Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Calgary, Alberta, Canada, June 19–22, 2005
Systematic Variations in the Capitan Depositional System: McKittrick Canyon, West Texas and New Mexico
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.