Age and Petroleum Potential of Rocks Exposed on Hay Creek Anticline, Jefferson County, Oregon

Stephen I. Wareham, Lanny H. Fisk

For many years, the rocks exposed along the eroded axis of the Hay Creek anticline were reported to be metamorphic and Mesozoic, or even Paleozoic in age. Outcrops consist of black to dark-gray siltstones and sandstones with less abundant chert pebble conglomerate and recrystallized limestone. We refer to these strata informally as the "Hay Creek formation." Various thermal maturation indicators document that the sequence is only slightly overmature for petroleum generation. Calcareous nannofossils recovered from the limestone yield an age of early to middle Eocene. Thus, the Hay Creek outcrops are the first marine Tertiary rocks identified east of the Cascade Range in either Oregon or Washington.

The depositional environment of the Hay Creek formation is interpreted to be a submarine turbidite fan. Siltstones were deposited as pelagic rain and turbidite fallout. Sandstones and conglomerates were deposited by turbidite flows, as evidenced by flute casts and graded bedding. Limestone pods possibly represent shelf calcareous oozes transported to deeper water.

The petroleum source rock potential of the Hay Creek formation is good. High total organic carbon in the siltstones, type II and III kerogens, and the maturity indicators suggest that hydrocarbons have been produced. Oil and gas shows in nearby wildcat wells are further indications of the potential for production from these prevolcanic rocks.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.