OSLEGER, DAVID A., and ISABEL P. MONTANEZ, Department of Earth Sciences, University of California, Riverside CA

ABSTRACT: Criteria for the Recognition of Sequence Stratigraphic Components within a Mixed Carbonate-Siliciclastic System, Middle Cambrian of the Southern Great Basin

Many siltstones interbedded within carbonate successions are interpreted to be lowstand deposits that bypassed a subaerially-exposed carbonate shelf during a relative fall in sea level. The Middle Cambrian Bonanza King Formation of the southern Great Basin contains an areally extensive interval of mixed carbonates and siliciclastics that shows a great deal of internal complexity and requires a more sophisticated interpretation of its origin. The mixed interval is primarily composed of two distinct lithofacies: (1) silty dolomitic laminites that cap shallowing-upward carbonate cycles and clearly record deposition on tidal flats, and (2) overlying calcareous siltstones interbedded with nodular lime mudstones interpreted to have been deposited in moderately deep water.

Two surfaces about 2 to 10 m apart that show extensive evidence of subaerial exposure can be traced 250 km across the platform. This thin interval is interpreted to represent a Type 2 sequence boundary zone because it cannot be traced onto outermost platform locations where the interval appears relatively conformable. Above a well-defined shelf margin wedge, the entire style of sedimentation abruptly changes across a single transgressive surface from one of prograding tidal flat and shallow subtidal deposition to one of deeper water drowning and retrogradation, manifesting deposition within a transgressive systems tract. This mixed carbonate-siliciclastic system illustrates a much greater degree of internal complexity than most generalized models for mixed systems, and cautions agains calling all siliciclastics interbedded within carbonate successions "lowstand deposits" without dissecting them in greater detail.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.