--> ABSTRACT: Facies Architecture of Tocito Shelf Sandstone, Northwest New Mexico, by Ivo Bergsohn and Dag Nummedal; #91030 (2010)

Datapages, Inc.Print this page

Facies Architecture of Tocito Shelf Sandstone, Northwest New Mexico

Ivo Bergsohn, Dag Nummedal

Outcrops along the northwest margin of the San Juan basin reveal a Tocito sandstone ridge complex that overlies a transgressive surface. The ridge complex contains laterally extensive pebble lags that traverse facies boundaries. These lags distinguish three stages of ridge growth interrupted by two erosional hiatuses. During stage one, bioturbated muddy fine sandstone, thin ripple-bedded sandstone, and sandy mudstone were deposited. In stage two, bioturbated muddy sandstone interfingered with interbedded muddy sandstone and sandy mudstone capped by cross-bedded sandstone. In stage three, cross-bedded sandstone with rare basal bioturbated muddy sandstone was deposited. Collectively, these deposits compose an upward-shoaling sequence inferred to have formed at "paracycle" s illstand of the Coniacian transgression.

Exposures of strata below the transgressive surface and subsurface data from Bisti oil field reveal evidence for earliest Coniacian(?) relative sea level fall prior to "Tocito" transgression. In outcrop, this fall is expressed by incision of fine-grained Gallup "A" tongue shoreface sandstone by coarser Torrivio fluvial sandstone. Downdip at Bisti field, examination of well cores linked to over 200 electric logs revealed a terraced shelf topography. An upward increase in maximum and median grain size across this terraced surface is inferred to result from the distribution of coarse Torrivio sediments distributed onto the shelf at lowstand. These shelfal sediments are inferred to be the source for the sandstone ridges at Bisti field. Integral sandstone ridges (2.5 km2) are en echelon with an across-ridge spacing of 1,500 m between crests, and form a linear sandstone ridge complex (250 km2) on isopach maps of crossbedded sandstone facies.

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