Abstract: Accommodation Control on the Facies Distribution within High-Frequency Cycles of a Mixed Carbonate and Siliciclastic System (Upper Carboniferous, Paradox Basin, USA)

Olivier Lerat, Frans Van Buchem, Remi Eschard, Mike Grammer, Gregor Eberli, Peter Homewood

The Upper Carboniferous mixed carbonate and siliciclastic shelf exposed along the San Juan River in southeastern Utah shows a distinct hierarchy of two scales of cycles: medium-scale cycles (4th order; tens of meters thick) bounded by major, regional flooding surfaces, which are composed of a varying number of small-scale cycles (5th order; meter scale). The interaction of relative sea level change, existing or created topography, the sedimentary processes, and the sediment supply by the carbonate factory and influx of siliciclastics has produced a wide variety of geometries and facies types in these cycles. The ultimate control, however, that dictates both the distribution of facies and their geometries is the available accommodation space and it's rate of variation. To illustrate th s control, one medium-scale cycle (composed of five small-scale cycles) of the upper Ismay Member (Desmoinesian) has been studied in an area 4 × 4 km along the superb exposures of the San Juan River.

The deepening phase (highest accommodation) is characterized by a large variety of facies and faunal elements: soils, Chaetetes-packstones (very shallow subtidal), fusulinid-wackestones (deep shelf/normal marine), laminated shale and sponge-mud/wackestones (deepest environment). Carbonate and siliciclastic facies interfinger in the shallower domain. The deeper water facies are continuous over the entire area. The shallowing phase (lowest accommodation) is characterized by little variation of facies types, and a more restricted fauna: low diversity brachiopod mud/wackestones (restricted environment), a crinoidal grainstone bar (high-energy tidal shoal), bioturbated carbonate mud/wackestones and siltstones with scarce fauna and exposure horizons, alternating with shoreface sands. These acies can be traced laterally with generally little change in thickness or facies type. Carbonate and siliciclastic facies are separated vertically.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994