EBTAG Annual Workshop and Field Trip
May 20-21, 2013

¹N.M. Bureau of Geology and Mineral Resources, N.M. Institute of Mining and Technology, 801 Leroy Place, Socorro, NM, 87801, dkoning@nmbg.nmt.edu

Because the local geology controls locations of springs and wetlands in the La Cienega area, we summarize the rock units found here and their stratigraphic relations. The older bedrock includes the Galisteo and Espinaso Formations (ca. 55-37 and 36-28 Ma, respectively). The Galisteo Formation consists of reddish, typically cemented sandstone, pebbly sandstone, and mudstone. The Espinaso Formation is composed of light gray, well-cemented sandstone and conglomerate interbedded with minor latite + andesite flows. Overlying these older units is a package of sedimentary strata (layered rock) called the Tesuque Formation (25-8 Ma),which was deposited in a subsiding basin during Rio Grande rift extension. These strata are less cemented than the older bedrock and are generally moderately to well consolidated. Deformation associated with basin subsidence caused the Tesuque Formation here to be tilted 5-8° to the east-northeast. In the La Cienega area, the Tesuque Formation contains two, partly interfingering, stratigraphic components. Lithosome E, on the west, consists of volcaniclastic sediment comprised largely of brown, gravelly sandstone, sandstone, and clayey-silty sandstone. It is interbedded with at least four volcanic flows of the Cieneguilla basanite (26-25 Ma). Lithosome S, on the east, is composed of arkosic sediment, mainly sandstone with subordinate mudstone and lesser conglomerate, eroded from the granite-cored Sangre de Cristo Mountains. Lithosome S progrades westward over lithosome E.

A younger, relatively non-tilted, sediment package called the Ancha Formation was laid down between 3 and 1.5 Ma, after a long period of uplift and erosion that produced up to 150 ft of local paleotopographic relief. The Ancha Formation is coarser, less consolidated, and less cemented than the underlying Tesuque Formation. These properties explain why Ancha Formation hydraulic conductivity values are one to four orders of magnitude higher than older strata. The Ancha Formation, whose lower contact is a scoured angular unconformity, thickens over paleovalleys created by pre-Ancha erosion. The paleovalleys are hydrologically important because they may hold thick accumulations of saturated Ancha sediment. We mapped two noteworthy paleovalleys, one of which parallels the modern Santa Fe River 1-2 mi to the southeast and another which trends west-northwest from Eldorado.

The Ancha Formation can be subdivided into two interfingering stratigraphic components. The first is associated with a southwest-flowing, ancestral Santa Fe River and is relatively coarse-grained in a downstream direction. Santa Fe River deposits are as much as 220 ft thick 1 mi south-southwest of Agua Fria, but thin westward to 60-100 ft under the Caja del Rio Plateau. North and south of the ancestral Santa Fe River deposits are alluvial slope deposits associated with smaller, west-flowing, ephemeral streams. The northern alluvial slope deposits pinch out northward and northeastwards against the Santa Fe uplands. The southern alluvial slope deposits are generally 60-150 ft thick but thicken to 200-295 ft about 5 mi east of La Cienega. The lower 30-130 ft of the southern alluvial slope deposits is generally coarse-grained (sand and gravel). In overlying strata, sandy gravel dominates near the Sangre de Cristo Mountain front but gross texture fines westward to clayey-silty fine sand.