SEDIMENTARY FACIES ARCHITECTURE, DEPOSITIONAL MODEL AND SEQUENCE STRATIGRAPHY OF A NEARSHORE, MIXED CARBONATE-SILICICLASTIC SYSTEM OF MIDDLE/UPPER CAMBRIAN OF SOUTHEASTERN ARIZONA

The middle and upper Cambrian Abrigo Formation of southeastern Arizona is a mixed carbonate–siliciclastic unit that records deposition in a subequatorial location in what was western Laurentia during the Sauk transgression. In addition to clay, silt, and sand derived from the adjacent land surface, the carbonate constituents include lime mud, bioclasts, ooids, oncoids, and intraclasts deposited in a shallow-marine setting controlled by storm-induced wave action and offshore flows. The formation consists of 15 facies comprising 8 facies associations representing lower to upper offshore, offshore transition, and lower, middle, and upper shoreface. The stratigraphic succession can be divided into six distinct phases. An initial transgressive systems tract was terminated by maximum flooding, and a subsequent highstand systems tract developed during Bolaspidella Biozone time. The second sequence starts with another transgressive systems tract, and is overlain by a final highstand systems tract during the Cedaria and Crepicephalus biozones. The uppermost part of the second sequence represents a falling stage systems tract developed during Aphelaspis Biozone time. The presence of Elvinia Biozone trilobites near the base of the highest sandstone unit suggests that delivery and deposition of these sands took place during the lowstand that followed the widespread Sauk II–Sauk III hiatus. The difference between the Abrigo Formation and other Cambrian inner-detrital-belt examples concerns the relative dominance of carbonate versus siliciclastic strata in the offshore transition setting. Carbonate production may have been promoted by decreased clay input, increasing accommodation, and possibly changing nutrient supply. Input and dispersal of siliciclastic sediment were not restricted to the falls in sea level, but appear to have dominated the transgressive systems tract and late phase of the highstand as well. By contrast, carbonate sedimentation was dominant mostly during the middle phase of the highstand. Moreover, the stratigraphic distribution of facies indicates that there were two separate carbonate factories: one in the nearshore immediately seaward of the siliciclastic shoreface and the other in the distal offshore area, possibly the inner side of the middle carbonate belt.