ABSTRACT: High-Frequency Carbonate/Siliciclastic Reciprocal Sedimentation Within the Upper San Andres Depositional Sequence (Permian, Guadalupian), Last Chance Canyon, Guadalupe Mountains, New Mexico

Mark D. Sonnenfeld

The upper San Andres Formation and Cherry Canyon Sandstone Tongue in Last Chance Canyon form a third-order depositional sequence comprising at least 12 smaller scale, time-bounded depositional units. The intertidal through toe-of-slope facies associations, volumetric proportions of carbonate/siliciclastic sedimentation, and depositional topography change progressively throughout the sequence. These variations reflect the controls that produce the landward-stepping, vertically stacked, and seaward-stepping arrangements of units constituting the sequence. In Last Chance Canyon many of the small-scale depositional units have stratal geometries and carbonate/siliciclastic relationships that are analogous to third-order sequences.

The oldest depositional unit is a point-sourced turbidite lobe that onlaps the underlying sequence boundary. Carbonate-rich slope sandstones form the next two units, which also onlap the sequence boundary. These three landward-stepping to vertically stacked units compose the transgressive systems tract (TST). Units within the TST record a progressive decrease in sedimentation rate, depositional energy, and siliciclastic content that reflects the sequence-scale transition from toe-of-slope siliciclastic turbidites to an increasingly carbonate-dominated ramp. Within each of the two youngest units, a basal sandstone hemicycle is overlain by a mounded fusulinid-rich carbonate hemicycle. The maximum flooding surface caps the TST and is overlain by the most carbonate-rich interval of the se uence. From basal to upper slope, the facies in this unit are laminated mudstones and wackestones, crinoid-bryozoan bafflestone bioherms, brachiopod-sponge reefs, and fusulinid mounds.

Seaward-stepping units compose the middle highstand systems tract (HST). These are characterized by strongly progradational fusulinid bank margins, recording a change from the late TST to earliest HST hummocky, aggradational fusulinid ramps. The middle HST marks the resumption of siliciclastic input to the outer shelf and slope. These depositional units begin with basal turbidite channels overlain by bioturbated sands and form basinally restricted sand wedges. The sandstone wedges are capped by transgressive surfaces that are locally colonized by sponge-brachiopod mini bioherms and are downlapped by prograding fusulinid shoals.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990