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Delaware Basin Stratigraphy



Gulf Research PDB-03 serves as a type log
through the Delaware Basin section.

 

This brief discussion is for the purpose of setting the context for understanding the relationship of the Midland Basin salts to the Delaware Basin adjacent to the study area. More detailed descriptions are presented elsewhere (for example, Adams, 1944; Anderson and others, 1972; Snider, 1966; Lowenstein, 1988; and Hovorka, 1990). The upper Guadalupian section is composed of the Bell Canyon Formation, capped by the Lamar limestone, a finely laminated, organic-rich, silty limestone deposited prior to evaporite precipitation. The Bell Canyon Formation is the deep-water basinal equivalent of the Seven Rivers, Yates, and Tansill Formations on the Platform (Garber and others, 1989). Because of its high gamma-ray-log response and sharp contact with overlying Castile Anhydrite I, this contact serves as an excellent stratigraphic marker.

Rustler Formation
The two regionally traceable anhydrite-dolomite beds of the Rustler Formation are tentatively correlated with the two anhydrite-dolomite beds of the Alibates Formation, and the siliciclastics of the Dewey Lake with upper Rustler siliciclastics. In the Delaware Basin, insoluble residue is commonly included within the lower clastic unit of the Rustler Formation (Holt and Powers, 1987). Additional stratigraphic complexity observed elsewhere in the Rustler Formation (Holt and Powers, 1987) may be important for resolving the evolution of this part of the section but is outside the scope of this study.

Salado Formation
The Salado Formation in the Delaware Basin was examined in the Gulf Research PDB-03 core has a log response similar to the Salado Formation of the Midland Basin. Cycles defined by anhydrite with or without polyhalite replacement define the base of cycles. Thick relatively pure halite (minor mud, polyhalite, and anhydrite) make up the upper part of cycles.

For this study, I used a unit tentatively correlated with the lower Salado MB 134 of Snider (1966) as a genetic break between the Salado and the Castile Formations. This unit was selected because, during my study of the PDB-03 core from Pinial Dome in Loving County,

Texas, Salado MB 134 was observed to be an inflection point in the gradual upward-shallowing facies observed in the upper part of anhydrite IV and the lower Salado Formation. Above this marker, fabrics indicating shallow-water deposition and intermittent exposure are dominant in the halite as well as the anhydrite. A dolomite and magnesite bed within Salado MB 134 provided a moderately traceable gamma-ray-log kick, but, in some logs close to the Capitan Reef, the position of this anhydrite had to be estimated.

Castile Formation
The Castile Formation (only partly shown on this log) has been divided into four anhydrite units designated with Roman numerals (Snider, 1966), separated by laminated halite having dominantly recrystallized cumulate textures (Hovorka, 1990). Anhydrite beds I, II, and III, and their overlying halite units, can be traced widely over the Delaware Basin (Snider, 1966; Anderson and others, 1972), but near the Capitan Reef in the study area, the halite units pinch out or are laterally equivalent to anhydrite. Anhydrite bed IV is a composite of multiple genetic units and, therefore, the stratigraphy and facies relationships are complex over much of the Delaware Basin as well as all of the study area (Hovorka, 1990); it is therefore difficult to identify and correlate a contact between the Castile and the Salado Formations.


Stratigraphic Units and Type Logs
Relationship between Midland and Delaware Basin Units