STRATIGRAPHIC EVIDENCE FOR LATE ORDOVICIAN (PRE-HIRNANTIAN) GLACIATION, CENTRAL NEVADA

Previous work suggests that there were two major pulses of glaciation in the Late Ordovician—the late Mohawkian (Chatfieldian) and the well-documented Hirnantian event. We examined a composite section of the Upper Ordovician Hanson Creek Formation in the Monitor Range in central Nevada to evaluate the paleoclimatic history between these two glacial intervals and identify and interpret the origins of 3^rd-order (1-5 My-scale) sequences.

Five 3^rd-order sequences (30-100 m thick) were identified between the Chatfieldian and Hirnantian events. The transgressive systems tract of each sequence is composed of basinal lime mudstone and shale and the highstand systems tracts, and sequence boundaries are composed of progressively shallower water wackestones (Sequences 1 and 2) to packestone-grainstones (Sequences 3, 4, and 5). No evidence of sequence boundary unconformities was observed. With cross-bedded skeletal grainstones and debris flow conglomerates, the top of Sequence 3 possesses a sedimentological imprint essentially identical to that observed at the Hirnantian glacial event, which overlies Sequence 5. Preliminary conclusions from these sequence stratigraphic trends suggest that extensive My-scale sea-level changes were occurring at least 8 My before the Hirnantian glaciation, which implies that Gondwanan glaciers were present and fluctuating in volume prior to the well known end-Ordovician glaciation. Samples obtained from these five 3^rd-order sequences are being processed to determine the O-isotope composition of apatitic conodonts to substantiate these preliminary glacial interpretations.