Shelf-to-basin facies distributions and sequence stratigraphy of a steep-rimmed carbonate margin; Capitan depositional system, McKittrick Canyon, New Mexico and Texas

Abstract

ABSTRACT Shelf-to-basin outcrop studies in steep-rimmed, shelf-margin settings are uncommon because continuous shelf-to-basin transects are rarely exposed in a single outcrop. Discontinuous or absent stratigraphic marker beds across the shelf margin further complicate outcrop studies in the shelf-margin setting. This paper discusses the results of a high-resolution investigation of the shelf-to-basin profile along the north wall of North McKittrick Canyon, New Mexico and Texas. In McKittrick Canyon, carbonate-dominated sedimentary rocks associated with the steep-rimmed, Upper Permian Capitan depositional system are exposed along a continuous 5-km outcrop face. Measured sections, lateral transects, scintillometer readings, and geochemical data were synthesized into a digital database and interpreted in conjunction with a digital photomosaic of the entire canyon wall. Results of this work include a shelf-to-basin facies map and sedimentologic interpretation of the north wall of North McKittrick Canyon, and indicate that the dominant bathymetric profile during Capitan deposition was a marginal mound. In this model, the Capitan reef facies was deposited at the shelf-slope break in water depths ranging from 15 to 75 m, but always downdip from the topographically higher shelf crest. This model is supported by the following observations and interpretations: (1) a facies progression from the shelf crest to the shelf margin interpreted to represent a shallow-to-deeper-water succession; (2) proportional expansion of beds in a downdip direction; (3) presence of oriented (transported) fusulinid grainstones downdip from in situ fusulinid wackestones and packstones updip; (4) siltstones that thin and pinch out towards the shelf margin; (5) a decrease in dolomite from the shelf crest to the shelf margin; and (6) the absence of true toplap stratal geometries. In reality, a static paleobathymetric model cannot characterize the depositional system, because the facies distributions, facies proportions, stratal geometries, and quantified depositional parameters vary systematically from the Seven Rivers through the Tansill. In order to understand the observed variations, emphasis was placed on quantifying key depositional parameters such as progradation, aggradation, offlap angle, outer-shelf dip, water depth, distance to the shelf margin and toe of slope, and facies-tract width. The systematic variations in these parameters, in conjunction with the facies distribution map and stratal geometries, helped to define the sequence-stratigraphic framework, and allowed for comparative evaluation of such things as sediment accumulation rates and sites, and stratigraphic evolution. The Capitan depositional system is represented by three composite sequences, each containing four high-frequency sequences. Two and one half of these composite sequences are exposed in McKittrick Canyon. The overall depositional system is interpreted to have evolved predictably from a deeper-water margin in the Seven Rivers composite sequence, to a shallow-water margin in the Tansill composite sequence. The subtidal outer-shelf and shelf-margin facies tracts were sites of major sediment production. Accumulation rates across the shelf margin indicate a relatively continuous growth history, with periods of nondeposition or erosion limited to the terminal phase of each composite sequence. As a result, the preserved sedimentary record of high-frequency and composite sequences in the outer-shelf to upper-slope position is equally proportioned between transgressive and highstand systems tracts. This symmetric outer-shelf to upper-slope record of carbonate accumulation is significantly different from the asymmetric, highstand-dominated middle-shelf accumulation record reported previously for this and many other carbonate shelves.