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Kelley, J.S., 1980

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Bibliographic Reference

Kelley, J.S., 1980, Environments of deposition and petrography of lower Jurassic volcaniclastic rocks, southwestern Kenai Peninsula, Alaska: University of California, Davis, Ph.D. dissertation, 304 p., illust.


Little-known Lower Jurassic volcaniclastic rocks in the southwestern Kenai Peninsula west of the Port Graham fault are critical to Mesozoic paleogeographical interpretation of south-central Alaska. The Lower Jurassic volcaniclastic rocks and depositionally underlying Upper Triassic-Lower Jurassic rocks are the only exposures on the Kenai Peninsula of Mesozoic rocks apparently equivalent to Mesozoic rocks exposed along the west side of the Cook Inlet. The Upper Triassic-Lower Jurassic rocks, the Port Graham formation, are correlative to coeval rocks in the Kamishak Formation exposed on the west Cook Inlet. The Lower Jurassic volcaniclastic rocks, the Pogibshi formation, range in age from early Hettangian to early Sinemurian, and are probably at least in part time-stratigraphically equivalent to the Talkeetna Formation on the west side of Cook Inlet. The upper portion of the Pogibshi formation is time-stratigraphically equivalent to the lower portion of the Talkeetna Formation in the Talkeetna Mountains. The Pogibshi formation consists of andesitic and dacitic pyroclastic and volcanolithic rock derived from a volcanic arc that probably lay to the northwest on continental or subcontinental crust. The Pogibshi formation consists of three members, in ascending order, the Dangerous member, the July member, and the Naskowhak member. Each member includes mappable units that correspond to differing lithofacies and rock types. The lower part of the Dangerous member consists principally of pyroxene andesite conglomerates and breccias emplaced by sediment gravity flows in a submarine canyon cut into the Port Graham formation. The middle part of the Dangerous member consists of chiefly dacitic pyroclastic debris mostly deposited by gravity settling and turbidity currents. The upper part of the Danger member largely consists of tuffaceous andesitic conglomerates and breccias emplaced by submarine sediment gravity flows. The lower part of the July member consists of dacitic turbidites with up to 30 percent framework quartz, and interbedded tuffaceous mudstone. The upper part of the July member consists of principally dacitic vitric pyroclastic debris flows emplaced at low temperatures in a marine environment. The lower part of the Naskowhak member consists mostly of dacitic and andesitic sandstones, tuffaceous mudstones, and coal. The upper part of the Naskowhak member consists of tuffaceous sandstones, tuffaceous mudstones, and vitric tuffs. The Pogibshi formation consists of two depositional successions. The Dangerous and July members consist of strata largely deposited as resedimented debris in subwave base marine environments that ranged from abyssal to neritic depths. The upper succession, the Naskowhak member, consists of volcaniclastic debris reworked in environments that ranged from transitional-marine to neritic. Greatly different depositional environments such as those recorded in the Pogibshi formation are spatially associated by submarine canyons on the flanks of modern volcanic arcs.

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