DEER TRAIL GROUP IN NORTHEASTERN WASHINGTON: AN INTERMEDIATE MESOPROTEROZOIC AGE UNIT BETWEEN THE WESTERNMOST MESOPROTEROZOIC BELT SUPERGROUP AND THE NEOPROTEROZOIC WINDERMERE GROUP

The westernmost Mesoproterozoic Belt Supergroup dips west beneath and is overthrust by the Proterozoic Deer Trail Group in northeastern Washington in the Mesozoic, E-vergent Colville River fold-thrust belt near Chewelah, WA. Here we characterize detrital zircon (DZ) age populations of several formations of the Deer Trail, its stratigraphically overlying Neoproterozoic Windermere Gp, and the structurally underlying Belt Supergroup. The lowest sampled Belt unit is Revett Fm, with a major DZ age population peak of about 1580 Ma, within the North American Magmatic Gap (NAMG), which has been interpreted to have a western source later rifted away in late Neoproterozoic time. The overlying Snowslip Fm (Missoula Gp) still shows a prominent NAMG peak as well as a youngest DZ age peak (MDA) about 1390 Ma, the youngest known W-sourced Belt unit. Missoula Gp farther east lacks the NAMG peak and was derived from Mazatzal-Yavapai domains in SW Laurentia (SWL). Just below the Cambrian unconformity in the footwall east of Chewelah, the Argillite of Half Moon Lk overlies the Snowslip and has a SWL source with no NAMG grains and an MDA of about 1375 Ma, the age of Belt deformation in the East Kootenay orogeny. The lowest sample from the structurally overlying Deer Trail Gp (Togo Fm) has a similar SWL source and MDA to that of the Argillite of Half Moon Lk., suggesting correlation of the two and an unconformable relationship to the Belt Supergroup. Higher in the Deer Trail Gp, the Wabash-Detroit Fm has a similar SWL source and an MDA of about 1300 Ma, clearly post-Belt in age. A flood of quartzose sand and conglomerate begins in the Buffalo Hump Fm and Huckleberry Cgl with SWL sources and Grenvillean MDAs of about 1150 Ma. The Buffalo Hump was previously considered to be part of the underlying Deer Trail Gp, but its contrasting young MDA suggests that it is part of the overlying Neoproterozoic Windermere Gp (Huckleberry Cgl). The thin basal Deer Trail Gp (0.7 km) overlying the Belt in the footwall contrasts with the thick Deer Trail Gp section (5 km) and overlying Windermere Gp (2 km) within the Colville River fold-thrust belt, both beneath Cambrian unconformities. Deer Trail-Windermere Gps were preserved by down-faulting to the west in late Neoproterozoic time, and the W-dipping normal fault was reactivated as the east-verging thrust decollement in Mesozoic time.