ANATOMY OF THE SAUK II-III SEQUENCE BOUNDARY: UNCONFORMITY IN THE CORSET SPRING SHALE AND STEAMBOAT PASS SHALE MEMBERS AND CORRELATIVE CONFORMITY IN THE UPPER DUNDERBURG SHALE, GREAT BASIN, USA

Sequences are defined as packages of rock bound by unconformities or their correlative conformities. The Sauk II-III sequence boundary has been defined for more than four decades but few investigations have focused on the details of this (or these) surfaces. Previous investigators recognized unconformable surfaces in Upper Cambrian Dunderburgia and lower Elvinia trilobite zones. The Dunderburgia zone unconformity is defined from areas of Wyoming and Montana. The lower Elvinia zone unconformity is present in the eastern Great Basin of west-central Utah. The latter, the focus of this study, is characterized as a disturbed carbonate interval in the Corset Spring Shale of the central House Range and the upper Steamboat Pass Shale in the southern House Range. The correlative conformable interval is present in Elvinia zone in the Dunderburg Shale in the Duck Creek Range of eastern Nevada. Both Dunderburgia and lower Elvinia intervals likely were unconformable but due to rapid subsidence along the shelf margin of western Laurentia, only the lower Elvinia interval was preserved in western Utah.

The lower Elvinia zone unconformity can be traced throughout the central and southern House Range, indicating missing time in the rock record. It is characterized by paleokarst pockets, internal sediment fills, fractures, small faults, and erosional surfaces. This disrupted interval features upright, dipping and overturned second-generation flat-pebble conglomerate clasts up to 45 cm in length, disturbed thrombolite beds and a rare local hardground on a thrombolite adjacent to the interval. Collections of trilobite and brachiopod faunas of other investigators were used to determine biostratigraphic ages. Collected gamma-ray profiles also permit detailed correlation of successions from onshore (Utah) to offshore, deep water, sections (Nevada). The lower Elvinia unconformity is significant because it is near the peak of the SPICE event and also the base of the Jiangshanian Stage, recognized by the FAD of Irvingella angustilimbata, a proxy species for Agnostotes orientalis, which defines the base of the Jiangshanian. Thus, a physical stratigraphic surface may be useful for recognizing this interval globally.