Permian Reef Complex Virtual Field Trip
Stop II-7: Castile Formation – Ochoan EvaporitesCarbonate Rock
BE CAREFUL TRAFFIC IS ESPECIALLY HAZARDOUS AT THIS OUTCROP!
Excellent exposures of the Castile Formation in deep roadcuts. This unit is the oldest true Ochoan sediment in the region and conformably overlies the Guadalupian Bell Canyon Formation. The Castile is entirely confined to the Delaware basin and does not extend onto the adjacent shelf areas. It overlies a thin, limestone and siltstone/shale zone which may be a lateral facies equivalent of the very youngest part of the Capitan and Tansill Formations. The bulk of the Castile Fm. itself consists of a thick section of laminated anhydrite with intervals of laminated halite. The Castile Formation has been reported to reach a maximum thickness of 470 to 600 m (1,550 to 2,000 ft) in subsurface sections in the northeastern part of the Delaware basin (King, 1948, p. 89).
The Castile grades conformably upward into the Ochoan Salado Formation; the Salado contains laminated halite, anhydrite, sylvite, polyhalite, and even more soluble evaporite minerals (see mine photo). The extreme solubility of its components means that the Salado does not generally appear in outcrop. Indeed, in this area, much (or all) of the Salado may have been removed by erosion. The Salado does, however, form a wedge of sediment which thickens toward the northeast to a maximum of greater than 600 m (2,000 ft) (Anderson et al., 1972, p. 82). In the northeastern part of the Delaware basin, the Salado is extensively mined for potash minerals. Unlike the Castile, the Salado Formation extends beyond the borders of the Delaware basin onto the surrounding shelf areas where it generally lies directly on Guadalupian carbonate rocks. The Salado, in turn, is unconformably overlain by the dolomitic Upper Permian Rustler Formation, the Dewey Lake Redbeds, and younger units. The pre-Rustler unconformity shows extensive Permian tilting and erosion for, in places (particularly the southwestern part of the region), it has completely removed the Salado, allowing the Rustler to lie directly on the Castile Formation or Guadalupian carbonate rocks.
The onset of Castile evaporite deposition coincided closely with the termination of reef growth around the Delaware basin margin. It is not entirely clear whether this is a causal or coincidental relationship. Eustatic sea level drop, tectonic movements, reef growth, or other factors could have increased the restriction of influx of normal marine water into this already partially barred basin. This, coupled with the extreme aridity and high evaporation rates in the area, may have led to drastic increases in the salinity of basin water, with the associated killing of the salinity-sensitive reef organisms and the eventual start of evaporite deposition. It must be emphasized, however, that although the changes in depositional patterns at the Guadalupian-Ochoan transition were dramatic, the causes of these changes may have been considerably more subtle. Strongly evaporitic conditions existed throughout Guadalupian time, as apparently did hypersaline stagnant bottom waters in the basin. Marine influx from the south was certainly present during Guadalupian time to maintain normal or near-normal marine conditions in the surface waters of the Delaware basin.
This influx must have continued through much of Ochoan time, if in a somewhat more restricted form, to supply the salts of the Castile and Salado Formations. Thus, it appears most likely that it was a gradual change in marine water supply versus evaporative water removal which led to the abrupt shift from carbonate to evaporite sedimentation, presumably when a critical salinity level was reached. This gradual (but not perfectly continuous) salinity transition apparently continued through Ochoan time, leading to deposition of anhydrite, then halite and sylvite, and eventually the true bittern salts found in the northeastern Delaware basin.
The Castile Formation, then, represents an evaporite filling of the approximately 550 m (1,800 ft) deep basin left at the end of Guadalupian time. Although there may have been some drop in basinal water levels, most of the Castile clearly was deposited in deep water (at least below wave base) as indicated by the absence of shallow-water sedimentary structures in most intervals and the presence of fine-scale lamination. The laminae consist of regular (although variable thickness) alternations of white anhydrite laminae and darker laminae containing a mixture of organic matter (circa 1.5 percent average) and calcite (see photo below). The anhydrite-calcite couplets average 1-2 mm in thickness throughout the Castile Formation (Anderson et al., 1972, p. 73). On outcrop, the anhydrite may have been altered to gypsum (this locality has both gypsum and anhydrite exposed according to S. D. Kerr in Dunham, 1972). The laminations have remarkable lateral continuity, as one might expect for deeper-water evaporites, and individual laminae have been traced for more than 70 miles (Anderson et al., 1972; Dean and Anderson, 1978). In a few instances, however, layers of coarse, reworked sulfate, nodular sulfate, or bottom-nucleated crusts were noted (Kendall and Harwood, 1989; Leslie et al., 1993). These were interpreted as shallow-water deposits that may have formed during times of extreme evaporative drawdown. Other nodular zones, however, may reflect recrystallization during burial and uplift (see photo below).
The laminations of the Castile Formation (as well as those in the uppermost Bell Canyon and Salado Formations) have been interpreted as annual varves (Udden, 1924; Anderson and others, 1972). The calcite and organic-matter layers represent periodic (annual?) freshening of the water and the development of plankton blooms. The anhydrite layers represent restricted, more evaporitic conditions. Considering the basinal nature of the depositional setting, the obvious aridity, and the prior influx of clastic terrigenous material, it seems odd that the Castile-Salado evaporites contain virtually no detrital windblown silt. It is possible that the alkaline waters may have dissolved much of the original silt influx.
Approximately 260,000 calcite-evaporite cycles have been counted in the uppermost Bell Canyon-Castile-Salado sequence. This implies extremely rapid deposition of thousands of feet of evaporites in the Delaware basin, a common situation with major evaporite deposits. The Castile cycles provide one of the longest continuous climatic records from any time interval in the Phanerozoic. The record of "varve" thicknesses has been analyzed recently (Anderson, 1982, 1984, 1988, 1991) for larger-scale cyclicities. Major peaks were found in the range of 20,000 and 100,000 years, indicating that Milankovitch-scale climatic variations were active during deposition of the unit.
The microfolding which has contorted a number of intervals in the Castile at this outcrop (see photo) clearly is post-depositional and been interpreted to represent volume changes due to hydration and/or dehydration reactions during burial and/or uplift. Recent, detailed measurements on these structures have indicated that both small- and large-scale fold axes trend N30-50°W. This direction is parallel to the trend of Tertiary faulting in this area (Anderson and Kirkland, 1988) and the folds, therefore, are likely to be late-stage, compressional, structural features rather than early, soft-sedimentary or later hydration structures. See and Alexander and Watkinson (1989) and Watkinson and Alexander (1993) for additional discussion of possible causes.
The evaporite filling of the Delaware basin is largely responsible for the spectacular exposures of the Guadalupian facies which we are sing on this trip. The complete plugging of the "hole" left at the close of Capitan reef growth and the subsequent, Tertiary, removal of that plug has left us with resurrected Guadalupian topography and facies relations in this area.
The Castile and Salado evaporites may also have had a major impact on the oil and gas distribution in the Permian basin. The rapid burial of basinal source rocks to depths sufficient for oil and (or) gas generation is one probable effect. It is quite possible that compactional geopressuring of the basinal sediments resulted from the rapid deposition. This may have eventually aided the early migration of hydrocarbons from the basin, before deep burial and destruction of porosity in potential shelf reservoirs. Overpressuring and early oil migration may have been significant factors in the excellent hydrocarbon productivity of the Permian basin region. The early oil movement may also explain why primary porosity and early diagenetic porosity modifications, rather than later diagenetic porosity types, are so important in many Permian basin reservoirs. Finally, the extensive blanketing of both shelf and basin by an impermeable cover of evaporites clearly provided an outstanding hydrocarbon seal for the entire region.