Exit 149 to Paseo del. Volcan. We will now drive across the Llano de. Albuquerque. 5.2. Road crests at Ceja del Rio Puerco to begin its descent into the valley of ...
Lucas, S.G. and Spielmann, J.A., eds., 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40.
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FIRST DAY: MIDDLE AND UPPER TRIASSIC STRATIGRAPHY, SEDIMENTOLOGY AND PALEONTOLOGY OF WEST-CENTRAL NEW MEXICO SPENCER G. LUCAS, ANDREW B. HECKERT, JUSTIN A. SPIELMANN, LAWRENCE H. TANNER AND ADRIAN P. HUNT Assembly Point: New Mexico Museum of Natural History and Science, 1801 Mountain Rd. NW, Albuquerque, New Mexico Departure Time: 8 AM Distance: 146.4 Stops: 3 SUMMARY Today’s trip is a traverse of an east-west swatch of the southern Colorado Plateau in west-central New Mexico (Fig. 1.1). Here, cuestas, mesas and arroyos on the northern dipslope of the Zuni Mountains, leading into the San Juan Basin, expose a rugged topography developed primarily in Triassic and Jurassic strata. The three stops of the trip afford us a complete examination of the Triassic section in west-central New Mexico—Middle Triassic Moenkopi Formation and Upper Triassic Chinle Group (Fig. 1.2). The stops in these strata provide an excellent introduction to the lithostratigraphy, sedimentology, ichnology and vertebrate paleontology of this classic section of the nonmarine Triassic. Mileage
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From parking lot of New Mexico Museum of Natural History and Science, turn right onto 18th Street to proceed south to stop sign at intersection of 18th and Mountain Road; turn right at stop sign. 0.3 Traffic light at intersection of Mountain Road and Rio Grande Blvd.; turn right to proceed north FIGURE 1.1. Map of route of first-day road log. on Rio Grande Blvd. 0.5 tensive upland plain prior to the incision of the I-40 overpass; turn left to enter I-40 westbound. Rio Grande Valley during the middle Pleistocene. 1.2 This complex surface is now preserved as a Bridge over Rio Grande. Average annual flow tableland between the Cejita Blanca (little white past this point is about one million acre-feet. 0.1 rim) and the Ceja de Rio Puerco scarps; it is The “Adobe Cliffs” to the right are developed in about 105 km long north-south and up to 13 km the Pleistocene Los Duranes Formation. 0.7 wide east-west. 4.0 Mile post 155. The Albuquerque volcanoes on 8.1 Top of “9 Mile Hill.” Exit 149 to Paseo del the skyline at 2:00 are about 150,000 years old Volcan. We will now drive across the Llano de and are a classic example of basaltic volcanism Albuquerque. 5.2 in the Rio Grande rift. 1.3 13.3 Road crests at Ceja del Rio Puerco to begin its Unser Blvd. exit (exit 154). We will now climb descent into the valley of the Rio Puerco. Ahead “9 Mile Hill,” the long slope up to the Cejita you can see Mesa Gigante at the edge of the Blanca scarp. This scarp forms the eastern edge Colorado Plateau and Mount Taylor beyond it. of the Llano de Albuquerque, which was an ex3.7
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FIGURE 1.2. Generalized stratigraphy of the Upper Triassic Chinle Group in west-central New Mexico; total Chinle thickness is approximately 600 m.
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Rio Puerco exit (140) to right. 0.5 Cross Rio Puerco. The Rio Puerco is an entrenched meandering river system with intermittent flow that originates about 100 km north of here and drains an area of fine-grained Mesozoic strata along the southeastern side of the San Juan Basin. The river has a high suspended load 26.5 and enters the Rio Grande 72 km south of here 31.1 at Bernardo. 1.6 Coal-bearing strata to right are Upper Cretaceous Crevasse Canyon Formation. 0.9 Mile marker 138. Good view of Mesa Gigante from 1:00 to 2:00 with Mount Taylor behind it.
The highway now descends into the Apache graben. This structural feature is part of the Rio Puerco fault zone, which is a major structural element of the Rio Grande rift between the Nacimiento uplift to the north and the Lucero uplift to the south. 6.5 Tohajiilee exit (131). 5.6 Los Lunas exit (126). Red bluffs to right are Upper Triassic Chinle Group strata, the upper part of the Painted Desert Member of the Petrified Forest Formation with the local, ledge-forming Correo Bed (Fig. 1.2); overlying gray and red beds on skyline are Jurassic strata capping
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Mesa Gigante. 12.4 Laguna exit (114). 6.6 Casa Blanca exit (108). Mount Taylor on skyline to right is a Miocene volcano. 3.2 Cubero exit (104). Note intertongued Cretaceous Dakota-Mancos succession here. Mount Taylor at 2:00. 15.4 Quemado exit (89). The McCartys lava flow we have just passed is only about 5,000 years old; we will now pass through the El Malpais lava field just east of Grants. 4.4 Exit 85 (to Grants). 3.4 Exit 81 (to Grants). 2.7 Milan exit (79). 7.0 Bluewater Village exit (72). Note dipslope of Permian strata in Zuni Mountains to left. 9.0 Prewitt exit (63); exit here to NM Highway 412. 0.3 Stop sign at top of offramp. Turn left. We will now drive up the Sonsela dipslope. The Sonsela Member of the Petrified Forest Formation forms the long cuesta south of Interstate 40 that stretches from the Fort Wingate Military Depot east to the community of Bluewater. This sandstone- and conglomeratedominated unit thus forms the south wall of the Chinle strike valley between Gallup and Grants. Following Dutton (1885), Darton (1928) mistakenly thought that the Sonsela represented the base of the Upper Triassic section in the Zunis, but subsequent workers, including Cooley (1957), Foster (1957), Stewart et al. (1972a,b), Ash (1978), Lucas and Hayden (1989), Heckert and Lucas (1996, 2002a), Lucas et al. (1997a,b) and Heckert (1997a,b), all identified it as a medial sandstone in the Upper Triassic section (Fig. 1.3). The Sonsela Member rests disconformably on the Blue Mesa Member of the Petrified Forest Formation. This is the Tr-4 unconformity of Lucas (1993), and Heckert and Lucas (1996) documented as much as 20 m of erosional relief on this contact between thicker Blue Mesa Member sections to the west near Fort Wingate and thinner sections to the east near Prewitt. The FIGURE 1.3. Representative section of the Sonsela Member in upper contact of the Sonsela with the overlying west-central New Mexico. Painted Desert Member is almost always cov-
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ered at the base of the Sonsela dipslope, but appears to be gradational. Sandstones dominate the Sonsela lithosome, although conglomerates are also common. A persistent mudstone interval is sometimes exposed in the middle of the unit as well. Basal conglomerates and conglomeratic sandstones of the Sonsela Member contain many pebble-to cobble-sized clasts. These clasts may include mudstone rip-ups but more commonly are siliceous extraformational clasts, apparently derived from Paleozoic limestones presumably exposed to the south during Late Triassic time. Higher in the Sonsela section, reworked calcrete nodules are more common. Sandstones are medium- to coarse-grained subarkoses, sublitharenites, and, rarely, quartzarenites. Mudrocks are not wellexposed, but are often bluish-purple and bentonitic, thus closely resembling the mudstones of the underlying Blue Mesa Member. Trough-crossbedding is the most common bedform in the Sonsela, with some subordinate, principally low-angle, planar crossbeds present as well. Individual sets are typically 1-1.5-mthick, with much scour-and-fill within the bed. Most of the conglomerates are concentrated at the bases of the sets. The predominance of scour-and-fill and laterally extensive coalesced sandstone bodies strongly suggests that Sonsela deposition was by low-sinuosity streams (Deacon, 1990). The Sonsela Member is the principal petrified wood-bearing unit at the Petrified Forest National Park (PFNP) in Arizona (Heckert and Lucas, 1998, 2002b), and logs are relatively common in the Sonsela throughout the Zuni Mountains of New Mexico. These logs tend to be shades of yellow, white, and gray, and are neither as abundant, nor as large and colorful as the logs at the PFNP. Still, trunks up to 10 m long have been recovered in places. Heckert and Lucas (2002b) recognized three, bed-level units within the Sonsela Member at PFNP. They termed these the Rainbow Forest, Jim Camp Wash, and Agate Bridge beds, in ascending stratigraphic order. The Rainbow Forest and Agate Bridge beds are both predomi-
nantly sandstone and conglomerate. Both bear petrified logs, especially the Rainbow Forest Bed, host to most of the major “forests” in the Park. The Jim Camp Wash bed is a mudstone-dominated interval that separates the two sandstones. Akers et al. (1958) noted a similar interval at the type section of the Sonsela in the Chuska Mountains to the northwest (see also Lucas et al., 1997b). In the Zuni Mountains, the three units are only apparent on fairly close examination, as the Jim Camp Wash Bed strata are thin and typically covered. The Rainbow Forest Bed in the Zunis tends to bear most of the petrified wood and is dominated by siliceous conglomerate clasts. The Agate Bridge Bed has less wood and more intraformational clasts. Deacon (1990) reported paleocurrents in the Sonsela Member in the Zuni Mountains that are predominantly to the north or northeast at Fort Wingate, Continental Divide, and Bluewater. A section Deacon (1990) measured at Thoreau has easterly to southeasterly paleocurrents. Within our broader understanding of the Chinle depositional system, Sonsela strata represent the deposits of fluvial systems draining highlands to the south (e.g., Stewart et al., 1972a; Lucas, 1993; Heckert and Lucas, 1996). These systems were themselves tributaries to a trunk drainage running southeast to northwest across the Four Corners area, where intraformational conglomerate is more common in the Sonsela interval, and correlative strata are termed the Moss Back Formation (Stewart et al., 1972a; Lucas, 1993; Lucas et al., 1997b). Diverse lines of evidence, including palynology (Litwin et al., 1991) and tetrapod biostratigraphy (e.g., Lucas and Hunt, 1993; Lucas, 1997, 1998; Heckert and Lucas, 2002a; Hunt et al., 2005) indicate that the Sonsela is Lamyan (earlymid Norian in age), and the transition between the lower, St. Johnsian fauna and the Lamy fauna occurs low in the Jim Camp Wash Bed in Arizona (Heckert and Lucas, 2002a; Parker, 2006b). Underlying Blue Mesa Member strata are St. Johnsian (Adamanian:latest Carnian). The contact between the Blue Mesa and Sonsela members is sharp and, locally, 1-2 m of ero-
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sional relief are visible. Heckert and Lucas (1996) and Heckert (1997a) documented as much as 23 m of erosion by comparing a 44-m-thick Blue Mesa section on the Fort Wingate Military Depot in the west to a 21-m-thick section of equivalent strata near Bluewater in the eastern Zuni Mountains. Farther to the east, in the Lucero uplift, the Blue Mesa Member was apparently removed during the development of the Tr-4 unconformity (Lucas and Heckert, 1994; Heckert and Lucas, 1996; Heckert, 1997a). This transect is one of the few well-constrained indications of the degree of erosion associated with the development of the Tr-4 unconformity. 3.3
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Begin descent down section. 1.1 Turn left on improved dirt road/cattleguard; take hard left at road fork. 0.6 Road forks; stay left. 0.5 Moenkopi Formation roadcuts. 0.2 STOP 1. Walk 0.25 miles to the north to outcrop. The section exposed here presents an excellent opportunity to examine the San Andres (Early Permian-Leonardian) – Moenkopi (Middle Triassic) – lower Chinle Group (Upper Triassic-Carnian) sequence. The San Andres Formation is pale yellowish brown, light olive gray and light brownish gray limestone that locally contains brachiopods and crinoid debris.
FIGURE 1.4. Selected tetrapod footprints from the Moenkopi Formation at Bluewater Creek. A, Swimming traces. B-C, Chirotherium. D-E, Therapsipus (see Lucas et al., 2003 for more details)
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FIGURE 1.5. Lower Chinle Group stratigraphic sections in the Zuni Mountains (see Heckert and Lucas, 2002a for details).
About 14.6 m of siliciclastic red beds of the Moenkopi Formation (Anton Chico Member) disconformably overlie the San Andres and are mostly grayish red lithic wackes and siltstones. These Moenkopi strata are typical of a thin (up to 68 m thick) Moenkopi section between the San Andres and Chinle in west-central New Mexico. Earlier workers, especially of the U.S. Geological Survey, were heavily influenced by McKee’s (1954) opinion that the Moenkopi Formation pinched out in eastern Arizona and thus was not present in New Mexico. However, stratigraphic position, lithology and paleontology indicate Moenkopi strata (correlative with the uppermost, Holbrook Member in northeastern Arizona) are present across northern and cen-
tral New Mexico. At Bluewater Creek, these Moenkopi strata contain tetrapod footprints and isolated bones and teeth of capitosaurid amphibians. The footprints are those of chirotheres, dicynodonts (Therapsipus) and indeterminate swimming traces (Lucas et al., 2003) (Fig. 1.4). By walking a short distance west up the creek, the mottled strata of the Zuni Mountains Formation, the basal unit of the Chinle Group in western New Mexico, can be seen (Heckert and Lucas, 2002a, 2003). Here, the Zuni Mountains Formation is almost 8 m of color-mottled sandstone and siltstone. Overlying red beds (52 m thick) are the type section of the Bluewater Creek Formation) (Lucas and Hayden, 1989). About 21 m of grayish purple bentonitic mud-
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stone follow and pertain to the Blue Mesa Member of the Petrified Forest Formation. Yellowish orange, trough-crossbedded, locally conglomeratic sandstone of the Sonsela Member of the Petrified Forest Formation cap the bluff (Fig. 1.5). After stop, return to paved highway. 1.1 Turn right on NM-412 and retrace route to I-40. 4.5 Turn left on I-40. 4.2 Sonsela roadcut. 5.4 Thoreau exit (53); continue west on I-40. Thoreau is a critical location for the Upper Triassic in this area because workers of the U.S.
Geological Survey long claimed that the upper Chinle Group – Owl Rock and Rock Point formations – were not present east of this point. This is one of the reasons Stewart et al. (1972a) were so reluctant to equate the “siltstone member” in the Chama basin with the Rock Point to the west. Maxwell (1988a, b), however, mapped Rock Point Formation at Petaca Pinta (SW1/4 T6N, R6W and SE1/4 T6N, R7W) about 100 km southeast of Thoreau. Lucas and Hunt (1990) examined these rocks and concluded that they consist of 36.5 m of Owl Rock Formation overlain by 69.8 m of Rock Point Formation (Lucas and Heckert, 1994). The supposed thin-
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ning and pinchout of these strata around and to the west of Thoreau is not real. Instead, the subEntrada unconformity (J-2 unconformity of Pipiringos and O’Sullivan, 1978) has much greater stratigraphic relief than previously expected; this relief accounts for the local absence 139.3 of upper Chinle Group strata in west-central New Mexico. Dubiel (1989a, p. 182) attributed deposition of the Rock Point Formation to “marginal-lacustrine mudflats, eolian sand sheets and eolian dune strata in the same subsiding basin, centered in the Four Corners area, that localized Owl Rock deposition…persistent subsidence along the Zuni lineament continued to localize deposition and preservation of the Owl Rock Member and the 140.8 Rock Point facies.” The Rock Point facies, however, has a much broader distribution than Dubiel (1989a, b) recognized, well east of the Zuni lineament. Indeed, our lithostratigraphy suggests that the Rock Point facies is one of the most widely distributed of the Chinle Group, being present in Wyoming, Idaho, Utah, Nevada, Colorado, Arizona and New Mexico (Lucas, 1993). Furthermore, the Rock Point overlies the Owl Rock Formation; the two units are nowhere lateral equivalents (correlations in the Comb Ridge of northeastern Arizona by O’Sullivan, 1972 are erroneous). Thus Dubiel’s (1989a, b) interpretation of Rock Point deposition is rejected. 5.8 Cross Continental Divide. 6.7 Dipslope is on Painted Desert Member of Petrified Forest Formation, which has been estimated to be very thick (330 m) here (Repenning et al., 1969). 4.5 Iyanbito exit (36). 3.1 Exit 33 to NM Highway 400 to McGaffey; leave I-40 via this exit. 0.2 Turn left at stop sign. 0.3 Turn left to McGaffey at bridge over I-40. 1.4 Sonsela cuesta to right. The Sonsela Member of the Petrified Forest Formation in this area is lightcolored, fine-grained to conglomeratic, crossbedded sandstone with some fossil logs. It ranges in thickness from 15 to 61 m in this area
and disconformably overlies the Blue Mesa Member of the Petrified Forest Formation (Cooley, 1957, 1959; Repenning et al., 1969; Deacon, 1990; Heckert and Lucas, 1996, 2002a, 2003; Anderson et al., 2003). 0.9 Enter modern Fort Wingate. To right is historic Fort Wingate established in 1860 as Fort Fauntleroy. The garrison was withdrawn in 1861 because of the invasion of New Mexico by Confederate forces, and only a mail station was maintained throughout the Civil War. When the post was reoccupied in 1868 it was designated Fort Wingate for Captain Benjamin Wingate, who died of wounds received in the Battle of Valverde, south of Socorro, in 1862. 1.5 Turn left on dirt road and stop. STOP 2. At this stop, we focus on the early depositional history of the Chinle Group. Here, thick deposits of pedogenically-modified strata overlie the unconformable surface between the Upper Triassic Chinle Group and the underlying Middle Triassic Moenkopi Formation (Fig. 1.5). These basal Chinle strata encompass a wide range of lithotypes, including conglomerates, sandstones, and mudrocks, some of which have been altered to the point where they have become “porcellanites.” These strata are intensively color-mottled and turbated. Historically, these strata have been called the “mottled strata,” both here and throughout their outcrop distribution from eastern Arizona to the Lucero uplift in central New Mexico (e.g., Stewart et al., 1972a; Ash, 1978; Lucas and Hayden, 1989; Heckert and Lucas, 2002a). Similar strata in east-central Utah were termed the Temple Mountain Member of the Chinle Formation by Robeck (1957). The turbation has been attributed to lungfish (Dubiel et al., 1987), crayfish (Hasiotis et al., 1993), or pedogenesis (Lucas and Hayden, 1989; Lucas and Anderson, 1993; Heckert and Lucas, 2002a; Tanner and Lucas, 2006). Heckert and Lucas (2003) introduced the term “Zuni Mountains Formation” for these strata. Here, these strata are as thick or thicker than on any other locality on the southern Colorado Plateau (~21 m). Indeed, these strata are so
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thick (and the underlying Moenkopi Formation so thin) that they are mapped with the Moenkopi Formation on Anderson et al.’s (2003) geologic map of the Fort Wingate quadrangle (Fig. 1.6). Our emphases here are threefold: (1) a thin remnant of the Moenkopi Formation is present; (2) above the Moenkopi Formation an unconformable surface (Tr-3 unconformity of Pipiringos and O’Sullivan, 1978) is overlain by a complex array of deposits; (3) these deposits themselves have a complex depositional and post-depositional history. Regarding the Moenkopi Formation, these strata are thinly bedded siliciclastics, principally sandstones and siltstones with minor intraformational conglomerates. Moenkopi Formation strata are relatively thin (generally
