Accommodation Cycles in Peritidal Carbonate and Continental to Shoreface Siliciclastic Facies, San Andres-Grayburg Formations, Eddy County, New Mexico

C. Robertson Handford, Magell P. Candelaria, and Steve Lafollette

Cyclic siliciclastic-carbonate facies of the San Andres-Grayburg formations permit analysis of stacking patterns to determine accommodation history and how systematic shifts in cycle thickness correlate with facies in landward settings.

San Andres cyclic-peritidal strata include burrowed, subtidal wackestones to fenestral, algal-laminated, peloid-intraclast packstones and grainstones of intertidal- supratidal origin. The San Andres-Grayburg contact is marked by paleokarst and an abrupt downward-facies shift from peritidal carbonates to redbed siliciclastics (distal desert wadi to eolian sand flat and siliciclastic sabkha) of the overlying lower Grayburg Formation.

An accommodation plot shows a strong relationship between facies and cycle thickness trends. San Andres ooid-grainstones are associated with thickening-upward cycles. There is an abrupt downward shift of facies at the San Andres-Grayburg sequence boundary, which lies midway-through a succession of thinner-than-normal cycles. Nonmarine, fining-upward siliciclastic cycles make up the lower Grayburg above the sequence boundary. Thicker-than-normal cycles follow, signaling an increase in accommodation and a change from continental to marine shoreface siliciclastic and peritidal carbonates.

Locating sequence boundaries within peritidal carbonates through the use of accommodation plots is subject to debate because facies shifts are not always abrupt. However, this study, which documents abrupt facies shifts from peritidal carbonates to nonmarine clastics and which also demonstrates a remarkable correlation between depositional facies and position along the accommodation plot further corroborates the utility of accommodation plots for sequence stratigraphic studies.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California