EVIDENCE FOR GROWTH OF SYN-DEPOSITIONAL STRUCTURES IN THE MIDDLE CRETACEOUS OCHOCO BASIN, CENTRAL OREGON

Cretaceous (Albian) marine sedimentary rocks of the Ochoco basin are exposed in the Mitchell Inlier of central Oregon. The Mitchell anticline is a doubly plunging, NE-trending fold that exposes Paleozoic metamorphic basement rocks in the core and over 1500 m of Cretaceous strata on its limbs. The oldest Cretaceous unit is the Basal Member of the Hudspeth Formation, which thickens southeastward from 80 to 280 m, and consists of: (1) green sandstone derived mainly from local basement rock; (2) distally derived conglomerate with clasts of chert, volcanic and plutonic rocks; and (3) turbiditic sandstone. The Basal Member is gradationally overlain by the Main Mudstone Member of the Hudspeth Formation, whose upper ~60 m consists of thin-bedded turbidites. The Gable Creek Formation has a sharp base and consists of ~1000 m of conglomerate and sandstone deposited by debris flows, grain flows, and turbidity currents in a submarine fan setting, with paleo-transport generally to the southwest.

The Main Mudstone Member displays a dramatic change in thickness from 200 m in the NW to 930 m in the SE, across the axis of the Mitchell anticline. Conglomerate and sandstone of the basal Gable Creek Formation rest on basement rock in the core of the anticline at the north end of Toney Butte, resulting in a sizeable unconformity. Available stratigraphic constraints show that these relationships are not the result of post-Gable Creek fault juxtaposition. The data are therefore interpreted as recording either growth of the Mitchell anticline or broad southeastward tilting during middle Cretaceous sedimentation. A syn-sedimentary normal fault that cuts the lower part of the Gable Creek Formation is recognized by thickness differences between footwall and hangingwall strata and a continuous conglomerate unit that caps the fault. Attempts to reconstruct a detailed sequence of depositional and deformational events are hampered by poor exposure in key locations as well as structural complexities resulting from a second, superposed NW-trending anticline.