DEVONIAN PHACOPID TRILOBITES AND LINGULOID BRACHIOPODS AT THE BASE OF THE KASKASKIA SEQUENCE IN THE MIOGEOCLINAL LIPPINCOTT SANDSTONE, BASAL LOST BURRO FORMATION, NORTHERN NOPAH RANGE, EASTERN CALIFORNIA, USA

In eastern California and southern Nevada, a cratonal to miogeoclinal transect of Paleozoic carbonates is punctuated with significant siliciclastic stratigraphic marker beds that bracket sequence boundaries. In the northern Nopah Range (NNR), the Tippecanoe-Kaskaskia Sequence Boundary (T-KSB) occurs in the Lippincott Sandstone, the lowermost member of the Devonian Lost Burro Formation. Here the T-KSB sharply separates 3 meters of regressive interbedded quartz sandstones and dolostones from 29 meters of overlying cross-bedded, massive quartz arenite. The cessation of Lippincott siliciclastic deposition is no younger than Givetian, based on conodont elements of Polygnathus parawebbi and P. linguiformis sensu lato recovered just above the significant T-KSB paleokarst in Lucky Strike Canyon to the east in the Spring Mountains. The only previous report of Lippincott fossils include a pteraspidid (heterostracan) fish fauna from Death Valley, which tentatively correlates with Sevy Dolmite agnathans of eastern Nevada and the purported Emsian age of the Beartooth Butte Formation in Montana (Ilyes & Elliot, 1991).

Fossils discovered in talus stratigraphically channeled between very steeply dipping beds of the NNR Lippincott include molds and carapace fragments of phacopid trilobites, and disarticulated valves of linguloid brachiopods. Phacopid specimens are represented by disarticulated pygidia and thoracic segments—no cephalic segments have been identified. Both linguloids and phacopids are normally restricted to marine paleoenvironments and provide evidence for marine deposition of Lippincott siliciclastics. Depositional interpretation of widespread sand sheets associated with sequence boundaries has included suggestions of nonmarine deposition, ranging from aeolian dunes to “fluvial” braidplains. Devonian trilobites are virtually unknown from miogeoclinal sediments of the conterminous western US—a few rare phacopids occur near the Devonian shelf-slope break in central Nevada. The phacopid fauna is of particular paleobiogeographic importance in establishing a post-Ordovician trilobite record for the western US miogeoclinal platform.