Triassic fossils found stratigraphically above ‘Jurassic’ eolianites

Rocky Mountain Geology, v. 47, no. 1, p. 37–53, 5 figs., 1 appendix, June 2012 37

Triassic fossils found stratigraphically above ‘Jurassic’ eolianites necessitate the revision of lower Mesozoic stratigraphy in Picket Wire Canyonlands, south-central Colorado

Andrew B. Heckert1*, Eric J. Sload1, Spencer G. Lucas2, and Bruce A. Schumacher3

1Department of Geology, Appalachian State University, ASU Box 32067, Boone, North Carolina 28608-2067, U.S.A. 2New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104, U.S.A.3USDA Forest Service, 1420 E. 3rd Street, La Junta, Colorado 81050, U.S.A.*Correspondence should be addressed to: [email protected]

ABSTrACT

The recent discovery of Triassic tetrapod fossils in the Picket Wire Canyonlands of southeastern Colorado necessitates large-scale modification of the currently accepted stratigraphy of the area. The bone-bearing strata lie stratigraphically above a thick (~80 meter [m]) eolianite historically identified as the Middle Jurassic Entrada Sandstone. The identifiable fossils include teeth and bone fragments of Late Triassic tetrapods, including metoposaurs, phytosaurs, and aetosaurs, recovered from thin (m-scale) discontinuous channels of limestone-pebble conglomerate deposited in a high-energy f luvial environ-ment. Metoposaur bones consist of characteristically textured dermal bone fragments of the skull and pectoral elements, as well as a tooth. Phytosaur fossils consist of type C and B teeth, skull and jaw frag-ments, and some osteoderms. Aetosaurs are represented by several distinctive osteoderms, including some with evidence of prominent eminences and anterior bars. All identifiable tetrapods pertain to taxa known only from strata of Late Triassic age elsewhere, but none constrains the age of the fossil assem-blage more precisely, although the assemblage is similar to lower Chinle Group assemblages of Carnian age (Otischalkian–Adamanian). The two most reasonable solutions to the discovery of Late Triassic index fossils stratigraphically above “Jurassic” beds are that the Triassic strata of this area have been mistakenly correlated with the Middle Jurassic Entrada Sandstone, or else the fossils are reworked into dramatically younger (Middle to Upper Jurassic) beds. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to Upper Triassic conglomerates of Wyoming (Gartra Formation) and New Mexico (Cobert Canyon Bed). Therefore, we consider the fossils to be in Upper Triassic strata. New lithostratigraphic data, including a composite measured section from the Picket Wire Canyonlands—as well as analysis and correlation of newly measured sections and others in the literature from south-central Wyoming, Colorado, Oklahoma, and New Mexico—suggest that the eoli-anite below the bone-bearing horizon and the finer clastic strata directly beneath the eolianite are best correlated to the Red Draw Member of the Jelm Formation. We correlate the bone-bearing conglom-erates with the Cobert Canyon Bed at the base of the Chinle Group, described by previous authors as limestone and lithic-pebble conglomerate underlying the Travesser Formation in northern New Mexico. The gypsiferous and clastic strata overlying the conglomerates and below the Morrison Formation, ~30 m higher in Picket Wire Canyon, are referred to the Middle Jurassic Ralston Creek (= Bell Ranch) Formation, a correlative of the Summerville Formation. These correlations extend the known distribu-tion of Jelm Formation strata southeastward from north-central Colorado and south-central Wyoming and highlight the need for a major, modern restudy of this unit.

KEy WordS: amphibian, Chinle Group, Colorado, correlation, Entrada Sandstone, fossils, Jelm Formation, Mesozoic, Middle Jurassic, Picket Wire Canyonlands, reptile, stratigraphy, Upper Triassic.

38 Rocky Mountain Geology, v. 47, no. 1, p. 37–53, 5 figs., 1 appendix, June 2012

INTrodUCTIoN

The fossils whose discovery initiated this project come from the Picket Wire Canyonlands, a 23,000-acre unit within the Comanche National Grassland (administered by the United States Forest Service [USFS]) in southeastern Colorado (Fig. 1). As shown by Schumacher (2002), the Picket Wire Canyonlands are critical to resolving stratigraphic correlations between extensive Mesozoic exposures in the front range of central Colorado and those in more deeply dissected canyon country in northeastern New Mexico and the panhandle of Oklahoma. Although the exposed section in Picket Wire Canyonlands is known to range from Permian to Cretaceous in age, the majority of the vertebrate fossils, includ-ing the famous Purgatoire River tracksite (Lockley et al., 1986; Schumacher, 2002) and other Morrison Formation localities (Schumacher, 2002, 2008) are from Jurassic deposits. In 2001, one of us (BAS) discovered some of the fossils we describe here. Since then, we have added to these collections with fossils discovered by members of the 2009 and 2010 Appalachian State University “Triassic Trip” field courses. In this paper, we dis-cuss these fossils and their stratigraphic position, and we use this information to present a major revision of lower Mesozoic stratigraphy in the Picket Wire Canyonlands that necessitates a critical reappraisal of our understanding of the correlation of Triassic strata from Wyoming, through and under the Denver Basin, to the High Plains of Oklahoma and New Mexico. This paper represents an update and cul-mination of research presented by Schumacher et al. (2009) and Sload et al. (2011).

Throughout this paper, sedimentary rock classi-fication follows standard schemes (e.g., Prothero and Schwab, 2003) generally and Folk (1974) in particu-lar for sandstones. Rock colors are after Goddard et al. (1984).

GEoLoGIC SETTING

The Picket Wire Canyonlands are approxi-mately 47 kilometers (km) south of La Junta, Colorado, in the Chaquaqua Plateau, itself part of the High Plains physiographic province (Howard and Williams, 1972). The combination of a regional uplift (the Apishapa Arch), local structure (Black

JC

B

TBPR

T

BC

Bravo Canyon 1C

Bravo Canyon 1B

Bravo Canyon1A

DMNS 4125

DMNS 4124

DMNS 4126

DMNS 4127 DMNS 4128

A

B

C

Figure 1. Geographic setting of study area in southeastern Colorado. A, Index map showing location of Picket Wire Canyonlands and other localities described in text. B, Physiographic map of Colorado showing greater region including locations of measured sections in Figure 5. C, Google Earth® image of greater Bravo Canyon area, Picket Wire Canyon, Comanche National Grassland. DMNS num-bers refer to fossil localities. Bravo Canyon A, B, and C, refer to mea-sured sections in Figure 3. Sections in Figure 5: BC = Bravo Canyon; B = Bobcat Mountain, Colorado; C = Centennial, Wyoming; J = Type Jelm Formation, Wyoming; PR = Purgatoire River, Kansas; T = Type Travesser Formation, New Mexico; TB = Two Buttes, Colorado.

A. B. HECKERT, E. J. SLOAD, S. G. LUCAS, AND B. A. SCHUMACHER


Hills Monocline), and incision by the Purgatoire River exposes Permian, Triassic, and Jurassic rocks in a region otherwise dominated by broad plains underlain by flat-lying Cretaceous rocks (e.g., Duce, 1924; Weist, 1965; Scott, 1968; Tweto, 1979; see also Mallory, 1972). As a consequence, Triassic and Jurassic strata exposed in the Picket Wire Canyonlands are the only outcrops of this age for great distances, often more than 100 km in any direc-tion (e.g., McKee et al., 1956, 1959), rendering corre-lation difficult. Duce (1924) mapped and described pre-Morrison strata in this region and considered the strata described here to be equivalent to the Lykins Formation, which he thought was of Pennsylvanian–Permian age. More modern studies recognize that some of these strata are of Triassic or Jurassic age, but some of the pre–Morrison Formation beds in the Picket Wire Canyonlands have been incor-rectly assigned to either the Upper Triassic Dockum Formation (Group) or to the Middle Jurassic Entrada Sandstone (e.g., McKee et al., 1956, 1959; Oriel and Mudge, 1956; Weist, 1965; Scott, 1968; Tweto, 1979; Kauffman, 1986; Schumacher, 2002), based on superficial lithologic similarity to other distant out-crops of these units. The fossils we describe here are the first age-diagnostic remains recovered from the lower Mesozoic strata in this region, and they con-firm a Triassic age for strata some previously corre-lated to the Middle Jurassic Entrada Sandstone.

PALEoNToLoGy

The fossils described and illustrated here were recovered from lenticular conglomeratic deposits we refer to the Cobert Canyon Bed of the Baldy Hill Formation (Chinle Group). The distinct lithologic appearance and character of these lenticular con-glomerates in Picket Wire Canyon is indistinguish-able from the Cobert Canyon Bed of the type area in northeastern New Mexico. All localities are on a northwest-facing dip slope developed on the upper-most strata of the Red Draw Member of the Jelm Formation, but the fossils are clearly associated with the lenticular limestone-pebble conglomerate, which is the only unit in which we have observed fossil bone in the area. Detailed locality information, including Global Positioning System (GPS) coordinates, is on file at the USFS office in La Junta and at the Denver Museum of Nature and Science (DMNS), and the

stratigraphic assignments we describe are discussed later in the paper. We have identified five discrete, bone-producing areas in these outcrops (DMNS localities 4124–4128; Fig. 1C), although all of the fossils illustrated here come from a single, general locality (DMNS 4125). All are from the same stratigraphic horizon and derived from lenses of the limestone-pebble conglomerate described previously. Recovered fossils were surface collected, with only a few in situ specimens removed from out-crops. These fossils consist primarily of centimeter (cm)-scale, non-diagnostic bone fragments, but sev-eral teeth and some bone fragments are diagnostic of Triassic tetrapod taxa and in a single instance include a ganoid osteichthyan scale. Significantly, no bone frag-ments are either (1) large enough to pertain to a large Jurassic dinosaur, or (2) diagnostic of any taxon known from the Jurassic. Therefore, in spite of the fragmen-tary nature of the material, we reject the hypothesis that these are reworked Triassic fossils in Jurassic deposits. In the following paragraphs we describe the most diag-nostic fossils in systematic fashion.

SySTEMATIC PALEoNToLoGy

Class AMPHIBIA Linnaeus, 1758order TEMNoSPoNdyLI von Zittel, 1887

Suborder CAPIToSAUroIdEA Watson, 1920Family METoPoSAUrIdAE Watson, 1920

Metoposauridae (?) indet.

Many of the recovered fossils clearly pertain to labyrinthodont amphibians, including a tooth (Fig. 2A–B) and several skull (Fig. 2C–D) and pectoral girdle (Fig. 2E–F) fragments. The tooth (Fig. 2A–B) is labyrinthodont, moderately large, conical, and very weakly curved. The labyrinthodont infolding of the enamel and its sheer size suggest that it is a palatal tooth (fang) of a large labyrinthodont amphibian. Although there are many large amphibians in the Middle Triassic of North America (e.g., Welles and Cosgriff, 1965; Morales, 1987; Schoch and Milner, 2000; see faunal review by Heckert et al. [2005]), this tooth is consistent with the large palatal fangs or caniniform lower teeth of a metoposaurid amphibian (e.g., Colbert and Imbrie, 1956; Hunt, 1993), although we cannot rule out other capitosauroids on tooth morphology alone. The skull fragments, two of which are illustrated here (Fig. 2C–D), are clearly capitosauroid, bearing

TRIASSIC FOSSILS CLARIFY ‘JURASSIC’ STRATIGRAPHY


Figure 2. Selected vertebrate fossils from DMNS locality 4125 in Cobert Canyon Bed, Bravo Canyon area, Comanche National Grasslands. A–B, Metoposaur tooth (DMNS 61220) in labial (A) and lingual (B) views. C–D, Metoposaur skull(?) fragments (DMNS 61221) in dorsal view. E–F, Metoposaur girdle element fragments (DMNS 61222) in ventral view. G–H, Phytosaur “caniniform” tooth (DMNS 61223) in labial (G) and lingual (H) views. I–J, Phytosaur “blade” tooth (DMNS 61224) in labial (I) and lingual (J) views. K–L, Phytosaur squamosal (DMNS 61225) in medial (K) and lateral (L) views. M, Phytosaur lower jaw fragment (DMNS 61226) showing tooth sockets in side view. N–P, Aetosaur caudal(?)-lateral osteoderm (DMNS 61228) in lat-eral (N) dorsal (O) and medial (P) views. Q–S, Aetosaur osteoderm fragments (DMNS 61229) in dorsal view. Scale bars = 1 cm (A–B, I–J) or 2 cm (all others).



a “waffle iron”–like texture of grid-like ridges and pits on the skull roof. The exceptionally square and even nature of these ridges and the corresponding valleys is strongly reminiscent of the deep, regular texture that characterizes metoposaurid skulls (com-pare to Colbert and Imbrie, 1956) and is distinct from the shallower, more reticulate ornamentation that typifies capitosaurids (e.g., Howie, 1970). Thus, like the tooth, we tentatively refer this material to “Metoposauridae(?) indet.” Finally, some of the textured bone fragments more closely resemble pectoral girdle fragments. In particular, these pieces bear elongate grooves, flanked by tall ridges in outer (ventral) view. Again, this pat-tern is, in itself, only characteristic of capitosauroids generally, but the depth of the grooves, combined with the boxlike tall ridges, is strongly reminiscent of metoposaurids (e.g., Colbert and Imbrie, 1956). Importantly, metoposaurids became extinct by the end of the Triassic; although some temnospon-dyls persisted until the middle of the Cretaceous in Gondwana, there are no post-Triassic temnospondyls known in North America, and they are extremely rare in Laurasia after the Triassic (Schoch and Milner, 2000). Thus, we consider these fossils probable meto-posaurs and diagnostic of a Late Triassic age. The presence of metoposaurs (a Late Triassic group) is consistent stratigraphically with the other fossils, which include numerous fossils identifiable as either phytosaurs (Fig. 2G–M) or aetosaurs (Fig. Fig. 2N-S)—taxa also known from only Upper Triassic strata in the American West and elsewhere (e.g., Long and Murry, 1995; Heckert and Lucas, 2000). The fossils described here are relatively large, and large metoposaurids are most common in the lower part of the Chinle Group (Hunt and Lucas, 1993), but they occasionally occur in the upper Chinle.

Class rEPTILIA Laurenti, 1768Superorder ArCHoSAUrIA Cope 1869Suborder PArASUCHIA Lydekker 1890Family PHyToSAUrIdAE Jaeger 1828

Two teeth (Fig. 2G–J ) and several bone frag-ments (Fig. 2K–M) pertain to phytosaurs. The larger tooth (Fig. 2G–H ) is approximately 50 millime-ters (mm) tall, conical, and serrated. Its overall size and shape is strongly reminiscent of the large teeth near the tip of the snout in the upper and lower

jaws of phytosaurs, termed “tip of snout” teeth by Hungerbühler (2000) or “type C” (caniniform) teeth by Hunt (1989). The second, smaller tooth is coni-cal, laterally compressed, serrated mesially and dis-tally, and nearly as long as it is tall. These are char-acteristics of posterior teeth in the jaws of hetero-dont phytosaurs, termed “type B” (blade) teeth by Hunt (1989) and “maxillary” teeth by Hungerbühler (2000). Teeth such as these are common fossils in Upper Triassic deposits of the American Southwest (e.g., Heckert, 2004), but many phytosaurs possess a heterodont dentition that includes both of these mor-photypes (Hunt, 1989; Long and Murry, 1995), so they are not diagnostic to a lower taxonomic level. One of the larger bone fragments (Fig. 2M) pre-serves three grooves, each nearly 1 cm wide. These grooves are clearly tooth sockets of a large archosau-riform reptile exhibiting thecodont tooth implanta-tion. Among archosauriforms, several heterodont phytosaur taxa, including Angistorhinus, possess lower jaws where the splenials participate in the tooth margins medially, so that portions of both the sple-nials and dentaries possess grooves, rather than full sockets, such as those seen in Figure 2M (ABH, personal observation). To our knowledge, this is a uniquely phytosaurian trait. Finally, a relatively large, curved bone fragment (Fig. 2K–L) also pertains to a phytosaur. The curved, thickened outer margin of this bone well matches the pattern seen on the squamosals of phytosaurs (many examples illustrated by Long and Murry (1995) and Stocker [2010]). Indeed, the squamosal probably is the single most diagnostic skull bone in the phytosaurian skeleton (Hungerbühler, 2002; Stocker, 2010). Were this specimen more complete, a subfamilial or even generic identification would be possible. We inter-pret this specimen as the dorsal-most part of a right squamosal. Some rugosity on the dorso-lateral sur-face helps to orient the specimen, as does the fact that posteriorly the bone is less robust than anteriorly. This suggests that the descending process of the squamo-sal (squamosal–opisthotic process of Stocker [2010]) was relatively gracile. All of these fossils are diagnos-tic of phytosaurs, but we can be no more specific than “heterodont phytosaurs,” which includes most of the known taxa (Long and Murry, 1995; Hungerbühler, 2000, 2002; Stocker, 2010). Phytosaurs are reliably known from strata of Late Triassic age across Pangea, and there are no confirmed reports of phytosaurs of



Jurassic or Middle Triassic age (Long and Murry, 1995; Hungerbühler, 2002).

Suborder AEToSAUrIA Marsh 1884Family STAGoNoLEPIdIdAE Lydekker 1887Subfamily dESMAToSUCHINAE Huene 1942

desmatosuchinae? indet.

Among the remaining bone fragments are several that clearly pertain to osteoderms (armor) of aetosaurs (Fig. 2N–S), a clade of archosaurs known only from Upper Triassic strata in the Americas, Greenland, Europe, Africa, and India (Heckert and Lucas, 2000). Aetosaur armor consists of two columns each of para-median and lateral armor over the dorsal and lateral sides and a variable number of columns in the ventral carapace. The most complete osteoderm from Bravo Canyon (Fig. 2N–P) is either a caudal paramedian or a lateral osteoderm, probably from a relatively derived taxon (Desmatosuchinae of Heckert and Lucas, 2000; Parker, 2007). Key features preserved are the promi-nent spine as well as a finished edge. If this is a para-median osteoderm, its apparently narrow width and prominent dorsal eminence (spine) indicate that it is a caudal paramedian. If it is a lateral osteoderm, the finished surface is likely the medial margin, and the prominent spine suggests affinity with more derived aetosaurs (Heckert and Lucas, 2000; Parker, 2007). Several other fragments (Fig. 2Q–S) also belong to aetosaurs. The most diagnostic feature preserved in these osteoderm fragments is the smooth anterior surface, first termed the “anterior bar” by Long and Ballew (1985) and characteristic of all aetosaurs except Desmatosuchus (Heckert and Lucas, 2000; Parker, 2007). This feature, as well as the remnants of the typ-ical pattern of pits, grooves, and ridges, indicates that these bones are osteoderms of aetosaurs. More com-plete material almost certainly would have been diag-nostic to genus, and possibly species. Significantly, this armor is diagnostic of aetosaurs and is distinct from any armored Jurassic taxon, including either Jurassic crocodilians or thyreophoran dinosaurs. Thus, while we can only refer these fossils to Stagonolepididae indet., they demonstrate a Late Triassic age.

STrATIGrAPHIC PoSITIoN oF FoSSILS

In order to ascertain the stratigraphic position of the recovered fossils, we measured three sections

that stratigraphically bracket the localities in Bravo Canyon, a tributary to the Purgatoire River in the Picket Wire Canyonlands (Figs. 3 and 4). These sec-tions demonstrate that the fossiliferous conglomer-ate is underlain by more than 80 m of fine- to very fine–grained, well-sorted quartz arenites that are fre-quently cross-bedded and almost certainly primar-ily eolian in origin (Figs. 3 and 4A–C). Importantly, there is extensive color mottling and other indica-tions of pedogenesis in the uppermost eolianite beds underlying the bone-bearing conglomerate (Fig. 4F), features we interpret as indicative of a hiatus in depo-sition. Overlying the conglomerate are approximately 25-m gypsiferous mudstone and gypsum depos-its (Figs. 3 and 4C–D) with some local thin sand-stone beds. These overlying deposits have typically been assigned to the Middle–Upper Jurassic Ralston Creek Formation (McKee et al., 1956; Schumacher, 2002), a correlative of the Summerville Formation in New Mexico (Lucas, 2004). We see no need to change that assignment here. The strata bearing the fossils we describe are len-ticular deposits we interpret as small, isolated chan-nels resting disconformably upon a massive eoli-anite in the general vicinity of Bravo Canyon (Fig. 1C). Unfortunately, the conglomerate is not pres-ent in the steepest terrain (e.g., Fig. 4D), so it is dif-ficult to establish the nature of its upper and lower contacts with certainty. The best outcrops are found “floating” on the dip slope developed on the under-lying eolianite (Fig. 4E, G). Because the eolianite is strongly color mottled and bears reduction spots superficially resembling coarse grains (Fig. 4F), we interpret this contact as disconformable. This is cer-tainly the case where the overlying, gypsiferous strata lie directly on the eolianite, but this relationship is only readily seen on the eastern side of the Purgatoire River. Where present, the fossiliferous strata are len-ticular, with channel-fills of conglomerate overlain by finer-grained, sandy strata (Fig. 4G). Bones occur throughout the unit, but they are often broken and abraded (e.g., Fig. 4H). The fossiliferous deposits are clearly distinct from the overlying strata, which are inferred to be signifi-cantly younger. They also rest disconformably on the underlying eolianite; thus, they must be from a dif-ferent stratigraphic unit. Based on detailed lithologic similarity, we identify the bone-bearing conglom-erate as a correlative of the Cobert Canyon Bed. As



described by Lucas et al. (1987), the Cobert Canyon Bed is a thin intraformational limestone-pebble con-glomerate that marks the contact between the under-lying Upper Triassic Baldy Hill Formation and the overlying Upper Triassic Travesser Formation in Union County, northeastern New Mexico, and Cimarron County, Oklahoma, which is about 140 km east-southeast of the Picket Wire section. In

New Mexico–Oklahoma, the Cobert Canyon Bed is as much as 3 m thick and consists of limestone and other lithic pebbles within a subarkosic matrix (Lucas et al., 1987). Lucas et al. (1987) chose from a mapping perspective to include the Cobert Canyon Bed in the Baldy Hill Formation, noting the distinc-tive change in color from grayish red Baldy Hill–Cobert Canyon deposits to reddish brown Travesser Formation deposits. Lucas et al. (1987) ascertained that the fossil locality described by Stovall (1943) is actually in the Cobert Canyon Bed and that this assemblage was of Late Triassic age. Indeed, the fauna Stovall (1943) described (fragmentary metoposaurid and phytosaurian fossils) is essentially identical to the fauna documented here. Vertebrate fossils, principally of phytosaurs, also demonstrate a Late Triassic age for the Travesser Formation (Lucas et al., 1987). Points of detailed lithologic similarity between the type Cobert Canyon Bed and the fossiliferous outcrops in Picket Wire Canyon include (1) pebble-sized clasts of intraformational limestone and other lithics includ-ing abundant bone fragments (e.g., Fig. 4H); (2) sub-angular, subarkosic sandy matrix; (3) bedforms of len-ticular, cross-bedded channels (Fig. 4G); and (4) total thickness of ≤ 3 m (Fig. 4E, G). This combination of features is unique within the ~100 m of measured sec-tion reported here and is also highly distinctive within the Chinle lithosome. Therefore, we propose that the fossiliferous unit exposed in Picket Wire Canyon is a correlative of the Cobert Canyon Bed and thus repre-sents an unusual outlier of the Upper Triassic Chinle Group in southeastern Colorado. This conglomerate may also be correlative with the Gartra Formation of south-central Wyoming and north-central Colorado, and this regional relationship is pursued in the follow-ing section.

rEGIoNAL CorrELATIoN

Recognition of an Upper Triassic unit imme-diately below the Ralston Creek Formation in the Picket Wire Canyonlands requires a reexamina-tion of the stratigraphic assignment of the eolianite beneath the Cobert Canyon Bed. In this section, we

B

A

C

1

2

3

4

5

6

West East

5m

Bravo Canyon

1

2

4

5

6

78

9

10

11

3

123

45678

9

10

11

12131415

17

sandstone

sandstone, cross-bedded

siltstone

mudstone

gypsum

sandstone, laminar

agate nodules

covered interval

fossil bone

Cobert Canyon Bed(Upper Triassic)

Jelm

For

mat

ion

Red

Dra

w M

embe

r (M

iddl

e Tr

iass

ic)

Rals

ton

Cree

k Fo

rmat

ion

(Mid

dle

Jura

ssic

)Figure 3. Correlated measured sections in vicinity of Bravo Canyon, Picket Wire Canyonlands, encompassing fossiliferous localities (top of Bravo Canyon B) described here. See Figure 1C for location of sections.



Figure 4. Outcrop photographs of fossiliferous localities and measured sections described here. A, Overview of the Red Draw Member of Jelm Formation in Bravo Canyon—note interbedded fine-grained strata and sandstones at base of section here and in next photograph; TRj = Jelm Formation eolianites. B, View to northwest of measured section Bravo Canyon A, all strata are of Jelm Formation; note students for scale at base of section. C, Overview of the measured section Bravo Canyon B (white line shows transect); K = Cretaceous strata (on skyline); Jm = Morrison Formation (slopes on far mesa); TRj = Jelm Formation. D, Overview of measured section Bravo Canyon C; most strata are covered, but distinctive bands of gypsiferous Ralston Creek Formation (Jrc) are evident beneath resistant Morrison Formation (Jm) layers; note students for scale in center of photograph. E, Typical out-crops of Cobert Canyon Beds at mouth of Bravo Canyon (DMNS locality 4126). F, Color mottling and ?pedogenesis at top of Bravo Canyon section B in uppermost Red Draw Member of Jelm Formation. G, Lens of conglomerate and sandstone in Cobert Canyon Bed (DMNS locality 4127). H, In situ bone fragments in Cobert Canyon Bed near outcrop in G.



describe this eolianite and advance our stratigraphic hypothesis to correlate both it and the overlying Cobert Canyon Bed regionally (Fig. 5). Because the eolianite in the Picket Wire Canyonlands is overlain by deposits bearing in situ fossils of late Triassic age, it cannot be the Middle Jurassic Entrada Sandstone, as it was identified by some previous authors (e.g., as “interval B” of the Jurassic system by McKee et al., 1956, pl. 9). We note that the lower part of the eolianite consists of alternating beds of finer-grained, laminar, presum-ably water-lain (or reworked) siltstone interbedded with coarser-grained fluvial and/or eolian sandstone before the final switch to the massive eolianite (see Figs. 3A–B and 4A–C). This gradational change from interbedded water-lain and eolian beds to mas-sive eolianite is unknown in the Entrada lithosome, and the lower beds are lithologically dissimilar to the Dewey Bridge Member (the lower member of the Entrada across much of the southern Colorado Plateau; Lucas and Heckert, 2003; Lucas, 2004), so we are confident that the massive eolianite is not the Slick Rock Member of the Entrada. Because the eolianite is overlain disconformably by rocks of Late Triassic age, it appears likely that it is of Middle Triassic (or older) age. In the Mesozoic of the Rocky Mountain Region, the most extensive pre–Late Triassic eolianites have been assigned tradition-ally to the Jelm Formation, which is best known from the type area in southeastern Wyoming (see Lee, 1927; Pipiringos, 1957, 1968, 1972; Pipiringos et al., 1969; Blakey et al., 1988; Lucas, 1992, 1993, 1994). Potentially correlative strata to the south and east are much less studied but include the “unnamed unit” of the Dockum Group in southeastern Colorado (Oriel and Mudge, 1956) and the Anton Chico Member of the Moenkopi Formation (e.g., Lucas, 2004). The Baldy Hill Formation of northeastern New Mexico of Baldwin and Muehlberger (1959), as redefined by Lucas et al. (1987), lies at a similar stratigraphic posi-tion as the eolianite, but it is of Late Triassic age. Given the great distance from the Picket Wire Canyonlands to these Middle Triassic outcrops, any correlation requires particularly careful consider-ation. As presently recognized, the Jelm Formation consists of two members, the lower Red Draw and the overlying Sips Creek Member (Pipiringos, 1968, 1972; Blakey et al., 1988). As described by Pipiringos (1968), the lower Red Draw Member is as much as

43 m thick and consists of eolian sandstone beds with cross-bed dips generally to the southeast. The overlying Sips Creek Member is as much as 96 m of fine-grained sandstone and siltstone. Correlation of the eolian sandstone interval of the Jelm Formation (Red Draw Member) from southern Wyoming to northern Colorado has been advocated on a lithostratigraphic basis (based on lithology and strati-graphic position) by Pipiringos (1953, 1957, 1968), Hubbell (1954), Pipiringos et al. (1969), Shropshire (1974), and Lucas (1992, 1993). Thus, they correlate the Jelm Formation from near Laramie, Wyoming, across the Rocky Mountain Front Range to the vicin-ity of Boulder, Colorado (Lykins Gulch and Bobcat Mountain). At Boulder, the Jelm Formation lies between the Lower Triassic Lykins Formation and the Middle Jurassic Entrada Sandstone (Fig. 5). As Lucas (1992, 1993) noted, the eolian strata of the Jelm Formation near Boulder are remarkably similar to those exposed along the Purgatoire River and at Two Buttes in southeastern Colorado (Fig. 5). The fossils described here demonstrate that these eolianites are stratigraphically below Upper Triassic conglomerates, as they are at Red Mountain in Wyoming. Therefore, we correlate the Jelm Formation into southeastern Colorado, as was advocated by Lucas (1992, 1993) and as is implicit in the sedimentological summary of Blakey et al. (1988).

In their review of late Paleozoic and Mesozoic eolian deposits of the American West, Blakey et al. (1988) recognize the Jelm Formation eolianites as one of two major Triassic erg systems, the other being the stratigraphically higher Rock Point–Wingate litho-some, which straddles the Triassic–Jurassic bound-ary. In this > 100 page summary of North American Mesozoic eolianites, Blakey et al. (1988, p. 31) sum-marize the known extent and relationships of the Jelm in just two paragraphs, concluding that “clearly this unit is in need of major regional study”—words that remain true 24 years later. This is especially true farther south, where the equivalents of the Jelm Formation as recognized here are the Baldy Hill Formation (Baldwin and Muehlberger, 1959; Lucas et al., 1987) and/or the “unnamed unit” of the Dockum Formation (Oriel and Mudge, 1956). We note here that the “Entrada” in the fence diagram of Oriel and Mudge (1956, pl. I) is part of the Jelm Formation, as are at least some of the finer-grained strata they assign to the “unnamed unit” of the Dockum Formation (=



22-2

3 21 20 19 18

1716 15

14

1312

11 10 9 8 7 6 5 43 2

24 1

Jurassic Triassic

12345678

91011

1213141516171819202123242526

2728

29

3031

5mSand

ston

e

Sand

ston

e,

Cros

s-be

dded

Mud

ston

e

Gyp

sum

Lam

inar

San

dsto

ne

Agat

e N

odul

es

Cove

red

Inte

rval

Cong

lom

erat

e

Lim

esto

ne

NW

SE

5m

sand

ston

e

sand

ston

e,

cros

s-be

dded

, eol

ian

silts

tone

mud

ston

e

gyps

um

lam

inar

san

dsto

ne

agat

e no

dule

s

cove

red

inte

rval

cong

lom

erat

e

limes

tone

foss

il bo

ne

sand

ston

e,

cros

s-be

dded

, �uv

ial

12

3456

78 0

5 4 32

6 17-8

12

3456

78 0

234

56

789

10111213

1415161718

1920

1

12

345678910111213

Cent

enni

al, W

Y

Jelm

Mtn

./ Re

d M

tn.,

WY

Bobc

at

Mou

ntai

n, C

O

Brav

o Ca

nyon

, CO

Purg

atoi

re

Rive

r, CO

Two

Butt

es, C

O

8

1234567910111213

Cobe

rt C

anyo

n/Ba

ldy

Hill

, NM

Ralston Creek Fm

Entr

ada

Fm

Perm

ian

stra

ta

Entr

ada

Fm

Baldy Hill FmTravesser Fm

CobertCanyon Beds

CobertCanyon Bed

Figu

re 5

. Ten

tativ

e cor

rela

tion

of T

riassi

c str

ata i

n Pi

cket

Wire

Can

yonl

ands

to o

ther

loca

litie

s in

Wyo

min

g, C

olor

ado,

and

New

Mex

ico.

See

Fig

ure 1

for i

ndex

map

of s

ec-

tion

loca

litie

s.



“Group”). This is similar to the usage of McKee et al. (1959, pl. 3), which indicates several hundred feet of Triassic strata of their “interval C” in the vicin-ity of the Purgatoire River. However, McKee et al. (1959, pl. 6) also follow McKee et al. (1956) in show-ing Middle Jurassic Entrada Sandstone above Triassic strata in this region.

It should be noted that the Red Draw Member of the Jelm often has been considered Late Triassic in age, based in part on fossil vertebrates (e.g., Pipiringos, 1968; assignment followed by Blakey et al., 1988). Such fossils are fragmentary, and only Lucas (1994) has described, illustrated, and provided provenance data of vertebrates from the Red Draw Member. Lucas (1994) demonstrated that the only age-diagnostic vertebrate fossils from the Red Draw Member were of capitosaurid amphibians, which in the American West are only known from strata of Middle Triassic or older age. Lucas (1993, 1994) therefore considered the Red Draw Member to be likely of Middle Triassic age, although he did not rule out an Early Triassic age. We concur with his assessment, and because the fauna we describe here is distinct from the Red Draw fauna Lucas (1994) described, we correlate the fossiliferous conglom-erate in Picket Wire Canyonlands with the Cobert Canyon Bed, not to the Jelm Formation.

The bone-bearing Cobert Canyon Bed in the Picket Wire Canyonlands also lie at the same strati-graphic position as the Gartra Formation of southern Wyoming (e.g., High and Picard, 1969; Picard, 1978; Fig. 5). Significantly, at Red Mountain near Laramie, Wyoming, Lucas (1994) documented bone fragments of phytosaurs and metoposaurs from a conglom-eratic interval (the “Jelm conglomerate” of Knight, 1917) above the eolian strata at the top of the Jelm Formation. Lucas (1994) correlated these conglomer-atic, bone-bearing strata to the Upper Triassic Gartra Formation, which is at the base of the Chinle Group in southern Wyoming, northwestern Colorado, and northeastern Utah (Lucas, 1993). Neither the Gartra Formation nor the Cobert Canyon Bed yield a suf-ficiently age-diagnostic fauna to confirm their age to a finer resolution than “Late Triassic,” so we cannot ascertain if they are direct correlatives of each other.

Importantly, this outcrop is one of the few local-ities where Chinle Group strata are known in east-ern Colorado. Generally, Colorado was relatively high during Chinle deposition, with fewer and

thinner exposed Upper Triassic sections than those of the Popo Agie to the north (High and Picard, 1969; Picard, 1978) or the Chinle–Dockum basins to the south (e.g., Lucas, 1993; Fig. 5). Indeed, in western Colorado some Chinle rocks lie directly on Precambrian basement that has been interpreted as remnants of Ancestral Rocky Mountain highlands (e.g., Lucas et al., 1997). The thin and discontinuous nature of Cobert Canyon Bed deposits in the Picket Wire Canyonlands also suggests ephemeral deposi-tion in relatively higher terrain.

In Picket Wire Canyonlands, approximately 20 m of alternating red siltstone and mudstone bear-ing numerous sedimentary structures (ripple marks, mud cracks, salt casts) exist below the Jelm and above a distinctive 1.5 m bed algal-laminated dolomite. This dolomite bed is a resistant bench former that is easily recognized and serves as a reliable marker bed, capping the exposures of probable Permian age. The bed has been variously assigned to both the Forelle Limestone Member of the Lykins Formation (Kauffman, 1986) and the Day Creek Dolomite (Maughan, 1980; Scott, 1968). Maughan (1980) referred to this unit as the Day Creek Dolomite for southeastern Colorado and demonstrated that it cor-relates regionally with other units of Middle Permian (Guadalupian) age, including the Alibates Dolomite lentil of the Quartermaster Formation of northeast-ern New Mexico. Thus, accepting that the algal-laminated bed is Permian in age, the entire Triassic section of Picket Wire Canyonlands is no more than 100 m thick. This is striking, as the Triassic section of northeastern New Mexico is more than 250 m thick (Lucas et al., 1987). This indicates that the Apishapa Arch of southeastern Colorado was topographically high during much of the Triassic, resulting in an area of non-deposition and erosion. Mallory (1972) indicates that this region also was strongly elevated during the Pennsylvanian. Further stratigraphic anal-ysis below the Jelm in Picket Wire Canyon may pro-vide much-needed information for enhanced strati-graphic correlation of Permian rocks.

CoNCLUSIoNS

The discovery of age-diagnostic vertebrate fossils from ephemeral conglomeratic deposits above the mas-sive eolianite in the Picket Wire Canyonlands requires a revision of the stratigraphic assignment and our



understanding of both the bone-bearing horizon and the underlying eolianite. Significantly, the eolianite cannot be of Jurassic age, because it is overlain by a unit bearing fossils of unambiguous Late Triassic age. The most parsimonious interpretation is that the eolianite is referred to the Red Draw Member of the Middle Triassic Jelm Formation, a unit otherwise known from south-central Wyoming and north-central Colorado. As noted by Blakey et al. (1988), the Jelm requires major restudy of its distribution, sedimentology, and stratigraphic relationships. The fossiliferous strata are best correlated to the Cobert Canyon Bed, previously known only from northeastern New Mexico and the panhandle of Oklahoma. These beds also may be cor-relative with the Gartra Formation of Wyoming. The presence of both the Jelm and the Cobert Canyon Bed in Picket Wire Canyonlands highlights the impor-tance of these isolated outcrops, especially when fos-siliferous, in understanding regional stratigraphic rela-tionships.

ACKNoWLEdGMENTS

Volunteers with Passport in Time, an archaeol-ogy and historic preservation program of the USDA, helped to locate and collect some of the fossils from 2001 through 2006. Participants in the 2009 and 2010 Appalachian State University (Appalachian) “Triassic Trip” field courses helped collect additional fossils and the stratigraphic data presented here. The Department of Geology at Appalachian supported travel costs asso-ciated with these trips and presentation of results at the Southeast Section of the Geological Society of America meeting in 2011. One of us (EJS) was supported by an Undergraduate Research Assistantship and Student Travel Grants from Appalachian’s Office of Student Research. Reviewers L. H. Tanner and M. D. Picard provided thoughtful reviews of an earlier version of this manuscript.

rEFErENCES CITEd

Baldwin, B., and Muehlberger, W. R., 1959, Geologic studies of Union County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin, v. 63, 171 p.

Blakey, R. C., Peterson, F., and Kocurek, G., 1988, Synthesis of late Paleozoic and Mesozoic eolian deposits of the western interior of the United States: Sedimentary Geology, v. 56, p. 3–125.

Colbert, E. H., and Imbrie, J., 1956, Triassic metoposau-rid amphibians: American Museum of Natural History Bulletin, v. 110, p. 403–452.

Cope, E. D., 1869, Synopsis of the extinct Batrachia, Reptilia, and Aves of North America: American Philosophical Society Transactions, v. 14, p. 252.

Duce, J. T., 1924, Geology of parts of Las Animas, Otero, and Bent Counties: Colorado Geological Survey Bulletin, v. 27, p. 72–102.

Folk, R. L., 1974, Petrology of sedimentary rocks: Hemphill Publishing Co., Austin, Texas, 184 p.

Goddard, E. N., Trask, P. D., DeFord, R. K., and three others, 1984, Rock-color chart: Geological Society of America.

Heckert, A. B., 2004, Late Triassic microvertebrates from the lower Chinle Group (Otischalkian–Adamanian: Carnian), southwestern U.S.A.: New Mexico Museum of Natural History and Science Bulletin, v. 27, 170 p.

Heckert, A. B., and Lucas, S. G., 2000, Taxonomy, phylog-eny, biostratigraphy, biochronology, paleobiogeography, and evolution of the late Triassic Aetosauria (Archosauria: Crurotarsi): Zentralblatt für Geologie und Paläontologie, Teil I, v. 1998, p. 1539–1587.

Heckert, A. B., Lucas, S. G., and Hunt, A. P., 2005, Triassic vertebrate fossils in Arizona: New Mexico Museum of Natural History and Science Bulletin, v. 29, p. 16–44.

High, L. R., Jr., and Picard, M. D., 1969, Stratigraphic rela-t ions within upper Chugwater Group (Triassic), Wyoming: American Association of Petroleum Geologists Bulletin, v. 53, p. 1091–1104.

Howard, A. D., and Williams, J. W., 1972, Physiography, Rocky Mountain Association of Geologists, Geologic atlas of the Rocky Mountain region: Denver, A. B. Hirschfeld Press, p. 29–31.

Howie, A. A., 1970, A new capitosaurid labyrinthodont from East Africa: Palaeontology, v. 13, p. 210–253.

Hubbell, R. G., 1956, Upper Triassic facies relations in north-ern Carbon County, Wyoming: American Association of Petroleum Geologists Bulletin, v. 40, p. 2743–2748.

Huene, F. V., 1935–1942, Die fossi lien Reptilien des südamerikanischen Gondwanalandes, Ergebnisse der Sauriergrabungen in Südbrasilien 1928/29: Munich, C. H. Beck’sche Verlagsbuchhandlung, 332 p.

Hungerbühler, A., 2000, Heterodonty in the European phy-tosaur Nicrosaurus kapffi and its implications for the taxo-nomic utility and functional morphology of phytosaur den-titions: Journal of Vertebrate Paleontology, v. 20, p. 31–48.

____2002, The late Triassic phytosaur Mystriosuchus west-phali, with a revision of the genus: Palaeontology, v. 45, p. 377–418.

Hunt, A. P., 1989, Cranial morphology and ecology among phytosaurs, in Lucas, S. G., and Hunt, A. P., eds., Dawn



of the age of dinosaurs in the American Southwest: New Mexico Museum of Natural History, Albuquerque, New Mexico, p. 349–354.

____1993, Revision of the Metoposauridae (Amphibia: Temnospondyli) and description of a new genus from western North America: Museum of Northern Arizona Bulletin, v. 59, p. 67–97.

Hunt, A. P., and Lucas, S. G., 1993, Taxonomy and strati-graphic distribution of late Triassic metoposaurid amphibi-ans from Petrified Forest National Park, Arizona: Arizona–Nevada Academy of Science Journal, v. 27, p. 89–96.

Jaeger, G. F., 1828, Aber die fossilien Reptilien, welche in Würtemberg aufgefunden worden sind: Stuttgart, Metzler, 48 p.

Kauffman, E. G., 1986, Geological and paleontological site analysis of the Pinon Canyon Maneuver Site, Las Animas County, Colorado: United States Army.

Knight, S. H., 1917, Age and origin of the Red Beds of southeastern Wyoming: Geological Society of America Bulletin, v. 28, p. 168–169.

Laurenti, J. N., 1768, Classis Reptilium. Specimen medicum, exhibens synopsis Reptilium emendatum, cum experi-mentis circa venena et antidote Reptilium Austriacorum: Vienna, J. Thom. Nob. et Trattnern, 214 p.

Linnaeus, C., 1758, Systema Naturae: Stockholm, Salvius.Lee, W. T., 1927, Correlation of geologic formations between

east-central Colorado, central Wyoming, and southern Montana: U.S. Geological Survey Professional Paper, v. 149, 80 p.

Lockley, M. G., Houck, K. J., and Prince, N. K., 1986, North America’s largest dinosaur trackway site: Implications for Morrison Formation paleoecology: Geological Society of America Bulletin, v. 97, p. 1163–1176.

Long, R. A., and Ballew, K. L., 1985, Aetosaur dermal armor from the late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park: Museum of Northern Arizona Bulletin, v. 47, p. 45–68.

Long, R. A., and Murry, P. A., 1995, Late Triassic (Carnian and Norian) tetrapods from the southwestern United States: New Mexico Museum of Natural History and Science Bulletin, v. 4, 254 p.

Lucas, S. G., 1992, Stratigraphy and age of the Triassic Jelm Formation, Wyoming–Colorado: Geological Society of America Abstracts with Programs, v. 24, no. 6, p. 49.

____1993, The Chinle Group: Revised stratigraphy and bio-chronology of Upper Triassic nonmarine strata in the western United States: Museum of Northern Arizona Bulletin, v. 59, p. 27–50.

____1994, The beginning of the age of dinosaurs in Wyoming: Casper, Wyoming Geological Association, 44th Annual Field Conference Guidebook, p. 105–113.

____2004, The Triassic and Jurassic systems in New Mexico, in Mack, G. H., and Giles, K. A., eds., The geology of New Mexico: A geologic history: New Mexico Geological Society, v. 11, p. 137–152.

Lucas, S. G., and Heckert, A. B., 2003, Jurassic stratigraphy in west-central New Mexico: New Mexico Geological Society Guidebook, v. 54, p. 289–301.

Lucas, S. G., Heckert, A. B., Estep, J. W., and Anderson, O. J., 1997, Stratigraphy, biostratigraphy, and sequence stratig-raphy of the Upper Triassic Chinle Group, Four Corners region: New Mexico Geological Society Guidebook, v. 48, p. 81–107.

Lucas, S. G., Hunt, A. P., and Hayden, S. N., 1987, The Triassic system in the Dry Cimarron Valley, New Mexico, Colorado, and Oklahoma: New Mexico Geological Society Guidebook, v. 38, p. 97–117.

Lydekker, R., 1887, The fossil Vertebrata of India: Records of the Geological Survey of India, v. 20, p. 51–80.

____1890, Catalogue of the fossil Reptilia and Amphibia in the British Museum (Natural History). Part IV: London, British Museum (Natural History), London, 295 p.

Mallory, W. W., 1972, Pennsylvanian arkose and the ancestral Rocky Mountains: Denver, Rocky Mountain Association of Geologists, Geologic atlas of the Rocky Mountain region: A. B. Hirschfeld Press, p. 131–132.

Marsh, O. C., 1884, The classification and affinities of dino-saurian reptiles: Nature, v. 31, p. 68–69.

Maughan, E. K., 1980, Permian and Lower Triassic geol-ogy of Colorado, in Kent, H. C., and Potter, K. W., eds., Symposium on Colorado geology, Rocky Mountain Association of Geologists Geologica l Conference Guidebook, p. 103–110.

McKee, E. D., and eight others, 1956, Paleotectonic maps of the Jurassic system: Washington, D.C., U.S. Geological Survey, Miscellaneous Geologic Investigations Map I-175, 6 p.

McKee, E. D., Oriel, S. S., Ketner, K. B., and four others, 1959, Paleotectonic maps of the Triassic system: U.S. Geological Survey, Miscellaneous Geologic Investigations, Map I-300, 32 p.

Morales, M., 1987, Terrestrial fauna and flora from the Triassic Moenkopi Formation of the southwestern United States: Arizona–Nevada Academy of Science Journal, v. 22, p. 1–19.

Oriel, S. S., and Mudge, M. R., 1956, Problems of Lower Mesozoic stratigraphy in southeastern Colorado: Rocky Mountain Association of Geologists, Guidebook to the geology of the Raton Basin, Colorado, p. 19–24.

Pa rker, W. G., 2007, Rea sse s sment of the aetosaur 'Desmatosuchus' chamaensis with a reanalysis of the phy-logeny of the Aetosauria (Archosauria: Pseudosuchia): Journal of Systematic Palaeontology, v. 5, p. 41–68.



Picard, M. D., 1978, Stratigraphy of Triassic rocks in west-cen-tral Wyoming, in Boyd, R. G., ed., Resources in the Wind River Basin: Casper, Wyoming Geological Association, 30th Annual Field Conference Guidebook p. 101–130.

Pipiringos, G. N., 1953, Correlation of marine Jurassic and related rocks in the Laramie Basin, Wyoming, in Blackstone, D. L., and Gilder, H. R. V., eds., Laramie Basin, Wyoming and North Park, Colorado: Casper, Wyoming Geological Association, 8th Annual Field Conference Guidebook, p. 34–39.

____1957, Stratigraphy of the Sundance, Nugget, and Jelm Formations in the Laramie Basin, Wyoming: Wyoming Geological Survey Bulletin, v. 47, 63 p.

____1968, Correlation and nomenclature of some Triassic and Jurassic rocks in south-central Wyoming: U.S. Geological Survey Professional Paper 594-D, p. D1–D26.

____1972, Upper Triassic and pre-Morrison Jurassic rocks, in Segerstrom, K., and Young, E. J., eds., General geol-ogy of the Hahns Peak and Farwell Mountain quadran-gles, Routt County, Colorado: U.S. Geological Survey Bulletin, v. 1349, p. 18–29.

Pipiringos, G. N., Hail, W. J., Jr., and Izett, G. A., 1969, The Chinle (Upper Triassic) and Sundance (Upper Jurassic) formations in north-central Colorado: U.S. Geological Survey Bulletin, v. 1274-N, p. D1–D26.

Prothero, D. R., and Schwab, F., 2003, Sedimentary geology: 2nd ed., New York, W. H. Freeman, 600 p.

Schoch, R. R., and Milner, A. R., 2000, Sterospondyli: Stem-Sterospondyli, Rhinesuchidae, Rhytidostea, Trematosauroidea , Capitosauroidea : München, Encyclopedia of Paleoherpetology, Verlag Dr. Friedrich Pfeil, 203 p.

Schumacher, B. A., 2002, Stratigraphy and paleontol-ogy of Picket Wire Canyonlands, Comanche National Grassland, Pike/San Isabel National Forest, southeast-ern Colorado, in New Concepts in Global Tectonics Symposium, Otero Junior College, La Junta, Colorado, p. 319–332.

____2008, The Last Chance site, a new sauropod quarry from the Upper Jurassic Morrison Formation, southeast-ern Colorado, in Farley, G. H., and Choate, J. R., eds.: Unlocking the unknown: Papers honoring Dr. Richard J. Zakrzewski: Fort Hays Studies Special Issue, v. 2, p. 77–88.

Schumacher, B. A., Heckert, A. B., and Lucas, S. G., 2009, Vertebrate fossi l discoveries prompt major strati-graphic shift in Mesozoic stratigraphy of Picket Wire Canyonlands, Comanche National Grassland, southeast-ern Colorado: Geological Society of America, Abstracts with Programs, Portland, Oregon, v. 41, no. 7, p. 546.

Scott, G. R., 1968, Geologic and structure contour map of the La Junta Quadrangle, Colorado and Kansas: U.S

Geological Survey, Miscellaneous Geologic Investigations Map, I-560, scale 1:24,000.

Shropshire, K. L., 1974, The Chinle and Jelm Formations (Triassic) of north-central Colorado [Ph.D. dissert.]: Boulder, Colorado, University of Colorado, 229 p.

Sload, E. J., Heckert, A. B., Lucas, S. G., Schumacher, B. A., 2011, Triassic tetrapod fossils found above “Jurassic” strata in Picketwire Canyon, southeastern Colorado, require major revision of lower Mesozoic stratigraphy across the southern High Plains: Geological Society of America, Abstracts with Programs, v. 43, no. 2, p. 3.

Stocker, M. R., 2010, A new taxon of phytosaur (Archosauria: Pseudosuchia) from the late Triassic (Norian) Sonsela Member (Chinle Formation) in Arizona, and a critical reevaluation of Leptosuchus Case, 1922: Palaeontology, v. 53, p. 997–1022.

Stovall, J. W., 1943, Stratigraphy of the Cimarron Valley (Mesozoic rocks), in Schoff, S. L., ed., Geology and ground water resources of Cimarron County, Oklahoma: Oklahoma Geological Survey Bulletin, v. 64, p. 43–100.

Tweto, O., 1979, The geologic map of Colorado: U.S. Geological Survey, scale 1:500,000.

Watson, D. M. S., 1920, The structure, evolution and origin of the Amphibia: the “orders” Rachitomi and Stereospondyli: Royal Society of London Philosophical Transactions, Series B, v. 209, 73 p.

Weist, W. G., Jr., 1965, Geology and occurrence of ground-water in Otero County and the southern part of Crowley County, Colorado: U.S. Geological Survey Water-Supply Paper, v. 1799, 90 p.

Welles, S. P., and Cosgriff, J., 1965, A revision of the laby-rinthodont family Capitosauridae and a description of Parotosaurus peabodyi, n. sp. from the Wupatki Member of the Moenkopi Formation of northern Arizona: University of California Publications in Geological Sciences, v. 84, p. 1–150.

Zittel, K. A., von, 1887–90, Handbuch der Paläontologie: Oldenbourg, München und Leipzig Abtei lung 1. Paläozoologie Band III. Vertebrata (Pisces, Amphibia, Reptilia, Aves).

Manuscript Submitted September 13, 2011

Revised Manuscript Submitted December 12, 2011

Manuscript Accepted January 6, 2012



Appendix 1. Description of measured sections presented in Figure 3. Bravo Canyon C

Section base begins on the road at notch where stream crosses it at UTM Zone 13S 0617698E., 4158627N. Strata strike N. 45–60˚ W., and dip as much as 15˚ to NE. Students measured this section using dip angles ranging from 5–15˚ under the direction of ABH. Thicknesses represent composite values, typically ~10˚. Unit Description Thickness (m) Morrison Formation (Upper Jurassic)

11 Nodular layer of agates. Agates are pale yellowish brown (10YR6/2) to yellowish gray (5Y8/1) fresh, weather to medium bluish gray (5B5/1). Locally used as the base of the Morrison Formation. Gypsiferous.

Top of section (not measured)

Ralston Creek Formation (Middle Jurassic)

10 Mostly covered interval. Available outcrops are laminar, micritic to grainstone limestones. These are yellowish gray (5Y7/2) when weathered and light olive gray (5Y5/2) when fresh.

5.5

9 Ledge-forming gypsiferous outcrop; ranges in color from pale greenish yellow (10Y8/2) to grayish yellow green (5GY7/2). Consists mostly of heavily weathered gypsum particles.

0.6

8B Gypsiferous unit ranging from grayish orange pink (10R8/2 to 10R7/4) to moderate orange pink (5YR8/4). Breaks off in larger nodules than gypsum of unit 9.

Combined thickness of unit 8 (A+B): 2.0

8A Flaggy silt/sandstone; pale brown (5YR5/2) with fine to very fine grains; sub-angular quartz arenite.

7 Gypsiferous unit, ranging from yellowish gray (5Y7/2) to light olive gray (5Y5/2) when weathered, and grayish orange pink (10R8/2) to grayish pink (5R8/2) to moderate orange pink (5YR8/4) when fresh. Also very pale orange (10YR8/2) and pale yellowish orange (10YR8/6). Breaks off in larger (~8 cm) nodules like those of unit 8B.

1.1

6 Gypsiferous unit of white color (N9) with very slight pink hue (N9 or color of higher value than grayish pink (5R8/2)). Poorly indurated—breaks apart into fine powder easily.

3.8

5 Heavily weathered gypsiferous unit with some clay. Color ranges from almost white (N9) to yellowish gray (5Y7/2). Contains more clay minerals than higher gypsiferous units.

6.2

4 Base of unit is gray and gypsiferous with light olive gray color (5Y5/2). Heavily weathered and appears to have larger translucent grains. Some gypsiferous material is very pale orange (10YR8/2) or grayish orange pink (5YR7/2). Top of unit is gray/green sandstone with grain sizes ranging from 2.5–1.5 phi. Color is light olive gray (5Y5/2) when weathered and the same when fresh.

4.5

3 Largely covered interval—very heavily weathered with average color pale yellowish brown (10YR6/2). Grain size ranges from 3.0–2.0 phi (fine-grained). Sub-angular to sub-rounded clasts. Calcareous.

5

Cobert Canyon Beds(?) 2 Sandstone; grayish red (5R5/2) when fresh and pale brown (5YR5/2) when weathered.

Ranging in grain size from > 4–3 phi (fine- to very fine–grained). Well indurated and cemented. Not as heavily weathered as higher units.

Lens-like unit, thickness ranging from 0.3 to 1.5



Jelm Formation (Middle Triassic) Red Draw Member 1 Sandstone; mottled; pale yellowish brown (10YR6/2) when weathered and pale yellowish

orange (10YR8/6) when fresh. Grain size ranges from 3–2 phi. 3+ (Basal Unit)

Total Thickness: 29.6 m Bravo Canyon B Base of section begins at UTM zone 13S, 0618540E, 4157992 N. Strata strike N. 53˚ W. and dip approximately 8˚ to NE. Students measured using dips of 7˚, 8˚, and 10˚, thicknesses compiled. Some thicknesses measured directly from cliff-forming outcrops. Top of section is at UTM zone 13S 0618620E, 4158209N. Section measured 20 May, 2010, by students under the direction of ABH. Unit Description Thickness

(m) Jelm Formation (Middle Triassic) Red Draw Member 6 Color mottled sandstone. Appears coarse-grained due to color mottling, but is actually fine-

grained, rounded to well-rounded, well-sorted quartz arenite. Color: moderate reddish brown (10R4/6) and grayish orange (10YR7/4) caused by diagenesis. Some patches are even lighter very pale orange (10YR8/2) and darker dark reddish brown (10R3/4). Locally calcareous. Top is a stripped dip slope surface.

Not measured (top of section)

5 Sandstone; light brown (5YR5/6) to very pale orange (10YR8/2). Fine grained, rounded, well-sorted quartz arenite.

8.5

4 Lithology similar to unit 3; less massive. 6.5 3 Sandstone; grayish orange (10YR7/4) to light brown (5YR5/6); very fine– to fine-grained,

subrounded to rounded, well-sorted quartz arenite. 17.25

2 Color mottled sandstone similar in lithology to unit 3; light brown (5YR6/4 to 5YR5/6). Slightly calcareous.

41.5

1 Sandstone; grayish red (10R4/2) and pale red (10R6/2). Very fine–grained, sub-angular to sub-rounded, moderately well-sorted quartz arenite. Well indurated and cemented. This unit is equivalent to unit 17 of the Bravo Canyon A section.

Basal Unit

Bravo Canyon A Base of section begins at UTM zone 13S, 0618009E, 4157929 N. Strata strike N. 60˚ W. and dip less than 5˚ to SW., so treated as flat lying. Top of section is at UTM zone 13S 0617958E, 4157857N. Section measured 20 May, 2010, by students under the direction of ABH. Unit Description Thickness

(m) Jelm Formation (Middle Triassic) Red Draw Member 17 Sandstone; moderate brown (5YR4/4 to 5YR3/4). Very fine– to fine-grained color mottled

sandstone, rounded to sub-rounded, well-sorted quartz arenite. 10+ (top)

15 Sandstone; light brown (5YR5/6). Very fine– to fine-grained, sub-rounded to rounded, well-sorted quartz arenite.

1.5

14 Sandstone; light brown (5YR6/4). Very fine–grained, sub-angular to sub-rounded, well-sorted quartz arenite. Calcareous.

1.0

13 Sandstone; pale reddish brown (10YR5/4). Very fine–grained, finer than unit 14, sub-rounded to rounded, well-sorted quartz arenite.

1.2

12 Sandstone; moderate brown (5YR3/4). Very fine–grained, sub-rounded to rounded, well-sorted quartz arenite. Well cemented.

1.0



11 Sandstone; light brown (5YR6/4). Very fine–grained, almost silty; rounded to sub-rounded, well-sorted to very well–sorted, quartz arenite. Locally calcareous.

7.0

10 Sandstone; pale reddish brown (10R5/4). Very fine–grained, rounded, well-sorted, micaceous sandstone. Calcareous.

2.0

9 Sandstone; light brown (5YR5/6), very fine– to fine-grained, sub-rounded to rounded, well-sorted to very well–sorted. Very calcareous.

5.3

8 Sandstone; moderate reddish brown (10R4/6) and dark reddish brown (10R3/4). Very fine–grained (almost silt), sub-rounded to rounded, very well–sorted, slightly micaceous. Locally calcareous.

1.5

7 Sandstone and siltstone; similar to unit 5; forms a ledge. 1.5 6 Sandstone; pale reddish brown (10R5/4) and moderate reddish brown (10R4/6). Very fine–

grained (almost silty), sub-angular to sub-rounded, well-sorted, quartz, some feldspar. Locally calcareous.

2.0

5 Sandstone; light brown (5YR6/4 to 5YR5/6). Very fine-grained sandstone-siltstone, sub-angular to sub-rounded, very well–sorted quartz arenite. Weakly calcareous.

2.0

4 Sandstone; pale reddish brown (10R5/4). Fine-grained to very fine–grained, angular to sub-rounded, well-sorted, quartz arenite. Abundance of calcite cement. Locally calcareous.

2.5

3 Siltstone; moderate brown (5YR4/4); well indurated, mildly calcareous. 2.4 2 Sandstone; moderate brown (5YR4/4); very fine–grained, moderately well-sorted (some clasts

are 5x larger than others), quartz rich. Locally calcareous. 0.3

1 Siltstone; moderate brown (5YR3/4); similar to unit 3, well-sorted, weakly calcareous. 3.3 Total thickness: 34.6 m !


Date post:	10-Jan-2023
Category:	Documents
Upload:	fed
View:	0 times
Download:	0 times

Triassic fossils found stratigraphically above ‘Jurassic’ eolianites

Documents