Geologic Occurrence and Regional Assessment of Evaporite-Hosted Native Sulfur, Trans-Pecos Texas

In the western Delaware Basin and southern Central Basin Platform of Trans-Pecos Texas, Upper Permian (Ochoan) evaporite strata host locally voluminous but notoriously unpredictable, sporadic deposits of biogenic native sulfur. The two sulfur-bearing regions, the Rustler Springs (Culberson and Reeves Counties) and Fort Stockton (Pecos County) sulfur districts, encompass about 2,000 State-owned tracts that are widely distributed throughout the two districts. This study was initiated to provide the General Land Office of Texas (GLO) with a regional assessment of sulfur potential of the Trans-Pecos, thus enabling more effective land management. Specific goals were to (1) investigate the presumed geologic controls on sulfur occurrence by detailing the structural and stratigraphic context of sulfur within the sulfur districts, (2) provide details about biogenic sulfur formation, Ochoan evaporite lithology, radiation log expression, and geology of prominent mines---subjects either discussed in diverse sources or in industry sources not readily accessible to the public, (3) construct a computerized data base documenting formation tops and sulfur-bearing zones, and (4) provide a tract-by-tract sulfur assessment of State-owned lands. Approximately 1,450 well logs from GLO files were the primary source of subsurface data. Stratigraphic correlation of Ochoan evaporites in the Rustler Springs sulfur district indicates that component formations (Castile, Salado, and Rustler) dip gently eastward toward the center of the Delaware Basin. However, strata are locally disrupted in the central and eastern parts of the district by small, uniformly northeast-trending graben systems and high-angle normal faults that probably formed during late-phase Basin and Range extension. Mineralization in two of the largest sulfur deposits in the district, the Pennzoil Culberson mine and the Texasgulf Phillips Ranch deposit, is restricted to dissolution-enhanced fault zones within the downfaulted blocks. Underexplored grabens detected on regional cross sections and structure-contour maps potentially contain commercial sulfur deposits. Photogeologic inspection of the Gypsum Plain in the western part of the Rustler Springs sulfur district reveals (1) a concentration of exposed northeast-trending grabens and normal faults representative of the subsurface structures and (2) east-trending, fracture-controlled dissolution troughs that are locally associated with commercial sulfur deposits. Computer-contoured isopach and structure maps of the districtwide sulfur-bearing zone reveal no distinct regional trends in orebody geometry but are invaluable in delineating areas of sulfur concentration. In contrast, isopach and structure-contour maps of the Fort Stockton sulfur district define a trend of sulfur concentration in aligned culminations along the axis of a northwest-oriented anticline that locally affects bedding of Guadalupian and Ochoan evaporitic formations (Seven Rivers, Yates, Tansill, Salado, and Rustler) at the southern end of the Central Basin Platform. Associated faults, especially along the southwestern edge of the anticline, have high sulfur potential but have been poorly explored. Summary maps of the two sulfur districts identify areas where significant sulfur potential exists, as determined by depth (shallowest sulfur occurrence), thickness (thickest sulfur-bearing zones), and concentration (densest surficial fractures and lineaments) limits. Areas where these criteria overlap define regions of highest sulfur potential.