4 New Mexico Geological Society, Special Publ~cationNo. 10,1981,pp. 133-145. Y 1 4 DEEP-SEATED SALT DISSOLUTION IN THE DELAWARE BASIN, TEXAS AND NEW MEXICO ROGER Y. ANDERSON Department of Geology University of New Mexico Albuquerque, New Mexico 87131 ABSTRACT Hiss's (1976) map of the structure of the Rustler Formation pro- Patterns of salt dissolution in the Delaware Basin are related to vided a well-defined picture of the location and configuration of the bedrock geometry and hydrology that developed following the depressions formed by this type of deep dissolution and re uplift, tilting, and erosion in the late Cenozoic, and the greatest solved them into smaller isolated but interconnecteddepressions. volume of salt has been removed since that time. During the Per- Hiss's (1975) report on the hydrology of the Delaware Basin pro- mian, some salt was dissolved from the to^ of the Castile Forma- vides a basis for interpreting a relationship between these patterns tion before deposition of the Salado orm mat ion and from the top of dissolution and the regional basin and reef aquifers. These rela- of the Salado before deposition of the Rustler Formation. In addi- tionships, as well as the occurrence of a number of smaller more tion, some salt dissolution occurred after the Permian and before localized dissolution features, suggest how regional dissolution the Cretaceous. Post-uplift surface dissolution has progressed has progressed throughout the basin and how it is likely to pro- across the Delaware Basin from south to north and west to east ceed in the future. Additionally, these relationships suggest some and generally down the regional dip. Deep-seated dissolution has mechanisms that might account for the patterns of deep-seated occurred around the margin of the basin where the Capitan Lime dissolution in the basin. stone aquifer is in contact with the Permian evaporites and within the basin where selective dissolution in the lower Salado has WENT OF DISSOLUTION undercut the overlying salt beds of the middle and upper Salado. Dissolution has not advanced down regional dip uniformly but has ' Extensive dissolution in the Delaware Basin has long been left outliers of salt and has progressed selectively into structurally recognized by geologists studying the more extensive Permian predisposed areas. This selective advance has significance for the Basin. Hills (1968) suggested that river underflow had worked stability of the U.S. Department of Energy's Waste Isolation Pilot down dip and reached far into the subsurface and that the dis- Plant (WIPP) site. turbed hydrodynamic balance following uplift may have reached as far as the eastern edge of the Delaware Basin. The amount of INTRODUCTION salt dissolution in the Delaware Basin, however, has been difficult The Delaware Basin of southeastern New Mexico and western to document because the original extent of the salt beds was not Texas contains thick carbonate and evaporite sequences depos- known or recognized. The problem was simplified with the discov- ited during the Permian (Table 1). The upper Permian carbonate ery that each salt bed in the Castile Formation (Table 1) in the rocks of the Capitan reef form the margins of the Delaware Basin. eastern part of the basin had a correlative bed of dissolution brec- Within and overlying this reef are thick evaporite deposits which cia in the western part of the basin (Anderson and others, 1978). are being considered as the host rock for the Department of Dissolution breccias have also been recognized within 6 mi (10 km) of the western edge of the basin (Castile outcrop along U.S. Energy's Waste Isolation Pilot Plant (WIPP). The dissolution history of these evaporite rocks is of primary importance for the stability Highway 621180, near Texas State line; shown as H-Ill breccia on of th~srepository site, and this paper discusses recent studies of Figure 2). These breccias showed that the western edge of salt in dissolution in the basin. the basin for all units is the result of dissolution, and original salt The principal areas of salt dissolution in Cenozoic time within the thickness followed trends that were approximately east-west in Delaware Basin (fig. 1) were identified by Maley and Huffington the basin. These breccias also showed that removal of salt and the (1953) who recognized that a number of deep structural depres- subsequent uniting of anhydrite beds previously separated by salt sions on the Permian Rustler Formation were filled with Cenozoic beds were responsible for many of the difficulties in log (sub- deposits. One major trend of these depressions has developed surface) correlation and interpretation within the evaporites. Re- along the inner margin of the buried Permian Capitan reef (a major vised correlations, based on this new information, showed that aquifer) along the eastern side of the basin (fig. 2) in areas of thin or only a relat~velynarrow band of salt along the eastern side of the missing Permian Salado Formation salt. A second trend follows the basin has escaped the effects of regional dissolution (fig. 2). axis of the Pecos River in the central part of the basin, but the The original thickness of salt beds in the basin was reconstructed stratigraphic units affected by dissolution were not recognized using the trends established in the undissolved areas (see Ander- until Anderson and others (1972) found that large quantities of son and others, 1972; figs. 13, 15, 16, 17), and the original volume bedded salt were missing from the middle of the evaporite se of salt in the major units was estimated (fig. 3). A comparison with quence near the center of the basin. Further work (Anderson and the postdissolution volume shows that approximately 50 percent others, 1978) demonstrated that large depressions in the basin of the salt originally in the basin now remains. The lower Salado were the result of selective dissolution of lower Salado salt beds has had the greatest proportion of salt removed, with about 27 and that a (previously unsuspected) variety of deep-seated dasolu- percent of the original salt remaining reflecting the dissolution tion processes had undermined the overlying salt beds in the that has undercut overlying salt beds recognized by Anderson and others (1972). [NEW MEXICO GEOLOGICAL SOCIETY I 0; Fig. 2- 0- 100 200- 300 KILOMETERS \ TEXAS ----- I 0 10 20 30 40 50 KILOMETERS - I F~gureI. Index map showing location and outline of Dela ware Basin and locations of Figures 2,5, and 6. PRE-CENOZOIC DISSOLUTION high as the lower Salado and that, clearly, were implaced by col- Adams (1944) was correct in identifying a significant break in lapse after late Cenozoic uplift (Castile A of Kirkland and Evans, deposition between the Castile and Salado formations. Correla- 1976). Lang (1947) described how similar patches of Cretaceous tions of laminae with records of geophysical logs indicate that this rocks had been trapped in sinks or cavernous channels and were break was accompanied by dissolution of salt from thesalt beds in stratigraphically out of place. Also, Udden (1915) originally as- the Anhydrite IV and Halite Ill of the Castile Formation (fig. 3) from cribed a Cretaceous age to the Castile Formation because he around the margins but not in the center of the basin. The original found Cretaceous age cuttings 265 ft (76 m) below the top of the delineation of a wedge of undercutting dissolution within the Castile in an area overlain by the Rustler Formation and which evaporites (Anderson, 1978) incorporated some of the missing salt could only have been emplaced by a deep collapse. beds from the upper part of the Castile, and it is now believed that A truncation surface sufficient to allow precretaceous dissolu- some of this material was removed before the Salado was depos- tion to reach into the Castile Formation would require a regional ited, leaving a dissolution residue. The salt that was removed dip in the Cretaceous of 100 ftlmi (19 mlkm) to account for even before Salado deposition was not used in estimating the percent the present thickness of thestratigraphic section farther east in the of salt later removed from the basin (fig. 3). basin. Inasmuch as this is the present regional dip following uplift, Baltz (1959), King (1942), and Adams (1944) recognized an angu- extensive pre-Cretaceous truncation and dissolution would allow lar contact between the Rustler and Salado formations in the no space for the recognized post-Cretaceous uplift. western part of the basin. The amount of salt removed cannot be The Salado and Rustler formations are now stripped from the estimated, but the unconformity is not considered major and the western half of the basin, so it is not possible to determine if salt contact between the two units within the basin is conformable was dissolved before Cretaceous time. lsopach maps of the mid- (Kroenlein, 1939; Pierce and Rich, 1962). dle and upper Salado (Anderson, 1978, figs. 9, 10) show regionally Triassic rocks were eroded and stripped from an area west of the normal thickness trends extending westward to the area of pres- present Delaware Basin (McKee and others, 1959; Bachman, 1974), ent truncation indicating that dissolution, if it took place, was con- and Cretaceous beds rest directly on the Permian. Some disso- fined to the upper part of the Salado in the western part of the lution of salt from the upper part of the evaporites may have basin. occurred during this interval. More recently, Bachman (1980) has CENOZOIC DISSOLUTION suggested that small patches of Cretaceous rocks resting on the Castile Formation inside the western part of the basin indicate a Structural and Stratigraphic Controls more profound episode of dissolution extending down into the Large-scale dissolution of salt from the Delaware Basin occurred Castile.