Sequence Stratigraphic Control from Alluvial Architecture in from Upper Cretaceous Fluvial System — Wahweap Formation, Southern Utah, U.S.A

Zubair A. Jinnah
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa

The Wahweap Formation (Fm) is a ~400 m thick package of fluvial/estuarine channel sandstones and floodbasin mudstones deposited adjacent to the Western Interior Seaway (WIS) during the Late Cretaceous. It contains four informal members: the lower, middle, upper and capping sandstone members. The study presented here aims to test existing tectono-eustatic basin models and reconstruct the sedimentary and tectonic history of the southern Cordilleran foreland basin during this important interval.

A radiometric date of 80.1 Ma has been obtained from the lower part of the middle member, indicating that the middle and upper members were deposited during the Claggett transgression, a eustatic rise in the WIS that occurred at ~79.6 Ma.

Sedimentological analysis and sand:mud ratios suggest that the lower member, which contains two laterally extensive sandstone bodies, with internal erosive surfaces and intraformational rip-up clasts, was deposited in a low accommodation space setting.

The middle member is dominated by grey mudrocks with minor carbonaceous shale and fine-grained sandstone. It represents the fastest sediment accumulation rate in the Wahweap Fm and records a shift to a high accommodation space setting at the same time as the initiation of the Claggett transgression in the WIS. The upper member consists of several thick sandstone bodies interbedded with mudrocks. Tidally influenced inclined heterolithic strata and the brakish water trace fossil Teredolities are present suggesting, along with its appropriate age, that the upper member corresponds to the sea level highstand in the WIS during the Claggett transgression. Sedimentological evidence and new age data therefore suggest that stratigraphic changes in the Wahweap Fm are eustatically rather than tectonically driven, and support previous placement of a sequence boundary at the upper/capping sandstone contact.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery