ORIGIN AND EVOLUTION OF PALEOKARST FROM THE LOWER ORDOVICIAN POGONIP GROUP NEAR BEATTY, NV

The lower to mid-Ordovician Pogonip Group (Upper Ibexian-Lower Whiterockian) contains multiple generations of dolostone breccias that were interpreted by Cooper and Keller (1995; 2001) to have formed as the result of platform exposure and paleokarstification. Dolostrone breccias from a single paleokarst horizon within the Lower to Middle Ordovician Goodwin Formation (Pogonip Group) were examined from a single outcrop (located near Beatty, Nevada) in order to produce a detailed model for formation of the paleokarst horizon. Detailed outcrop and petrographic examination of the dolostone breccias and host rock suggests that the paleokarst was formed and affected by two distinct cycles of sea-level change. An initial relative transgression resulted in deposition of subtidal and intertidal facies that comprise the host rock. Exposure of the carbonate platform generated multiple phreatic caves below the water table, and numerous vadose conduits above the water table as meteoric waters percolated downwards. Subsidence of subsurface karst led to the formation of a paleodoline at the exposure surface. A second relative transgression deposited lagoonal sediments over the older karst, while subsequent re-exposure of the carbonate platform resulted in the development of small breccia pockets and grikes within the recently deposited lagoonal sediments. The distal location of the study area within the carbonate platform suggests karst formation was the result of a substantial drop in relative sea-level; the presence of multiple generations of paleokarst imply that at least 2 higher-frequency cycles of sea-level change overprint the larger regression.