PRELIMINARY PROVENANCE IDENTIFICATION OF MUDROCK WITHIN THE POJOAQUE MEMBER (MIOCENE) OF THE TESUQUE FORMATION IN THE EASTERN CENTRAL ESPAÑOLA BASIN, NEW MEXICO

The Española Basin is a continental rift basin within the Rio Grande Rift located in north-central New Mexico bordered by the Jemez Mountains and Sangre de Cristo Mountains, west and east respectively. This study focused on the Pojoaque Member (Miocene) of the Tesuque Formation in a rich vertebrate fossil locality near Espanola, NM. During deposition this area was part of the gently sloping plains directly west of the Sangre de Christos. Immediately to the northeast was the Penasco Embayment, a broad shallow bay reaching far to the east of this locale. Previous authors designated two paleodrainage systems based on petrographic, lithologic, and paleocurrent analyses of coarse clastics, which resulted in the identification of two Lithosomes: A and B. The source area of Lithosome A was attributed to the Precambrian-core Santa Fe block of the Sangre de Cristo Mountains, while some contributions for Lithosome B sediments came from the Latir volcanic fields to the northeast.

In the study area, the bulk of the Pojoaque Member is composed of mudrock with some channel sandstones, thin nonmarine limestones, and scattered interbedded tuff layers. Geochemical analyses (XRD and ICP-OES) were used to test the source areas for Lithosomes A and B by identifying distinct minerals from whole mudrock samples. Twenty-six samples were tested in two different labs. Chlorite and chlorite clay group minerals were present in several samples and others contained Vermiculite and trace amounts of Kaolinite, which could be weathered decay products of the original chlorites. These samples were attributed to Lithosome B during field collection. Considering basin geometry these sediments were possibly transported across the Penasco Embayment from the metamorphic regions of the emerging Picuris Mountains to the northeast. A small sediment contribution may also be from basalts far to the north; Mordenite, a clay mineral associated with weathering of mafic volcanic rock, was found in trace amounts in one sample. The changing sedimentation patterns have been attributed to tectonic or climactic changes. Analysis of one bed indicated strongly evaporative conditions were present during deposition.Using geochemical analysis to determine source areas and paleoenvironmental conditions is key to better understanding the extensive fossil deposits here.