New Mexico Geological Society Annual Spring Meeting — Abstracts

We conducted detailed stratigraphic studies as part of a larger geologic mapping project to assess seismic hazards to the north-central to northeast portion of LANL. Units Qbt1g-Qbt3 of the Tshirege Member of the Bandelier Tuff are differentiated by the degree of welding, crystallization, weathering characteristics, and distinctive mineralogy. To better understand lateral variations in stratigraphy, we measured four stratigraphic sections and collected samples for petrographic and x-ray diffraction analysis. Nonwelded Qbt1g is vitric (50-70% glass). The top of Qbt1g is marked by a "vapor-phase notch", above which glass is altered to cristobalite, tridymite, and alkali feldspar in units Qbt1v through Qbt3. The non-welded unit Qbt1v-c ("colonnade") lies directly above the vapor-phase notch and has a distinctive columnar appearance, due to cooling joints and/or tectonic fractures where the unit is indurated by devitrification and vapor-phase alteration minerals. We hypothesize that the main cementing agents in Qbt1 v-c are cristobalite and Fe-oxide. Devitrification is recognized by axiolitic and minor spherulitic intergrowths of cristobalite and feldspar. Cristobalite content decreases as tridymite content increases in overlying non-to moderately-welded unit Qbt1v-u ("upper"). Qbt1v-u and moderately-welded Qbt2 contain tridymite, alkali feldspar, and lesser cristobalite as crystals in pore spaces, suggesting an increase in vapor-phase alteration. In the western portion of the study area, closer to the source of the Bandelier Tuff, the upper part of Qbt1v-u is welded together with unit Qbt2. In the eastern portion of the study area, nonI welded Qbt1v-u is overlain by partially to moderately welded Qbt2.