Abstract: Paleokarst and Sequence Boundaries in L.-M. Ordovician Dolomites South of the Las Vegas Valley Shear Zone, California and Nevada

COOPER, JOHN D., MARTIN KELLER and OLIVER LEHNERT

Lithoclast breccias within the dolomitic Lower to Middle Ordovician Pogonip Group in the Nopah Range, eastern California, previously interpreted as Ordovician debris flows or as post-Ordovician colluvial or tectonic deposits, are reinterpreted as Ordovician karst breccias. These include solution collapse breccias, cave roof crackle breccias, and doline and cave-fill breccias, commonly with reddish quartz sand and detrital dolomite matrix. These breccias also occur in association with solution karren, terra rossa sediment, and quark sand-filled grike systems. As such they define a succession of exposure surfaces that are disconformities and thus sequence boundaries. This is somewhat of a departure from conventional sequence stratigraphic studies in platform carbonates where parasequence stacking patterns are employed in the search for subtle sequence boundaries.

With the Nopah Range section serving as an anchor, these karst- and hardground-expressed sequence boundaries/disconformities provide a high-resolution chronostratigraphic framework that can be correlated to craton-margin sections to the east in the Spring Mountains of southern Nevada, and more distal platform sections to the west. Systems tract and other sequence architectural element

analysis supports integrity of the defined sequences. Preliminary new as well as reappraised conodont biostratigraphic analysis suggests encouraging verification for the coeval nature of several of the sequences between sections. Recognition and regional correlation of these surfaces have important implications for the paleogeography, paleotectonic setting, relative sea-level changes, and top of the Sauk Supersequence.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California