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Chronostratigraphic Correlation and Interpreted Depositional Environments of the Miocene Bear Lake Formation, Bristol Bay Basin, Alaska Peninsula

Emily S. Finzel1, Kenneth D. Ridgway2, Rocky R. Reifenstuhl1, James White3, and Robert B. Blodgett4
1 Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK
2 Purdue University, West Lafayette, IN
3 Geological Survey of Canada, Calgary, AB
4 U.S. Geological Survey, Anchorage, AK

The Miocene Bear Lake Formation (BLF) is exposed along the coast near Port Moller on the Alaska Peninsula, in the mountains to the northeast, and extends offshore into the Bristol Bay Basin. The BLF has the highest reservoir potential in this frontier foreland basin.

New measured sections with additional palynological and megafossil data have been used to establish a stratigraphic framework for the BLF in the Port Moller area. The lowest part of the BLF is characterized by fluvial trough cross-stratified sandstone, interbedded with coal and mudstone, including occasional plant fossils. The middle part of the BLF is dominated by tidally influenced bioturbated sandstone and shale, and wavy- and flaser-bedded sandstone and shale. The upper part of the BLF consists of flaser- and wavy-bedded sandstone and conglomerate, bioturbated sandstone locally rich in marine trace and megafossils, and coarse-grained, bioturbated, channelized sandstone interbedded with discrete horizons rich in marine megafossils. In this upper part of the BLF, there is a transition from brackish to marine fauna upsection.

The BLF represented by our measured sections, approximately 1,200 m, exhibits an upward deepening succession that is interpreted as a relative sea level transgression. It is unlikely that the relative rise in sea level was a result of a rise in eustatic sea level, because sea level curves for the Miocene indicate a fall in eustatic sea level during that time. The relative sea level rise could be a result of subsidence on the southeastern margin of the basin caused by a northward prograding thrust belt or emplacement of intrusive rocks in the arc.