REGIONAL SIGNIFICANCE OF THE TIPPECANOE-KASKASKIA SEQUENCE BOUNDARY IN THE SOUTHERN GREAT BASIN

A miogeoclinal to cratonal transect south of the Las Vegas Valley Shear reveals that the Tippecanoe-Kaskaskia Sequence Boundary (T-KSB) truncates major stratigraphy in the upper half of the Tippecanoe Sequence (TS). From the northern Nopah Range, CA, cratonward to the eastern Spring Mountains, NV, the T-KSB progressively eliminates the lower part of the Lippincott Sandstone member of the Devonian Lost Burro Formation, distinctive facies in the Silurian Hidden Valley Dolomite (HVD), distinctive bioclastic, dolograinstones in the Silurian Laketown Dolomite (LD), and Upper Ordovician, upper Ely Springs Dolomite.

The northern Nopah Range section includes a thick (200+ m) interval of the combined upper Ely Springs Dolomite, LD, HVD, and Lippincott Sandstone. Here the T-KSB lies within the Lippincott Sandstone, separating regressive interbedded quartz sandstones and dolostones from a sharply overlying 30 m-thick, cross-stratified, thick-bedded transgressive sandstone package. Cratonward, in the upper plate of the Wheeler Pass thrust, northwestern Spring Mountains, NV, the Lippincott Sandstone is 20 m thick and rests disconformably on upper HVD. Eastward, at McFarland Peak, the HVD is absent and the Lippincott Sandstone is a <1 m-thick transgressive quartz arenite that rests disconformably on LD skeletal rudstones containing Llandovery conodonts, and pentamerid brachiopods.

Further eastward, at Lucky Strike Canyon, the upper Ely Springs Dolomite, LD, HVD, and Lippincott Sandstone are missing. However, quartz sand is present in karst pipes that cut downward into the middle Ely Springs Dolomite, which lies below a 10 m-thick karsted interval. Late Ordovician (Aphelognathus shatzeri) conodont faunas occur below, and Middle Devonian elements of Polygnathus parawebbi and P. linguiformis sensu lato (no younger than Givetian) were recovered from just above the paleokarst. In the upper plate of the Keystone thrust, eastern Spring Mountains, the T-KSB and top Sauk Sequence boundary have reduced the TS to <30 m.

These stratigraphic relationships suggest that the T-KSB expresses a major readjustment in the tectonic subsidence history along this part of the early Paleozoic Cordilleran margin.