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ABSTRACT: Similarities Between Third and Fourth-Order Stratigraphic Sequences: An Example from the Permian Upper San Andres: Last Chance Canyon, Guadalupe Mountains, New Mexico

Mark D. Sonnenfeld

The upper San Andres/Cherry Canyon tongue in Last Chance constitutes a seismic-scale, unconformity bounded (third-order) depositional sequence containing mixed carbonate and siliciclastic lithologies deposited in intertidal through toe-of-slope environments. This sequence is composed of more than 12 clinoform packages that are smaller scale, higher frequency (fourth-order) depositional units or parasequences. Although shelf parasequences are asymmetric because flooding surfaces are coincident with maximum regressive surfaces, slope parasequences are relatively symmetric because transgressive deposits separate these two key surfaces. Slope parasequences within the highstand systems tract have bounding surfaces, stratal geometries, facies distributions, and siliciclastic/ca bonate interrelationships that show similarities, independent of scale, to the lowstand, transgressive, and highstand systems tracts constituting the third-order sequence.

Erosional surfaces and intermittent turbidite-filled channels form the basal portion of slope parasequences. Waning rates of siliciclastic sedimentation from suspension resulted in a basinally restricted, bioturbated sandstone wedge. Bedding planes within the wedge onlap the underlying parasequence boundary. These fourth-order geometries are similar to those of the third-order lowstand through transgressive systems tracts. The sandy wedge is capped by a transgressive surface. This surface is sporadically colonized by sponge-brachiopod communities, which are fourth-order analogs to the larger crinoid-bryozoan bioherms and brachiopod-sponge reefs developed on the third-order maximum flooding surface. Prograding fusulinid shoals downlap the fourth-order wedge, have fourth- and fifth-orde toplap surfaces, and may be capped by regressive shelf sands. This mixed lithology package is a fourth-order analog to the third-order highstand systems tract.

Sigmoidal, symmetric slope parasequences can be traced shelfward to individual asymmetric topsets where the deepest facies of one unit directly overlies the shallowest facies of the preceding unit. Last Chance Canyon documents the basinward transition from asymmetric shallowing-upward parasequences to more symmetric parasequences composed of inferred deepening-upward phase characterized by a waning siliciclastic influence, followed by a shallowing-upward phase.

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