SUBSURFACE MIDDLE PERMIAN (GUADALUPIAN) EVAPORITE - SILICICLASTIC INNER SHELF OF THE DELAWARE BASIN AREA, SOUTHEAST NEW MEXICO: A GREAT PLACE TO EVALUATE OUTCROP-DEFINED COMPOSITE AND HIGH FREQUENCY SEQUENCES

The Artesia Group is present throughout the Northwest Shelf of SE New Mexico and adjacent Texas. It includes the Tansill, Yates, Seven Rivers, Queen and Grayburg Formations which are equivalent to the shelf margin Capitan Reef, Goat Seep Reef, and Getaway carbonate bank. In a 200 square kilometer subsurface study area in Lea County (T16S, R34 and 35E) it averages 540 m (1775 ‘) thick. The sequence stratigraphy of the Artesia Group here is correlated and compared to sequence studies by others in the classic Guadalupe Mountains. There, Kerans and Tinker (1999) identify four composite sequences (CS 11 - 14) and nineteen high frequency sequences (HFS G 10 - 28) in outcropping middle shelf, shelf crest, and outer shelf facies tracts. At the surface the equivalent evaporite-rich, inner shelf facies tract is generally eroded or poorly exposed, however it is well preserved and completely penetrated by thousands of wells in the subsurface of the Northwest Shelf east of the Pecos River.

More than 50 well sections in the study area are used to prepare sequence cross-sections and isopach maps of the important CS and HFS within the Artesia Group inner shelf facies tract. Here the lithologies consist mostly of cyclic anhydrites alternating with feldspathic siltstones and very fine-grained sandstones which can be recognized using Gamma Ray and other wireline logs. The siliciclastic units generally are interpreted as shallow marine transgressive deposits comprising reworked relict eolian sands and silts related to sea level lowstands. The overlying evaporite units represent highstands.

The Artesia Group CS and HFS show little variation in thickness (520 - 560 m) in the study area nor does the character of the wireline logs change significantly, suggesting uniform depositional environments in the backreef, evaporite dominated, inner shelf during middle and late Guadalupian time. All four of the CS and most of the important HFS in the more lithologically variable outcrops of the Guadalupe Mountains can be correlated to and identified in the relatively simple evaporite-siliciclastic inner shelf facies tract in the subsurface study area.