New Mexico Geological Society Annual Spring Meeting — Abstracts

Lithologic similarities between the uppermost sandstones of the Morrison Formation (Jurassic fluvial), the Lytle Sandstone (Cretaceous fluvial), and the Mesa Rica Sandstone (Cretaceous fluvial) have led to stratigraphic confusion in northeastern, New Mexico. Each unit is composed of recycled sediments and is classified as quartzarenite.

Petrographic point counts indicate subtle differences in framework composition between the three units. The uppermost Morrison sandstones contain slightly more feldspars and show greater variability in relative framework concentration. However, considerable overlap among samples prevents this from being a useful criterion in distinguishing one unit from another. Heavy and trace mineral occurrences, were also studied and appear useful ina semiquantitative way in distinguishing between units. Heavy minerals which may prove useful include opaques (hematite, magnetite,ilmenite), zircons, hornblendes, pyroxenes, epidotes (Morrison only) and glaucophane (Mesa Rica only). Matrix calcite was also observed in the Morrison Formation, but not in the overlying Cretaceous sandstones. X-ray diffraction analysis revealed that the predominant clay type in all three units is kaolinite, with lesser amounts of smectite/illite in the Morrison and Mesa Rica. Textural differences (grain size, sorting, rounding) are also subtle and often overlapping. Quartz overgrowths are present in all three units, but are more abundant in the Lytle (where they are sometimes cherty) and in the Mesa Rica than in the Morrison.

In general, no single petrographic criterion is suitable forI distinguishing between these three units, due to overlap between the units. However, several criteria may be combined in a general way to characterize these units.