CROSS-PLATFORM ARCHITECTURE OF A SEQUENCE BOUNDARY IN MIXED SILICICLASTIC-CARBONATE LITHOFACIES, MIDDLE CAMBRIAN, SOUTHERN GREAT-BASIN, USA

Stratigraphic analysis of mixed siliciclastic-carbonate lithofacies wi thin the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range o f depositional environments from semi-arid tidal flats to deeper subti dal, restricted lagoons. Stratigraphic trends, cross-platform facies v ariations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in thre e temporally distinct phases. (1) Extensive progradation of mixed peri tidal environments culminated in a prolonged episode of subaerial expo sure marked by an areally extensive intraclast breccia (0.5-1.2 m thic k) that we interpret to be a major Type 1 sequence-bounding disconform ity. (2) Abrupt flooding of the exposed platform resulted in the depos ition of mixed deeper subtidal lithofacies, including a condensed inte rval of fissile, fossiliferous shale. (3) Progressive shallowing and a ggradational accumulation was accompanied by a decrease in siliciclast ics and a shift to pure carbonate deposition. Deep-water siliciclastic s and megabreccias record deposition along the base-of-slope off the M iddle Cambrian shelf-edge, and are interpreted to represent lowstand d eposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf. The presence of fine siliciclastics in both periti dal facies and sharply overlying deeper subtidal facies of the study i nterval within the Bonanza King suggests a variable, but relatively co ntinuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the prim ary siliciclastic source changed with relative sea-level position. Dur ing lowered sea level, aeolian processes acting upon the unvegetated C ambrian craton transported fine siliciclastics onto peritidal and shal low-subtidal environments. During higher sea level, coastal siliciclas tic reservoirs supplied sediment that was transported for long distanc es by geostrophic currents flowing along the submerged platform. As op posed to many Cambro-Ordovician grand cycles that are commonly interpr eted to consist of a transgressive shaly half-cycle grading upward int o a regressive carbonate half-cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic-rich, mixed li thofacies rather than in adjoining purer carbonates, implying that som e 'grand cycles' should not be considered synonymous with 'sequences'. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecis ion of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continen t-wide during the Ehmaniella trilobite biochron.