Virgin Limestone Research Articles

Detailed performance analysis of the wet extractive grinding process for higher calcium yields from steelmaking slags

In this paper, the wet extractive grinding method for efficient recovery of calcium from steelmaking slags was studied. This work supports the Sustainable Development Goal of UN, SDG12: Responsible Consumption and Production by reducing the need for virgin limestone and aiming to circular solutions. The extraction is the first step in our pH-swing process, wherein CaO from the slag using a NH4Cl-water solvent is dissolved. After filtering the solid phase, CO2 is bubbled into the Ca-rich solution to produce precipitated calcium carbonate (PCC). Traditional mechanical mixing for extraction stage is found problematic since it requires energy intensive preliminary fine-grinding to be effective. Effects of the processing time (1.25–20 min), and the slag-to-solvent ratio (SLR = 30–120 g/L) were studied here in detail in the extractive grinding process. Inductively coupled plasma-atomic emission spectroscopy (ICP-OES), energy-dispersive X-ray spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses were used for the detailed physical and chemical characterization of raw and residual slag powders. Results showed that Ca-yield increases from 35 to ~70% via the EG process as comparison with mechanical mixing. Results also show that the rate of the process also significantly increased.

Stromatolites from the Lower Triassic Virgin Limestone at Blue Diamond, NV USA: The role of dysoxia, enhanced calcification and nutrient availability in the growth of post-extinction microbialites

Abstract Microbialites undergo a well – documented resurgence following the Permian – Triassic mass extinction. This comeback has been attributed to several causes, including the emptying of Early Triassic ecosystems by the severity of the extinction, a decline in burrowing activity and intensity, and the unusual chemistry of Early Triassic oceans. A laterally – continuous stromatolite horizon found within the Lower Triassic Virgin Limestone was examined at the Blue Diamond, NV U.S.A. locality in order to determine the factors that led to growth of the stromatolites, and by extension, the environmental conditions that led to the formation of widespread microbialites in the aftermath of the Permian-Triassic mass extinction. The Virgin Limestone at the study locality consists of ~6 m – thick cycles of shale and siltstone that pass upwards into bioturbated lime mudstone, and are capped by oolitic packstone or grainstone. Each ~6 m – thick cycle represents a rapid sea level rise that led to the deposition of fine – grained clastics below storm wave base, followed by deposition of coarsening – upwards carbonates that were deposited in progressively shallower environments. Analysis of redox-sensitive trace metals (U. V and Mo) from the shale and siltstone interbeds indicate that the fine – grained clastics were deposited under dysoxic waters, however, extensive bioturbation and macrofossils from the carbonate units indicates better aeration of shallower settings. The stromatolites occur within a single, laterally extensive horizon, and 4 distinct stromatolite morphologies are observed along a 3 km long transect, including: 1) aggregates of high-relief hemispherical domes and columnar stromatolites with well-developed laminae that are up to 0.75 m thick; 2) lozenge-shaped aggregates of intergrown columnar stromatolites with dimpled top surfaces; 3) isolated hemispheroids; and, 4) meandering ridges of intergrown domes that are small, low – relief (usually

An Early Triassic small shelly fossil-style assemblage from the Virgin Limestone Member, Moenkopi Formation, western United States

Small shelly fossils (SSFs) are minute fossils moulded or replaced by apatite, and less commonly, other minerals like glauconite and iron oxides. This taphonomic mode is best known from Cambrian deposits, though some occurrences occur across geological time. Instances of small shelly-style preservation were found in insoluble residues from the Lower Triassic Virgin Limestone Member exposed in southern Nevada, the western United States, a second such occurrence known from this unit. Fossil steinkerns of tiny brachiopods, echinoid spines and ophiuroids are fluorapatite, with scarce phosphatic internal moulds of bivalves and two replaced ostracods. In contrast, the crinoid ossicles, almost all of which are >1000 µm, are preserved as stereomic moulds of silica or dolomite. Though the style of preservation is similar to another Virgin Limestone small shelly fossils-style assemblage, this assemblage preserves greater diversity, likely reflecting the variation in palaeocommunities across the shelf. The size selectivity of phosphatization is clear, as the majority of the fossils <1000 µm are phosphatized. Importantly, the original skeletal material does not exert the strongest control on style of preservation: crinoid ossicles are replaced or moulded by silica whilst ophiuroid and echinoid fragments are phosphatized. It is likely that the underlying phosphatization mechanisms are related to the small particle size of the skeletons or skeletal elements coupled with the local pore water environment. Early Triassic equatorial seas characterized by warm temperatures and lower oxygen levels likely fostered small shelly fossil-style preservation across the shelf during this time.

SMALL SHELLY FOSSIL PRESERVATION AND THE ROLE OF EARLY DIAGENETIC REDOX IN THE EARLY TRIASSIC

Minute fossils from a variety of different metazoan clades, collectively referred to as small shelly fossils, represent a distinctive taphonomic mode that is most commonly reported from the Cambrian Period. Lower Triassic successions of the western United States, deposited in the aftermath of the end-Permian mass extinction, provide an example of small shelly style preservation that significantly post-dates Cambrian occurrences. Glauconitized and phosphatized echinoderms and gastropods are preserved in the insoluble residues of carbonates from the Virgin Limestone Member of the Moenkopi Formation. Echinoderm plates, spines and other skeletal elements are preserved as stereomic molds; gastropods are preserved as steinkerns. All small shelly style fossils are preserved in the small size fractions of the residues (177 to 420 μm), which is consistent with the size selection of small shelly fossils in the Cambrian. Energy-dispersive X-ray spectra of individual fossils coupled with X-ray diffraction of residues confirm that the fossils are dominantly preserved by apatite and glauconite, and sometimes a combination of the two minerals. The nucleation of both of these minerals requires that pore water redox oscillated between oxic and anoxic conditions, which, in turn, implies that Lower Triassic carbonates periodically experienced oxygen depletion after deposition and during early diagenesis. Long-term oxygen depletion persisted through the Early Triassic, creating diagenetic conditions that were instrumental in the preservation of small shelly fossils in Triassic and, likely, Paleozoic examples.

Carbon Footprint of Recycled Aggregate Concrete

Carbon footprint is one of the most widely used tools for assessing the environmental impacts of the production and utilization of concrete as well as of the components derived from it, representing the amount of carbon dioxide and other greenhouse gases associated with this product, expressed as CO2 equivalents. In this paper, carbon footprint was used to compare the environmental performance in the production phase of a concrete made with both recycled and crushed virgin limestone aggregates, using a life cycle analysis methodological approach. Research outcomes revealed, as expected, that carbon dioxide equivalent emissions decreased slightly as the use of recycled aggregates increased. Emissions for concrete with 0.5 w/c were between 347 and 351 kg of CO2-e/m3. It was also corroborated that cement is the material with the greatest influence on greenhouse gas emission generation in the concrete’s production phase, regardless of the use of recycled or virgin aggregates.

Mix design considerations of foamed bitumen mixtures with reclaimed asphalt pavement material

In the present work, a mix design parametric study was carried out with the aim of proposing a practical and consistent mix design procedure for foamed bitumen mixtures (FBMs). The mix design parameters that were adopted in the study are mixing and compaction water content (MWC), compaction effort using a gyratory compactor and aggregate temperature. This parametric study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material and a mix design procedure was proposed. This proposed methodology was also found to apply to FBMs with RAP. A detailed consideration was also given to characterising the RAP material so as to understand its contribution to the mechanical properties of FBMs. Optimum MWC was achieved by optimising mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75–85% of optimum water content obtained by the modified Proctor test was found to be the optimum range of MWC that gives optimum mechanical properties for FBMs. It was also found that the presence of RAP influenced the design foamed bitumen content, which means that treating RAP as black rock in FBM mix design is not appropriate. To study the influence of bitumen and water during compaction, modified Proctor compaction and gyratory compaction were employed on mixes with varying amounts of water and bitumen. By this, the work also evaluated the validity of the total fluid (water + bitumen) concept that is widely used in bitumen–emulsion-treated mixes, and found it not to be applicable.

TAPHONOMIC BIAS OF SELECTIVE SILICIFICATION REVEALED BY PAIRED PETROGRAPHIC AND INSOLUBLE RESIDUE ANALYSIS

Abstract Silicification is an important mode of fossil preservation but the extent to which silicified material represents an unbiased sampling of the total fossil assemblage within a given rock sample remains poorly quantified. Here, we use paired analyses of thin sections and acid-extracted silicified specimens from the same samples to examine the biases introduced during silicification of Lower Triassic Virgin Limestone carbonates preserved in the Muddy Mountains of southern Nevada. Bivalves dominate most thin sections in the point count data, but rarely silicify completely enough to be recognized in residue. Echinoderms and gastropods are less abundant in thin section but dominate the residues. The abundances of these groups in thin section and residue are only weakly correlated. These findings suggest that although silicification generally captures relative trends in proportional abundance of higher taxa among samples, the silicification process can be taxonomically biased. Given the biases that can occur during silicification, it should not be assumed that silicified collections present a pristine picture of taxonomic or paleoecologic composition. Petrographic analysis has the potential to illuminate the reliability of paleontological data based on silicified collections.

High temperature and low oxygen perturbations drive contrasting benthic recovery dynamics following the end-Permian mass extinction

The end-Permian mass extinction event was the greatest loss of biodiversity ever experienced on the planet. The event is thought to have been triggered by the initiation of the volcanic eruptions of the Siberian Traps. The five million year recovery interval that followed the extinction event was strongly influenced by the environmental effects of sustained volcanic eruptions including extreme temperature events and persistent global and regional oxygen minimum zones. The effects of these environmental perturbations on the paleoecological recovery of the benthic marine fauna were studied in two depositional units from the Southwestern United States representing two substages during the Early Triassic recovery. The Smithian Sinbad Limestone was influenced by high sea surface temperatures and contains a relatively high diversity fauna that exhibits extremely small body size. Gastropods that lived in this environment were size-limited, a possible result of metabolic stress due to extreme temperatures. These microgastropods were able to become dominate components of the benthic fauna by occupying niche space vacated by other taxa that were excluded by high temperatures. The Spathian Virgin Limestone shows evidence for low oxygen conditions. The resulting low diversity benthic fauna had a more ecologically complex community structure than the Smithian Sinbad Limestone including the occupation of epifaunal tiering space by crinoids. As the prevalence of aerobic facies increased through time, diversity, body size, and the complexity of faunal interactions also increased suggesting that low oxygen conditions were the limiting factor for the benthic recovery in that region. The differences in diversity and community structure between the two units highlight the importance of environmental and temporal differences in driving the recovery patterns of the benthic fauna following the extinction event. High temperature events and low oxygen conditions restricted the benthic fauna in different ways but both contributed to the delay in recovery from the end-Permian mass extinction.

Laboratory Mix Design Procedure for Foamed Bitumen Mixtures

The primary objective of this study was to propose a practical and consistent mix design procedure for foamed bitumen mixture (FBM). The main focus of the proposed methodology was the use of the gyratory compaction method. The study maximized the mix design parameters, such as mixing water content (MWC) and compaction effort. The study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material, and a mix design procedure was proposed. The proposed methodology was validated on FBMs with 50% RAP and 75% RAP. Efforts were also made to optimize the foamed bitumen (FB) content in FBMs with and without RAP. Optimum MWC was achieved by optimizing mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75% to 85% optimum water content was obtained by the modified Proctor test and found to be the optimum range of MWC that gave optimum mechanical properties for FBMs. The study used a gyratory compactor for FBM compaction and found that a unique, mixture-specific number of gyrations that was independent of the FB content could be established. The study also found that the presence of RAP influenced the design's FB content: treating RAP as black rock in FBM mix design was not appropriate.

Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematics and evolutionary significance

The recovery from the end-Permian mass extinction event was a key interval in the history of life, but few modern studies provide systematic data on benthic marine faunas from the epoch immediately following the crisis. Here, the bivalve fauna from the early Spathian (Olenekian, late Early Triassic) Virgin Limestone Member of the Moenkopi Formation is comprehensively documented for the first time. The new genus Sementiconcha (Myophoricardiidae), type species Sementiconcha recuperator sp. nov., and the new species Leptochondria nuetzeli, Eumorphotis ericius, E. virginensis and Pleuromya prima, are described. Leptochondriidae is placed in synonymy with Asoellidae, which is revised. With 27 species belonging to 18 genera, the Virgin Limestone Member records the highest bivalve diversity reported so far from this time interval, questioning previous claims that the recovery from the end-Permian mass extinction was delayed until the Middle Triassic. The two bivalve subclasses (Pteriomorphia and Heteroconchia) that are present in the Virgin Limestone Member clearly differ in their evolutionary contexts. Pteriomorphs of the Virgin Limestone are nearly exclusively composed of genera that survived the end-Permian mass extinction event, whereas heteroconchs are highly dominated by genera that evolved in the Early Triassic. This contrasting evolutionary background probably reflects differential effects of the end-Permian mass extinction event and subsequent crises on these two subclasses, possibly related to differences in filter feeding efficiency and shell mineralogy. The high proportion of infaunal heteroconchs, including deep-infaunal Pholodomyoida, is an additional indicator of a relatively advanced recovery stage, further corroborating that recovery of benthic organisms was well underway during the late Early Triassic. http://zoobank.org/urn:lsid:zoobank.org:pub:EC9802EA-F1D5-4066-A608-2B8DECE7C10D

Microbial ooids and cortoids from the Lower Triassic (Spathian) Virgin Limestone, Nevada, USA: Evidence for an Early Triassic microbial bloom in shallow depositional environments

Lower Triassic sedimentary rocks contain a variety of unusual facies and fabrics, with microbialites being a distinctive component of many carbonates deposited following the Permian–Triassic mass extinction. Coated grains are common in shallow water facies from the upper Lower Triassic (Spathian) Virgin Limestone (Moenkopi Formation) in southern Nevada, and were investigated in order to determine their origin. Petrographic analysis reveals that the majority of the coated grains found within the Virgin Limestone are micritic ooids with a concentric fabric, or with a homogenous fabric composed of dense, often cloudy micrite. In addition, asymmetric ooids, aggregate grains, and distorted ooids are also locally common in some oolitic units; low-Mg calcite ooids and bimineralic ooids composed of low-Mg calcite and dense, cloudy micrite are less commonly found, but are also documented from the Virgin Limestone. Cortoids (i.e., grains that are coated with constructive micrite envelopes) are a minor component of oolitic grainstones and packstones (typically 10–15% of the grains), although they may also comprise entire beds. The cortoids are coated with micrite similar to that which comprises the ooid cortices, and may be finely laminated or dense and cloudy in nature. The micrite ooids and constructive micrite envelopes are interpreted as microbial in origin based on the finely laminated or cloudy, dense nature of the micrite, as well as coatings that are uneven, or often of greater thickness on one side of elongate nuclei, such as bivalve shells or phylliod algae blades. The origin of the low-Mg calcite ooids and layers is less certain, but may also be microbial. The results of this study suggest that a microbial bloom occurred in shallow water environments, which was the result of 3 factors: (1) the unusual chemistry of Early Triassic oceans; (2) runoff of nutrient-rich waters, which enhanced microbialite growth; and, (3) wave agitation and warm waters that led to CO2 degassing and further supersaturation of shallow waters with respect to calcium carbonate.

Paleoecology and geochemistry of Early Triassic (Spathian) microbial mounds and implications for anoxia following the end-Permian mass extinction

Large microbialite mounds (1–2 m in height) have previously been reported from two units within the Spathian section of the Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Spring, Nevada (United States). Previous investigations led to the interpretation that the mounds were formed under anoxic and alkaline conditions that suppressed metazoan grazers and delayed the biotic recovery from the end-Permian mass extinction. Here we report low organic carbon and total sulfur abundances throughout the section that suggest that anoxia was not prevalent during deposition. We also report that the upper mound-bearing unit contains stromatolitesponge patch reefs in which mutual encrustation between stromatolites and sponges contributed to the building of a reef framework. The stromatolite-sponge patch reefs contain discrete burrows within stromatolitic laminations, suggesting that there was suffi cient oxygen for grazing during the formation of the upper unit mounds. The enhanced ecological complexity of the upper unit mounds leads us to conclude that the mounds represent the transition to biotic recovery following the end-Permian mass extinction.

Utilisation of steel slags as neutralising agents in biooxidation of a refractory gold concentrate and their influence on the subsequent cyanidation

A study on the possibilities to utilise steel slag as neutralising agent in biooxidation of a refractory gold concentrate has been done with reference to commercial grade slaked lime. The idea has been to reduce the operating costs for neutralisation in the biooxidation plant, which is known to be the second largest operating cost. Other benefits would be savings in cost for landfilling of slag, possibilities to recycle elements present in the slag and savings of virgin limestone deposits. The slags used were an EAF slag and a slag from ladle refining; both originating from Swedish scrap based steel-making. Continuous biooxidation of the refractory gold concentrate was conducted in a single-stage 5 L reactor at a retention time of 56 h. The neutralisation capacity was determined by comparing the amount needed, per ton of feed concentrate added, to maintain the desired pH of 1.5 during steady state operation. Slaked lime had the highest neutralisation capacity with 110 kg/ton feed followed by ladle slag and EAF slag with values of 152 and 267 kg/ton feed, respectively. Sulphide mineral oxidation was similar and high in all cases although the ladle slag results were slightly better. Gold recoveries after cyanide leaching on the residues obtained were also similar and were in the range of 86–89%. However, the cyanide consumption expressed as kilogram cyanide per ton of concentrate fed to biooxidation, was double in the case of ladle slag and three times as much for the EAF slag compared to the slaked lime experiment. The increased cyanide consumption could not be explained only by the increased amount of elemental sulphur obtained in the slag experiments. The elemental sulphur formed had different reactivities as seen from the thiocyanate formation and cyanide losses due to thiocyanate formation were 16%, 32% and 40% for EAF slag, slaked lime and ladle slag, respectively. It is concluded that the ladle slag could be a possible replacement for limestone if they are mixed in proper proportions so that the microbial carbon dioxide demand is met whereas the EAF slag is less suitable due to the very fine reaction products obtained which gave operational problems with filtration and washing. To come further, experiments with the normal multi-stage biooxidation set-up with total retention time of 120 h should be performed which would increase the sulphur oxidation and eventually also reduce the cyanide consumption.

Origin of Lower Triassic microbialites in mixed carbonate-siliciclastic successions: Ichnology, applied stratigraphy, and the end-Permian mass extinction

Studies aimed at understanding the recovery from the end-Permian mass extinction have focused on the distribution of faunas, trace fossils, and microbialites across depositional environments, but few have examined how these individual ecologic components relate to one another, especially the latter relationship between microbialites and trace fossils, which appear to be mutually exclusive. Microbialites occur throughout Lower Triassic strata primarily in two different forms depending on the depositional system in which they were preserved in. In carbonate-dominated settings, microbialites can commonly take the form of large stromatolitic patch reefs, while in siliciclastic settings microbialites are found primarily as wrinkle structures. This study focuses on the palaeoenvironmental distribution of microbial structures and trace fossils from an uppermost Lower Triassic deposit, the Virgin Limestone Member of the Triassic Moenkopi Formation, which is comprised of mixed carbonate-siliciclastic sedimentary rocks, and is known to contain both stromatolites and wrinkle structures, in addition to trace fossils.Results show that the highest trace fossil diversities are found in lower shoreface environments, while offshore environments contain the lowest diversity. Strata deposited in the offshore transition, separating lower shoreface and offshore environments, contain moderate diversity trace fossil assemblages. Microbialites, in the form of either stromatolites or wrinkle structures, appear to emerge only following transgression, and are commonly found across and following marine flooding surfaces. Wrinkle structures appear to have formed as low-oxygen waters encroached upon nearshore settings, suppressing bioturbation and allowing for microbial mat development. Stromatolites likely formed due to the upwelling of anoxic alkaline waters during transgression, which may have generated a firm substrate for colonization, as well as fostering cementation of the microbialite. While most studies examining the aftermath of the end-Permian mass extinction focus on either carbonate- or siliciclastic-dominated settings, this study reports the environmental distribution of microbialites in both carbonate and siliciclastic facies, and unites them into a single depositional model that accounts for stratigraphic distribution, relation to trace fossils, and the unique environmental conditions present during the aftermath of the end-Permian mass extinction.

Development of Lower Triassic Wrinkle Structures: Implications for the Search for Life on Other Planets

Wrinkle structures are microbially mediated sedimentary structures that are a common feature of Proterozoic and earliest Phanerozoic siliciclastic seafloors on Earth and occur only rarely in post-Cambrian strata. These macroscopic microbially induced sedimentary structures are readily identifiable at the outcrop scale, and their recognition on other planetary bodies by landed missions may suggest the presence of past microbial life. Wrinkle structures of the Lower Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation in the western United States record an occurrence of widespread microbialite formation in the wake of the end-Permian mass extinction, the largest biotic crisis of the Phanerozoic. Wrinkle structures occur on proximal sandy tempestites deposited within the offshore transition. Storm layers appear to have been rapidly colonized by microbial mats and were subsequently buried by mud during fair-weather conditions. Wrinkle structures exhibit flat-topped crests and sinuous troughs, with associated mica grains oriented parallel to bedding, suggestive of trapping and binding activity. Although Lower Triassic wrinkle structures postdate the widespread occurrence of these features during the Proterozoic and Cambrian, they exhibit many of the same characteristics and environmental trends, which suggests a conservation of microbial formational and preservational processes in subtidal siliciclastic settings on Earth from the Precambrian into the Phanerozoic. In the search for extraterrestrial life, it may be these conservative characteristics that prove to be the most useful and robust for recognizing microbial features on other planetary bodies, and may add to an ever-growing foundation of knowledge for directing future explorations aimed at seeking out macroscopic microbial signatures.

Faunal diversity, heterogeneity and body size in the Early Triassic: testing post-extinction paradigms in the Virgin Limestone of Utah, USA*

The recovery intervals after mass extinction events are portrayed as times of ecological stress, with effects including reduced body size (Lilliput Effect), reduced taxonomic diversity and lowered ecological complexity. As the end-Permian mass extinction is the largest of the ‘Big Five’ events, with a prolonged recovery interval postulated during the Early Triassic, we expect these effects to be most pronounced in Early Triassic faunas. We have used new data and analytical techniques on the fauna of the Spathian Virgin Limestone to test whether the Lilliput Effect can be observed, whether the fauna is vertically and laterally homogeneous, and whether taxonomic diversity is significantly lower than that of comparable nearshore marine deposits transgressive over redbed sequences from other deposits not regarded as falling into post-extinction recovery intervals. The Lilliput Effect can be observed in some cases, but no uniform trend in body size reduction emerged. When the distributions of the body size across a range of marine invertebrate taxa was compared between the Virgin Limestone and the latest Triassic Penarth Group of Britain there was no support for a significant reduction in body size among the Virgin Limestone fauna. Evidence for faunal homogeneity was also limited. The alpha diversity of individual sites in the Middle Triassic Muschelkalk of central Europe was generally higher than Virgin Limestone sites, after accounting for differences in sample size using rarefaction, but there was some overlap. Although the beta diversity of the Muschelkalk is also apparently higher, a number of biases may be elevating Muschelkalk diversity. Future comparisons between recovery and ‘normal’ faunas need to ensure that as many confounding factors as possible are controlled for and that multiple samples are tested.

Anatomy of an anachronistic carbonate platform: Lower Triassic carbonates of the southwestern United States

Examination of Lower Triassic carbonate rocks from around the world has revealed the resurgence of atypical carbonates that were last common during the Proterozoic (‘anachronistic facies’). Anachronistic facies are attributed to a combination of post-extinction paleoecologic factors and widespread, harsh environmental conditions, particularly the presence of anoxic, alkaline waters in the global ocean. The marine rocks of the Moenkopi Formation (Virgin Limestone) of southern Nevada and laterally equivalent deep-water limestones of the Union Wash Formation of east-central California provide a means to reconstruct depositional environments across a Lower Triassic carbonate ramp and place the atypical facies and fabrics that occur within those rocks into a larger ‘Anachronistic Platform Model.’ Examination of shallow subtidal and intertidal facies reveal widespread ooid shoals in both the Union Wash Formation and Virgin Limestone, as well as the presence of small, fringing stromatolite bioherms in shallow subtidal environments of the Union Wash Formation. Inner to middle ramp environments of the Virgin Limestone contain aggregates of cabbage-head stromatolites that formed 1–2 m-high patch-reef bioherms, in addition to flat-pebble conglomerates, mud-chip facies and wrinkle structures. Deep-water facies from the Union Wash Formation contain synsedimentary calcium carbonate precipitates that form laterally extensive crusts in basinal environments, or interlocking mosaics of hemispheres and fans in outer ramp to slope environments. The results of this study reveal that carbonate sedimentation during the Early Triassic was controlled by a combination of biotic factors, environmental stresses, unusual oceanic chemistry and normal carbonate sedimentary processes that resulted in the development of carbonate platforms that had affinities to both Phanerozoic and Proterozoic depositional systems.

EARLY TRIASSIC MICROBIAL SPHEROIDS IN THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION, NEVADA, USA

Abstract Lower Triassic carbonate sedimentary rocks exhibit fabrics and facies indicative of reduced bioturbation and reduced abundance of skeletal animals and algae relative to their Permian counterparts. Widespread microbial mounds are one widely cited example. Micritic spheroids of probable microbial origin occur at a few horizons in the Spathian (uppermost Lower Triassic) Virgin Limestone Member of the Moenkopi Formation, exposed in southern Nevada. These spheroids (3–12 mm) consist of an irregular framework of micrite clots up to 100 μm in diameter and micrite-walled filaments ∼10 μm wide and <500 μm long. Based upon their microstructure, we suggest that these microbial spheroids formed through the rapid microbial precipitation of calcium carbonate from seawater. As such, they probably represent unattached analogues of Early Triassic microbial mounds. The microbial spheroids formed and lithified rapidly in shallow-water, high-energy settings. Their presence, in concert with other carbonate microbiali...

Stromatolites of the Lower Triassic Union Wash Formation, CA: Evidence for continued post-extinction environmental stress in western North America through the Spathian

Recent studies have documented the complexity of biotic recovery from the Permian–Triassic mass extinction by revealing that recovery began quickly in some regions while the biota in other areas continued to suffer as the result of widespread harsh environmental conditions throughout much of the Early Triassic period. Examination of Lower Triassic rocks from the southwestern U. S. has produced a variety of sedimentologic and biotic evidence that point to persistent environmental stress following the extinction (specifically anoxic, alkaline waters in the deep ocean, and possible high carbon dioxide levels in the surface ocean) and have also documented trends in the long-suffering biota present in that region. The shoaling facies of the upper member of the Union Wash Formation at the Cerro Gordo, CA USA locality contain stratiform stromatolites that suggest harsh environmental conditions in the area may have lingered until the late Spathian (latest Early Triassic). The stromatolites are laterally extensive (some layers persist for at least 1 km) and typically exhibit planar to wavy laminations with some development of small domes (up to 10 cm in height). A shallow subtidal depositional environment is suggested by evidence of eroded or planed surfaces within the stromatolites, as well as channelization of the stromatolites, and the occurrence of ooids and coated grains within the interval. The stromatolites documented here occur stratigraphically higher than previously documented inorganic calcium carbonate precipitates from the Union Wash Formation [e.g., Woods, A.D., Bottjer, D.J., Corsetti, F.A., 2007. Calcium carbonate seafloor precipitates from the outer shelf to slope facies of the Lower Triassic (Smithian–Spathian) Union Wash Formation, California, U.S.A.: Sedimentology and palaeobiologic significance. Palaeogeography, Palaeoclimatology, Palaeoecology 252, 281–290; Woods, A.D., Bottjer, D.J., Mutti, M., Morrison, J., 1999. Lower Triassic large seafloor carbonate cements: Their origin and a mechanism for the prolonged biotic recovery from the end-Permian mass extinction. Geology 27, 645–648] and microbialites from the Virgin Limestone [e.g., Schubert, J.K., Bottjer, D.J., 1992. Early Triassic stromatolites as post-mass extinction disaster forms. Geology 20, 883–886; Pruss, S.B., Bottjer, D.J., 2004. Late Early Triassic microbial reefs of the western United States; a description and model for their deposition in the aftermath of the end-Permian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology 211, 127–137]. Therefore, these stromatolites may represent the last-known occurrence of anachronistic facies and associated unusual palaeoceanographic conditions in eastern Panthalassa following the Permian–Triassic mass extinction.

Calcium carbonate seafloor precipitates from the outer shelf to slope facies of the Lower Triassic (Smithian-Spathian) Union Wash Formation, California, USA: Sedimentology and palaeobiologic significance

Formerly aragonite seafloor fans from the Lower Triassic (Smithian-Spathian) Union Wash Formation of east-central California were examined to determine the processes active during their growth. The seafloor fans were deposited in an outer shelf to slope environment along the western margin of Pangaea, and are contained within two stratigraphic intervals. The lower precipitate-bearing interval is 130 m thick, comprised of micritic limestone, and contains precipitates that fall into 2 end-member morphologies: 1) laminated precipitate masses that contain distinct hemispheres or fans; and 2) precipitate masses that are composed of a complex interlocking mosaic of hemispheres or fans. Precipitates found in the bedded, lower half of the interval are typically 2–3 cm tall and 10–20 cm wide, and are commonly undercut, disturbed, and, in the case of the interlocking mosaics of hemispheres, show evidence of entrainment and transport by density currents. Precipitates are larger in the upper, massive portion of the lower precipitate-bearing interval and occur only as interlocking hemisphere/fan mosaics. Fans and hemispheres attain heights as tall as 30 cm, and precipitate bodies, made of interlocking mosaics of hemispheres and fans, attain thicknesses of up to 50 cm and widths of 1–2 m. Chaotic bedding, broken precipitates, and brecciation of precipitate bodies imply an unstable and mobile substrate. The upper precipitate-bearing interval is 30 m thick, comprised of bivalve-, ammonoid- and microgastropod-bearing wackestone, and is separated from lower precipitate-bearing strata by 23 m of interbedded shale and calcareous siltstone. Much of the upper precipitate-bearing unit was deposited by subaqueous debris flows, as suggested by multiple breccias contained within the interval. Seafloor cements are found in undisturbed units and rarely within breccia clasts, suggesting that precipitates formed locally as well as higher on the shelf. Precipitate growth was likely the result of an increase in the saturation state of calcium carbonate in Early Triassic oceans, related to widespread oceanic anoxia. The presence of unusual carbonates during the period, and their correlation with depauperate faunas in the equivalent shallow facies of the Virgin Limestone, suggest that the remarkable oceanographic conditions associated with precipitate growth likely dampened the biotic recovery in the region until those conditions disappeared in the Middle Triassic.