New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping and sedimentologic study of the Tesuque Formation in the Espanola 7.5-minute quadrangle elucidates middle Miocene (~11-16 Ma), rift-related deposition and tectonics in the central Espanola Basin. This work also demonstrates the utility of lithosomes A and B of Cavazza (1986). Lithosome B consists of clastic sediment containing a mixed assemblage of gravel (sandstone, siltstone, limestone, quartzite, tuff, and granite) derived from the northeast across the Penasco Embayment. Lithosome B was deposited by a relatively large, broad fluviatile system that trended south-southwest on the basin floor. Near the eastern quadrangle boundary, Lithosome B interfingers with, and grades into, piedmont deposits of sand, silt, and granite-bearing gravel (Lithosome A) derived from the local Sangre de Cristo Mountains to the east. Mapping of the contact between these two lithosomes and study of well cuttings indicate that this contact was probably near the eastern quadrangle boundary for at least 1 m.y. prior to the deposition ofthe No.4 White Ash (15.3-15.5 Ma). After 15.3 Ma, this contact shifted more than 7 km to the west throughout the remainder of the middle Miocene (11-15 Ma) as the piedmont (Lithosome A) alluvium prograded westward. It is assumed that such a significant progradation was due to either a climate change or a change in rift tectonic style or rate. Westward progradation probably occurred slightly before a sharp marine δ¹⁸O increase at 13-15 Ma. This observation and comparison of approximate accumulation rates suggest that a change in rift tectonism was at least partly responsible for this extensive and prolonged progradation. This change in tectonism may have influenced rift volcanism, which perhaps explains the deposition of coarse, locally derived basaltic lapilli shortly after 13.7 Ma.