The northern volcanic fields include the Sischu, Nowitna, Dishna, and Yetna volcanic fields (Fig. 1). The Sischu volcanic field (71-66 Ma) consists of a narrow belt of poorly exposed rhyolite and dacite domes, flows, and tuff that extends from the Sischu Mountains to the southern Chitanatala Mountains (Moll and others, 1981; Moll-Stalcup and Arth, 1989). The main volcanic field covers an area of more than 725 km2, is at least 500 m thick, and is fault bounded on the southeast side. A felsic pluton that crops out just east of the volcanic rocks has an age of 64 Ma (Silberman, and others, 1979). The Nowitna volcanic field consists of more than 1500 m of chiefly andesitic flows preserved in a gently folded northeast-trending syncline that is fault-bounded on the southeast side. The field covers more than 2700 km^, and overlaps the suture between the Innoko and Nixon Fork terranes. At least seven highly altered rhyolite domes overlie the andesite flows. K-Ar ages on three whole-rock andesite samples collected near the top of the section are 64 to 63 Ma (Silberman and others 1979). The Dishna volcanic field consists of calc-alkalic dacite, rhyolite, and minor andesite poorly exposed in a series of isolated ridges and hills that rise above the alluvium in the Innoko-Dishna Rivers area (Chapman and others, 1985). These undated rocks are presumed to be Late Cretaceous or early Tertiary in age. Volcanoplutonic complexes occur in the McGrath area at Page Mountain, Cloudy Mountain, Candle Mountain, Takotna Mountain, Mount Joaquin, the Beaver Mountains, and in the Lonesome Hills. These complexes have circular-shaped outcrop areas that consist of andesite flows and shallow hypabyssal rocks intruded by small granitic stocks. Most of the volcanic rocks are highly altered by the intrusions. The margins of many of the complexes appear to be fault-bounded and the complex at Page Mountain is down-faulted against the surrounding sedimentary rocks. Date volcanic and intrusive rocks from these complexes yield K-Ar ages ranging from 73 to 65 Ma (Moll and others, 1981; Bundtzen, and Laird, 1982; 1983a; 1983b; 1983c) and are interpreted as being deeply eroded volcanic centers.
We thank A.B. Till and J.A. Dumoulin for providing unpublished data on the Seward and York terranes on the Seward Peninsula, and J.H. Dover for providing unpublished data and interpretations for east-central Alaska. We also thank J.P. Calzia and N.J. Silberling for their constructive and thoughtful reviews, C.J. Nokleberg for computer drafting of the stratigraphic columns, and J.A. Nokleberg for coloring of the map. DESCRIPTION OF STRATIFIED MAP UNITS POST-ACCRETION SEDIMENTARY ROCKS Qs Sedimentary rocks (Quaternary) Chiefly Quaternary continental sedimentary rocks and lesser volcanic rocks, and unconsolidated silt, sand, and gravel. Depositionally overlie older overlap assemblages and terranes. Locally folded and faulted Ts Sedimentary rocks (Tertiary) Chiefly Tertiary continental sedimentary rocks and lesser volcanic rocks. Depositionally overlie older overlap assemblages and terranes. Locally folded and faulted Czs Sedimentary rocks (Cenozoic) Chiefly Tertiary continental sedimentary rocks and lesser volcanic rocks, and Quaternary (including Holocene) sedimentary rocks and unconsolidated silt, sand, and gravel. Depositionally overlie older overlap assemblages and terranes. Locally intensely folded and faulted Ks Sedimentary rocks (Cretaceous) Chiefly deep marine shale anft minor conglomerate deposited by turbidity currents. Local coarse-grained sandstone, and fine-grained sedimentary rocks deposited in deepmarine conditions to shallow-marine to nonmarine conditions. Local interlayered volcanic rocks chiefly rhyolite, dacite, and andesite tuff, flows, and breccia. Depositionally overly older Mesozoic overlap assemblages, .and terranes. Locally intensely folded and faulted. In southwestern Alaska, consists chiefly of Upper Cretaceous Kuskokwim Group (Cady and others, 1955; Box and Elder, 1992) that is mainly shale and fine-to medium-grained sandstone, and lesser siltstone and conglomerated that formed as deep-water marine turbidites with abundant, immature, lithic-rich components. Kuskokwim Group forms major overlap assemblage on Dillinger, Goodnews, Nixon Fork, Ruby, Tikchik, and Togiak terranes. In west-central Alaska in the Yukon-Koyukuk basin unit consists mainly of Lower Cretaceous, deep-marine turbidite deposits and Upper Cretaceous, shallow marine and non-marine deltaic sedimentary rocks that overlie Angayucham, Nixon Fork, and Ruby terranes (Nilsen, 1989) GN Gravina-Nutzotin overlap assemblage (mid-Cretaceous to Late Jurassic) Chiefly argillite, graywacke, and conglomerate, with lesser andesitic and basaltic volcanic, and volcaniclastic rocks of the Chisana Formation, Douglas Island Volcanics, and similar unnamed volcanic units. Sedimentary rocks range from deep marine turbidites to shallow-water and nonmarine deposits. In eastern-southern Alaska, coarse clastic rocks in Nutzotin part of assemblage derived locally from the underlying Wrangellia terrane and from unknown metamorphic source terranes. In southeastern Alaska, coarse clastic rocks in Gravina part of assemblage derived from stratigraphically un...
Qal Alluvium along streams (Holocene)-Mainly in active flood plains of major and some minor streams. Locally, includes outwash related to older phases of the Alaska glaciations. Chiefly stratified boulders, cobbles, gravel, and sand; silt locally common. Qaf Alluvium in fans (Holocene)-Mainly large active fans and cones on steep to gentle slopes adjacent to the broad glaciated valley of the Copper River. Chiefly poorly stratified boulders, cobbles, gravel, and sand. COLLUVIAL DEPOSITS Qc Colluvium, undifferentiated (Holocene and Pleistocene)-Chiefly talus but also includes deposits of small landslides, rock glaciers, and other mass-wasting processes; in places includes large proportion of alluvium in small fans and cones and locally includes remnants of morainal deposits. Chiefly unsorted boulders, cobbles, gravel, and sand. GLACIAL DEPOSITS Qrg Rock glacier deposits (Holocene and Pleistocene)-Deposits in both active rock glaciers, which have well-defined lobate forms, and inactive rock glaciers, which have smooth forms. Chiefly angular blocks and diamicton. Qag Drift of Alaskan glaciation (Holocene)-End, lateral, and ground moraine of the Alaskan glaciations which were deposited during the recession of existing glaciers. In the extensive areas of glacial drift west of Copper Glacier three stages of Alaskan glaciation are recognized (H. Schmoll, written comm., 1994). The youngest stage includes ground, lateral, and end moraines that have been deposited near the margins of present-day glaciers. The two older stages, related to an older phase of Alaskan glaciation and marked by prominent and well-defined lateral moraines, occur further beyond the present-day glacier margins. Dashed line with hachures indicates extent of youngest stage of Alaskan glaciation and locally includes areas within which glaciers have been advancing and retreating during the late 20th century. Plain dashed line separates the two older stages of Alaskan glaciation. Drift is locally modified by colluvial processes, especially along steep eastside of the large nunatak west of Copper Glacier. Diamicton and rubble; local gravel and sand. Qwg Drift of Wisconsin glaciation (Pleistocene)-Chiefly lateral and ground moraine of both younger and older stages of Wisconsin glaciation. Diamicton and rubble; local gravel and sand. Qog Drift of older glaciations (Pleistocene)-Glacial and fluvioglacial deposits observed only in the north wall of the Jacksina Glacier valley where unit is overlain by an agglutinate flow (unit Qaa) that may be as old as 1.38 Ma (Richter and Smith, 1976). Chiefly diamicton; local sand, gravel and boulders. VOLCANIC ROCKS Wrangell volcano-Mount Wrangell (el. 14,163 ft [4317 m]) is a large shield volcano, whose summit area lies mostly to the southwest in tMe contiguous Gulkana A-l and Valdez D-l quadrangles. Only a part of the northeast flank of the shield is exposed in the quadrangle. A porphyritic high-silica andesite lava, similar to that found elsewhere on the Wrangell shield (Richter and others, 1994) is the predominant rock ...
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