The spectacular mixed siliciclastic/carbonate exposures of the Guadalupe Mountains include 30 high-frequency sequences (HFS) that stack together to form six composite sequences (CS), the CS9 through CS14. These sequences include carbonate ramps and reef-rimmed platforms as well as basin-restricted lowstand sequences. The Capitan Formation represents the shelf-margin and slope facies tracts of the upper 12 HFS. The Capitan is examined in the context of this late Leonardian-Guadalupian ramp-to-rimmed-shelf system by focusing on extrinsic controls on platform development.
Eustatic changes initiate and punctuate larger scale changes in platform evolution. Rapid shifts of large magnitude, such as the latest Leonardian (L7-L8 HFS) eustatic rise, are a first-order control on platform architecture and reef formation. The model for the late Permian eustatic curve based on the present stratigraphic framework suggests that by the time the Capitan was established, eustatic amplitudes were in the range of 20 m or less. This amplitude variation does not cause major shifts in the shelf-margin location but is sufficient to affect critical accommodation factors that influence reef depth and faunal composition.
Antecedent topography, whether of tectonic, depositional, erosional, or compactional origin, is the critical parameter in controlling the timing and development of the Capitan and other buildups in the Leonardian-Guadalupian sequences, as well as the primary control driving the ramp-to- rimmed-shelf transition. The shelf-slope break, whether a ramp or rimmed shelf, is only one of numerous geometric parameters that can be used to describe the dynamic evolution of carbonate platforms. Changing styles of carbonate-platform progradation and aggradation, which are responses to changes in platform and basin accommodation and sediment-supply, can be captured using P/A (ratio of progradation to aggradation for a given chronostratigraphic unit) and SMP/A (shelf-margin progradation/aggradation ratio) ratios. P/A values > 25 are characteristic of ramps and lowstand wedges, whereas P/A values < 25 are indicative of either transgressive-dominated ramps or reef-rimmed margins. SMP/A values within the Capitan- equivalent sequences can be used to document the complex but systematic and predictable progradational-aggradational-progradational response of the shelf margin to changing base level.
Within the high-frequency sequence framework, other analytical tools, including facies tract substitution and facies proportions, can be used to better constrain interpretations of the dynamic water-depth setting of the Capitan margin and factors controlling its position on the profile. This holistic approach, which draws on relationships from outer-shelf and shelf-crest facies tracts in the interpretation of the Capitan margin, demonstrates the power of a stratigraphic framework for sedimentologic analysis.
Figures & Tables
Geologic Framework of the Capitan Reef
The Capitan Formation of southeast New Mexico and west Texas contains one of the world's best exposed and most famous reefs. Depositional and diagenetic models derived from the Capitan have been used to interpret carbonate strata throughout the world. This volume contains 12 state-of the- art papers summarizing major new research on the Capitan, putting the Capitan into a modern statrigraphic, depositional, paleontologic, and diagenetic framework.