LONGLEY, ANDREW, and GILL M. HARWOOD, School of Environmental Sciences, University of East Anglia, Norwich, UK

ABSTRACT: Yates Formation Small-Scale Cyclicity (Permian, Guadalupe Mountains): An Alternative Hypothesis

The Yates Formation, the back-reef facies of the middle Capitan Reef, contains alternating carbonates and siliciclastics. This cyclicity has been interpreted as the stacking of depositional systems in response to three orders of orbitally-forced, low-amplitude, eustatic variations. Five major cycles have been defined. The shorter duration, shallowing-upward parasequences are, however, most numerous in the back-reef strata closest to the reef. Their occurrence corresponds to a lateral thickening and splaying of beds towards the reef, coupled with a steepening of the basinward dip of the parasequences. This has been interpreted as the depositional profile, steepening towards a 'deep-water' reef.

An alternative hypothesis is that the shorter duration cycles and the basinal dip of the parasequences are controlled by episodic differential subsidence between the back-reef and basin, and not by eustatic sea-level variation. This differential subsidence was partially accommodated by minor faults which contain syn-sedimentary fills. Further accommodation was generated by continued warping during, and immediately after, sedimentation. Field measurements have demonstrated that shallow-water and thus originally horizontal features, such as erosion surfaces, fenestral fabrics and laminites, were subsequently warped as the progressive basinwards dip developed. Differential syn-sedimentary subsidence occurred at, and near, the platform margin, with the belt of maximum subsidence migrating basinward as the reef prograded. Interpretation of the inclined back-reef strata as result of basinward syn-sedimentary warping revives the shallow-water, and perhaps the barrier, hypothesis for the Capitan Reef.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.