Advances in depositional models of the Permian Rustler Formation, southeastern New Mexico
— Dennis W. Powers, Robert M. Holt, Richard L. Beauheim, and Ron G. Richardson

The Permian Rustler Formation in southeastern New Mexico was deposited in complex depositional environments that ranged from saline marine lagoon to salt pan-mudflats with subaerial exposure. Two significant transgressive freshening events deposited thin carbonate beds, each about 8 m thick. The Culebra Dolomite Member is the more permeable of the carbonate units. One of the geological factors influencing permeability is the presence or absence of halite cements in both fractures and other porosity. These cements underlie halite in the halite pan to mudflat deposits in a higher unit, and they confirm an earlier prediction that diagenetic halite would fill Culebra porosity near the depositional center. The Magenta Dolomite Member follows the basic Culebra pattern, with halite-filled porosity in a drillhole nearer the depositional center.

Cores recently obtained corroborate interpretations that mudstones in the Rustler were generally deposited without significant halite, or it was removed by syndepositional dissolution; they are not residues after later dissolution of halite. A laminar gray claystone underlying the Culebra is continuous across the study area, overlying mudflat as well as halite pan deposits. It indicates that the underlying mudstones are not dissolution residues. The uppermost mudstone-halite bed of the Rustler in a new drillhole shows a division between lower halite pan deposits and upper mudflat deposits that is also reflected in more distal mudflat deposits of the entire unit. These findings increase confidence that mudstones will yield additional information about the history of the mudflat to halite pan beds of the Rustler. One new core from the Tamarisk Member shows brecciation consistent with earlier predictions that halite has been dissolved in a limited area within this unit. The lower Rustler now reveals wider distribution of halite cements, which are not easily distinguished in available geophysical logs.

These data help refine ideas about the effects of halite cements and dissolution on porosity and on geochemistry of water or brine recovered from Rustler carbonates. Halite in these units also limits vertical Culebra recharge by limiting permeability, and its very presence indicates that little fresh water can be percolating into and through halitic beds.