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Base-of-Cycle Dissolution


Dissolution also occurs at the base of high-frequency cycles and at sequence boundaries. Influx of marine water during short- or long-term sea-level rise partly or completely dissolves the salt from the top of the previous cycle, and forms an insoluble residue at the base of the transgressive deposit (panel a).

Wavy-laminated base-of-cycle insoluble residue. This is one of the lowest Salado cycles in the Delaware Basin to exhibit base-of-cycle residue and indicates that sediment accumulation has shallowed that basin to the depth at which dissolution can occur. Gulf Research PDB-03 core, 2,360 ft below datum.

Insoluble residues are composed of disseminated impurities and mudstone and anhydrite interbeds from halite (Hovorka, 1994). As halite is dissolved from the top of the bed by undersaturated water, impurities accumulate first as a lag on the floor of the water body, and then as dissolution proceeds downward, as wavy-laminated impurities accreted to the bottom of the insoluble residue bed. Criteria for recognizing base of cycle dissolution are (1) a concentration of insoluble impurities at the base of a transgressive deposit and (2) distinctive accreted wavy-laminated texture. Under ideal circumstances, a relationship can be observed between the residue thickness and the amount and duration of freshening in the overlying cycle, so that thick residues are found downdip beneath thick carbonate beds, and thin residues are found updip beneath thin anhydrite beds (Hovorka, 1994). Dissolution of halite during transgression increases accommodation and bed thickness for the sediments deposited during transgression.

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The mudstone bed at the base of Salado anhydrite 20 in the Midland Basin is tentatively identified as a base-of-cycle insoluble residue. Across the Central Basin Platform, base-of-cycle dissolution during transgression is the probable mechanism for forming abundant, relatively thick anhydrite beds in the Salado Formation as seen in the north–south and east–west cross sections. Multiple episodes of base-of-cycle dissolution is the mechanism proposed for reducing the percent halite to <70 across the Central Basin Platform. This is an area where subsidence during Salado time created high accommodation as apparent in the isopach of the salt section.

Freshening of ground water at the base of a regressive depositional sequence can also result in dissolution of halite (panel b). A probable example of this process may be seen in the upper part of the Salado Formation above bed 60. Several episodes of accumulation of anhydrite beds and thick halite units along the structural axis of the Midland Basin are seen in the north–south and east–west cross sections. Salado facies equivalent to this interval along the north and east parts of the Midland Basin are thin mudstone beds or muddy insoluble residue. Marginal areas may have had salt dissolved while thick salt accumulated in the basin center. This interpretation is made uncertain by the probability that this interval has been attacked by undersaturated water at later times, during Alibates or Dockum deposition.


Burial Dissolution