About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1702

Last Page: 1702

Title: Nonmarine Lithofacies Included In Scappoose Formation, Northwest Oregon: ABSTRACT

Author(s): Robert O. Van Atta, Kevin B. Kelty

Article Type: Meeting abstract

Abstract:

Discovery of commercial quantities of gas near Mist, in northwest Oregon, has stimulated exploration activity and renewed interest in Tertiary stratigraphy of the region. The youngest rocks include the Scappoose Formation and the overlying Grande Ronde Basalt (middle Miocene, Columbia River Basalt Group). As originally described, the Scappoose Formation was said to be of late Oligocene to early Miocene age, based on fossils of "Blakeley age." However, recent work shows that basaltic conglomerate lenses in fluvial arkosic sandstone with interbedded marine siltstone and sandstone in the Scappoose Formation are coeval with the Grande Ronde Basalt. Chemical analyses of basalt clasts in the conglomerates and of flows of overlying Grande Ronde Basalt have the same diagnostic ra ios of (Na2O + K2O)/P2O5, (CaO + MgO)/P2O5, and TiO2/P2O5 and very similar trace-element compositions. Older basalts in the region, middle Eocene Tillamook Volcanics and late Eocene Goble Volcanics, which are other possible sources of the conglomerate clasts, have very different major oxide and trace-element compositions.

Identical conglomerates containing Grande Ronde basalt clasts are found overlying older, pre-Scappoose formations as well as between flows in Grande Ronde Basalt sections. The conglomerates and associated cross-bedded arkosic sandstones with mudstone rip-ups were deposited in channels and valleys eroded into and through the Scappoose Formation.

The Scappoose Formation was apparently deposited as a wave- and tide-dominated delta. Marine regression or progradation of shoreline was followed by development of a fluvial valley system which was filled and partially buried by Grande Ronde Basalt.

End_of_Article - Last_Page 1702------------

Copyright 1997 American Association of Petroleum Geologists