ABSTRACT: Mixed Carbonate-Siliciclastic Deposition on a Wave-Swept Carbonate Ramp: Lower Carboniferous Mount Head Formation, Southwestern Alberta, Canada

BRANDLEY, RICHARD T., and FEDERICO F. KRAUSE, University of Calgary, Calgary, Alberta, Canada

The Mount Head Formation contains mixed carbonate-siliciclastic sediments deposited in a broad embayment of the Carboniferous shoreline on a gently sloping ramp that deepened to the west.

Inner-ramp, restricted-shelf and sabkha deposition, similar to that of the modern Persian Gulf, occurred in peritidal areas and formed sabkha cycles in mixed carbonate-siliciclastic sediments.

Mid-ramp, carbonate-sand shoal complexes blanketed much of the 150+ km wide ramp. Shoal complexes consist of (1) turbulent-water, millimeter- and centimeter-sized bioclast, ooid and peloid accumulations, and (2) quiet-water, lenticular, silty-carbonate intershoal deposits. Shoal lithofacies exhibit distinct bathymetric signatures ranging from near-surface ooid shoals to peloid-bioclast shoals that formed near wave base. Shoal lithofacies also reveal complex 3-D spatial distributions that shift and vary with time. Shoal complexes developed above fair-weather wave base in response to long-period waves rather than tidal currents. Large volumes of carbonate sand were produced and swept either seaward or shoreward depending on relative sea level trends and available accommodation space.

Outer-ramp deposition was dominantly open marine, and resulted in thick accumulations of skeletal wackestone and packstone, and whole-fossil mudstone to wackestone. Relative sea level fluctuations influenced deposition, producing common ooid grainstone beds and eventually semirestricted conditions in outer-ramp areas.

Sedimentological interpretations are constrained by biostratigraphic and sequence stratigraphic data.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)