Age of "Goose Rock Conglomerate," Wheeler and Grant Counties, North-Central Oregon

Martin R. Aguirre, Lanny H. Fisk

A reconstruction of the geologic history and synthesis of the tectonic history of the Blue Mountains and Columbia Basin require that we know the age of key outcrops. The geological record in this entire area has been largely hidden by a thick blanket of Tertiary volcanic rocks. Only a few erosional inliers are available from which to interpret the pre-volcanic tectonic and sedimentation history.

The "Goose Rock Inlier," exposed along the John Day River in the Butler basin and Turtle Cove, is one such important outcrop. These exposures, consisting of a few hundred feet of fluvial sandstones and conglomerates, informally called the "Goose Rock Conglomerate," have received only limited attention from previous workers. Estimates of this unit's age range from Early Cretaceous to early Tertiary, on the basis of lithologic similarities with other named and unnamed formations in the area. The terrestrial nature of these strata and the apparent lack of fossils cause considerable difficulty in correlation with other Cretaceous sequences, which are largely marine. In the absence of scarcity of other fossils, palynology offers a unique alternative for making biochronological and paleoenv ronmental interpretations.

Samples from a lens of stratified carbonaceous mudstone interbedded with the conglomerate exposed along Deer Creek yielded a datable palynomorph assemblage. The absence of tricolpate pollen and the presence of distinctive trilete spores suggest a pre-middle Albian and probable Aptian (Early Cretaceous) age for the palynoflora. The overall composition of the palynomorph assemblage is distinct from and older than that of the primarily middle Albian "Mitchell Formation" (previously Hudspeth and Gable Creek formations) exposed in the Mitchell inlier, to the west, although further study may show that the "Goose Rock Conglomerate" is equivalent to the "Basal Member" of the Mitchell sequence.

The revised age allows the tectonic history of the Blue Mountains to be more clearly resolved. The "Goose Rock Conglomerate" rests unconformably on Permian-Triassic rocks of the Blue Mountains tectonic melange and, thus, probably represents the first deposition after the accretion of this allochthonous terrane.

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