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.

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 Sims and Murrell (unpublished) have obtained a well defined isochron age of 79 ka using the U-series method (Fig. 3).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.

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.

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

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.