New Mexico Geological Society Annual Spring Meeting — Abstracts

Our research in the Tesuque quadrangle, Santa Fe County, New Mexico has produced insights to controversial stratigraphic and structural relationships along the Sangre de Cristo Mountain front.

Volcaniclastic strata of the Bishop's Lodge Member have been variously placed within or unconformably below the Tesuque Formation. Our mapping clearly substantiates early workers' choice of including these strata within the Tesuque. The volcaniclastic layers are interbedded with and overlie characteristic arkosic, conglomeratic sandstone of the Namb6 Member ofthe Tesuque. Within the older, arkosic strata of the Nambe Member lie several olivine basalt flows, likely similar in age to a nearby 24.9 ± 0.6 Ma basalt (Baldridge et al.; 1980, EPSL v. 51, p. 30921). We submit that this relationship is evidence that subsidence of the Espanola Basin was underway prior to 25 Ma and that the Bishop's Lodge Member correlates regionally to the Oligocene-lower Miocene Abiquiu and Picuris Formations. Previous restriction of the Nambe Member to the middle Miocene overlooked the presence of diagnostic fossils only in the upper part of the member.

The contact between the Tesuque Formation and the mostly Proterozoic rocks of the Sangre de Cristo Mountains is both faulted and depositional. Field and geophysical data do not support a single, range-bounding fault with possible Quaternary movement, an interpretation proposed by some workers for the eastern margin of the Espanola basin. Near Namb6 Lake, the Tesuque Formation is clearly in depositional contact upon the Paleozoic and Proterozoic rocks. Farther south, a fault along the contact juxtaposes granite and Tesuque Formation along one segment, but offsets different beds of the Nambe Member along another, demonstrating that the mountain-front faults are of comparable magnitude to the myriad, minor faults found farther west into the basin. Most Pliocene and Quaternary stream deposits close to the mountain front have not been offset, precluding significant Quaternary fault movement.