LOWER–MIDDLE ORDOVICIAN PALEOKARST ARCHITECTURE: A NEW OUTCROP ANALOG FROM THE NOPAH RANGE, CALIFORNIA, USA

The Nopah Range contains one of the most laterally and vertically extensive sections of exhumed Ordovician paleokarst in North America. Barren exposures display a wide range of vadose karst features and fill processes providing a new and exceptional opportunity to document a super-sequence scale, ancient karst system. Karst breccias in the Nopahs were commonly misinterpreted as fault breccias and Quaternary talus until the late 1990s and, to date, little has been published on the paleokarst architecture. The Ordovician Pogonip Group consists of shallow-marine carbonate-platform strata that contain multiple, paleokarst intervals. Discrete karst events range in scale from incised breccia-capped cycles to sequence-scale collapsed paleocaverns. Karst events show evidence of increasing magnitude and likely record terminal Sauk–Tippecanoe sea-level lowstand superimposed over higher frequency, sea-level oscillations. This ongoing field-oriented project will provide the Kansas Geological Survey with a 7-km outcrop window to constrain geocellular modeling of the karst-modified Arbuckle (Cambro-Ordovician) saline aquifer system as part of a DOE-sponsored assessment of CO₂ sequestration potential.

Presented here are the initial mapping results from a 0.5-km outcrop window along Cooper Canyon that illustrate vadose karst architecture and clastic infill. Steep-sided sinkholes are recorded in the overlying Eureka Quartzite, which funnel downward into a narrow vertical shaft (< 2 m) before opening into a collapsed paleocavern (~ 60 m thick) situated approximately 100 m below the Pogonip–Eureka contact. Above the polymict-breccia-filled paleocavern, crackle breccia developed within mechanically compromised roof strata. Roof sagging and wholesale collapse is evident along the Pogonip–Eureka contact. Finely laminated cave sediments also record compaction-induced fracturing. Cemented fractures (~ 1 cm wide) adjacent to and on-trend with vertical, polymict-breccia-filled shafts may indicate incipient wall failure along preexisting joints. These joints often tip-out into steeply dipping (~30º) breccia-filled passageways. It's rich variety of karst features—and their attendant fill record—should make the Nopahs a classic field area and an important Ordovician paleokarst analog.