Depositional environments, diagenesis, and conodont biostratigraphy of the Montoya Group (Late Ordovician), Sacramento Mountains, New Mexico

The Montoya Group represents the southwestern exposure of sheet-like carbonate remnants of late Ordovician cratonic sediments. The central geologic location of the Sacramento Mountains in the Montoya Group distribution and excellent exposures of the strata provided an opportunity to analyze and interpret the depositional fabric, conodont biostratigraphy, and diagenetic sequences of the pervasively dolomitized and siliceous carbonates.

The Montoya Group consists of three formations: Second Value, Aleman, and Cutter Dolomite. The Second Value is subdivided into the Cable Canyon and Upham Members. Conodont faunas aided in the interpretation and dating of depositional events for these formations.

After an extended erosional period, middle Edenian transgressing seas deposited coarse-grained quartzose sand across the southwest sloping carbonate ramp. This thin sand bed, the Cable Canyon Member of the Second Value, was deposited in the upper wave-tidal, lower peritidal facies. The overlying Upham Member of the Second Value rests conformably on the Cable Canyon and was deposited in deepening, well-agitated water of the wave-tidal facies. Water depths continued to deepen in the Maysvillian as grain supported sediments were replaced by mud-dominated sediments in the Aleman Formation. Depositional environments shifted downdip to the open shelf facies. Shoaling conditions ended Aleman deposition with the early Richmondian south and west of the study area at this time. Following the short exposure, peritidal facies conditions developed and deposited the Cutter Dolomite during the Richmondian. A second exposure of the ramp ended Montoya deposition in the late Richmondian.

Diagenetically, the Montoya underwent cyclic alteration after deposition. Dolomitization and sulfate emplacement affected the Second Value-Aleman succession during early burial and the Cutter penecontemporaneous with deposition. Exposure of these sediments at the end of the Second Value-Aleman and Cutter deposition respectively resulted in silicification, dissolution, and meteoric cementation. A second burial further dolomitized the units and emplaced sulfates. Recrystallization of the dolomite and cementation eliminated the early secondary porosity in the Montoya. Recrystallization and silicification were less pervasive in the Cutter Dolomite. Cenozoic deformation raised the Montoya from deep burial into another phase of freshwater diagenesis in meteoric phreatic and vadose environments. Most of the porosity found in the Montoya is a product of these diagenetic processes still active today.