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<jats:title>Abstract</jats:title> <jats:p>Understanding the flow of carbon through hyperalkaline lakes is a key means of understanding their biogeochemistry, sedimentology, and their paleoenvironmental and paleoclimatic records. Furthermore, understanding how mineral precipitation is regulated in these lakes can provide insights into how their sequestration of carbon can be managed. We report geophysical surveys of Mono Lake, California, USA, which show unanticipated geomorphological control on the recent/contemporary formation of lacustrine carbonate formations (“tufa”). Acquired shallow-penetration seismic data show a fault zone below the lake floor, but despite the regional evidence for geothermal waters rising up these fractures, we find no evidence for tufa precipitation at the surface exposure of this structure, either in the seismic data or in the swath bathymetry. However, we do find sub-lacustrine tufa columns in these data elsewhere, which is the first time these have been reported directly. We find and report on a strong link between column location and meteoric Ca supply to the lake, with the latter sourced either through surface runoff or groundwater. For example, a region close to a creek inlet has more frequent and larger tufa bodies, which grow at a wider depth range than another region far from an inlet but close to the fault. This demonstrates the importance of meteoric water ingress in regulating carbonate mineral formation in these basins, and raises the possibility that management of water within the catchment could be a means to enhance carbon capture in natural and artificial hyperalkaline lakes.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Although intensified work on the volcaniclastic-rich sediments of the fossil-bearing Mussentuchit Member (uppermost Cedar Mountain Formation, Utah) has provided a refined chronostratigraphic framework, paleoenvironmental interpretations remain cryptic. To resolve this, we performed facies analysis and architectural reconstruction on exposed Mussentuchit Member outcrops south of Emery, central Utah, USA. Contrary to previous interpretations (fluvial, lacustrine), we identified a broad suite of facies that indicate that deposition occurred on the landward part of a paralic depocenter, influenced by both distal alluvial and proximal coastal systems. We conclude that the Mussentuchit Member was a sink for suspension-settling fines with most undergoing pedogenic alteration, analogous to the modern coastal plain of French Guiana (Wang et al. 2002; Anthony et al. 2010, 2014). However, this landward paralic depocenter was not uniform through time. Sedimentological evidence indicates landscape modification was ongoing, influenced by an altered base-level (high groundwater table, long residency of water in sediments, shifts in paleosol types, heavier to lighter δ18O, and distinct shifts in relative humidity (ε); common in coastal settings). If the above data is coupled with recent age data, we interpret that the Mussentuchit Member correlates to the S.B. 4 Greenhorn Regression (Thatcher Limestone) of the adjacent Western Interior Seaway to the east. As a landward paralic depocenter, the Mussentuchit would have been sensitive to base-level conditions in response to ongoing tectonic processes pushing the foredeep east, and lower paleo-CO2 levels coupled with a minor global sea-level fall (brief glacial phase) just before to the Cenomanian–Turonian Thermal Maximum. Altogether, our results not only strengthen linkages in the central Western Interior Seaway, but simultaneously results in novel linkages to near-coeval paralic depocenters across mid-Cenomanian North America.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>The processes that transport sediment from the coastline to the shelf edge are key components of the sedimentary source-to-sink system, determining basin-margin building, deepwater deposition, organic-material accumulation, and the long-term carbon cycle. Research on shelf sediment transport has been aided recently by advances in modeling and marine technology. In this study we provide a much needed review of up-to-date findings on how sediment moves from the outer shelf onto the upper slope, and we summarize four dominant shelf-to-slope drivers: 1) river currents, 2) reworking storm waves and longshore currents, 3) strong tidal currents supplementing river outflow, and 4) small-scale to very large-scale gravity collapse of the shelf-edge area.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>The presence of microbialites in the hypersaline lagoons of Rio de Janeiro is especially important in the study of recent analogs of carbonate rocks with microbial origins, mostly after the discovery of giant petroleum reservoirs in the Brazilian pre-salt section and their similarities with stromatolites from Lagoa Salgada (Rio de Janeiro State). Many studies have been conducted to analyze the biology, geochemistry, mineralogy, and geomicrobiology of these microbialites. This paper, however, focuses on the petrography, sedimentology, and geochemistry of recent and superficial microbial mats from Lagoa Vermelha to understand the interaction of carbonate and siliciclastic grains with an organic matrix and discuss their similarities and differences with pre-salt rocks. A sedimentologic description was performed to understand the sediment dynamics in microbial mats. A petrographic description involved the characterization of components and textures in microscale. Furthermore, geochemical analyses were performed using scanning electron microscopy and X-ray diffraction for detailed mineralogical characterization. This multitechnique study showed the lamellar and cracked texture of the matrix being displaced by biologically induced carbonate growth and siliciclastic grains. In addition, chemical analysis showed the concentration of magnesium and silica in the matrix, with the absence of Mg-clay minerals. Even though the studied microbial mats present relevant similarities with some pre-salt facies, a microbially dominated genesis for the pre-salt limestones cannot be supported by the studied data.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Saline currents (SCs) have commonly been used to model muddy turbidity currents (TCs) in a laboratory. However, little is known about the limitations of this proxying, in particular when concerning processes and products related to their sedimentologic and stratigraphic imprints. The present study is aimed at investigating experimental hydraulic and sedimentological conditions and processes involved in the generation and development of bedforms by both SCs and TCs, when similar input conditions are applied (discharge, slope, densimetric Froude number). In all performed runs reported herein, only ripples were observed to form, and were identified and classified using known criteria such as their dimensions, near-bed shear stresses (), shear velocities (), and grain Reynolds values (Re*). Turbidity currents were observed to deposit sediments carried by the flow predominantly in the upstream section of the flume, increasing bed slope and thus increasing , , and near-bed concentrations (cb). This resulted in longer-wavelength bedforms compared to those generated by SCs under similar input conditions in those sections of the flume. On the other hand, along the downstream sections of the flume, bed slopes were observed to remain similar for all experiments, and both types of currents showed similar vertical distribution of velocities, concentrations, and stable stratification. Measured bedform wavelengths and heights were slightly higher when generated by SCs, due to the coarser bed material observed in SCs (which influenced the increase in near-bed turbulent intensities). Moreover, TCs presented a slight decrease in turbulence intensities due to their observed high near-bed suspended-sediment concentration. Spatial and temporal changes in several hydraulic parameters in both SCs and TCs highlight the role of sediment suspension in modifying turbulent processes and vertical stratification of these flows, depending on their concentrations. Both hydraulic and sedimentologic observations of this study support in principle the assumption that SCs can be used experimentally as a surrogate for diluted TCs to reproduce bedforms classified as ripples, as long as both type of currents reach similar hydraulic and sediment-transport conditions, in particular for depth-averaged concentration Cvol &amp;lt; 1% and near-bed concentration cb &amp;lt; 2%.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Contemporary microbialite formation has been documented on rock coasts in a variety of geomorphic, oceanographic, and climatic settings. Based on a synthesis of these diverse occurrences plus new observations, a generalized model is presented. At each locality microbialite development is associated with discharge of mineralized freshwater in the coastal zone. Microbialite formation in the high intertidal and supratidal zones of rock coasts occurs in a variety of sub-environments (cliff face, shore platform surface, platform surface pools, boulder beach, and sand beach) and forms a variety of laminated rock encrustations and oncoids. Allochthonous microbialites occur on the backshore as breccias of reworked microbialite clasts, oncoids transported from rock pools, and partly encrusted boulders. The microbialite-influenced rock coast is a distinct type of siliciclastic environment that offers potential comparison for ancient microbialite occurrences. It has preservation potential in both transgressive and regressive settings. Potential ancient examples are suggested.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability.</jats:p> <jats:p>Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall.</jats:p> <jats:p>Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Landslide deposits are common in basin fill of tectonically active areas, including the Basin and Range province of western North America. However, interpreting and reconstructing ancient landsliding events from these deposits is challenging, but can be aided by detailed field analysis. Remnants of the Blue Diamond landslide breccia are exposed capping hills and ridges in the foothills of the eastern Spring Mountains near Blue Diamond, Nevada, USA. Uncertainties surrounding the origin and emplacement of the ancestral Blue Diamond landslide have emerged based on the disparate distribution of landslide outcrops. Therefore, in this study we used detailed sedimentological data and observations to interpret a two-phase emplacement history for the Blue Diamond landslide.</jats:p> <jats:p>Sedimentological observations are consistent with Blue Diamond landslide breccia emplacement as a rock avalanche. The presence of clastic dikes and flame structures and negligible incorporation of bedrock substrate material suggest that runout occurred over a saturated substrate. Flow transformation into a debris avalanche is ruled out because clast-count data show that debris entrainment was not sufficient to act as the sole mechanism behind the excessive mobility experienced by the Blue Diamond landslide. Instead, we propose that the excessive mobility was driven by flow entrainment of large Aztec Sandstone boulders and interaction with a saturated runout path substrate that caused a reduced basal frictional resistance, enabling initial emplacement onto Blue Diamond Hill. We therefore suggest that the Blue Diamond landslide was derived from a source area about 8.5 km northwest of the Blue Diamond townsite and flowed into the Blue Diamond Hill site where it was emplaced onto Moenkopi Formation atop the hill during the Miocene. Due to loading by this new overburden, incompetent gypsum horizons failed in the upper Kaibab Formation stratigraphically below the Moenkopi Formation. These failed gypsum horizons then served as a compound landslide rupture surface, transporting the overlying Moenkopi Formation and landslide breccia. This secondary emplacement likely ceased by late Miocene to Pliocene time.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, &amp;lt; 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Lateral migration is a key process shaping sinuous rivers and controlling sediment exchange with floodplains. The rate at which channels migrate is affected by bend curvature, bank erodibility, and sediment supply. The relationship between migration rate and sediment supply is poorly understood in dryland regions, where direct measurements are scarce. Here we propose a simple mass-balance model to estimate the sediment flux of ephemeral streams in North America's Great Basin and establish a comparison with timelapse photogrammetric data of lateral migration. The model takes into consideration variables such as long-term hillslope erosion, transient sediment storage in intra-catchment lowlands, and sediment bypass to depocenters. Our results point to first-order similarities in how sediment supply drives channel migration across diverse hydro-climatic regimes. However, we find that, for a given sediment supply and channel width, and despite their ephemeral discharge, dryland streams with minimal bank vegetation migrate about three times faster than humid-climate, vegetated ones. This difference in migration pace likely results from the compound effect of bank erodibility and bend geometry. Our model sheds new light on the driving mechanisms of channel mobility in dryland streams and may find application in assessing the sediment budgets of ungauged streams, reservoir trapping, and morphodynamic adjustments in stressed watersheds.</jats:p>