--> ABSTRACT: Architecture of a Transgressive Systems Tract: Tocito Sandstone, New Mexico, by Dag Nummedal, Donald J. P. Swift, and Bruce Kofron; #91030 (2010)

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Architecture of a Transgressive Systems Tract: Tocito Sandstone, New Mexico

Dag Nummedal, Donald J. P. Swift, Bruce Kofron

Coniacian-age marginal marine strata in the San Juan basin illustrate well the three-dimensional architecture of the transgressive component of a third-order depositional sequence. The predictive potential of the model presented here lies in its ability to relate marine, shale-enclosed reservoir sand bodies, in themselves below seismic resolution, to the depositional sequence boundary and condensed section, which normally are very distinct seismic stratigraphic markers.

A sequence boundary is identified at the erosional base of the seaward units of the Tocito Sandstone by (1) a hiatus spanning three Inoceramus faunal zones, (2) basal onlap in the sequence above the hiatus as mapped in a pattern of progressively farther landward extent of younger bentonites, and (3) abrupt shallowing of facies across the hiatus. Traced up the depositional dip, the sequence boundary continues beneath a coarse fluvial sandstone (Torrivio sandstone) as demonstrated by incision of this unit into an underlying strand plain, the Gallup Sandstone.

Individual Tocito sand bodies climb successively higher in the stratigraphic section when traced updip. Underlying these inferred shelf sand ridges is a landward-thickening wedge of fluvial strata (Dilco Coal Member of the Crevasse Canyon Formation), the expected consequence of trapping of siliciclastics on the transgressive coastal plain. The retrogradational deposits (Borrego Pass sandstone) constitute parasequences within the overall transgressive systems tract of the "Tocito depositional sequence."

The Tocito sand bodies are underlain by a stair-stepping set of ravinement surfaces, cut during episodes of shoreface retreat. They are overlain by parasequence condensed sections which appear to merge seaward into a major regional condensed section within the calcareous lower part of the Mulatto shale member of the upper Mancos Shale.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.