Goodnews Terrane and Kuskokwim Group, Eek Mountains, Southwest Alaska: Open Marine to Trench-Slope Transition: ABSTRACT

Abstract

In the Eek Mountains, southwest Alaska, four depositional settings record Permian to mid-Cretaceous marine sedimentation and post-Valanginian crustal shortening. Sequence 1: pillowed basalt flows (SiO2 = 44%) and volcaniclastic sediments intercalated with Permian Atomodesma-bearing sandy limestones grade upward through volcanic debris flow deposits and tuffaceous argillites into sequence 2: thin-bedded chert with argillite partings. Sequence 3 rests conformably(?) on sequence 2 and consists of undated argillite grading upward into Early Cretaceous (Valanginian) thin-bedded Tcde/Tde turbidites and thick-bedded conglomeratic grain flow deposits. Sequence 4 consists of late Early Cretaceous (Albian) thick-bedded conglomeratic grain flow deposits and minor Tcde/Tde and Ta/Tab turbidites. Sequence 4 (Kuskokwim Group) overlies sequences 1, 2, and 3 (Goodnews terrane) with an angular unconformity preserving submarine canyon cut-and-fill. Sequence 1 requires an ocean island(?), and sequence 2 an open marine origin. Sequences 3 and 4 are inner-fan turbidite deposits; conglomerate and sandstone compositions indicate recycled orogen, arc orogen, and collision orogen provenances. This suggests preaccretion Valanginian clastic input from the now-adjacent Kilbuck terrane. All four sequences record southeast over northwest imbrication along southeast-dipping thrusts. Deformation and metamorphic intensity increases with age/depth; a maximum is recorded in pervasively foliated Permian greenstone crosscut by prehnite and pumpellyite veins. Sequences 1 and 2 are highly disrupted, with local blocks-in-matrix fabric. Sequence 3 shows a similar, less pervasive style. Sequence 4 is locally disrupted and overturned along faults but lacks penetrative deformation. Deformation began between Valanginian and Albian times, and either progressively penetrated upsection with time or was multiphased with consistent recurring structural style. End_of_Article - Last_Page 672------------