ORDOVICIAN SEQUENCE BOUNDARIES IN EASTERN CALIFORNIA AND SOUTHERN NEVADA

A swarm of disconformities punctuates the Lower-Upper Ordovician succession in an outer miogeocline to craton-margin stratigraphic transect south of the Las Vegas Valley Shear. Most of these disconformities have karstic expression, dramatically displayed in our baseline inner miogeoclinal Nopah Range, CA section. Here at least nine disconformities have been identified in the stratal interval from the top of the Lower Ordovician Goodwin Formation (GF) through the Ninemile Formation, the Middle Ordovician Antelope Valley Formation (AVF), the Middle Ordovician Eureka Quartzite, to the top of the Upper Ordovician Ely Springs Dolomite (ESD). Four of these surfaces have been correlated to the outer platform section in the Talc City Hills, six have been correlated into mid-platform sections in the Cottonwood and Funeral Mountains, Death Valley region, CA, and all the surfaces have been correlated into the slightly more outboard section in the Montgomery Mountains, NV.

Inboard from the Nopah Range, correlations to sections in various thrust plates in the Spring Mountains, NV, show merging of some disconformities across the miogeoclinal cratonal hinge. For example, disconformities that bracket the Eureka Quartzite progressively merge cratonward into one master surface that represents the Sauk-Tippecanoe Supersequence boundary in the upper plate of the Keystone thrust, eastern Spring Mountains. Other recognizable disconformities include the top of the GF, the contact between the lower and upper AVF, a surface within the middle part of the upper AVF, and a surface in the lower part of the ESD.

These regionally expressed disconformities provide a preliminary sequence-stratigraphic framework across the Ordovician platform in this region. The Talc City Hills section, in particular, holds much promise for bridging correlations from inboard platform-interior to outboard platform-margin sections, especially upon completion of a high-resolution conodont biostratigraphic framework. Evidence for periodic platform-wide exposure and the merging of disconformities cratonward has strong implications for the Ordovician tectono-eustatic history of the early margin of SW Laurentia.