Depositional Dynamics In A Mixed Carbonate–Siliciclastic System: Middle–Upper Cambrian Abrigo Formation, Southeastern Arizona, U.S.A.

Abstract

Abstract The Middle and Late Cambrian mixed carbonate–siliciclastic Abrigo Formation of southeastern Arizona was deposited during the Sauk transgression in the craton interior landward of the passive margin of Laurentia. It overlies shallow-marine sandstone of the Bolsa Quartzite, which mantled the Precambrian land surface. The Abrigo Formation consists of ten distinct rock types that comprise fifteen lithofacies, which are grouped into eight facies associations. The strata represent an array of shallow-marine environments that were dominated by wave and storm activity. The interpreted paleoenvironments include lower offshore, upper offshore, offshore transition, and lower, middle, and upper shoreface. These environments migrated laterally as a function of relative sea-level changes along with input of siliciclastic sediment and its effect on carbonate deposition. 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. Correlation across 170 km of the study area suggests that these lithofacies were deposited in six temporally distinct phases involving two transgressive–highstand systems tract couplets during the Bolaspidela through Crepicephalus biozones, a falling-stage systems tract in the Aphelaspis Biozone, and a lowstand systems tract in the Elvinia Biozone following the Sauk II–Sauk III hiatus. In general, the mixed carbonate–siliciclastic depositional environment of the Abrigo Formation shows that fine-grained siliciclastic facies dominate the transgressive systems tract. By contrast, carbonate sedimentation was dominant mostly during the middle to late phase of the highstand. The upper part of the highstand systems tract records progradation of the sandy shoreface. Nevertheless, the ratio between siliciclastic and carbonate sediment in various bathymetric zones differs from previously described inner-detrital-belt examples of Cambrian age. In the Abrigo Formation, some bioclastic grainstone in shoreface deposits contains siliciclastic sand, indicating that the two were deposited together until sand was dominant. However, carbonate production and siliciclastic mud sedimentation were for the most part mutually exclusive, suggesting that the shallow-water carbonate factory during the Middle and Late Cambrian was vulnerable to poisoning from clay or nutrient input. Consequently, carbonate sedimentation in the offshore transition, located between sand-dominated shoreface and mixed carbonate–siliciclastic offshore facies, reflects the bypassing of siliciclastic mud.