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AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 7. (July)

First Page: 1066

Last Page: 1080

Title: Depositional Relations of Cretaceous and Lower Tertiary Rocks, Northeastern Alaska

Author(s): C. M. Molenaar (2)

Abstract:

Analysis of depositional environments, new paleontologic data, and analogy with depositional patterns observed in areas to the west all indicate the need for revision of Cretaceous and lower Tertiary stratigraphy in northeastern Alaska. In the Sadlerochit Mountains area of the Arctic National Wildlife Refuge, the northern-derived (Ellesmerian), late Neocomian Kemik Sandstone Member and organic-rich pebble shale member of the Kongakut Formation unconformably overlie Jurassic and Triassic rocks. The unconformity, which is of mid-Neocomian age, is present throughout northernmost Alaska and passes southward into a conformable shelf sequence. After pebble shale deposition, the depositional pattern is simply one of progradational basin filling from a southern (Brookian) provena ce. This pattern is represented in vertical sequence initially by deep-marine basinal deposits succeeded by prodelta slope shales, and ultimately by deltaic deposits that prograded to the east or northeast in a predictable fashion over most of the area. The better known, thick, shallow-marine and nonmarine units in the central North Slope grade to thin prodelta or basinal turbidites; some units are entirely missing owing to nondeposition on the south-dipping flank of the basin. No evidence exists for subaerial erosion to explain these marked stratigraphic changes.

In the Sadlerochit Mountains area, which may be an extension of the Barrow arch, the pebble shale (Neocomian) is overlain by about 1,600 ft (500 m) of deep-water deposits of Late Cretaceous age; Aptian and Albian rocks are either absent (by nondeposition) or are represented by a thin, condensed section. Here the Upper Cretaceous to lower Tertiary section consists of basinal shale, bentonite, and thin-bedded prodelta turbidites. Two noteworthy features in this sequence are an interval of organic-rich shale of probable Turonian to Coniacian age just above the pebble shale, and the placement of the Cretaceous-Tertiary boundary within deep-marine deposits. Unlike the western part of the North Slope, where deltaic deposition occurred in the Early Cretaceous, prograding deltaic deposition d d not reach the western part of the Wildlife Refuge until early in the Tertiary. The consistent pattern of easterly deltaic progradation is complicated at Igilatvik (Sabbath) Creek (in the north-central Wildlife Refuge) by the occurrence of thick, regressive, dominantly nonmarine deposits of early Tertiary age, which apparently prograded into the basin from the south or southeast.

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