EBTAG Annual Workshop and Field Trip
October 12-13, 2017

¹New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining & Technology, 801 Leroy Place, Socorro, NM, 87801, dan.koning@nmt.edu

The Santa Fe River flows over a variety of geologic formations exhibiting unique hydrogeologic characteristics. In the Sangre de Cristo Mountains, its headwaters are underlain by foliated granitoids and granitic gneiss. West of McClure reservoir, this granitic rock exhibits northwest-striking foliation and is interlayered with northwest-trending layers of amphibolite and granite. Between Nichols Reservoir and the western foot of the mountains, Pennsylvanian strata (mainly limestone, siltstone, and shale) is preserved overlying the crystalline bedrock -- both of which have been folded into NNE-trending antiforms and synforms. Fracture-flow, likely enhanced by brittle faulting, is the dominant groundwater pathway.

Between the mountain front and the town of Cieneguilla, the Santa Fe River flows on clastic strata of the Tesuque and Ancha Formations of the Santa Fe Group. The 26-13 (Ma) Tesuque Formation in this area thickens westward (to 1500 m) and has been gently folded into a NW-plunging syncline, whose axis is located 9 km SW of Santa Fe. Stratal dips decrease westward from 10-15° W near downtown Santa Fe to 5-6° W near Agua Fria. Near the river the Tesuque Fm is differentiated into lithosome S overlying thinner lithosome A, and its upper two-thirds coarsens upwards. Lithosome S consists of reddish sandstone and pebbly sandstone (with minor conglomerate, siltstone, and mudstone) deposited on a westward-sloping, large alluvial fan by an ancestral Santa Fe River (ASFR). Its gravel assemblage (mostly granitic with 3-40% Paleozoic rocks, 5-30% quartzite, 1-8% chert) indicates that its headwaters extended into what is now the upper Pecos River watershed. The tannish lithosome A is slightly finer grained than lithosome S, and its predominately granitic gravel assemblage indicates deposition on an alluvial slope by smaller drainages than the ASFR. In lithosome S, important considerations for hydrogeology include: 1) anisotropy due to coarse channel fills interbedded with finer-grained inter-channel deposits, both of which are laterally discontinuous and dipping; 2) local groundwater barriers caused by basin fill faults, which is especially evident at the San Isidro Crossing fault.

Overlying a scoured unconformity, the 3-1.3 Ma Ancha Formation is relatively thin (<80 m), undeformed and non-cemented. It can be divided into a coarser Santa Fe River lithofacies (near the present-day river) and finer alluvial slope lithofacies. Much of the groundwater flow in the Ancha Formation occurs near its base in relatively coarse gravel associated with buried valleys. The Ancha Formation is mainly found south of the river but is more extensive west of Agua Fria.

Near the town of La Cienega, pre-Tesuque Fm units include the grayish, largely volcaniclastic Espinaso Formation overlying the reddish, clastic sedimentary package of the Galisteo Formation. Uplifted to the surface on the east-dipping limb of the aforementioned syncline, these low-permeability units cause upwelling of west-flowing groundwater. This groundwater locally emerges from buried valleys of the Ancha Formation, creating seeps and wetlands.