Abstract

The following paper concerns strata in the Artesia group and the San Andres formation and in their lateral equivalents.

Cyclic alternations of time-synchronous “carbonate” and “clastic” units characterize shelf and basin sediments of Artesia-Delaware Mountain age. This cyclicity is thought to be related to alternating periods of deposition at high and low stages of sea-level. During periods of high sea-level, relatively thick “carbonate” units were deposited on the shelf and especially the shelf margin, while thin dark limestones were deposited in the basin. During periods of low-sea level, thin “clastic” units were deposited on the shelf, while thick sandstones and siltstones were deposited in the basin. Terrigenous clastics were not deposited on the steeply sloping shelf margin. The essential character of these two types of cyclic units persists through several major facies changes.

Evidence of exposure between clastic and carbonate units on the shelf suggests the possibility of short intervals of extremely low sea-level between high “carbonate stage” sea-levels and intermediate “clastic stage” sea-levels. This pattern of sea-level fluctuation appears compatible with Simpson’s theory of glacial cyclicity. Glacial control for cyclical sedimentation in Middle Permian strata appears reasonable in view of world-wide pale-ogeological considerations.

Time transgressive lithofacies patterns in the San Andres formation of the shelf area suggest that the main part of the formation represents a single sedimentary cycle deposited during a transgressive rise in sea level to a high “carbonate” stage followed by regressive basinward lithofacies prograding. A minor transgressive-regressive cycle in the uppermost San Andres is associated with the Lovington sand and its overlying carbonate unit. Because of the concept of cyclic reciprocal carbonate and clastic sedimentation demonstrated by the overlying Artesia group and the basinal Delaware Mountain group, it appears that the San Andres should be time-equivalent to the basinal upper Bone Spring limestone.

Cyclic sedimentation appears to be characteristic of all Middle Permian strata deposited on the broad cratonic shelf area which occupied the western-central United States. Regional correlations between the Permian Basin and more distant areas suggest that major transgressions during San Andres, Seven Rivers and Tansill time may correspond to similar transgressive sequences in other portions of the shelf.