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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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THE TYPE SECTION AND AGE OF THE TIMPOWEAP MEMBER AND STRATIGRAPHIC
NOMENCLATURE OF THE TRIASSIC MOENKOPI GROUP IN SOUTHWESTERN UTAH
SPENCER G. LUCAS1, KARL KRAINER2 AND ANDREW R. C. MILNER3
2
1
New Mexico Museum of Natural History, 1801 Mountain Rd. NW, Albuquerque, New Mexico 87104;
Institute for Geology and Paleontology, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria;
3
St. George Dinosaur Discovery Site at Johnson Farm, 2180 East Riverside Dr., St. George, Utah 84790
Abstract—The name Timpoweap Member of the Moenkopi Formation has long been applied to the lowermost
stratigraphic interval (mostly limestone and shale of marine origin) of the Moenkopi Formation in southwestern
Utah. We describe in detail a section (lectostratotype) of the Timpoweap Member in Timpoweap Canyon near
Hurricane, Utah. At this section, the Timpoweap Member is ~ 20 m thick and is mostly limestone and shale.
Petrographic study of the lectostratotype indicates that Timpoweap deposition mostly took place in shallow
marine and restricted tidal flat paleoenvironments. Ammonoid assemblages from the Timpoweap near Hurricane
and Cedar City include Anasibirites kingianus (Waagen) and Wasatchites sp. and indicate it is of late Smithian age.
We endorse removal of the basal, conglomeratic interval long included in the Timpoweap and its recognition as the
Rock Canyon Conglomerate. The Timpoweap Member has long been recognized as laterally equivalent to the
Sinbad Member of the Moenkopi Formation to the east, and both units are in a similar stratigraphic position, of
similar lithology, and are the same age. Therefore, two names are not needed for this stratigraphic unit, the older
name Sinbad should be used, and the younger name Timpoweap should be abandoned. The nearly 700 m thick
section of Moenkopi Formation strata in southwestern Utah includes seven readily mappable stratigraphic units,
so we endorse Poborski’s suggestion that Moenkopi be elevated to group rank. However, we only include
conglomeratic and red-bed siliciclastic strata in the Moenkopi Group (Rock Canyon, lower red, middle red and
upper red formations), and assign carbonate- and evaporite-dominated intervals (Sinbad, Virgin and Shnabkaib
formations) to the Thaynes Group.
INTRODUCTION
The thick and extensively exposed Triassic section in southwestern Utah encompasses marine and nonmarine strata assigned to the LowerMiddle Triassic Moenkopi Group (Formation) and nonmarine strata of
the Upper Triassic Chinle Group (e.g., Gregory, 1948, 1950; Poborski,
1954; McKee, 1954; Stewart et al., 1972). Near the base of this Triassic
section is a limestone-dominated interval that rests disconformably on
the Permian Kaibab Formation or overlies a conglomeratic unit (Rock
Canyon Conglomerate) that fills incised valleys that were cut into the
Kaibab (Nielson, 1991). The Triassic limestones are marine rocks of
Early Triassic (Smithian) age that Gregory (1948, 1950) named the
Timpoweap Member of the Moenkopi Formation. This unit has been
recognized and mapped across much of southwestern Utah, especially in
Washington and Iron counties, as well as in northwesternmost Arizona
(Mohave County) (Fig. 1). Here, we describe a lectostratotype section
of the Timpoweap Member in detail and use its petrography to identify
microfacies and interpret depositional environments. We also present
the first published documentation of Timpoweap ammonoids to establish its precise age. Finally, we advocate what we believe are obvious and
needed changes to the stratigraphic nomenclature of the Moenkopi Formation in southwestern Utah and adjacent areas.
PREVIOUS STUDIES
Although G. K. Gilbert, as early as 1875, described Triassic strata
in southwestern Utah, it was not until the early 1900s that some of these
strata were assigned to the Moenkopi Formation (Huntington and
Goldthwait, 1904). Reeside and Bassler (1922) first proposed subdivisions of the Moenkopi Formation in southwestern Utah (Fig. 2). These
subdivisions included the Rock Canyon conglomeratic member, to which
Reeside and Bassler assigned limestone and conglomerate of what they
perceived to be the lower part of the Moenkopi interval.
However, Gregory (1950, p. 54) concluded that at its type locality the Rock Canyon conglomeratic member is equivalent to strata of the
FIGURE 1. Index map of southwestern Utah showing location of Timpoweap
Canyon (modified from Nielson, 1991). Hatchured areas show general
distribution of “Timpoweap” strata. Note that Timpoweap strata are
apparently not present at Harrisburg Dome and that they are absent or very
thin at Washington Dome (R. Biek, written commun., 2007).
Permian Kaibab Formation, and he recommended that the name Rock
Canyon conglomeratic member be abandoned. The U. S. Geological Survey accepted Gregory’s recommendation (Keroher et al., 1966) but a
comprehensive study by Nielson (1991) provides a sound basis for
using the name Rock Canyon Conglomerate for the basal unit of the
Moenkopi in southwestern Utah.
Gregory (1950, p. 60) named the Timpoweap Member of the
Moenkopi Formation as 24-70 m of “yellow, red, and gray gypsiferous
sandy shales; lenticular brown sandstones, mostly in hard, thin sheets;
sandy limestones, massive and shaly, some minutely brecciated with
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FIGURE 2. Development of stratigraphic nomenclature of the Moenkopi Group and related strata in southwestern Utah.
angular black cherts. Most beds fossiliferous and oil-bearing. Weathers
as a flight of firm steps.” Gregory (1948, p. 227) had earlier provided a
more detailed description of the Timpoweap Member, pointing out that
it generally consists of a basal conglomerate and breccia overlain by
limestone and capped by variegated shales. Gregory (1948) also stated
that the Early Triassic ammonoid Meekoceras was found in the
Timpoweap Member at Kaibab Gulch, and Gregory (1950, p. 63) listed
the following ammonite taxa from the Timpoweap Member:
“Meekoceras” micromphalus, Meekoceras aff. M. mushbachanum and
Wasatchites cf. W. seeleyi. He also described three measured stratigraphic
sections of the Timpoweap Member and indicated its distribution in
southern Utah from the Hurricane Cliffs to the Paria River.
Poborski (1954, p. 972), in his study of the Virgin Limestone
Member of the Moenkopi Formation, stated that it
....forms an easily mappable unit. The writer suggests it
should be given formational rank. The term “Moenkopi
Group,” therefore, should be used for all lithologic units
that have been known collectively as the “Moenkopi formation.” The stratigraphic units above and below the Virgin Formation should be considered as distinct formations.
In the Pavant Range, Crosby (1959) assigned a limestone interval
of the lower Moenkopi Formation that yielded Meekoceras to the Virgin
Limestone Member. Davis (1983) also showed this same unit as a 55-mthick limestone interval bearing Meekoceras assigned to the “Virgin Limestone Member” of the Moenkopi Formation. However, these Smithian
strata are older than the Virgin Limestone (it yields a Spathian ammonoid
assemblage: Poborski, 1954), so they should have been assigned to either
the Timpoweap or Sinbad members of the Moenkopi Formation. Indeed, Hintze (1993, p. 176), in a generalized stratigraphic section entitled “Fillmore-Cove Fort Area,” situated at the southern end of the
Pavant Range, shows “Meekoceras” coming from the Sinbad Limestone
Member, overlain by the lower red member (“formerly middle red”) and
Virgin Limestone Member (“formerly Shnabkaib Member”), although he
does not provide a source for the information and name changes indicated.
Poborski’s (1954) suggested elevation of the Moenkopi to group
status found no followers in workers of the U. S. Geological Survey (e.g.,
McKee, 1954; Stewart et al., 1972), who continued to use the stratigraphic nomenclature of the Moenkopi Formation in southwestern Utah
developed by Gregory (1948, 1950) (Fig. 2). Subsequent workers have
also used that nomenclature (e.g., Blakey, 1979; Nielson and Johnston,
1979; Higgins and Willis, 1995; Higgins, 1997; Willis and Hylland, 2002;
Biek, 2003; Hurlow and Biek, 2003; Hayden, 2004, 2005; Biek et al.,
2007).
ROCK CANYON CONGLOMERATE
A careful reading of Reeside and Bassler (1922) indicates that the
unit they named “Rock Canyon conglomeratic member” includes a basal
conglomeratic interval and an overlying limestone-dominated interval
that yields the ammonoid Meekoceras. Thus, the term Timpoweap
Member as introduced by Gregory and used by subsequent workers
(e.g., McKee, 1954; Stewart et al., 1972; Blakey, 1979) either is equivalent to the upper part (limestone and shale) of the original Rock Canyon
Conglomerate (Gregory’s, 1948, 1950 original usage), or is completely
equivalent to the Rock Canyon Conglomerate (Blakey, 1979 is the clearest
example of this). Nielsen (1991) presented a comprehensive and convincing study of the Rock Canyon Conglomerate and recognized it as a
distinct, conglomerate-dominated unit that fills incised paleovalleys that
were cut into the Kaibab Formation prior to deposition of the overlying,
Meekoceras-bearing limestone interval. We endorse Nielsen’s (1991)
stratigraphic conclusion to restrict the name Timpoweap to that limestone interval, and recognize the Rock Canyon Conglomerate as the
(locally) underlying basal unit of the Moenkopi in southwestern Utah
(Fig. 2).
TIMPOWEAP LECTOSTRATOTYPE SECTION
Gregory (1950) named the Timpoweap Member after Timpoweap
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upper half is mostly shale. The contact with the overlying “lower red
member” of the Moenkopi Formation is where red siltstones and fine
sandstones rest on grayish-yellow calcareous shale. Choosing the color
change from grayish-yellow to red as the Timpoweap-lower red contact
identifies the contact used by most previous workers, and this contact
was mapped by Biek (2003) and Hayden (2005) as the Timpoweaplower red contact. However, Nielson and Johnson (1979) suggested including all clastic beds regardless of color in the lower red member, so
their Timpoweap-lower red contact would correspond to our units 1920. Nevertheless, thin limestone beds are present throughout the yellowish shale interval at the top of the Timpoweap, and the yellow shale to
red siltstone contact is much easier to map than the yellowish limestone
to yellowish shale contact (R. Biek, written commun., 2007).
Petrography
Kaibab Formation
At the lectostratotype section, the base of the section we measured (in the Harrisburg Member of the Kaibab Formation) includes a
silicified limestone (unit 1: Fig. 3), which contains rare ostracods and is
overlain by a 0.4-m-thick-chert layer (unit 2). This chert layer is partly
indistinctly laminated, probably representing silicified stromatolitic limestone. A few voids are filled with authigenic quartz crystals.
The next limestone (unit 3) of the Harrisburg Member is medium
bedded and contains a few thin shale interbeds. The microfacies is indistinctly laminated mudstone containing a few ostracods. The mudstone is
recrystallized to microsparite and contains small, irregular voids filled
with chalcedony and quartz, and a few larger voids are filled with coarse
calcite cement. The overlying limestone of unit 4 is also a recrystallized
mudstone, which is indistinctly laminated and contains very small
authigenic crystals. Unit 5 is medium-bedded limestone composed mostly
of mudstone.
FIGURE 3. Lectostratotype section of Timpoweap Member. See Appendix
for description of numbered stratigraphic units.
Canyon, which is the canyon of the Virgin River just northeast of Hurricane, Utah (Fig. 1). However, no specific designation or description of a
type section of the Timpoweap Member was presented. To remedy
this, Nielson and Johnston (1979) described 14 measured stratigraphic
sections of the Timpoweap Member in the Virgin River Canyon. They
designated four reference sections, but no particular section as the type
section (or principal reference section). Here, we designate and redescribe
their measured section 1 (Nielson and Johnston, 1979, fig. 3) as the
lectostratotype section of the Timpoweap Member in Timpoweap Canyon (Fig. 3, Appendix).
At the lectostratotype section, the Timpoweap Member is about
30 m thick. Almost all of the type section is limestone and shale. The
base of the Timpoweap Member is a sharp surface where limestone sits
on chert breccia of the Rock Canyon Conglomerate, which, in turn, rests
on chert at the top of limestone of the Harrisburg Member of the Permian
Kaibab Formation (Fig. 3). The lower half of the Timpoweap Member is
mostly bedded limestone of various types (see below), whereas the
Rock Canyon Conglomerate
A thin chert breccia represents the Rock Canyon Conglomerate
(Fig. 3, unit 6). It is composed of reworked, angular chert clasts up to 2
cm in diameter. The matrix of the breccia is composed of coarse-grained
sandstone containing abundant reworked chert grains mostly up to 2
mm, rarely up to 5 mm in diameter, and well-rounded quartz grains that
are mostly monocrystalline quartz (Fig. 3A-B). This sandy matrix is
poorly sorted. Quartz grains display well-developed authigenic
overgrowths. The remaining pore space is filled with coarse calcite cement, rarely with chalcedony. Locally, the matrix is also composed of
well-sorted and rounded to well rounded sandstone having a grain size of
mostly 0.2 to 0.5 mm. Grains are mostly monocrystalline quartz, and,
subordinately, reworked chert grains are present. Rare reworked fragments of chalcedony occur. Quartz grains commonly display authigenic
overgrowths, and the remaining pore space is filled with sparry calcite
cement.
Timpoweap Member
The chert breccia is overlain by medium-bedded limestone at the
base of the Timpoweap that is similar to unit 5 below the chert breccia.
Above that is a 0.4-m-thick limestone bed unit 8) that is thinly laminated
and composed of laminae of fine-grained peloidal wackestone alternating
with laminae of mudstone containing sponge spicules and rare ostracod
shells. This bed is stromatolitic (laminated bindstone) and locally silicified (Fig. 4C). The overlying limestone units 9-11 are medium bedded
and display a muddy texture. Unit 10 is dolomitic.
The overlying limestone (unit 12: Fig. 4D) is bioturbated and
contains shell debris (bioclastic, quartz-peloid packstone). This limestone is composed of mixed siliciclastic-carbonate sandstone,
nonlaminated, poorly sorted, and fine-grained, with larger grains floating
in the fine-grained sandstone (up to 5 mm). Siliciclastic grains are almost
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entirely monocrystalline quartz, and rare chert fragments are present.
Most grains are of silt to fine sand size (mostly around 0.1 mm, up to
about 0.5 mm); a few larger, reworked chert fragments are present. The
most abundant carbonate grains are gray micritic grains (peloids), 0.1 to
0.3 mm in size, and a few larger intraclasts are present. A few fossil
fragments are present—single calcite crystals (indicating the presence of
echinoderm fragments), a few shell fragments (mostly ostracods, a few
bivalves), and rare phosphoritic bioclasts (probably fish teeth and/or
scales), all embedded in micritic matrix; rarely, some calcite cement is
present.
A cherty, bedded limestone (unit 13) overlies this unit (Fig. 4E)
and is composed of fine-grained, nonlaminated wackestone containing
abundant peloids, mostly 0.1 to 0.2 mm in diameter and a few quartz
grains (5-10%; 0.1 to 0.3 mm; angular), abundant fragments of thinshelled bivalves, a few small gastropods and some ostracods. A few
reworked fragments of chalcedony are present. The grains are embedded
in micritic to microsparitic matrix, with rare calcite cement.
The medium-bedded limestone ledge of unit 15 (Fig. 4F), intercalated in mostly covered shale, is a grainstone (tempestite) that overlies a
stromatolitic limestone bed with a sharp erosional contact. The grainstone
is indistinctly laminated (crossbedding) and shows some normal grading.
It is composed of abundant thin fragments of bivalve shells, a few small
gastropods, and ostracods and abundant spherical grains (sparitic and
micritic) that probably represent recrystallized and micritized ooids.
The spherical grains and bioclasts commonly display thin micritic envelopes. This indicates that the original aragonitic shells were dissolved,
and the porosity was later filled with sparry calcite. The outline of thin
shell fragments is indicated by the stable and thin micritic envelopes. The
pore space is filled with calcite cement. Locally, some micritic matrix is
present. The grainstone also contains a few small detrital quartz grains
and micritic intraclasts.
The ledgy limestone of unit 17 (Fig. 4G) is medium bedded and
contains thin shale partings. The microfacies is mixed siliciclastic-carbonate siltstone that is nonlaminated and bioturbated. This lithotype
contains abundant small, micritic, gray carbonate grains (partly peloids),
mostly 0.1 to 0.2 mm in diameter, and a few larger micritic intraclasts.
Also present are abundant angular detrital quartz grains, 0.1 to 0.3 mm in
diameter. A few bioclasts are present, including echinoderm fragments,
shell fragments mostly derived from bivalves, some ostracods and bioclast
fragments that cannot be identified. Some opaque grains are also present.
Rare phosphoritic bioclasts (fish remains) occur. The matrix is recrystallized micrite.
The uppermost intercalated limestone (unit 19) is 0.6 m thick and
forms a cuesta. This limestone (Fig. 4H) is composed of grainstone that
is indistinctly laminated and poorly sorted, containing abundant bivalve
shell fragments, some small gastropods, a few ostracods, and abundant
small spherical grains (recrystallized and micritized ooids) mostly 0.2
mm in diameter, and a few micritic intraclasts that are up to 7 mm in size,
some of which contain bivalve shells (reworked bioclastic wackestone).
The grain size is mostly up to 2 mm (arenitic). Bioclasts and ooids
commonly display thin micritic envelopes. The pore space is filled with
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calcite cement; rarely, some micrite is present. The rock contains a few
small, detrital quartz grains. This grainstone is overlain by 9 m of yellowish-gray shale (mostly covered), followed by red sandy siltstone that
forms the base of the lower red member of the Moenkopi Formation.
Sedimentation
The chert layer in the Kaibab Formation (unit 2) probably formed
during subaerial exposure of the limestone under the influence of meteoric water. Chert formed during early diagenesis as pore filling and as a
replacement product of carbonate allochems. Namy (1974) described a
similar chert layer at an unconformity within the Marble Falls limestone
of central Texas that he interpreted to have originated during an episode
of Early Pennsylvanian subaerial exposure.
The lower 18 m of the studied section (up to unit 11: Fig. 3),
composed of bedded mudstone containing only a few fossils, particularly ostracods (referred to standard microfacies type SMF 23 according
to Wilson, 1976 and Flügel, 2003), is interpreted to have been deposited
in a low-energy, restricted tidal-flat environment. The few irregular voids
within the mudstone probably resulted from the replacement of evaporite minerals by calcite. The dolomitic mudstone in the upper part (unit
10) is similar to the recrystallized dolomite described by Blakey (1979).
The laminated bindstone (referring to SMF 19) of unit 8, intercalated in the tidal flat mudstone, is typical of a restricted tidal flat environment, most likely an intertidal environment, although such microbial
mats may also occur in subtidal environments where high salinity or
other factors prevent destruction of the mats from grazing organisms.
The chert-pebble conglomerate of unit 6 shows a typical channelfill geometry but lacks internal sedimentary structures. This conglomerate has the same petrographic composition as at other outcrops of the
Rock Canyon Conglomerate—the angular chert clasts are mostly derived
from the underlying silicified top of the Kaibab limestone (Nielsen, 1991).
The poor rounding of the clasts indicates short distances of transport.
The well-rounded quartz grains of the sandy matrix are probably derived
from older sedimentary rocks. This thin conglomerate bed with its channel fill geometry, and calcite cemented grains, is interpreted as a tidal
channel deposit.
The fine-grained, bioclastic quartz-peloid packstone of unit 12,
comparable to SMF 2, is interpreted to have been deposited in somewhat deeper water below wave base. The overlying bioclastic wackestone
(SMF 9) of unit 13 also indicates deposition in a shallow marine environment with more open circulation at or just below the fair weather wave
base. The covered interval of unit 14, representing grayish orange shale,
points to a regressive event causing increased fine-grained siliciclastic
input. The intercalated grainstone (unit 15), composed of thin grainstone
beds (SMF 12), some of them displaying indistinctly developed normal
grading, formed in a high energy environment during storms, representing
storm layers (tempestites) on shoals within the muddy tidal flat environment. The grainstone is similar to the skeletal and oolitic calcarenite of
Blakey (1979) and the bivalve oolitic grainstone subfacies of the foreshoal/
shoal facies of the Sinbad Formation described by Godspeed and Lucas
(this volume).
FIGURE 4.Thin section photographs showing different microfacies of the Timpoweap lectostratotype section (all photographs taken under plane
polarized light). A, Sandy matrix of chert breccia composed of reworked chert grains (CH) and quartz grains. The detrital grains are cemented by calcite
and rarely by chalcedony (C; upper left). Unit 6, sample TP 6. B, Sandy matrix of chert breccia composed of well sorted and well rounded detrital quartz
grains which display authigenic quartz overgrowths. Remaining pore space was filled with calcite cement. Unit 6, sample TP 6.C, Laminated fine grained
stromatolite composed of thin peloidal wackestone layers interlaminated with thin micritic layers which we interpret as microbial crusts. Some sponge
spicules are present. Unit 8, sample TP 8. D, Mixed siliciclastic-carbonate sandstone composed of quartz grains, gray to dark gray micritic grains and
micritic matrix. Few larger intraclasts are floating in the sandstone. Few bivalve shell fragments and echinoderms are present. Unit 12, sample TP 12. E,
Wackestone composed of abundant recrystallized mollusc shells (bivalves, rare small gastropods) and peloids, embedded in micritic matrix. Unit 13, sample
TP 13. F, Grainstone, overlying dark gray stromatolitic limestone with a sharp erosional contact. The grainstone contains abundant recrystallized mollusk
shell fragments with thin micritic envelopes, cemented by calcite. Unit 15, sample TP 15. G, Mixed siliciclastics-carbonate siltstone, bioturbated,
composed of small detrital quartz grains and micritic grains (mostly peloids). Bioclasts such as bivalve fragments, echinoderms and ostracods are rare. Unit
17, sample TP 17. H, Grainstone composed of abundant mollusk shell fragments derived mainly from bivalves, subordinately from small gastropods, and
abundant spherical grains (recrystallized ooids) cemented by calcite. Rarely larger micritic intraclasts (lower middle) are present. Unit 19, sample TP 19.
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Unit 17, which is similar in composition to unit 12, indicates a
slight deepening, followed again by shallowing of the sea and fine
siliciclastic sedimentation of a tidal flat environment. Intercalated beds of
high-energy grainstone (unit 19), similar to unit 15, are interpreted as
storm layers (tempestites).
Deposition of the Timpoweap followed an episode of regional
base-level fall that resulted in subaerial exposure and incision of the
Kaibab Limestone. Subsequent base-level rise resulted in deposition of
the Rock Canyon Conglomerate in paleochannels and as thin regolithic
breccia (Blakey, 1979; Nielsen, 1991). A marine transgression then resulted in deposition of the Timpoweap limestones in a shallow marine to
restricted tidal flat environment. During sedimentation of the Timpoweap
Member, sea level also fluctuated; at the lectostratotype sea-level was
highest during sedimentation of units 12, 13 and 17 in the middle of the
succession. A slight regression is indicated by mudstones (covered intervals) of units 14 and 16. The thicker mudstones in the upper part of the
section indicate the regressive event that led to deposition of overlying
nonmarine red beds of the lower red formation of the Moenkopi Group.
AGE AND CORRELATION
Various workers (e.g., Reeside and Bassler, 1922; Gregory, 1948,
1950; Stewart et al., 1972; Blakey, 1979; Nielson and Johnston, 1979;
Nielson, 1991) have mentioned the occurrence of the ammonoid
Meekoceras in the Timpoweap Member, which indicates an Early Triassic (Smithian) age. Specific Timpoweap ammonoid localities in the literature include “Kaibab gulch” (Gregory, 1948, 1950) and a site along the
Hurricane Cliffs in the SE ¼ sec. 27, T42S, R13W located by Blakey
(1979, fig. 5). This site is NMMNH locality 5323 and is in pinkish-gray
bioclastic limestone of the uppermost Timpoweap and yields many
preserved casts and steinkerns of ammonoids that may be Meekoceras
but are too poorly preserved for definite identification. However, a few
specimens from this locality can be assigned to Anasibirites kingianus
(Waagen), and this indicates a precise correlation to the late Smithian (see
below)
One of us (ARCM) has discovered extensive ammonoid assemblages in the Timpoweap Member east of Cedar City, Iron County,
Utah, that were reported on briefly by Lucas et al. (2004). At this
section, the upper part of the Timpoweap Member is exposed just east
of the Hurricane fault on the Squaw Creek anticline and the western
margin of the Markagunt Plateau (Hintze, 2005) and yields ammonoids
from multiple levels (Fig. 5). As is the case at the Hurricane Cliffs locality, most of the ammonoid fossils are casts and steinkerns that may be
Meekoceras but are too poorly preserved for identification. However,
some of the better preserved specimens can be identified as Anasibirites
kingianus and Wasatchites sp. (Fig. 5).
Ammonoids we assign to Anasibirites kingianus (e.g., Fig. 5) are
moderately involute, somewhat compressed laterally, have a rounded
venter except for larger (outer) whorls where the venter is flattened with
distinct shoulder angles, and ornamented with numerous ribs that run
nearly straight up the flanks and cross the venter. Thus, they show
characters diagnostic of that genus (see especially Mathews, 1929; Smith,
1932; Kummel and Erben, 1968), and, pending a revision of the genus,
we assign the specimens to A. kingianus as used by Kummel and Erben
(1968).
Shells we assign to Wasatchites (Fig. 5) are moderately evolute,
subquadrate in cross section and have an ornamentation of distinct, welldefined, radial ribs that originate near the umbilical shoulder. These ribs
form distinct bullae near the umbilical shoulder towards the body chamber. At the shoulder, the ribs form a second set of weaker bullae and cross
the tabulate venter in a straight to arcuate path. These specimens thus
show diagnostic features of Wasatchites (cf. Mathews, 1929; Smith 1932;
Tozer, 1994), but we do not make a species-level assignment pending a
needed revision of Wasatchites.
The co-occurrence of Anasibirites and Wasatchites in the
Timpoweap Formation indicates that the unit is of late Smithian age, in
the Anasibirites kingianus zone. This makes it a precise correlative of the
Sinbad Formation to the east (see Lucas et al., this volume).
From this section near Cedar City (Fig. 5), three bulk samples
were processed for conodonts by M. J. Orchard of the Canadian Geological Survey in 2005. His official report (MJO-2005-19) identified the
conodonts as: Ellisonia? and Parachirognathus? from unit 6, Ellisonia?,
Guangxidella? cf. G. bransoni (Muller) and Scythogondolella milleri
(Muller) from unit 8 and Ellisonia? from unit 10. All three samples also
yielded ichthyoliths, microbivalves and microgastropods.
TIMPOWEAP AND SINBAD
In the San Rafael Swell of east-central Utah, the Sinbad Member
of the Moenkopi Formation of Gilluly and Reeside (1928) is as much as
18 m of limestone-dominated strata that yield ammonoids of the
Anasibirites kingianus zone (Lucas et al., this volume). The Sinbad has
long been recognized as the thinner, landward marine equivalent of the
Timpoweap (e.g., Stewart et al., 1972; Blakey, 1974, 1989; Blakey et al.,
1993; Goodspeed, 1996). Indeed, within the limits of outcrop continuity, it is easily seen that the Timpoweap and Sinbad are continuous
across southern Utah. Thus, Stewart et al. (1972) and Blakey (1989)
drew the southern edge of the Timpoweap outcrop as confluent with the
southern edge of the Sinbad outcrop belt, and Blakey (1989) referred to
the lower “carbonate package” of the Moenkopi Formation across Utah
as “Sinbad-Timpoweap.”
Given the demonstrated equivalence of the Timpoweap and Sinbad,
only one name is needed for the lithosome they represent (Lucas et al.,
2004). Therefore, the older name Sinbad should be used, and the junior
synonym Timpoweap should be abandoned.
MOENKOPI AND THAYNES GROUPS
The stratigraphic section assigned to the Moenkopi Formation in
southwestern Utah is about 700 m thick. As currently used, seven members of the Moenkopi Formation are all mappable units that have been
readily and repeatedly mapped at the 7.5 minute scale (e.g., Averitt and
Threet, 1973; Higgins and Willis, 1995; Higgins, 1997; Hayden, 2004;
Biek, 2003; Hurlow and Biek, 2003; Biek et al., 2007). Therefore, we
endorse Poborski’s (1954) elevation of the Moenkopi to group status in
southwestern Utah. Subdivisions of the Moenkopi Formation to the
east in Utah, such as in the San Rafael Swell, where the Moenkopi
section is about 300 m thick, are also mappable units that are routinely
mapped at the 7.5 minute scale and should be regarded as formations of
the Moenkopi Group (Black Dragon, Torrey, Moody Canyon, Sewemup,
etc.). However, to the southeast of the Colorado River in much of northern Arizona and in northern New Mexico, the Moenkopi section is much
thinner (100 m or less) and it should still be considered a formation rank
unit containing one or more members (Wupatki, Moqui, Holbrook, Anton
Chico).
In addition to raising Moenkopi to group rank in Utah, we advocate removing the carbonate- and evaporite-dominated units from the
Moenkopi Group and, instead, assigning them to the Thaynes Group
(Goodspeed, 1996). Thus, we elevate the term Thaynes Formation of
Thomas and Krueger (1946) to group rank to encompass the limestonedominated section of Lower Triassic strata recognized across much of
Utah, western Wyoming, eastern Idaho and eastern Nevada (e.g., Kummel,
1954; Clark, 1957; Stewart et al., 1972; Paull and Paull, 1994). In eastern
Nevada, for example, the Thaynes Group is at least 500 m thick and is a
marine, limestone-dominated unit with distinctive “members” that yielded
ammonoids and conodonts of Smithian and Spathian age (e.g., Clark,
1957). Indeed, it has long been known that the Sinbad (Timpoweap),
Virgin and Shnabkaib formations of Utah are simply tongues of the
Thaynes offshore lithosome that were deposited landward and are
interbedded with red-bed siliciclastics of the Moenkopi (e.g., McKee,
115
FIGURE 5. Measured section of Timpoweap strata just east of Cedar City showing ammonoid localities and selected ammonoids in the NMMNH (New
Mexico Museum of Natural History and Science) collection. Base of section is a fault.
116
1954; Clark, 1957; Bissell, 1969). Therefore, a stratigraphic nomenclature that accurately reflects well-established regional stratigraphic relationships should assign the Sinbad, Virgin and Shnabkaib as formationrank units in the Thaynes Group and would restrict the Moenkopi
Group to underlying, laterally equivalent and overlying conglomerate
and red-bed siliciclastic-dominated units (Fig. 2).
ACKNOWLEDGMENTS
Garrett Vice and Vincent Morgan assisted in the field. Bob Biek,
Jim Jenks and Norman Silberling reviewed the manuscript. We are particularly grateful to Bob Biek for clarifying Timpoweap stratigraphy
based on his extensive mapping experience.
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117
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TIMPOWEAP MEMBER—TYPE SECTION
Measured on the north bank of the Virgin River just east of La Verkin Overlook in the NE ¼ NE ¼ sec. 25, T41S, R13W, Washington County,
Utah. Base of section at UTM zone 12S, 299682E, 4119159N and top at 299578E, 4119188N (NAD 27). Bed thicknesses are in meters.
Moenkopi Group
Lower Red Formation:
21
Siltstone; pale reddish brown; sandy.
Not measured
Thaynes Group:
Sinbad Formation:
20.
Covered; yellowish-gray shale.
9.0
19.
Limestone; light gray (N7); weathers light brownish gray (5YR6/1); thinly bedded wackestone;
brachiopods; forms a cuesta.
0.6
18.
Covered; shale.
2.7
17.
Limestone; yellowish gray (5Y7/2) and very pale orange (10YR8/2); weathers to dark yellowish orange
(10YR6/6); ledgy, with shale partings.
3.8
16.
Covered; grayish orange shale?
0.9
15.
Limestone; pale yellowish brown (10YR6/2); weathers grayish orange (10YR7/4); medium bedded; ledge.
0.8
14.
Covered; grayish orange shale.
1.2
13.
Limestone; medium light gray (N6) to light gray (N7); cherty grainstone; forms top of cliff.
1.3
12.
Limestone; pale yellowish brown (10YR6/2) with some moderate brown (5YR3/4) mottles; very bioturbated
grainstone; shell debris.
1.5
11.
Limestone; same color and lithology as unit 5.
1.1
10.
Limestone; medium gray (N5); dolomitic?
1.0
9.
Limestone; same color and lithology as unit 5.
1.6
8.
Limestone; grayish orange (10YR7/4) and medium gray (N5); thinly laminated; algal.
0.4
7.
Limestone; same color and lithology as unit 5.
3.1
Rock Canyon Conglomerate:
6.
Chert breccia in limestone matrix.
0.8
Kaibab Formation (Harrisburg Member):
5.
Limestone; grayish orange (10YR7/4) and medium gray (N5); medium bedded.
8.6
4.
Calcarenite; grayish orange (10YR7/4); weathers to pale yellowish orange (10YR8/6).
0.4
3.
Limestone; light gray (N7); weathers to medium light gray (n6); medium bedded; a few thin shale interbeds. 0.8
2.
Chert; light gray (N7).
0.4
1.
Limestone; white; brachiopods.
118