Biostratigraphy and carbonate facies of the Mississippian Arroyo Penasco Formation, north-central New Mexico
Authors: Armstrong Augustus K. 
Published: March 2023

The Late Osage to Meramec Arroyo Penasco Formation, 10-130 ft thick, rests on a peneplained surface of Precambrian rocks and is overlain unconformably by sediments of Pennsylvanian age. The Arroyo Penasco Formation crops out in the San Pedro, Nacimiento, Jemez, Sandia, Manzanita, Manzano, and Sangre de Cristo Mountains of north-central New Mexico.

The basal unit, 2-60 ft thick, is transgressive and is composed of quartz conglomerate, sandstone, and thin shale. Three incomplete carbonate depositional cycles were recognized. The lowest, Cycle 1, consists of dolomite, dedolomite, and coarse-grained poikilotopic calcite with corroded dolomite rhombs. These rocks contain gray nodular chert with a microfauna of Late Osage age: Endothyra spinosa Chernysheva, E. skippae n. sp., and Septabrunsiina parakrainica Skipp, Holcomb, and Gutschick. The sediments of the earliest cycle show initial deposition as shallow-marine lime mudstone followed by stromatolitic intertidal to supratidal carbonate rocks.

Cycle 2 is shallow-marine to intertidal echinoderm wackestone to lime mudstone and dolomite containing a sparse fauna of Endothyra aff. E. spinosa Chernysheva, Endothyra aff. E. irregularis (Zeller), E. irregularis(Zeller), and E. spiroides Zeller of Early Meramec age. Cycle 3 is shallow-marine wackestone to arenaceous oolitic to ooid-echinoderm packstone ending as subtidal lime and mudstone to intertidal dolomite. The ooid facies contains a rich microfauna of Early to Middle Meramec age: Endothyra prodigiosa Armstrong, E. macra Zeller, E. irregulari Endothyra, E. (Zeller) aff. E. omphalota Rauser-Cernoussova and Reitlinger, and Tournayella sp.

Late Mississippian and Early Pennsylvanian uplift and erosion resulted in extensive erosion and removal of the Arroyo Penasco Formation. A solution limestone collapse breccia, 5-30 ft thick, rests on smooth surface of stromatolite dedolomite in the Sangre de Cristo Mountains. The breccia resulted from movement of meteoric ground waters in Late Mississippian or Early Pennsylvanian time. These dissolved a 5-30 ft thick gypsum bed and caused subsequent collapse of adjacent overlying Lower Meramec carbonate rocks. Solution activity was extensive, and sinkholes developed.

This study, hopefully, will provide a regional understanding of the biostratigraphic and facies relationship of the Mississippian strata in north-central New Mexico with the Leadville Limestone of the San Juan Basin and the San Juan Mountains of Colorado and the Mississippian carbonate rocks of south-central and southern New Mexico. The known outcrops of Mississippian strata in north-central New Mexico were examined and measured, and lithologic and micropaleontologic samples were collected at five-ft intervals.

The Mississippian and Devonian rocks of the San Juan Mountains of southwestern Colorado were examined and two sections carefully measured and studied: the classical section at Rockwood Quarry above Durango and the section at Davis' Creek in the Piedra River Canyon. The Kelly and Caloso Formations in the southern Ladron Mountains were also sampled and studied in thin sections. All shale samples were treated with sodium bicarbonate, washed and screened, and then picked under a binocular microscope for microfossils. A number of carbonate-rock suites were dissolved in formic acid, screened, and separated in heavy liquids for conodonts. The lithologic samples were cut for petrographic thinsections that were made for carbonate and foraminifera studies.