Geology and Revised Stratigraphic Interpretation of the Miocene Sucker Creek Formation, Malheur County, Oregon

Miocene volcanic and sedimentary rocks originally mapped as Sucker Creek Formation near Adrian, Oregon and Succor Creek State Park includes a stratigraphic section of at least 1,539 meters of westward-tilted and faulted deposits. Mapping indicates a stratigraphic section that can be divided into four mappable units in T. 23 S., R. 45 E., T. 23 S., R. 46 E., and T. 24 S., R. 46 E. The main sedimentary section overlies basalts and silicic volcanic rocks, at least 200 meters thick, exposed along Succor Creek. Starting with the basal unit, the section consists of: 198 meters of bentonitic claystones containing a white volcanic ash and a prominent orange sandy siltstone; 31 meters of alternating thinly-bedded diatomite (partly altered to porcellanite) and bentonitic claystones; 279 meters of olive-gray bentonitic claystones with several layers of white volcanic ash; and, in the upper part, bentonitic claystones are interbedded with 38 meters or more of conglomerates and gravels. Above this dominantly clay-stone basal unit is an 594 meter sequence of palagonite tuffs containing minor basalt flows, dikes and sills, overlain by a rhyolite dome complex that is locally 107 meters thick, then a, 19 meters thick, zone of thinly-bedded pumice lapilli and ash. The uppermost unit in the section is a 92 meter thick pale greenish rhyolite tuff. The basalt and palagonite unit is apparently localized near and north of Devils Gate, but the underlying lacustrine claystone section is more widespread in the region. The section is broken and partially repeated in several normal fault blocks. Because the stratigraphic section includes considerable amounts of claystone, this section contrasts markedly with the type section, described by Kittleman & others (1965) about 9 km south of this study area. That type section is 178 m of mostly volcanoclastic sandstone. The differences in stratigraphic sections indicate that, in this report and the section described by Kittleman & others, beds and no one type section can be considered typical. Similar rocks occur at depths of 2865 m to 3828 m beneath the Snake River Plain near Meridian, Idaho in the J.N. James well, in a section containing about 950 m of claystone. Preliminary synthesis of detailed sections of the formation mostly indicates that the Sucker Creek Formation indicates bimodal volcanic rocks, and sedimentary facies rocks. Volcanic ash layers in the lacustrine claystones are the best marker beds for regional correlation, and then recognition may provide a basis for understanding the basin evolution