New Mexico Geological Society Annual Spring Meeting — Abstracts

The Chacra Mesa and La Ventana fields are in the southeastern San Juan Basin, west of the Nacimiento uplift and north-northeast of the Mt. Taylor volcanics. These fields are defined by the outcrop of the Cliff House Sandstone and the La Ventana Tongue of the Cliff House on the north and the Point Lookout Sandstone on the south, encompassing the entire stratigraphic sequence of the Cretaceous Menefee Formation. Along the southern edge, the basal Cleary Coal Member of the Menefee Formation is exposed and the upper coal member crops out along the northern edge. These coal-bearing sequences represent transitional phases during the fourth regressive and subsequent fifth transgressive shift in the Late Cretaceous shoreline in the San Juan Basin.

Although comprehensive geologic mapping and some coal quality work has been available for the La Ventana and Chacra Mesa fields for several years, there have been few extensive coal investigations involving drilling and coal characterization in both the Cleary and upper coal members of the Menefee. As a result of a recent coal quality drilling and sampling project at the New Mexico Bureau of Mines and Mineral Resources, a larger base of lithology and quality data is now available for these fields. These data, merged with previous investigations, provide the basis for stratigraphic and geographic comparison of the coal and coal-bearing sequences in the La Ventana-Chacra Mesa area.

From lithologic data, differences in the coal thickness and number of seams between the two coal-bearing sequences are apparent. Thickness of coal and number of seams is dependent on the frequency and duration of oscillations in the shoreline, in both the regressive and transgressive sequences. Two areas, the west edge of the La Ventana field and the western Chacra Mesa field, have relatively thick coals and numerous seams in the Cleary Coal Member of the Menefee. These coal-rich sequences probably developed during gradual oscillations in the shoreline, allowing for moderately thick peat accumulations. The thickest coals in the upper coal member of the Menefee are associated with massive sandstone buildups of the La Ventana Tongue on the east side of the La Ventana field. This stacking of paralic sandstones is indicative of a stillstand in the generally transgressive shoreline.

Coal characteristics vary stratigraphically and geographically in the study area. The average ash yield of the upper member coals is significantly lower (5-6%) than the Cleary coals (14%) for both fields. This may, in part, be due to the greater number of thin partings in the Cleary coals. Both coal members in the La Ventana field contain greater than 1% total sulfur, predominantly of organic origin. The Cleary coals in the Chacra Mesa field have a low average total sulfur content (0.41%) compared to the upper member coals (0.88%) in the same field. The Cleary coals upper member in the Chacra Mesa field contain higher percentages of organic than pyritic sulfur. The relatively high total sulfur content of the Cleary and upper member coals in the La Ventana field may indicate a greater frequency of flooding during peat development in comparison to the Chacra Mesa coals.