SEDIMENTARY PROVENANCE OF AN EVOLVING FOREARC REGION: LATE TRIASSIC HURWAL FM., EXCELSIOR GULCH UNIT, SOUTHERN WALLOWA MOUNTAINS, NORTHEASTERN OREGON

Deep-marine clastic rocks in the Late Triassic Hurwal Formation, Wallowa terrane, northeastern Oregon, contain an important record of an evolving forearc. We are investigating subsidence mechanisms using the framework composition of an intraformational boulder conglomerate, the Excelsior Gulch unit (~30–80m). We conducted clast counts (n=200) in the field at eight stations covering two locations and identify the following major clast types: (1) dark gray lime mudstone with trace skeletal material, (2) skeletal wackestone (gastropods, algae, sponges, bivalves), (3) dark gray siliciclastic mudrock/argillite, (4) light gray, structureless chert, (5) chert-grain lithic arenite, and (6) hornblende dacite. We identify subordinate clasts of red, white, and green “chert” that may be of sedimentary origin, or may represent siliceous volcanic clasts (i.e., felsite). Dark gray lime mudstone is the dominant clast type (~25–50%) at each location. Lesser clast types vary up-section and between locations. At location 1 (Red Gulch), near the base of the unit, red, green, black, and gray chert clasts are dominant. Near the middle, dark gray mudrock/argillite clasts dominate. Skeletal wackestone clasts are the dominant lithology near the top. At location 2 (Excelsior Gulch), green, light gray, black, and white chert clasts are abundant near the base of the unit. Near the middle, skeletal wackestone, white, black, gray, and green chert, and chert-grain, chert-grain lithic arenite clasts dominate, and chert and hornblende dacite clasts are abundant near the top of the section. Co-occurring sandstone is dominated by volcanic lathwork and felsite grains; however, samples include a significant component of non-volcanic lithic grains including marble, recrystallized lime mudstone, skeletal wackestone, quartzarenite, lithic wacke, siltstone, undulatory polycrystalline quartz, and quartz-mica tectonite(?). Numerous previous interpretations have suggested that Hurwal Formation deposition took place in an intraoceanic island arc, where post-magmatic thermal decay drove subsidence. We suggest that the variety and type of framework grains in the Excelsior Gulch unit indicates the presence of an extrabasinal source during Late Triassic time (ca. 215 Ma) and may signify an orogenic flexural-subsidence mechanism.