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Depositional relations within strata of Cretaceous age in the lower Cook Inlet area have been clarified by recent drilling and high quality seismic data. Lower Cretaceous (Neocomian) strata are preserved along the Cook Inlet basin axis and are correlated with the Herendeen Formation of Port Moeller and the Nelchina beds in the Matanuska Valley. The Kaguyak Formation of Cape Douglas is expanded to include strata which range in age from Albian through Maestrichtian. Three members are proposed, which are (in ascending order) the Unnamed Albian-Cenomanian, the Middle Member, and the Saddle Mountain Member. Depositional relations within the Kaguyak Formation were strongly controlled by tectonic events and resultant basin configuration.
The Unnamed Albian-Cenomanian Member (up to 1,400 ft [425 m] thick) is composed of carbonaceous sandstone and shale. It is correlated with similar-age deposits in the Matanuska Valley and the Mt. Katmai area, and represents shallow to marginal marine deposition. The Middle Member is up to 4,400 ft (1,350 m) thick and consists of a southeastward-thickening wedge which is predominantly marine siltstone and claystone. Sandstone is significant only at Kaguyak Bay where proprietary studies indicate submarine fan deposition, and in the Socal Anchor Point well where a similar environment is likely. Middle Member strata lap-up on and over-top a topographic shelf, which began to develop at about Turonian time and was a prominent feature in the area east of the Iniskin Peninsula by Campanian ti e.
Significant shallowing at the beginning of Maestrichtian time was accompanied by an outpouring of coarse clastics of the Saddle Mountain Member. Up to 2,000 ft (600 m) of mostly nonmarine sandstone, siltstone, and conglomerate extends seaward (southeastward) to the shelf edge. We suggest that this shelf edge was the paleolimit of Saddle Mountain Member deposition and that marine reworking and washing fluvially supplied sand along this shelf edge provided the only Cretaceous sandstone with reservoir potential in this area.
Overlying and possibly continuous with the Saddle Mountain Member is a Paleocene sequence of sandstone and conglomerate, about 600 ft (185 m) thick in the COST well. This interval is seismically and lithologically distinct from the underlying Cretaceous and overlying West Foreland Formation rocks and is named the Silver Salmon Formation. This formation showed the best reservoir properties of the Tertiary rocks in lower Cook Inlet.
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