) is visible from the far eastern end of

the described field-trip route. This broad syncline is in turn approximately 3–6 km

south of the east-west trending Caribou Fault, which separates the Cretaceous

Matanuska Formation (to the south) from a sequence of Jurassic (Naknek

Formation) and Cretaceous sedimentary rocks, and Tertiary basalt flows and

pyroclastic deposits. The Caribou Fault exhibits right-lateral strike-slip

displacement that occurred before emplacement of the Eocene basalts, whereas

7
later motion on this fault system has been largely vertical (up-to-the-north; Grantz

1961). Right-lateral strike-slip faulting along the Caribou and Castle Mountain

Faults and their associated splays has offset volcanic exposures in the Talkeetna

Formation, most notably in the East Boulder Creek area (Clift and others, 2005a).

At Sheep Mountain, the base of the Talkeetna Formation exposures

contain rare carbonate rocks and shales, and are dominated by medium-grained

volcaniclastic sandstones that are interbedded with thick, matrix-supported tuff

breccias. Sedimentary structures are rare but some shales are parallel-laminated

and sandstones commonly grade upward into shale, suggesting possible

deposition by turbidity currents. Higher in the section, volcaniclastic sandstones

and shales are interbedded with very thick-bedded, clast-supported breccias,

which are up to 20 m thick with clasts >20 cm in diameter. Farther up-section, ~3

km from the section base, volcaniclastic rocks are interbedded with massive

lavas and volcanic breccias that locally appear to be autobrecciated lavas. Lava

units in this same area (the approximate middle of the Sheep Mountain

stratigraphy) have amygdaloidal tops. These volcanic rocks in the middle of the

section are typified by thickly bedded, matrix-supported, ungraded breccias up to

5 m thick, separated by minor shales each up to 30 cm thick. Clasts within the

breccias are all angular, volcanic, and can be up to 70 cm in diameter. The

uppermost 1 km of the section includes structureless, coarse-grained andesitic

tuffs (Clift and others, 2005a) interbedded with 1–2-m-thick, massive vesicular

basalts and basaltic andesite lavas. These, in turn, are overlain by a 12-m-thick

matrix-supported tuff breccia, with vesicular clasts up to 60 cm across. A 10-mthick columnar-jointed basaltic unit caps this Sheep Mountain exposure,

interpreted as a sill because of its geometry (Clift and others, 2005a).

The general depositional environment of the Sheep Mountain volcanic

facies is interpreted to be proximal to volcanic centers. This proximal setting is

inferred based on the high proportion of primary volcanic material (such as lavas

and ignimbrites) the coarse-grained facies of breccia clasts and tuff deposits, and

the lack of sorting evident in the abundant breccias. The paucity of sedimentary

structures throughout most of these volcaniclastic units complicates interpretation

of their depositional environment, though rare crossbedding indicates reworking

by currents. Bivalve fossils in the westernmost and easternmost shale-rich

exposures of the Talkeetna Formation on Sheep Mountain suggest that, where

marine deposition occurred, water depths were likely no deeper than ~500 m

(outer shelf to upper bathyal environments); one such fossiliferous exposure is

listed as the third stop on this field trip. Fossil terrestrial-plant leaves (including

cycads) have also been found in isolated areas in the middle of the Sheep

Mountain section, indicating some subaerial exposure and likely shallow water

depths of sedimentation for much of the unit.

Figures 3 and 4 show stratigraphic logs made from the Sheep Mountain

area; these logs represent five stratigraphic sections measured from canyons cut

into the southwest, south-central, and east side of Sheep Mountain, as well as

mapping work done along the southern ridge crest facing the Matanuska Valley,

whereas Figure 4 represents one continuous section >260 m thick measured in a

canyon cut into the northeast side of Sheep Mountain. The total thickness of the

8
Talkeetna Formation exposed on Sheep Mountain is estimated to be >4 km, of

which ~560 m are included in the detailed logs of Figures 3 and 4. Sections

logged on the north and south sides of Sheep Mountain contain similar

proportions of primary volcanic material (73 percent overall, by bed thickness; the

remainder consists of volcaniclastic material that has undergone some

reworking). The Sheep Mountain sections are offset by multiple small-scale (<10

m offset) northwest-trending faults and are intruded by occasional dikes that may

be part of a peperite complex (Busby-Spera and White, 1987); these complexes

are associated with volcanism interacting with thick sedimentary sequences

where magma has encountered water-saturated sediment and intrude into the

sediment. The Talkeetna Formation at Sheep Mountain is dominated by proximal

tuffs and matrix-supported breccias (Fig. 5), with minor lavas and volcaniclastic

deposits (including occasional cross-bedded sandstones). Primary volcanic

deposits include, on the south side of Sheep Mountain, thick (up to ~20 m)

volcanic breccias and amygdaloidal, vesicular (zeolite-filled) lava flows. Some of

these facies are visible in the area of Stop 2 on this field-trip route, on the south

side of Sheep Mountain. Exposures on the north side of Sheep Mountain include

structureless coarse-grained andesitic tuffs interbedded with basaltic and

basaltic-andesite lavas (Fig. 5) that are overlain by a 12-m-thick matrix-supported

breccia composed of clasts of tuff up to 60 cm in diameter. The uppermost unit of

the Talkeetna Formation at Sheep Mountain is a basaltic lava flow up to 10 m

thick with columnar jointing structures.

In addition to inferring a generally proximal volcanic depositional

environment for the Talkeetna Formation at Sheep Mountain, Clift and others,

(2005a) and Draut and Clift (2006) placed other Talkeetna Formation exposures

into submarine depositional contexts based on comparison with modern arc

analogs in the western Pacific. Figure 6 shows the inferred depositional settings

for volcanic products of the Talkeetna Arc south of the Matanuska River, south of

the Copper River basin (the basin being east of this field-trip area), at Sheep

Mountain, and in the East Boulder Creek and Horn Mountains areas (both north

of Sheep Mountain; see Fig. 1).

Geology Visible from the Glenn Highway near Sheep Mountain

Driving northeast on the Glenn Highway, nearing Sheep Mountain, the

route will pass the Matanuska Glacier and its outwash plain. These will be visible

on the right (south of the road) beginning around mile marker 101. There is a

parking area with views of the glacier at mile marker 107.8.

After passing the Matanuska Glacier the road turns northeast near mile

marker 110, and there will be a view of the rock promontory that reaches 3185

feet known as the Lion Head (it looks more like a reclining lion when viewed from

the east). The Lion Head is a Tertiary intrusion, one of several in the area. Two

more similar intrusive bodies are visible to the east (elevation 3400 feet) and

southeast (4032 feet), both due east of the Matanuska Glacier’s outwash plain.

9
These Tertiary intrusive bodies comprise felsic plugs and dikes of quartz

porphyry and feldspar porphyry (Grantz, 1961); the Lion Head is surrounded by

sediments of the Cretaceous Matanuska Formation and by Quaternary gravels.

Looking south across the Matanuska River near mile marker 110 the

Chugach Mountains are visible. To the south in the foreground, the lower part of

the Jurassic Talkeetna Formation arc volcanic stratigraphy is exposed; here the

volcanic material is dominated by basaltic andesite lava (Fig. 3). At the western

end of the exposures visible from this section of the highway, the Talkeetna

Formation is in faulted contact with Cretaceous sediments. Some intrusive

plutonic rocks (primarily intermediate but with minor felsic and mafic areas) of

Jurassic-Cretaceous age have been mapped in this northern part of the Chugach

Mountains.

Around mile marker 111 the Glenn Highway begins to traverse along the

south/southeast side of Sheep Mountain. The southwesternmost of three

summits on Sheep Mountain, reaching 6209 feet, is visible opposite mile marker

111. The road follows the northeast trend of the mountain along its base for

approximately the next eight miles to its western end. The turn-off to Sheep

Mountain Lodge is at mile 113.5. The summit north of Sheep Mountain Lodge

reaches 6223 feet. The easternmost and highest peak on Sheep Mountain, the

summit known as Gunsight Mountain, reaches 6441 feet and is visible opposite

mile marker 116. At its eastern end (beginning around mile marker 117) black

shales of the Matanuska Formation (Cretaceous) are visible, lying in faulted

contact with the Talkeetna Formation.

After passing Sheep Mountain, the Glenn Highway continues northeast,

entering the Copper River Basin approximately ten miles to the east.

Field-Trip Stops

Stop 1. Matanuska glacier overlook, near mile marker 108 on the Glenn

Highway.

Stop 2. Approximately 1 km east of Sheep Mountain Lodge there is a long

exposure above the road of well bedded volcaniclastic materials. Vehicles may

be left in the gravel parking area on the north side of the highway. Upon climbing

the slope above this parking area, a series of medium-bedded, coarse-grained

mass-flow deposits becomes visible. These units comprise volcaniclastic

sandstone and angular, poorly sorted breccias. The depositional environment is

likely to be marine as evidenced by the presence of shales within sequences of

coarser sandier breccias. Progressing east (up-section) across the ridge there is

a 6-m-thick exposure of massive volcanic breccia, including blocks up to 1 m

10
across. The angular character of the clasts, suspended in a sandy matrix,

suggests a proximal origin, likely close to the volcanic center. Clasts include both

fresh and weathered lavas, as well as volcanic sandstones. Continuing to the

east, the slope exposes a series of well bedded shales and parallel laminated

sandstones, which show normal grading. These are interpreted as turbidites,

probably part of a proximal slope apron around the volcanic center. The

sandstones contain grains of volcanic material, with no apparent quartz, mica, or

other minerals of possible continental origin. Primary volcanic material includes

airfall tuffs, largely of andesitic composition, although some darker colored units

hint at a basaltic component as well. Lava flows are rare but one 8-m-thick

massive flow is interbedded with the volcaniclastic rocks. Debate continues as to

whether this unit might be an intrusive sill within a peperite complex (Clift and

others, 2005a).

Moving further east and thus up-section the route comes to a prominent

ridge of welded tuffs and sandy breccias. The sequence is well bedded, with a

series of tuffs each ranging from 20–50 cm thick, and rare more massive lava

flows. The ridge itself consists of a series of 4- to 5-m-thick dark-colored basaltic

tuffs and crystalline tuffs. Tuff bed bases are typically sharp and commonly

underlain by a conglomeratic layer, parallel-laminated sandstone, or breccia. The

thickest tuff is 8 m thick and shows a columnar jointing pattern and curious

vertical welded tubes that may be formed by fluid escape after rapid

emplacement. Above this interval the Talkeetna Formation exposes a more

reworked facies style, dominated by thin- to medium-bedded, commonly graded

sandstones and laminated shales, consistent with deposition from turbidity

currents and debris flows.

Stop 3. The second set of Talkeetna Formation outcrops that will be visited on

this field trip is located on the east side of Sheep Mountain, just south of an

unnamed drainage that lies directly south of Camp Creek. Sediment-rich units

within the Talkeetna Formation contains many known fossil localities. See Martin

(1926) and the Alaska Paleontological Database website

(http://www.alaskafossil.org) for complete listing of all localities. Most of the

localities contain marine fauna (i.e., ammonites; Imlay, 1983) and brachiopods

(Sandy and Blodgett, 2000), although non-marine facies locally present that are

rich in the remains of terrestrial plant fossils (Knowlton, 1916). Stop 3 is a fossil

locality in the Talkeetna Formation that is relatively easily accessible by

automobile, exposed approximately 400 feet above the road base of an

abandoned section of the old Glenn Highway. It consists of sedimentary pockets

(mostly mudstone and siltstone) located interstitially within a volcanic

agglomerate. The locality can be accessed by driving roughly 6 km east of Sheep

Mountain Lodge along the Glenn Highway, and then turning left onto an old

abandoned stretch of the old Glenn Highway (the currently available 1:63,360

scale USGS topographic map shows only this old roadway), and then driving SW

to where the old road crosses Camp Creek. Vehicles can be parked on the east

side of the creek crossing, as the road is no longer passable at this point without

a 4-wheel-drive vehicle. The access to the outcrop then requires walking

11
approximately 400 m to the southwest, crossing another smaller unnamed creek,

then following a clearing on the north side of the creek that appears to represent

an old game trail. This game trail leads northwest until it reaches the base of a

prominent exposure of volcanic and agglomeratic rocks of the Talkeetna

Formation. Careful searching of the exposure will show scattered pockets of

sedimentary rocks that contain numerous brachiopods (roughly five or six

species present, mostly belonging a single small rhynchonellid species, but also

include three or four species of terebratulids and one spiriferoid), lesser bivalves,

and rare ammonoids. Small outcrops of the Cretaceous Matanuska Formation

containing large inoceramid bivalves are exposed along the game trail on the

approach to the Talkeetna Formation outcrop, separated by a major high-angle

fault according to the geologic map of Grantz (1961). The Talkeetna Formation

fossil locality has the following GPS coordinates: Latitude 61o 50.137’ N.,

longitude 147o 25.364’ W with an approximate elevation of 3,900 ft.

Acknowledgements

Funding for Draut and Clift to study the Talkeetna Formation was provided by a

grant from the National Science Foundation, Continental Dynamics program. Jeff

Trop (Bucknell University) and Thomas Moore (USGS) provided constructive

reviews of this manuscript.

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Figure 1. Regional map of the area northeast of Anchorage along the Glenn

Highway where the Jurassic arc volcanic materials of the Talkeetna Formation

are exposed. Locations of the cross section shown in Figure 2 is indicated, as are

locations of stratigraphic sections shown in Figures 3 and 4. Locations of photographs that appear in Figure 5 are also noted.
Jurassic Talkeetna Formation exposed

Figure 2. Cross section of the Sheep Mountain area, oriented northwest- southeast

(Clift and others, 2005a).
10

Section A

40

50

60

70

80

90

100

110

120

130

140

20 m unexposed

Tuffaceous

breccia

Massive tuff

Massive

volcanic breccia

Tuffaceous sands

Vesicular lava

Welded

tuffaceous breccia

Welded tuffs

Tuffs with conglomerate

bases

Basaltic breccia

Massive

brecciated

tuffs

Volcanic

breccias

Massive sandy tuff

Bedded, graded,

and laminated tuffs

10

20

30

40

50

60

Volcanic sands and

thin tuffs

Section I

10

Brecciated tuff

Massive sandy tuff

Graded tuff

Section J

10

20

Section H Section V

Thinly bedded

sandstone/ calcareous

couplets

Calcareous

Sands grade up

into shales

Bivalve fossils

Sand fines up

Pebbly sand

fines upward

to siltstone

Increasing grain size

Tuff

Lava

Breccia

Conglomerate

Sandstone

Siltstone

Shale

Parallel lamination

Cross-lamination

Figure 3. Stratigraphic sections on the south side of Sheep Mountain mapped by Clift and Draut

during 2002 and 2003 (locations shown on Figure 1). These measured sections do not form one

continuous section; several faults within the Sheep Mountain massif, combined with difficulties of

access, preclude generation of one comprehensive stratigraphic column for the Talkeetna Formation at Sheep Mountain.
Plag-phyric lava flow, chilled top

Dark tuffs and

breccias

Dark massive tuff

Volcanic sandstone

Brecciated tuff

Tuffaceous

sandstone

Volcanic breccia

Volcanic sand

Basaltic lava

Thinly bedded

fine sands and

siltstones

Massive plag- phyric lava

Lava cross-cut by dike

Bedded

tuffs

Laminated, sandy

graded tuff

Thickly

bedded tuffs

10

20

30

40

50

60

70

80

90

100

Thickly bedded

lavas

Vesicular massive lavas

Graded volcanic sandstone

Volcanic breccia

Massive plag- phyric andesite

Conglomerate with sharp erosive base

Massive volcanic breccia

Massive

lavas with minor thin volcanic

breccias

Bedded sandy

tuffs

Massive lavas

110

120

130

140

150

160

170

180

190

200

Massive volcanic breccia

Basaltic

lava

Thick

bedded

breccias and sandstones

Thick bedded laminated and massive sandstones

Massive vol- canic breccia

Tuffaceous

breccia

Tuffaceous

breccia

210

220

230

240

250

260

Tuff

Lava

Breccia

Conglomerate

Sandstone

Siltstone

Shale

Parallel lamination

Cross-lamination

Increasing grain size

Figure 4. Continuous stratigraphic section measured on the north side of Sheep

Mountain by Clift, Hanghoj, and Draut in 2002.
A B

C D

Figure 5. A: Proximal volcanic breccia unit within the Talkeetna Formation, mapped on

the south side of Sheep Mountain. B: Volcanic breccia from the north side of Sheep

Mountain. C: Volcaniclastic deposit on the south side of Sheep Mountain showing sedimentary structures that indicate current reworking. D: Light-colored coarse-grained tuff

unit on Sheep Mountain, underlain and overlain by darker lava flows.
Lavas and

mass-flow

deposits

Air-fall tephra

Sheep Mountain &

South of Matanuska

Tectonic erosion

of forearc?

Little Oshetna

Backarc

S of CRB

Figure 6. Schematic reconstruction of depositional environments for the arc volcanic and volcaniclastic

Talkeetna Formation, from Draut and Clift (2006). Not to scale or actual compass orientation.