Canyon Formation Research Articles

The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation

Research Article| July 01, 2011 The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation Charles Verdel; Charles Verdel † 1Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA †Current address: Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA; e-mail: cverdel@ku.edu Search for other works by this author on: GSW Google Scholar Brian P. Wernicke; Brian P. Wernicke 1Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Samuel A. Bowring Samuel A. Bowring 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1539–1559. https://doi.org/10.1130/B30369.1 Article history received: 08 Jul 2010 rev-recd: 10 Oct 2010 accepted: 12 Oct 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Charles Verdel, Brian P. Wernicke, Samuel A. Bowring; The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation. GSA Bulletin 2011;; 123 (7-8): 1539–1559. doi: https://doi.org/10.1130/B30369.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Current understanding of secular changes in the carbon isotopic composition of mid- to late Ediacaran carbonates suggests a relatively long, steady recovery of the global ocean from the Shuram negative excursion, followed by a smaller negative excursion at the Precambrian-Cambrian boundary. New radiometric, stratigraphic, and carbon isotope data from thick exposures of the upper Johnnie Formation in the Panamint Range of eastern California, combined with data from carbonate-rich facies of the Stirling Quartzite in the Funeral Mountains, confirm an Ediacaran age for these strata and provide a more complete record of isotopic variations during this time interval than previously determined from SW Laurentia and other key sections around the globe. A siltstone in the lower part of the Johnnie Formation yielded a detrital zircon grain with an age of 640.33 ± 0.09 Ma, lowering the maximum radiometric age constraint on the Johnnie Formation by >400 m.y., consistent with an Ediacaran age based on chemo- and biostratigraphic data. In contrast to previous C isotope compilations from this region, which were generally based on relatively thin portions of the Cordilleran miogeocline near its depositional hinge, the more basinward exposures exhibit a recovery from values near –12‰ to 0‰ within the upper part of the Johnnie Formation. Details in the shape of the chemostratigraphic profile through the upper Johnnie Formation closely match those in profiles through the Wonoka Formation in South Australia (which lies above the basal Ediacaran global stratotype section and point) and the Shuram-Buah interval in Oman, confirming temporal correlation and suggesting genesis through changes in the isotopic composition of the global ocean. The Shuram excursion in SW Laurentia is followed by at least three smaller Ediacaran to earliest Cambrian isotopic excursions recorded within, from oldest to youngest, the uppermost Johnnie Formation, the middle Stirling Quartzite, and the lower Wood Canyon Formation. These data indicate that the negative excursion associated with the base of the Cambrian is not a unique post-Shuram event, and that post-Shuram, pre-Cambrian animal evolution occurred in an environment of repeated large-magnitude fluctuations in the carbon isotopic composition of the global ocean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Merychyus calaminthus (Mammalia, Artiodactyla, Oreodontidae) of probable early late Arikareean (late Oligocene to late early Miocene) age from the lower part of the Chalk Canyon Formation, Maricopa and Yavapai counties, central Arizona

ABSTRACT Late Oligocene and early Miocene, nearly flat-lying and dominantly volcaniclastic, continental strata in the lower part of the Chalk Canyon Formation are exposed in central Arizona along the southwestern margin of the Transition Zone and the adjacent northeastern margin of the Basin and Range Province. The strata have yielded both of the two known oreodont records reported from Arizona. These occurrences represent the Cave Creek and Black Canyon Local Faunas. Both records are assigned tentatively to the early late Arikareean (Ar3), small-bodied oreodontid species Merychyus calaminthus, based primarily on their occlusal and basicranial anatomies and on the comparatively small sizes of the individuals they represent. Both fossil sites are in discontinuous exposures separated by about 28 km (17 mi). Overlying olivine-rich basaltic lava flows still in the lower part of the formation in the Cave Creek area are as old as 23.3 ± 2.7 Ma in age, based on whole-rock Potassium-Argon radiometric age determinations. On the other hand, a pumice-rich air-fall tuff bed at the base of the formation is only 20.98 ± 0.06 and 21.03 ± 0.06 Ma old, based on 40Argon/39Argon (Ar/Ar) analysis. A blue ash layer in the Black Canyon City area and at the approximate level of the oreodont site is 23.7 ± 0.9 Ma old, based on the 238Uranium-206Lead analysis of zircon crystals. The oreodont records and nearly all available radiometric data suggest that the fossil-bearing strata in the lower part of the Chalk Canyon Formation are correlatives of the Harrison Formation in the northwestern Nebraska Panhandle. The Agate Ash in the Harrison Formation is 23.23 ± 0.13 Ma in age on the basis of Ar/Ar analysis.

Submarine canyon formation and evolution in the Argentine Continental Margin between 44°30'S and 48°S

In the framework of the Vulnerable Marine Ecosystems (VME) of the High Seas of the South West Atlantic, large areas of the Argentine Continental Margin (ACM) between 44°30'S and 48°S have been swath-mapped for the first time, obtaining full data coverage of the seafloor in this region between the outermost continental shelf and the middle slope down to 1600 m water depth. The slope is characterized by the presence of smooth terraces (Nagera, Perito Moreno and Piedra Buena) that widen towards the south, limited by morphological steps with evident signs of erosion in the form of scours. These terraces form part of the Argentine contourite depositional systems, generated by the interaction of the northwards flowing Antarctic water masses with the seafloor. Within the studied area, seven canyons and their multiple branches dissect the upper and middle continental slopes, from west to east, across the terraces and the steps. These canyons, which belong to the Patagonia submarine canyon system and are collected at a depth of ~ 3.5 km by a slope-parallel, SSW–NNE-oriented channel known as the Almirante Brown transverse canyon, display a large variety of morphologies. These include incisions from just a dozen of metres to 650 m, straight to highly meandering sections with sharp bends, well-developed levees and walls that reach 35° in slope gradient, hanging branches, conspicuous axial incisions and multiple knickpoints. Only the northernmost canyon indents in the continental shelf, whereas the others start at the limit between the upper and middle slopes, and are often fed by small, straight, leveed gullies. The action of both across-slope processes represented by submarine canyons and along-slope processes represented by terracing and scouring conform the ACM as a peculiar mixed margin, with the presence of both contour and gravity currents at the same place at the same time. We propose that at present, along-slope erosion and transport mainly occurs along the Perito Moreno terrace, whereas across-slope processes are much more dominant in the Nagera terrace. Erosive bedforms such as crescent scours, generated by contour currents, contribute to the progressive bottom-up erosion of the Nagera terrace and act as an initial collector of across-slope transported sediment, that later, due to flow focusing and recurrence, incise and interconnect creating definitive canyons that progress upslope by retrogressive erosion until their head indents the shelf break. Changes in the balance between across-slope and along-slope transport would imply a disequilibrium in the combination of processes leading to canyon formation, producing canyon abandonment, and partial or total filling. These changes could be produced by a variation in the depth of the main interfaces of Antarctic water masses leading to either an increase or a decrease in the erosion and transport capacity of contour currents, and/or by an enhancement of across-slope transport related to an increase of sediment availability.

Northwest McGregor field CO2 Huff ‘n’ Puff: A case study of the application of field monitoring and modeling techniques for CO2 prediction and accounting

Northwest McGregor field CO2 Huff ‘n’ Puff: A case study of the application of field monitoring and modeling techniques for CO2 prediction and accounting

Paleogeographic isolation of the Cretaceous to Eocene Sevier hinterland, east-central Nevada: Insights from U-Pb and (U-Th)/He detrital zircon ages of hinterland strata

The Late Cretaceous to Paleogene Sevier hinterland of east-central Nevada is widely regarded as an orogenic plateau that has since undergone topographic collapse. New U-Pb detrital zircon age data consisting of 1296 analyses from the Lower Cretaceous Newark Canyon Formation and the Upper Cretaceous to Eocene Sheep Pass Formation indicate that Precambrian detrital zircon populations recycled from local Paleozoic strata are dominant. Subordinate Mesozoic zircon populations are derived mainly from local backarc volcanic centers of Late Jurassic and Early Cretaceous age, while ca. 38–36 Ma detrital zircon age peaks record the local onset of Eocene volcanism. Sevier hinterland deposits of east-central Nevada lack significant Triassic, Early Jurassic, and Late Cretaceous populations common in terranes of western Nevada and the Sierra Nevada magmatic arc. These data suggest that long-term evolution of the Sevier Plateau involved geographic isolation through a combination of high relief and rugged topography related to Early Cretaceous shortening, and continued isolation through development of latest Cretaceous to Eocene internally drained, extensional basins. The (U-Th)/He zircon ages obtained from the Sheep Pass Formation record late Paleo zoic, Early Cretaceous, and Late Cretaceous cooling through 180 °C. Preservation of late Paleozoic (ca. 265 Ma) cooling ages indicates that much of the Upper Paleozoic section within east-central Nevada that contributed detritus to the Sheep Pass basin was unaffected by deep thrust burial, or by burial beneath thick Mesozoic sedimentary cover. Early Cretaceous (ca. 135 Ma) cooling ages are potentially coeval with shortening along the central Nevada fold-and-thrust belt, although ca. 80 Ma cooling ages within the Sheep Pass Formation are coeval with hinterland midcrustal extension. Together, these new data provide support for previous interpretations that the Sevier hinterland represents an ancient high-elevation orogenic plateau, and that the latest Cretaceous locally marks a transition from contraction to extension.

The youngest rostroconch mollusc from North America, Minycardita capitanensis n. sp.

Rostroconchs are an extinct class of mollusc that lived worldwide through most or all of the Paleozoic Era (Runnegar 1978). They were most diverse in the early Paleozoic (Pojeta 1985), perhaps due to a lower rate of evolution in the rostroconch clade that survived the end-Ordovician mass extinction event (Wagner 1997). Rostroconchs have a univalved larval shell and a pseudo-bivalved adult shell. Their ecology appears to have ranged from infaunal to rarely epifaunal, and from deposit to suspension feeding (Pojeta et al. 1972, Pojeta & Runnegar 1976, Runnegar 1978, Pojeta 1987). A single but well-preserved, essentially complete rostroconch specimen was recovered from the region surrounded by the famous Capitan Reef of the Permian of West Texas (Newell et al. 1953). In particular, the specimen is from the upper scaphopod bed of the Reef Trail Member of the Bell Canyon Formation exposed in the Guadalupe Mountains National Park (Rigby & Bell 2005: fig. 2). The fusulinid biostratigraphic zonal indicator Paraboultonia splendens Skinner & Wilde, 1954 occurs both below the upper scaphopod bed and above it, indicating that this horizon was deposited during the latest Guadalupian (late middle Permian) (Rigby & Bell 2006), and thus is the youngest known deposit in North America to contain a rostroconch. Most known Permian rostroconchs have been recovered from rocks in North America, and only one occurrence stands out as clearly younger than this one: Pseudoconocardium Zavodowsky, 1960 from the Late Permian of Siberia, a large rostroconch that differs significantly in shape and ornamentation from rostroconchs of the late Paleozoic in North America (Hoare et al. 2002). Permian rostroconchs are rare, having been described from only sixteen localities worldwide (listed in Hoare & Plas 2003). In contrast, rostroconchs were relatively abundant during the preceding Pennsylvanian (Hoare et al. 2002). Thus the occurrence described herein represents the youngest known record of the North American rostroconch lineages, and is one of the last snapshots of a major molluscan clade.

On gregarious behavior inAlbertosaurusThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

One century ago, a field party from the American Museum of Natural History discovered a bonebed in the Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada. Excavations by that museum, the Royal Tyrrell Museum of Palaeontology, and the University of Alberta have revealed the presence of at least a dozen individuals — represented by articulated partial skeletons, associated skeletons, and disarticulated isolated elements — of Albertosaurus sarcophagus . Tyrannosaurids dominate the bonebed assemblage, which also includes an adult Hypacrosaurus altispinus , two individuals of Albertonykus borealis , and numerous other, predominantly terrestrial, vertebrates. Skeletal morphology, phylogenetic inference, monodominant bonebeds, trackway sites, and ecological inferences support the notion that some non-avian theropods were gregarious animals. And specifically in the Albertosaurus bonebed, associated geologic and taphonomic evidence do not rule out a behavioural component in this catastrophic, mass-death assemblage.

A new articulated freshwater fish (Clupeomorpha, Ellimmichthyiformes) from the Horseshoe Canyon Formation, Maastrichtian, of Alberta, CanadaThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

Horseshoeichthys armaserratus , gen. et sp. nov., (Clupeomorpha: Ellimmichthyiformes: Sorbinichthyidae) is described from the Horseshoe Canyon Formation (Maastrichtian), Albertosaurus bonebed locality, Alberta, Canada. Horseshoeichthys armaserratus is classified as an ellimmichthyiform based on the following characters: the presence of a sixth infraorbital with a sensory canal that leads to the fifth infraorbital, absence of a supraorbital bone, subrectangular predorsal scutes, parietals in contact with each other at the midline, and two supramaxillae. The specimen is classified in the Sorbinichthyidae Family as it has abdominal ribs articulating in pits on the centra, posterior spines on predorsal scutes, and absence of a median spine on predorsal scutes. A new genus and species is proposed based on the presence of (anteriorly) Y-shaped mesethmoid, supraorbital, subrectangular predorsal scutes with coarse, rounded serrae on the posterior margin and a large anterior projection, scales with serrae on the circuli, and two postcleithra. This specimen represents the first freshwater ellimmichthyiform from the Upper Cretaceous of North America and the highest paleolatitude (59°N) occurrence known for the family. Furthermore, the dentary and centra have distinctive morphologies that are matched by specimens in microvertebrate localities from three underlying formations, including the Milk River Formation (Santonian), which indicates at least a 14 million year history for this lineage.

A history of Albertosaurus discoveries in Alberta, CanadaThis article is one of a series of papers published in this Special Issue on the theme Albertosaurus.

After many years of taxonomic uncertainty, Albertosaurus was established as a new genus in 1905, the year Alberta became a province of Canada. Gorgosaurus is a closely related tyrannosaurid from the Judithian beds of southern Alberta that was subsequently synonymized with Albertosaurus. Although most researchers consider the genera as distinct, there has been considerable confusion over the temporal and geographic range of Albertosaurus. Albertosaurus sarcophagus is only known from 13 skulls and (or) skeletons of varying completeness, and one (possibly two) bonebeds, all from the Horseshoe Canyon Formation (Campanian–Maastrichtian) of Alberta. Many of the major Albertosaurus specimens are scientifically compromised due to poor collection techniques, incomplete locality and stratigraphic information, politics, vandalism, accidents, gunplay, and landowner issues. The background of each specimen is discussed to eliminate some of the sources of confusion and to document how much of each specimen is preserved.

Palaeopathological changes in a population of Albertosaurus sarcophagus from the Upper Cretaceous Horseshoe Canyon Formation of Alberta, CanadaThis article is one of a series of papers published in this Special Issue on the theme Albertosaurus.

Over the past decade, excavations of a monodominant theropod bonebed from the lower Maastrichtian beds of the Horseshoe Canyon Formation have recovered a minimum number of 26 individuals of the tyrannosaur, Albertosaurus sarcophagus . Examination of skeletal elements from the bonebed revealed a small number of abnormalities from at least two individuals. These include bony spurs (enthesophytes) of unknown origin on three pedal phalanges. Well-healed fracture calluses are present on two rib shafts and a gastralia element. The left dentary of one adult preserves both healed and unhealed parallel bite marks and a perforative lesion attributable to a partially healed, mechanically induced puncture wound. Unfortunately, the limited range in the types and frequencies of pathological changes provides only little information on the distribution of such phenomena but may be suggestive of the overall “health” of the population.

Faunal assemblages from the upper Horseshoe Canyon Formation, an early Maastrichtian cool-climate assemblage from Alberta, with special reference to theAlbertosaurus sarcophagusbonebedThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

The faunal assemblage from the early Maastrichtian portion of the Horseshoe Canyon Formation is described on the basis of four new vertebrate microfossil localities and remains from the Albertosaurus bonebed. All of the localities sampled were deposited during a cool, dry climate at a palaeolatitude of ∼58°N. Thus, these assemblages provide insight into a northern cool-climate assemblage in the early Maastrichtian of western North America. This fauna is characterized by the presence of taxa with more northern affinities, such as Holostean A, champsosaurs, Troodon , and toothed birds. Warm-climate taxa, such as crocodylians, large and diverse turtles, and albanerpetontids are notable in their absence. The Albertosaurus bonebed locality at the top of unit 4 of the Horseshoe Canyon Formation was deposited during the initial stages of a trend to a warmer and wetter climate that is represented in unit 5. The bonebed shares many taxa with the underlying vertebrate microfossil localities. However, a notable difference is the presence of Atrociraptor marshalli from the Albertosaurus bonebed but not the other localities in the upper Horseshoe Canyon Formation. The presence of Atrociraptor may be attributable to this change in climate rather than local ecological conditions. Also, the assemblages are different in the paucity of fish remains in the bonebed, and the relative rarity of shed hadrosaur teeth. The low abundance of aquatic taxa and rarity of shed teeth of hadrosaurs indicate that the locality is largely autochthonous, with little material being transported into the site.

A taxonomic assessment of the type series of Albertosaurus sarcophagus and the identity of Tyrannosauridae (Dinosauria, Coelurosauria) in the Albertosaurus bonebed from the Horseshoe Canyon Formation (Campanian–Maastrichtian, Late Cretaceous)This article is one of a series of papers published in this Special Issue on the theme Albertosaurus.

The type series of Albertosaurus sarcophagus , consisting of a type (CMN 5600) and a paratype (CMN 5601) is inadequately established, since it has not been demonstrated that the specimens represent the same taxon. This problem has arisen because the original diagnostic characters have a wider distribution among Tyrannosauridae or they are in error, and both specimens consist of incomplete and damaged skulls. The type series was studied to test for the presence of diagnostic characters in the type specimen and, if so, to provide a rationale for referring the paratype to the taxon. Of the bones shared between the skulls, only five could be compared. One character, the enlarged posterior pneumatic recess of the palatine, is shared between the two specimens; this condition differs from the situation seen in other tyrannosaurids. This character provided the rationale for assessing the identity of the specimens collected from the Albertosaurus bonebed, from which one palatine was collected that exhibits the diagnostic recess. Isolated cranial bones from the bonebed were compared with those preserved in the type series to evaluate their referral to A. sarcophagus ; additional characters shared between the type series and the specimens from the bonebed were identified in the maxilla, lacrimal, and palatine. This evidence supports the hypothesis that A. sarcophagus is the tyrannosaurid preserved in the bonebed. A hypothesis of the principal growth changes in the skull in A. sarcophagus, which includes the type series and bonebed material, is proposed.

Stratigraphy, sedimentology, and taphonomy of the Albertosaurus bonebed (upper Horseshoe Canyon Formation; Maastrichtian), southern Alberta, CanadaThis article is one of a series of papers published in this Special Issue on the theme Albertosaurus.

The Albertosaurus bonebed provides evidence for the mass-mortality of at least 12 Albertosaurus . Albertosaurus, other vertebrates, and trees are concentrated on upper point bar surfaces of a small meandering paleochannel. Throughout the field area, Albertosaurus, other vertebrates, and coalified plants are also preserved patchily in overbank mudstones at the same horizon. Although the bonebed is dominated by Albertosaurus, there are also sparse macrofossil remains of Hypacrosaurus and Albertonykus . The basal lag yields vertebrate microfossils containing at least 19 additional taxa. Freshwater invertebrates are preserved at and below the base of the paleochannel. Large elements are over represented at the site, whereas smaller elements are under represented, suggesting hydraulic sorting. The taphonomic signature of the site suggests that death, disarticulation, and burial took place within one year. We conclude that one or more storms of large intensity downed trees and temporarily drowned the paleolandscape, ultimately killing the tyrannosaurids directly or indirectly. Logs and carcasses were reworked on a point bar during the waning stages of the storm and over subsequent years. Scavenging and breakage of remains by large carnivores was probably inhibited by the mixed log and bone jam. Remains of smaller scavengers (small theropods and snails) are plentiful suggesting these forms were more successful at accessing remains. Juvenile Albertosaurus are under represented at the site suggesting the possibility of age segregation. Because the group of Albertosaurus may have been forced together by rising floodwaters, it is not possible to assess the degree to which the taxon was gregarious from these data alone.

Upper Cretaceous palynostratigraphy of the Dry Island areaThis article is one of a series of papers published in this Special Issue on the themeAlbertosaurus.

The overall palynology of the upper Horseshoe Canyon Formation to lower Scollard Formation within the Edmonton Group is reviewed providing new insights into biostratigraphic relationships for the interval, including the recognition of unconformities in the stratigraphic package. The palynomorph recovery from the Horseshoe Canyon Formation and overlying units documents a number of significant events that allow the subdivision of the section. The Albertosaurus bonebed occurs within the lower Maastrichtian portion of the Edmonton Group. The bonebed is situated just below the base of the Scollardia trapaformis Palynomorph Zone, an important marker assemblage that has been documented from areas of the Yukon, Northwest Territories, Alberta, and Saskatchewan. This indicates that the bonebed lies at the top of the Mancicorpus vancampoi Zone. The transition between the zones is marked by a colour change in the sediments, the appearance of coals and organic-rich sediments above the transition, a decrease in numbers of vertebrate fossils, and the appearance of new pollen species. It is inferred that the observed changes reflect a climatic switch from dry to wetter conditions. At a large scale, palynomorph assemblages across this transition do not clearly indicate marked differences that can be attributed to the climatic change, with the exception of the appearance of Scollardia trapaformis and Mancicorpus gibbus in the assemblages. On a microscale of a few metres, there are changes in relative abundance of plant groups.

Shelf erosion and submarine river canyons: implications for deep-sea oxygenation and ocean productivity during glaciation

Abstract. The areal exposure of continental shelves during glacial sea level lowering enhanced the transfer of erodible reactive organic matter to the open ocean. Sea level fall also activated submarine canyons thereby allowing large rivers to deposit their particulate load, via gravity flows, directly in the deep-sea. Here, we analyze the effects of shelf erosion and particulate matter re-routing to the open ocean during interglacial to glacial transitions, using a coupled model of the marine phosphorus, organic carbon and oxygen cycles. The results indicate that shelf erosion and submarine canyon formation may significantly lower deep-sea oxygen levels, by up to 25%, during sea level low stands, mainly due to the supply of new material from the shelves, and to a lesser extent due to particulate organic matter bypassing the coastal zone. Our simulations imply that deep-sea oxygen levels can drop significantly if eroded shelf material is deposited to the seafloor. Thus the glacial ocean's oxygen content could have been significantly lower than during interglacial stages. Primary production, organic carbon burial and dissolved phosphorus inventories are all affected by the erosion and rerouting mechanisms. However, re-routing of the continental and eroded shelf material to the deep-sea has the effect of decoupling deep-sea oxygen demand from primary productivity in the open ocean. P burial is also not affected showing a disconnection between the biogeochemical cycles in the water column and the P burial record.

Rapid formation of a modern bedrock canyon by a single flood event

Deep river canyons are thought to form slowly over geological time (see, for example, ref. 1), cut by moderate flows that reoccur every few years. In contrast, some of the most spectacular canyons on Earth and Mars were probably carved rapidly during ancient megaflood events. Quantification of the flood discharge, duration and erosion mechanics that operated during such events is hampered because we lack modern analogues. Canyon Lake Gorge, Texas, was carved in 2002 during a single catastrophic flood. The event offers a rare opportunity to analyse canyon formation and test palaeo-hydraulic-reconstruction techniques under known topographic and hydraulic conditions. Here we use digital topographic models and visible/near-infrared aerial images from before and after the flood, discharge measured during the event, field measurements and sediment-transport modelling to show that the flood moved metre-sized boulders, excavated ~7 m of limestone and transformed a soil-mantled valley into a bedrock canyon in just ~3 days. We find that canyon morphology is strongly dependent on rock type: plucking of limestone blocks produced waterfalls, inner channels and bedrock strath terraces, whereas abrasion of cemented alluvium sculpted walls, plunge pools and streamlined islands. Canyon formation was so rapid that erosion might have been limited by the ability of the flow to transport sediment. We suggest that our results might improve hydraulic reconstructions of similar megafloods on Earth and Mars.

Major ion and isotope geochemistry of fluids and gases from coalbed methane and shallow groundwater wells in Alberta, Canada

The production of coalbed methane (CBM) represents a vital new source of natural gas supply in Western Canada. There are, however, concerns over potential negative environmental impacts on shallow groundwater resources in the hypothetical case that leakage of fluids and gases from CBM operations occurs. This paper compares major ion and isotope geochemistry data for produced fluids or gases from two major coal deposits in Western Canada (Mannville Formation and the Horseshoe Canyon/Belly River Group) with similar data collected for shallow groundwater in south-central Alberta. The objective was to generate comprehensive baseline geochemical data to determine the key geochemical characteristics and differences of produced fluids and gases from two coal deposits and shallow groundwater in Alberta and to find parameters that are suitable for identifying potential leakage of fluids or gases into shallow groundwater. Shallow groundwater had average total dissolved solids (TDS) of 1037 mg/L. Most samples belonged to the Na–HCO3–SO4 water type and average SO42- concentrations were 185 mg/L. The Horseshoe Canyon/Belly River Group swabbing fluids had average TDS of 5427 mg/L, a Na–HCO3 water type, and average SO42- concentrations of 47.7 mg/L. The produced fluids from the Mannville Formation had average TDS contents of 74,500 mg/L, negligible SO42- and a Na–Cl water type. Shallow groundwater and produced fluids from the Horseshoe Canyon Formation and the Mannville group had distinct δ2H and δ18O values and plotting δ18O values versus total dissolved solids was found to be an effective approach to distinguish the waters. Sulfur isotope data revealed the occurrence of bacterial (dissimilatory) SO4 reduction in some shallow groundwater samples and in the produced fluids from the Horseshoe Canyon/Belly River Group. Methane was found in several shallow groundwater samples and its average δ13C (−72.1 ± 6.8‰) and δ2H values (−297 ± 17‰) indicated a biogenic origin predominantly from CO2 reduction. Dissolved gases from the Horseshoe Canyon Formation fluids with average δ13C values of methane of −54.0 ± 4.1‰ and ethane of −36.5 ± 2.4‰ and traces of higher alkanes suggest a mixture of predominantly biogenic and some thermogenic gas. Dissolved hydrocarbon gas from the Mannville Formation had the highest average δ13C values of −49.4 ± 3.6‰ for methane, −28.8 ± 2.1‰ for ethane and −26.9 ± 1.1‰ for propane. The presence of higher alkanes suggests that the Mannville produced fluids contain an appreciable thermogenic gas component. The biogenic gas component in the Horseshoe Canyon and the Mannville Formation was mainly formed via acetate fermentation according to αCO2–CH4 values between 1.02 and 1.07. It is concluded that δ18O values of the fluids in concert with total dissolved solids, and the isotopic compositions of methane and ethane are sufficiently distinct in shallow groundwater and produced fluids from the Horseshoe Canyon and the Mannville Formations that they may serve as tracers for evaluating potential contamination of shallow groundwater with produced fluids or gases.

Articulated skeletons of the aetosaur Typothorax coccinarum Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA

ABSTRACT We report two nearly complete, articulated skeletons of the crurotarsan archosaur Typothorax coccinarum from the Upper Triassic Bull Canyon Formation of east-central New Mexico. These are the most complete, articulated aetosaurs from North America and provide a wealth of new anatomical and paleobiological data, including articulated presacral armor that confirms the distinctiveness of T. coccinarum from the closely related T. antiquum and from Redondasuchus. Cervical vertebrae are small, but the corresponding reduction in armor is accomplished by a reduced number of cervical osteoderms. The third row of osteoderms includes a thin, elongate, lateral spike. The ventral armor consists of 10 thoracic columns and four caudal columns of osteoderms. Spiked osteoderms near the cloacal vent are the first spikes reported in aetosaurian ventral osteoderms. The forelimb of T. coccinarum was very short, only ∼0.65 the length of the hind limb, possesses some adaptations found in digging taxa, and was held in a sprawling or ‘semi-erect’ position. In contrast the hind limb is much more robust, ‘pillar erect,’ and functionally mesotarsal. The articulated pes, including unguals, has, minimally, the phalangeal formula 2-3-3?-4?-3? with relative digit lengths III > II > IV > I > V, digits I–IV equally as wide as long and other characteristics of the footprint ichnogenus Brachychirotherium, often attributed to an aetosaurian trackmaker. Both specimens are ∼2.5 m long and the preserved armor and limb bones are as large or larger than known Typothorax fossils, suggesting that this approximates the upper size limit of T. coccinarum, and we calculate body mass estimates of ∼100–104 kg for both specimens.

New Early Permian colonial rugose corals from the central Cordilleran miogeocline, U.S.A.

Five new species of Early Permian (late Sakmarian to Kungurian) colonial corals from eastern Nevada and southeastern California, one assigned to a new genus, are described and illustrated. These includeHeintzella playfordin. sp. from the Arcturus Formation in Nevada and the Darwin Canyon Formation in California,Paraheritschioides fergusonensisn. sp. from the Ferguson Mountain and Bird Spring formations in Nevada, andWendoverella arcan. gen. and n. sp.,Permastraea nevadensisn. sp., andPararachnastraea moormanensisn. sp. from the Pequop Formation in eastern Nevada. These new taxa are distinct from all previously described species, but most are related to other species in the North American miogeocline.Wendoverella arcan. sp., however, is unlike any other species described from North America but is quite similar to a Russian species, indicating faunal communication between the Ural Mountains and the Cordilleran miogeocline at least into Artinskian time.

Characterization of paralic paleoenvironments using benthic foraminifera from Lower Cretaceous deposits (Scotian Shelf, Canada)

Abstract The foraminifera from a Lower Cretaceous cored interval of Cohasset A-52 well (located on the Scotian Shelf) were used to interpret the depositional environments of these sediments. Paleoenvironmental interpretation was based on the analysis on 41 samples from four cored intervals of the well collected between 2069 and 2219 m below seafloor. The interval consists of 25 m of interbedded gray-black shale, mudstone and sandstone belonging to the Cree Member of the Logan Canyon Formation. The foraminiferal association recorded in the studied samples is comprised mainly of agglutinated specimens of Trochammina, Haplophragmoides, Ammobaculites and Verneuilinoides . A scattered occurrence of calcareous benthic foraminifera and thecamoebians is also recorded. A comparison of these microfauna with modern foraminiferal associations from paralic environments suggests that these sediments were deposited in a Cretaceous marsh–estuarine environment.