Moenave Formation Research Articles

On the cranial structure of a new protosuchid (Archosauria: Crocodyliformes) from the McCoy Brook Formation (Lower Jurassic) of Nova Scotia, Canada

ABSTRACT Well-preserved skull bones referable to a new protosuchid, Protosuchus micmac, are common in the lower portion of the McCoy Brook Formation (Lower Jurassic: Hettangian) of Nova Scotia, Canada. P. micmac is closely related to P. richardsoni (Brown, 1933) from the Moenave Formation (Lower Jurassic) of Arizona and to P. haughtoni (Busbey et Gow, 1984) from the upper Stormberg Group (Lower Jurassic) of southern Africa. It differs from the latter taxa in several craniodental features, most notably in the presence of two large caniniform teeth in the dentary. The geographic range of protosuchid crocodyliforms provides further confirmation of a cosmopolitan distribution of many groups of terrestrial tetrapods during the Early Jurassic.

Posterolateral markings on dinosaur tracks, Cameron Dinosaur Tracksite, Lower Jurassic Moenave Formation, northeastern Arizona

Posterolateral markings, appearing as short, rectilinear grooves projecting from the posterior margins of digit IV, occur on tracks in five of 34 trackways at the Cameron Dinosaur Tracksite in the Dinosaur Canyon Member of the Lower Jurassic Moenave Formation, northeastern Arizona. A new ichnospecies, Grallator (Anchisauripus) madseni, is described from these five trackways made by bipedal, tridactyl dinosaurs. The size and morphology of the tracks suggest that they may represent trackmakers similar to Coelophysis and Syntarsus, two dinosaurs described from Triassic and Jurassic deposits in northeastern Arizona and northwestern New Mexico.Among the working hypotheses cited as possible causes of the posterolateral markings are: 1) they were made by the dragging of digit III or of the metatarsus; 2) they are marks produced by a feature similar to a horny excrescence or cornified ridge on digitometatarsal pad IV; and 3) they are related to a peculiarity that may have resulted from sexual dimorphism.

Paleomagnetism of the Chinle and Kayenta Formations, New Mexico and Arizona

Paleomagnetic data were obtained from 22 sites (6–10 samples/site) in the Upper Shale Member of the Chinle Formation, 43 sites in the Owl Rock Member of the Chinle Formation, and 35 sites in the Kayenta Formation. Thermal demagnetization and data analyses indicate that within‐site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization (ChRM). Site‐mean directions define at least four antipodal polarity zones within each member/formation, suggesting the ChRM was acquired soon after deposition. Fifteen site‐mean virtual geomagnetic poles (VGPs) from the Upper Shale Member of the Chinle Formation yield an early Norian paleomagnetic pole position of 57.4°N, 87.8°E (K = 60, A95 = 5.0°). Eighteen site‐mean VGPs from the Owl Rock Member of the Chinle Formation yield a middle Norian paleomagnetic pole position of 56.5°N, 66.4°E (K = 183, A95 = 2.6°). Twenty‐three site‐mean VGPs from the Kayenta Formation yield a Pliensbachian pole position of 59.0°N, 66.6°E (K = 155, A95 = 2.4°). Combined with paleomagnetic poles from the Moenave Formation and the Shinarump Member of the Chinle Formation, these data record ∼30 m.y. of North American apparent polar wander (APW) within a regional stratigraphic succession. During the Camian and Norian stages of the Late Triassic, Chinle poles progress westward. During the Hettangian through Pliensbachian stages of the Early Jurassic, the pattern of APW changed to an eastward progression. Even after correction for 4° clockwise rotation of the Colorado Plateau, a sharp comer in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (∼200 Ma) Moenave Formation (59.4°N, 59.2°E). Amongst other implications, the sharp change in the APW path at the J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter.

Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah

The Moenave Formation (Dinosaur Canyon Member) and the closely associated Wingate Sandstone in southern Utah represent the southwestern erg margin of the Lower Jurassic Wingate erg. The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies. The sediments of the basal association are composed of three drying-upward or locally drying-wetting-upward sequences that can be traced from the erg margin 100 km slightly upwind (that is, toward the west-northwest) into the outer erg-margin zone. Sequences of the inner erg-margin zone contain dune facies in their upper parts, sequences in the intermediate erg-margin zone contain sand-sheet facies, and those in the outer erg-margin zone contain sabkha facies. The basal association thins markedly toward the west-northwest. Sediments of the overlying association contain up to six drying-upward or drying-wetting-upward sequences but are apparently only fully preserved in the outer erg-margin zone. Individual sequences show much lateral variation but characteristically contain lower, muddy, ephemeral-stream deposits and upper, sandy, ephemeral-stream deposits; sand-sheet deposits; or incipientdune deposits. The general time trend is interpreted as a gradual change toward a more humid climate. The stacked drying-upward or drying-wetting-upward sequences suggest that this long-term climatic change was characterized by a number of fluctuations between relatively arid and humid conditions. These climatic fluctuations apparently controlled both sediment yield to the basin and the depositional conditions in the erg-margin region. During arid intervals, ephemeral braided streams transported sandy bed-load into the erg-margin area, where eolian reworking took place. During humid intervals, sheet-floods and high-sinuosity rivers transported a more mud-rich sediment load into the basin, where eolian activity was minimal.

Sandstone-body structures and ephemeral stream processes in the Dinosaur Canyon Member, Moenave Formation (Lower Jurassic), Utah, U.S.A.

Studies of fluvial sandstone-body structures in the Lower Jurassic Dinosaur Canyon Member suggest a threefold subdivision of the ephemeral stream deposits. Sandstone-sheets with interbedded siltstones are less than 1 m thick and laterally extensive for hundreds of metres. They are interpreted as sheetflood deposits. Simple channel sandstone-bodies are a few metres thick and a few tens of metres wide. They reflect solitary channel incision, episodic migration and plugging. Multistorey channel sandstone-bodies are a few metres thick and laterally extensive for hundreds of metres. They are composed of several channel-shaped storeys and exhibit only local incision. The multistorey sandstone-bodies are interpreted as braided ephemeral stream deposits. Two sandstone-sheet subtypes with grooves and mounds, respectively, are interpreted as intermediate between the sheetflood deposits and solitary incised channel deposits on one hand and between sheetflood deposits and braided stream deposits on the other hand. The solitary channels and braided streams are accordingly interpreted to be initiated from sheetfloods through differential erosion and differential deposition, respectively. This model of channel evolution from sheetfloods is probably applicable to other semiarid and arid fluvial environments dominated by surface runoff.

Paleomagnetism of the Moenave Formation: Implications for the Mesozoic North American apparent polar wander path

Previous syntheses of the Mesozoic North American apparent polar wander (APW) path have suggested the presence of a sharp corner or cusp (the J-1 cusp) in the Late Triassic to Early Jurassic. Paleomagnetic data from the Lower Jurassic (Sinemurian) Moenave Formation clarify details of this important feature in the APW path. A total of 220 oriented cores were collected from 31 levels in four stratigraphic sections of the Moenave Formation in northern Arizona and southern Utah. Thermal demagnetization in multiple steps, generally between 600 and 660 OC, was successful in revealing the characteristic component of natural remanent magnetization at 23 sites. The paleomagnetic pole is lat = 58.2ON, long = 51.9OE; radius of 95% confidence (ag5) = 4.5'. The Moenave pole position verifies the existence of the J-1 cusp, and thus supports the use of paleomagnetic Euler pole analysis in modeling the pattern of APW.