UNROOFING SEQUENCE IN THE OROGRANDE BASIN: PRELIMINARY ASSESSMENT OF THE RATE OF DENUDATION OF THE PEDERNAL UPLIFT, SOUTHERN NEW MEXICO

Unroofing sequences are valuable stratigraphic tools to assess the denudation of ancient mountains. Here we provide modal petrographic data that demonstrates inverted stratigraphy in sandstone beds of the Orogrande Basin of New Mexico; namely, inversion of lower Paleozoic strata (quartz-rich), the Mesoproterozoic Debaca sequence (metamorphosed sedimentary-volcanic and gabbroic), and basement granite (potassium feldspar-rich). Pennsylvanian strata of the Orogrande Basin are exposed in the modern Sacramento Mountains of southern New Mexico and preserve limestone beds with biostratigraphically well-dated successions. Coupling the thicknesses of these units with constrained chronology allows assessment of the erosion rate of the Pedernal Uplift, adjacent to the Orogrande Basin.

Modal petrographic analysis was conducted via the Gazzi-Dickinson method on sandstone samples collected from a stratigraphic section in the Sacramento Mountains. Three distinct units with varying modal clast compositions were defined in the late Paleozoic Gobbler and Holder formations. The lowest is a quartz-rich interval (Atokan-lower Missourian) with rare feldspar that approximates lower Paleozoic sandstone compositions. The middle interval (lower Missourian-middle Missourian) is denoted by an abrupt increase in plagioclase with rare volcanic clasts inferred as Debaca-sourced. The upper interval (post-middle Missourian) is indicated by a strong increase in potassium feldspar content and inferred as the eroded granite core of the Pedernal Uplift.

Dividing thickness and duration of the lower two intervals produces accumulation rates that are also inferred erosion rates if the thicknesses of the lower Paleozoic and Debaca sequence did not vary. However, the Debaca sequence varies in thickness, and the calculated minimum age range for the lower and middle units is 41 m/my and 570 m/my respectively. Study data indicates that lower Paleozoic sandstone was atop the Pedernal Uplift in the Atokan, but by the Missourian the Debaca sequence was exhumed at the surface. The Missourian presents a drastic increase in erosion rate, and may imply coeval peak regional subsidence. After the middle Missourian, the granitic core of the Pedernal Uplift was exhumed and subsequent erosion/accumulation rates remain unconstrained.