Abstract

Cyclic siliciclastic-carbonate facies of the San Andres-Grayburg formations in Grayburg-Jackson Field, Eddy County, New Mexico, record deposition in coastal desert, intertidal-supratidal flats, grain shoals, siliciclastic shoreface, and shallow, restricted to open subtidal environments. A long continuous core of these cyclic strata permits 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. Some siliciclastic shoreface facies (Lovington member) are also present but carbonate facies dominate throughout the overlying Vacuum member. The San Andres-Grayburg contact is marked by paleokarst and an abrupt basinward-facies shift from peritidal carbonates to nonmarine redbed siliciclastics (distal desert-ephemeral stream to eolian sand flat and siliciclastic sabkha) of the Premier member of the lower Grayburg Formation. These nonmarine siliciclastic facies pass upward into marine shoreface, shallow-marine carbonate shelf, shoal, and tidal flat facies of the overlying lower Grayburg, Metex, and Loco Hills members.

An accommodation plot shows a strong relationship between facies and cycle thickness trends. San Andres ooid-grainstones are commonly associated with thicker-than-normal cycles. There is an abrupt basinward shift of facies at the San Andres-Grayburg 3rd-order sequence boundary, which lies midway through a succession of thinner-than-normal cycles. Nonmarine, fining-upward siliciclastic cycles make up the lower Grayburg Formation above the sequence boundary. Thicker-than-normal cycles follow, signaling an increase in accommodation and a change from continental to marine shoreface siliciclastics and peritidal carbonates.