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The San Andres (G9) — Grayburg (G10) Forced Regressive Turnaround, Brokeoff Mountains, New Mexico, U. S. A.


Outcrops in the Brokeoff Mountains provide valuable insights into diagnostic stratal architecture and potential seismic geometries of mixed siliciclastic-carbonate, forced regressive depositional systems. Here, stratigraphic relationships indicate 75-m of lower Grayburg (G10) onlap across the basinward-dipping upper San Andres (G9) bounding surface. LiDAR, drone imagery, GigaPans, behind-outcrop cores, and over sixty measured sections combined with 30-km of GPS-constrained bed tracing along multiple ridges and canyons reveal a late G9 forced regression that produced significant down-to-the-basin accommodation. Carbonate-dominated sigmoidal cycles (early G9) record 1 km of progradation and 25 m of aggradation before transitioning into sigmoid-oblique, siliciclastic-dominated cycles that ultimately downstep and prograde an additional 5 km seaward. The early G9 ramp margin is intensely eroded and karsted with sand-filled fissures penetrating to depths of 30 m. The depth and extent of karst diminishes basinward. Near the terminal G9 margin along Cutoff Ridge, a slope channel has incised basin-floor siltstones suggestive of process disequilibrium driven by base-level fall. Diminishing karst, increasing siliciclastics, downstepping, and basinal incision records downward and basinward translation of the G9 shoreline indicative of forced regression. Overlying the G9 terminal ramp margin are 75 meters of thin-bedded (0.1-0.5 m thick), cyclic peritidal strata consisting of backstepping siliciclastic-dominated algal laminites, shallow marine sands, eolian sands, and fenestral-pisolite-tepee facies. G10 strata record persistent peritidal to supratidal conditions seaward and downdip from the early G9 ramp margin indicating significant sea-level fall (>75 m) at end-San Andres time. A 2-m thick ooid grainstone overlies the G10 and thins shelfward across the extensively karsted, early G9 ramp margin. Landward of this position, the G10 is absent due to onlap or erosion. The antecedent and early G9 ramp margin formed a current-swept, barren shelf with a prominent shelf break as recorded by 2.5-m thick, offlapping foreshore and shoreface oolitic grainstones that ultimately prograded (090°-140°; n=62) 4-km along a horizontal trajectory. Shoreface grainstones were shed locally into lowstand-incised subaqueous channels (<6-m deep by <200-m wide) cut into the top of the G10. Seaward, channels open into heterolithic ebb-tide deltas characterized by complex cross-stratification and diverse ichnofacies and fossil assemblages. This laterally extensive oolitic unit, which directly overlies the G10, is situated 75 m above the terminal G9 margin effectively bracketing the updip limit of the onlapping and aggradationally stacked, siliciclastic-dominated G10 lowstand wedge. Similar stratal attributes are expected in Midland Basin and Middle Eastern Shuaiba-Bab fields.