NEOGENE RETROARC BASIN DEVELOPMENT, BRISTOL BAY BASIN, ALASKA PENINSULA

FINZEL, Emily S., Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, [email protected], RIDGWAY, Kenneth D., Dept. Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, REIFENSTUHL, Rocky, Alaska Division of Geological & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709, DECKER, Paul, Alaska Div of Oil & Gas, 550 W 7th Avenue, Anchorage, AK 99501, BLODGETT, Robert B., US Geological Survey - Contractor, 4200 University Drive, Anchorage, AK 99508, and WHITE, James M., Geological Survey of Canada, 3303 - 33 Street North West, Room 2203, Calgary, AB T2L 2A7, Canada

The Miocene Bear Lake (BLF) and Pliocene Milky River Formations (MRF) provide a stratigraphic record of retroarc basin development along the Alaska Peninsula. These formations are considered potential reservoirs in the subsurface of the Bristol Bay basin.

New measured sections with additional palynological and megafossil data have been used to establish a stratigraphic framework for the BLF and overlying MRF in the Port Moller area. The BLF represents a transgressive marine succession that grades from nonmarine strata at its base to shallow shelfal strata near the top, with the majority of the measured thickness characterized by shallow estuarine strata. In much of this area, the BLF contains folds and low- and high-angle slip surfaces that show normal, reverse, and strike-slip motion, recording varying degrees of gravitationally driven deformation. The lower MRF consists locally of a ‘chaotic zone' characterized by immature volcaniclastic sandstone, conglomerate, and megabreccias containing large blocks of primary lavas. Megafossil data suggest that the ‘chaotic zone' formed over a relatively short time period and that it spans the transition from dominantly estuarine environments of the BLF to an open shelf environment of the MRF, as well as a change in provenance from primarily clastic sedimentary and plutonic sources to volcanic-rich detritus.

The BLF and MRF represented by our measured sections, approximately 1,300 m, exhibit 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 Neogene indicate a fall in eustatic sea level. The Middle to Late Miocene was marked by a hiatus in volcanic arc activity, which has been attributed to the subduction of remnants of the Kula-Pacific plate under the Alaska Peninsula. The relative sea level rise could be a result of subsidence of the southeastern margin of this retroarc basin, related to the subduction of these remnants by a northward prograding thrust belt, and/or by emplacement of intrusive rocks in the arc.