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1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-2051; efinzel{at}purdue.edu
2 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-2051
3 Alaska Division of Geological and Geophysical Surveys, 3354 College Road, Fairbanks, Alaska 99709
4 U.S. Geological Survey, Contractor, 4200 University Dr., Anchorage, Alaska 99508
5 Geological Survey of Canada, 3303-33 Street NW, Calgary, Alberta T2L 2A7, Canada
6 Alaska Division of Oil and Gas, 550 W. 7th Avenue, Suite 800, Anchorage, Alaska 99501-3560
Emily Finzel is a Ph.D. student at Purdue University. She holds an M.S. degree in sedimentology and structural geology from the University of Alaska-Fairbanks. From 2003 to 2006, she worked as a field geologist for the Alaska Division of Geological & Geophysical Surveys, focusing on the North Slope and Bristol Bay Basin. Her current research addresses the geodynamic development of sedimentary basins along convergent plate margins.
Ken Ridgway is a professor at Purdue University. His research group addresses a broad spectrum of questions concerning crustal tectonics. Much of their research has focused on the sedimentary record in basins of southern Alaska to understand how this continental margin developed through time and how it is currently deforming. For more information on the basin analysis group, see www.eas.purdue.edu/basin.
Rocky Reifenstuhl began his Alaska work in 1977, which included 18 months with Marline Oil Corporation. In 1981, he began working as a field geologist for the Alaska Division of Geological & Geophysical Surveys, and in 1983, he received a B.S. degree in geology from University of Alaska Fairbanks. He has published some 80 geologic maps and reports from every region of Alaska.
Robert Blodgett is a stratigrapher and paleontologist with more than 30 years of experience on Alaskan rocks. He received his Ph.D. from Oregon State University. His research interests cover Phanerozoic biostratigraphy of western North America, and he recently coedited Geological Society of America Special Paper No. 442 "The Terrane Puzzle: New Perspectives on Paleontology and Stratigraphy from the North American Cordillera."
James White is a Geological Survey of Canada palynologist, specializing in Cenozoic and Early Cretaceous palynostratigraphy in western and northern Canada and the adjacent United States. His recent work is on modeling biostratigraphy from a literature database and on the palynostratigraphy of the Mallik gas hydrate research borehole, Mackenzie delta.
Paul Decker is a geologist with the Alaska Division of Oil and Gas engaged in petroleum systems research integrating subsurface and outcrop data. From 1988 to 2004, he worked in Alaskan exploration and development for ARCO, Phillips, and ConocoPhillips. Paul holds a Ph.D. and an M.S. degree in structural geology from the University of Wisconsin, Madison, and a B.S. degree in geology from Fort Lewis College.
ABSTRACT
The Miocene Bear Lake Formation is exposed along the coast and mountains of the central Alaska Peninsula and extends offshore as part of the Bristol Bay Basin. The Bear Lake Formation is up to 2360 m (7743 ft) thick in an offshore well and is considered to have the highest reservoir potential in this gas-rich frontier basin. Our new macrofossil and palynological data, collected in the context of measured stratigraphic sections, allow us to construct the first chronostratigraphic framework for this formation. Biostratigraphic age assignments for the numerous, commonly isolated, onshore exposures of the Bear Lake Formation show that deposition initiated sometime before the middle Miocene (15 Ma) and extended to possibly the earliest Pliocene. The bulk of the Bear Lake Formation, however, was deposited during the middle and late Miocene based on our new findings. We interpret the Bear Lake Formation as the product of a regional transgressive estuarine depositional system based on lithofacies analysis. The lower part of the formation is characterized by trough cross-stratified sandstone interbedded with coal and pedogenic mudstone deposited in fluvial and swamp environments of the uppermost parts of the estuarine system. The lower-middle part of the formation is dominated by nonbioturbated, wavy- and flaser-bedded sandstone and siltstone that were deposited in supratidal flat environments. The upper-middle part of the Bear Lake Formation is characterized by inclined heterolithic strata and coquinoid mussel beds that represent tidal channel environments in the middle and lower tracts of the estuarine system. The uppermost part of the formation consists of tabular, bioturbated sandstone with diverse marine invertebrate macrofossil faunas. We interpret this part of the section as representing the subtidal tract of the lower estuarine system and possibly the adjacent shallow inner shelf. A comparison of our depositional framework for the Bear Lake Formation with core and well-log data from onshore and offshore wells indicates that similar Miocene depositional systems existed throughout much of the Bristol Bay Basin. The documented changes in depositional environments within the Bear Lake Formation are also important for understanding upsection changes in the geometries of potential reservoirs.
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