Mileage 26.3. STOP 4: Twin Craters flow
Lat. 34° 59.51' N
Long. 108° 02.04' W

Because of problems in separating flows from different vents, Maxwell (1986) included as one unit, Qbt, flows from the Twin Craters and the Lost Woman and La Tetera (Tetra) vents. He considers these flows older than the Oso Ridge flows and younger than the El Calderon flows. The Twin Craters flow at this stop is fine-grained and microporphyritic with olivine phenocrysts in a groundmass of plagioclase, clinopyroxene, olivine, and opaque oxides. It is tholeiitic in composition (Table 1).

At this stop, the Los Alamos group was able to collect charcoal from the soil beneath the flow and overlying the Precambrian gneiss. This sample yielded a radiocarbon date of 15,800±90 years B.P. No other dates are available for this flow. After looking at the charcoal in the soil, climb on top of the outcrop to look at surface features of the flow. The surface features here appear much more degraded than the ~10 ka Bandera flow (Stops 5, 6), suggesting that the 15.8 ka age may be in error (anomalously young).

Mileage 29.8. STOP 5: Bandera Crater Trading Post
Lat. 34° 59.64' N
Long. 108° 05.22' W

The Bandera flows originated from the Bandera Crater, a double cinder cone about 150 m (492 ft) high and 1 km (0.6 mi) in diameter (Figs. 7a, b, and 8). The eruption of the Bandera Crater and its associated flows was the second youngest volcanic event in the Zuni-Bandera volcanic field. Like many other cinder cones in this field, the Bandera Crater is breached to the southwest. A large lava tube, intermittently collapsed, extends about 29 km (18 mi) south from the breach in the crater wall and a commercial ice cave is located in a collapsed portion of the tube near the Candelaria Trading Post. Causey (1971) recognized six stages in the development of the crater and its associated flows, culminating in the eruption of the black cinders that cap the cinder cone and blanket the hills to the north. Two small commercial cinder pits have been opened in the cinders covering the hills to the north of NM Highway 53 where the cinder blanket is thickest. A variety of crustal and mantle xenoliths and anorthoclase megacrysts have been found in these cinder pits (Laughlin et al., 1971, 1974; Gallagher, 1973).

FIGURE 7a—Aerial photo showing the numerous volcanic vents near Bandera Crater. (click on image for an enlargement in a new window)

FIGURE 7b—This portion of the geologic map of Maxwell (1986) covers the area shown in Fig. 7a. (click on image for an enlargement in a new window)

FIGURE 8—Aerial photo looking south toward Bandera Crater. Cerro Rendiga is the low shield volcano in the backgroun.

The Bandera lavas are nepheline normative, holocrystalline, microporphyritic, and vesicular near the surface. Both aa and pahoehoe surfaces are common on the flows. The whole-rock chemistry of a representative sample of the Bandera flows is given in Table 1.

Four dating techniques have been used to date the Bandera flows or to constrain their ages: conventional K-Ar, ¹⁴C, ³He, and ⁴⁰Ar/³⁹Ar. At the time of preparation of this road log only the ¹⁴C, ³He, and conventional K-Ar results were available. Two K-Ar dates (Laughlin et al., 1979, 1993) have been obtained on flows that immediately underlie flows from the Bandera Crater. These dates of 0.199±0.042 and 0.148±0.87 Ma (Table 2) provide maximum ages for the Bandera flows. A minimum age of 3166±77 years B.P. for the Bandera activity is provided by a radiocarbon date on a twig enclosed in laminated ice from the Candelaria Ice Cave, which is located in the main lava tube extending from the Bandera Crater (Thompson et al., 1991).

In May 1992 the U.S. National Park Service provided a backhoe to excavate through the cinders on the hillside north of the Bandera Crater in an attempt to find charcoal for radiocarbon dating. In the course of two days, 10 short backhoe trenches were excavated. The first seven trenches did not penetrate the cinders and it was only when the backhoe was moved about 200 m (656 ft) to the north of the present commercial cinder pit that the trenches encountered soil below the cinders (Fig. 9). The sample site is about 1 km (0.6 mi) northeast of the crater rim in the axis of a shallow swale, with an upstream drainage area of about 0.1 km². The bottom of the swale is presently unchanneled, accumulating fine-textured loamy soils derived from the adjacent sandstone slopes. The scoria deposit may also have mantled a shallow valley.

FIGURE 9.—This map shows the geochronology sampling sites near Banderia Crater. (click on image for an enlargement in a new window)

The stratigraphy at the charcoal sample site consists of 0.4-1.1 m (1.3-3.6 ft) of fine-textured cumulative soil overlying 0.9-1.2 m (3-4 ft) of scoria. The scoria deposits include two distinctly different layers, a 0.55-0.75 m (1.8-2.5 ft) thick upper layer of frothy scoria and a 0.35-0.50 m (1.1-1.6 ft) thick lower layer of rounded scoria with abundant lithic fragments. Beneath the scoria deposit are sandstone boulders with intervening pockets of sandy clayey soil up to about 0.30 m (1 ft) thick. The charcoal samples were collected from these pockets of soil among the sandstone boulders. Sample Beta-53845 consisted of charcoal within patches of darker soil that possibly represented burnt roots. The charcoal was separated in the laboratory by hand-picking. Sample AA-9075 consisted of small (1-2 mm) fragments of disseminated charcoal in the soil matrix. The charcoal was hand-picked in the field. Sample Beta-53845 yielded a radiocarbon date of 9170±70 years B.P. and sample AA-9075 gave an age of 9810±60 years B.P. Because of the possibility of sample contamination by modern rootlets not removed during sample preparation of Beta-53845, we conclude that the date of 9810 years B.P. provides the most reliable maximum-limiting age for the eruption of the Bandera cinders. This sample yields a calibrated age of 10,990 cal. years B.P.

Two samples of the surface of the Bandera flow were collected for ³He dating (Fig. 9). The first sample was collected along the west side of the trail from the Bandera Crater to the Ice Cave on the east side of the lava tube (Stop 5). The sample consisted of a slab of the pahoehoe surface of the flow. The second sample was an in-place block of aa lava from the west edge of the flow at the foot of Cerro Bandera (Stop 6). These samples yielded an average age of 10.4±1.2 ka.

A sample of the Bandera flow from near the vent (Stop 6) has been collected for U-series dating and both basalt and anorthoclase megacryst samples have been collected for conventional K-Ar and ⁴⁰Ar/³⁹Ar dating. No results are available as yet for these samples.

Two stops will be made at the Bandera Crater. At Stop 5, you can park in the parking lot of Candelaria Trading Post and, after paying the admission fee, walk first to the site where a sample of the pahoehoe surface of the Bandera flow was collected for ³He dating. This sample yielded an average (two aliquots) age of 10,500 years. Because the trenches through the cinders were refilled after sample collection, a stop will not be made at the cinder pit. From here proceed to the Ice Cave where the twig was collected from the laminated ice for radiocarbon dating.

Mileage 32.6. STOP 6: West side of Bandera flow
Lat. 34° 59.57' N
Long. 108° 05.48' W

Stop 6 is on the west side of the Bandera flow, at the base of Cerro Bandera (Figs. 8 and 9). At this site, Poths collected her second sample for ³He dating (12,500 years). A short walk across the aa surface of the flow will take you to the main lava tube from the Bandera Crater. Samples have been collected here for conventional K-Ar and ⁴⁰Ar/³⁹Ar dating. Results are not yet available for these samples.

Laughlin et al. (1993) report a conventional K-Ar date of 0.148±0.087 Ma on a basalt flow from Cerrito Arizona collected about 4 km (2.5 mi) southwest of this stop (Table 2). This flow is also from the second pulse of volcanic activity recognized in the Zuni-Bandera volcanic field.

Approximately 8 km (5 mi) south of this stop a plagioclase phyric basalt, "the Big Plag Basalt," is exposed south of Cerro Rendija. Stratigraphic relations suggest that eruption of this basalt should fall within the second phase (about 0.150 Ma) of volcanic activity. Conventional K-Ar dating, however, yields results of 3.7±0.4 Ma and 5.92±0.14 Ma. A very pure plagioclase separate, enriched in the phenocryst plagioclase component, was prepared to emphasize the effect of excess ⁴⁰Ar. This sample gave an apparent K-Ar age of 19.5±2.2 Ma (WoldeGabriel and Laughlin, unpublished data), suggesting a large amount of excess ⁴⁰Ar was incorporated in the phenocrysts during crystallization.

Mileage 54.1. STOP 7: Ramah Navajo flow
Lat. 35° 01.58' N
Long. 108° 24.51' W

This is a very brief stop to look at a basalt flow that has given an extremely anomalous apparent age of 7.65±0.08 Ma. It is a tholeiitic flow (Table 1), holocrystalline and equigranular. It consists of plagioclase, olivine, clinopyroxene, and opaque oxides. About two feet of eolian sands and silts cover the flow and only the tops of pressure ridges are exposed above the alluvial surface. This flow is probably coeval with the approximately 0.700 Ma flows of the North Plains (Stop 10) and the Fence Lake flow (Stops 8 and 9). Return to NM 53.

Mileage 103.2. STOP 8: North edge of Fence Lake flow
Lat. 34° 44.37 N'
Long. 108° 40.60'

The Fence Lake flow is one of the oldest and probably the largest basalt flow in the Zuni-Bandera volcanic field. Its vent, which cannot be identified, was probably located somewhere along the present Continental Divide where it is now covered by the Chain of Craters or flows from them. From this general area, lava flowed both to the north and to the west for distances of up to about 100 km (62 mi). North of the present town of Fence Lake, the lava was apparently confined by a preexisting drainage to a width of 2-3 km (1.2-1.9 mi). Twenty to 30 m (66 to 98 ft) of post-flow erosion has left the flow remaining as a prominent ridge. Further west, the lava turned and flowed southwestward into the valley of the Zuni River.

The Fence Lake flow is tholeiitic (Table 1) and is chemically very similar to basalts of the North Plains. Because of differences in flow thickness, there is considerable variability in the petrography of samples from this flow. Most samples are relatively coarse grained, however, with phenocrysts of olivine, plagioclase, and, in some cases, clinopyroxene in a ground mass of plagioclase, clinopyroxene, and opaque oxides. Where the flow is thick, the texture is commonly subophitic to ophitic.

The Fence Lake flow has been dated by both the conventional K-Ar and ⁴⁰Ar/³⁹Ar methods with mixed results (Table 3). Laughlin et al. (1979) reported a K-Ar date of 1.41±0.29 Ma for a sample collected from the north side of the flow (sample FL-3-74, Stop 8). This date was believed to be anomalously old and another sample was collected from the south side of the flow (AWL-4-90, Stop 9). A date of 0.184±0.036 Ma was obtained on this sample (Laughlin, Perry, Damon, and Shafiqullah, unpublished data). Because of the discrepancy between these two apparent ages, additional material was collected from the north side of the flow (sample AWL-14-91) and both conventional K-Ar and ⁴⁰Ar/³⁹Ar dates were run on both this sample and on sample AWL-4-90 from the south side of the flow. Results of these analyses are given in Table 3. We conclude that the age of the Fence Lake flow falls between 0.6 and 0.7 Ma.

TABLE 3—Conventional K-Ar and ⁴¹Ar/³⁹Ar dates for the Fence Lake flow and North Plains basalts.

Mileage 106.1. STOP 9: South side of Fence Lake flow

New exposures have been cut by the Highway Department while widening the road.

Mileage 136.9. Stop 10: South of Cerros de los Gatos
Lat. 34° 37.22 N
Long. 108° 14.72 W

These plugs may have served as vents for some of the North Plains basalts which are tholeiitic and generally similar to the Fence Lake flow in composition (Table 1). These flows have not been adequately mapped and, because of eolian sand cover, it is difficult to determine how many flows are present in the area. They comprise the southern part of the Zuni-Bandera volcanic field and are overlain in the north by the Hoya de Cibola flows and the McCartys flow. Their source vent or vents cannot be located because of probable burial by younger flows from the Chain of Craters and associated cinder cones. In many places these flows are covered by eolian sands and silts, and only the tops of pressure ridges protrude above the alluvium.

The North Plains basalts typically are relatively coarse-grained and porphyritic, with phenocrysts of both plagioclase and olivine. Groundmass constituents include plagioclase, clinopyroxene, opaque oxides and, in some cases, minor glass and olivine.

Several attempts to date these flows have been made using the conventional K-Ar method (Table 3). Ander et al. (1981) reported an age of 3.8 Ma for one of the North Plains basalts, but this date appears to be anomalously old. More recent dates fall between 0.593 and 0.724 Ma (Laughlin et al., 1993), suggesting that these flows are coeval with the Fence Lake flow.

Mileage 175.3. STOP 11: Sandstone Bluffs Overlook
Lat. 34° 58.53' N
Long. 107° 49.03' W

This stop provides an excellent overview of the younger basalts of the El Malpais National Monument. From this point you can see the McCartys flow in the foreground and, in the background to the west, older flows from the Hoya de Cibola, Bandera Crater, and El Calderon. A summary of the volcanic stratigraphy of the Zuni-Bandera volcanic field is presented in Table 4.

Table 4—Summary of Zuni-Bandera volcanic field stratigraphy.

This is the last stop. From here you may proceed north on NM 117 to Interstate 40 and then east to Albuquerque. As you drive east on the interstate, note basalts in the valley east of Laguna Pueblo. These flows are generally believed to be the distal end of the Laguna flow that were examined at Stop 2 and that originated from El Calderon. Detailed geochemical/geochronological/paleomagnetic investigations by the University of New Mexico/Los Alamos National Laboratory staff have been started to test the El Calderon/Laguna flow/Laguna Pueblo flow correlation.