16-Lucas et al (NMAZ day 1).p65

Lucas, S.G. and Spielmann, J.A., eds., 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40.
169
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
0.0
0.3
0.8
2.0
2.1
2.8
4.1
Comments
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
170
FIGURE 1.2. Generalized stratigraphy of the Upper Triassic Chinle Group in west-central New Mexico; total Chinle thickness is
approximately 600 m.
17.0
17.5
19.1
20.0
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
171
43.5
50.1
53.3
68.7
73.1
76.5
79.2
86.2
95.2
95.5
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-
172
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-
173
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
98.8
99.9
100.5
101.0
101.2
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)
174
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-
175
102.3
106.8
111.0
116.4
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-
176
122.2
128.9
133.4
136.5
136.7
137.0
138.4
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
177
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 <10
m) throughout the western Zuni Mountains, and
typically overlie an erosional surface developed
on underlying Permian strata. Indeed, there is a
significant amount of paleotopography developed
on the unconformable surface below Moenkopi
strata regionally, so that, although the Moenkopi
overlies the San Andres Formation throughout
much of the Zunis, the San Andres is locally absent and Moenkopi strata rest on the underlying
Glorieta Formation as well (Heckert and Lucas,
2002a, 2003). Moenkopi Formation red beds
typically weather to grayish red, with some unweathered outcrops light greenish gray. These
strata are flaggy to ledgy with minor, low-angle
crossbeds. Moenkopi red beds are readily differentiated from the overlying Chinle red beds
by the absence of bentonitic mudstone in the
Moenkopi. Locally, uppermost Moenkopi strata
are pedogenically modified, with color bleaching and reduction spots, but little change in lithology or sedimentary fabric. Where changes
are more profound, these strata are usually assigned to the overlying Zuni Mountains Formation.
At this stop, complex valley-fill deposits overlie the Tr-3 unconformity. These strata are
siliciclastics that were extensively pedogenically
modified. In many places this modification destroyed the primary sedimentary fabric, sometimes overprinting it with abundant rhizoliths,
some of which are up to 2 m in length and as
much as 20 cm in diameter (Fig. 1.7). Dubiel
(1987, 1989a) argued that these structures are
lungfish burrows, but we follow McCallister
(1988) and Lucas and Hayden (1989) and consider them predominantly rhizoliths. Hasiotis et
al. (1993) posited that they were instead crayfish burrows linked to the fluctuating water tables
believed responsible for the color mottling. Notably, however, fossils of the supposed burrowmakers are lacking at this location. Although the
simple (generally non-branching) nature of these
structures is more similar to that of some burrows, Lucas and Hayden (1989) noted that they
could well represent the non-branching roots of
primitive tree-like plants such as Neocalamites.
Where most stratigraphers agree, however, is
that whatever one terms the post Tr-3 deposits,
they represent a complex valley infill of
paleotopography generated between the end of
Moenkopi deposition in the early Anisian and
the onset of Chinle deposition during the Carnian,
approximately 8 to 10 million years later. Accordingly, primary (unmodified) channel-fill fluvial deposits at the base of the Chinle are generally assigned to the Shinarump Formation
(present in thin, discontinuous ribbons of conglomerate in the Fort Wingate area), and similar
lithologies with extensive color mottling and pedogenic modification are assigned to the Zuni
Mountains Formation (“mottled strata” of previous usage). There is little doubt that much of
the Zuni Mountains Formation represents
pedogenically modified Shinarump strata. At
Nazlini near the type section of the Chinle on the
Navajo Reservation there are outstanding outcrops documenting that pedogenic and diagenetic alteration of the Zuni Mountains Formation
took place in Shinarump Formation deposits.
Mehl et al. (1915) were the first to report vertebrate fossils from the Fort Wingate area. These
specimens included the holotype (now lost) of
the aetosaur Acompsosaurus wingatensis,
which is assignable to cf. Stagonolepis (Heckert
and Lucas, 2002a). Charles Camp visited this
area in 1924 but only found fragmentary
phytosaur specimens. From the early 1960s,
178
FIGURE 1.6. Geological map of the northeastern part of the Fort Wingate 1:24,000 quadrangle (after Anderson et al., 2003). Map units
are: Psa = Permian San Andres Formation, Pg = Permian Glorieta Sandstone; Trmm = Triassic Moenkopi Formation mapped with Zuni
Mountain and Shinarump formations of the Chinle Group (undivided); Trbc = Triassic Bluewater Creek Formation of Chinle Group
divided into lower (Trbcl), McGaffey (Trbcm) and upper (Trbcu) members; Trp = Petrified Forest Formation of Chinle Group, divided
into Blue Mesa (Trpb) and Sonsela (Trps) members; Qa = Quaternary alluvium; Qcl = Quaternary colluvium.
179
FIGURE 1.7. What are these vertical structures? Lungfish burrows,
crayfish burrows or rhizoliths?
141.3
141.6
144.5
144.7
144.8
145.9
S.R. Ash has collected fossil plants from localities in the Bluewater Creek Formation, particularly in the lacustrine facies of the “Ciniza beds.”
“Lake Ciniza” has also produced some of the
few described Chinle conchostracans (Tasch,
1978) as well as fish scales and coprolites. More
recently, R.F. Dubiel and his colleagues have
found vertebrate footprints in the area including
one assignable to the ichnogenus Grallator
(Lucas and Heckert, 2002).
After stop return to highway and turn right.
0.5
School zone in modern Fort Wingate. 0.3
FIGURE 1.8. Measured section of Owl Rock Formation at Stop 3.
Modern Fort Wingate. 2.9
The small hogback here is a Painted Desert
beds exposed on the southern Colorado PlaMember sandstone, the Perea Bed (Fig. 1.2);
teau. These limestones provided the primary
cross I-40. Cooley (1957) proposed, but did
basis for some workers (especially Blakey and
not publish, formal stratigraphic nomenclature for
Gubitosa, 1983; Dubiel, 1989a, b) to conclude
persistent sandstone beds in the Painted Desert
that the Owl Rock Formation was deposited in
Member like this one. Lucas et al. (1997a) foran extensive lake. Dubiel (1989a) even suggested
malized Cooley’s Perea Bed. 0.2
that subsidence along the “Zuni lineament” creTurn left onto frontage road, do not enter Iated the lake basin, and that the Rock Point For40! 0.1
mation, which overlies the Owl Rock, represents
Road to left to Fort Wingate. 1.1
the shoreline and landward facies that eventually
Stop at Owl Rock Formation outcrop on right
prograded over the lake as it disappeared (see
side of frontage.
earlier discussion).
STOP 3 One of the most distinctive units of the
Lucas and Anderson (1993), however, pointed
Upper Triassic Chinle Group is the Owl Rock
out that most Owl Rock limestone beds appear
Formation (Fig. 1.8), as much as 150 m of redto be pedogenic calcretes, not lacustrine carbed siltstone, sandstone and mudstone
bonates. Features indicative of pedogenesis in
interbedded with laterally persistent limestone
these limestones include great lateral persistence
180
and thickness (up to 4 m thick), extreme induration, tabular to platy structure, pisolitic and
multilaminar internal fabrics, common secondary silica and zones of dissolution, brecciation
and recementation. Many vertical, tube-like structures in the Owl Rock limestones claimed to be
lungfish burrows (e.g., Dubiel, 1989a, b) are
actually rhizoliths (Tanner, 2000). Most Owl
Rock limestones thus represent stage III to stage
VI calcretes (Gile et al., 1966; Bachman and 146.0
Machette, 1977) according to Lucas and Ander- 146.2
son (1993).
A more detailed analysis of Owl Rock deposition by Tanner (2000; Tanner and Lucas, 2006)
further undermined the model of Owl Rock
deposition in a large lake. Instead, Tanner (2000)
concluded that Owl Rock deposition took place
in a low-gradient floodbasin during a period of
increasing aridity. Owl Rock limestones are pedogenic calcretes or palustrine-lacustrine limestones formed in ponds and ephemeral lakes that
developed in topographic lows on the floodplain
that were subjected to intense pedogenesis.
Significantly, the Owl Rock Formation lacks a 146.4
fossil assemblage of a lacustrine megafauna; indeed, few body fossils are known from Owl
Rock limestones. In Moenkopi Wash, Arizona,
Kirby (1989, 1991, 1993) reported a tetrapod
fauna from clastic rocks in the Owl Rock Formation that is essentially identical to that of the
underlying Painted Desert Member of the Petrified Forest Formation. This fauna includes the
phytosaur Pseudopalatus and the aetosaur
Typothorax coccinarum, both of which indicate a Revueltian (early-mid Norian) age (Lucas,
1998).
The excellent outcrop of the Owl Rock Formation in the roadcut of the I-40 frontage road
just south of Red Rock Park (Fig. 1.8) was key
to the conclusions of both Lucas and Anderson
(1993) and Tanner (2000). The roadcut exposes
~ 8 m of Owl Rock strata that include four laterally persistent limestone beds. These beds display a brecciated fabric, are locally cherty and
contain root traces and channels, all diagnostic
features of pedogenic to palustrine carbonates.
After stop turn right on NM Highway 566,
paved road to Red Rock Park. 0.1
Bridge over railroad tracks. 0.2
Enter village of Church Rock (population 1077
by the 2000 census; post office from 1952);
chapter house on right. Church Rock, also known
as Navajo Church, has long been a local landmark, even appearing on Marcou’s (1858) geological map of New Mexico. Dutton (1885, fig.
12) provided a wonderful illustration of Church
Rock, noting that the rocks “are the upper members of the Jura-Trias, and strongly cross-bedded.” Some Diné people referred to it as
Tsé’ìì’ahi, Standing Rock (Van Valkenburgh,
1941); not to be confused with the community
of the same name farther north. 0.2
Turn left into Red Rock Park. The section
here exposes the top of the Rock Point overlain
by Wingate Sandstone on the way in to the amphitheater. The Rock Point here is a distinctly
“playa” facies of mudstone and muddy sandstone
with “pseudoanticline” structures formed by
evaporite crusts (“puffy grounds”) that formed
by interstitial precipitation of (most likely) gypsum below the sediment surface. Also present
are minor calcite pseudomorphs after gypsum.
Red Rock Park, which covers one square mile,
includes a large arena seating 8,000, an exhibition hall and arts and crafts pavilion, a convention center, auditorium, campground and information center.
End of first day road log.