Paper No. 7
Presentation Time: 2:55 PM


KONING, Daniel J., New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, BROXTON, David E., Earth Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 and JOHNSON, Peggy S., New Mexico Bureau of Geology, 801 Leroy Place, Socorro, NM 87801,

We investigate the relation between Plio-Pleistocene tectonic activity and sediment preservation in the largely west-tilted, southern Española Basin (SEB) of the Rio Grande rift, including the Santa Fe embayment (SFE). Here, an extensive angular unconformity underlies two coarse-grained, interfingering, clastic units: the Puye Fm (5.3-1.6 Ma), which includes an axial river facies but mostly consists of sediment shed from volcanic highlands west of the SEB, and the Ancha Fm (3?-1.3 Ma) to the southeast, eroded from the Sangre de Cristo Mtns. Tectonic subsidence in the hanging wall of the Pajarito fault (PF) near Los Alamos created a ~6 km-wide half-graben that preserved 300-600 m of Plio-Pleistocene deposits, including 332+ m of Puye Fm that thins to ~100 m towards the eastern margin of the half-graben. The Puye Fm thickens southward (from 98 to 274 m in 6 km) over a south-dipping ramp in the northern half of the half-graben. Puye Fm and interbedded basalts are subhorizontal and 150-300 m thick east of the half-graben, where the unconformity exhibits erosional relief. This package pinches out 19-22 km west of the PF, in an area where the unconformity is slightly tilted 0.5-2.0° W.

In its western extent, 4-30 m of Ancha Fm underlies exposed 2.7-2.4 Ma basalt flows, but it thickens eastward to 70-90 m in a synclinal axis near the center of the SFE. The synclinal axis parallels the La Bajada fault (LBF) and its western limb formed by flexural uplift along the LBF (Baldridge et al., 2001). The presence of 5-30 m-deep paleovalleys at the base of the Ancha Fm indicates erosion prior to ~3 Ma, when much of its corresponding area may have occupied the distal hanging wall ramp of the Santo Domingo Basin (SDB) to the southwest. Smith et al. (2001) interpret that the SDB "see-sawed" from west- to east-tilted ca. 2.6 Ma, after which faulting was concentrated along the LBF on its east side. This change likely enhanced tectonic subsidence along the syncline axis and explains the thickening and younging of the Ancha Fm eastward from the LBF into the syncline axis. West of Santa Fe, the NW strike of the Ancha Fm base contrasts with NE strikes of underlying Miocene strata. Other observations support the interpretation that Miocene structures in the SFE, such as the San Isidro Crossing fault and Rancho Viejo hinge zone, were relatively inactive in the Plio-Pleistocene.