New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Petrographic analysis of Early Permian, nonmarine strata of the Abo Formation (and equivalent strata) reveal a wide range in plagioclase and K-feldspar percentages and highly variable feldspar alteration trends throughout northern, central, and southern New Mexico. Sandstone modal composition trends show a significant decrease in detrital K-feldspar and increase in plagioclase as you compare percentages from northern (P=53%; K=47%), central (P=85%; K=15%), and southern New Mexico (P=97%; K=3%). In addition, elevated occurrences of diagenetic albite are in higher abundance in these strata in central and southern New Mexico compared to northern New Mexico.

Feldspar alteration is common during the diagenesis of felsic, arkosic sedimentary strata and often involves the process of sodium metasomatism associated with deep basin brines. At depth in a sedimentary basin, the onset of albitization of K-feldspar occurs at temperatures between 60–70°C (near depths of ~2500 m) when formation waters transition out of the stability field of K-feldspar and begin to approach the stability field of albite. At the basin surface and at shallower basin depths, evaporative concentration of salts in the groundwater of a closed basin will yield Na-rich brines that can react with detrital silicates to develop authigenic albite. In locations and basin conditions where felsic sedimentary strata react with available saline fluids, silt- and sand-size K-feldspar and plagioclase are partially-to-completely replaced by albite at a specific range of depth and temperature conditions in a basin. Feldspar can also undergo hydrothermal diagenesis that occurs in part during interaction of subsurface strata with magmatic systems and associated hydrothermal fluids.

In order to better constrain the extent and degree of feldspar alteration in Permian strata throughout New Mexico, data were collected from three field transects that were selected based on geologic and tectonic criteria that included (1) near and far from Laramide and Rio Grande rift structures, (2) near and far from shallow rift igneous systems, and (3) near and far from the Permian paleoshoreline and younger Permian evaporite strata throughout New Mexico.

In addition to applying traditional point-counting methods to determine standard detrital modes (e.g., Q-F-L, Qm-P-K, Lv-Lm-Ls), during the point-counting process, photomicrographs of all feldspar counts are collected in order to better isolate the degree and range of alteration in plagioclase and K-feldspar grains from each field locality. For this added approach, after completing a count for a sample, each feldspar photomicrograph is imported into ImageJ (Java-based image processing program) where the degree of alteration can be initially imaged and then quantified into a percent-of-alteration/grain value for each sample. Preliminary image analyses reveal a wide range of alteration in both plagioclase and K-feldspar from these strata.

The primary goal of this study is to constrain the extent and degree of feldspar alteration in Early Permian strata throughout New Mexico and observe whether trends are more regional and likely related to normal diagenesis, or whether trends are more isolated and focused in regions of high heat flow and near faults associated with the Rio Grande rift and/or Laramide orogeny in New Mexico.