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Evidence of Fault-Induced Oversteepening and Mass Wasting along the Tansill Platform Margin in Walnut Canyon, New Mexico

Rush, Jason *1; Kerans, Charles 2
(1) Kansas Geological Survey, The University of Kansas, Lawrence, KS.
(2) John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.

A substantial amount of literature exists on ancient slope apron and toe-of-slope, mass transport deposits. However, literature regarding ancient examples of process-dependent, platform margin collapse is rare. Detailed facies mapping at cycle-scale resolution permitted critical assessment of depositional processes operating along the structurally dynamic and aggradational Tansill margin exposed in Walnut Canyon, New Mexico. Stratigraphic architecture reveals an oversteepened carbonate platform that was prone to catastrophic, fault-induced, mass wasting. Results presented here make conclusions about the instability of the Tansill shelf margin and its effects on stratigraphy, which has implications for interpreting steep margins elsewhere.

Several near-vertical, depositionally strike-aligned, syndepositional fractures indicate structural instability across the Tansill platform margin as it built out over forereef/slope megabreccias and siliciclastics of the Delaware Basin. A facies disjunction and horizontally juxtaposed fusulinid zones were documented across a 3-m-wide by 30-m-high, 80° seaward-dipping dilational fracture. On the landward side of this fracture, sediments are dominantly peritidal and contain the fusulinid Paradoxiella pratti (G27-HFS). Seaward (<5 m) of this fracture, massive-bedded units contain small cement-rich Collenella-sponge bioherms and abundant Reichelina lamarensis (G28-HFS) indicating an open marine setting. The ranges of P. pratti and R. lamarensis do not overlap confirming a near-vertical unconformity, or paleoescarpment. The outermost G27 shelf margin likely ruptured in response to compaction-induced faulting and slid basinward along a 14° dipping décollement developed in unconsolidated Triplet sands. Mass wasting during lowstand is believed to have generated a steep-sided embayment while transferring large megabreccia complexes basinward. This embayment was completely filled with G28 outer shelf sediments. Within the fold axis of a small monocline situated ~20 m above the dilational fracture, calcite-cemented fractures are nested within earlier siliciclastic fracture fills indicating at least two stages of post-G28 deformation. Fine sands/silts and angular polymict breccia deposited along the floor of this dilational fracture indicate reactivation and subaerial filling during lowstand conditions associated with the seaward migration of eolian Ocotillo ergs (G29).


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California