Triassic Formation Research Articles

Electrical tomography and TDEM prospection in the Chianciano thermal basin (Siena, Italy)

Chianciano thermal basin in Tuscany belongs to a great structure that extends, from San Casciano Bagni to Rapolano, orientated SSE-NNW. Several springs are located in this thermal field: Acqua Santa, Macerina, Casuccini and Sillene well. They are located close to the contact between the lower complex of the «Tuscan series» (Triassic limestones and dolomites) and Pliocene formations (sands and clayey sands, clays and silty clays). According to the groundwater flow scheme, the former formations represent the geothermal reservoir and, where they outcrop, the recharge areas. Thermalized fluids rise through direct faults, having an apennine orientation. In order to locate a new production well, electrical tomography and TDEM prospection were performed. Geophysical interpretation led to the construction of 1D imaged and 2D sections showing the main tectonic features. The different electrical behaviour of the hydrogeological units disclosed a fault located near the old Sillene well; this tectonic feature caused the uplifting of Triassic formations towards the north-east. A new production well was located close to the fault in the uplifted sector with excellent results as it captured the thermal aquifer at a depth of 30 m with a discharge up to 70 l/s.

Mineralogy and fluid inclusions study of carbonate-hosted Mississippi valley-type Ain Allega Pb–Zn–Sr–Ba ore deposit, Northern Tunisia

The Ain Allega Pb–Zn–Sr–Ba ore deposit is located in the flysch zone on the Eastern edge of the Triassic diapir of Jebel Hamra. It is part of the extrusive Triassic evaporate formation along the Ghardimaou–Cape Serrat faults. The ore body consists of argilic–dolomite breccias surrounded by argilo–gypsum Triassic formation, which forms the hanging wall of the deposit, and rimmed by the Paleocene marls. The ore minerals show a cap-rock type mineralization with different styles particularly impregnation in dolomite, cement of breccias, replacement ore and open space filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite and pyrite. Principal gangue minerals are composed of barite, celestite, calcite, dolomite and quartz. The ore minerals are hosted by the Triassic carbonate rocks which show hydrothermal alteration, dissolution and brecciation. X-ray – crystallographic study of barite–celestite mineral series shows that pure barite and celestite are the abundant species, whereas strontianiferous barite (85–96.5% BaSO 4) and barian–celestite (95% SrSO 4) are minor. Primary and secondary mono-phase (liquid only) fluid inclusions are common in celestite. Microthermometric analyses in two-phases (liquid and vapour) fluid inclusions suggest that gangue and ore minerals were precipitated by a low-temperature (180 °C) saline (16.37 wt.% NaCl equivalent) solution originated possibly from a basinal brine with some input from magmatic or metamorphic fluid. Based on geology, mineralogy, texture and fluid characteristics, the Ain Allega deposit is classified as a carbonate-hosted Mississippi valley-type deposit.

Provenance and thermal history of the Bayan Har Group in the western‐central Songpan–Ganzi–Bayan Har terrane: Implications for tectonic evolution of the northern Tibetan Plateau

AbstractTriassic turbidites dominate the Songpan–Ganzi–Bayan Har (SGBH) terrane of the northern Tibetan Plateau. U‐Pb dating on single detrital zircon grains from the Triassic Bayan Har Group turbidites yield peaks at 400–500 m.y., 900–1000 m.y., 1800–1900 m.y., and 2400–2500 m.y., These results are consistent with recently published U‐Pb zircon ages of pre‐Triassic bedrock in the East Kunlun, Altyn, Qaidam, Qilian and Alaxa areas to the north, suggesting that provenance of the Bayan Har Group may include these rocks. The similarities in the compositions of the lithic arkosic sandstones of the Bayan Har Group with the sandstones of the Lower‐Middle Triassic formations in the East Kunlun terrane to the north also suggests a common northern provenance for both. A well exposed angular unconformity between the Carboniferous–Middle Permian mélange sequences and the overlying Upper Permian or Triassic strata indicates that regional deformation occurred between the Middle and Late Permian. This deformation may have been the result of a soft collision between the Qiangtang terrane and the North China Plate and the closure of the Paleo‐Tethyan oceanic basin. The Bayan Har Group turbidites were then deposited in a re‐opened marine basin on a shelf environment. Fission‐track dating of detrital zircons from the Bayan Har Group sandstones revealed pre‐ and post‐depositional age components, suggesting that the temperatures did not reach the temperatures necessary to anneal retentive zircon fission tracks (250–300°C). A 282–292 m.y. peak age defined by low U concentration, retentive zircons likely reflects a northern granitic source. Euhedral zircons from two lithic arkoses with abundant volcanic fragments in the southern area yielded a ∼237 m.y. zircon fission track (ZFT) peak age, likely recording the maximum age of deposition. A dominant post‐depositional 170–185 m.y. ZFT peak age suggests peak temperatures were reached in the Early Jurassic. Some samples appear to record a younger thermal event at ∼140 m.y., a short lived event that apparently affected only the least retentive zircons.

Facies Analysis of Triassic Formations of the Hassi R`Mel in Southern Algeria Using Well Logs: Recognition of Paleosols Using Log Analysis

Facies Analysis of Triassic Formations of the Hassi R`Mel in Southern Algeria Using Well Logs: Recognition of Paleosols Using Log Analysis

Origin of Upper Triassic formation water in middle Sichuan Basin and its natural gas significance

Abstract The formation water of Upper Triassic Xujiahe Formation in the middle Sichuan Basin is of high salinity in most areas and of low to medium salinity locally. All the formation water lack sulphate ion implying good reducing and sealing conditions, but are rich in barium ion. The forming environment of the Xujiahe formation water is similar to that of the Jurassic formation water, but very different from that of the underlying marine formation water. The Xujiahe formation water has lower sodium (potassium) chloride coefficients and higher metamorphism coefficients. The water shows the characteristics of original continental sedimentary formation water reworked by later marine water of high salinity. In reworking the reservoirs, surface water of high sulphate ion content should be avoided, and also quick changes of formation temperature, pressure and chemical equilibrium should be avoided to prevent precipitates from blocking pores and throats.

Tausonite and aluminum-fluorine titanite from the metamorphosed metalliferous sediments of the Triassic chert formation of the Sikhote Alin

Rare strontium mineral tausonite and a peculiar Al- and F-rich titanite variety were found in the metamorphosed metalliferous sediments of the Triassic chert formation of the Sikhote Alin, which are distinguished by the abundance of native elements, intermetallic compounds, and metal solid solutions, as well as the presence of diverse Au, Ag, and PGE minerals. Tausonite was documented in the manganese (metamorphosed siliceous-rhodochrosite) rocks of the Ol’ga mining district and in the “brown cherts” (siliceous rocks with manganese garnet and spessartine) of the Dal’nerechensk district, Primorye. It forms rather numerous grains 2–10 μm across usually occurring as inclusions in quartz or rhodonite. According to the electron microprobe analysis, in addition to Sr, Ti, and O, the mineral contains only Fe3+ (up to 0.20 a.f.u.). Aluminum-fluorine titanite was found in the “brown cherts” of the Dal’nerechensk district of Primorye (upper reaches of the Gornaya River). Its crystals are up to 200 × 200 μm in size. The recalculation of the microprobe analyses to crystal chemical formulas indicated that up to half of the Ti sites in the structure of this mineral may be occupied by Al. The decrease of the total positive charge owing to the Al3+ substitution for tetravalent Ti4+ is compensated for by a decrease in the total negative charge owing to F− substitution for O2− via the scheme Al3+F− → Ti4+O2−. The occurrence of considerable amounts of F substituting for oxygen in the titanite structure and, as a consequence (owing to the crystal chemical features of the mineral), the high Al content were related to the reduced character of the metamorphism of the metalliferous deposits.

Discovery of Fossil Charophytes from the Late Triassic Xujiahe Formation and Early Jurassic Lower Yimen Formation, Sichuan Province, and Its Significance

This paper describes the fossil charophytes collected from the Late Triassic Xujiahe Formation in Dazu and Early Jurassic Lower Yimen Formation in Xichang, Sichuan Province, and their significance. Up to now, papers dealing with fossil charophytes of the Late Triassic, especially of the Early Jurassic, have been rarely published. The description of the two new species of Porochara, P. dazuensis and P. xichangensis, and their fossil assemblage not only provides an important basis for classification and correlation of red beds in southwestern China but reveals for the first time the features of the charophyte flora of the Late Triassic-Early Jurassic in the area, thus offering valuable data for the establishment of Triassic and Jurassic charophyte fossil zones in China.

Primor’ye as a new promising region of Russia with gold-palladium-platinum ore mineralization of a nontraditional type

The following regionally distributed contact-metamorphosed metalliferous sediments occur in association with each other in the lower part (Upper Olenekian‐Ladinian) of the Triassic siliceous formation of the Sikhote-Alin, which is enriched in carbon: manganese‐silicate (metamorphosed siliceous‐rhodochrosite) rocks, “brown cherts” (cherts with pyrophanite and spessartine), tin‐iron ores, “ilmenite‐biotite‐feld spar” (metamorphosed argillous) rocks, and “itabirites” (gold-bearing jaspers). These rocks occur in association with the products of hydrothermal regeneration composing bodies of different morphology and composition, including deposits of manganiferous skarnlike (amphibole‐pyroxene) rocks and skarns [1‐3]. Metalliferous sediments and products of their regeneration contain nickel and cobalt sulfides, arsenides, antimonides, tellurides, sulfoarsenids, sulfoantimonides and sulfotellurides, minerals of rare earth elements, thorium, uranium, barium, vanadium, and other minerals. Furthermore, reduced forms of different metals are common: native elements, solid solutions, intermetallic compounds, phosphides, and silicides. Associations of the reduced forms occur in pores and microsize fissures with organic matter. They were formed during the contact-metamorphic process under high temperatures (according to the admixture concentration level of several elements in native metals) accompanied by hydrocarbon decay and hydrogen reducing of oxidized forms. In view of the degree of the metalliferous rock distribution and the usual presence in it of minerals of Au, Ag, Pt, and Pd (a separate publication devoted to these minerals has been prepared [5]), the question of the concentration level of noble metals in these rocks becomes very important. Analyses performed earlier [1, 3] could not give an unambiguous answer to this question. Although they revealed high concentrations of Au, Pt, and Pd in separate samples, they characterized only manganese‐silicate rocks, their amount was insufficient, and reproducibility was weak. The concentrations of Au and Pt were found to be lower (one order of magnitude or more) than was expected according to the results of the investigation of mineral forms and dissolution of samples in hydrofluoric acid. The following three analytical methods have been used in the present work: fire assay with atomic absorption (AA) finishing, ICP-MS, and AA. The results of the fire assay analysis were found to contradict the data from the microprobe investigation and dissolution of samples in hydrofluoric acid. As was established during the microprobe investigation of the cinder, the losses were connected with

Transfer zones within the Longmen Mountains thrust belt, SW China

The Longmen Mountains thrust belt contains complex structures resulting from multi-phase tectonic movements. Field investigations and interpretations of seismic profiles reveal two important transfer zones in the thrust belt: the Anxian and Guanxian transfer zones. Many thrust and strike-slip faults are found in the shallow part of the transfer zones, and duplexes and detachment structures in the deep part. The Anxian transfer zone is located between the Jiaoziding and Jiudingshan nappes, consisting mainly of the Lower Triassic Feixianguan Formation (T1f) and Jianglingjiang Formation (T1j), and Middle Triassic Leikoupo Formation (T2l). The Guanxian transfer zone is located between the Baoxing and Jiudingshan nappes, consisting mainly of Devonian, Carboniferous, Triassic, Jurassic, and Presinian rocks. This zone contains many thrust faults, strike-slip faults, and duplexes in its deeper part (where early structures have been almost completely overprinted), and many klippens are found at the surface.

東九州（臼杵-津久見地域）の黒瀬川帯 (1)ジュラ紀付加体に断層で挟まれた整然層の上部三畳系

東九州（臼杵-津久見地域）の黒瀬川帯 (1)ジュラ紀付加体に断層で挟まれた整然層の上部三畳系

Evolution of Middle America and the in situ Caribbean Plate model

Abstract Regional geological data and global analogues suggest Caribbean Plate geology continues that seen along the margin of eastern North America in a more extensional setting, between the diverging Americas. From west to east there are continental masses with Triassic rifts, proximal continental blocks with kilometres-thick Mesozoic carbonates, more distal areas of Palaeozoic horsts flanked by Triassic–Jurassic dipping wedges of sediments, including salt and overlain by Cretaceous basalts, and most distal areas of serpentinized upper mantle. Plate history began along with the Late Triassic formation of the Central Atlantic Magmatic Province and involved Triassic–Jurassic rifting, Jurassic–Early Cenozoic extension and Oligocene–Recent strike–slip. Great extension promoted volcanism, foundering, eastward growth of the plate by backarc spreading and distribution of continental fragments on the plate interior and along its margins. Hydrocarbons probably are present. Caribbean geology has important implications for understanding of oceanic plateaus, intra-oceanic volcanic arcs, the ‘andesite problem’ and genesis of ‘subduction’ HP/LT metamorphic rocks. The model can be tested by re-examination of existing samples and seismic data and by deep sea drilling.

Sedimentology and paleoecology of the Lower Member of the Lower Triassic (Smithian-Spathian) Union Wash Formation, east-central California

Most paleoecological and paleobiologic studies of the postextinction interval following the end-Permian crisis have been focused on rocks deposited in shallow shelf, open marine settings. Lower Triassic marginal marine environments have not been examined closely but represent a crucial piece of the postextinction puzzle since they provide an end member to which studies of open marine environments can be compared. The Lower Member of the Lower Triassic (Smithian-Spathian) Union Wash Formation is a mixed carbonate-siliciclastic succession deposited in environments ranging from intertidal to lagoonal and provides a glimpse into Early Triassic marginal marine settings and faunas. Fossil components of the member are dominated by echinoid spines and debris, which are interpreted to represent a parautochthonous fossil assemblage concentrated primarily in tidal-dominated distributary channels and as grain-rich mouth bars. This Early Triassic echinoid assemblage affords an opportunity to examine the paleoecology of echinoids during the postextinction interval and to speculate about how the extinction and recovery relate to long-term paleobiological trends. Echinoids of the Lower Member of the Union Wash Formation inhabited environments very proximal to the intertidal zone, and this distribution may have provided a survival mechanism in the wake of the end-Permian mass extinction by allowing echinoids to avoid shelfal stresses such as shallow-water anoxia and euxinia. These shallow-water echinoid forms would likely seed the diversification of all later echinoid taxa, and indeed, all later shelfal and deep-sea echinoids can be traced back to post-Paleozoic shallow-water forebears.

Re-Os Dating of Pyrrhotite from the Chaoshan Gold Skarn, Eastern Yangtze Craton, Eastern China

The Chaoshan gold skarn, located in the central part of the Tongling Cu-Au-Mo district in eastern China, consists of 10 ore bodies occurring along an intrusive contact between the Early Cretaceous Baimangshan pyroxene diorite (BPD) and limestones of the Middle Triassic Nanlinghu Formation. Less commonly, ores occur in interlayered fractures within the calcareous wall rocks. Endoskarn alteration is well developed and manifested by the presence of massive skarn bodies consisting of garnet, scapolite, vesuvianite, epidote, actinolite, plagioclase, and lesser amounts of diopside. Toward the intrusion, the endoskarn alteration grades into potassic alteration composed of K-feldspar and biotite, suggesting that skarn formation, and thus gold mineralization, is directly related to the BPD. Metallic minerals are mainly pyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena, and sphalerite, of which pyrrhotite, pyrite, and arsenopyrite are the dominant Au-bearing phases. Re-Os dating of auriferous pyrrhotite was performed to determine the age of mineralization of this deposit. Fourteen duplicate analyses from seven samples define a rough isochron on the 187Re/188Os versus 187Os/188Os diagram, yielding an age of 146 ± 47 Ma (2σ) and an initial 187Os/188Os of 0.97 ± 0.25 (MSWD = 17). A greatly improved Re-Os isochron was obtained by plotting seven analyses from four samples that have similar initial 187Os/188Os and thus are assumed to have been derived from the same source, yielding an isochron age of 141.7 ± 9.9 Ma (2σ) and an initial 187Os/188Os of 1.027 ± 0.058 (MSWD = 1.15). This age is consistent with the SHRIMP zircon U-Pb age (142.9 ± 1.1 Ma) of the BPD and molybdenite Re-Os ages (141.7 ± 2.5 to 136.9 ± 2.2 Ma) of Cu-Mo deposits throughout the Tongling district, and therefore is interpreted as the timing of gold mineralization. The extremely radiogenic initial 187Os/188Os implies a significant crustal component in the Os budget of the pyrrhotite samples, likely derived from an enriched mantle source and crustal contamination of the BPD magma during its ascent through the crust.

Stratigraphic relationships of the Triassic Halfway Formation in the Western Canada Sedimentary Basin

Although the lithostratigraphic definition of the Middle Triassic Halfway Formation at the type section is quite precise and should be a clear guide to identifying the formation, there has been some confusion in its usage in the Western Canada Sedimentary Basin (WCSB). Some of the problems were identified by Dixon (2005) and schematically illustrated in his figure 6 but his work focused more on the identification of an unconformity rather than on nomenclatural problems. A lithostratigaphic unit is a mappable, usually contiguous, lithological body, which may or may not have chronological significance. The Halfway Formation fits this definition, for the most part, but depending on how you interpret stratigraphic relationships within the Middle to lowermost Upper Triassic succession, there are significant chronostratigraphic implications about the position of strata commonly identified as Halfway Formation. After studying the Triassic in the WCSB for a number of years (Dixon, 2002a, b, 2004, 2005, 2007, 2008) I believe some light can be shed on the problem and it is the intent of this brief note to attempt some clarification by illustrating the stratigraphic relationships in the Doig to lower Charlie Lake succession and show that strata identified lithostratigraphically as Halfway Formation can be chronostratigraphically very different in various parts of the basin. ### Definition of Halfway Formation Hunt and Ratcliffe (1959) defined the Middle Triassic Halfway Formation from the Southern Production No. B-14-1 well, located at 1-12-84-23W6 in northeast British Columbia (Fig. 1⇓), between log depths 5000 and 5148 ft (equivalent to 1523.5 and 1567.9 m in the re-entered 2/1-12-84-23W6 well). In the type well the Halfway Formation is defined as the predominantly sandstone interval underlain by thinly interbedded mudstone, siltstone, and sandstone of the Doig Formation, and overlain by thinly to thickly interbedded mudstone, dolostone, anhydrite, and sandstone of the Charlie Lake …

First hexactinellid sponge reported from the Upper Triassic Nayband formation of central Iran

Sponges, along with scleractinian corals, are among the main reef-building organisms in Triassic reefs. Hypercalcified groups, including the chambered sphinctozoans, and the unchambered inozoans, chaetetids, and spongiomorphids, represent the most abundant Triassic reef-building sponges. Earlier workers have described elements of the latter group as “hydrozoans.” Hexactinellid sponges, abundant in some Permian reefs (e.g., in Texas, Finks, 1960), are rarely known from similar Triassic deposits, in general (Tichy, 1975), and particularly from Upper Triassic stratigraphic units. Hexactinellid sponges have been sporadically reported from well-investigated Upper Triassic reefs in the western Tethyan region (e.g., Keupp et al., 1989). However, a variety of hexactinellid sponges have been reported from Upper Triassic deposits and reefal limestones of the northern and central Tethyan realm (Boiko, 1990; Wu, 1989; Wu and Xiao, 1989; Rigby et al., 1998).

Cretaceous synfolding remagnetization components revealing tectonic rotation of the middle Yangtze fold belt

To reveal the deformation process of the middle Yangtze fold belt, we conducted a paleomagnetic study on Middle Triassic limestones and Middle to Late Jurassic sandstones from Wanzhou, Chongqing. Stepwise thermal and alternating field demagnetization were used to isolate the multi-component remanent magnetizations. The Jurassic samples were overprinted by recent geomagnetic field, while three magnetization components were isolated from the Middle Triassic samples. A low temperature component (LTC) was isolated at temperatures below 200 degrees C, an intermediate temperature component (ITC) at 200-360 degrees C and a high temperature component (HTC) at 400-460 degrees C. The LTC is distributed around the present-day Earth magnetic field, probably a viscous component. Stepwise unfolding indicates that the maximum precision parameters of ITC and HTC components are achieved at 33 +/- 8% and 50 +/- 27% (with 95% confidence) unfolding, respectively. The best-clustered ITC mean direction, Dec = 11.2 degrees, Inc = 45.2 degrees (alpha(95) = 4.5 degrees, N = 34), corresponding to a paleopole at 79.3 degrees N, 219.5 degrees E (d(p) = 3.6 degrees, d(m) = 5.7 degrees), is consistent with the Cretaceous reference direction of the South China Block (SCB). The best-clustered HTC mean direction (taking 70% unfolding), Dec = 24.2 degrees, Inc =49.0 degrees (alpha(95) = 3.6 degrees, N = 23), corresponding to a paleopole at 69.2 degrees N, 195.5 degrees E (d(p) =3.1 degrees, d(m) = 4.8 degrees), suggests a clockwise rotation of 12.8 degrees +/- 3.5 degrees. These synfolding remagnetization components clearly reveal that a clockwise rotation happened at the middle stage of folding, thus supporting that at least part of the variation in fold axis strikes is due to orocline rotation. Combined with published data, our analysis indicates that the Wanzhou-Xiangxi segment of the middle Yangtze fold belt experienced oroclinal bending. Furthermore, a published post-folding component isolated from the Middle Triassic Puqi Formation suggests a 27.5 degrees +/- 5.8 degrees clockwise rotation, confirming that at least 50% of the observed clockwise rotations in the eastern middle Yangtze fold belt can be attributed to oroclinal bending. The remagnetization data and geological evidence observed in the middle Yangtze fold belt suggest that collision between SCB and North China Block (NCB) probably lasted till the early period of Early Cretaceous.

Geochemistry and metal potential of triassic carbonaceous silicites in Sikhote Alin

In the Triassic siliceous formation of Sikhote Alin, carbonaceous silicites occur in the late Olenekian-middle Anisian member (4–20 m) of alternating cherts and clayey cherts (“phtanite member”) near the section base. The silicites are represented by radiolarian and spicule-radiolarian cherts alternating with clayey cherts. They contain up to 8.5% Corg. In the majority of sections, the rocks underwent structural and mineral transformation at the mesocatagenetic stage. The slightly oxidized organic (primarily, marine sapropelic) matter contains quinones, methyl, methylene, and ether groups. The content of neutral bitumens in rocks shows a wide variation range. The carbon isotopic composition in phtanites and clayey phtanites (δ13C from −27.3 to −30.2‰) is identical to that in many Paleozoic-Mesozoic bitumens and oils. As compared with other Mesozoic sedimentary rocks of Sikhote Alin, the carbonaceous silicites are enriched in V, B, Mo, Ni, Cu, and Ag. Anomalously high concentrations of Ba are recorded in phtanite rock sections at the Gornaya and Khor rivers and in the vicinity of Khabarovsk. Modal value of the Au content in phtanites and clayey phtanites is three or four times higher than the clarke value in carbonaceous silicites and reaches anomalous values in some sections (e.g., Ogorodnaya River section). Carbonaceous silicites of this section are also enriched in Pt. Positive Au-Corg correlation is recorded in clayey phtanites of the Ogorodnaya River section containing more than 0.5% Corg. In organic fractions, Au and Ag are concentrated in alcohol and alcohol-benzene bitumens, asphalt acids, and asphaltenes. Migration of bitumens from high-carbonaceous clayey phtanites to the pore-fissure space of cherts and phtanites also fostered the concentration of these metals in some low-carbonaceous layers of the member.

Identification and evaluation of low resistivity pay zones by well logs and the petrophysical research in China

This paper presents an overview of petrophysical research and exploration achievements of low resistivity pay (LRP) zone by well logs in China. It includes geological characteristics and characteristics of well log response of the low resistivity pay zones discovered and evaluated in recent years, as well as the problems in recognizing and evaluating low resistivity pay zones by well logs. The research areas mainly include the Neogene formations in the Bohai Bay Basin, the Triassic formations in the northern Tarim Basin and the Cretaceous formations in the Junggar Basin. The petrophysical research concerning recognition and evaluation of the low resistivity pays, based on their genetic types, is introduced in this paper.

Evidence for Prosauropod Dinosaur Gastroliths in the Bull Run Formation (Upper Triassic, Norian) of Virginia

Definitive criteria for distinguishing gastroliths from sedimentary clasts are lacking for many depositional settings, and many reported occurrences of gastroliths either cannot be verified or have been refuted. We discuss four occurrences of gastrolith-like stones (category 6 exoliths) not found within skeletal remains from the Upper Triassic Bull Run Formation of northern Virginia, USA. Despite their lack of obvious skeletal association, the most parsimonious explanation for several characteristics of these stones is their prolonged residence in the gastric mills of large animals. These characteristics include 1) typical gastrolith microscopic surface texture, 2) evidence of pervasive surface wear on many of these stones that has secondarily removed variable amounts of thick weathering rinds typically found on these stones, and 3) a width/length-ratio modal peak for these stones that is more strongly developed than in any population of fluvial or fanglomerate stones of any age found in this region. When taken together, these properties of the stones can be explained most parsimoniously by animal ingestion and gastric-mill abrasion. The size of these stones indicates the animals that swallowed them were large, and the best candidate is a prosauropod dinosaur, possibly an ancestor of the Early Jurassic gastrolith-producing prosauropod Massospondylus or Ammosaurus. Skeletal evidence for Upper Triassic prosauropods is lacking in the Newark Supergroup basins; footprints (Agrestipus hottoni and Eubrontes isp.) from the Bull Run Formation in the Culpeper basin previously ascribed to prosauropods are now known to be underprints (Brachychirotherium parvum) of an aetosaur and underprints (Kayentapus minor) of a ceratosaur. The absence of prosauropod skeletal remains or footprints in all but the uppermost (upper Rhaetian) Triassic rocks of the Newark Supergroup is puzzling because prosauropod remains are abundant elsewhere in the world in Upper Triassic (Carnian, Norian, and lower Rhaetian) continental strata. The apparent scarcity of prosauropods in Upper Triassic strata of the Newark Supergroup is interpreted as an artifact of ecological partitioning, created by the habitat range and dietary preferences of phytosaurs and by the preservational biases at that time within the lithofacies of the Newark Supergroup basins.

A new vetigastropod (Gastropoda Stomatellidae) from the Late Triassic Cassian Formation

A new vetigastropod (Gastropoda Stomatellidae) from the Late Triassic Cassian Formation