Cretaceous sedimentary rocks in the Mitchell inlier of central Oregon accumulated in a forearc basin within a poorly understood part of the North American Cordillera. Sedimentary units are broadly to tightly folded and include the Albian Hudspeth Formation (marine mudstone and turbidites) and the Cenomanian Gable Creek Formation (submarine-fan conglomerate and sandstone). Previous and new mapping in the Toney Butte area reveals interference of the NE-trending Mitchell anticline and the NW-trending, basement-cored Toney Butte anticline. The entire ∼1.2–1.4 km thick Main Mudstone member of the Hudspeth Formation pinches out across the Toney Butte anticline, and the overlying Gable Creek Formation is laterally continuous and rests depositionally on basement rock in the core of the fold. Two leading hypotheses are considered to explain these relationships: (1) Pinch-out of the Main Mudstone member is due to preexisting erosional paleotopography, and all folds are Tertiary in age; or (2) the Toney Butte anticline initially grew during deposition of the Main Mudstone member, was overlapped by the Gable Creek Formation, and was further folded and rotated after deposition of Cretaceous strata. Based on critical map and stratigraphic relationships, we favor the second hypothesis. Accounting for ∼37° of postbasinal clockwise rotation, the data are interpreted to record growth of a basement-cored anticline due to north-south crustal shortening during Albian sedimentation in a large forearc basin. We infer that this folding episode resulted from regional transpression associated with northward translation of accreted terranes in the Blue Mountains along the western U.S. Cordilleran margin.

Abstract The Allamoore talc district comprises about 100 square miles of west-central Culberson and east-central Hudspeth Counties in Trans-Pecos Texas. Talc-bearing rocks occur within the late Proterozoic Allamoore Formation and crop out in a discontinuous belt about 20 miles long from Tumbledown Mountain, about 8 miles north-northwest of Van Horn, to the Streeruwitz Hills, about 5 miles northwest of Eagle Flat (Fig. 1). Talc production began in 1952. Annual district production for the past decade has ranged from about 200,000 tons to about 400,000 tons of talc (Fig. 2) with the latest available data for 1988 showing 261,000 tons of talc with a value of almost $4.5 million (Virta, 1988). The Allamoore district generally ranks third among domestic talc producers, accounting for about 20% of total production. There are four active talc producers in the Allamoore district: Milwhite, Pioneer, Texas Talc (Dal Minerals), and Southern Clay. The Milwhite property is located at Tumbledown Mountain, whereas the other open pit operations are located in the Allamoore area, eight miles west of Van Hom. The Milwhite beneficiation plant is six miles east of Van Horn, and the Pioneer plant is located at Allamoore.

ABSTRACT Cretaceous forearc strata of the Ochoco basin in central Oregon may preserve a record of regional transpression, magmatism, and mountain building within the Late Cretaceous Cordillera. Given the volume of material that must have been eroded from the Sierra Nevada and Idaho batholith to result in modern exposures of mid-and deep-crustal rocks, Cretaceous forearc basins have the potential to preserve a record of arc magmatism no longer preserved within the arc, if forearc sediment can be confidently linked to sources. Paleogeographic models for mid-Cretaceous time indicate that the Blue Mountains and the Ochoco sedimentary overlap succession experienced postdepositional, coast-parallel, dextral translation of less than 400 km or as much as 1700 km. Our detailed provenance study of the Ochoco basin and comparison of Ochoco basin provenance with that of the Hornbrook Formation, Great Valley Group, and Methow basin test paleogeographic models and the potential extent of Cretaceous forearc deposition. Deposition of Ochoco strata was largely Late Cretaceous, from Albian through at least Santonian time (ca. 113–86 Ma and younger), rather than Albian–Cenomanian (ca. 113–94 Ma). Provenance characteristics of the Ochoco basin are consistent with northern U.S. Cordilleran sources, and Ochoco strata may represent the destination of much of the mid- to Late Cretaceous Idaho arc that was intruded and eroded during and following rapid transpression along the western Idaho shear zone. Our provenance results suggest that the Hornbrook Formation and Ochoco basin formed two sides of the same depositional system, which may have been linked to the Great Valley Group to the south by Coniacian time, but was not connected to the Methow basin. These results limit northward displacement of the Ochoco basin to less than 400 km relative to the North American craton, and suggest that the anomalously shallow paleomagnetic inclinations may result from significant inclination error, rather than deposition at low latitudes. Our results demonstrate that detailed provenance analysis of forearc strata complements the incomplete record of arc magmatism and tectonics preserved in bedrock exposures, and permits improved understanding of Late Cretaceous Cordilleran paleogeography.