--> Stratigraphic and Structural Interpretation of Willapa Basin, by Stephen P. Palmer; #91024 (1989)

Datapages, Inc.Print this page

Stratigraphic and Structural Interpretation of Willapa Basin

Stephen P. Palmer

Willapa basin is a middle Eocene to Holocene depocenter in southwestern Washington and on the adjacent continental shelf. Surface geology, biostratigraphy, and seismic and well data have been used to develop a stratigraphic and structural interpretation of the basin. Middle Eocene deep-water sediments were deposited on convergent margin and accreted to North America prior to the early Miocene. Turbidite sandstones interpreted to be the lower to middle Miocene Astoria(?) Formation overlie this Eocene sequence in onshore wells. Distal turbidites of lower to middle Miocene age (Hoh assemblage) were thrust beneath the Eocene and Astoria(?) Formation rocks during the middle to upper Miocene. Upper Miocene bathyal shales and siltstones unconformably overlie the older rocks. Thi sequence was then unconformably(?) covered by upper Miocene sandstones and siltstones deposited in a nearshore shelf environment. This extreme change in water depth is interpreted to be of tectonic origin. The occurrence of redeposited lower to middle Miocene deep-water fauna in the late Miocene shelf sequence is ascribed to erosion at the sea floor of mud diapirs rooted in Hoh assemblage rocks. Activation of these diapirs was probably associated with this tectonic uplift. Pliocene and Pleistocene conglomerates, sandstones, and siltstones were deposited in both marine and deltaic environments and are locally exhibited as an onlap sequence in marine seismic data. Diapiric growth has continued into the Holocene, resulting in both angular truncation of the Pliocene-Pleistocene sequence and diapiric breaching at the sea floor.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.