Abstract: New Miocene Formation at Coos Bay, Oregon

John M. Armentrout

Reexamination of Neogene strata in the South Slough synclinorium, Coos Bay, Oregon, has resulted in the recognition of previously undescribed middle Miocene rocks.

Rocks of Miocene age were unknown in the Coos Bay area until 1949 when dredging of the bay channel recovered blocks of sandstone containing a Miocene molluscan fauna. Outcrops of these rocks were located in 1966 in the Empire quadrangle at the SW¼, Sec. 30, T25S, R13W, where 9 m of fine-grained lithic graywacke is exposed seasonally between the sand beach and bay mud.

The middle Miocene rocks of Coos Bay unconformably overlie the middle to upper Eocene Bastendorff and Tunnel Point Formations, and are overlain unconformably by the upper Miocene Empire Formation.

The molluscan fauna collected from these sandstones consists of 19 species. Paleoecologic analysis suggests that deposition occurred on a muddy sand bottom in less than 60 m of water under warm-temperature climatic conditions. The molluscan fauna is assigned to the middle Miocene Newportian Stage of Addicott's provincial molluscan biochronology. Correlative formations are the Astoria Formation of Oregon, Astoria(?) Formation of Washington, Temblor Formation of California, Narrow Cape Formation of Kodiak Island, and the upper Poul Creek Formation of the Yakataga District, Alaska.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California