Abstract

Seismic-scale outcrops in the western Brokeoff and Guadalupe Mountains, New Mexico and Texas, expose a 20km dip-oriented transect through an upper San Andres (Middle Guadalupian) depositional sequence. The sequence is bounded by regional subaerial-to-submarine unconformities (and their correlative conformities), which lie at the top of the Cutoff, lower and middle San Andres Formation (Lower Guadalupian), and base of the Grayburg Formation (Middle Guadalupian). Systems tracts within the sequence were defined on the basis of seismic-scale bedding geometry, patterns of stratal termination (onlap, downlap, truncation), stacking patterns of meter-scale parasequences, and process-sedimentological considerations.

The lowstand systems tract (Brushy Canyon Formation) consists of a >300 m-thick wedge of basinal fine-grained siliciclastics and minor allochthonous carbonates that onlaps and pinches out against a submarine unconformity at the basin margin. Part of this wedge may be coeval with a thin (~ 10 m), mixed carbonate-siliciclastic bank that overlies and onlaps the middle San Andres bank margin.

The transgressive systems tract consists of mixed carbonate-siliciclastic basin (Cherry Canyon Tongue) and bank (medial upper San Andres Formation) facies. The Cherry Canyon Tongue is composed of very fine-grained sandstones and siltstones which are cut by carbonate-rich channels along the basin margin. Along the upper slope, siliciclastic beds of the Cherry Canyon Tongue interfinger with allodapic carbonate grainstones derived from the bank. The upper San Andres transgressive bank (~ 40 m-thick) is composed of aggradational grainstone-dominated parasequences which represent an increase in water depth over the lowstand bank. The upper 10-15 m of the transgressive bank contains several sandstone-based parasequences which correlate to the Cherry Canyon Tongue. The top of the Cherry Canyon Tongue marks a regional downlap surface.

The highstand systems tract consists of an early phase, characterized by sigmoid progradational, carbonate-dominated parasequences that prograded the bank ~ 1 km and aggraded the inner part of the bank 30 m, and a late phase, characterized by complex sigmoid-oblique to oblique progradational, mixed carbonate-siliciclastic parasequences which prograded the bank ~ 5-6 km. The upper sequence boundary is marked by a karst profile which extends >30 m into underlying highstand strata; this profile is best developed above early highstand strata, and grades basinward into a sharply erosional toplap surface above late highstand strata.