GEOLOGY OF THE LA CIENEGA AREA, SANTA FE COUNTY, N.M. Daniel Koning and Peggy Johnson
N.M. Bureau of Geology and Mineral Resources
GOALS OF TALK
• Describe geology of the La Cienega area, with much emphasis on the Ancha Fm
• A topic in next presentation: How does the geology directly influence the locations of springs and seeps in the La Cienega area?
Three rock packages:
Younger (late rift)
Middle (main rifting)
Older bedrock (pre-rift)
Three rock packages:
Younger (late rift)
Middle (main rifting)
Older bedrock (pre-rift)
Galisteo Formation
•Deposited in a basin formed during E-NE compression 55-37 million years ago.
•Composed of reddish, cemented sandstone, mudstone, and pebbly sandstone
Galisteo Basin
Three rock packages:
Younger (late rift)
Middle (main rifting)
Older bedrock (pre-rift)
Photos of lithosome E, volcaniclastic sediment
Three rock packages:
Younger (late rift)
Middle (main rifting)
Older bedrock (pre-rift)
The Ancha Fm is a good aquifer – when it is wet!
Base of Ancha Formation – a scoured unconformity; commonly cemented over bedrock
Alluvial slope deposits in Ancha Fm – lateral textural changes
downstream
W E
Proximal alluvial slope deposits in Ancha Fm (near Sangre de Cristo Mtns)
Exposure located along Lamy-Santa Fe railroad south of Eldorado. UTM coordinates of base of section: 3927340 N, 417770 E (NAD 83, zone 13)).
Distal alluvial slope deposits in Ancha Fm (southeast of La Cienega)
On La Cienega on-ramp to I-25. UTM coordinates of base of section: 934548 N, 398323 E (NAD 27, zone 13).
Santa Fe River deposits in Ancha Fm – lateral textural changes
downstream
W E
Outcrop of Santa Fe River deposits of Ancha Fm overlying lithosome S of the Tesuque Formation
Fining-upward trend in alluvial slope deposits of Ancha Fm
• The base of the Ancha Formation was mapped using interpretations of lithologic data from cores, cuttings, geophysical logs, exploration and water well logs, and outcrop exposures.
• The elevation of the Ancha Formation's base in wells and drill holes was calculated by subtracting the base's depth pick from the surface elevation.
• Base elevation contours were interpolated from point data using a kriging function in ArcGIS, and smoothed by hand.
• Saturated thickness estimates were calculated from a subset of wells that:
Thickness trends of the Ancha Formation
Methods
1) penetrate or nearly penetrate the base of the formation;2) location is well-constrained3) have an interpretable lithologic record4) have a reliable water level
• 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.
• To the south, piedmont deposits are generally 60-150 ft thick but thicken to 200-295 ft in the center of the embayment.
• Thickest strata lies near the Santa Fe embayment syncline, suggesting that tectonic subsidence accounts for the thicker preservation there.
• Ancha Formation also thickens over paleotopographic lows and in paleovalleys created by pre-Ancha erosion.
Thickness trends of the Ancha Formation
Results
1 mile
N
Elevation contour map of Ancha Fm base (ft above MSL)
La Cienega
1 mile
N
La Cienega
Isopach map of Ancha Fm (in ft)
1 mile
N
Ancha Fm saturated thicknesses (in ft)
La Cienega
1 mile
N
Formations lying beneath Ancha Fm
La Cienega
Tesuque Fm, lithosome S
Tesuque Fm, lithosome E interbedded with basalts
Espinaso Fm bedrock
Espinaso
Fm
Galisteo Fm bedrock
Geology of the La Cienega area
Take-away points:
•Ancha Fm thins to SW over much less permeable bedrock
•Ancha Fm is thicker in paleovalleys, where it is commonly saturated in its lower part
