Showing papers in "Journal of Sedimentary Research in 2003"

Seismic Geomorphology and Stratigraphy of Depositional Elements in Deep-Water Settings

Abstract: Analyses of 3-D seismic data in predominantly basin-floor settings offshore Indonesia, Nigeria, and the Gulf of Mexico, reveal the extensive presence of gravity-flow depositional elements. Five key elements were observed: (1) turbidity-flow leveed channels, (2) channel-overbank sediment waves and levees, (3) frontal splays or distributary-channel complexes, (4) crevasse-splay complexes, and (5) debris-flow channels, lobes, and sheets. Each depositional element displays a unique morphology and seismic expression. The reservoir architecture of each of these depositional elements is a function of the interaction between sedimentary process, sea-floor morphology, and sediment grain-size distribution. (1) Turbidity-flow leveed-channel widths range from greater than 3 km to less than 200 m. Sinuosity ranges from moderate to high, and channel meanders in most instances migrate down-system. The high-amplitude reflection character that commonly characterizes these features suggests the presence of sand within the channels. In some instances, high-sinuosity channels are associated with (2) channel-overbank sediment-wave development in proximal overbank levee settings, especially in association with outer channel bends. These sediment waves reach heights of 20 m and spacings of 2-3 km. The crests of these sediment waves are oriented normal to the inferred transport direction of turbidity flows, and the waves have migrated in an up-flow direction. Channel-margin levee thickness decreases systematically down-system. Where levee thickness can no longer be resolved seismically, high-sinuosity channels feed (3) frontal splays or low-sinuosity, distributary-channel complexes. Low-sinuosity distributary-channel complexes are expressed as lobate sheets up to 5-10 km wide and tens of kilometers long that extend to the distal edges of these systems. They likely comprise sheet-like sandstone units consisting of shallow channelized and associated sand-rich overbank deposits. Also observed are (4) crevasse-splay deposits, which form as a result of the breaching of levees, commonly at channel bends. Similar to frontal splays, but smaller in size, these deposits commonly are characterized by sheet-like turbidites. (5) Debris-flow deposits comprise low-sinuosity channel fills, narrow elongate lobes, and sheets and are characterized seismically by contorted, chaotic, low-amplitude reflection patterns. These deposits commonly overlie striated or grooved pavements that can be up to tens of kilometers long, 15 m deep, and 25 m wide. Where flows are unconfined, striation patterns suggest that divergent flow is common. Debris-flow deposits extend as far basinward as turbidites, and individual debris-flow units can reach 80 m in thickness and commonly are marked by steep edges. Transparent to chaotic seismic reflection character suggest that these deposits are mud-rich. Stratigraphically, deep-water basin-floor successions commonly are characterized by mass-transport deposits at the base, overlain by turbidite frontal-splay deposits and subsequently by leveed-channel deposits. Capping this succession is another mass-transport unit ultimately overlain and draped by condensed-section deposits. This succession can be related to a cycle of relative sea-level change and associated events at the corresponding shelf edge. Commonly, deposition of a deep-water sequence is initiated with the onset of relative sea-level fall and ends with subsequent rapid relative sea-level rise.

Bacterially Induced Mineralization of Calcium Carbonate in Terrestrial Environments: The Role of Exopolysaccharides and Amino Acids

Abstract: This study stresses the role of specific bacterial outer structures (such as glycocalix and parietal polymers) on calcium carbonate crystallization in terrestrial environments. The aim is to compare calcium carbonate crystals obtained in bacterial cultures with those obtained during abiotically mediated synthesis to show implications of exopolysaccharides and amino acids in the mineralogy and morphology of calcium carbonate crystals produced by living bacteria. This is done using various amounts of purified exopolysaccharide (xanthan EPS) and L-amino acids with a range of acidities. Amino acids and increasing xanthan content enhance sphere formation in calcite and vaterite. Regarding calcite, the morphology of crystals evolves from rhombohedral to needle shape. This evolution is characterized by stretching along the c axis as the amino acid changes from glutamine to aspartic acid and as the medium is progressively enriched in EPS. Regarding vaterite, the spherulitic habit is preserved throughout the morphological sequence and starts with spheres formed by the agglomeration of short needles, which are produced in a xanthan-free medium with glutamine. Monocrystals forming spheres increase in size as xanthan is added and the acidity of amino acids (glutamic and aspartic acids) is increased. At high xanthan concentrations, amino acids, and mainly aspartic and glutamic acids, induce vaterite precipitation. The role of the carboxyl group is also probably critical because bacterial outer structures associated with peptidoglycan commonly contain carboxyl groups. This role, combined with the results presented here, clearly demonstrate the influence of bacterial outer structure composition on the morphology and mineralogy of bacterially induced calcium carbonate. This point should not be neglected in the interpretation of calcite cements and carbonate accumulations in terrestrial environments.

Three-Dimensional Sedimentary Architecture of a Large, Mid-Channel Sand Braid Bar, Jamuna River, Bangladesh

Abstract: The three-dimensional subsurface alluvial architecture of a large (approximately 3 km long, 1 km wide, 12 m high), mid-channel sand braid bar in the Jamuna River, Bangladesh is described. Evolution of the bar and its depositional characteristics are assessed from a unique combination of ground-penetrating radar surveys, vibracoring, and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29-month period. This methodology permits identification of the formative processes of different packages of braid-bar sedimentation and provides a facies model for deposition within the entire bar. Mid-channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes. These dunes created a bar-core that grew by (i) propagation of a downstream-accreting slipface, (ii) vertical accretion through stacking of dunes on both bar stoss and top, and (iii) lateral accretion on the bar margins during recession of the flood hydrograph. Braid-bar sedimentation is dominated by four radar facies: (1) large-scale, predominantly planar, dipping reflections interpreted as cross-stratification, up to 8 m in height and greater than 100 m in width, that is produced by the cross-channel migration of bar margins, (2) medium-scale, trough-shaped and planar discontinuous reflections interpreted as cross-stratification up to 4 m in height and 300 m wide, that is deposited from large, sinuous-crested sand dunes, (3) discontinuous reflections, up to 2 m high and 30 m wide, interpreted as small-scale trough cross-stratification, that is the product of smaller sinuous-crested dunes, and (4) high-amplitude, undulating reflections interpreted as mud drapes, deposited in regions of slow flow, often in the bar-tail region at low stage. Dune size decreases vertically within the bar, in response to the progressively shallower flows on the emerging bar top. Later evolution of the bar, as one anabranch channel became dominant, created a 1.5 km extension to the bar tail with an 8 m high, angle-of-repose, bar-margin slipface, formed by flow transverse to the long axis of the bar. Seven styles of deposition can be defined that constitute the alluvial architecture: bar-margin slipface, vertical accretion in channel, bar-top vertical accretion, upstream accretion, lateral accretion, downstream accretion, and low-stage mud drapes. A model of braid-bar sedimentation is presented that shares many similarities with previous studies of smaller sand-bed braid bars with the dominance of dune-scale cross-stratification, the presence of large-scale, bar-margin cross-stratification, and the occurrence of lateral, vertical, upstream, and downstream accretion. However, the contribution of the bar-margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly. This study suggests a scale invariance in several aspects of mid-channel bar sedimentation in sand-bed rivers and proposes a model of braid-bar sedimentation that may be applied widely within studies of braided alluvial architecture.

Microbialite Formation in Seawater of Increased Alkalinity, Satonda Crater Lake, Indonesia

Abstract: The crater lake of the small volcanic island Satonda, Indonesia, is unique for its red-algal microbial reefs thriving in marine-derived water of increased alkalinity. The lake is a potential analogue for ancient oceans sustaining microbialites under open-marine conditions. Current reef surfaces are dominated by living red algae covered by non-calcified biofilms with scattered cyanobacteria and diatoms. Minor CaCO3 precipitates are restricted to the seasonally flooded reef tops, which develop biofilms up to 500 µm thick dominated by the cyanobacteria Pleurocapsa, Calothrix, Phormidium, and Hyella. Microcrystalline aragonite patches form within the biofilm mucilage, and fibrous aragonite cements grow in exopolymer-poor spaces such as the inside of dead, lysed green algal cells, and reef framework voids. Cementation of lysed hadromerid sponge resting bodies results in the formation of "Wetheredella-like" structures. Hydrochemistry data and model calculations indicate that CO2 degassing after seasonal mixis can shift the carbonate equilibrium to cause CaCO3 precipitation. Increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium. Therefore, photosynthesis-induced cyanobacterial calcification does not occur. Instead, passive, diffusion-controlled EPS-mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place. In contrast to extreme soda lakes, the release of Ca2+ from aerobic degradation of extracellular polymeric substances does not support CaCO3 precipitation in Satonda because the simultaneously released CO2 is insufficiently buffered. Subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous, unidentified Mg-Si phase. The microstromatolites probably formed when Lake Satonda evolved from seawater to Ca2+-depleted raised-alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong CO2 degassing. The latter effect caused rapid growth of fibrous aragonite, while Mg-Si layers replaced the initially Mg-calcite-impregnated biofilms. This could be explained by dissolution of siliceous diatoms and sponge spicules at high pH, followed by Mg-calcite dissolution and Mg-silica precipitation at low pH due to heterotrophic activity within the entombed biofilms.

A classification of metamorphic grains in sands based on their composition and grade

Abstract: An operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data reproducibility among operators and the potential of high-resolution bulk petrography in provenance studies. For each of four protolith compositions (metapelite, metapsammite/metafelsite, metacarbonate, metabasite), six archetype grains displaying increasing degree of recrystallization and foliation development are illustrated. Such a classification grid is specifically devised as a subsidiary tool for point counting with the Gazzi-Dickinson method. Traditional QFR parameters can also be easily recalculated from the data set obtained, thus meeting all possible needs (Decker and Helmold 1985; Suttner and Basu 1985). An experiment shows that usage of visual-comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way. A petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense-mineral data. The "metamorphic index" (MI) is introduced as an estimator of average metamorphic grade of source rocks. Our classification, an extension of concepts used first in the study of arc-continent collision in Taiwan (Dorsey 1988) and successfully expanded to interpret the evolution of continent-continent collision in the Himalayas (Najman and Garzanti 2000; White et al. 2002), proves to be fruitful in provenance analysis of foreland-basin sediments shed from Alpine-type, thick-skinned collision orogens, particularly when integrated with dense-mineral, geochemical, and geochronological data.

Maximizing Information from Fine-Grained Sedimentary Rocks: An Inclusive Nomenclature for Mudstones

Abstract: A simple nomenclature for fine-grained sedimentary rocks is proposed based on sedimentary structures present and abundance of all materials forming more than 10% of rock volume irrespective of their origin (allochthonous, autochthonous, and diagenetic). A mudstone (sedimentary rock that comprises > 50% grains 90% clay), "silt- and sand-bearing, clay-rich mudstone" (50 to 90% clay, 10 to 50% sand, 10 to 50% silt), and "calcite ce-ment-, calcareous nannoplankton-, and clay-bearing mudstone" (all components 10 to 50%). Textural information can and should be incorporated into this scheme by prefixing the rock name with descriptions such as "bioturbated," and "thin-bedded." Such a nomenclature is necessary to fully describe variability within fine-grained sedimentary rocks that hitherto have typically been described using selected constituents of varying significance such as total organic carbon, fissility, parting spacing, -ray signature, and lamination. Using the inclusive nomenclature proposed here will improve opportunities to compare all fine-grained sediments, aid interpretation of depositional and diagenetic controls of variability, and enable these rocks to be more easily integrated into basin-scale facies models.

The Interpretation of Vertical Sequences in Turbidite Beds: The Influence of Longitudinal Flow Structure

Abstract: Because turbidite beds aggrade progressively beneath a moving current, the vertical grain-size profile of a bed is generally an indication of the longitudinal velocity structure of the flow, and longitudinal gradients in suspended sediment concentration ("density"). A current is more likely to show a simple waning flow history farther from its source; this is because faster-moving parts of the flow overtake slower moving parts, and the flow organizes itself over time so that the fastest parts are at the front. Thus distal (e.g., basin plain) turbidites commonly show simple, normally graded profiles, whereas more proximal turbidites often show complex vertical sequences within a bed, related to unsteadiness. A turbidity current may deposit a structureless, poorly sorted bed where the capacity of the current is exceeded, i.e., where there is insufficient turbulent kinetic energy to maintain the entire suspended mass. Capacity-driven deposition may occur where the flow decelerates. Where flow nonuniformity is the cause of capacity-driven deposition, a massive interval will form the lowest part of the bed, and will have a flat base. Where flow unsteadiness is the cause, a normally graded massive interval may overlie erosional features or traction structures at the base of the bed. Based on the assumption of longitudinal gradients in velocity, density, and grain-size distribution, the longitudinal density structure of a current may induce a switch, at any given point, from capacity-driven deposition to either (1) bypass and resuspension, (2) bypass with traction, or (3) competence-driven deposition, each resulting in a characteristic upward change in deposit character. The temporal evolution of the flow at a point varies systematically in a streamwise sense. Taking account of these longitudinal variations permits predictions of complex vertical sequences within beds, and of their downstream relations.

Hydraulic-Jump and Hyperconcentrated-Flow Deposits of a Glacigenic Subaqueous Fan: Oak Ridges Moraine, Southern Ontario, Canada

Abstract: The Oak Ridges Moraine in southern Ontario is a poly- genetic moraine constructed of a number of coalesced deposits of gla- cifluvial and glacilacustrine origin. A detailed study of the facies ar- chitecture has been completed on a series of pit sections extending ; 300 m subparallel to the paleoflow direction. Eight major lithofacies and five facies associations have been described. These data have been interpreted to be upper-flow-regime hyperconcentrated-flood-flow de- posits emplaced under a regime of rapid flow expansion and loss of transport capacity within a plane-wall jet with an associated hydraulic jump. Deposition from the plane-wall jet with jump occurred in three zones of flow transformation: zone of flow establishment, transition zone, and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were de- posited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-walled scours infilled by diffusely graded sand define the transition zone, the zone of maximum vortex erosion, and the distal limit of deposits emplaced under upper-flow- regime conditions. These strata record rapid bed aggradation from sediment-laden supercritical flows that episodically were scoured by large vortices generated within migrating hydraulic jumps. Strati- graphically upward and downflow strata consist only of lower-flow- regime sedimentary structures. Medium-scale, planar cross-strata and small-scale cross-lamination related to migrating 2-D dunes and cur- rent ripples, respectively, characterize the zone of established flow. The facies and sediment architecture suggest that this fan was deposited during a relatively short period of time (days, weeks) by energetic sed- iment-laden floods.

Sequence Stratigraphy in Lacustrine Basins: A Model for part of the Green River Formation (Eocene), Southwest Uinta Basin, Utah, U.S.A.

Abstract: In the middle Green River Formation of central Nine Mile Canyon, Uinta Basin, Utah, several lacustrine-dominated intervals 10 m thick comprise aggradational carbonate parasequence sets and a progradational clastic parasequence. Maximum flooding surfaces are best identified within profundal oil shale that caps some of the clastic parasequences. These lacustrine transgressive systems tracts therefore exhibit parasequence stacking patterns unlike typical marine sequences. Two types of sequence boundary are identified. Type A sequence boundaries display evidence for a basinward shift in facies across a regionally mappable surface that is an angular or, rarely, parallel unconformity, and they typically juxtapose amalgamated braided fluvial channel sandstone (late lowstand systems tract) onto the profundal oil shale. They also bound depositional sequences that show a distinct asymmetry, being dominated by transgressive systems tracts 5-80 m thick. Highstand systems tracts are less than 4 m thick and may be removed completely, by erosion on overlying sequence boundaries. Other surfaces satisfy only some of the standard criteria of sequence boundaries and are termed type B sequence boundaries. Type A sequence boundaries mark pronounced base-level falls following times when the Uinta Lake had merged with a lake in an adjacent basin to form a much deeper lake. Such merging permitted the establishment of a new threshold at higher elevation following lake-level balancing. Type B sequence boundaries are interpreted as marking base-level falls from a barely merged lake or a lake that had an outflow. Over a 200 m stratigraphic thickness, type A sequence boundaries are more common upsection, indicating that, with time, a pluvial climate became more pronounced or that the adjacent lake was more easily filled. Type A sequence boundaries also become angular rather than parallel unconformities upsection, suggesting increased tilting of the basin margin over time.

Fluvial Architecture of the Hawkesbury Sandstone (Triassic), Near Sydney, Australia

Abstract: The Hawkesbury Sandstone has long been assumed to represent the deposits of a large braided river system, comparable in style and magnitude with the modern Brahmaputra River of Bangladesh. Such an interpretation is based mainly on the common occurrence of very large-scale crossbedding, but no architectural studies of the unit have hitherto been carried out. This paper represents a first attempt to estimate the magnitude of Hawkesbury channels and bars on the basis of the preserved architectural evidence. Photomosaics were constructed of two cliff sections south of Sydney, one 5.6 km in length. On the basis of these profiles we estimate that characteristic channel-scale architectural elements are at least 2.7 km wide, and individual macroforms are 5-10 m high, indicating the constructional depth of typical channels. Hollow elements (scoop-shaped units interpreted to have formed at channel confluences) are up to 20 m deep. These magnitudes are large, but measurably smaller that those of channels and bars in the modern Brahmaputra River of Bangladesh.

Composition and Discrimination of Sandstones: A Statistical Evaluation of Different Analytical Methods

Abstract: The discriminative power of four analytical approaches to sandstone composition is evaluated with respect to the separation of different formations and source areas. The case study is Cretaceous synorogenic sandstones (litharenites) from the Eastern Alps of Europe, which belong to four different formations and are derived from two source areas. Methods evaluated are light-mineral analysis (petrographic framework composition), heavy-mineral analysis, major-element XRF analysis, and trace-element XRF analysis. The statistical parameters calculated (percentages of well-classified samples, Mahalanobis distance) applying the logratio approach suggest that light-mineral analysis has a significantly lower discriminative power than the other three methods. Taking into account the analytical expenditure for data acquisition, trace-element analysis appears to be the most efficient method for discrimination of at least the sandstone units examined. Although based on a single case study, these results are interpreted to have a more general meaning with respect to sandstone discrimination based on composition. Concerning sandstone provenance, trace-element analysis provides a quick tool to estimate the discriminative potential of a sample suite, i.e., the potential to discriminate between contrasting source areas. If a provenance model already exists and discriminate functions between contrasting source areas are calculated, trace-element analysis is considered to be most efficient in correctly assigning an unknown sample to its source area. These results cannot be extended to all kinds of sands and sandstones, but they cast serious doubt on the belief that petrographic point-count methods are the best approach to discriminate between sandstones.

A Budget of Carbonate Framework and Sediment Production, Kailua Bay, Oahu, Hawaii

Abstract: Sediments of the bay and coastal plain of Kailua (Oahu, Hawaii) are > 90% biogenic carbonate produced by destruction of primary reef framework (coral and encrusting coralline algae) and by direct sedimentation through the biological activity of calcifying organisms (the green alga Halimeda, the branching coralline alga Porolithion gardineri, molluscs, and benthic foraminifera). Field measurements of benthic community structure, gross carbonate production, bioerosion, and direct sedimentation in 17 physiographic zones in Kailua Bay are used to calculate modern calcareous sediment production rates in the 12 km2 fringing reef system. Total gross carbonate productivity by corals and encrusting coralline algae (based on mapped percent cover and known growth rates) occurs at an average rate of 1.22 (± 0.36) kg m-2 y-1 over hard substrates of the reef platform, corresponding to 0.8 (± 0.2) mm y-1 (using a bulk density of 1.48 g cm-3, the average of coral and coralline algae). Coralline algae contributes 42% of the total gross productivity. Bioerosion of coral and coralline algae facies at and near the reef surface (estimated from slabbed reef samples) occurs at average rates of 0.10-1.15 kg m-2 y-1 and releases 1,911 (± 436) m3 of unconsolidated carbonate sediment annually. Mechanical erosion (coral breakage) likely contributes an additional 315 m3 y-1. Carbonate sediment is also produced directly by the green alga Halimeda, branching coralline algae, molluscs, and benthic foraminifera at a combined rate of 1,822 (± 200) m3 y-1 (using densities specific to sediment origin). The total rate of production of unconsolidated carbonate sediment in Kailua Bay is the sum of these sources, amounting to 4,048 (± 635) m3 y-1. Normalizing gross sediment production (in kg y-1) by reef habitat area (in m2) generates average rates of productivity (in kg m-2 y-1) that are directly comparable to each other and to the literature; such rates can also be employed and tested in other reef settings. In Kailua Bay, the total production of calcareous sediment corresponds to an average (normalized) rate of 0.53 (± 0.19) kg m-2 y-1, with 0.33 (± 0.13) kg m-2 y-1 contributed through erosion of the coralgal framework and 0.20 (± 0.06) kg m-2 y-1 contributed by direct sediment production on the reef surface. Applying these modern sediment-production rates over the 5,000 years that Kailua Bay has been completely inundated by postglacial sea-level rise, an estimated 20.2 (± 3.2) 106 m3 of unconsolidated carbonate sediment has been produced in the system. The volume of sediment stored in the various reef channels and holes in Kailua Bay is 3.7 (± 0.3) 106 m3, or 19% of that produced since 5,000 y BP. The volume of sand in the modern beach is 1.0 (± 0.1) 106 m3, or 5% of Holocene sediment production. The volume of carbonate sediment stored in the coastal plain is estimated, using core-log data and associated radiocarbon ages, to be 10.0 (± 1.8) 106 m3, or 51% of Holocene sediment production. The remaining 25% likely represents sediment loss due to the natural processes of dissolution, attrition, and transport offshore. These export terms are not well understood and emphasize the need for sediment dynamics to be incorporated into reef and sediment budgets. Although sediment production in this reef system is prodigious, the rate of "new" sediment supplied to the beachface is less than 2% of what moves on and off the beach seasonally.

The Effect of Sea Level, Climate, and Shelf Physiography on the Development of Incised-Valley Complexes: A Modern Example From the East China Sea

Abstract: Analysis of high-resolution seismic reflection and sonar data from the East China Sea (ECS) continental shelf reveals the presence of an extremely broad (> 330 km) and deep (maximum incision of 72 m) incised-valley complex whose development and subsequent fill is constrained to the end of the last glacial cycle ( 20,000 yr BP to present). Results from this study indicate that sea-level change, climate-controlled discharge and sediment supply, and shelf physiography were important factors in the development and subsequent fill of the ECS incised- valley complex. For example, wet climatic conditions during the initial Marine Oxygen Isotope Stage (MIS) 2 fall of sea level promoted fluvial erosion of the exposed shelf. However, the trend toward drier conditions during the MIS 2 fall of sea level slowed the overall development of the incised-valley complex to the point where only minor incision occurred during the maximum lowstand. Furthermore, the low-gradient morphology of the exposed outer shelf diminished the ability of fluvial systems to incise. Instead, fluvial systems migrated laterally, creating a shallow ( 300 km) incised-valley system on the outer shelf. During the late MIS 2 rise of sea level, accommodation was generated within the flooded reaches of the valley complex. However, arid climatic conditions that prevailed at this time resulted in minor sediment delivery to the transgressive shoreline via the incised-valley complex. Thus, the upper part of the drowned lowstand fluvial deposits were exposed at the sea floor and reworked into a valley-wide tidal-bar and tidal sheet complex. Wetter climatic conditions during middle to late MIS 1 resulted in abundant sediment delivery to the drowned parts of the valley complex (estuary) and buried lowstand fluvial deposits below the depth of tidal ravinement, where they remained undisturbed. Current sequence stratigraphic models can be applied to the late Quaternary stratigraphic secession of the ECS. However, these models are too simplistic because they rely largely on rates and directions of sea-level change to explain stratal architectures. Although the extent and rate of sea-level change is extremely important, this study shows that high-frequency climate change and shelf physiography also play important roles in the development of stratigraphic architectures. These factors are particularly important for the development of incised valleys, where they directly influence incised-valley morphology and facies distributions.

The Barremian-Aptian Evolution of The Eastern Arabian Carbonate Platform Margin (Northern Oman)

Abstract: Carbonate platform margins are sensitive recorders of changes in sea level and climate and can reveal the relative importance of global and regional controls on platform evolution. This paper focuses on the Barremian to Aptian interval (mid Cretaceous), which is known for climatic and environmental changes towards more intensified greenhouse conditions. The study area in the northern Oman mountains offers one of the very few locations where the Cretaceous carbonate margin of the Arabian Plate can be studied along continuous outcrops. Our detailed sedimentological and sequence stratigraphic model of the platform margin demonstrates how major environmental and ecological changes controlled the stratigraphic architecture. The Early Cretaceous platform margin shows high rates of progradation in Berriasian to Hauterivian times followed by lower rates and some aggradation in the Late Hauterivian to Barremian. High-energy bioclastic and oolitic sands were the dominant deposits at the margin. Turbidites were deposited at the slope and in the basin. The Early Aptian platform margin shows a marked change to purely aggradational geometries and a welldeveloped platform barrier that was formed mainly by microbial buildups. The sudden dominance in microbial activity led to cementation and stabilization of the margin and slope and, therefore, a decrease of downslope sediment transport by turbidites. In the Late Aptian, large parts of the Arabian craton were subaerially exposed and a fringing carbonate platform formed. Seven Barremian to Early Albian large-scale depositional sequences reflecting relative sea-level changes are identified on the basis of time lines constrained by physical correlation and biostratigraphy. The reconstruction of the margin geometries suggests that tectonic activity played an important role in the Early Aptian. This was most likely related to global plate reorganization that was accompanied by increased volcanic activity in many parts of the world. Along the northeastern Arabian platform the associated global changes in atmospheric and oceanic circulation are recorded with a change in platform-margin ecology from an ooid-bioclast dominated to a microbial dominated margin. Time-equivalent argillaceous deposits suggest an increase in rainfall and elevated input of nutrients onto the platform. This process contributed to the strongly diminished carbonate production by other organisms and favored microbial activity. The platform margin may thus represent a shallow-marine response to the Early Aptian global changes, commonly associated with an oceanic anoxic event in basinal environments.

Floodplain Formation and Sediment Stratigraphy Resulting from Oblique Accretion on the Murrumbidgee River, Australia

Abstract: Oblique accretion is a significant process of deposition along low-energy, mixed-load and suspended-load Australian rivers. Previously described as accretionary bank deposits sandwiched between well-developed point bars of sand and gravel and muddy overbank deposits, fine-grained oblique-accretion deposits dominate the floodplain stratigraphy of many inland Australian rivers. They contribute more than 65% of floodplain sediments along the Murrumbidgee River and almost all of the floodplain formed by bend migration on the suspended-load channels of the Darling and Cooper basins. Deposits consist mainly of alternating thin beds of sand and mud (inclined heterolithic stratification), with some plant litter, that form as drapes on the prograding bank. These beds dip mostly channelwards and quickly wedge out as they grade up and onto the floodplain. Because oblique accretion traps nearly all of the sediment deposited from suspended load near the channel margin, vertical accretion on distal areas of the floodplain is minimal. Where oblique accretion is associated with scroll formation, the resulting deposits are more complex, sometimes including a component that slopes away from the channel on the distal side of the first floodplain scroll. A model is presented showing how, with point bars or scrolls either present or absent, oblique accretion can make a significant contribution to the preservation of fine-grained within-channel deposits in contemporary floodplains. The examples presented here demonstrate that analogues to ancient point-bar deposits containing alternating sandstone and shale sequences are common in the low-energy fluvial environments of inland Australia.

From Cyclothems to Sequences: The Record of Eustasy and Climate on an Icehouse Epeiric Platform (Pennsylvanian-Permian, North American Midcontinent)

Abstract: The stratigraphic succession of the North American Midcontinent contains a high-resolution (< 100,000 year) record of sea level and climate change spanning the Pennsylvanian-Permian boundary. Outcrop-based sequence stratigraphic analysis on the upper Wabaunsee, Admire, and lower Council Grove groups in Kansas and Nebraska reveals a hierarchy of fifty-one fifth-order meter-scale cycles grouped into five fourth-order composite sequences. Fifth-order cycles are bounded by subaerial unconformities in nearshore settings and correlative conformities in offshore settings, and are therefore very thin depositional sequences. Lowstand systems tracts, observed only in nearshore deposits, are expressed as sandy, incised-valley fills in these meter-scale cycles. Transgressive systems tracts, which are dominated by carbonate deposition, include coastal evaporites indicating arid climatic conditions. Highstand systems tracts, which are dominated by siliciclastic deposition, include thin but persistent coals in muddy coastal successions indicating humid climatic conditions. Paleosols at sequence boundaries evolve from vertic to calcic, indicating climate change from relatively humid to relatively arid conditions during subaerial exposure. This indicates that relatively arid climate coincided with eustatic lows while relatively humid climate coincided with eustatic highs on this low-latitude platform. Composite sequences are bounded by subaerial, angular unconformities and display a transgressive-regressive stacking pattern of meter-scale cycles. The high-resolution stratigraphic record of the Midcontinent helps constrain the rate and magnitude of environmental change in ancient icehouse conditions and can serve as a baseline of environmental change for comparison with other coeval successions.

Information on grain sizes in gravel-bed rivers by automated image analysis

Abstract: The time required in the field to characterize textural variation over gravel surfaces can be reduced by taking vertical photographs for subsequent image analysis. We present modified edge-detection algorithms which combine edge seeding with an image porosity concept and partial watershed segmentation. The methods allow quick, reliable, and operator-independent size analysis from a wide range of vertical bed-surface images. They are tested using 24 naturally lit images of an exposed river bed with mixed lithologies and partial burial of gravel by sand. Grain-size percentiles derived by automated image analysis correlate closely with those from manual image analysis, with only small and consistent degrees of bias. They also correlate well with percentiles from field measurements with substantial bias, which, however, is consistent so that it can be corrected for, leaving a residual scatter of 0.25 (where = log2 mm = -) over a wide range of bed conditions. The bias depends somewhat on sand cover, and the biggest residual discrepancies are for tail percentiles.

Formation of Deep Incisions Into Tide-Dominated River Deltas: Implications for the Stratigraphy of the Sego Sandstone, Book Cliffs, Utah, U.S.A.

Abstract: The Upper Cretaceous Sego Member of the Mancos Shale in east-central Utah is composed of tidally deposited sandstones interbedded with intervals containing marine shales and thin wave-deposited sandstones. The tidal sandstones have been interpreted to comprise multiple amalgamated estuarine valley fills above a major sequence boundary that is incised into distal marine deposits of the underlying Buck Tongue Shale. According to this interpretation, the deposits constitute the base of a thick transgressive succession within the Sevier foreland clastic wedge. Alternatively, lower Sego sandstones have recently been interpreted by the authors to be tide-dominated river delta deposits in an overall regressive interval of the foreland succession. A cross section showing facies variations and stratigraphic surfaces within the lower Sego Member along 100 kilometers of the eastern Book Cliffs is used in this paper to refine depositional and sequence stratigraphic interpretations of these tidal deposits, particularly the origin of deep incisions within the Sego sandstones. A strike-oriented section of the lower Sego Member reveals tidal sandstone lenses, 40-50 kilometers wide and tens of meters thick, separated laterally by areas tens of kilometers wide that have no tidal sandstones. Facies variations within the tidal sandstones reflect depositional processes on tide-dominated deltas that prograded obliquely into the shallow intracratonic Western Interior Seaway of North America. Two major episodes of delta progradation and subsequent transgressions are interpreted to have formed tidal sandstone layers within the lower Sego Sandstone. Thinner tidal sandstone layers, of more local extent, are interpreted to reflect changes in tidal current strength but not necessarily shifts in shoreline position or major changes in water depth. Multiple origins are suggested for incised erosion surfaces within the lower Sego sandstones. Firstly, the tide-dominated deltaic sandstones are floored by laterally discontinuous erosion surfaces. These basal surfaces, marking abrupt vertical changes in depositional process and coarsening of grain size, are interpreted to record tidal scouring of the sea floor during shoreline progradation. Secondly, sharp-based upward-coarsening deltaic sandstones are cut by numerous deep incisions with tens of meters of relief, and locally they are entirely removed by erosion. Some of this deep erosion within the lower Sego Sandstone may reflect the upstream avulsion of incised distributaries during forced regression of deltas into the basin, whereas others may have formed by tidal erosion of abandoned distributaries during more gradual progradation and subsequent retrogradation of deltas. Highly variable facies trends within incision fills are related to pronounced changes in sediment supply to different distributaries on the delta, to differences between incisions cut during delta regression versus those cut during the start of transgression, and to varying distances from delta axes. When the entire coast was transgressed, the influence of tides waned and wave-generated currents ravined delta tops. Depositional interpretations of the Sego Member indicate that the Buck Tongue to Neslen interval lies in a broadly regressive part of the foreland basin fill.

Integrating Sandstone Petrology and Nonmarine Sequence Stratigraphy: Application to the Late Cretaceous Fluvial Systems of Southwestern Utah, U.S.A.

Abstract: Petrographic and dispersal data are essential to correct interpretation of mechanisms that create continental sequence-stratigraphic architecture. A case study from southern Utah demonstrates that Upper Cretaceous (upper Santonian-Campanian) alluvial successions in the southernmost part of the Cordilleran foreland basin were deposited by fluvial systems of contrasting drainage directions and provenance, and suggests that different mechanisms governed their sequence architecture. Most of the rivers flowed northeast, subparallel to the basin foredeep. Less common fluvial systems flowed to the east-southeast. The fluvial sandstones fall naturally into four petrofacies: (1) quartzofeldspatholithic (mean Qt61F19L20); (2) feldspatholithic (Qt29F19L52); (3) quartzolithic (Qt75F6L20); and (4) quartzose (Qt99F1L1). Petrofacies 1 and 2 were derived from mixed supracrustal and basement sources to the southwest and south, respectively, whereas petrofacies 3 and 4 were derived from uplifted thrust sheets of the Sevier orogenic belt to the southwest and west, respectively. Only the east-southeast-flowing rivers transported the quartzose petrofacies. The fluvial strata, which include the uppermost Straight Cliffs, Wahweap, and Kaiparowits formations, form two large-scale stratigraphic successions typically interpreted as continental stratigraphic sequences hundreds of meters thick. Each succession begins with an amalgamated braided-fluvial deposit, grades to mudstone-rich strata with low sandstone-body connectivity, and culminates in highly connected sandstone bodies with multistory stacking. The basal amalgamated deposits of each succession are architecturally similar, but their compositional and dispersal characteristics are different. Quartzofeldspatholithic, quartzolithic, and quartzose sandstones above the lower base-level shift are variable, but generally similar in compositional and dispersal characteristics to both underlying and overlying strata, a phenomenon termed here congruence. In contrast, quartzose amalgamated fluvial sandstone above the upper base-level shift differs sharply in composition and dispersal direction from underlying and overlying lithic-rich strata. The foredeep axis controlled the progradation direction of the congruent shift, which was likely driven by climatically induced sediment influx, a eustatic fall, or both. In the case of the incongruent shift, increased sediment supply permitted the rivers to cross the foredeep. Temporal association of the upper amalgamated deposit with active structures in the thrust belt and foreland basin indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz.

From Rifted Margins to Foreland Basins: Investigating Provenance and Sediment Dispersal Across Desert Arabia (Oman, U.A.E.)

Abstract: Arid Arabia, where chemical weathering and anthropic modifications are negligible, is a huge natural laboratory for the study of sediment provenance. Mafic and ultramafic detritus from ophiolite belts is found along the Oman Mountains and on Masirah Island ("obduction-orogen provenance"). Dominant cellular serpentinite grains from residual mantle harzburgites of the Sama'il ophiolite contrast with gabbroic, diabase, and basaltic grains from igneous crustal rocks of the Masirah ophiolite. Detritus from autochthonous basement and cover rocks is found along the Arabian Sea margin ("rift-shoulder provenance"). Transition from carbonate-dominated lithic sands ("undissected stage") to hornblende-rich plagioclase arkoses ("dissected stage") is best documented at the northern edge of the Gulf of Aden. Along the older, strike-slip Owen-basin margin, only locally are feldspars, hornblende, and garnet derived from small outcrops of Pan-African basement. Modern eolian sands from Yemen to the Arabian (Persian) Gulf display marked enrichment in quartz, low plagioclase/feldspar ratios, and depleted dense-mineral assemblages with rounded epidotes, hornblende, garnet, and ultrastables, pointing to extensive recycling also fostered by Quaternary changes. The Rub'-Al-Khali and Wahibah sand seas have distinct quartz/feldspar ratios, reflecting largely separate transport pathways. Dune fields, fed chiefly from rift highlands in the west and south, mix with orogenic detritus in the northeast. Carbonate-dominated sands in coastal UAE are mostly derived from the Zagros Mountains, blown by northerly Shamal winds across the Gulf during Pleistocene lowstands to reach the backbulge depozone and the foreland beyond. Contributions from the Oman Mountains reach only 5-10% of bulk sediment along their southern apron, and mostly indirectly through deflation of Plio-Pleistocene fans. The Sama'il ophiolite supplies enstatite and diopside to the eastern UAE and Wahibah sands; the Masirah ophiolite supplies amphiboles and clinopyroxenes locally along the Batain coast. The composition of desert sediments faithfully mirrors tectonic history of source terranes, deformation style during orogeny, and unroofing level during extension. Bulk petrography coupled with dense-mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale.

Quantification of Brittle Deformation in Burial Compaction, Frio and Mount Simon Formation Sandstones

Abstract: Brittle deformation, together with grain rearrangement and ductile grain deformation, is a key mechanism of compaction in sandstones. Quartz cementation, by virtue of its impact on sandstone mechanical properties, is expected to affect the compaction progress by these various mechanisms. Sandstone samples of different ages and compositions, taken from two basins with contrasting burial histories, are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction. Exponential trends of increasing deformation by microfracturing are observed in both the lithic-rich Frio Formation from the Gulf of Mexico basin (r2 = 0.81) and in the quartz-rich Mount Simon Formation from the Illinois basin (r2 = 0.78). The two formations contrast in terms of the observed rate of grain fracture increase with depth. A larger number of quartz grains in the Mount Simon Formation undergo fracturing at shallow burial ( 3 km) a larger number of quartz grains are fractured in the Frio sandstones. Cathodoluminescence (CL) imaging conducted on an SEM shows that grain fracturing and subsequent cementation of fractures by quartz can occur before, during, and after normal quartz overgrowth precipitation, although fracturing is generally concurrent with cementation. The intensity of brittle deformation for a single fractured grain is independent of depth, as illustrated by plotting the ratio of the number of particles in fractured grains versus depth (r2 = 0.21 for Frio and 0.03 for Mount Simon), suggesting that, once fractured, grains do not tend to become the locus of subsequent fracturing. Apparent fracture apertures in the Frio grains are slightly wider (average 5 µm) than in Mount Simon grains (average measurable aperture width 4 µm). Quartz grains in the Frio have a variety of fracture morphologies, including wedge-shaped apertures, intense comminution at grain contacts, and grains with exploded fabrics. Apparent fracture apertures in Mount Simon grains are thinner and transect grains as straight, uniform traces. Differences in fracture intensity and pattern are attributed to the fact that cementation in the Frio began at greater depth than cementation in the Mount Simon. Combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures (here referred to as Cf or volume of intragranular cement) allows calculation of the amount that brittle deformation influences compaction. In the Frio Formation, 0.12 to 8.37% of porosity loss due to compaction can be attributed to brittle deformation, whereas the values for the Mount Simon Formation lie between 0.25 to 2.16%. The larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing.

The Effect of Stylolite Spacing on Quartz Cementation in the Lower Jurassic Sto Formation, Southern Barents Sea

Abstract: The shallow marine Lower Jurassic quartz arenites of the Sto Formation in the southern Barents Sea comprise (1) intervals where dispersed detrital clay is absent, and where the spacing between clay-rich laminae that evolved into stylolites upon burial is exceptionally large, up to several meters, and (2) intervals where minor detrital clay matrix occurs, clay laminae are very common, and stylolite spacing is typically less than a centimeter. Point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large, 4-11%, versus 10-20% outside these intervals. There is also a correlation between distance to nearest stylolite and volume of quartz cement. Samples located a centimeter or less from a stylolite contain 10-20% quartz overgrowths, for distances of 3-20 cm quartz cement content is 4-10%, and only 3-8% when the closest stylolite is more than 20 cm distant. Modeling of quartz cementation with the ExemplarTM diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size, degree of grain coating, or content of quartz grains, i.e., the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites. On the contrary, the modeling suggests that the samples situated more than 20 cm from stylolites contain 5-8% less quartz cement than what would have been the case given a more normal stylolite abundance. This study indicates that sandstones with exceptionally few clay-rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories. However, such sandstones seem to be highly unusual on the Norwegian continental shelf. This suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures, and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation.

Facies Model of a Semiarid Freshwater Wetland, Olduvai Gorge, Tanzania

Abstract: The sedimentology and stratigraphy of a freshwater wet- land in early Pleistocene (; 1.75 Ma) volcaniclastic deposits, Olduvai Gorge, Tanzania, are characterized using texture, mineralogy, micro- fossils, and stratigraphic description to create a depositional facies model. The freshwater wetland was located on the margin of a semi- arid, closed basin containing a playa lake. The lake-margin deposits are dominated by two distinct lithologies: waxy claystones and earthy sediments. Waxy sediments are olive green, dense claystones that con- tain authigenic minerals such as trona, and represent a fluctuating saline, alkaline lake (Hay 1976). Earthy sediments are beige, friable siltstones that contain siliceous microfossils, bone fragments, and minor pebbles; they represent the freshwater wetland. Microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt-tolerant species. Associated lithofacies are: diatomite, carbonate, and sandstone. Interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland, based on ; 50 excavations, identified seven distinct sub- environments: spring deposits, perennial marsh, pool, ephemeral wet- land, fringing wetland, wetland channel, and ephemeral stream. The stratigraphic sequence, estimated to represent an ; 40-50 kyr long interval, indicates that persistent (; 10 3 yr) wet periods prevailed dur- ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands, and equally long intervening dry periods caused the lake to contract, allowing the flourishing of wetlands and accretion of earthy sediment. The lithofacies associations in the Olduvai paleo- wetland provides an important first step in the development of a de- positional environmental model for freshwater wetlands that can be tested elsewhere, particularly in arid and semiarid settings.

Numerical Forward Modeling of Carbonate Platform Dynamics: An Evaluation of Complexity and Completeness in Carbonate Strata

Abstract: It is clear that carbonate strata are complex and incomplete, but the nature and origins of the complexity, and the origin and distribution of hiatuses remain poorly understood. A three-dimensional numerical forward model of carbonate platform systems has been used to investigate how variations in complexity and completeness arise in carbonate strata. The model includes depth-dependent carbonate production occurring as a stochastic mosaic of producing elements across a topographic surface. It also includes depth-dependent sediment erosion, transport, and deposition governed by a regional transport direction that can vary stochastically through time, and by wave refraction that causes local variations in transport direction. Combined mosaic production and variable sediment transport create a complex distribution of prograding islands, generating self-organizing, shallowing-upward parasequences of variable thickness and variable lateral extent. Hiatuses occur throughout the parasequences, not just at the base and top, generated by nondeposition due to mosaic production, and by erosional scour. This erosion is triggered by variations in water depth and fetch distance, created by complex island morphologies. Preserved strata represent between 15 and 60% of total elapsed time measured at a 2 ky resolution, and much of the preserved thickness represents punctuated deposition, with depositional events spanning only a few thousand years. Stratigraphic completeness depends on subsidence rate and sediment transport rate, so that completeness generally increases with increasing subsidence rate and decreasing transport rate, but shows a nonlinear relationship with subsidence rate. This suggests that reliable estimates of completeness of ancient carbonate strata require more than knowledge of subsidence rate and sea-level change, particularly because the real situation is likely to be considerably more complex than represented in this model. The stochastic production mosaic and stochastic variations in regional transport direction create complex strata, whereas the self-organizing island progradation process creates simpler, ordered strata. A combination of the two processes creates an intermediate level of complexity, producing strata consisting of laterally impersistent parasequences with numerous erosion surfaces and complex stacking patterns and complex thickness distributions. This has important implications for parasequence correlation in outcrop and the subsurface.

Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A.

Abstract: Spheroidal (cannon ball) calcite-cemented concretions, some of gigantic size, were studied from three Cretaceous shelf sandstone units: Ferron, Frontier, and Second Frontier. The concretions have diameters between 2 cm and 6 m; those larger than 40 cm in diameter in the Ferron and Frontier have septarian structure. Rare concretions in the Second Frontier with cone-in-cone also have septaria. The pattern of carbon and oxygen isotopic data and intergranular volume (IGV) data across concretions indicate that concretion growth was complex; not all concretions grew progressively from their centers toward their edges. In many concretions displacive fascicular-optic high-Mg calcite (FOC) precipitated simultaneously throughout the concretions (pervasive growth pattern), although the earliest cementation was concentrated in and near their centers. Remaining pores were subsequently filled by poikilotopic calcite of lower Mg content. Displacive FOC cement in the bodies of concretions and in cone-in-cone structure, and the large IGV values, indicate that these carbonate phases formed at burial depths no greater than tens of meters. IGV values indicate that spar cement continued to depths of at least 50 m and possibly deeper. The association of organic matter and microbial structures, or microbial byproducts, with displacive FOC and cone-in-cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite. The oxygen isotope composition of pore waters was fairly uniform (in the range of 18O of -4 to -6‰ SMOW) during 80% of concretion growth but became even more depleted in 18O during the last stage of growth and during septarian fracture filling, ranging into values as depleted as -14‰ SMOW. 13C in calcite is negative (-4 to -18‰ V-PDB) but variable, reflecting a range of microbial processes under both oxidizing and reducing conditions. Earlier calcites fall mostly in the range of -10 to -16‰ V-PDB; relatively later calcite, in veins and on concretion margins, is more enriched in 13C. Fe and Mn concentrations in calcites have a rough positive correlation with higher 13C values. The concentration of Mg, derived from seawater and altered volcanic rock fragments, covaries positively with the degree of meteoric influence: lower Mg contents correlate positively with relatively depleted 18O values. 87Sr/86Sr values are typical of Sr derived from coeval seawater or marine skeletal debris. Growth of concretions 4 to 6 m in diameter, apparently in < 5 My, requires some spatial localization of growth sites by a self-organization process, and probably a large supply of carbonate shells in the original sediment. Elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history. Concretions formed close to the tops of parasequence boundaries.

Early Marine Lithification and Hardground Development on A Miocene Ramp (Maiella, Italy): Key Surfaces to Track Changes in Trophic Resources in Nontropical Carbonate Settings

Abstract: Phosphatic and carbonate hardgrounds occur in a Lower Miocene nontropical carbonate succession in the Maiella carbonate platform margin, located in the Central Apennines. Multiple diagenetic events occurred at or near the sea floor before the deposition of the overlying sediment and included precipitation of inclusion-rich calcite, micrite, iron oxides, and phosphates. Later diagenetic features are limited to chemical compaction and precipitation of clear calcite cement. We relate the development of these features to a two-step model, involving progressive intensification of upwelling on this carbonate margin, which was triggered by regional changes in water circulation and modulated by sea-level changes, leading initially to precipitation of inclusion-rich calcite. With an increase in trophic resources related to the paleoceanographic conditions on the ramp, increased flux of organic matter to the sea floor led to temporary formation of microbial biofilms. These conditions were associated with extensive precipitation of micrite and phosphate microspherules in the uppermost sediment. The lack of sedimentation provides the precondition to accumulate and preserve evidence for organic-matter utilization in the uppermost sediment layer. Our study suggests that hardgrounds in nontropical carbonates might be used as indicators of circulation changes and can provide a useful link to major environmental changes in the ocean-margin environment. The occurrence of microbial micrite and phosphate microspherules precipitated in the absence of sedimentation near the sea floor as a response to higher nutrient supply provides a new and important criterion to differentiate nontropical carbonate facies deposited under the influence of higher nutrient supplies rather than temperature alone. Furthermore, our study shows that local depletion in 13C at a key stratigraphic surface does not necessarily reflect meteoric exposure but may be related to microbial activity at the sea floor.

An Assessment of Stratigraphic Completeness in Climate-Sensitive Closed-Basin Lake Sediments: Salar de Atacama, Chile

Abstract: Dated subsurface cores, 40 m, 100 m, and 200 m in length, from the Salar de Atacama, Chile, record changes in climate and tectonics over the past 325 ka. These cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change. The same basic facies, efflorescent halite crust and cement (subaerial halite), and chevron halite (saline lake) occur in each core. The dominant facies is efflorescent halite formed in a subaerial environment like that in the Atacama basin today. Thick, well-preserved efflorescent crusts in the Salar de Atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines. Similar pedogenically formed evaporites may be pre served in the geologic record. The N-S trending Salar Fault System has downdropped the eastern block of the Salar de Atacama (core 2002) relative to the western block (cores 2005 and 2031). Relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium-series ages from core 2005. Major faulting along the Salar Fault System occurred between 16.5 and 5.4 ka, with offset of 26.5 m and faulting rates of 2.4 m/kyr. Aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting. Although stratigraphic units in core 2002 shifted down > 30 m over the past 60 ka, the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores. Faulting along the Salar Fault System, despite significant offset, did not alter the basic stratigraphic record. Temporal completeness of the stratigraphic record, within the limits of the age dates and their errors of greater than ± 3 ka, was examined qualitatively by comparing long-term sedimentation rates (0.6-0.9 m/kyr) and shorter-term sedimentation rates (0.3-3.6 m/kyr). The similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of Salar de Atacama sediments, at least at the 10 kyr scale. Although hiatuses, long periods of subaerial exposure, and dissolution of evaporites may have occurred, the main climate fluctuations at Salar de Atacama over the last 100 ka are recorded in all three subsurface cores. Paleoclimate records obtained from such cores are therefore representative of basin-wide climate change and are valid for regional paleoclimate reconstructions.

Sandstone petrography of continental depositional sequences of an intraplate rift basin: western Cameros Basin (North Spain)

Abstract: The Cameros Basin in Central Spain is an intraplate rift basin that developed from Late Jurassic to Middle Albian time along NW–SE trending troughs. The sedimentary basin fill was deposited predominantly in continental environments and comprises several depositional sequences. These sequences consist of fluvial sandstones that commonly pass upward into lacustrine deposits at the top, producing considerable repetition of facies. This study focused on the western sector of the basin, where a total of seven depositional sequences (DS- 1 to DS-7) have been identified. The composition of sandstones permits the characterization of each sequence in terms of both clastic constituents and provenance. In addition, four main petrofacies are identified. Petrofacies A is quartzosedimentolithic (mean of Qm85F2Lt13) and records erosion of marine Jurassic pre-rift cover during deposition of fluvial deposits of DS-1 (Brezales Formation). Petrofacies B is quartzofeldspathic (mean of Qm81F14Lt5) with P/F > 1 at the base. This petrofacies was derived from the erosion of low- to medium-grade metamorphic terranes of the West Asturian–Leonese Zone of the Hesperian Massif during deposition of DS-2 (Jaramillo Formation) and DS-3 (Salcedal Formation). Quartzose sandstones characterize the top of DS-3 (mean of Qm92F4Lt4). Petrofacies C is quartzarenitic (mean of Qm95F3Lt2) with P/F > 1 and was produced by recycling of sedimentary cover (Triassic arkoses and carbonate rocks) in the SW part of the basin (DS-4, Pen˜ - acoba Formation). Finally, depositional sequences 5, 6, and 7 (Pinilla de los Moros–Hortiguela, Pantano, and Abejar–Castrillo de la Reina formations, respectively) contain petrofacies D. This petrofacies is quartzofeldspathic with P/F near zero and a very low concentration of metamorphic rock fragments (from Qm85F11Lt4 in Pantano Formation to Qm73F26Lt1 in Castrillo de la Reina Formation). Petrofacies D was generated by erosion of coarse crystalline plutonics located in the Central Iberian Zone of the Hesperian Massif. In addition to sandstone petrography, these provenance interpretations are supported by clay mineralogy of interbedded shales. Thus, shales related to petrofacies A and C have a variegated composition (illite, kaolinite, and randomly interlayered illite–smectite mixed-layer clays); the presence of chlorite characterizes interbedded shales from petrofacies B; and Illite and kaolinite are the dominant clays associated with petrofacies D. These petrofacies are consistent with the depositional sequences and their hierarchy. An early megacycle, consisting of petrofacies A and B (DS-1 to DS-3) was deposited during the initial stage of rifting, when troughs developed in the West Asturian–Leonese Zone. A second stage of rifting resulted in propagation of trough-bounding faults to the SW, involving the Central Iberian Zone as a source terrane and producing a second megacycle consisting of petrofacies C and D (DS-4, DS-5, DS-6, and DS-7). Sandstone composition has proven to be a powerful tool in basin analysis and related tectonic inferences on intraplate rift basins because of the close correlation that exists between depositional sequences and petrofacies.

The Recycled Orogenic Sand Provenance from An Uplifted Thrust Belt, Betic Cordillera, Southern Spain

Abstract: The Betic Cordillera of southern Spain represents an uplifted foreland fold–thrust belt. Source rock types of the Betic Cordillera include metamorphic (mainly phyllite, schist, quartzite, and gneiss), sedimentary (siliciclastic and carbonate), volcanic (felsic to intermediate pyroclasts), and mantle-derived (peridotite, gabbro, serpentinite, and serpentine schist) rocks. The fluvial systems range that transect the Betic Cordillera are the major detrital source of sediment along the southern Spanish coast, supplying sand to beaches and offshore depositional systems in the Alboran Sea basin. Three key sand petrofacies derived from the Betic mountain belt reflect the main clastic contribution of known source rocks. All the sands are quartzolithic, ranging from quartz-rich to lithic-rich. Fluvial systems draining the Sierra de Los Filabres, the Sierra Nevada, the Sierra de Gador, and the Ma´laga Mountains, and their related beaches constitute a metamorphic–sedimenticlastic quartzolithic sand petrofacies (Qm34610 F463 Lt6269; Lm72614 Lv264 Ls26613), derived dominantly from the Nevado–Fila´bride, Alpuja´rride, and Mala´guide complexes. This quartzolithic petrofacies extends from northeast of Almeria to Torremolinos (southwest of Ma´laga), and northeast of Algeciras. Only one beach sand sample, east of Cabo de Gata, is volcanolithic. Volcanic detritus (mainly having felsitic textures) is derived from Miocene (15–7 Ma) pyroclastic sequences cropping out in the southeast of the chain. This metamorphic–sedimenticlastic quartzolithic petrofacies changes in the coastal stretch from Torremolinos to Marbella, where drainage systems cut across the Serrania de Ronda. Here source rock types include peridotite, gabbro, and serpentinite of the Ronda Peridotite Massif, and metamorphic rocks of the Ma´laguide and Alpuja´rride units. The fluvial and beach sands of this area are quartzolithic (Qm32612 F1063 Lt58611), and include abundant peridotite and serpentinite grains. The latter quartzolithic petrofacies changes abruptly from Algeciras to Ca´diz, where the sand becomes quartz-rich (Qm7765 F462 Lt1964). This sand petrofacies is derived predominantly from recycling of sedimentary sequences, mainly the quartzarenite turbidite units of the Gibraltar Arc (the Algeciras Flysch). This petrofacies is characterized by higher proportion of quartz grains and abundant sedimentary lithic fragments (Lm163 Lv161 Ls9863). The three onshore petrofacies plot in the recycled-orogen provenance compositional field and the lithic to transitional to quartzose recycled subfields of Dickinson (1985). They vary from lithic, to transitional and quartzose depending on their source lithologies in the Betic foreland fold–thrust belt. These actualistic petrofacies best describe the nature and distribution of sand petrofacies derived from a collisional fold–thrust belt where primary and recycled source rocks are interfingered. Deep-marine turbidites of the Alboran Basin have basinwide quartzolithic sands having close compositional relations with Betic Cordillera onshore sand petrofacies. Comparison of detrital modes from mainland to deep-marine environments provides a suitable basis for interpreting the Miocene to Pleistocene sand dispersal history in the Alboran Basin. These modern quartzolithic petrofacies are used to interpret analogous ancient collisional sandstone petrofacies of the Alpine orogenic belt of the western-central Mediterranean region and of other collisional orogenic systems, as a broader point of view.

Unraveling the Origin of Carbonate Platform Cyclothems in the Upper Triassic Dürrenstein Formation (Dolomites, Italy)

Abstract: Facies analysis of the Durrenstein Formation, central-eastern Dolomites, northern Italy, indicates that this unit was deposited on a carbonate ramp, as evidenced by the lack of a shelf break, slope facies, or a reef margin, together with the occurrence of a "molechfor" biological association. Its deposition following the accumulation of rimmed carbonate platforms during the Ladinian and Early Carnian marks a major shift in growth mode of the Triassic shallow marine carbonates in the Dolomites. The Durrenstein Formation is characterized by a hierarchical cyclicity, with elements strongly suggestive of an allocyclic origin, including (a) subaerial exposure features directly above subtidal facies within meter-scale cyclothems, (b) purely subtidal carbonate cyclothems, (c) symmetric peritidal carbonate cyclothems, and (d) continuity of cyclothems of different orders through facies boundaries. The Durrenstein cyclothems are usually defined by transgressive and regressive successions, and so most of them probably originated from sea-level oscillations. Their allocyclic origin allows their use for high-resolution correlations over distances up to 30 km. A stratigraphic section in the Tre Cime di Lavaredo area, encompassing the upper part of the Durrenstein Formation and the lower part of the overlying Raibl Formation (Upper Carnian) was studied using time-frequency analysis. A strong Milankovitch signal appeared when interference arising from a variable sedimentation rate was estimated and removed by tuning the short precession line in a spectrogram. All of the principal periodicities related to the precession index and eccentricity, calculated for 220 Ma, are present: P1 (21.9 ky); P2 (17.8 ky); E1 (400 ky), E2 (95 ky), and E3 (125 ky), along with a peak at a frequency double that of the precession, which is a predicted feature of orbitally forced insolation at the equator. Components possibly related to Earth's obliquity at ca. 35 ky and ca. 46 ky are present as well. The recovery of Milankovitch periodicities allows reconstruction of a high-resolution timescale that is in good agreement with published durations of the Carnian based on radiometric ages. The recognition of a Milankovitch signal in the Durrenstein and lower Raibl formations, as well as in other Mesozoic carbonate platforms, strongly supports a deterministic and predictable--rather than stochastic--control on the formation of carbonate platforms. Carbonate platforms might thus be used in the future for the construction of an astronomical time scale for the Mesozoic.