--> ABSTRACT: Fan-Delta Deposition in the Cretaceous (Albian) Fortress Mountain Formation, Central North Slope, Alaska, by Timothy J. Ryherd; #91003 (1990).

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ABSTRACT: Fan-Delta Deposition in the Cretaceous (Albian) Fortress Mountain Formation, Central North Slope, Alaska

Timothy J. Ryherd

The Albian Fortress Mountain Formation at Atigun syncline is interpreted to be a proximal fan-delta depocenter. This interpretation is based on the absence of shale and marine indicators, the presence of indicative sedimentary features, and the discontinuous occurrence of this conglomerate in thick localized accumulations.

The Fortress Mountain Formation crops out discontinuously across the Brooks Range foothills belt from west of the Kukpowruk River to the Sagavanirktok River. It is among the first sediments shed north into the Colville trough during early uplift of the Brooks Range orogen. In the central North Slope area, the Fortress Mountain Formation overlies deformed sediments of the pebble shale, Okpikruak Formation, and Konqakut Group. This study examines the Fortress Mountain Formation at Atigun syncline, near the eastern limit of its outcrop belt, where a 1700-m section was measured and described during the 1989 field season.

The Fortress Mountain Formation at Atigun syncline is composed of interbedded conglomerate (20%) and pebbly sandstone (80%). Significantly, shale is almost completely absent. The conglomerate is clast supported and consists dominantly of rounded and spherical chert pebbles and cobbles in a sandy matrix. Conglomerate beds are commonly ungraded and unimodal, with occasional crude imbrication. Most beds have planar bases and tops, but some channeling does occur. While most beds are featureless, some sandstone beds are planar and trough cross-bedded. Plant fragments are common on bedding surfaces.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990