ABSTRACT

Current models for steep forereef facies give a primary role to talus and rock fall processes during both sea-level highstands and lowstands. Highstand sedimentation is also believed to include abundant basinal turbidites, whereas basin starvation is likely during extensive lowstand talus formation. However, the forereef facies of the Capitan Formation (Permian, west Texas and New Mexico) largely lacks the characteristic matrix-poor carbonate breccias formed by talus processes and provides an alternative model appropriate to carbonate margins that produce a wider mixture of coarse- to fine-grained debris. Such a margin not only has different depositional processes but can also have a significantly different response to sea-level fluctuations.

The Capitan forereef can be divided into an upper, middle, and lower forereef on the basis of primary dip and lithologic variations. The upper forereef has primary dips of 30° and is composed of rudstones with minor grainstones. The middle forereef is distinguished from the upper forereef by a decrease in primary dip (to 15-30°) and a change to a mixture of rudstones and packstones with minor wackestones. Dips decrease to 510° in the lower forereef accompanied by an increase in packstones to grainstones, but rudstones are still significant. The lower forereef interfingers with packstones to wackestones and sandstones of the basin-margin facies.

During sea-level highstands, the Capitan forereef was supplied with abundant very fine to coarse debris from the wide reef facies. Most of this was deposited on the upper to middle forereef by debris flows and on the lower forereef to basin by debris flows and high- to low-density turbidity currents. During the deposition of the lower Capitan only, siliciclastic sands were also present, primarily as matrix in debris-flow units. Lowstand deposits, in contrast, are almost entirely basinal siliciclastics because production of carbonate sediment was minimal. Transgressive deposits are composed of minor forereef matrix-poor debris flows and a few major debris flows with a siliciclastic matrix that extend far out into the basin.

Although the large-scale stratal patterns may appear similar, a comparison of the Capitan forereef facies with the Triassic Latemar margin and the Quaternary Tongue of the Ocean reveals contrasting details, with the Capitan debris-flow-dominated and the others rock-fall-dominated. This study emphasizes the role of primary sediment production in determining the depositional processes in forereef and foreslope settings, and, as a result, the stratal patterns commonly used to interpret the history of relative sea level.