Abstract

Complete understanding of outcrop-scale stratal architectures requires extrapolation of two-dimensional depositional cross sections into three dimensions. This study integrates a high-resolution digital outcrop model with outcrop observations to create a three-dimensional geologic model of distal outer-ramp carbonate stratigraphy. Data were taken from sinuous canyon-wall exposures of the Permian San Andres Formation of Last Chance Canyon, New Mexico. A series of laterally extensive carbonate benches previously interpreted as constructional sponge mud mounds are here modeled as a channel–levee complex that is characterized by an alternating history of aggradation punctuated by erosional sediment bypass. Levee mudstones have an erosional base, contain dolomitized peloidal thin beds, and have limited faunal constituents. Primary levees were initiated as a carbonate apron and transformed into a low-relief channel–levee system as a result of deposition by dilute turbidity currents. Subsequent sediment bypass incised through the pre-established channels and into the underlying facies. The fill consists of skeletal packstone lags and fine peloidal packstones, which are correlated to coral- and sponge-bearing secondary levee deposits that drape the primary levees. Debrites, also found within some channelized areas, are thought to represent channel-margin oversteepening and slumping. Channel widths range from approximately 300 to 800 m, and channel-to-levee relief reaches up to 40 m.

Three-dimensional surface models representing architectural elements of the channel–levee complex confirm outcrop observations. Surfaces that were modeled as a best fit to the three-dimensional outcrop tracings reveal development of low-sinuosity channels in previously established bathymetric lows created by earlier phases of sediment bypass and erosion. Sequence stratigraphic interpretations suggest that the channel–levee complex developed during the highstand part of a long-term transgression. The overall transgressive setting led to generation of substantial quantities of carbonate mud in more proximal areas of the ramp. Accumulated mud in the source region was transported down depositional dip during highstand parts of high-frequency cycles and resulted in growth of the levees. Carbonate channel–levee complexes are not likely to form during highstand sequences in which the source sediment is grain-dominated. Leveed channels are a viable component of deep-water carbonate settings and may be more common than has been previously recognized. Complex stratal architectures found in this channel–levee complex could be analogous to apparently gullied slopes of muddy carbonate ramps, especially those developed in transgressive sequences.