Depositional environments along a carbonate shelf to basin transect in the Silurian of Nevada, U.S.A.

Abstract

During the Llandovery (Silurian) in central Nevada foundering of the shelf margin and drowning of a homoclinal carbonate ramp produced a rimmed carbonate shelf. Westward the shelf was bordered by a slope which passed into a deep-water basin. At the shelf margin, carbonate sand shoals (Lone Mountain Dolomite) aggraded and prograded westwards over the slope. The spatial and temporal facies variation in a transect across the slope (Roberts Mountains Formation) records the history of a prograding shelf margin.Initial slope sedimentation adjacent to the carbonate shelf margin was lime mudstone hemipelagically derived from the shelf. Westwards and downslope dilute carbonate turbidites and hemipelagic lime mudstone (laminated facies) were deposited. Contemporaneous slope-rise sediments (Fourmile Canyon Formation) consist primarily of siliciclastic mudstone with minor dilute carbonate turbidites and turbiditic siliciclastic siltstone, the latter essentially being part of the deep-water basin. Subsequent shelf margin progradation and increasing slope differentiation is reflected in the downslope introduction of calcarenite and conglomerate derived from the shelf margin. Slope sedimentation records an overall coarsening- and thickening-upwards cycle. Superimposed on this, in some areas, are small-scale coarsening- and thickening-upwards cycles reflecting deposition on carbonate debris aprons. Abandonment and subsequent build-out of the aprons reflects the pulsatory and uneven progradation of the outer shelf margin. Simultaneous with these depositional patterns, the immediate shelf margin periphery (top-of-slope) received hemipelagic lime mudstone sediments. Many rotational slides, some generating debris flows, indicate upper-slope instability. Most allochthonous carbonate debris from the shelf margin by-passed the top of slope and was deposited further downslope. With the westward progradation of the shelf margin the fringing facies migrated basinward.