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.