RHYOLITE VOLCANISM IN THE TRANS-MEXICAN VOLCANIC BELT WITH IMPLICATIONS FOR GROUNDWATER FLOW IN THE VILLAGE OF GARBANZO

Our research is part of an interdisciplinary service project to locate the best potential site for a culinary water well to be drilled in the village of Garbanzo, which is located within the Trans-Mexican Volcanic Belt, 23 km NE of Irapuato. Studying the lithology of the area allows for a better understanding of groundwater flow. Over 95% of the rocks in the area surrounding Garbanzo are rhyolite, with the remainder being vesicular olivine basalt. The absence of intermediate rocks indicates that the area experienced bimodal volcanism. Our mapping revealed a steeply dipping fault with a strike of approximately N80E that was not visible from aerial photography. The high ground north of the fault, including the northern half of Garbanzo, is underlain by white to light-gray rhyolite with abundant geodes; we refer to this unit as the “geode rhyolite.” It is rich in rounded sanidine and rounded and embayed quartz phenocrysts and is highly porous due to the geode cavities. Garbanzo sits on the rim of a steep 40m deep valley cut by a nearby stream. The bottom of the stream valley consists of an exposure of white to light-gray rhyolite that is almost completely silicified and contains few vugs, which greatly reduces its porosity. This rhyolite constitutes the bulk of the rock around Garbanzo, and despite having fewer sanidine phenocrysts than the geode rhyolite, we interpret it to be the lower portion of the geode rhyolite. It is flow banded, and flow-band layers are folded into gentle to tight folds with heights up to 5m. The groundmass texture is not pyroclastic. These structures and the groundmass texture lead us to interpret the geode rhyolite as a thick lava flow. The geode rhyolite also occurs south of the fault, although it has been downdropped relative to the geode rhyolite to the north and underlies four other much thinner volcanic layers, including the olivine basalt, an unwelded rhyolite tuff, and two layers of welded rhyolite tuff. The fact that the geode rhyolite is much more porous near its top has potential implications for groundwater recharge because precipitation entering the more porous upper part can then slowly percolate through fractures in the lower layer.