Light-colored, parallel-stratified, non-reef marine limestone and dolomite (Victorio Peak Formation) grade southeastward abruptly into dark cherty limestone (Bone Spring) along the northwestern margin of the Delaware basin (King, 1948). Study of the well-exposed transition zone in the Guadalupe Mountains suggests the presence of three major contemporaneous environments along a gentle basinward slope: bank, and bank margin (0.5-1 mile wide), both Victorio Peak; and euxinic basin, Bone Spring. Basinward regression of transition facies was 2-3 miles during accumulation of 1,500 feet of section. Middle Permian erosion truncated the transition strata creating basin-sloping unconformities on which limestone, megabreccia, and sandstone (Cutoff and Brushy Canyon Formations) were eposited.

Principal lithologic facies of the transition zone are successively: carbonate grainstone and dolomite packstone (bank); dolomite wackestone and mudstone (bank margin); and carbonate mudstone (basin). Paucity of desiccation or solution features, algal laminates, oolites, and coated or composite grains suggests prevalence of sub-tidal bank environments. Allochthonous channel fills and sheet deposits of skeletal packstone and wackestone occur in bank-margin and basin-edge facies. Sand-size skeletal grains and carbonate silt are dominant constituents in bank and bank-margin rocks. Carbonate silt predominates in basin facies. Characteristics of bank relative to basin strata are lighter color, dominance of grain-supported rocks, coarser grain size, normal-marine fauna, more dolomitization, less chert (but larger, more rounded nodules), and thicker, more massive beds. Undulatory bedding and disturbed laminations characterize many basin strata adjacent to the bank margin.

Two major controls on sedimentation and subsequent diagenesis were diminished turbulence with depth, and an abrupt change along the depositional slope from normal-marine to euxinic water at inferred depths of a few hundred feet. Bank proximity and finer particle size favored dolomitization. Low permeability of organic-rich basin facies apparently inhibited downdip dolomitization. Cementation obliterated most depositional porosity.

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