Interpretation of the Lower Mississippian Joana Limestone and implications for the Antler orogenic system.

Abstract

Late Devonian through Early Mississippian thrust loading by the Roberts Mountains allochthon (Antler orogeny) in eastern Nevada flexurally warped the lithosphere into a foreland basin, forebulge, and back-bulge basin. Depositional facies patterns in the Antler foreland were controlled predominantly by migration of the flexural features and modified by the effects of eustasy. Flexural downwarping controlled the geometry and location of depositional basins. In relatively shallow water, eustasy controlled the overall nature of local depositional systems. Carbonate depositional systems were generally associated with major sea level highstands (Joana Limestone) whereas siliciclastic depositional systems were associated with lowstands (Pilot Shale). Lithologic, stratigraphic and paleontologic studies of the Joana Limestone allowed delineation of two distinct facies assemblages, the lower Joana Limestone and the upper Joana Limestone. The lower Joana Limestone represents a regionally extensive, transgressive-regressive, middle shelf sequence. The overlying upper Joana Limestone represents a rapid facies shift to shelf margin deposition on the east and to slope and basin margin deposition in the western portion of the study area. The depositional facies of the Joana Limestone document a progressive eastward shift of the shelf margin during Early Mississippian (late Kinderhook to early Osage) time as part of a larger-scale retrogradational sequence that developed since Late Devonian time. Retreat of the shelf margin was associated with eastward migration of the flexural forebulge. Migration of flexural features across the Antler foreland was recorded by strata deposited during the transition from a passive to a collisional margin regime. Relatively deep-water siliciclastic strata of the Pilot Shale were deposited in the downwarped back-bulge basin. The deepening back-bulge basin substrate was subsequently uplifted differentially and partially eroded as the forebulge migrated through the region during Early Mississippian time. Eustatic sea level rise during this time allowed carbonate deposition over the uplifted forebulge area (lower Joana Limestone). Continued eastward migration of the forebulge resulted in downwarping of the western portion of the study area as it was incorporated into the foreland basin and upwarping over the forebulge in the easternmost portion of the study area. Forebulge upwarping resulted in shoaling of the substrate and corresponded to the position of the shelf margin (eastern facies belt of upper Joana Limestone). Downwarping resulted in deepening of the substrate and corresponded to westward deepening slope and basinal sedimentation (central, western, southern, and far western facies belts of the upper Joana Limestone) followed by turbidite fan deposition (Chainman Shale) derived from the Roberts Mountains allochthon.