geology of the la cienega area, santa fe county, n.m. · geology of the la cienega area, santa fe...

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