Late Paleozoic spatial and temporal subsidence patterns in intracratonic southwest Laurentia: Assessment of uplift, denudation, and demise of the Ancestral Rocky Mountains in modern south-central New Mexico

The Ancestral Rocky Mountains (ARM) represent an intraplate deformational event that resulted in a series of Precambrian-cored basement uplifts with adjacent basins that accumulated Pennsylvanian to early Permian strata. Tectonic models for the event are debated largely because of the lack of robust age control in the basin fill. In New Mexico, the ARM event resulted in a series of basins with some of the best biostratigraphic records across the orogenic province. The first part of this study utilizes published biostratigraphic and lithostratigraphic data to (1) reconcile the onset of subsidence by first accumulation of Pennsylvanian strata, (2) establish a period of peak subsidence estimated by maximum accumulation rate, (3) correlate estimated peak subsidence with the first appearance of arkose derived from the adjacent denuded Precambrian-cored block, and (4) demarcate synorogenic strata from Permian strata that are postorogenic. Results demonstrate that within New Mexico ARM basins, (1) onset was relatively synchronous, predominantly beginning in early Atokan time; (2) peak subsidence, while potentially younging southward, was relatively coeval in the Middle to Late Pennsylvanian; (3) first occurrence of arkose either predates the period of peak subsidence or is coeval with peak subsidence; and (4) early Permian strata across the study area onlap preexisting faults and folds, and/or form a buttress unconformity with Precambrian basement. Intracratonic western Laurentia experienced deformation consisting of N-NW trending Precambrian-cored uplifts that are termed the ARM, which were structurally offset from adjacent basins. ARM tectonic models remain unresolved, and debate over tectonic origins has continued largely because definitive ARM structures are scarce and detailed quantitative subsidence studies are lacking. The Orogrande basin of modern southern New Mexico provides this key missing data lacking elsewhere in the ARM province because it features deformed strata with structures that are unequivocally of Pennsylvanian-early Permian origin. This second part of this study examines the tectonic subsidence of the Orogrande basin through backstripping analysis of three Pennsylvanian to early Permian sections along the basin margin adjacent to the Pedernal uplift of the ARM, and generated tectonic subsidence curves demonstrate initiation of basin subsidence, period of peak basin subsidence, and demise of the basin. Additionally, analysis of well-constrained folding and faulting enhances assessment of Orogrande basin tectonism. Magnitudes of tectonic subsidence are plotted against decompacted time units determined by conodont or fusulinid biostratigraphy to illustrate basin evolution over time. Results show pulses in tectonic activity, and a stepwise tectonic subsidence pattern. Subsidence along the eastern margin of the Orogrande basin was roughly synchronous, with a first pulse of subsidence lasting from late Morrowan through early Desmoinesian time. Following a quiescent subsidence pulse, geohistory curves indicate a second intense subsidence pulse from late Desmoinesian through Missourian time. Virgilian to early Wolfcampian time marks a transition to a different phase of basin evolution, with faulting and folding of Orogrande basin strata evident during this interval. By mid-Wolfcampian time, classical ARM tectonism had ceased in the region, and the Orogrande basin and former Pedernal uplift subsided in unison to accommodate early Permian sediment deposition. Peak tectonic subsidence intervals were mid-Missourian in the northern part of the study area and early Desmoinesian southward. The geometry of tectonic subsidence curves suggests that the earlier, intense pulses of classical ARM tectonism here represent flexural foreland basin development in response to a migrating tectonic load. In conjunction with structural data for the eastern Orogrande basin shelf, results suggest that east-west shortening produced folding and faulting evident in Pennsylvanian strata to destroy this shelf as a depocenter. This occurred in response to northeast directed convergence along the southwestern margin of Laurentia. Early Permian subsidence appears epeirogenic and is not related to the Pennsylvanian phases of tectonism that represent ARM deformation in the region. The third part of this study uses modern analytical techniques to enhance understanding of the conflicting ARM tectonic models by quantifying uplift and erosion/denudation of the Pedernal uplift, specifically through examining provenance of uplift-derived siliciclastics. The deposition of eroded clastic material on the biostratigraphically well-dated eastern shelf of the Orogrande basin, and known thicknesses of eroded strata, allow erosion rates of the uplift to be calculated. This is facilitated by defining an unroofing sequence through identification of reverse stratigraphy. Three measured sections along strike of the basin margin adjacent to the uplift are utilized to assess lateral variation in uplift denudation. Samples from these three sections were subjected to modal petrographic analysis and whole-rock geochemical analysis in order to assess clastic sediment provenance and define sediment unroofing as a result of ARM tectonism. Both petrographic and geochemical results reveal an unroofing sequence present in the Pennsylvanian stratigraphy of the Sacramento Mountains, and unroofing of uplifted earlier Paleozoic cover strata to the Precambrian core of the Pedernal uplift by the time of deposition of the Beeman Formation in the Late Pennsylvanian. Erosion rates calculated for the Pedernal uplift range from 70.6 to 76.1 m/my, comparable to modern Colorado Plateau erosion rates. Characterizing the clastic sediment of the eastern basin margin, in conjunction with erosion rates, indicates the paleogeography of the Orogrande basin and Pedernal uplift as this part of the ARM province evolved during the Pennsylvanian.

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