Abstract

Depositional cycles present in a mixed carbonate and quartz gravity-flow sequence exposed in the Death Valley-Owens Valley of southeastern California may record the response of this system to third-order (∼0.7-1.4 m.y.) sea-level changes during Late Pennsylvanian time. Time-equivalent outer-shelf and basin-margin deposits elucidate the following key elements of this depositional system: (1) a distally steepened ramp profile, (2) an unchanneled basin-margin sequence which accumulated as a line-sourced apron system, and (3) penecontemporaneous deposition of carbonate and fine-grained quartz sediment in both the shallow- and deep-water regimes.

Basin-margin facies consist of mudstones, turbidites, debris flows, and megabreccias organized into thickening- and coarsening-upward packages. Lime mudstone intervals (3-7 m thick) alternate with mixed-carbonate-siliciclastic, gravity-flow intervals (10-30 m thick). Time-equivalent outer-ramp facies show a similar organization of alternating silty, bioclastic wackestones/grainstones (10-40 m thick) containing a diverse open-marine fauna and rhythmically bedded lime mudstone (15-60 m thick) which is barren of fauna.

An upward increase in debris flows and megabreccias within basin-margin cycles indicates the involvement of large-scale ramp processes and cannot be explained by simple progradation and abandonment of depositional lobes. The absence of megabreccia caps on many of the cycles argues against autocyclic progradation and collapse of the platform, although larger scale coarsening- and thickening-upward trends within the basin-margin succession are attributed to autocyclic processes.

Instead, we propose that cycles develop in both the shallow- and deep-water regimes by changes in relative sea level. Drowning of the outer shelf during relative highstands (rhythmically bedded lime mud stone) would yield starved-basin deposits (lime mudstone). As sea level falls, both carbonates and siliciclastics prograde closer to the shelf edge (silty, bioclastic wackestones/grainstones), thus activating the mass-transport system (mixed-clastic, gravity-flow packages). Increased storm and wave influence on the outer ramp/upper slope triggers debris flows and occasional large-scale collapses, producing megabreccias.

Most models of turbidite deposition have stressed reciprocal sedimentation with sea-level change: bioclastic turbidites during highstands and silicilclastic turbidites during lowstands. Instead, our model for mixed-clastic deposition emphasizes the importance of the build-up profile, the position of sea level relative to the shelf margin, and the rate of change of sea level.

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