Late Cretaceous to Early Tertiary Deltaic to Marine Sedimentation, North Slope, Alaska

Lawrence R. Phillips

Along the lower Colville River near Ocean Point, Alaska, Late Cretaceous to early Tertiary sediments (Colville Group and the Sagavanirktok Formation) record depositional environments from delta plain to prograding delta to shallow marine shelf. The unit hosts the northernmost known dinosaur remains and is less than 100 m thick with numerous tephra deposits in its lower sections. Furthermore, it is characterized by cyclic, relatively fine-grained sediments indicating mostly depositional and few erosional events.

The major depositional elements of the delta plain are 3.5 to 5.45-m thick tabular fining-upward cycles that are cut by sand-filled fluvial channels (up to 10 m thick). The cyclic sediments contain abundant roots and grade upward from small-scale cross-beds to laminated and then structureless silt and clay terminating in organic-rich layers. The channel-fill sequences fine upward and change vertically from large to small-scale cross-beds. Over-bank flooding as well as lateral migration of small meandering fluvial channels formed the cyclically interbedded deposits, meandering rivers deposited the thick cross-bedded sands, and soil development or marsh deposits formed the organic-rich horizons that cap each cycle. Plant debris, nonmarine invertebrates, and vertebrate fossils are locall concentrated in the delta plain sediments.

Subsidence related to compaction of the deltaic sediments along with possible delta lobe switching resulted in repeated progradation of the delta front over the delta plain. Delta front sediments are 3 to 10-m thick tabular deposits of large and small-scale cross-bedded sands and silt bounded by organic-rich beds. Also, there are abundant roots, rare channels and invertebrate fossils that suggest a transitional environment from sand-flats to estuarine or bay.

Cross-bedded fossiliferous marine sandstone containing hummocky cross-strata overlies the delta front deposit. This shallow storm-dominated shelf deposit records a marine transgression that locally terminated deltaic sedimentation.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.