No abstract

No abstract

No abstract

Surticial deposits, primarily of late Pleistocene and Holocene age, cover about 60 percent of the Iliamna quadrangle, which is in southwestern Alaska. The deposits are thickest in the western part, where bedrock is mostr ly covered, In the mountainous eastern part the surface is largely bedrock, but unconsolidated surficial materials occur locally. The surficial deposits are primarily the result of glaciation, with subsequent modification by glaciofluvial, lacustrine, and marine processes. The oldest deposits are correlated with the Mak Hill Glaciation of early Wisconsin age. The glaciers coalesced to form a piedmont lobe that covered much of the quadrangle during the herein newly named Kukaklek Stade. The deposits are found only on hilltops 600-1,000 feet above deposits of the next glaciation in the western part of the quadrangle. Most of the surficial materials, as well as the present topography of the quadrangle, resulted from four stades of the Brooks Lake Glaciation that are correlated with the late (classical) Wisconsin Glaciation of the conterminous United States. During the two oldest stades, herein named the Kvichak and Iliamna, the glaciers coalesced to form piedmont lobes that covered most of the quadrangle; Iliarnna Lake and other large lake basins were formed. The two youngest stades, Neiwhalen (new) and Iliuk, were minor glacial advances of the alpine valley glacier type. Moraines of the Iliuk Stade locally divide lake basins into two parts. Two stades of the Alaskan Glaciation, of Holocene age, are recognized in the quadrangle. Glaciers of the Tustumena Stade advanced 1-3 miles from the cirques, and locally three advances can be mapped. During the subsequent Tunnel Stade the ice rarely advanced more than 1 mile beyond the cirque threshold. The small modern glaciers are remnants of the Tunnel Stade. Iliamna Lake, originally dammed by a moraine of the Kvichak Stade, attained its present size and shape when dammed by a morine of the Iliamna Stade; i t formed major terrace levels a t above 40, 80, 100, and 130 feet above the present lake level. The highest stand of water was about 150 feet above the present level. Radiocarbon-dated beach deposits indicate that the 80-foot level was formed a t least 8,520 years ago. An intermediate 53-foot level was formed about 5,520 years ago. Elevated marine beach deposits and wave-cut bedrock platforms along the west coast of Cook Inlet indicate that the coast is rising. Radiocarbondated material from one locality a t Kamishak Bay suggests that the rate of uplift is about 2 feet per century.

The McKinley sequence of granitic rocks consists of several discrete plutons in the central Alaska Range. Most of these plutons crop out south of the Denali fault system (DFS) in the Talkeetna quadrangle. Plutons of the McKinley sequence largely intrude deformed upper Meszoic flysch between the DFS and the northern edges of Wrangellia and the Peninsular terrane, which jointly make up the Talkeetna superterrane. The average K‐Ar age of biotite from nine granites of the McKinley sequence is 57.3 Ma; Rb‐Sr data for whole rock samples indicate that the McKinley sequence cannot be older than 60 Ma. A selected suite of 20 samples of granite and granodiorite range in SiO2 from 65.9 to 77.6%. All 20 samples are corundum normative, and 18 are moderately peraluminous. Initial 87Sr/86Sr ratios range from 0.7054 to 0.7085. The σ18O values range from +11.2 to +14.6‰. These high and variable Sr isotopic ratios, peraluminous nature, rare earth element patterns, and high σ18O values suggest that granitic rocks of the McKinley sequence crystallized from hybrid magmas produced by assimilation of sedimentary rocks by a mantle‐derived melt. Mesozoic flysch is the likely source of the crustal component of the hybrid magmas. Geologic evidence suggests that the Talkeetna superterrane collided with stable Alaska after Early Cretaceous time. The flysch basin, lying south of stable Alaska, was closed by northward movement of the Talkeetna superterrane; maximum age for basin closure and terrane accretion is middle Cretaceous (Cenomanian). Paleomagnetic evidence indicates that all terranes north of the DFS have been part of stable Alaska since the Paleocene and that northward movement of Wrangellia was completed by 50 Ma. Granitic rocks of the McKinley sequence may be products of terrane accretion; the granitic rocks crystallized from hybrid magmas produced during terrane collision and deformation of the flysch basin. Isotopic ages of the McKinley sequence establish the time of final accretion of the Talkeetna superterrane as Paleocene.