Subaerial Exposure and Erosion in Capitan Reef (Permian), Guadalupe Mountains, New Mexico

Leslie A. Melim, Peter A. Scholle

A prominent subaerial exposure surface has been identified within the Capitan reef in Rattlesnake Canyon. Equivalent to the middle Yates interval in the back reef, the exposure surface records planation of the reef during a sea level drop to below the shelf edge. This is the first exposure surface recorded within the reef, although several are known within the back reef facies.

The surface is quite planar and can be traced shelfward for nearly 1 km before it is lost in near-back reef grainstones. The surface does not appear to correlate directly with one of the Yates sandstone beds. Channels cut into the surface have a maximum relief of nearly 2 m. Gypsum molds (now calcite filled) and mud cracks are found in the channel-filling sands. In nonchannel areas, a thin (1 to 5-cm) zone of reddened pebbles is sometimes present.

Although the exposure horizon has only a thin sediment veneer, the extensive truncation of the underlying beds implies significant sediment transport across the surface. This is presumably reflected in one of several prominent clastic lowstand wedges found in the Bell Canyon Formation of the Delaware basin.

The back reef facies prograded out over this surface during the upper Yates and lower Tansill intervals. During this period of progradation, the reef and forereef facies were minor or completely absent. Water depths over the surface reached approximately 70 m before the surface was completely buried during Tansill deposition.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.