LATERAL AND VERTICAL MARINE TO FLUVIAL TRANSITIONS IN THE UPPER PART OF THE UPPER CRETACEOUS TULUVAK FORMATION, NORTH SLOPE, ALASKA, INCLUDING A POSSIBLE EXAMPLE OF A MARINE GILBERT-TYPE DELTA

WILSON, Gregory C., ConocoPhillips Alaska, Inc, 700 G Street, Anchorage, AK 99501, [email protected], MORRIS, William R., ConocoPhillips Co, Houston, TX 77095, and REIFENSTUHL, Rocky, Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709

Upper Cretaceous rocks of the Tuluvak Formation (in the redefined nomenclature of Mull, Houseknecht, and Bird, 2003) generally record upward shoaling from nearshore marine to nonmarine fluvial depositional environments. Locally the transition is abrupt both laterally and vertically. A unique locality near the southern flank of Rooftop Ridge displays the outcrop-scale topset-foreset-bottomset morphology typical of Gilbert Deltas. However, this would imply the development of a Gilbert-type delta prograding into a marine environment. Marine Gilbert deltas occupy a much smaller share of the technical literature than the widely-described lacustrine variety. At the Rooftop Ridge locality, bottomset sandstone beds display characteristics of a nearshore marine setting, with abundant Inoceramus and marine trace fossils. Foreset beds, about 25 feet thick, are comprised of matrix-rich and matrix-supported pea-gravel to pebble conglomerates. These foreset beds are exposed across an outcrop width of about 400 feet before being obscured by cover. Foreset gravels appear to downlap over a narrow interval with, rather than erode into, the underlying bioturbated, massive, and locally trough cross-bedded marine sandstones. The topset beds record marine reworking of the uppermost gravel unit, with discrete truncation of the underlying foresets. These topset beds are comprised of a largely clast-supported massive to horizontally bedded, pea-gravel to pebble conglomerate (slightly less matrix). This unit records reworking in a foreshore setting during transgression. Marine trace fossils have been observed in float above the topset beds. Similar horizontally-bedded conglomerates with an open framework are interbedded with matrix-rich poorly sorted trough-crossbedded (fluvial?) conglomerates at several other Tuluvak localities. Conglomeratic Tuluvak occurs as much as 25 miles north of this locality. Further north, and in the subsurface, it appears mostly marine in origin. Development of this coarse-grained facies in a marine environment suggests an embayment local to high-sediment influx in the littoral zone, protected from reworking. If truly a Gilbert delta, it also implies homopycnal flow and the localized flushing of saltwater from the vicinity.