Canyon Formation Research Articles

The role of preexisting fractures and faults during multistage hydraulic fracturing in the Bakken Formation

We performed an integrated study of multistage hydraulic fracture stimulation of two parallel horizontal wells in the Bakken Formation in the Williston Basin, North Dakota. There are three distinct parts of this study: development of a geomechanical model for the study area, interpretation of multiarray downhole recordings of microseismic events, and interpretation of hydraulic fracturing data in a geomechanical context. We estimated the current stress state to be characterized by an NF/SS regime, with [Formula: see text] oriented approximately [Formula: see text]. The microseismic events were recorded in six vertical observation wells during hydraulic fracturing of parallel wells X and Z with three unusual aspects. First, rather than occurring in proximity to the stages being pressurized, many of the events occurred along the length of well Y, a parallel well located between wells X and Z that had been in production for approximately [Formula: see text] years at the time X and Z were stimulated. Second, relatively few fracturing stages were associated with an elongated cloud of events trending in the direction of [Formula: see text] as was commonly observed during hydraulic fracturing. Instead, the microseismic events in a few stages appeared to trend approximately [Formula: see text], approximately 30° from the direction of [Formula: see text]. Earthquake focal plane mechanisms confirmed slip on faults with this orientation. Finally, the microseismic events were clustered at two distinct depths: one near the depth of the well being pressurized in the Middle Bakken Formation and the other approximately [Formula: see text] above in the Mission Canyon Formation. We proposed that steeply dipping N75°E striking faults with a combination of normal and strike-slip movement were being stimulated during hydraulic fracturing and provided conduits for pore pressure to be transmitted to the overlaying formations. We tested a simple geomechanical analysis to illustrate how this occurred in the context of the stress field, pore pressure, and depletion in the vicinity of well Y.

New information on the rare horned dinosaurArrhinoceratops brachyops(Ornithischia: Ceratopsidae) from the Upper Cretaceous of Alberta, Canada

Arrhinoceratops brachyops is a poorly understood chasmosaurine ceratopsid from the Upper Cretaceous Horseshoe Canyon Formation of Alberta, previously described on the basis of only a single skull. Here, we report on a second specimen attributable to this species, including a relatively complete skull, syncervical, and partial left forelimb. This second specimen clarifies aspects of morphology not visible in the holotype, and also elucidates variation in A. brachyops. The species is distinguished by a square-shaped triangular process of the premaxilla, a steeply inclined triturating surface of the predentary, and a triangular nasal horncore in horizontal section. The dentary is also distinctive in bearing a bony lateral ridge similar to that of Anchiceratops ornatus, but more strongly developed. Phylogenetic analysis cannot resolve the relationships of Arrhinoceratops beyond the level of Chasmosaurinae, owing to both missing data and conflicting characters. However, we do find some support for a deep split within Chasmosaurinae, contrary to conventional topologies. We also report on other fragmentary specimens plausibly attributable to A. brachyops that suggest a minimum age range of approximately 750 ka for this species.

Early Cretaceous construction of a structural culmination, Eureka, Nevada, U.S.A.: Implications for out-of-sequence deformation in the Sevier hinterland

Assessing temporal relationships between foreland and hinterland deformation in fold-thrust belts is critical to understanding the dynamics of orogenic systems. In the western U.S. Cordillera, the central Nevada thrust belt (CNTB) has been interpreted as a hinterland component of the Sevier fold-thrust belt in Utah. However, imprecise timing constraints on CNTB deformation have hindered evaluation of space-time patterns of strain partitioning between these two thrust systems. To address this problem, new 1:24,000-scale geologic mapping and balanced cross sections are presented through the CNTB near Eureka, Nevada, in conjunction with industry drill-hole data, conodont age determinations, and 40 Ar/ 39 Ar and U-Pb ages from volcanic, intrusive, and sedimentary rocks. Our mapping redefines the first-order structures and deformation geometry of the CNTB at the latitude of Eureka. Contractional structures include two north-striking, east-vergent thrust faults, the Prospect Mountain thrust and Moritz-Nager thrust, which are connected as the same fault in cross section, several north-striking map-scale folds, and a Cambrian over Silurian relationship observed in multiple drill holes, corresponding to repetition of ∼2–2.5 km of stratigraphy, that defines the blind Ratto Canyon thrust. Two distinct sets of normal faults cut the contractional structures, and are overlapped by a regional late Eocene (ca. 37 Ma) subvolcanic unconformity. Retrodeformation of both sets of normal faults reveals the existence of the Eureka culmination, a 20-km-wide, 4.5-km-tall anticline with limb dips of 25°–35°, that can be traced for ∼100 km north-south on the basis of Paleogene erosion levels. The culmination is interpreted as a fault-bend fold that formed from ∼9 km of eastward displacement of the Ratto Canyon thrust sheet over a buried footwall ramp. The type exposure of the Early Cretaceous (Aptian) Newark Canyon Formation (NCF) is preserved on top of Mississippian, Pennsylvanian, and Permian rocks on the eastern limb of the Eureka culmination. We propose that the NCF was deposited in a piggyback basin on the eastern limb of the culmination as it grew, which is consistent with published east-directed paleocurrents and provenance data suggesting derivation from proximal late Paleozoic subcrop units. Syncontractional deposition of the NCF is used to define the probable Aptian construction of the Eureka culmination and associated slip on the Ratto Canyon thrust at depth. After deposition, the NCF continued to be folded during late-stage growth of the culmination. Aptian deformation in the CNTB at Eureka postdated migration of the Sevier thrust front into Utah by at least ∼10 m.y. and possibly as much as ∼30 m.y., and therefore represents out-of-sequence hinterland deformation. CNTB deformation was coeval with emplacement of the Canyon Range thrust sheet in the type-Sevier thrust belt in western Utah, and may represent internal shortening of this orogen-scale thrust sheet that acted to promote further eastward translation.

Formation of pockmarks and submarine canyons associated with dissociation of gas hydrates on the Joetsu Knoll, eastern margin of the Sea of Japan

This study, based on 3.5kHz SBP, 3D seismic data and long piston cores obtained during MD179 cruise, elucidated the timing and causes of pockmark and submarine canyon formation on the Joetsu Knoll in the eastern margin of the Sea of Japan. Gas hydrate mounds and pockmarks aligned parallel to the axis on the top of the Joetsu Knoll are associated with gas chimneys, pull-up structures, faults, and multiple bottom-simulating reflectors (BSRs), suggesting that thermogenic gas migrated upward through gas chimneys and faults from deep hydrocarbon sources and reservoirs. Seismic and core data suggest that submarine canyons on the western slope of the Joetsu Knoll were formed by turbidity currents generated by sand and mud ejection from pockmarks on the knoll. The pockmark and canyon formation probably commenced during the sea-level fall, lasting until transgression stages. Subsequently, hydropressure release during the sea level lowering might have instigated dissociation of the gas hydrate around the base of the gas hydrate, leading to generation and migration of large volumes of methane gas to the seafloor. Accumulation of hydrate caps below mounds eventually caused the collapse of the mounds and the formation of large depressions (pockmarks) along with ejection of sand and mud out of the pockmarks, thereby generating turbidity currents. Prolonged pockmark and submarine canyon activities might have persisted until the transgression stage because of time lags from gas hydrate dissociation around the base of the gas hydrate until upward migration to the seafloor. This study revealed the possibility that submarine canyons were formed by pockmark activities. If that process occurred, it would present important implications for reconstructing the long-term history of shallow gas hydrate activity based on submarine canyon development.

A new dissorophid (Temnospondyli, Dissorophoidea) from the Early Permian of New Mexico (United States)

An amphibian skull and partial skeleton found from the El Cobre Canyon Formation of New Mexico, USA, is identified as a new species of the dissorophoid temnospondyl genus Broiliellus. Significant anatomical features include: a relatively long, narrow preorbital region, with both nasals and vomers no more than half as wide anteriorly than posteriorly; extended ventral projection of the postorbital; a unilaterally retained lateral exposure of the ectopterygoid (in addition to the normally present lateral exposure of the palatine); supratemporal lacking a semilunar flange; highly vaulted vomers forming a median internasal septum; and an angular with a swollen posteroventral keel. Phylogenetic analysis strongly supports a sister group relationship with Broiliellus brevis and clearly distinguishes it from another dissorophoid, ‘B.’ novomexicanus from New Mexico. The medially restricted supraneural osteoderms are more similar to those seen in Cacops than Broiliellus, which reinforces the importance of avoiding using single ‘key features’ to discriminate taxa.

Tracing Clastic Delivery To the Permian Delaware Basin, U.S.A.: Implications For Paleogeography and Circulation In Westernmost Equatorial Pangea

Abstract The Guadalupe Mountains of the southwestern U.S. (New Mexico–West Texas) expose the shelf edge of the Permian Delaware Basin, which formed near the western edge of the Central Pangean suture in equatorial Pangea. Voluminous siliciclastic strata filled this basin in mid-Permian time, and compose the Delaware Mountain Group (Brushy Canyon, Cherry Canyon, and Bell Canyon formations). These units have long been recognized to record ultimate deepwater deposition of siliciclastic sediments that were pre-sorted via eolian transport as they progressed toward the shelf edge of the Delaware Basin in Permian time. The generally arkosic mineralogy led most previous researchers to identify crystalline basement uplifts of the Ancestral Rocky Mountains (ARM) of the western U.S. as the primary source for the large volumes of Permian siliciclastic material that fill this basin. New provenance data combining modal mineralogy with detrital-zircon geochronology, however, refute this interpretation. Rather than an ARM basement source, indicated by Paleoproterozoic ages, the predominant age populations, comprising 70–80% of the grains, reflect contributions from Paleozoic, Neoproterozoic, and Mesoproterozoic (Grenvillian) age basement. The combined (arkosic–subarkosic) mineralogy and provenance spectra of the siliciclastic strata from the Delaware Mountain Group, together with comparisons with coeval strata and available paleocurrent data indicate sources in the Ouachita system, including recycled Appalachian detritus, and terranes accreted and uplifted south of the Ouachita suture, now in Mexico and Central America. Dispersal pathways likely involved a component of fluvial transport draining the piedmont region of the Ouachita orogenic belt, and ultimate eolian deflation of these fluvial systems within an easterly to southeasterly and seasonal westerly to northwesterly atmospheric circulation. Monsoonal transport, especially northern excursions of the ITCZ during northern hemisphere summers, would have facilitated transport from terranes south of the Ouachita–Marathon suture in present-day Mexico and Central America. Source locations predominantly south-southeast of the study region potentially obviate the need to call upon transcontinental fluvial transport directly from the Appalachian orogen to the southwestern edge of Pangea, at least for this time period. Furthermore, these results place new constrains on the final docking, major uplift, and consequent erosion of the Yucatan–Maya terrane south of the Ouachita orogenic belt, which provided significant siliciclastic detritus to depocenters in the western U.S. during middle Permian time. In contrast, the uplifts of the ARM were either covered, or greatly diminished as sources by this time.

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along western Laurentia. The formation is divided into seven informal members, from bottom to top: (1) arkosic grit deposited as proximal turbidites during onset of rifting; (2) quartzite and grit deposited as medial turbidites; (3) pebbly slate with dropstones deposited during an older glacial episode; (4) slate with interbedded quartzite deposited mostly as distal turbidites; (5) mafic volcanic and intrusive rocks; (6) diamictite with basement and volcanic clasts deposited during younger glaciation; and (7) graywacke with rare carbonate layers. The formation has a basal nonconformity on Archean to Paleoproterozoic basement rocks of the Facer Formation and is overlain by arkosic quartzite of the Maple Canyon Formation and argillite of the Kelley Canyon Formation. Changes in member thicknesses reflect syndepositional faulting. Vertical and lateral changes in provenance and maximum depositional ages were evaluated from detrital zircon (DZ) U-Pb geochronology of five well-exposed stratigraphic sections across the sedimentary basin. DZ patterns for the arkosic grit record local sources during onset of rifting, patterns for the pebbly slate and slate members record distal Laurentian sources and possible sediment recycling, and patterns for diamictite and graywacke record basement sources along the rift margin and reworking of mafic to felsic volcanic rocks. Young zircon grains derived from volcanic rocks give maximum depositional ages of 703 ± 6 Ma and 667 ± 5 Ma for the lower part of the diamictite and graywacke members, respectively, suggesting a late Sturtian age for the younger glaciation. Associated cap carbonates, however, have Marinoan-style features, indicating that global correlations based only on lithology may not be valid. DZ data for the Kelley Canyon Formation record a change to distal Laurentian sources during final stages of early rifting. Mafic volcanic and intrusive rocks are subalkaline to alkaline and have trace-element ratios indicative of a rift setting. Volcanic clasts in diamictite include basalt, trachyte, and rhyolite. Volcanic zircon grains have e Hf = –1 to –21, which record crustal contamination during igneous activity. Rare mafic to ultramafic dikes in the upper part of the section were intruded during waning stages of igneous activity. Early rifting spanned ca. 720 Ma to 660 Ma but was incomplete and followed by deposition of mature quartzite of the overlying lower Brigham Group across a broad basin, indicating that Rodinia rift models showing pre–720 Ma separation of western Laurentia may need revision. Final rifting and transition to drift occurred ca. 550 Ma during deposition of the upper Brigham Group.

Dinosaur biostratigraphy of the Edmonton Group (Upper Cretaceous), Alberta, Canada: evidence for climate influence

A high-resolution biostratigraphic analysis of 287 dinosaurian macrofossils and 138 bonebeds in the Edmonton Group (Upper Cretaceous) of southern Alberta provides evidence for at least three dinosaurian assemblage zones in the Horseshoe Canyon Formation (HCFm). From bottom to top the zones comprise unique assemblages of ornithischians and are named as follows: (1) Edmontosaurus regalis – Pachyrhinosaurus canadensis (lower zone); (2) Hypacrosaurus altispinus – Saurolophus osborni (middle zone); and (3) Eotriceratops xerinsularis (upper zone). Whereas the lower and middle zones are well defined and based on abundant specimens, the validity of the uppermost zone (E. xerinsularis) is tentative because it is based on a single specimen and the absence of dinosaur taxa from lower in section. The transition from the lower to the middle zone coincides with the replacement of a warm-and-wet saturated deltaic setting by a cooler, coastal-plain landscape, characterized by seasonal rainfall and better-drained substrates. Whereas changes in rainfall and substrate drainage appear to have influenced the faunal change, changes in mean annual temperature and proximity to shoreline appear to have had little influence on faunal change. We speculate that the faunal change between the middle and upper zones also resulted from a change in climate, with ornithischian dinosaurs responding to the re-establishment of wetter-and-warmer climates and poorly-drained substrates. Compared with the shorter-duration and climatically-consistent dinosaurian assemblage zones in the older Dinosaur Park Formation of southern Alberta, HCFm assemblage zones record long-term morphological stasis in dinosaurs. Furthermore, the coincidence of faunal and paleoenvironmental changes in the HCFm suggest climate-change-driven dinosaur migrations into and out of the region.

A hierarchical approach to architectural classification in marginal-marine systems: Bridging the gap between sedimentology and sequence stratigraphy

A new hierarchical architectural classification for clastic marginal-marine depositional systems is presented and illustrated with examples. In ancient rocks, the architectural scheme effectively integrates the scales of sedimentology (core, outcrop) and sequence stratigraphy (wireline-log correlation, reflection seismic). The classification also applies to modern sediments, which allows for direct comparison of architectural units between modern and ancient settings. In marginal-marine systems, the parasequence typically defines reservoir flow units. This classification addresses subparasequence scales of stratigraphy that commonly control fluid flow in these reservoirs. The scheme consists of seven types of architectural units that are placed on five architectural hierarchy levels: hierarchy level I: element (E) and element set (ES); hierarchy level II: element complex (EC) and element complex set (ECS); hierarchy level III: element complex assemblage (ECA); hierarchy level IV: element complex assemblage set (ECAS); and hierarchy level V: transgressive-regressive sequence (T-R sequence). Architectural units in levels I to III are further classified relative to dominant depositional processes (wave, tide, and fluvial) acting at the time of deposition. All architectural units are three-dimensional and can also be expressed in terms of plan-view and cross-sectional geometries. Architectural units can be linked using tree data structures by a set of familial relationships (parent-child, siblings, and cousins), which provides a novel mechanism for managing uncertainty in marginal-marine systems. Using a hierarchical scheme permits classification of different data types at the most appropriate architectural scale. The use of the classification is illustrated in ancient settings by an outcrop and subsurface example from the Campanian Bearpaw–Horseshoe Canyon Formations transition, Alberta, Canada, and in modern settings, by the Mitchell River Delta, northern Australia. The case studies illustrate how the new classification can be used across both modern and ancient systems, in complicated, mixed-process depositional environments.

Spatial and temporal trends in pre-caldera Jemez Mountains volcanic and fault activity

New 40Ar/39Ar dates from the Jemez Mountain volcanic field (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the field includes two eruptive pulses: 10.8–9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12–9 Ma mafic to silicic volcanism in the southwestern JMVF. While 9–7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7–7.2 Ma), corresponding with a period of rift widening caused by reactivation of Laramide faults in this area. The older 8.7–7.8 Ma mafic lavas emitted from Encino Point and the younger 7.7–7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava flows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6–7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12–8 Ma Canovas Canyon Rhyolite and 7–6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1–6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5–2 Ma. The more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10–9 Ma) to west (9–7 Ma) to east (5–2 Ma) pattern that reflects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Cañones fault zone in the northern JMVF.

Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA

Few ankylosaurs are known from more than a single specimen, but the ankylosaurid Euoplocephalus tutus (from the Late Cretaceous of Alberta, Canada and Montana, USA) is represented by dozens of skulls and partial skeletons, and is therefore an important taxon for understanding intraspecific variation in ankylosaurs. Euoplocephalus is unusual compared to other dinosaurs from the Late Cretaceous of Alberta because it is recognized from the Dinosaur Park, Horseshoe Canyon, and Two Medicine formations. A comprehensive review of material attributed to Euoplocephalus finds support for the resurrection of its purported synonyms Anodontosaurus lambei and Scolosaurus cutleri, and the previously resurrected Dyoplosaurus acutosquameus. Anodontosaurus is found primarily in the Horseshoe Canyon Formation of Alberta and is characterized by ornamentation posterior to the orbits and on the first cervical half ring, and wide, triangular knob osteoderms. Euoplocephalus is primarily found in Megaherbivore Assemblage Zone 1 in the Dinosaur Park Formation of Alberta and is characterized by the absence of ornamentation posterior to the orbits and on the first cervical half ring, and keeled medial osteoderms on the first cervical half ring. Scolosaurus is found primarily in the Two Medicine Formation of Montana (although the holotype is from Dinosaur Provincial Park), and is characterized by long, back-swept squamosal horns, ornamentation posterior to the orbit, and low medial osteoderms on the first cervical half ring; Oohkotokia horneri is morphologically indistinguishable from Scolosaurus cutleri. Dyoplosaurus was previously differentiated from Euoplocephalus sensu lato by the morphology of the pelvis and pes, and these features also differentiate Dyoplosaurus from Anodontosaurus and Scolosaurus; a narrow tail club knob is probably also characteristic for Dyoplosaurus.

Cunninghamia taylorii sp. nov., a Structurally Preserved Cupressaceous Conifer from the Upper Cretaceous (Campanian) Horseshoe Canyon Formation of Western North America

The fossiliferous ironstone deposits of the Upper Cretaceous Horseshoe Canyon Formation in the Drumheller badlands, Alberta, Canada, contain a diverse, anatomically preserved flora. Within this assemblage occur abundant stems, leafy shoots, leaves, ovuliferous cones, and pollen cone clusters that are here described as a new species of Cunninghamia. The stems have persistent leaf bases and plagiotropic branching. Some stems show several growth increments, and the cortex and pith are made up of resin canals, large parenchymatous cells, and lysigenous cavities. Leafy shoot axes have parenchymatous cells, resinous cells, lysigenous cavities, and resin canals within the cortex and pith. The leaves are helically arranged, lanceolate to falcate, with two abaxial stomatal bands, a large central resin canal, and two smaller lateral canals. The ovuliferous cones have numerous helically arranged bract scale complexes. The bracts and three-lobed ovuliferous scale are variously ornamented. Anatomical features of bract...

Palaeoenvironmental and palaeoclimatic reconstruction of the Upper Cretaceous (late Campanian–early Maastrichtian) Horseshoe Canyon Formation, Alberta, Canada

Pedogenic features and geochemical signatures of paleosol profiles exposed in the Horseshoe Canyon Formation (HCFm) were studied to reconstruct the ancient environments and climates that prevailed at high latitude (~60°N) in the North American Western Interior during the late Campanian to early Maastrichtian. Eight pedotypes representing hydromorphic, moderately-drained and well-drained paleosols were recognized. When considered within their stratigraphic context, these paleosols show that palaeoenvironmental and palaeoclimatic changes occurred either gradually between major lithological subdivisions of the HCFm or relatively rapidly within subdivisions. Hydromorphic paleosols, predominant in the coal-bearing stratigraphic interval at the base of the HCFm, were gradually replaced by well-drained paleosols in the overlying coal-poor interval. While climate remained warm and humid during this change in soil drainage conditions, mean annual temperature and precipitation dropped rapidly higher in section, shortly prior to the Campanian–Maastrichtian boundary, without concomitant lithological or pedological changes. A brief return of poorly-drained paleosols in the middle of the HCFm coincides with a regional marine transgression (Drumheller Marine Tongue). Well-drained palaeoenvironments, associated with a warm and dry climate, persisted through the upper part of the HCFm until a gradual return to poorly-drained conditions and to a warm, humid climate occurred at the top of the formation. An important decline in turtle diversity through the HCFm, previously attributed to climate, is shown to coincide instead with changes in soil drainage conditions, where aridity, landscape instability, and migratory barriers possibly acted as secondary limiting factors to faunal recovery.

LOWER CAMBRIAN ANEMONE BURROWS FROM THE UPPER MEMBER OF THE WOOD CANYON FORMATION, DEATH VALLEY REGION, UNITED STATES: PALEOECOLOGICAL AND PALEOENVIRONMENTAL SIGNIFICANCE

Large (up to 7 cm in diameter) and deep-penetrating (up to 30 cm) vertical burrows are described from the lower Cambrian Wood Canyon Formation of the Death Valley region, United States, and their paleoenvironmental and paleoecological implications are addressed. Trace fossils occur as dense populations that precede the earliest occurrences of Skolithos pipe rock in the region. These trace fossils are assigned to the ichnogenera Bergaueria, Conichnus, and Dolopichnus, and each represents the burrowing behavior of an anemone-like organism. These ichnotaxa occur within oolitic and sandy dolostones of a flood-tidal delta and lagoonal environment, respectively, and burrow fill is dominated by echinoderm ossicles. Ichnofabric indices of 4–5 and a tidally influenced position place these ichnotaxa within the same nearshore locus of bioturbation as Skolithos pipe rock; however, unlike Skolithos pipe rock, the tracemaker of these trace fossils can be identified. This study reveals that anemone-like animals were responsible for extensive modification of the marine substrate and this style of deep-penetrating bioturbation appears first in nearshore environments. This is in accord with the observation that many evolutionary novelties originate in nearshore environments, and specifically that deep-penetrating bioturbation originated nearshore before expanding offshore. This study also provides insight into the paleoecology of earliest Cambrian noncalcified cnidarians and their behavior and paleoenvironmental distribution during the Cambrian radiation.

A revised stratigraphy and depositional history for the Horseshoe Canyon Formation (Upper Cretaceous), southern Alberta plains

The Upper Cretaceous paralic to nonmarine Horseshoe Canyon Formation (HCFm) of southern Alberta is divided into seven mappable members: Strathmore, Drumheller, Horsethief, Morrin, Tolman, Carbon, and Whitemud (bottom to top). This subdivision, based on combined outcrop and subsurface analyses, reflects lithostratigraphic variations related to changes in sea level (previously recognized) and newly documented changes in climate, volcanism, and orogenesis in an evolving foreland basin. Million-year-scale cycles of orogenesis resulted in changes in sediment supply and rates of subsidence in the basin and are interpreted in the context of a simple, two-phase foreland-basin sequence stratigraphic model: (i) overthrust loading resulting in reduced rates of sediment supply and subsidence in the most distal portions of the Alberta foredeep (our field area); (ii) tectonic quiescence leading to increased rates of sediment supply and subsidence during proximal-foredeep rebound. During the first ∼2.5 Ma of its history (Strathmore and Drumheller members), the HCFm was tectonically and climatically “stable”, and depositional style and stratigraphic architecture were influenced by vertical aggradation and modest progradation of shorelines. During the remaining ∼4.5 Ma (Horsethief, Morrin, Tolman, Carbon, and Whitemud members), there were more complex and frequent changes in climate, volcanism, orogenesis, landscape weathering, and soil formation. Understanding this previously unrecognized complexity is critical for correctly assessing hydrocarbon resource distributions and biostratigraphic and taphonomic patterns.

Cranial anatomy of the Late Triassic phytosaurMachaeroprosopus, with the description of a new species from West Texas

ABSTRACTThe skull anatomy of a new species of the phytosaurMachaeroprosopusis described for the first time on the basis of two specimens from the Upper Triassic Cooper Canyon Formation of Texas. Additional information is provided by a third specimen referred toMachaeroprosopussp. A paranasal bone, an additional paired element of the narial region, is identified. Important new data are presented for the braincase, including the morphology of the epipterygoid and presphenoid, an anterior process of the prootic, an anteroventral process of the laterosphenoid, and a parasphenoid process.Machaeroprosopus lottorumn. sp. is characterised by four apomorphies: a supratemporal fenestra closed on the skull roof with bevelled anterior rim, a comparatively short squamosal, a flat and rugose narial rim, and medially extended palatines that come close to form an ossified secondary palate. With respect to the supratemporal fenestra, the supraoccipital–parietal complex and several features of the squamosal,Machaeroprosopus lottorumn. sp. bridges the morphological gap between species previously referred to the generaPseudopalatusandRedondasaurus. A parsimony analysis of known species ofMachaeroprosopussupports the hypothesis that the development of the rostral crest inMachaeroprosopusis a sexually dimorphic feature, and questions the validity of the genusRedondasaurus. Consequently,Redondasaurusis here considered a junior synonym ofMachaeroprosopus.

A taxonomic and biostratigraphic re-evaluation of the Post Quarry vertebrate assemblage from the Cooper Canyon Formation (Dockum Group, Upper Triassic) of southern Garza County, western Texas

ABSTRACTThe Post Quarry, within the lower part of the type section of the Upper Triassic Cooper Canyon Formation in southern Garza County, western Texas, contains a remarkably diverse vertebrate assemblage. The Post Quarry has produced: the small temnospondylRileymillerus cosgriffi; the metoposauridApachesaurus gregorii; possible dicynodonts and eucynodonts; a clevosaurid sphenodontian; non-archosauriform archosauromorphs (Trilophosaurus dornorum, simiosaurians, and possiblyMalerisaurus); the phytosaurLeptosuchus; several aetosaurs (Calyptosuchus wellesi,Typothorax coccinarum,Paratypothorax, andDesmatosuchus smalli); the poposauroidShuvosaurus inexpectatus(“Chatterjeea elegans”); the rauisuchidPostosuchus kirkpatricki; an early crocodylomorph; several dinosauromorphs (the lagerpetidDromomeron gregorii, the silesauridTechnosaurus smalli, a herrerasaurid, and an early neotheropod); and several enigmatic small diapsids. Revised lithostratigraphic correlations of the lower Cooper Canyon Formation with the Tecovas Formation, the occurrence ofLeptosuchus, and the overall composition of the assemblage indicate that the Post Quarry falls within the Adamanian biozone, and not the Revueltian biozone. Stratigraphic subdivision of the Adamanian biozone may be possible, and the Post Quarry may be correlative with the upper part of the Adamanian biozone in Arizona. The age of the Post Quarry assemblage is possibly late Lacian or earliest Alaunian (late early Norian or earliest middle Norian), between 220 and 215 Ma.

New jalodontid chondrichthyans from the Middle Permian of West Texas, USA

The teeth of two new genera of jalodontid sharks have been found in the strata of the Middle Permian in the Guadalupe Mountains area of West Texas. Isacrodus marthae gen. et sp. nov. occurs in the Williams Ranch Member, Cutoff Formation, Roadian. A second taxon Texasodus varidentatus gen. et sp. nov. is found in beds of the Hegler and Pinery Members, Bell Canyon Formation, Upper Wordian–Lower Capitanian. The family Jalodontidae includes Jalodus, Adamantina and the two new genera; Isacrodus and Texasodus.

Lithostratigraphy, Paleontology, Biostratigraphy, and Age of the Upper Paleozoic Abo Formation Near Jemez Springs, Northern New Mexico, USA

In the Jemez Springs area of Sandoval County, northern New Mexico, siliciclastic red beds of the upper Paleozoic Abo Formation are well exposed and yield fossil plants and vertebrates. The local Abo Formation section is more than 190 m thick and rests disconformably on the Upper Pennsylvanian Guadalupe Box Formation and is conformably overlain by the Lower Permian DeChelly Sandstone (Yeso Group). Abo sandstone sheets are low sinuosity river deposits, and intercalated sandstone beds and lenses represent sheet splays and minor channel fills that formed during overbank flooding. The dominant Abo lithofacies is mudstone, which represents floodplain deposits, many with calcareous paleosols. Fossils are present in three stratigraphie intervals of the lower to middle Abo Formation. All three intervals yield eupelycosaur-dominated vertebrate fossil assemblages of Coyotean age (Coyotean = late Virgilian-Wolfcampian on the North American provincial marine timescale: Lucas 2006). The lowest interval also yields the Spanish Queen Mine paleoflora of pteridosperms and conifers. Strata of the Guadalupe Box Formation disconformably below the Abo Formation contain late Virgilian fusulinids. We correlate the Abo Formation fossil assemblages in the Jemez Springs area to the Coyotean-age fossil assemblages in the upper part of the El Cobre Canyon Formation in the Arroyo del Agua area and in the Canon del Cobre in the Chama basin of northern New Mexico. This suggests a middle Wolfcampian age for the Jemez Springs area fossil assemblages, an age very close to the Pennsylvanian-Permian boundary.

Early Ordovician mitrates and a possible solute (Echinodermata) from the western United States

Two new kirkocystid mitrate stylophorans (Echinodermata, Homalozoa) and a new possible solute (Echinodermata, Homalozoa) are described from the Early Ordovician of the western United States. The mitrates are among the earliest members of their clade to appear near the beginning of the Ordovician Radiation.Anatifopsis ninemilensisnew species comes from the Ninemile Shale in central Nevada and the McKelligon Canyon Formation in west Texas.Anatifopsis fillmorensisnew species comes from the middle Fillmore Formation in western Utah and a Ninemile Shale equivalent limestone bed in southern Nevada. The possible soluteDrepanocystis dubiusnew genus new species from the lower Wah Wah Limestone in western Utah, shows unusual morphology with an elongate theca and a long arm shaped like a sickle.