ALTERNATING PERIODS OF OROGENIC AND VOLCANIC SOURCES IN THE LATE TRIASSIC–EARLY JURASSIC ALDRICH MOUNTAINS GROUP, BLUE MOUNTAINS PROVINCE, EASTERN OREGON

The sedimentary provenance of late Paleozoic and Early Mesozoic basins in the Blue Mountains Province of eastern Oregon changed from (1) Permian and Late Triassic chert-rich, subduction-related sources (Baker terrane and Vester Fm.; megasequence 1), to (2) Early and Middle Jurassic volcanic and syn-orogenic sources along the western margin of Laurentia (Mowich Group; Izee basin megasequence 2). Intervening rocks of the Late Triassic–Early Jurassic Aldrich Mountains Group (AMG) represent a series of fault-bounded successor basins and provide a window into plate-margin tectonics during this time. Sedimentary framework grains in the AMG alternate between (1) quartzolithic-metasedimentary and (2) volcaniclastic. The basal unit of the AMG, the Fields Creek Fm. (Late Triassic) contains two petrofacies: volcaniclastic and quartzolithic. Volcaniclastic samples contain abundant felsite and feldspar grains whereas quartzolithic samples contain variably deformed volcanic and quartz-mica tectonite grains with lesser amounts of shale, argillite, and bioclastic limestone. The overlying Laycock Fm. (Late Triassic) is primarily volcaniclastic and contains feldspar, pyroxene, and polycrystalline volcanic lithic grains. The overlying Murderers Creek Fm. (Early Jurassic) is quartzolithic with siltstone, limestone, slate, and quartzite grains with secondary volcanic and meta-volcanic grains. The youngest rocks in the AMG, the Keller Creek Fm. (Early Jurassic), are entirely volcaniclastic.

These results indicate the end of chert-rich, subduction-related sources in the Blue Mountains during Late Triassic (Carnian) time. Instead, sediments of the AMG reflect alternating periods of orogenic versus volcanic sources: quartzolithic petrofacies in the Fields Creek and Murderers Creek fms. record the uplift and erosion of sedimentary and metamorphic sources during Late Triassic (Carnian) and Early Jurassic time (Hettangian). In contrast, the Laycock and Keller Creek Fms. record two major pulses of volcanic activity during Late Triassic (Norian) and Early Jurassic time (Sinemurian). The presence of a variety of metamorphic lithic grains suggests that the onset of collisional tectonics in the Blue Mountains began in Late Triassic time and that magmatism continued during syn-orogenic sedimentation.