ⓘ Arroyo Penasco Group. The Arroyo Penasco Group is a geologic group exposed in the Nacimiento, Jemez, Sandia, and Sangre de Cristo Mountains of New Mexico. It pr ..

                                     

ⓘ Arroyo Penasco Group

The Arroyo Penasco Group is a geologic group exposed in the Nacimiento, Jemez, Sandia, and Sangre de Cristo Mountains of New Mexico. It preserves fossils dating back to the late Mississippian, including the foraminifera Endothyra and Plectogyra. The discovery of these fossils in 1955 showed that the massive limestone beds formerly included as the base of the Pennsylvanian Sandia Formation were actually Mississippian in age. The fossil assemblage suggests an age of Osagean to Meramecian.

The group rests everywhere on Precambrian basement that shows remarkably little relief, suggesting that the group was deposited on a peneplain. Its outcrops are spotty and its upper contact shows indications of well-developed karst topography, often filled in by the iron-rich sediments of the Log Springs Formation, indicating that it was heavily eroded before deposition of the Sandia Formation.

Originally described as a formation in the Nacimiento Mountains, the Arroyo Penasco was later found to correlate to the Espiritu Santo Formation and Tererro Formation in the Sangre de Cristo Mountains, and it was promoted to group rank. The lower Espiritu Santo Formation is designated the Del Padre Member and is a transgressive siltstone, sandstone, and shale unit, which interfingers with the carbonate rocks of the upper Espiritu Santo Formation. A similar sequence is seen in the Terrero Formation, whose base is a collapse breccia.

The Espiritu Santo Formation has undergone dolomitization, dedolomitization, and calcitization of gypsum and the Macho Member has experienced dissolution and brecciation. Together with the presence of paleosilcretes in the Manuelitas Member, this suggests a history of at each four episodes of sea level fluctuation in the Mississippian in northern New Mexico. Diagenetic fabrics can be traced across the region, demonstrating that the group was deposited across an extensive surface of low relief.

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