Characterizing facies and clay mineralogy of Guadalupian tidal deposits in SE New Mexico: Implications of dolomitization
Bagnall, Dillon John
The Queen and Seven Rivers formations represent back reef deposits on the northwest shelf of the Delaware Basin. The Queen Formation is dominated by sandstone, limestone, and dolomite, whereas the Seven Rivers Formation consists dominantly of gypsum that is interbedded with clastic and thin dolomite intervals. XRD results of the clay fraction demonstrated differences between the Queen and Seven Rivers formations with the most important being the loss of chlorite/smectite and kaolinite in the Seven Rivers Formation, which is more illite/smectite dominated. Whole-rock XRD and thin section analysis demonstrated that dolomite occurred preferentially near occurrences of chlorite and chlorite/smectite and was mostly absent in red clastic facies, but persistent in green clastic facies. The isolated occurrence of dolomitized beds indicates that Mg-rich dolomitizing fluid preferentially altered certain carbonate and clastic beds. To explain these observations, I propose a dolomitization model that transitions from an open marine to semi-restricted environment that deposits clastic and carbonate sediments. A fully restricted environment occurs thereafter allowing gypsum to precipitate resulting in a Mg-rich fluid that dolomitizes the underlying limestone and clastic units to various degrees also providing magnesium for chlorite formation. A red clastic facies then forms on top of the gypsum which represents oxidizing conditions, potentially terrestrial. The transition from sandstone, limestone, and dolomite to a more gypsum-rich deposit is thought to be related to an overall drying sequence through the Permian. The disappearance of kaolinite at the boundary of the Queen and the Seven Rivers formations indicates a wetter environment during Queen deposition to increased aridification during Seven Rivers deposition.