NM Dept. Logo New Mexico Bureau of Mines & Mineral Resources Bulletin 149
Field-trip guide to the geochronolgy of El Malpais National Monument and the Zuni-Bandera volcanic field, New Mexico

0.0 Best Western Motel The Inn, Grants, New Mexico. Turn left from the parking lot at The Inn and proceed west along old Highway 66 through the town of Grants. During the uranium "boom" days the Grants area was a major producer of uranium. The first discovery of uranium in this region was in 1950 in the Jurassic Todilto Formation exposed in Haystack Butte west of town. With the declining demand for uranium the economy of this region has suffered and the population of Grants has dropped. Formation of the new El Malpais National Monument is helping the local economy by drawing more tourists to the region.
0.9 Note prairie-dog town on right-hand side of the street. Residents of Grants feed them with stale vegetables from the local supermarkets.
1.6 El Malpais National Monument Tourist Center.
2.4 Grants Mining Museum on Iron Avenue. The museum provides an outstanding perspective on the history of uranium mining in the Grants area.
3.4 Proceed west on NM 122 toward Milan. Black Mesa, on the right-hand side of the road, is one of the low mesas that surround Mount Taylor. The basal flow capping the mesa has been dated at 2.57 ±0.13 Ma (Laughlin et al., 1993). Go through the town of Milan. Continue past the intersection of NM 605 (6.0 mileage) on NM 122. El Tintero cinder cone is at 2 o'clock. This cone is the source for the Bluewater flow. Beyond El Tintero is Haystack Butte where the first uranium discovery was made.

Mileage 13.0. STOP 1: Bluewater flow
Lat. 35° 15.80' N
Long. 107° 58.23' W

The source of the Bluewater flow is the El Tintero cinder cone (Fig. 2) about 20 km (12.4 mi) west of Grants. Except where it has been quarried for road material, the El Tintero is well preserved and maintains its original shape. Original surface features are also well preserved on the Bluewater flow except where it is covered by eolian sands and silts.

FIGURE 2.—El Tintero cinder cone is the source for the Bluewater flow. Cinders are being quarried in this site for use as road material. A late, stubby basalt flow from El Tintero can be seen in the foreground.

The Bluewater flow is a tholeiite, very similar in composition to the Laguna and McCartys flows (Table 1). Most samples of the flow are holocrystalline and microporphyritic with both olivine and plagioclase present as phenocryst phases. The groundmass is dominated by plagioclase and clinopyroxene with minor opaque oxides and perhaps some olivine.

TABLE 1—Representative chemical compositions of dated basalts.

BluewaterLaguna McCartysTwin CratersBandera Ramah NavajoFence LakeNorth Plains


Attempts to date the Bluewater flow have been made using three methods: conventional K-Ar, U-series, and the 3He surface exposure dating technique. All samples used for dating were collected near Stop 1. Convention al K-Ar dates of 5.69±0.12 and 2.23±0.24 Ma were obtained on two different samples collected by Laughlin and Perry, respectively. Based on the degree of preservation of this flow and its position on the present valley floor, these apparent ages are believed to be anomalously old because of the presence of excess 40Ar, i.e. 40Ar incorporated into minerals from gases in the magma. Sims and Murrell (unpublished) have obtained a well defined isochron age of 79 ka using the U-series method (Fig. 3). Poths (Los Alamos National Laboratory) has also dated two samples from this outcrop using the 3He surface dating method. The average for these samples was 57±6 ka (uncertainty in production rates is ±30%). During crushing of olivine, Poths has detected large amounts of excess 40Ar in olivine and clinopyroxene from the Bluewater flow.

FIGURE 3—230Th/232Th versus 238U/232Th for the Bluewater flow from the Zuni-Bandera field on the Jemez lineament in New Mexico. The isochron, shown by the dashed line, gives a 2σ age of 79 ka+40 ka/-30 ka.

Continue west on NM 122.

13.1 Turn left on access road (NM 606) to Exit 72 on Interstate 40.
13.3 Return to Interstate 40 and proceed east.
23.8 Zuni Canyon flow is on the right. This flow, which originated from the Paxton Springs volcano, is an alkaline basalt. Based upon the degree of surface preservation, it is older than the Bandera flows and younger than both the Bluewater and Laguna flows. Although this flow has not been dated, there are several potential sites to look for charcoal beneath the flow within Zuni Canyon. The small hill on the right surrounded by lava is the Chinle Formation. It is capped and ringed by travertine deposits of Quaternary age.
27.1 There are high-mesa basalts to the north on the southern flank of Mount Taylor; these range from 3.7 to 2.9 Ma (Perry et al., 1990; Laughlin et al., 1993). We are now traveling across the Laguna flow which has been dated at 110-128 ka (Laughlin et al., 1993).
30.8 Exit Interstate 40 at the NM 117 interchange toward Quemado (Exit 89), cross over the interstate and proceed east on frontage road (NM 124) to the right. McCartys flow is on the left. Geomorphic features of the McCartys flow were first described by Nichols (1946), and Carden and Laughlin (1974) described the chemical variations along the length of the flow.

Mileage: 32.0. Stop 2: Laguna flow
Lat. 35° 04.53' N
Long. 107° 45.22' W

The Laguna flow (Qbc of Maxwell, 1986) is well exposed in the Rio San Jose valley east of Grants (Fig. 4). According to Maxwell, it originated from the El Calderon volcano at the southern end of the Zuni Mountains. From its source it flowed northward around the east side of the Zuni Mountains before it turned eastward to flow down the Rio San Jose valley. Within the valley it is overlain in places by the McCartys basalt. Drake et al. (1991) concluded from water-well data that it can be traced eastward down the valley beneath alluvium to the vicinity of the Laguna Pueblo, where it is again exposed.

FIGURE 4—This photo shows the ~3 ka McCartys flow overlying the older (110-128 ka) Laguna flow near the intersection of Interstate 40 and NM 117 about 6 mi east of Grants.

The Laguna flow is an olivine tholeiite (Table 1). It is typically holocrystalline, but small amounts of glass containing opaque oxides are locally present. All thin sections show olivine phenocrysts in a groundmass of plagioclase, clinopyroxene, olivine, and opaque oxides. Sample 63, collected where the flow is relatively thick, is characterized by an ophitic texture with plagioclase and clinopyroxene, as well as olivine appearing as phenocryst phases.

Several attempts have been made to date the Laguna flow by the conventional K-Ar method (Laughlin et al., 1979, 1993; Lipman and Mehnert, 1979; Champion and Lanphere, 1988) with ambiguous results. Laughlin et al. (1979) reported an apparent age of 1.57±0.26 Ma for a sample collected along Interstate 40 west of the intersection with NM 117. Because it was believed that this age was anomalously old, the flow was resampled at two new sites. Laughlin et al. (1993) report ages of 0.054±050, 0.110±0.076, and 0.120±0.73 Ma for these samples (sample 63 was analyzed twice) (Table 2). Lipman and Mehnert (1979) reported an apparent age of 0.28±0.25 Ma for the flow near Laguna Pueblo, and Champion and Lanphere (1988) reported an apparent age of 0.128±0.033 Ma for a sample collected by Laughlin at Stop 2.

TABLE 2—K-Ar dates on second pulse basalts in the Zuni-Bandera volcanic field.

Sample IDLocation MaterialK2O
(10-12 mg/)
Age (Ma)

B-1-74West of Bandera CraterBasalt1.320.37870.199±0.042
BR-2-74Black Rock Zuni PuebloBasalt0.480.48310.70±0.55
153Black Rock Zuni PuebloBasalt0.590.13960.164±0.035
AWL-2-89Cerrito ArizonaBasalt0.980.20910.148±0.087
AWL-6-90East of El MorroBasalt0.480.0760.50.109±0.044
AWL-10-80Laguna FlowBasalt1.130.0880.20.054+0.050
63Laguna FlowBasalt0.840.13210.110+0.076


Return to Interstate 40/NM 117 intersection and proceed south on NM 117.

43.4 Turnoff to Sandstone Bluffs overlook.

Mileage 45.8. STOP 3: McCartys flow at "Little Narrows"
Lat. 34° 56.01' N
Long. 107° 50.33' W

The McCartys flow is the youngest basalt flow in the Zuni-Bandera volcanic field. Its source is a low shield volcano located about 40 km (24.8 mi) south of the intersection of Interstate Highway 40 and NM 117. A small cinder cone about 8 m (26 ft) high sits on top of this broad shield. Although some of the lava flowed southwestward 8 to 9 km (5 to 5.6 mi), most followed the preexisting drainage and flowed northward about 40 km (24.8 mi) before turning eastward 10 km (6.2 mi) down the Rio San Jose valley. The McCartys flow overlies older basalts of the Zuni-Bandera volcanic field and Holocene alluvium.

The McCartys flow is typically a vesicular, porphyritic basalt. Carden and Laughlin (1974) examined chemical and petrographic variations along the length of the flow and reported that within 4 km (2.5 mi) of the source the basalt is characterized by plagioclase phenocrysts 0.20 to 1.5 cm in length. At greater distances from the source, large plagioclase phenocrysts are absent and olivine phenocrysts are present. Plagioclase is the dominant mineral in samples of McCartys flow.

Prior to the work of Laughlin et al. (unpublished), the age of the McCartys flow was poorly constrained. Nichols (1946) concluded on the basis of Indian legends and archeological and faunal evidence that the McCartys eruption probably took place after 700 A.D. During the summer of 1992, two charcoal samples were collected from a baked soil beneath the flow at the "Little Narrows," including burnt roots 1-2 mm in diameter (Stop 3, Figs. 5a, b, and 6). Accelerator mass-spectrometer radiocarbon dates of 2970±60 and 3010±70 years B.P., which yielded an average, calibrated age of 3160-3200 years B.P., were obtained on these samples. Poths has analyzed three aliquots of a sample of the surface of the flow using the 3He method. An average age of 2450 years has been obtained on these samples. An uncertainty in the production rate for 3He of ±30% may contribute to the discrepancy with the radiocarbon dates.

FIGURE 5a—Aerial photo of the eastern edge of McCartys flow. Stop is near the center of the photo. (click on image for an enlargement in a new window)

FIGURE 5b—This p ortion of the geologic map of Maxwell (1986) covers essentially the same area as the photograph in Fig. 5a. (click on image for an enlargement in a new window)

FIGURE 6.—Geologist Steven Reneau of the Los Alamos National Laboratory is collecting charcoal for 14C dating from the baked soil beneath McCartys flow.

Leave Stop 3 and return to The Inn in Grants via NM 117 and Interstate 40.

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Last Updated: 28-Mar-2006