AN OUT-OF-SYNCLINE THRUST AND OTHER GEOLOGIC DISCOVERIES ON THE LYLE QUADRANGLE, OREGON

A combination of surface mapping and extensive subsurface material analyses has helped define and explain structural features and stratigraphic relations within the Columbia River flood basalt (CRB) on the Lyle Quadrangle, Oregon. A thrust fault within the Mosier Syncline, was first identified during regional mapping by the U.S.G.S. in 1980. This fault, referred to here as the Rocky Prairie thrust, is an ENE-WSW oriented structure parallel to the Yakima Folds of the surrounding region including the Mosier Syncline (MS). It is an out-of-syncline thrust that is structurally smaller than the major anticlinal folds and associated thrusts to the north (Bingen Anticline) and south (Columbia Hills Anticline). But it is otherwise similar in geometry to these larger structures, with imbricated thrusts, cross-faults, and southward anticlinal fold asymmetry, indicating south vergence/transport. In thrust belts, structures of this type are commonly considered to have formed by fold accommodation (FA), and may be rootless. Significantly, the origin of these (FA) faults is strongly influenced by interbedded sediment which provides a competence contrast permitting layer-parallel flexural slip. The presence of such sediment has been clearly documented in a subsurface sampling data set for the Mosier area. These data show that interbedded Ellensburg-equivalent sediment are prevalent, with thicknesses of the Selah Member of up to 40 meters (m), Quincy-Squaw Creek Member up to 20 m, and the Vantage Member 6-12 m. Thick CRB flows are also common. The Pomona Member is up to 62 m thick, the Rosalia flow is up to 76 m, and the Ginkgo flow is 85-90 m. The prevalence of thick sequences of interbedded sediment and thick lava flows indicates an area of recurring paleo-drainage development within an actively subsiding syncline. A fine-grained micaceous sand facies, in the Quncy-Squaw Creek Member, occurs on the upper limb of the Columbia Hills Anticline (CHA), suggesting that much of the uplift of the CHA and subsidence of the adjacent MS, occurred in post-Quincy-Squaw Creek time. Syndepositional subsidence of the MS is consistent with the documented history of growth of anticlines, in this region, during CRB emplacement.