Showing papers in "Journal of Sedimentary Research in 2010"

Large Distributive Fluvial Systems: Characteristics, Distribution, and Controls on Development

Abstract: An analysis of remotely sensed imagery reveals that fluvial planform geometries within aggrading continental areas are dominated by distributive fluvial systems (DFSs). We documented the gradient, length, apex location, planform, termination type, and tectonic and climatic setting of 415 examples of fluvial systems which in planform display a radial, distributive channel pattern and have an apex–toe distance > 30 km (large DFSs). The longest of these DFSs is 704 km in length, with the majority (72%) ranging between 30 and 100 km in length. Gradients on individual systems range from 0.00003 (0.0018°) to 0.02656 (1.5°). Six planform types are recognized, those with: (1) a single braided channel that bifurcates downstream into braided and/or straight channels, (2) a single dominant braided channel, (3) a single dominant braided channel which becomes sinuous downstream often bifurcating, (4) a single dominant sinuous channel, (5) a single sinuous channel that bifurcates downstream into smaller sinuous channels, and (6) multiple sinuous channels. Of the studied examples 58% occur within exorheic basins and 42% in endorheic basins, with seven different termination types recognized. In many examples, channel planform changes downstream from a distributive pattern to a contributory pattern. In others, channels terminate at an axial fluvial system, at the coast, in eolian dune fields, playa lakes, permanent lakes, or wetlands. Large DFSs and their catchments are developed in all climatic regimes, including drylands, tropical, subtropical, continental, and polar climates. Large DFSs occur in all tectonic settings, including extensional, compressional, strike-slip, and cratonic tectonic regimes. General trends and relationships between the different studied parameters can be observed, leading to a broad understanding of the main controls on large DFS development. All of the planform types occur in all tectonic settings and all climate zones. Braided planforms dominate all tectonic settings, but particularly compressional regimes. High-gradient braided systems tend to be associated with areas of high relief and are well developed in dryland climates where discharge is inferred to be intermittent in comparison to tropical climates. Large DFSs with sinuous planforms do occur in dryland climates but tend to predominate in wetter, more tropical climates where discharge is more constant and the fluvial systems can distribute bedload more efficiently. Sinuous systems also tend to have significantly lower gradients than braided systems. Although these general observations can be made, there are significant variations from these trends, which are inferred to be controlled by variations in (1) discharge related to climate and (2) sediment supply, which is a function of climate, catchment size, catchment lithology, and catchment relief. Large DFS length is controlled by the available horizontal accommodation space, which in turn is strongly related to tectonic setting. The longest DFSs occur in peripheral foreland basins and cratonic settings where lateral systems can develop across an extensive basinwards slope. Extensional, strike-slip, and piggy-back basins are narrower and have much more limited horizontal accommodation space. Consequently DFSs developed in these settings are shorter, with radii often less than 30 km. DFSs dominate in aggradational settings such as actively subsiding sedimentary basins and therefore form a significant proportion of alluvial sedimentary successions preserved in the rock record.

Lenticular Shale Fabrics Resulting from Intermittent Erosion of Water-Rich Muds—Interpreting the Rock Record in the Light of Recent Flume Experiments

Abstract: Lenticular lamination is a fabric that is known from shales of all ages, but its origin and paleoenvironmental significance is poorly understood. We have successfully reproduced this fabric in flume experiments. Beds of water-rich mud were eroded in a flume and yielded sub-millimeter to centimeter-size fragments that can be transported in bedload for distances of ten kilometers or more. Upon redeposition and compaction, these deposits have the same textural qualities as lenticular laminated shales from the rock record. Although accumulation of fecal pellets or abundant burrow tubes in a shale may produce comparable fabrics upon compaction, these can be distinguished from erosion-produced lenticular lamination via petrographic criteria. Lenticular lamination in shales that is due to deposition of water-rich mud fragments most likely records intermittent erosion and transport of surficial muds by currents.

Algal Blooms and “Marine Snow”: Mechanisms That Enhance Preservation of Organic Carbon in Ancient Fine-Grained Sediments

Abstract: Combined petrographic and geochemical methods are used to investigate the microfabrics present in thin sections prepared from representative organic carbon-rich mudstones collected from three successions (the Kimmeridge Clay Formation, the Jet Rock Member of the Whitby Mudstone Formation, and the pebble shale and Hue Shale). This study was initiated to determine how organic carbon-rich materials were being delivered to the sediment–water interface, and what happened to them after deposition, prior to deep burial. Analyses of the fabrics present shows that they exhibit many common attributes. In particular they are all: (1) highly heterogeneous on the scale of a thin section, (2) organized into thin beds ( The organomineralic aggregates present in these mudstones are interpreted to be ancient examples of marine snow. This marine snow likely formed in the water column, particularly during phytoplankton blooms, and was then transported rapidly to the seafloor. The existence of the thin beds with homogenized tops and an in-situ infauna indicates that between blooms there was sufficient oxygen and time for a mixed layer to develop as a result of sediment colonization by diminutive organisms using either aerobic or dysaerobic metabolic pathways. These textures suggest that the constituents of these mudstones were delivered neither as a continuous rain of sediment nor were the bottom waters persistently anoxic. In addition, the presence of thin lags and sharp-based beds suggests that the seafloor was being episodically reworked during deposition. These fabrics indicate that conditions in the water columns and at the seafloors while these rocks were being deposited were very dynamic, and episodic fluxes of high concentrations of organic carbon to the seafloor, during phytoplankton blooms, likely enhanced preservation of organic carbon.

Three-Dimensional Connectivity of Point-Bar Deposits

Abstract: The geometry of heterogeneities within fluvial channel-belt deposits is predicted using an existing model of flow and sediment transport in river channel-meander bends. The thickest and coarsest-grained sediment accumulations are deposited near a channel-bend apex, and finer-grained sediments accumulate higher on the downstream end of a channel bar. Shale drapes deposited on individual beds during low flows are preserved mostly on downstream-dipping surfaces in finer-grained bar-top deposits. Extensive finer-grained deposits also accumulate in concave-bank areas in the lee of point bars when meanders migrate downstream. The coarsest deposits occur as elongate bodies parallel to the channel-belt axis when channel bends migrate mostly by downstream translation and are more circular when channel bends simply increase in sinuosity. The character of deposits preserved in concave-bank areas of the channel, the style of channel cutoff and filling during abandonment, and the amount of channel-belt aggradation during bar migration influence whether coarser-grained bodies of adjacent bar deposits are well connected. This 3D gridded model allows reservoir simulation studies that predict effects of types of deposit heterogeneity on patterns of subsurface flow through reservoirs and aquifers. Subsurface flow simulations suggest that channel belts with coarser-grained abandoned fills and significant vertical aggradation during bar migration constitute reservoirs with appreciably higher recovery factors than those with finer-grained channel abandoned fills and no vertical aggradation. Model results highlight the need to better understand connections between different types of facies associations in fluvial channel belts to improve predictions of fluvial-reservoir behavior.

Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

Abstract: Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a “reservoir” scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels.

High-Relief Slope Clinoform Development: Insights from Outcrop, Magallanes Basin, Chile

Abstract: The Cretaceous–Paleogene Tres Pasos and Dorotea formations of the Magallanes Basin, Chile record the filling of a deep-water foreland setting. Slope clinoforms with at least 700–900 m relief (compacted) prograded southward along the foredeep axis, which was oriented parallel to the adjacent Patagonian Andes. Fluvial- and wave-influenced deltaic deposits of the Dorotea Formation represent the upper, flat portions of the sigmoidal slope profiles. The paleo-shelf edge is estimated where shelf sandstones pinch-out basinward. Mudstone, siltstone, and a notable paucity of sandstone characterize upper slope strata. Further down-slope, conduits are evidenced by sedimentary bodies associated with cross-stratified or normally graded sandy conglomerate and local mudstone rip-up clasts, interpreted to indicate that considerable sediment bypassed the slope. Turbiditic sandstones and mass-transport deposits of the Tres Pasos Formation characterize the lower to base of slope setting. Numerous examples of slope clinoforms have been recognized in the rock record, with the majority characterized by 200–500 m of estimated paleo-relief. Higher relief examples include those mapped in outcrop from the Magallanes Basin documented here, and comparable clinoforms from the subsurface Cretaceous Brookian succession of the North Slope, Alaska. In the Magallanes Basin, numerous factors contributed to the development of high-relief clinoforms, including generation of substantial basin margin relief, the absence of mobile substrata, adequate sediment supply, and the elongate basin shape. The slope that built and maintained the relatively smooth clinoform profile was narrow, and thus, sediment that was transported across the shelf was focused as it passed into deeper water. In general, the development of slope clinoforms, including high-relief examples like those of the Magallanes Basin, is facilitated when the rate of sediment input onto the slope is higher than the rate at which rugose slope topography is generated from mass wasting or substrata remobilization.

Microbes and Ooids

Abstract: The Bahamian Archipelago is one of the few locations in the world where ooid formation is actively occurring. Ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14C, δ18O, δ13C, Mg/Ca, and Sr/Ca ratios. Ooids were examined under SEM after each step in the incremental analyses to characterize the nature of dissolution. Radiocarbon dating indicates that ooids presently at the sediment–water interface began forming across the Archipelago between 1000 and 2800 yr BP and continue to form today. The ooids have the same pattern of microboring alteration across the region. The surface and outer cortex of the ooids are punctuated with unfilled microborings, whereas the inner cortex contains two morphologies of aragonite cement filling the microborings. The two morphologies of cement form in association with two different species of cyanobacteria, one is Solentia sp. the other is interpreted to be Hyella sp. The chemistry of ooids from across the region is remarkably similar. δ18O and δ13C values for all samples vary directly, having a slope of approximately 1. The outer cortex has low δ18O and δ13C values of −3.4‰ and 0.2‰ respectively, whereas the δ18O and δ13C values of the inner cortex are high with values of 1.9‰ and 6.8‰ respectively. The presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid, causing it to appear close to equilibrium with seawater. The isotopic variation in δ18O and δ13C within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings. The most exterior portion of the ooid has very high Mg/Ca values and is interpreted as an amorphous calcium carbonate (ACC) coating. There are two other phases in the cortex, both being aragonite. The outer cortex has a higher Mg/Ca ratio and lower Sr/Ca ratio than the inner cortex. This difference in chemistry is a result of the presence of aragonite cement in the inner cortex. Stable isotopic and trace element results coupled with SEM investigations indicate that microbes do not play a role in ooid formation, but instead alter the texture and chemistry of ooids after they have formed. A new model of ooid formation is proposed whereby a veneer of ACC precipitates on an ooid while it is at the sediment–water interface (the active phase). This veneer of ACC later recrystallizes to aragonite needles and a new cortex layer is formed (the stationary phase). Observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis, which allow a better understanding of paleoenvironments hosting ooid formation.

Planform and Facies Variability in Asymmetric Deltas: Facies Analysis and Depositional Architecture of the Turonian Ferron Sandstone in the Western Henry Mountains, South-Central Utah, U.S.A.

Abstract: Facies analysis of the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains of south-central Utah, U.S.A., indicates sediment accumulation in a series of flood-dominated, marine current- and wave-influenced, deltas. Twelve lithofacies are recognized: 1. erosionally based, thick cross-bedded sandstone bodies (Distributary Channels), 2. similar though thinner bodies containing common bioturbation (Marine-Influenced Distributary Channels), 3. root-penetrated, plant fossil bearing siltstone with minor sandstone (Coastal Floodplain and Floodbasin), 4. coal and carbonaceous shale (Coastal Mire), 5. thin-bedded, carbonaceous and bioturbated sandstone–siltstone (Coastal Lagoon), 6. erosionally based sandstone with large- and small-scale cross-bedding and bioturbation (Mouth-Bar Complex), 7. sharply based sandstone bodies internally dominated by hummocky cross-stratification, soft-sediment deformation, or lacking structure (Proximal Delta Front), 8. sharply bounded, calcareous, fossiliferous and bioturbated sandstone sheets (Abandoned Delta Lobe), 9. thickly interbedded sandstone, coarse- and fine-grained siltstone (Medial Delta Front), 10. thinly interbedded sandstone, coarse- and fine-grained siltstone (Distal Delta Front), 11. mainly siltstone with minor thin-bedded sandstone (Prodelta) and 12. fine-grained siltstone with bentonite beds (Offshore). Deltas prograded into shallow water, forming sharp-based mouth-bar sand bodies. The upper delta front was evidently fluidal and prone to failure, leading to the development of rotational slope failures, debris–flow filled gullies, and, in places, growth faults. Paleocurrent data indicate that the regional sediment dispersal direction was eastward. Data from delta-front facies, however, suggest that outflow plumes and associated bottom currents were deflected towards the southeast, giving rise to an asymmetric delta planform. The Holocene and modern Burdekin River Delta of NE Australia is considered a close planform, process, and facies analog for the Ferron Notom deltas. The Burdekin Delta facies assemblage is vertically and laterally heterogeneous, despite being the product of a consistent array of environmental controls. Adopting a model that incorporates such a degree of heterogeneity negates the need for multiple depositional models for complex stratigraphic intervals such as the Ferron Sandstone. The facies model also suggests that asymmetric deltas may be produced by directional growth of delta lobes, rather than by deflection of beach ridges. Stratigraphic stacking patterns strongly suggest that sediment accumulation in the Ferron Sandstone of the western Henry Mountains was forced by a regime of progressively more limited accommodation through time.

Concepts Learned from a 3D Outcrop of a Sinuous Slope Channel Complex: Beacon Channel Complex, Brushy Canyon Formation, West Texas, U.S.A.

Abstract: Sinuous channels are common bathymetric features on Earth’s continental margins. Until now, the 3D stratigraphy of these features has primarily been inferred from 3D seismic studies and from limited 2D outcrop exposures of ancient successions. The Beacon Channel Complex of the Permian upper Brushy Canyon Formation is an exceptionally well-exposed example of a 3D exposure of a sinuous slope channel system. The Beacon Channel Complex crops out on five cliff facies in an area of approximately 1 km2 (0.625 mi2). Nearly one complete wavelength of sinuosity is recorded in the outcrop.

Acoustic Properties of Ancient Shallow-Marine Carbonates: Effects of Depositional Environments and Diagenetic Processes (Middle Jurassic, Paris Basin, France)

Abstract: Examination of petrophysical properties (acoustic velocity, porosity, permeability, and density) and petrographical characteristics (texture, facies composition, and diagenesis) of more than 250 core plugs from the Middle Jurassic carbonates of the eastern Paris Basin provides insights into the parameters controlling acoustic velocities in relatively low-porosity carbonate rocks (Φ < 20%). The pore-type observations reveal distinct acoustic velocities in samples with intergranular macropores and samples with micropores in subhedral micrite, such that velocities in microporous mudstone–wackestone (lagoonal) deposits are greater than in macroporous grainstone (shoal) samples, at a given porosity range (15–20%). The standard Wyllie and Raymer transforms fit very well with the linear regression between acoustic velocity and porosity from mudstone or lagoonal facies. Marls and fine-grained deposits interpreted as lagoonal facies include statistically significant correlation (r = 0.9) between velocity and porosity. However, the data suggest that the wide scatter in velocity–porosity relationship from grainstones are not the result of different sorting, grain size, pore type, dolomite content, or clay content. Instead, early cementation greatly influences acoustic properties during diagenesis, and are interpreted to account for the high variability of velocities over a given porosity range. Specifically, at a given porosity, acoustic velocities in compacted grainstone that did not undergo early cementation are higher than in early-cemented grainstone. Petrographic observations suggest that early cementation limits mechanical compaction, creating a heterogeneous medium from the earliest stages of diagenesis (non-touching grains, preservation of intergranular macropores that are partially to totally filled by later blocky calcite cement). The abundant interfaces between micritized ooids, early cement fringes, and blocky calcites in grainstones may induce significant wave attenuation. As a result, the standard time-average equations fail to predict the effect of diagenetic features such as early cementation on sonic velocity. Conversely, an absence of early cementation favors mechanical compaction, grain-to-grain contact, and suturing. The result is a homogeneous micritized grain-supported network that may facilitate wave propagation. Through demonstration of the key role of early cementation in the explanation of variability in acoustic properties, the results of this study illustrate the complicated factors influencing velocity transforms in carbonates (Wyllie and Raymer), i.e., classical tools for predicting reservoir properties. These insights on the interpretation of Vp and the refinement of velocity–porosity transforms in grainstone units may be broadly applicable to enhancing seismic-based exploration in carbonate successions.

Fluvial Sequence Stratigraphy: The Wapiti Formation, West-Central Alberta, Canada

Abstract: Outcrop exposures and high-resolution subcrop data allow the description and mapping of four stratigraphic discontinuities in the fully nonmarine strata of the Campanian–Maastrichtian Wapiti Formation (Western Interior foreland basin, Alberta, Canada). This framework defines four unconformity-bounded depositional sequences (A–D), based on sequence stratigraphic methodology, well-log patterns, and facies analysis. In ascending order, sequence A records the transition from the underlying marine facies of the Puskwaskau Formation into the Wapiti fluvial facies, and consists of strongly progradational and aggradational stacking patterns. Sequences B and C show a similar pattern of basal amalgamated channel-fill deposits that grade upwards into floodplain-dominated strata. Finally, sequence D is dominated by channelized sediments and extensive overbank facies. An aggradational stacking pattern suggests deposition under high-accommodation conditions. Maximum flooding surfaces are interpreted within fine-grained deposits in the upper portions of sequences C and D. They are tied to regionally extensive coals that accumulated more than 250 km away from the coeval shoreline. This study provides new evidence that major coal seams may represent the extension of marine maximum flooding surfaces into the downstream-controlled fluvial realm. Furthermore, the new stratigraphic framework of subaerial unconformities and maximum flooding surfaces provides the means of subdividing the previously undifferentiated fully nonmarine Wapiti Formation into depositional sequences and component systems tracts.

Flow–Deposit Interaction in Submarine Lobes: Insights from Outcrop Observations and Realizations of a Process-Based Numerical Model

Abstract: Sediment gravity flows have a propensity to infill lows and build depositional relief, which influences subsequent flows. This flow–deposit interaction is intrinsic to the evolution of submarine fans at a range of scales. A novel approach is presented that assesses the interaction of turbidity currents with a subtle but evolving depositional topography. Conceptual models developed from outcrop observations are tested with a process-based numerical model. The outcrop dataset was collected from submarine lobe deposits extensively exposed in the Tanqua depocenter, SW Karoo Basin, South Africa. The process-based numerical model of turbidity-current flow and sedimentation (FanBuilder) is used to mimic the process sedimentology. Input parameters of flows are constrained by observations of the outcrop geology (sedimentology and depositional architecture). Modeling results are analyzed and compared with outcrop observations and, where necessary, lead to iterative refinement of the underlying conceptual process-sedimentological model. The model successfully developed characteristic features of the depositional architecture, such as finger-like geometries and stacking patterns, which are comparable in scale and geometry to those observed in outcrop. The results highlight that lobe deposits have intricate geometries that, when stacked, form a complicated internal stratigraphy, in contrast to simple models of lobes consisting of laterally extensive sheets. In addition, new insights into the processes of lobe growth have come from the modeling that can be tested at outcrop in the future. Process modeling indicates that the stratigraphic complexity can be controlled by a subtle and dynamic depositional surface that drives instability in the position of distributive channels and the site of deposition. As such, this research emphasizes the importance of autogenic controls on the depositional architecture of submarine fan systems.

Log Jams and Flood Sediment Buildup Caused Channel Abandonment and Avulsion in the Pennsylvanian of Atlantic Canada

Abstract: Accumulations of logs and flood sediment frequently block modern channels and may trigger avulsion, but these effects are difficult to demonstrate for the ancient record. Braided-fluvial channels in the Pennsylvanian South Bar Formation of Atlantic Canada contain sandstone successions up to 6 m thick of sigmoidal cross-beds, plane beds, and antidunes, deposited rapidly at high-flow-stage. These strata are commonly capped by accumulations up to 2.5 m thick of flattened, coalified logs and coal intraclasts (originally peat fragments), many of which are overlain by mudstone laid down in abandoned channels. The logs include lycopsids, calamiteans, tree ferns, pteridosperms and cordaitaleans, inferred to have grown on inactive braided tracts near the channels. A compaction estimate suggests that one log accumulation was originally more than four times its present thickness. Most accumulations are interpreted as stable “transport log jams” formed during floods, although some may have been “unstable jams” stranded on bars during peak-flow recession. Associated with the logs are extrabasinal gravel and intraclasts of mudstone and coal, which suggest that floods in sediment-choked channels undercut banks of gravelly sand capped by mud and forested peat, widened the channels, and toppled riparian vegetation. An estimated blockage ratio of 8% for one accumulation (ratio of the cross-sectional areas of the log jam and host channel) is close to the 10% value considered to cause substantial blockage in some modern rivers. In two instances, a radical change in paleoflow between pre- and post-abandonment channels is consistent with an interpretation that log jams and flood sediment buildup promoted channel-belt avulsion. Although large trees had evolved by Middle to Late Devonian times, it is unlikely that riparian plants occurred in stands that were sufficiently dense to exert a major influence on river dynamics until the Pennsylvanian. Thus, we report some of the earliest evidence for the effects of woody debris on ancient fluvial systems.

Correlation of Aptian-Albian Carbon Isotope Excursions in Continental Strata of the Cretaceous Foreland Basin, Eastern Utah, U.S.A.

Abstract: Nodular carbonates (“calcretes”) in continental foreland-basin strata of the Early Cretaceous Cedar Mountain Formation (CMF) in eastern Utah yield δ13C and δ18O records of changes in the exogenic carbon cycle related to oceanic anoxic events (OAEs), and terrestrial paleoclimate. Chemostratigraphic profiles of both forebulge and foredeep sections show two prominent positive δ13C excursions, each with a peak value of −3%0 VPDB, and having background δ13C values of about −6%0 VPDB. These excursions correlate with the global early Aptian (Ap7) and late Aptian–early Albian (Ap12-Al1) carbon isotope excursions. Aptian–Albian positive δ13C excursions in the CMF also correspond to 3–4 per mil increases in carbonate δ18O. These phenomena record local aridification events. The chemostratigraphic profile on the thinner forebulge section of the CMF is calibrated, for the first time, by a radiogenic U-Pb date of 119.4 ± 2.6 Ma on a carbonate bed, and by detrital zircon U-Pb dates on two bounding sandstone units (maximum depositional ages of 146 Ma and 112 Ma). Petrographic observations and diagenetic analyses of micritic to microsparitic carbonates from nodules indicate palustrine origins and demonstrate that they crystallized in shallow early meteoric phreatic environments. Meteoric calcite lines derived from CMF carbonates have δ18O values ranging between −8.1 to −7.5%0 VPDB, supporting an estimate of zonal mean groundwater δ18O of −6%0 VSMOW for an Aptian–Albian paleolatitude of 34° N. Furthermore, our two chemostratigraphic profiles exhibit a generally proportionate thinning of correlative strata from the foredeep on to the forebulge, suggesting that there were consistently lower rates of accumulation on the forebulge during the Aptian–Albian. Identification of the global Aptian–Albian δ13C excursions in purely continental strata, as demonstrated in this paper, opens a new avenue of research by identifying specific stratigraphic intervals that record the terrestrial paleoclimatic impacts of perturbations of the global carbon cycle.

Facies and Architectural Asymmetry in a Conglomerate-Rich Submarine Channel Fill, Cerro Toro Formation, Sierra Del Toro, Magallanes Basin, Chile

Abstract: Cross-sectional asymmetry is characteristic of sinuous channels in both fluvial and submarine settings. Less well documented are the facies distributions of asymmetric channels, particularly in submarine settings. Exposures of the axial submarine channel-belt in the Magallanes retro-arc foreland basin on Sierra del Toro represent the fill of a 3.5 km wide, 300 m thick channel complex, here termed the “Wildcat,” that displays an asymmetric cross section and facies distribution. Measured sections and mapping demonstrate that facies proportion, degree of amalgamation, and margin architecture vary laterally from east to west across the Wildcat channel complex. The eastern side is characterized by thick-bedded, amalgamated sandstone and clast- and matrix-supported conglomerate that onlap a steep, simple margin adjacent to sandy overbank deposits. The western side contains thin-bedded, sandy and muddy strata that onlap a shallow composite margin adjacent to mud-rich out-of-channel strata. The observed asymmetry is likely due to centrifugal flow forces and was caused by a low-sinuosity right-hand meander bend of the Cerro Toro axial channel-belt. The facies and architecture of the opposing margins indicate that the eastern and western sides constitute the outer and inner bends of the Wildcat channel complex, respectively. The modest cross-sectional asymmetry of the Wildcat complex is likely a product of the low channel-belt sinuosity. The absence of lateral accretion surfaces and deposits suggests that the channel did not migrate during filling. Flows depositing the uppermost channel fill were only weakly confined, resulting in flow divergence and overbank deposition. A depositional model that incorporates the asymmetric facies distributions and the contrasting outer-bend and inner-bend architecture of the Wildcat channel complex is also presented. Similar facies distributions exist in other low-sinuosity submarine channels, and even more extreme facies and cross-sectional asymmetry probably characterize more highly sinuous channels. Data on facies distributions presented here represents a useful resource for constraining numerical and experimental models of the evolution of sinuous submarine channels as well as reservoir models of sinuous submarine channels.

Temporal Evolution of Fluvial Style in a Compound Incised-Valley Fill, Ferron “Notom Delta”, Henry Mountains Region, Utah (U.S.A.)

Abstract: Nonmarine sequence stratigraphic models hypothesize systematic changes in fluvial style within individual sequences and across sequence boundaries, based largely on mudstone–sandstone ratios. The main purpose of the paper is to evaluate the validity of these models by documenting facies relationships and detailed bedding architecture of bar and channel deposits as well as mudstone–sandstone ratio within a compound incised-valley fill at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation in the Henry Mountains region, Utah, U.S.A. Field photomosaics, walking out of beds, and 42 measured sections document the stratal and facies organization in the incised-valley fill. Two erosional surfaces with erosional relief up to 17 m are traced continuously for up to 7 km. These erosional surfaces are defined by onlap and truncation and show a marked basinward shift in facies, with pebbly coarse-grained and conglomeratic fluvial sandstones overlying highly burrowed marine mudstones and sandstones. These major erosional surfaces partition the valley fill into two sequences, marked as V2 and V1 in ascending order. Each sequence comprises a multistory fill that reaches a maximum thickness of about 30 m. Within each sequence there is a vertical transition in facies from fluvial, to tidal, and finally back into fluvial facies. In the younger V1, there is a systematic vertical evolution of fluvial style from braided, to single-thread meandering, and finally to a low-sinuosity river system. In contrast, formative rivers in the older V2 were always meandering and show more marine and tidal influence. A major change in valley sedimentology is recognized from V2 to V1, as shown by a 30° eastward shift in main flow direction, overall increase in grain size and river discharge, and a clear change in fluvial style from meandering into braided streams. This is also shown by the increase in preserved dune height in fluvial bar and channel deposits across the boundary, which suggests that deeper rivers cut the younger V1 valley. The change in river style from braided to meandering in V1 is interpreted to reflect a gradual decrease in slope and discharge as the valley was filled. The change in fluvial style across the sequence boundary reflects an increase in grain size and discharge, probably driven by a high-frequency climate change, as well as local increase in valley slope.

Rhythmic Climbing-Ripple Cross-Lamination in Inclined Heterolithic Stratification (IHS) of a Macrotidal Estuarine Channel, Gomso Bay, West Coast of Korea

Abstract: Well-developed rhythmic climbing-ripple cross-lamination (RCRL) was described from estuarine tidal channels in Gomso Bay, west coast of Korea. Associated with upper intertidal point bars of closely spaced meandering channels, RCRL occurs between mean sea level and mean neap high-water level. RCRL is typically less than 40 cm thick, and constitutes the upper part of fining-upward channel-fill successions that are capped by intensely bioturbated mud. RCRL consists of mud-draped climbing-ripple cross-laminae that are continuous along strike direction of the channel for 10–20 m. Climbing patterns are dominantly supercritical and less commonly subcritical. Flood-oriented RCRL is developed in the landward part of the meander bend, while ebb-oriented RCRL is developed in the seaward part. RCRL demonstrates rhythmic change in cross-lamination thickness, which resembles various hierarchical tidal cycles, such as diurnal inequality, synodic neap–spring tidal cycle, and anomalistic tidal cycle. Truncated tidal rhythmicities registered in RCRL and fine-grained textures are consistent with an upper intertidal origin. Common links of RCRL with inclined heterolithic stratification (IHS) and tight meander bends as well as high sedimentation rate is suggestive that RCRL can serve as a diagnostic indicator of active channel migration in the fluvio-estuarine transition. This study highlights the significance of RCRL bearing well-preserved tidal rhythmicities in the reconstruction of paleodepositional environment, paleoelevation in terms of tidal frame, and paleogeography.

Depositional Environments and Sequence Stratigraphy of an Exhumed Permian Mudstone-Dominated Submarine Slope Succession, Karoo Basin, South Africa

Abstract: The physical stratigraphy of a 470-m-thick, claystone-dominated exhumed middle to upper submarine slope succession was constrained within an area of 400 km2 in which five sand-prone units were characterized (Units D/E, E, F, G, and H). Units D/E to Unit F show an overall pattern of thickening upward and basinward stepping. This stacking pattern is reversed from the top of Unit F to the base of Unit H, above which basinward stepping is again observed. Different architectural styles of sand-prone deposits occupy predictable stratigraphic positions within the basinward-stepping section, starting with intraslope lobes through channel–levee complexes to entrenched slope valleys. Sandstone percentage is highest in the intraslope lobes and lowest in the slope valley fills, reflecting a change from depositional to bypass processes. The landward-stepping stratigraphy is dominated by claystone units with thin distal fringes of distributive deposits. The upper basinward-stepping succession (Unit H) is a distributive system possibly linked to a shelf edge delta. Across-strike complexity in the distribution of sand-prone units was controlled by cross-slope topography driven by differential compaction processes. Hemipelagic claystones separating the sand-prone units represent shutdown of the sand delivery to the whole slope and are interpreted as relative sea-level highstand deposits. Eleven depositional sequences are identified, nine of which are arranged into three composite sequences (Units E, F, and G) that together form a composite sequence set. The highly organized physical stratigraphic stacking suggests that glacioeustasy, during the Late Permian icehouse period, was the main driving process for the analyzed succession.

Arkose, Subarkose, Quartz Sand, and Associated Muds Derived from Felsic Plutonic Rocks in Glacial to Tropical Humid Climates

Abstract: In the range of climates from high-latitude glacial to equatorial tropical, the mineral and chemical compositions of over 600 modern nonmarine, first-cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied. Under glacial conditions, sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks. Humid tropical climates yield quartz-rich sands, kaolinite-rich muds, along with Si, Mg, Ca, Na, and K in solution. Between these extremes, a wide range of detrital sediment compositions are developed in arid to wet climates. In arid-climate and semi-arid-climate sands, feldspar is commonly more abundant than quartz, and plagioclase > K-feldspar (except in the cases of granite source rocks); the muds are feldspathic with smectite dominant in the clays. With increasing temperature and moisture in temperate to subtropical climates, mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant K-feldspar. Through this range of climate, muds become more kaolinitic and thus more aluminous with increasingly severe weathering. Typically, feldspathic sands are associated with feldspathic muds; with decreasing feldspar in sands, there is less feldspar in muds. With increasingly intense chemical weathering, rare earth elements (REE) are more fractionated into muds. Typically sands contain 150 ppm REE with much of the REE in heavy minerals. Abundance differences are greatest between humid tropical sands and muds. In many samples the REE are mostly in monazite, allanite, sphene, and apatite. More intensely weathered samples contain more xenotime and zircon, evidenced by Yb enrichment.

Diagenesis of Flint and Porcellanite in the Maastrichtian Chalk at Stevns Klint, Denmark

Abstract: Flint is common in many successions, but since the nodules can be the result of a series of replacement and void filling processes, their origin and broader paleo-oceanographic significance can remain enigmatic. This study addresses the origin of flint, in the Campanian–Maastrichtian chalk of Denmark using the 450-m-long Stevns-1 core as a natural laboratory. Four types of siliceous nodules occur in the core: porcellanite, white flint, white flint with a core of dark flint, and dark flint. All nodules consist mainly of microquartz with subordinate lutecite, spherulitic chalcedony, megaquartz, and dolomite. The lutecite replaces macrofossil fragments, mainly bryozoans and shells, microquartz replaces coccoliths, and the chalcedony and megaquartz are pore-filling. The preservation of the external shape of bryozoans, shells, and coccoliths suggest that siliceous nodules were formed early by a one-by-one volume replacement of chalk. Values of δ18Osmow of microquartz and lutecite around 33‰ indicate temperatures of 15–17°C at the time of precipitation; in contrast, the δ18Osmow values of megaquartz indicate precipitation at temperatures up to 48°C. The occurrence of chalcedony together with more stable and ordered megaquartz in irregular voids indicates that silica gel and/or opal-CT transformed to the α-quartz phases, microquartz, lutecite, chalcedony, and megaquartz, through the dissolution–precipitation process (Ostwald ripening). These results motivate a conceptual model in which the replacement process was initiated by microbial decomposition of organic matter and took place during periods of low rate of, or even stopped, sedimentation which fixed the redox boundary and the microbial metabolic zones at a specific depth below the sea floor. Biogenic opal-A incorporated in the sediment column below the sea floor was dissolved and precipitated at the redox boundary as silica gel and/or opal-CT, possibly aided by sulfide-oxidizing bacteria. During burial, the dissolution–precipitation process transformed the silica gel and opal-CT to the α-quartz phases. The results of this study indicate that precipitation of the flint precursor can be controlled by a combination of an abundant biosiliceous source and high microbial activity in the sediments immediately below the seafloor. Flint is therefore likely to reflect deposition in a narrow interval at relatively shallow depths and therefore might be useful in studies of shallow-water carbonates.

The Geometry and Internal Architecture of Stream Mouth Bars in the Panther Tongue and the Ferron Sandstone Members, Utah, U.S.A.

Abstract: Clinothems are seaward-dipping beds which are deposited during the progradation of delta-front mouth bars. Measurements taken from clinothems in the field and from geospatially constrained, photo-realistic virtual outcrops were used to record clinothem dip, maximum bed thickness, and bed thickness every 10 m along a series of depositional-dip-oriented profiles from two ancient river-dominated delta successions which crop out in central Utah, USA. These data show systematic changes in bed dip and bed thinning which highlight discrete packaging of the beds into bedsets, interpreted to represent the progradation of stream mouth bars in the delta front. Individual bedsets are characterized by a progressive steepening of beds coupled with a more rapid thinning. Systematic collection of 2800 locations along 73 separate clinothems from the Panther Tongue Sandstone Member and parasequence 1f from the Ferron Sandstone were used to study the relationship between bed thickness, dip, and decay gradient parameter (τ), which describes the down-dip bed thinning. Clinothems in the Panther Tongue are much longer and more gently dipping than the clinothems in the Ferron Sandstone. In both systems, beds can be clustered into groups (bedsets) which show a systematic decrease in τ-values and an increase in dip angle. Boundaries between the groups are defined by a sudden increase in τ-values and a concurrent decrease in dip angle. These bedsets are interpreted to represent individual stream mouth bars which amalgamate to form mouth bar complexes within the delta-front sand bodies. The use of virtual outcrops was essential for the collection of the large volumes of thickness data and the recognition of very subtle (< 1°) changes in dip angle which have made these interpretations possible.

How Did Thin Submarine Debris Flows Carry Boulder-Sized Intraclasts for Remarkable Distances Across Low Gradients to the Far Reaches of the Mississippi Fan?

Abstract: Submarine density flows dominate sediment transport into many parts of the ocean and form submarine fans, which are some of the largest sediment accumulations on Earth. Previous studies often assumed that the distal fringes of submarine fans would be dominated by extensive sheet-like deposits from dilute and expanded turbidity currents, because only turbidity currents would transport large volumes of sediment for long distances across such low gradients. Deposits with a remarkable frondlike shape occur at the furthest fringe of the Mississippi submarine fan. Understanding the emplacement process of these deposits is important because their frondlike shape suggests deposition from debris flows rather than turbidity currents. Previous analyses concluded that these deposits comprise a complex arrangement of thin interbedded turbidity current and debris flow deposits. Here we propose a different internal geometry for the frondlike deposits. Intervals previously described as in place turbidites are interpreted to be clasts within a single debrite interval that is ~ 1 to 2 m thick. This debrite interval is underlain by clean sand. The debrite interval could have been emplaced by multiple debris flows, or one event comprising multiple pulses. Clasts in the debrite interval are much bigger than previously thought, including boulders with diameters that can exceed 50 cm. We show that thin (~ 2 m) and extremely fluidal debris flows could have carried these boulders across gradients of only 0.06° to the fringes of one of the world's largest submarine fans, without hydroplaning. This long transport distance could occur if the density of the clasts was less than that of the surrounding debris flow. Emplacement of the basal clean sand layer appears closely linked to debris flow deposition because the clean sand layer pinches out in a location similar to that of the overlying debrite. We therefore suggest that the basal clean sand most likely settled out from the overlying debris flows, and that the clean sand is not a fore running turbidity current deposit. Shearing at the base of a mud-rich debris flow would most likely produce a muddy basal sand interval, rather than the clean sand that is observed. This basal sand interval is therefore less likely to be a record of a lubricating layer with high-pore-fluid-pressure on which the debris flows moved. This study shows how thin and highly mobile debris flows can redistribute large volumes of sediment including bouldersized clasts long distances into the deep ocean across remarkably low gradients.

Cross-Basin Variations in Magnetic Susceptibility Influenced by Changing Sea Level, Paleogeography, and Paleoclimate: Upper Devonian, Western Canada Sedimentary Basin

Abstract: Upper Devonian rocks in western Canada were deposited as carbonate-platform and surrounding slope and basinal rocks during a second-order sea-level cycle, characterized by long-term climatic warming, that ended with cooling associated with the Frasnian–Famennian mass-extinction event. In this contribution, we assess the hypothesis that high-resolution magnetic susceptibility (MS) data records cross-basin variations that are intimately tied to sea level and associated climatic change, the depositional environment, and the paleogeographic location of siliciclastic sediment sources. Four main trends are apparent in the MS data: (1) the magnitude of the MS signature increases from west to east across western Canada; (2) platform facies generally have low MS signatures compared with off-platform facies; (3) the overall MS signature increases and then decreases throughout the stratigraphic profile, largely in response to changes in depositional setting (i.e., item 2); (4) MS is generally high during third- and fourth-order sea-level lowstand and early transgression and lower during late transgression and highstand. During the late Middle (Givetian) to early Late (Frasnian) Devonian, siliciclastic sediments (silt and fine sand size) sourced from the west Alberta arch and Peace River arch, contributed to relatively high MS signatures in facies deposited proximal to these source areas. Other important detrital sediment sources were the Ellesmerian orogenic belt in northern Canada and the Laurussian continental landmass to the east. These latter sources supplied fine-grained sediments that were dispersed basinwide and resulted in a nearly order-of-magnitude increase in MS values to the east. This trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the “Old Red Continent.” By the early Frasnian, the west Alberta arch was submerged, and by middle to late Frasnian time the Peace River arch was largely covered. During the late middle Frasnian a pronounced rise in MS signatures can be tied directly to deposition of the clay- and locally silt-rich Mount Hawk Formation. During the late middle to late Frasnian, carbonate-ramp facies prograded from east to west, resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in MS signatures to the east. By the late Frasnian, cross-basin MS profiles record uniformly low susceptibilities indicating that high MS siliciclastic detritus was either bypassed seaward, was swamped by carbonate input, or was dominated by diamagnetic quartz. Individual MS events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature. Variations in the MS signature can thus be explained partly by depositional environment (platforms with low MS, basins with higher MS) and partly by third- and fourth-order sea-level and associated climatic changes. Fourth-order changes in sea level are routinely interpreted to result from variations in paleoclimate in the Milankovitch band and recent work indicates that third-order sea-level changes may be linked to longer-term modulation of Milankovitch-band orbital variations. The increase in MS during prolonged Frasnian warming thus appears to be directly linked to paleoclimatic change, and comparison of the MS signature with more direct measures, like oxygen isotopes, would serve as a test of its utility as a paleoclimate proxy.

Factors controlling peat compaction in alluvial floodplains: a case study in the cold-temperate cumberland marshes, Canada.

Abstract: Subsidence due to peat compaction may have important implications in alluvial floodplains, because it leads to wetland loss, land inundation, and damage to buildings and infrastructure. Furthermore, it potentially influences spatial and temporal river sedimentation patterns, and hence the evolution of alluvial floodplains. As the vast majority of peatlands occur in cold temperate regions, alluvial plains situated in such areas are most susceptible to these implications. To determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction-induced subsidence, it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction. For this, we carried out field work in the Cumberland Marshes (east-central Saskatchewan, Canada) guided by the following two questions: (1) how much peat compaction has occurred over the past decades to centuries, and (2) which factors control the amount and rate of peat compaction at the same timescale. To address these questions, we used methods involving construction of stratigraphic cross sections, organic-matter content and bulk-density measurements, and radiocarbon dating. A new sampling device was devised to sample uncompacted peat in the field. Results show that peat layers have compacted up to 43% within a few centuries, with compaction rates of up to 6.08 mm/yr. The dominant factors influencing peat compaction are: (1) organic-matter content, (2) stress imposed on a peat layer, and, to a lesser extent, (3) plant species composition. In an alluvial setting, crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales (100–102 years). Sheet-like splay deposits initially compact underlying peat uniformly, whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness. Subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates.

Application of Quartz Sand Microtextural Analysis to Infer Cold-Climate Weathering for the Equatorial Fountain Formation (Pennsylvanian–Permian, Colorado, U.S.A.)

Abstract: Climatic interpretations of the upper Paleozoic (Permo-Pennsylvanian) Fountain Formation, a coarse-grained fan-delta system that formed in western equatorial Pangea, are difficult to constrain owing to a general lack of climatic indicators so typical of coarse clastic systems. We applied scanning electron microscopy (SEM) to analyze quartz grains in this system in an attempt to test the hypothesis of a glacial influence on these strata. SEM observations of first-cycle quartz grains from these strata reveal microtextures formed from fracturing during grain transport, even after diagenetic overprinting occurred under moderate burial conditions (up to 3.5 km depth and 100°C). Transport-induced microtextures can be grouped based on inferred fracture process into: (1) high-stress fractures, consisting of fractures created through sustained high shear stress, such as grooves, deep troughs, and gouges, and are inferred to occur predominantly during glacial transport; (2) percussion fractures, consisting of fractures created by grain-to-grain contact during saltation or traction flow, such as randomly oriented v-shaped cracks and edge rounding; and (3) polygenetic fractures, such as conchoidal fractures, arc-shaped steps, linear steps, and linear fractures, that occur under a wide range of transport processes and thus possess no environmental significance. Delineation of high stress, percussion, and polygenetic fracture types demonstrate that the Fountain Formation quartz grains exhibit microtextures similar to both till and glaciofluvial deposits, suggesting that periods of upland glaciation occurred in the source region of the Fountain Formation (Ute Pass uplift). The abundance of high-stress fractures peaks at two stratigraphic intervals. These intervals are inferred to record the presence of ice in the Ute Pass uplift and are correlative with polygonally fractured paleosurfaces in the Fountain Formation that are interpreted to reflect cold-temperature weathering. Moreover, the peak intervals are approximately coeval with inferred episodes of ice maxima from high-latitude localities, as well as other low-latitude localities. Geologically reasonable stream gradients and estimated transport distance suggest a best-estimate elevation of the ice terminus of ~ 1500 m, but possibly ranging to 3000 m. These data suggest that upland glaciers episodically existed within this equatorial setting and that further use of this technique may reveal more evidence of ice in other proximal deposits of the ancestral Rocky Mountains, as well as other systems of various geologic ages.

Architectural Style and Quantification of a Submarine Channel–Levee System Located in a Structurally Complex Area: Offshore Nile Delta

Abstract: A Pleistocene channel–levee system located in a structurally complex area of the offshore Nile Delta is studied using a high-resolution three-dimensional (3D) seismic dataset. The seismic facies and stacking patterns are characterized and quantitative analysis of its morphology (e.g., thalweg longitudinal profile, relief, width, and levee thickness) is utilized to understand the controls on channelized-flow processes and resultant architecture. Overall a change downslope from a degradational to aggradational style is observed, which can be related to the concave-up slope profile. In comparison to other channel–levee systems the Noor has a shorter length scale and displays a steepened thalweg slope. This is interpreted to be influenced by structural movements and an associated knickpoint, which controlled a change in flow processes (e.g., velocity, turbulence, and thickness) and the associated location of the canyon to channel–levee transition zone. An unusual feature of downslope levee thickening is observed. Instead of the normal reduction in overbank sedimentation downslope, it is suggested that structural modification causing relative steepening of seabed topography resulted in increased turbidity-current velocity, fluid volume discharge, and associated sediment load, which overcame the normal downslope loss of sediment due to overbank sedimentation. This study shows that architectural style is strongly linked to slope topography, and that structural movements can influence the slope profile and flow process, resulting in modification of the morphology and dimensions of channel–levee systems.

Use of Low-Cost Multistripe Laser Triangulation (MLT) Scanning Technology for Three-Dimensional, Quantitative Paleoichnological and Neoichnological Studies

Abstract: The purpose of this paper is to test the application of a new, low-cost ($2995.00 US), multistripe laser triangulation (MLT) scanner and three-dimensional (3D) software for semiquantitative and quantitative analyses of ichnofossils and modern traces. The goal of this research is to improve on existing analytical techniques and apply new methods to 3D digital models of ichnofossils and modern traces. Objectives are to (1) provide researchers with new ways to develop and test hypotheses quantitatively in the fields of paleoichnology, neoichnology, sedimentology, and soil science, and (2) discuss uses, advantages, and limitations of MLT technology related to ichnology. We scanned and created digital models of a variety of mostly continental ichnofossils and modern terrestrial traces produced by invertebrates and vertebrates. Visual methods applied include making uniformly colored specimen surfaces, stereo pairs, anaglyph stereo images, animations, and cross sections. Quantitative methods applied include measuring distances, tortuosity indices, and angles, and producing contour maps of tracks. Two of the most useful properties measurable from digital models are surface area (SA) and volume (V); these are used rarely in ichnology because they are difficult to measure with traditional methods. We use SA to calculate area exploited and introduce a method of quantifying surface roughness adapted from research on soil surfaces. We measure V of burrows, tracks, and coprolites, as well as introduce a new measure termed “volume exploited.” We hypothesize that different tracemakers make burrows with characteristic V to SA ratios; this is partially supported by statistical tests of previously published data. We also use V to SA ratios to determine relative compactness—a metric adapted from building physics. Digital models ensure perpetuity of specimens because they preserve 3D data that can be used to make physical copies, placed in museums, and disseminated easily to researchers and educational institutions of all levels. Data from digital models can be used to interpret ichnocoenoses, bioturbation rates, and pedogenic properties and processes in soils and paleosols. Note that MLT scanning digitizes only surfaces of objects, so it is best suited for exogenic traces and casts of endogenic traces.

Increased Production of Calcite and Slower Growth for the Major Sediment-Producing Alga Halimeda as the Mg/Ca Ratio of Seawater is Lowered to a “Calcite Sea” Level

Abstract: Laboratory experiments were conducted to subject the calcareous alga Halimeda, a prolific producer of aragonite in the modern ocean, to a variety of ambient Mg/Ca ratios. These experiments were based on three predictions: (1) that Halimeda would calcify at a lower rate when the ambient Mg/Ca ratio was reduced from its high level in modern seawater (5.2); (2) that this alga would grow more slowly under these conditions because its calcification yields as a byproduct CO2, which is available for photosynthesis; and (3) that the calcite content of Halimeda’s skeleton would increase as a percentage of total CaCO3. As predicted, reduction of the Mg/Ca ratio of ambient artificial seawater from its modern level to estimated early Oligocene and early Eocene levels (2.5 and 1.5, respectively) resulted in progressively lower rates of calcification, production of organic matter, and linear growth for Halimeda incrassata colonies. The ambient Mg/Ca ratio exerted a stronger influence over the rate of calcification than [Ca2+], which in the geologic past has risen in seawater whenever the ambient Mg/Ca ratio has declined. Thus, although elevation of [Ca2+] at such times would have promoted the calcification and growth of Halimeda, the concomitant decline of the Mg/Ca ratio would have had a stronger negative effect. In addition, the percentage of skeletal material consisting of calcite rose from 8% in modern-day seawater to 16% in artificial early Oligocene seawater and 46% in artificial early Eocene seawater. Magnesium was incorporated into this calcite as a function of the ambient Mg/Ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite. Halimeda exerts less biological control over its biomineralization than the related genera Penicillus and Udotea, which in previous experiments produced only 22–25% of skeletal CaCO3 as calcite in seawater having a very low Mg/Ca ratio (1.0). Nonetheless, it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low-Mg CaCO3 as calcite in ancient seas, such as those of the Cretaceous, whose Mg/Ca ratios were close to unity.

Paleosols in Late Moscovian (Carboniferous) Marine Carbonates of the East European Craton Revealing “Great Calcimagnesian Plain” Paleolandscapes

Abstract: Late Paleozoic strata commonly include lithologic cycles (“cyclothems”), driven by sea-level fluctuations that repeatedly flooded and exposed broad expanses of epeiric shelves. Such successions, commonly with multiple unconformities, are well documented in Permo-Pennsylvanian sedimentary successions, but remain less understood in the East European Craton (EEC), part of the Euramerican continental shelf rim. This study documents unconformities and paleosols from the Podolskian to Myachkovian (middle Pennsylvanian) epeiric-carbonate succession of the EEC and explores the non-actualistic soil types, landscapes, and the character and density of vegetation in this Pennsylvanian calcimagnesian dryland environment. Thin (mostly < 1 m) paleosols developed at seven successive unconformities consist of an upper terrestrial clay layer or “topclay”, a crust of beta calcrete beneath the topclay, and a weakly karsted substrate limestone with rare rhizocretions. Solution pits and small (< 3 m deep) karst sinkholes are rare. Nontruncated topclays commonly consist of two layers: in situ clayey epipedons and resedimented terrestrial clays. The latter are interpreted as playa sediments. The Myachkovian unconformities of the northernmost localities studied, near Arkhangelsk, lack beta calcrete and root traces, and the limestone beneath these unconformities is sepiolitized. In Podolskian paleosols of the southern Moscow Basin, the topclays are predominantly palygorskitic. The Myachkovian paleosol clays are smectitic–illitic with occasional chlorite admixture. The majority of late Moscovian paleosols surveyed formally conform to lithic (rendollic) haplocalcids within the aridisol class or rendzic calcic leptosols. A detailed study of a representative Podolskian paleo-pedon reveals development of shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter in the form of covalently bound organomineral complexes, and is fulvate-dominated resembling organic matter from extant dryland soils. The palygorskitic composition of Podolskian topclays and shallow (< 30 cm) pedogenic carbonates is interpreted to reflect hot, well-drained semidesert conditions with precipitation less than 300 mm/yr. The transition from palygorskitic to smectitic soil clays across the Podolskian–Myachkovian boundary, apparently deeper soil carbonate, and an increase in the degree of karstification record a general increase in humidity such that annual precipitation may have exceeded 300–400 mm. The available data reconstruct the “great calcimagnesian plain” landscapes formed during low sea level, supposedly glacial phases, across tens of thousands of square kilometers of the EEC. This non-actualistic system apparently lacked a fluvial network and was covered by uniform soils and playas. The primary productivity of this early (310–305 Ma old) dryland ecosystem inferred from high TOC and rhizogenic beta calcretes was not markedly different from present-day steppe or savanna grasslands and semideserts. In the northernmost outcrops surveyed, near Arkhangelsk, drier conditions probably limited colonization by higher plants. The available data open a “hidden chapter” in the Paleozoic evolution of terrestrial ecosystems which is not recorded by paleobotany and urges modifications in the concept of low-productivity character of pre-Miocene dry landscapes.

A Test of Initiation of Submarine Leveed Channels by Deposition Alone

Abstract: Leveed submarine channels play a critical role in the transfer of sediment from the upper continental slopes to intraslope basins and ultimately deeper marine settings. Despite a reasonable understanding of how these channels grow once established, how such channels are initiated on previously unchannelized portions of the seafloor remains poorly understood. We conducted a series of experiments to test whether leveed channels can start by deposition on a planar rigid bed. We systematically varied the current density and outlet velocity to explore the relative influence of inertia and excess density on the depositional dynamics of currents entering a basin and undergoing abrupt unconfinement. Under flow conditions ranging from supercritical to subcritical (bulk Richardson numbers of 0.02 to 1.2) our experiments failed to produce deposits resembling or exhibiting the potential to evolve into levees needed to create a self-formed channel. In the absence of excess density, a submerged sediment-laden flow produced sharp-crested lateral deposits bounding the margins of the flow for approximately a distance of two outlet widths down-basin. These lateral deposits terminated in a centerline deposit that greatly exceeded marginal deposits in thickness. As excess density increased relative to the outlet velocity, the rate of lateral spreading of the flow increased relative to the downstream propagation of the density current, transitioning from a narrow flow aligned with the channel outlet to a broad radially expanding flow. Coincident with these changes in flow dynamics, the bounding lateral deposits extended for shorter distances, had lower, more poorly defined crests that were increasingly wider in separation than the initial outlet, and progressively became more oblong rather than linear. Based on our results and a review of previous experimental and numerical models, we suggest that initiation of leveed channels from sediment-laden density currents traversing non-erodible beds is unlikely. Partial confinement of these currents appears to be necessary to establish the hydrodynamic conditions needed for sediment deposition along the margins of a density current that ultimately may create confining levees. We suggest that erosion into a previously unchannelized substrate is the most likely source of this partial confinement.