11-Lucas et al (Sinbad ammos).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.
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AMMONOID BIOSTRATIGRAPHY OF THE LOWER TRIASSIC
SINBAD FORMATION, EAST-CENTRAL UTAH
SPENCER G. LUCAS1, THOMAS H. GOODSPEED2 AND JOHN W. ESTEP1,3
New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico;
2
Kohala Mountain Road, Kamuela, Hawaii 96743; 3deceased
1
Abstract—The Lower Triassic Sinbad Formation of the Thaynes Group in the San Rafael Swell of east-central
Utah represents the maximum flooding zone and at least part of the highstand systems tract of a 185+ m thick
depositional sequence. An extensive but low diversity ammonoid assemblage dominated by Meekoceras (sensu
lato) and Anasibirites was collected from the Sinbad Formation. The occurrence of Anasibirites kingianus (Waagen)
and Wasatchites sp., in particular, indicate that the Sinbad Formation belongs to the Early Triassic (Smithian)
Anawasatchites tardus Zone. The Tardus Zone thus is the precise time of maximum flooding of the North
American craton during the Early Triassic.
INTRODUCTION
The Sinbad Formation of Utah records the most extensive Triassic marine depositional event in western North America (Figs. 1-2). At
its maximum extent, this Triassic sea reached east-central and southeastern Utah. Thus, in this region the Sinbad Formation occurs as a tongue of
the dominantly marine, carbonate rocks of the Thaynes Group (to the
north and northwest) within the dominantly siliciclastic rocks of the
Moenkopi Group (to the east, southeast and south: Fig. 2). Ammonoids
of the Sinbad Formation are useful biostratigraphic indicators, constraining the timing of maximum flooding in western North America during the
Early Triassic. Here, we document ammonoids from the Sinbad Forma-
tion in the San Rafael Swell and discuss their biostratigraphic significance. NMMNH refers to the New Mexico Museum of Natural History
and Science, Albuquerque.
STRATIGRAPHIC FRAMEWORK
In the western USA, the dominantly siliciclastic Moenkopi Group
(long called Moenkopi Formation by most workers) is exposed in portions of Wyoming, Idaho, Utah, Colorado, New Mexico and Arizona.
The State Bridge Formation in Colorado and the Chugwater, Red Peak
and Crow Mountain formations in Wyoming are parts of the same
lithosome. These dominantly terrestrial units interfinger with the marine
carbonate rocks of the Thaynes Group (formerly the Thaynes Formation) exposed to the west and northwest and the Sinbad and Virgin
formations of Utah (Fig. 2).
Lower Triassic marine deposits of the Moenkopi and Thaynes
Group were deposited on a pericratonic ramp that deepened to the
northwest. This pericratonic ramp extended approximately 650 km northsouth between northern Mexico and British Columbia, was between 500
and 160 km wide, and was part of an inland sea that extended, at maximum transgression, from southeastern Utah into British Columbia (e.g.,
Blakey et al., 1993; Paull and Paull, 1994). The ramp was bordered on
the east by the Uncompahgre highland and the Defiance uplift, to the
west by a topographic barrier in western Nevada, and to the south by the
Mogollon highland of Arizona.
The Sinbad Formation is a tongue of the lower Thaynes Group
intercalated with the lower Moenkopi Group in southern Utah (Fig. 2).
It represents part of the most extensive flooding of the Early Triassic
sea, which had an eastern shoreline located in southeastern and eastcentral Utah. In the San Rafael Swell, the Sinbad Formation thickens
from 10 to 16 m to the north and northwest. Eighty km to the south and
southwest, it thickens slightly in Capitol Reef National Park. To the
southeast the unit thins to 1 m and pinches out in Canyonlands National
Park (Blakey, 1974; Goodspeed et al., 1992; Blakey et al., 1993;
Goodspeed, 1996; Lucas et al., 1997; Lucas, 2001).
AMMONOID PALEONTOLOGY
Previous Studies
FIGURE 1. Location map of San Rafael Swell and of NMMNH ammonoid
fossil localities in the Sinbad Formation (geology modified from Hintze,
1980).
Stewart et al. (1972) listed all published (though not documented)
ammonoid records (beginning with Gilluly and Reeside, 1928, who named
the Sinbad) from the Sinbad Formation in the San Rafael Swell as
Meekoceras gracilitatis (?), Meekoceras(?) sp., Parannanites sp.,
Anasibirites sp., Xenoceltites sp., and Hemiprionites sp. Blakey (1974)
reported Wyomingites cf. W. whiteanus, Dieneroceras knechi, D. spathi,
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FIGURE 2. Time-rock north-northwest to south-southeast cross section of the Lower Triassic of Idaho, Wyoming and Utah.
and Arctoceras tuberculatum. Dean’s (1981) collection from the Capitol
Reef National Park is “dominated by Meekoceras sp., probably M.
gracilitatis, followed by Wyomingites sp. [and] a coarsely ribbed conch
similar to Wasatchites sp.” Batten and Stokes (1986) mentioned specimens of Meekoceras gracilitatis and Arctoceras mushbachanum (identified by Bernard Kummel) from the Sinbad Formation in the northern San
Rafael Swell. We cannot evaluate these reports because they lack documentation, and some of these identifications are questionable because
they identify ammonites (such as Meekoceras gracilitatus and
Wasatchites) that do not normally co-occur.
Specimens
The ammonoids of the Sinbad Formation collected during this
study comprise about 100 complete specimens, but only 68 of these
could be identified to the generic or species level (Goodspeed, 1996).
Most ammonoids from the Sinbad Formation were found preserved in
skeletal packstone. Many ammonoids seen on outcrop were either recrystallized cross-sections on bedding planes or fragmented specimens.
High-energy, post-mortem transport evidently caused abrasion and redistribution of many of the ammonoids, and diagenesis dissolved away
their shells. Thus, preservation of specimens is generally of incomplete
steinkerns (mostly half whorls) that lack original shell, poorly preserve
surface ornamentation and display few, if any, sutures.
Specimens identified here as Meekoceras came from localities
2642, 2643, 2644, 2645, and 2670. Anasibirites specimens were collected at localities 2642, 2643, 2644, 2645, 2647, 2664, 2665, 2668 and
2670. Specimens of Wasatchites were collected at localities 2644, 2645,
2665 and 2668. A single specimen of Lanceolites was collected at locality
2645. All of the localities (Fig. 1) are in the lower 3 m of the Sinbad
Formation in what Goodspeed (1996; Goodspeed et al., this volume)
termed an offshore facies and interpreted as the maximum flooding zone
of the depositional sequence that encompasses the Black Dragon, Sinbad
and lower part of the Torrey formations.
Ammonoid Taxa
Meekoceras sp.
Referred specimens: From NMMNH locality 2642: NMMNH
P-22215.
From NMMNH locality 2643: NMMNH P-18759, 18741 (Fig.
3A), 22197, 22287.
From NMMNH locality 2644: NMMNH P-18767, 20853, 20854,
20855, 20856, 20859, 20860, 20863, 20864.
From NMMNH locality 2645: NMMNH P-18728, 18729, 18730
(Fig. 3B-C), 18732, 18733, 18734, 18735, 18737, 18744, 18746, 18754,
18755, 18756, 18757, 18758, 18766, 18771, 22218.
From NMMNH locality 2670: NMMNH P-22216.
Description: Most specimens have a compressed, involute, moderately embraced and discoidal shape, are free of ornamentation and have
narrow, sharp-edged tabulate to rounded venters. Some specimens have
slightly corrugated flanks (Fig. 3A). The sides are moderately convex
from the sloping umbilical shoulder to the pronounced ventral shoulder.
Preserved sutures have septa that are ceratitic, with rounded saddles and
serrated lobes.
Discussion: Meekoceras is a genus much in need of revision, and
we assign these specimens to the genus as construed by Smith (1932),
which we consider a sensu lato use of Meekoceras. It is important to
note that none of our specimens has a concave venter, so they are not
Meekoceras gracilitatis. However, our specimens of “Meekoceras” represent more than one species and probably encompass more than one
genus. Clearly, Meekoceras and the entire Prionitidae sensu Tozer are in
need of taxonomic revision.
Anasibirites kingianus (Waagen)
Referred specimens: From NMMNH locality 2642: NMMNH
P-22195.
From NMMNH locality 2643: NMMNH P-22204.
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FIGURE 3. Selected ammonoids from the Sinbad Formation in the San Rafael Swell of Utah. A, Meekoceras sp., NMMNH P-18741. B-C, Meekoceras sp.,
NMMNH P-18730. D-G, Anasibirites kingianus (Waagen), NMMNH P-20853. H-J, Anasibirites kingianus (Waagen), NMMNH P-20859. K-M,
Lanceolites sp., NMMNH P-18756.
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FIGURE 4. Selected ammonites from the Sinbad Formation in the San Rafael Swell of Utah. A-B, Anasibirites kingianus (Waagen), NMMNH P-18731. CD, Wasatchites sp., NMMNH P-20893. E-H, Anasibirites kingianus (Waagen), NMMNH P-20869. I, Wasatchites sp., NMMNH P-22196. J, Anasibirites
kingianus (Waagen), NMMNH P-20873. K-L, Anasibirites kingianus (Waagen), NMMNH P-20870.
From NMMNH locality 2644: NMMNH P-20853 (Fig. 3D-G),
20859 (Fig. 3H-J).
From NMMNH locality 2645: NMMNH P-18731, 18736, 18742.
From NMMNH locality 2647: NMMNH P-22259.
From NMMNH locality 2664: NMMNH P-18748, 18749, 18751,
20897.
From NMMNH locality 2665: NMMNH P-18731 (Fig. 4A-B),
22248, 22254, 22255, 22261.
From NMMNH locality 2668: NMMNH P-18769, 20869 (Fig.
4E-H), 20870 (Fig. 4K-L), 20871, 20872, NMMNH P-20873 (Fig. 4J),
20875, 20888.
From NMMNH locality 2670: NMMNH P-22211.
Description: Shells 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. The finer, secondary ribs originate at the umbilical seam
and cross the umbilical shoulder and venter. The inner whorls of these
specimens have coarser ribs that become finer in the outer whorls toward
the body chamber. As the coarser ribs become less abundant in the outer
whorls, the finer ribs bundle into groups of seven to nine between the
coarser ribs. The umbilical wall is fairly steep, and the umbilicus is
deeply embraced by the outer whorls.
Discussion: These specimens unquestionably belong to
Anasibirites, as they show characters diagnostic of that genus (see, especially, Mathews, 1929; Smith, 1932; Kummel and Erben, 1968). In their
ribbing, species of Anasibirites show a wide range of coarseness which
also varies through the ontogeny of an individual
Mathews (1929) described 33 species of Anasibirites from Fort
Douglas, Utah. However, Smith (1932) revised the taxonomy, synonymizing many of the American taxa into “only” 12 species. Kummel and
Erben (1968) proposed that all Anasibirites are part of one species complex. Using variation in ornamentation as the principal criterion, they
concluded that the multiple Anasibirites “species” from western North
America, as well as those from other localities, should be considered one
taxon, Anasibirites kingianus (Waagen).
Wasatchites sp.
Referred specimens: From NMMNH locality 2644: NMMNH
P-20854, 20857.
From NMMNH locality 2645: NMMNH P-22196 (Fig. 4I).
From NMMNH locality 2665: NMMNH P-22247.
From NMMNH locality 2668: NMMNH P-20893 (Fig. 4C-D).
Description: Specimens are moderately evolute, subquadrate in
cross section and have sharp, undulose ribs that begin at nodes on the
umbilical shoulder and run up the flanks to cross the venter with a
reduction of size. Specimen P-20893 (Fig. 4C-D) is characteristic and
has a diameter of 44 mm, a width of 12 mm and an umbilical diameter of
11 mm. The sides are flat, widest at the umbilical shoulder, and taper
from the umbilical shoulder to the ventral shoulder. Though filled with
matrix, the umbilicus is wide and fairly deep. The surface ornamentation
107
consists of distinct, well-defined, radial ribs. Towards the body chamber,
these ribs form distinct bullae near the umbilical shoulder. The ribs rapidly increase in intensity at the ventral shoulder, forming weak nodes and
cross the tabulate venter in a straight to arcuate path. The cross section
of the last whorl is trapezoidal. No sutures are discernible.
Discussion: Specimens of Wasatchites are not common in the
Sinbad Formation, and all in the collection are incomplete. We judge them
to be too fragmentary and poorly preserved to assign to a species. However, they show the diagnostic features of the genus (cf. Mathews, 1929;
Smith 1932; Tozer, 1994).
Lanceolites sp.
Referred specimen: From NMMNH locality 2645: NMMNH
P-18756 (Fig. 3K-M).
Description: This specimen is small, very involute with a deeply
embraced and closed umbilicus, laterally compressed with flat flanks and
a wide, bicarinate venter with a shallow furrow, and has no ornamentation.
Discussion: NMMNH P18756 well matches Lanceolites (cf.
Hyatt and Smith, 1905; Smith, 1932) and is readily assigned to that
taxon. Preservation is not good enough to permit a species-level identification.
AGE AND CORRELATION
The ammonoid assemblage from the Sinbad Formation documented
here is dominated by Meekoceras and Anasibirites and also contains less
common Wasatchites and rare Lanceolites. These taxa come from a thin
stratigraphic interval, and all co-occur at NMMNH locality 2645. The
presence of Wasatchites and Anasibirites indicates the Sinbad ammonoid
assemblage belongs to the Anawasatchites tardus zone of Tozer (1967;
also see Tozer, 1994), which is the youngest Smithian ammonite zone.
Some taxa previously reported from the Sinbad Formation (such as
Meekoceras gracilitatis) are characteristic of the older Romunderi Zone
of the Smithian, and are not found in our collection. We can thus say with
certainty that the Sinabad Formation ammonite assemblage, which represents a relatively thin stratigraphic interval near the base of the formation, is assignable to the Tardus zone, so this is the time of maximum
flooding of the North American craton by the Smithian sea.
ACKNOWLEDGMENTS
Todd Lamaskin, John Rogers and Amy Thompson assisted in the
field. The Department of Earth and Planetary Sciences, University of
New Mexico, and the New Mexico Museum of Natural History and
Science provided financial support of this research. Jim Jenks and Norm
Silberling provided helpful reviews of the manuscript. We dedicate this
paper to the late John Estep, to honor a good friend and a keen student of
ammonoids.
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