NORTH of DENALI FAULT Pediment Surfaces

&' ?L DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-1 174 UNITED STATES GEOLOGICAL SURVEY GEOLOGIC MAP OF THE TALKEETNA QUADRANGLE, ALASKA By Bruce L. Reed and Steven W. Nelson DESCRIPTION OF MAP UNITS SURFICIAL DEPOSITS moraine have subdued geomorphic expres- sion and include alluvial, swamp, marsh, and ALLUVIAL AND MASS-WASTING DEPOSITS bog deposits. North of the Alaska Range, Q1 Young and old landslide deposits. - Chiefly end and lateral moraines are relatively fresh debris avalanches and large block glides in and include well-preserved pot-and-kettle areas of steep slopes. Particularly well topography exposed along headwaters of Dillinger and West Fork of Yentna Rivers Qpe Drift of pre-Eklutna age.-Recognized only as subdued lateral(?) and ground moraines on Qau Alluvium, undivided.-Includes alluvium on north side of Alaska Range, although glacial active flood plains, older alluvium on erratics and well-modified drift on top of terraces of major streams, alluvial fans, Yenlo Hills (T. 24 N., R. 11 W.) are prob- outwash from both Alaskan and Wisconsin ably deposits of this extensive glaciation in Glaciations, swamp, marsh, and bog deposits the Cook Inlet region (Karlstrom, 1964) along the larger streams in the southeast part of quadrangle, and alluvial deposits on broad NORTH OF DENALI FAULT pediment surfaces. It locally'includes collu- Sedimentary and volcanic rocks vium and other deposits on valley walls and hill slopes. The unit is chiefly boulders,gravel, Tc COAL-BEARING ROCKS.-A cumulative and sand with local areas of silt and clay thickness of about 30 m of subbituminous coal, with individual beds up to 9 m thick, Qr Active and recently active rock glaciers and crops out on the west bank of the Little associated talus aprons.-Chiefly rubble and Tonzona River (T.31 N., R. 20 W.). The diamicton; only larger rock glaciers shown coal-bearing rocks, which contain interbeds of ferruginous siltstone and silty shale, dip GLACIAL DEPOSITS about 55" to the north. The contact with Qa Drift of the Alaskan Glaciation.-Includes end unit Pzsl is covered but in this area is prob- and lateral moraines left after recession of ably a fault. These rocks are tentatively existing glaciers (Tunnel and Tustumena correlated to the east with the coal-bearing Stades); chiefly rubble and diamicton and group (Oligocene and Miocene) in the locally covered by vegetation; only larger Nenana coal field (Wahrhaftig and others, deposits shown 1969) and to the west with a sequence of middle or late Oligocene conglomerate, Qn Drift of the Naptowne Glaciation.-Includes sandstone, and shale (Reed and Elliott, end, lateral, and ground Moraine, postglacial 1968) alluvial, pond, and swamp deposits, fluvial- glacial deposits of all stages of Naptowne Ts CONTINENTAL SEDIMENTARY ROCKS.- Glaciation, and glaciolacustrine deposits Chiefly medium- to dark-gray phyllitic shale, formed during Knik and Naptowne Glacia- shale, sandstone, grit, and conglomerate with tions. At higher elevations, deposits merge minor carbonaceous shale and tuffaceous with and are covered by colluvium and sandstone. Shales are lithologically similar to locally contain drift of the Alaskan dark-gray shale in unit KJs, but close exam- Glaciation ination generally reveals a few Metasequoia fragments and carbonaceous debris. Light- to Qk Drift of the Knik Glaciation.-Well-developed medium-brown and maroon, fine- to coarse- end and lateral moraines shown only in grained sandstone beds 1-3 m thick typically northwest part of quadrangle. Possible drift are gradational into and interbedded with of this glaciation on south side of Alaska conglomerate and, in many places, contain Range is included with Naptowne and fragments of large broadleaf plant fossils. Eklutna deposits Resistant conglomerate beds generally form lenslike units from 2 to more than 200 m Qe Drift of the Eklutna Glaciation (Illinoian).- thick; clasts range in size from 0.5 to 20 cm, South of Alaska Range, lateral and ground are moderately well rounded, and consist chiefly of white quartz. Clasts of'qartzite, contact with unit KJs, nowhere within the quartz semischist, and varicolored phyllite quadrangle is the nature of the contact and chert suggest derivation from Pzm and between these two units precisely known. KJs terrane. Crossbedding and channel scour Owing to the extreme difference in compe- features are locally well developed. Lack of tence between the two units and to their granitic clasts in Ts indicates that Tonzona proximity to the Denali fault zone, the pluton (55 m.y.) was either not emplaced or contact is sheared. The basalt is, however, not unroofed during deposition. Poorly inferred to be part of a sequence which preserved plant fossils are best compared to includes unit KJs and to represent local species indicative of a Paleocene agd (J. A. extrusion within a marine basin. It is discon- Wolfe, written commun., 1977). The unit is tinuously exposed northeast to Straightaway believed to be an extensive continental Glacier and southwest to Farewell. Pillow sequence deposited during the Late Creta- basalts south of the Denali fault (Rbs) have a ceous and early Tertiary that now is similar major oxide content but occur within preserved in fault slivers chiefly along Denali a Paleozoic terrane fault zone. It is correlated with the Cantwell Formation (Paleocene) as restricted by KJs TJNDIVIDED MARINE SEDIMENTARY Gilbert and others (1976, p. 1). Tuffaceous ROCKS.-Moderate- to deep-water, inter- sandstone may reflect distal portions of the tonguing marine sedimentary rocks consist- Teklanika Formation of Gilbert and others ing chiefly of gray to black phyllite, argillite, (1976) and the onset of early Tertiary graywacke, and siltstone which are, in part, plutonism in this part of the Alaska Range. turbidite deposits; lesser amounts of gray, (Reed and Lanphere, 1 973a) green, and red chert, conglomerate, grit, and thin beds ~f limy mudstone and impure KJb PILLOW BASALT.-Dark-greenish-gray limestone. The conglomerate contains clasts elongate bodies of massive-weathering; of medium- to dark-gray chert, phyllite, and resistant pillow basalt. The unit includes sandstone in a fiae-grained sheared gray- interbeds and lenses of mudstone, shale, and wacke matrix. Clasts range in size from 6 to siltstone. Irregular patches and lenses of 25 mm. Fossils are extremely rare, but a few light-gray-weathering sparry calcite and fragments of belemnites, Buchiu, and reddish jasperoid breccia fillings are spor- occasional Inoceramus shell fragments adically developed. The basalt locally (T. 30 N., R. 19 W.) are Valanginian to includes gabbro which may represent its Barremian (Early Cretaceous) in age (D.L. feeder source. Pillows range in size from Jones, written commun., 1976). Fossils of 15 cm to 2 m in maximum dimension, are Jurassic age have not been found; thus, the generally porphyritic, and contain pheno- Jurassic age of its lower part is uncertain. crysts of altered plagioclase, subordinate The unit is in fault contact with units Rsv, clinopyroxene, and rare olivine in an inter- Pzsl, and Ts. Where observed, the contact granular to intersertal groundmass of plagio- with KJb is sheared, although bedding clase and pyroxene. Groundmass plagioclase attitudes and pillow flows appear conform- occasionally displays swallowtail and belt- able. The thickness of the unit is unknown buckle crystal shapes. Rare amygdules but is probably more than 1500 m. Rare consist of quartz, calcite, chlorite, and groove casts suggest a source either to the epidote. Chemically, the rocks range from north or south. These sedimentary rocks are hypersthene-olivine normative to hyper- intruded by units Tmt and TKi. Thermal sthene-quartz normative tholeiitic basalt. metamorphism has produced andalusite Trace element analyses indicate that the hornfels adjacent to larger intrusive bodies. basalts can be classified as ocean-floor basalt Sparse fossils indicate that this unit is, at (Pearce and Cann, 1973). The copper con- least in part, a time equivalent of unit KJs tent of nine samples of pillow basalt ranges south of the Denali fault. Lithologies of the from 200 to 500 ppm and averages about two units are broadly similar, but unit KJs 280 ppm. West of Pingston Creek, the flows south of the fault contains significantly generally face north, have a steep to sub- more sandstone, occasional plant fragments, vertical dip, are at least 1300 m thick, and and sedimentary structures that suggest local are thought to have considerable vertical shallow-water deposition. Unit KJs north of extent. In T. 32 N., R. 15 W. the pillow the Denali fault contains more chert and basalt appears to overlie unit KJs and has an shale and represents deeper marine deposi- apparent thickness of about 750 m. Tops tion. Northeast of T. 32 N., R. 14 W. the and bottoms of flows are locally brecciated; unit is chiefly dark-gray phyllitic shale and columnar joints are also present. Although argillaceous siltstone and is 1ithologicaUy the basalt is shown as having a normal similar to dark-gray phyllite (Pzp) Rsl SHALE AND LIMESTONE.-Marine sedi- bsu QUARTZITE, SEMISCHIST, AND META- mentary rocks that typically form a thin- to VOLCANIC ROCKS.-Tan to light-greenish- medium-interbedded sequence of black, gray, low-grade (greenschist facies), region- sooty, carbonaceous, and calcareous shale, ally metamorphosed quartzite, quartz semi- dark-gray calcareous siltstone, medium- to schist, quartz grit, metavolcanic rocks, and dark-gray limestone, and minor gray quartz- minor conglomerate, limestone, phyllite, and ite. Shale locally amounts to as much as 70 quartz-muscovite schist.The unit is isoclinally percent of the unit, is generally calcareous, folded and there is evidence of multiple and contains thin laminae of darkgray folding. Moderate to well-developed schistos- siltstone. Semiquantitative spectrographic ity is common. Schistosity and metamorphic analyses of eight samples of black shale from grade increase to northeast. The base is not the northwestern part of the quadrangle exposed; the unit is in fault contact with show the following average metal contents younger sedimentary rocks.

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