THE TECTONIC IMPLICATIONS OF THE STRATIGRAPHY OF THE BLACK MARMOT VALLEY, KHARKHIRAA UUL, MONGOLIA

The stratigraphy in the vicinity of the Black Marmot Coal Mine makes up part of a poorly studied section of Devonian and Carboniferous rocks overlying the Cambrian to Ordovician Lakes Island Arc Terrane in Northwestern Mongolia and may be significant in understanding regional Mid-Paleozoic Tectonic History. In the summer of 2004 an 8 km2 area in the vicinity of the mine was mapped and sampled. The stratigraphy of the study area is as follows: 1.) turbidites with individual beds 0.1-1m thick; 2.) organic-rich, thinly bedded shale; 3.) interbedded dolostone, mudstone, and limestone; 4.) litharenite to sublitharenite sandstone and conglomerate; 5.) quartz-rich conglomerate with interbeds of coal and tuff. Basalt flows, ranging from 15 to 50 meters in thickness, occur within units 3 and 4. Hyaloclastites and pillows in the basalt indicate shallow subaqueous eruptions. This stratigraphy represents a transition from a deep marine fan environment through lower energy shallower marine environments to a high energy fan delta or alluvial fan environment, a series of progressively shallower settings that indicate a local marine regression. The presence of carbonate rocks marks a period of relatively little clastic sedimentation, although thin section petrography reveals that the limestone contains significant amounts of clastic material. Significant amounts of preserved organic matter are preserved in units 1 and 2. Whole rock basalt XRF and ICP-MS geochemistry indicates a relatively primitive magma source particularly low in potassium typical of extensional settings. Petrographic analyses of sandstone from units 1 and 4 indicate a provenance of mixed source rocks, with a significant, but not dominant, component of volcaniclastic sediment. Both the geochemical and petrologic data suggest that the units described were deposited in an extensional basin, perhaps created by back-arc rifting related to a nearby convergent margin.