Evolution of the Ordovician through Middle Pennsylvanian carbonate shelf in east-central California

Abstract

Carbonate shelf rocks marked by various sedimentary features suggesting intertidal to supratidal deposition or bearing rich, benthic faunas and their corresponding slope and basin rock sequences containing planktic fossils and sediment-gravity-flow deposits have been identified in the Inyo Mountains–Argus Range region of east-central California for all of the Paleozoic from Ordovician through Middle Pennsylvanian. Four major depositional sequences, each beginning with a marine transgression and all but the third ending with a progradation, were developed representing (1) Early Ordovician into Early Silurian time, (2) Early Silurian into earliest Middle Devonian time, (3) Middle to Late Devonian time and (4) Early to middle Late Mississippian time. A period of basinal infilling in the Late Mississippian was followed by a fifth major marine transgression in the Early Pennsylvanian. The carbonate shelf itself evolved from a homoclinal ramp in the Ordovician to an accretionary rimmed margin by the Early Devonian. This margin was drowned, probably in the Middle Devonian; by the Early Mississippian, a new homoclinal ramp had been developed. Before the end of the Mississippian, this ramp became distally steepened and then was transformed into an accretionary rimmed margin. The Early to Middle Pennsylvanian marine transgression gave rise to still another homoclinal carbonate ramp. Subsidence of ∼2,300 m occurred between Early Ordovician and Middle Pennsylvanian time in the area immediately east of the Inyo Mountains. It increased westward to probably >3,500 m at the edge of the Owens Valley as the continental margin subsided and bent downward ∼1° between Early Ordovician and middle Late Mississippian time. As a result of subsidence and relatively low rates of sedimentation, the water depth in the western part of the study area increased from close to sea level at the beginning of the Ordovician to as much as 3,000 m in the Late Mississippian. Because of higher rates of sediment accumulation and lower rates of subsidence in the eastern part of the region, shallow-water environments were maintained throughout most of this time interval; even so, the shelf-slope break retreated at least 45 km southeastward from Middle Ordovician to middle Late Mississippian time and at least 35 km farther southeastward by Middle Pennsylvanian time.