THE FOREREEF FACIES OF THE PERMIAN CAPITAN FORMATION - THE ROLE OF SEDIMENT SUPPLY VERSUS SEA-LEVEL CHANGES

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 lowst and talus formation. However, the forereef facies of the Capitan Forma tion (Permian, west Texas and New Mexico) largely lacks the characteri stic matrix-poor carbonate breccias formed by talus processes and prov ides an alternative model appropriate to carbonate margins that produc e a wider mixture of coarse- to fine-grained debris. Such a margin not only has different depositional processes but can also have a signifi cantly different response to sea-level fluctuations. The Capitan forer eef can be divided into an upper, middle, and lower forereef on the ba sis of primary dip and lithologic variations. The upper forereef has p rimary dips of approximate to 30 degrees 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 degrees) and a c hange to a mixture of rudstones and packstones with minor wackestones. Dips decrease to 5-10 degrees in the lower forereef accompanied by an increase in packstones to grainstones, but rudstones are still signif icant. 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 up per to middle forereef by debris flows and on the lower forereef to ba sin by debris flows and high- to low-density turbidity currents, Durin g the deposition of the lower Capitan only, siliciclastic sands were a lso present, primarily as matrix in debris-flow units. Lowstand deposi ts, in contrast, are almost entirely basinal siliciclastics because pr oduction of carbonate sediment was minimal. Transgressive deposits are composed of minor forereef matrix-poor debris flows and a few major d ebris flows with a siliciclastic matrix that extend far out into the b asin. Although the large-scale stratal patterns may appear similar, a comparison of the Capitan forereef facies with the Triassic Latemar ma rgin and the Quaternary Tongue of the Ocean reveals contrasting detail s, with the Capitan debris-flow-dominate and the others rock-fall-domi nated. This study emphasizes the role of primary sediment production i n determining the depositional processes in forereef and foreslope set tings, and, as a result, the stratal patterns commonly used to interpr et the history of relative sea level.