New Mexico Geological Society Annual Spring Meeting — Abstracts

Correlation of 72 well logs along three intersecting cross sections provides a stratigraphic framework for Paleogene strata in the central portion of the San Juan Basin (Cereza, Gobernador, and La Jara canyons). Subsurface data tie to surface sections and contacts.

The San Jose Formation is subdivided into the Cuba Mesa (oldest), Regina, and Llaves Members. The Cuba Mesa Member is a 100 m-thick sheet sandstone with minor mudrock along latitude 36°30'. The unit intertongues with mudrock to the north, pinching out by latitude 36°37'30", longitude 107°19'. Because the Cuba Mesa occurs in outcrop and subsurface west of this point, the Member pinches out to the northeast, toward the central basin. The Regina Member is 430 m of mudrock with subordinate disconnected channel sandstone; it is indistinguishable from the underlying Nacimiento Formation north of the Cuba Mesa pinchout. The Llaves Member is represented by a 100+ m-thick sequence of sheet sandstones and red mudrock.

Paleocurrents, bedding, and stratification suggest that channel sandstones in the San Jose were deposited along southflowing mobile channel belts. The basal contact of the Cuba Mesa grades from an unconformity on the Chaco slope of the San Juan Basin where relatively slow subsidence allowed reworking along the channel belts, producing a sheet geometry, to a conformable contact in the basin center, where higher subsidence rates allowed less reworking of floodplain deposits. Basin-wide aggradation of Regina floodplain deposits marks a period of increased subsidence that was accompanied by west-southwest progradation of the Llaves.