STRUCTURAL CONTROLS ON MID-CRETACEOUS (ALBIAN) SEDIMENTATION IN THE TONEY BUTTE AREA, MITCHELL INLIER, CENTRAL OREGON

Middle Cretaceous marine sedimentary rocks are exposed in the Mitchell inlier of central Oregon where they rest unconformably on metamorphic rocks of the Baker Terrane. Strata in the Toney Butte area include: (1) Hudspeth Formation (mostly mudstone) that accumulated in a deep-marine basin plain; and (2) Gable Creek Formation (conglomerate and sandstone), which accumulated by coarse-grained sediment gravity flows in a large submarine fan system. The Hudspeth Formation consists of the Basal Member (sandstone and conglomerate) which is directly overlain by the Albian-age Main Mudstone Member, and younger intervals of marine mudstone that alternate with thick units of Gable Creek Formation. The Toney Butte area reveals a complex map pattern produced by two phases of folding. The younger, NE-trending Mitchell anticline is Tertiary in age. It overprints an older, NW-trending basement-cored anticline that shows systematic large-scale variations in stratigraphy across its axis: (1) the entire thickness (~1,500 m) of the Main Mudstone Member pinches out against the SW limb of the older anticline; (2) the Gable Creek Formation and younger Hudspeth units are laterally continuous across the old structure; and (3) these younger units have been tilted by post-Albian growth of the NW-trending anticline and by the Tertiary Mitchell anticline. Smaller-scale folds on the SW limb of the restored older anticline are either parasitic tectonic folds or slump folds that formed on the tilting SW limb of the anticline. Accounting for ~35 degrees of post-depositional clockwise rotation, the above data are interpreted to record ~N-S crustal shortening that produced growth of a basement-cored anticline and related syn-basinal tilting during deposition of the Main Mudstone Member. The overlying Gable Creek Formation post-dates the earliest phase of fold growth and reflects voluminous input of sediment from a partially dissected volcanic-arc source (probably Wallowa Terrane) in the north. Middle Cretaceous N-S shortening in central Oregon is tentatively attributed to regional contraction in a large W or NW bend in the Sr-isotope .706 line (continental-margin suture) in SE Washington, and may be related to Cretaceous dextral transpression and translation along the western Cordilleran margin. This inference is speculative and requires further testing.