The Mitchell inlier in north-central Oregon contains the largest exposure of Cretaceous marine sedimentary rocks in this region. Nearly 9,000 ft of Albian-Cenamanian rocks are exposed along the flanks of the Mitchell anticline. The Cretaceous section rests unconformably overlain by Tertiary volcanic rocks. The Cretaceous rocks have previously been divided into the Hudspeth and Gable Creek Formations. The Hudspeth Formation consists of thick sequences of hemipelagic mudstone that contain subordinate siltstones and thin beds of turbiditic sandstone. The Gable Creek Formation is composed of numerous isolated sequences of coarse conglomerate, pebbly sandstone, and sandstone. This study concentrates on the stratigraphy and petrologic composition of the Gable Creek conglomerates. The Gable Creek conglomerates are composed of a heterogeneous assemblage of volcanic rocks, chert, plutonic rocks, and lesser amounts of sedimentary and metamorphic rocks. The petrologic composition of the conglomerates can be accurately reflected by means of pebble count analysis. Pebble counts provide a valuable tool for quantifying conglomerate composition and documenting compositional variations within the 70-square-mile Cretaceous outcrop area. Cluster analysis of the pebble count data confirms the presence of two major conglomerate petrofacies within the inlier. The striking compositional contrasts between the two petrofacies may be due to primary differences in sediment composition or secondary, post-depositional changes. Statistical correlation values reveal that conglomerate composition is fairly uniform within each of the two petrofacies. Petrologic composition also remains nearly constant within each of the major conglomerate units but exhibits random variations upward through the stratigraphic section. The Cretaceous rocks at Mitchell were deposited in a deep marine basin sometimes referred to as the Ochoco Basin (Odiorne, written comm., 1985). The Ochoco Basin may have extended into southwestern Oregon where Cretaceous rocks of the Hornbrook Formation are exposed. Several small Cretaceous inliers in central Oregon represent nonmarine margins. This study supports the interpretation of the Cretaceous rocks at Mitchell as submarine turbidites as suggested by Kleinhans (1984). The Gable Creek rocks were deposited in submarine channels by various sediment gravity flow processes in a base-of-slope or proximal fan setting. The Gable Creek units are arranged into thinning- and fining-upward sequences that may be the result of progressive channel abandonment. The correlative Hudspeth rocks are interpreted as submarine levee, overbank, and interchannel deposits associated with the
Gable Creek channels. The geometry of the Cretaceous rocks is defined by a series of stacked channel-levee-interchannel sequences. Paleocurrent data from the Cretaceous section yield a dominant southwesterly direction of sediment transport and a subordinate northwesterly trend. Variations in paleocurrent orientation may reflect several directions of sediment input, overbank deposition, and channel abandonment. The Gable Creek conglomerates have source areas located to the southeast, east, and northeast, of Mitchell within the Late Paleozoic and Early Mesozic accreted terranes in eastern Oregon. The provenance of the congolmerates is widespread and includes the island arc rocks of the Seven Devils Group, rocks of the dismembered oceanic crustal terrane, forearc strata of the John Day inlier, and the Jurassic plutons in northeastern Oregon. A previously undescribed tuffaceous unit within the Gable Creek Formation probably marks a short-lived episode of mid-Cretaceous volcanic activity in the source areas.