GEOCHEMICAL CHARACTERIZATION AND MINERAL COMPOSITION OF DEVONIAN AND MISSISSIPPIAN CARBONATES, SACRAMENTO MOUNTAINS, NEW MEXICO

The Sacramento Mountains in southern New Mexico preserve the record of equatorial, shallow-marine, carbonate-dominated deposits of the Tobosa ‘shelf’ during the middle and upper Paleozoic. Numerous recent studies have focused on the carbonate stratigraphy of Mississippian ramp deposits, but little work has focused on temporal changes in mineralogy, geochemistry, and organic matter content in this succession. This study presents initial results of a larger study focused on understanding temporal changes in mineralogy and organic matter content during the deposition of carbonate ramp deposits. A 21-sample transect that spans the Devonian and Mississippian was collected in Alamo Canyon, with samples from the Sly Gap, Caballero, and Lake Valley formations. We are particularly interested in temporal changes within the Lake Valley Formation, and thus sampled several bed types and depositional elements within each member (Andrecito, Alamogordo, Nunn-Tierra Blanca, Arcente, and Doña Ana). To constrain the mineralogy, total organic content (TOC), and paragenesis of each sample, we conducted Leco, scanning-electron-microscope energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and x-ray fluorescence (XRF) analyses.

According to XRD, calcite (CaCO3) is the main mineral almost in all samples accounting for 67-99 % of all minerals which is also consistent with the results of SEM and XRF analysis where oxygen, calcium and carbon are shown to be the main elements present in all samples. Three samples derived from Devonian black shales contain higher concentrations of quartz (SiO2, 30-40%), dolomite (CaMg(CO3)2, 23.9-51.3%), and potassium feldspar (10-15.7%), which are supported by the results of XRF analysis. Interestingly, Lake Valley samples with only 1-3% clay show weathering patterns quite different from pure carbonates, but are not as ‘argillaceous’ as they seem in outcrop exposure.

According to Leco analysis, extremely low TOC (<0.5) and S1(<0.25) values were observed in all samples except a black shale in the Devonian Sly Gap Formation has relatively higher TOC (2.7%) and Tmax (458C) values. Extremely low hydrogen Index (HI) values ranging from 0 to 20 attest to the absence of hydrogen in all samples whereas high values of Oxygen Index (>50) in all samples indicate thermal immaturity, which is consistent with low Tmax values. High-resolution gas chromatography shows oxic and marine conditions for most deposits, with the exception of the Devonian black shale that has evidence of anoxia. Ongoing analysis is focused on contextualizing these results with the observed depositional architecture as well as temporal changes in relative sea level, paleo-oceanographic conditions, and sediment supply to the Tobosa shelf. We anticipate that these results will improve the regional understanding of intrinsic and extrinsic controls on sediment routing and deposition on carbonate ramps, and lead to better predictive modeling of carbonate systems around the world.