From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

AAPG Annual Convention, Calgary, Alberta, Canada, June 19–22, 2005

Systematic Variations in the Capitan Depositional System: McKittrick Canyon, West Texas and New Mexico

Scott W. Tinker

Abstract:

The shelf to slope Capitan depositional system reflects a high degree of stratigraphic order in facies distributions, facies proportions, stratal geometries, and progradation:aggradation ratios. The stratigraphic evolution can be examined by quantifying and visualizing several key depositional parameters, including progradation and aggradation of the shelf-crest and shelf-margin facies tract; distance from the shelf crest to reef; reef depth; outer-shelf dip angle; and lateral distance and depth from the shelf crest to the toe of slope. These parameters indicate that depositional styles varied systematically in time and space as a function of position within the composite stratigraphic hierarchy.

In a general sense, during marine transgression (TST) at the composite sequence (3rd order) scale, shelf-crest deposits were thinner and retrogradational, outer-shelf deposits expanded in width and shelf-margin deposits aggraded and prograded to “keep up” with a rising sea level. Sediment accumulation rates were greatest during TST deposition, which contrasts with many reports of HST-dominated production for other carbonate shelves. Oftentimes there was simultaneous retrogradation of the shelf crest facies tract and progradation of the shelf margin facies tract.

During highstand at the composite sequence scale, shelf-crest deposits amalgamated and prograded as they filled available space, outer-shelf deposits narrowed in width, and shelf-margin deposits prograded. Sediment accumulation rates in the outer-shelf and shelf margin (20-80 m water depth) were greater than those in the middle shelf and shelf crest (<10 m water depth), which differs from other reports of the greatest accumulation rates in the shallow waters of the inner to middle shelf. The composite sequence patterns are repeated at the high frequency sequence (4th order) scale, and vary predictably as a function of position within the composite sequence.