Depositional dynamics in a mixed carbonate–siliciclastic system, trilobite fauna, biostratigraphy and biofacies: middle–upper Cambrian Abrigo Formation, southeastern Arizona.

Abstract

The mixed carbonate–siliciclastic Abrigo Formation of middle and late Cambrian age, which crops out in southeastern Arizona, was deposited during the Sauk transgression in the craton interior, landward of the passive margin of Laurentia. The Abrigo Formation consists of ten basic rock types: claystone, siltstone, sandstone, lime mudstone, wackestone, bioclastic grainstone, packstone, oolitic packstone, oncolitic packstone, and intraclastic conglomerate. These comprise fifteen lithofacies, which are grouped into eight facies associations. They 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. One hundred eighty-two collections, yielding 940 trilobite remains have been found in the Abrigo Formation. They represent 69 species and 42 genera. Eight of the species are new. The fossil age ranges from early Marjuman to late Steptoean. Eight trilobite biofacies are defined from the generic relative abundance data: Ehmaniella, Olenoides–Bolaspidella, Blairella, Eldoradia, Modocia–Paracedaria, Cedaria, Coosella–Coosina, and Camaraspis. Trilobites collected and identified in this study are assigned to five biostratigraphic zones: Bolaspidella, Cedaria, Crepicephalus, Aphelaspis, and Elvinia zones. In addition, two subzones had been defined. Cedaria eurycheilos Subzone recognized in the upper part of Cedaria Zone and Coosella helena Subzone recognized in the upper part of Crepicephalus Zone. The stratigraphic succession was divided into six distinct phases associated with large-scale relative sea-level fluctuations. An initial flooding over the Bolsa Quartzite forming the 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 that 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 protracted and widespread Sauk II–Sauk III hiatus. Sedimentary dynamics were controlled by storm-induced wave action and offshore flows. There are two carbonate factories that operated simultaneously in this Cambrian inner shelf region. Dominance of carbonate versus siliciclastic strata in the offshore transition setting is interpreted to reflect periods when siliciclastic input was depleted, such that increasing accommodation and reduction of clay and possibly nutrients promoted carbonate production. Clay and silt bypassed the nearshore carbonate-depositing zone. Siliciclastic sediment input and dispersal were not only restricted to the falls in sea level, but appear to have dominated the transgressive systems tract and late phase of the highstand. Thus, carbonate sedimentation does not dominate the entire highstand systems tract as is commonly held but, rather, only during the late phase of the transgressive and early highstand phase. The comparison of this Cambrian model with younger mixed carbonate-siliciclastic units will help reveal the subtleties of the carbonate factory and how it operated in response to biotic evolution.