Interrelationships between the upper coal member of the Menefee Formation, the La Ventana Tongue, and the Lewis Shale in the southeastern San Juan Basin

Abstract

-The La Yentana Tongue of the Cliff House Sandstone has been the subject of several studies in the northwest and central San Juan Basin, in particular for oil and gas potential. The lack of drill-hole dala in the southeastern San Juan Basin had restricted detailed studies in this area to outcrops and a few oil and gas test logs. Logs from the New Mexico Bureau of Mines and Mineral Resources coal-quality drilling program ( 1985-1988), additional oil and gas drilling completed since the late 1970s, together with the older data provide sufficient subsurface data for detailed interpretations of stratigraphic relationships in the southeastern San Juan Basin. A 27-km cross section based on 24 geophysical logs was constructed north-northeastward from the outcrop area, 8 km east of Torreon Trading Post. The cross section transects the upper coal member of the Menefee, the La Ventana Tongue and the Lewis Shale. The maximum development of the La Ventana Tongue is clearly seen on the cross section, as is the relationship of the La Ventana to the coals within the upper coal member sequence. After a sudden landward shift of the shoreline that terminated the La Ventana buildup, a secondary stacking of barrier beach sandstones developed further inland. This unit is informally referred to in this report as the Chacra Mesa tongue. The intertonguing between the upper coal member of the Menefee and the La Ventana Tongue can be related to the principle of stratidynamics. This concept assumes continuing subsidence in a depositional environment and deals with transitional units. such as the upper coal member and the La Yentana, between the nonmarine and marine. The variables introduced are rate of subsidence and rate of shoreline shift. The La Ventana-upper coal member sequence developed during a time of minimal shoreline movement, with minor oscillations and variable sediment