Megafossil Biostratigraphy and T-R Cycles of the Shublik Formation in the Phoenix #1 Well , Northern Alaska

By

R.B. Blodgett (Oregon State University) and K. Bird (U.S. Geological Survey)

The Phoenix #1 well, situated offshore about 100-km northwest of Prudhoe Bay, exposes includes a superb, nearly completely cored interval of Middle-Upper Triassic Shublik Formation,the subject of detailed geochemical and sequence stratigraphic studies (Robison et al., 1996, Hulm, 1999, and Parrish et al., 2001). This section is 84.0 m thick and ranges in age from Ladinian to late Norian. Four major transgressive-regressive (T-R) cycles are identified, each characterized by a distinctive suite of flat clams (halobiid and monotid bivalves) which permit refined age assignment. Sequence boundaries are marked by disconformities in which significant biostratigraphic gaps are apparent; superjacent beds commonly contain concentrations of phosphatic nodules and/or oyster banks comprised of the genus Gryphaea. The lowest cycle is 13.5 m thick and contains the Ladinian bivalve Daonella frami. The second cycle is 24.2 m thick and is of early Carnian age. This shaly interval is particularly organic rich, as indicated by abundant traces of oil staining. The third cycle is 15.9 m thick and consists primarily of limey siltstone and shale of late Carnian age. The bivalve genus Halobia is abundant in both the second and third cycles. The final cycle is 29.8mthick and comprised primarily of limestone containing middle and late Norian age monotid bivalves; it appears to be correlative with the limestone member of the Otuk Formation in the Brooks Range. The boundary between the third and fourth cycles is marked by a major missing time interval, the early Norian, a time gap documented throughout much of Arctic Alaska. 

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.