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