Showing papers in "Journal of Sedimentary Research in 2007"

Diagenesis, Porosity Evolution, and Petroleum Emplacement in Tight Gas Reservoirs, Taranaki Basin, New Zealand

Abstract: Tight gas reservoirs have become popular targets in petroleum exploration in recent years, due largely to the increasing market demand for gas and also to technology advances used in extraction. Reservoir quality is typically poor due to deep-burial diagenesis, resulting in significant compaction, cementation, and illitization. However, analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality, and these results can potentially be used to help predict reservoir quality at other sites. The use of integrated analysis of diagenetic and burial history is here shown for the Eocene, K3E Kapuni Group reservoir in Cardiff-1, Taranaki Basin, New Zealand. The K3E belongs to a deep gas play that was considered uncommercial in the early 1990's due to poor reservoir quality associated with extensive authigenic illite. Potassium-argon dating has shown that destruction of the reservoir through illitization occurred in the Pliocene, associated with a late-stage heating and/or fluid-flow event. Timing of illite authigenesis in the K3E reservoir postdates the likely circulation of CO2-rich fluids that caused reaction of feldspar to form kaolinite and quartz. Homogenization temperatures from two-phase aqueous fluid inclusions within quartz cements, together with 1D basin models of CO2 expulsion from coaly source rocks, indicate that feldspar reactions may have started in the mid Miocene. The CO2-rich fluids are thought to have been generated by thermal decarboxylation of intraformational Paleocene–Eocene coals. At the same time, oil was being expelled from older, Cretaceous coals and migrated into the mid-late Eocene reservoirs; evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil-bearing fluid inclusions. The products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar, suggesting a relatively "closed" diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved. Petrographic evidence of reservoir sandstones at Cardiff-1 demonstrates the presence of some illite-free secondary macropores lined by residual oil. This local preservation of porosity may be replicating the original distribution of oil-saturated pores in the Pliocene, at the time when authigenic kaolinite underwent reaction to form illite. Remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c. 5 Ma. The data presented in this study are consistent with redistributed liquid petroleum into up-dip traps and represent the potential for a new exploration play. The Eocene reservoir at Cardiff-1 is currently gas-charged, with the pore system composed of approximately one third large macropores connected by a tortuous, microporous network. However, it is possible that, with the currently buoyant gas market and modern hydraulic fracturing techniques, it may now be economic to flow gas from the locally large pores identified in the K3E reservoir.

Meander-Bend Evolution, Alluvial Architecture, and the Role of Cohesion in Sinuous River Channels: A Flume Study

Abstract: Realistic physical models of meandering rivers have proven extremely difficult to produce, particularly in comparison to the formation of braided rivers in the laboratory. Here we address the question of why such realistic model meanders are so difficult to reproduce, through the most realistic physical modeling of meandering channels yet achieved. This paper demonstrates that cohesion is a key variable in the development and maintenance of single-thread channels. In particular, cohesion must be sufficient to force the planform away from a braided state but low enough for active migration to continue and for the avoidance of a gradual reduction and eventual cessation of planform movement (ossification). The enhanced realism of the experiments also enables the processes of meander evolution, and critically the resultant alluvial architecture, to be examined in a physical model for the first time. Planform history can be linked to deposits, and this process–product linkage enables the depositional development of the experimental deposits to be compared against, and to test, existing models of bedding geometries within point bars. Here we document three mechanisms for bend cutoff, provide new process explanations for certain modes of bend evolution in coarse-grained meandering rivers, examine the geometries and spatial distribution of alluvial architecture, and demonstrate that existing models of point-bar geometry successfully reproduce the larger-scale aspects of point-bar accretion in rivers dominated by episodic unit-bar accretion.

Seasonal Variations in Modern Speleothem Calcite Growth in Central Texas, U.S.A.

Abstract: Variations in growth rates of speleothem calcite have been hypothesized to reflect changes in a range of paleoenvironmental variables, including atmospheric temperature and precipitation, drip-water composition, and the rate of soil CO2 delivery to the subsurface. To test these hypotheses, we quantified growth rates of modern speleothem calcite on artificial substrates and monitored concurrent environmental conditions in three caves across the Edwards Plateau in central Texas. Within each of two caves, different drip sites exhibit similar annual cycles in calcite growth rates, even though there are large differences between the mean growth rates at the sites. The growth-rate cycles inversely correlate to seasonal changes in regional air temperature outside the caves, with near-zero growth rates during the warmest summer months, and peak growth rates in fall through spring. Drip sites from caves 130 km apart exhibit similar temporal patterns in calcite growth rate, indicating a controlling mechanism on at least this distance. The seasonal variations in calcite growth rate can be accounted for by a primary control by regional temperature effects on ventilation of cave-air CO2 concentrations and/or drip-water CO2 contents. In contrast, site-to-site differences in the magnitude of calcite growth rates within an individual cave appear to be controlled principally by differences in drip rate. A secondary control by drip rate on the growth rate temporal variations is suggested by interannual variations. No calcite growth was observed in the third cave, which has relatively high values of and small seasonal changes in cave-air CO2. These results indicate that growth-rate variations in ancient speleothems may serve as a paleoenvironmental proxy with seasonal resolution. By applying this approach of monitoring the modern system, speleothem growth rate and geochemical proxies for paleoenvironmental change may be evaluated and calibrated.

Autostratigraphy: A Framework Norm for Genetic Stratigraphy

Abstract: Autostratigraphy is the stratigraphy generated by large-scale autogenic processes, and needs to be heeded because of a current overreliance on allogenic controls in sequence stratigraphy Key principles of autostratigraphy, emerging from the theory of autoretreat and a new understanding of alluvial grade, derive from the non-equilibrium stratigraphic response, ie, the general lack of equilibrium configuration of depositional systems The non-equilibrium behavior of fluvial deltas during times (T) of steady dynamic forcing leads to variable stratigraphic response that is the inevitable result of length (D) and time (τ) scaling particular to the depositional system, rather than necessarily reflecting any sudden or unsteady change in the rate of allogenic forcing Some abrupt breaks in the stratigraphic record are not necessarily associated with changes in allogenic conditions but can result from purely autogenic processes of the system When T is comparable to or longer than τ, (1) the depositional system takes the non-equilibrium, large-scale autogenic response, (2) the superposition of autogenic and allogenic components of the forcing is prominently nonlinear, and thus (3) sequence stratigraphic models that have been built on the assumption of equilibrium response are incorrect Autostratigraphic analysis makes it possible to detect and identify complex autogenic responses and unsteady allogenic events in the stratigraphic record by quantifying a temporal change in the magnitude of D Autostratigraphy thus functions as a "norm" for genetic stratigraphy

Basic Building Blocks and Process Variability of a Cretaceous Delta: Internal Facies Architecture Reveals a More Dynamic Interaction of River, Wave, and Tidal Processes Than Is Indicated by External Shape

Abstract: Although delta-building processes and the resultant facies and facies associations have been studied for over a century, there remains a lack of well-documented facies architectural studies of ancient deltas. In this study, architectural-element analysis is applied to determine the basic building blocks of a Cretaceous delta at the top of the upper Turonian Wall Creek Member exposed along the western flanks of the Powder River Basin, Wyoming. We document the bed-scale basic building blocks of this ancient delta, and show them in the context of their facies and stratal variability, and their hierarchical spatial arrangement within a single deltaic parasequence. We also test the idea that external sand-body shape may not reflect the internal facies complexity, as has recently been suggested in studies of several modern deltas, including the Burdekin in Australia, the Brazos in the Gulf of Mexico, and the Baram–Trusan in Borneo. Five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits. These elements are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally modulated deposit (TM), and bar accretion (BA). Seasonal to decadal river floods are thought to represent the main building phases of the delta, producing channels, bars, and frontal splay elements. During intervening periods, the delta was reworked by waves, storms, and tides, producing tidally modulated and storm sheet elements. Due to the complex interactions of river effluents with waves, storms and tides, the delta is interpreted as mixed-influenced. Regional sandstone isolith, facies, and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front. The channels, bars, frontal splays, and tidally modulated elements are found only near the distributary mouth, whereas the storm sheets occur extensively away from the distributary mouth. The plan-view morphology of the studied delta shows a smooth-fronted, arcuate to cuspate shape, which should indicate wave dominance. In contrast, our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking. Additional outcrop mapping shows that storm-wave-reworked sands are attached to the flanks of the system. Previously published delta models predict incorrectly that the tidal reworking of a river-flood dominated system should result in a more bird-foot shape versus the lobate geometry that is actually mapped. Our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity.

Lower Cretaceous Strata in the Lhasa Terrane, Tibet, with Implications for Understanding the Early Tectonic History of the Tibetan Plateau

Abstract: Sedimentary strata in southern Tibet indicate that upper crustal deformation occurred throughout the region during Early Cretaceous time, suggesting that construction of the Tibetan plateau commenced tens of millions of years before the Late Cretaceous–early Tertiary Indo-Asian collision. Lower Cretaceous strata in the northern portion of the Lhasa terrane are characterized by lithic-rich conglomerate beds deposited in shallow marine and meandering-river fluvial environments. Sediments in these units were derived from two primary sources: volcanic rocks associated with Early Cretaceous intrusions, and sedimentary strata eroded from the northern Lhasa and southern Qiangtang terranes. The majority of detrital zircons from Lower Cretaceous fluvial conglomerate beds in northern Lhasa have U–Pb ages between 125 and 140 Ma and provide a maximum depositional age for these units of 125 ± 2 Ma. Lower Cretaceous strata in the southern portion of the Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments. Populations of detrital zircons in Lower Cretaceous conglomerate beds in southern Lhasa have U–Pb ages between 140 and 150 Ma, 500 and 600 Ma, and 850 and 950 Ma, and provide a maximum depositional age for these units of 143 ± 2 Ma. Both the modal composition and detrital-zircon U–Pb ages of the Lower Cretaceous conglomerate exposed in northern and southern Lhasa suggest different source areas, diachronous deposition, and possibly distinct genetic histories. Throughout most of the Lhasa terrane, the Lower Cretaceous clastic strata are overlain by a widespread limestone of Aptian–Albian age that was deposited in a shallow carbonate sea containing rudist patch reefs and muddy inter-reef zones. With respect to the tectono-sedimentary setting of the Lhasa terrane during Early Cretaceous time, the sedimentological and stratigraphic data are most consistent with a peripheral foreland basin model, which is interpreted to have resulted from the collision between the northern margin of the Lhasa terrane and the southern margin of Asia (the Qiangtang terrane). Several characteristics of the Aptian–Albian succession can be attributed to a peripheral foreland basin setting, although deposition within the region may have been influenced by a combination of mechanisms. Sedimentary characteristics of Lower Cretaceous rocks in the Lhasa terrane are consistent with recent ideas suggesting that portions of southern and central Tibet were deformed and above sea level before the Indo-Asian collision.

Changes in Depositional Processes—An Ingredient in a New Generation of Sequence-Stratigraphic Models

Abstract: Existing sequence-stratigraphic models for shoreline deposits commonly assume a constant process regime throughout the relative-sea-level (RSL) cycle over a wide range of timescales (third-order cycles as well as high-frequency, fourth- and fifth-order cycles), with the possible exception that tidal processes may be more important during transgressions. However, the dominant process affecting the coastal zone is a function of multiple interdependent factors and can change at any time during high- or low-frequency RSL cycles; indeed, changes are possible on timescales as short as 1,000 years. Thus, the relative intensity of wave and tidal processes may change gradually or abruptly on a regional scale because of changes in bathymetry or coastal morphology caused by rising or falling water levels, and/or changing shelf width. The specific nature of the response varies as a function of the physiographic and tectonic setting because the attenuation or amplification of wave and tidal action is strongly dependent on local and regional bathymetry and coastal morphology. Fluvial energy may also vary with sea-level change, as a result of climate change. Moreover, whereas most facies and sequence-stratigraphic reconstructions are based on river-, wave- or tide-dominated end-member environmental models, the majority of real-world environments are of mixed-energy character where these processes coexist in subequal proportions. In such mixed-energy coastal environments, changes in the relative intensity of the depositional processes on a local scale can also cause stratigraphic variations in the nature of the deposits as the environments migrate laterally. Reconstructions of regional or local process changes are complicated by the fact that changes in the grain size delivered to the coast, as a result of systematic variations in fluvial accommodation and the degree of bypass, may cause product changes without any change in the processes. Fine and very fine sand, such as is dominantly delivered to the transgressive and highstand coastlines favors the preservation of wave-generated hummocky and swaly cross stratification, whereas medium and coarse sand, as is delivered to the falling stage and early lowstand coastlines, permits the development of current-generated (including tide-driven) cross stratification. Future sequence-stratigraphic models need to incorporate and be more sensitive to such process and product changes.

High-Resolution Facies Analyses of Mudstones: Implications for Paleoenvironmental and Sequence Stratigraphic Interpretations of Offshore Ancient Mud-Dominated Successions

Abstract: The extent of lithofacies variability in fine-grained sedimentary rocks is very poorly known in comparison to that present in coarser grained clastic and carbonate successions. The absence of this information means that sediments present on continental shelves are rarely considered as integrated systems because fine-grained facies (such as shales) are mostly excluded from sophisticated, regional facies models. To shed light upon lithofacies and grain size in shale-dominated successions, so that they can be incorporated into shelf-wide depositional models, the Blackhawk Formation of the Mancos Shale (Campanian age), exposed in the Book Cliffs, Utah was investigated using combined field, whole-rock geochemical, optical, and electron optical methods. These sediments show systematic grain-size variations, are intensely bioturbated, and composed predominantly of detrital clays (mainly dioctahedral micas), quartz, and feldspar with minor pyrite and organic matter. They are organized into very thin (< 10 mm), upward-fining, genetic beds; they exhibit both systematic lateral (103 m scale), and vertical (10−2 to 100 m scales) lithofacies variability. Preferentially cemented units occur close to sequence boundaries (unconformities). These cemented units typically contain a very different detrital assemblage (including significant quantities of chlorite) from the rest of the succession and are located close to levels where there are marked stacking-pattern discontinuities. Overall, lithofacies variability is interpreted in terms of deposition occurring on an oxic continental shelf, with the coarser-grained facies being deposited in proximal settings (offshore-transition zone), in contrast to the finer-grained units that were deposited in more distal environments (offshore zone). Storms are interpreted to have been the dominant mechanism dispersing the sediment. Once deposited, the surface layers of the sediment were intensely reworked by burrowing organisms. Preferential preservation of ichnogenera from mid- and lower tiers suggest that the depositional environment was energetic and erosion commonly removed the surface sediment layers, prior to deposition of thin (10−2 m) storm beds. The larger scale (100 m) upward-coarsening units that are composed of stacked beds, are interpreted to be parasequences. Preferential cementation, at levels where large-scale stacking patterns change, is interpreted to occur at horizons where there were breaks in sediment accumulation, and bacterial metabolic processes were able to supply sufficient solutes to fill uncompacted pore space with cement. The presence of a very different detrital assemblage in these units suggests that winnowing occurred. Despite their relatively innocuous appearance in the field, these surfaces are likely to be significant bypass surfaces, over which sediment was delivered to the deeper parts of the basin during times of relative lowstand of sea level.

Sedimentology of Acid Saline Lakes in Southern Western Australia: Newly Described Processes and Products of an Extreme Environment

Abstract: Naturally acid saline systems with pH values between 17 and 4 are common on the Yilgarn Craton of southern Western Australia A combination of physical and chemical processes here yield a previously undescribed type of modern sedimentary environment Flooding, evapoconcentration, desiccation, and eolian transport at the surface, as well as influx of acid saline groundwaters, strongly influence these lakes Halite, gypsum, kaolinite, and iron oxides precipitate from acid hypersaline lake waters Shallow acid saline groundwaters affect the sediments of the lakes and associated mudflats, sandflats, channels, and dunes by precipitating early diagenetic halite, gypsum, iron oxides, clays, jarosite, and alunite These modern environments would likely yield a rock record composed mostly of bedded red siliciclastic and reworked gypsum sand, alternating with less common beds of bottom-growth gypsum and halite, with alteration by early diagenetic features diagnostic of acid saline waters This documentation of sedimentary processes and products of modern acid saline environments is an addition to the comparative sedimentology knowledge base and an expansion of the traditional models for classifying brines Implications include better interpretations of terrestrial redbeds and lithified martian strata, improved acid remediation methods, new models for the formation and occlusion of pores, and the new setting for finding previously undescribed extremophiles

A Review and Synthesis of Glendonites (Pseudomorphs after Ikaite) with New Data: Assessing Applicability as Recorders of Ancient Coldwater Conditions

Abstract: Calcite pseudomorphs after ikaite (glendonite) are associated with coldwater depositional systems, including glaciomarine and deepwater settings, as dictated by the limited stability field of ikaite. Ikaite precipitation is favored by elevated alkalinity and dissolved phosphate, conditions encountered commonly in association with organic-rich marine sediments where methane oxidation is occurring. The rapid recrystallization of ikaite to calcite during slight warming or pressure release results in considerable solid volume loss, producing a highly porous crystal mesh. Preservation of the original ikaite crystal form requires precipitation of diagenetic calcite cement during early burial to prevent compaction and collapse of pseudomorph structures. During later burial diagenesis remaining pore space may be filled with deeper burial calcite cement. Glendonites from the Permian of the Sydney Basin occur in subtidal shelf facies containing glacial dropstones and a normal marine fauna. Stable oxygen isotope signatures of modern ikaite suggest carbonate precipitation in equilibrium with ambient seawater; carbon signatures are usually strongly negative relative to normal marine carbonate, consistent with derivation of carbonate from methane oxidation. Review of published data suggests that while Holocene glendonite may provide reliable isotopic records of the conditions of ikaite precipitation, precipitation of later calcite cement within the glendonite structure reduces the significance of the isotopic signature as an indicator of primary depositional conditions. Bulk glendonite samples from the Permian Sydney Basin, Australia, have a broad range of δ18 O and δ13C (δ18OPDB = −5 to −15‰; δ13C = −8 to −16‰), in contrast to the narrow range for brachiopod carbonate (δ18OPDB = +1 to −5‰; δ13C = +5 to +7‰) from the same strata. Handpicked separates of "primary" glendonite and secondary calcite also have a wide range of stable isotope values. The data from Sydney Basin glendonites indicate that diagenetic precipitation of calcite has blurred the isotopic signature of primary ikaite replacement calcite at the scale of micosampling done in this study.

Age Controls on the Tanqua and Laingsburg Deep-Water Systems: New Insights on the Evolution and Sedimentary Fill of the Karoo Basin, South Africa

Abstract: Improved constraints on the timing of Permian clastic sedimentation in the Karoo basin of South Africa have been revealed by new U-Pb single-grain zircon ages determined using the Sensitive High Resolution Ion Microprobe Reverse Geometry (SHRIMP-RG). Analyses were performed on 180 zircon grains recovered from six ash beds exposed in the southwestern part of the Karoo basin. Samples were collected from two distinct loci of deep-water basin-floor and submarine-slope deposition within the Permian Ecca Group: the Tanqua and Laingsburg depocenters. Previous work constrained depositional ages of the 1800-m-thick deep-water to fluvial succession to 270–255 Ma. This poor chronostratigraphic resolution precluded robust stratigraphic correlation between the depocenters and impeded interpretation of basin evolution. The new data suggest that deep-water siliciclastic deposition began ~ 275 Ma (the lower Permian Collingham Formation), while the youngest ages are latest Permian. The Tanqua and Laingsburg depocenters began sand deposition at different times. Sandstone deposition in the Laingsburg depocenter started sometime after 275 Ma, and deposition of the first thick turbidite-dominated succession (Fan A) is bracketed into a period of less than 20 My. The Tanqua depocenter was sand starved during this interval. Both the uppermost sample from the Laingsburg depocenter and the lowermost from the Tanqua depocenter yield similar maximum depositional ages. This indicates, within the resolution of the age data, synchronous submarine sandstone deposition in the two loci at this time (~ 255 Ma). These data indicate that submarine deposition was still ongoing at a time when fluvial floodplain conditions are interpreted elsewhere in the Karoo basin. All the ash samples contain late Paleozoic zircon-grain age populations, while three samples contain small numbers of early Paleozoic, Proterozoic, and Archean zircon grains. Paleozoic zircon-grain age populations are interpreted as a record of arc activity linked to subduction. Increasing numbers of Precambrian zircons stratigraphically upward provide insight into changing source terranes during deposition into the Karoo basin.

Seismic Geomorphology and Evolution of Submarine Channels from the Angolan Continental Margin

Abstract: Three-dimensional seismic data from the shallow subsurface of the continental margin offshore Angola reveal two end-member morphological styles of submarine channel: (1) high-gradient, low-sinuosity, narrow channels with gull-winged levees, and (2) lower-gradient, deeply incised systems with moderate- to high-sinuosity channel axes. A third, and rare, channel form has moderate incision, low to medium sinuosity, and a moderate long-profile gradient. Based on channel parameters (incision depth, long-profile gradient, channel-axis sinuosity) and crosscutting relationships, we suggest that the channels evolved from initially steep and straight, with low sinuosity, to highly sinuous and deeply incised with lower channel-axis gradients. Correlation of long-profile gradient with both incision and sinuosity suggests that incised channels appear to remove convex-up curvature from the original slope as the channel axis evolves toward an equilibrium profile. Localized changes in channel planform, gradient, sinuosity, and incision reflect the complex morphology of the slope associated with growth of salt-related structures. Linear, high-amplitude seismic features, which correspond to weakly incised striations, or rills, on the open slope are considered to be precursors of submarine channels.

Continental Sequence Stratigraphy of the Upper Triassic (Norian–Rhaetian) Chinle Strata, Northern New Mexico, U.S.A.: Allocyclic and Autocyclic Origins of Paleosol-Bearing Alluvial Successions

Abstract: Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior of the southwestern United States were evaluated using an adapted marine stacking-pattern analysis methodology. A three-tier cyclic hierarchy is present in the strata at both study areas. Meter-scale fining-upward fluival aggradation cycles (FACs) comprise fluvial aggradational cycle sets (FACSETs) 4–15 m thick (avg. 8.4 m). FACSETs in turn stack into four fluvial sequences 26–48 m thick (avg. 41 m). Within these sequences, transgressive-systems-tract equivalents (TE) are characterized by channel sands and associated minor overbank deposits and relatively immature paleosols (i.e., high rates of deposition), whereas highstand- to falling-stage-systems-tract equivalents (HFE) are dominated by overbank muds and relatively well-developed paleosols (i.e., lower rates of deposition). These two fluvial successions, which are 200 km apart, contain age-equivalent fluvial sequences that record similar histories of deposition and pedogenesis: Sequence 1 contains only an incomplete HFE; Sequence 2 includes both the TE and HFE; Sequence 3 is an HFE; and Sequence 4 contains only a TE. Fluvial sequences likely accumulated in response to pulses of source area uplift and/or basin subsidence, which resulted in changes in accommodation. Conversely, higher-frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the products of autocyclic processes, such as channel avulsion, floodplain aggradation, and channel migration. These results suggest that regionally significant tectonic episodes may be discernible in suspended-load fluvial deposits that accumulated over a broad area.

Fluvial Fans: Myths, Misconceptions, and the End of the Terminal-Fan Model

Abstract: We propose that the so-called "terminal fan" facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and poor communication. Rivers in drylands may experience excessive down stream discharge reduction such that they terminate subaerially rather than reach the sea or a lake. The facies model predicts that the distal reaches of such rivers form a network of bifurcating distributary channels producing a fan-shaped sediment body, with downstream thinning and fining of sedimentary units, ending in sand-filled ribbons encased in mud. Extensive review of modern rivers has failed to turn up convincing examples that fit the model. Rivers in drylands do not ubiquitously end in fans. Fan-shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion. Channels on such fans generally do not repeatedly bifurcate downstream. Where they are seen to do so, it can usually be shown they are lacustrine deltas inherited from wetter times. The term "distributary" is being used carelessly and is conveying incorrect understanding of sediment geometry and architecture. The proposed synonym of "fluvial distributary systems" is unsatisfactory as it perpetuates the same misunderstandings. Reliance on planform alone in analogue selection is highly risky. The fluvial fan is a composite sediment body resulting from frequent nodal avulsions in a setting without horizontal constraints. Channels on fans range in planform as much as any other river. The resultant sedimentary record differs little from that expected from non-fan fluvial systems except having a regionally radiating orientation when viewed over geological time scales. Contrary to the implications of the facies model, there is no distinctive "terminal fan" sedimentary succession.

Quantitative Seismic Geomorphology of Pliocene and Miocene Fluvial Systems in the Northern Gulf of Mexico, U.S.A.

Abstract: Quantitative seismic geomorphology (QSG) is a new direction in the analysis of seismic data that will create a step-change in our knowledge, characterization, and understanding of older clastic environments. QSG is defined as a quantitative analysis of landforms, imaged in 3-D seismic data, for the purposes of understanding the history, processes, and fill architecture of a basin. Built upon a foundation of seismic stratigraphy and sequence stratigraphy, QSG uses 3-D seismic data integrated with core and logs to investigate the nature and architecture of reservoirs through quantitative data collection on the morphometrics, and through analyses of the spatial and temporal variability of reservoirs. In the northern Gulf of Mexico (Vermillion Island and South Marsh Island) shelf study area, 902 km2 of 3D seismic and 156 well-logging suites were analyzed to examine environments that included fluvial, deltaic, and shallow marine, as well as deeper-water shelf-edge, slope, and fan systems. Parameters measured include element sinuosity, meander wavelength and arc height, channel, and valley width:thickness, meander-belt width, and rates and directions of meander migration. Lithologic data from well logs and core, as well as relationships derived through experimental and modern-systems study of similar deposits, have been used not only to ascertain relationships between morphic and petrophysical character but also to predict spatial distribution of reservoir elements. Three specific fluvial-incision classes—aggradational fluvial systems (Class 1), bypass fluvial systems (Class 2), and creeks and distributaries (Class 3)—were examined through QSG methods and were found to show their own unique sinuosity, channel widths, meander lengths and meander-belt widths. Volume of shale calculated from well data shows Classes 1 and 2 to each be unique in their fill type and show the width:depth ratios to have some correspondence to net sand. These results suggest an ability to utilize seismic facies morphometrics to identify fill type within fluvial incisions, similar to techniques employed in the classification modern fluvial systems.

Evolution of Turbidity Currents Deduced from Extensive Thin Turbidites: Marnoso Arenacea Formation (Miocene), Italian Apennines

Abstract: Previous work has shown that large-volume (> 100 km3) turbidity currents are capable of depositing extensive (> 100 km) turbidites in non-channelized settings. This study of the Miocene Marnoso Arenacea Formation demonstrates that smaller-volume (< ~ 0.5 km3) turbidites can also be remarkably extensive. Some thin (< 40 cm) turbidites extend for 120 by 30 km within this non-channelized sequence. Flows that produced these thin turbidites were capable of traversing the basin plain in opposing directions, despite being too weak to cause significant substrate erosion. Some flows traveled upslope for at least 60 km. Long run-out distances result from the relatively large fraction of fine particles in these highly efficient flows. Sandstone intervals thin in an approximately exponential fashion, across the proximal 20 to 30 km of outcrop. Similar sandstone shapes are observed for flows that traversed the basin plain in opposite directions. This sandstone shape is a generic property of spatially depletive flows; it is consistently reproduced by laboratory and numerical models in which velocity, sediment concentration, and deposition rates decrease down-flow. Rates of sandstone thinning are orders of magnitude smaller than those observed in deposits from laboratory experiments. This discrepancy primarily reflects a low ratio of flow thickness to sediment settling velocity, and low rates of sediment suspension from the bed, for slower-moving laboratory flows. The distal part of the thin-bed sandstone interval is thinner (< 5 cm) and more tabular. Starved ripples suggest that this distal tabular sandstone shape results partly from bedload reworking. The thickest turbidite mudstone occurs in the same two locations, for flows that traveled across the basin in opposite directions. This suggests that substantial volumes of muddy fluid ponded within bathymetric lows. Abrupt lateral termination of two mudstone beds, which have a distinctly tabular shape, suggests that certain mudstone intervals were deposited by dense fluid mud layers rather than expanded turbulent suspensions.

A Comparison of Nanometer-Scale Growth and Dissolution Features on Natural and Synthetic Dolomite Crystals: Implications for the Origin of Dolomite

Abstract: This paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite was formed by replacement of calcite at 218°C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant, dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.

Cenomanian–Turonian Coastal Record in SW Utah, U.S.A.: Orbital-Scale Transgressive–Regressive Events During Oceanic Anoxic Event II

Abstract: The Cenomanian–Turonian interval of the Sevier foredeep, western U.S.A., is examined in order to (1) establish a high-resolution stratigraphic framework for marginal-marine strata of this interval and (2) test for the existence of high-frequency (tens of kyr-scale) cycles of continental runoff or sea-level change predicted by the hemipelagic record and climate models. High rates of sediment accumulation in marginal-marine environments of southwestern Utah (up to 210 m/Myr, compacted) and a northward translation of the major Sevier thrusting made possible the preservation of a highly detailed record of shoreline movements. The coeval Bridge Creek Limestone, linked with the study interval using biostratigraphic and bentonite-stratigraphic data of previous authors, provides an unprecedented, high-resolution orbital time scale. Three orders of transgressive–regressive cycles defined as genetic sequences are identified in the upper Cenomanian (S. gracile and N. juddii Zones) through lower Turonian (W. devonense through M. nodosoides Zones). The longest sequence (S. gracile Zone through V. birchbyi Zone) spans approximately 800 kyr and is penecontemporaneous with the δ13Corg positive excursion that defines Oceanic Anoxic Event II (OAE II). Medium-term and short-term sequences show durations of c. 65–160 kyr and c. 20–40 kyr, respectively. Features suggesting regression due to relative sea-level fall are described from some of the 20–40 kyr cycles in the lowermost S. gracile Zone (possibly including the uppermost M. mosbyense Zone). The data provide the first physical evidence globally of Cenomanian–Turonian changes in shoreline position and relative sea level, whose recurrence interval was as short as a few tens of kyr. These processes provide a viable depositional link between the rhythmic deposition of the Bridge Creek Limestone and the primary orbital forcing of insolation and climate. Although the possible tectonic influence is difficult to unravel, the study area represents an important reference point for climate and oceanographic modeling of the Cenomanian–Turonian greenhouse and OAE II.

Evolution of the Late Pleistocene–Holocene Dead Sea Basin from Sequence Statigraphy of Fan Deltas and Lake-Level Reconstruction

Abstract: Patterns of sediment transport to the Dead Sea basin during late Pleistocene and Holocene times were evaluated by comparing previously established lake levels with patterns of transgressions and regressions in fan deltas at the lake margins. While the lake-level history reflects the hydrological regime in the drainage basin, the transgression–regression patterns reflect the response of the clastic sediment transport to the regional hydrology and morphology of the basin. By using the two techniques we are able to demonstrate the time relationship between the factors controlling the deposition. Six sequences are exposed at the western margins of the basin. Their chronology was obtained by U–Th, radiocarbon, and luminescence ages on aragonite and quartz sand from the Samra, Lisan, and Ze’elim formations, which cover the past ~ 160 ka. Our data shows a lag between periods of high water inflow and clastic sediment flux to the basin. During highstands the sediments were stored in deep (100–200 m), long estuaries that formed in incised canyons into the east and west bounding escarpments. During lake-level falls the stored sediments were flushed out, forming prograding lowstand fan deltas. This leads to a high sediment flux during early stages of arid conditions following extreme high lake levels.

3-D Architecture and Sequence Stratigraphic Evolution of a Forced Regressive Top-Truncated Mixed-Influenced Delta, Cretaceous Wall Creek Sandstone, Wyoming, U.S.A.

Abstract: Ground-penetrating radar (GPR) is used to delineate the 3-D facies architectural elements and examine the evolution of a top-truncated, forced-regressive, mixed-influenced delta front in the Cretaceous Wall Creek Member of the Frontier Formation, Wyoming. The GPR data provide a bridge between outcrop facies architecture and recently published high-resolution 2-D seismic studies of Quaternary delta systems. The GPR data were integrated with outcrops, photomosaics, cores, and GPS data. Two orthogonal grids of 2-D GPR profiles provide information on the 3-D facies architecture and stratigraphy of the deltaic deposits. Three main GPR architectural elements are identified within the delta front: (1) mouth bars are characterized by seaward-dipping reflections (foreset beds) in dip view, and mounded bidirectionally downlapping reflections in strike view; (2) shallow delta-slope channels show truncation and onlap and also show low-angle landward (northwest) dipping reflections. The mouth bars are locally interrupted by (3) sub-horizontal radar reflections that are interpreted as tidally modulated bars. Within the mouth-bar radar facies, both top-preserved (proximal) and top-truncated (distal) examples are observed. Top-preserved proximal bars show evidence of mounded bar crests and landward accretion, as is observed in the upstream end of mouth bars on the modern Atchafalaya Delta in the Gulf of Mexico. Distal bar deposits primarily comprise the top-truncated, seaward-dipping foresets. The GPR interpretation shows two laterally overlapping delta lobes, interpreted to represent autocyclic delta switching. The older lower lobe forms a fan-shaped delta lobe composed of top-truncated coalesced mouth bars. The second phase of bars is largely top-preserved, building over the older bars. The presence of thicker bedsets and upstream accretion suggests that the younger lobe was quickly abandoned, as seen in the overlying transgressive ravinement. Although marine erosion during transgression was less effective on these later-stage bar deposits, ravinement increased landward. A comparison of the architectural elements at a more regional scale in two separate Wall Creek delta lobes (Murphy Creek and Raptor Ridge, ~ 30 km apart) indicates that the bar complexes were deposited on the delta fronts as accretionary forced-regressive deposits during a relative sea-level fall; the older river-dominated deltaic deposits at the Murphy Creek site prograded southeastward (~ 124°), whereas the younger and more tidally influenced Raptor Ridge bar deposits expanded southward (~ 171°) before final abandonment and regional transgression. The most distal lowstand lobe sediments within the offlapping parasequence at Raptor Ridge are more completely preserved and also show the greatest tidal influence, compared to the more severely top-truncated Murphy Creek, which is in an otherwise more landward location. This challenges the notion that tidal facies are predominantly associated with transgressive or highstand systems tracts. Greater preservation of deltaic facies in forced-regressive and lowstand deltas, as shown here, has also been documented in Quaternary shelf-edge deltas, such as the Lagniappe in the Gulf of Mexico and the Mahakham in Kalimantan.

Anatomy and Evolution of a Slope Channel-Complex Set (Neoproterozoic Isaac Formation, Windermere Supergroup, Southern Canadian Cordillera): Implications for Reservoir Characterization

Abstract: A detailed architectural analysis was conducted in Isaac Unit 5 of the Isaac Formation in the Castle Creek area (east-central B.C., Canada). Isaac Unit 5 developed within a turbidite-dominated slope system on the Neoproterozoic passive margin of western North America where sediment gravity flows and mass movements were common. Isaac Unit 5 crops out over a 3.5-km-long section oriented oblique to mean paleoflow direction and represents a long-lived pathway for transport and deposition that accumulated ~ 100 m of mostly sand in a deep-marine slope setting. It consists of three stacked, high net-to-gross channel-complex fills (8–30 m thick) that correspond to shorter-term flow conduits, which, in turn, are capped by mudstone-rich units. Fine-grained conglomerate and sandstone beds deposited from high-concentration sediment flows constitute most of channel-complex fills. In addition, muddy debrite and slump deposits occur, but although laterally extensive, are volumetrically minor. Laterally persistent, thin-bedded strata (4–20 m thick) composed mostly of Bouma Tc–e turbidites occur at the top of each channel complex and indicate episodes of local complex abandonment (interchannel complex deposition). Different kinds of channel-fill elements were identified within Isaac Unit 5, each characterized by a unique combination of facies assemblage, internal geometry, and bounding surfaces. Most commonly, channel fills consist of amalgamated, thick-bedded, normally graded sandstone (Bouma Ta and Tab divisions) associated with backfilling processes. Poorly stratified mudstone-clast breccia, associated with Ta beds and dune cross-stratified sandstones occur in channel fills that exhibit aggradational and laterally migrating stacking patterns. Inner-bend levee deposits are associated with this type of channel fill. Channel fills with inclined sandstone- and mudstone-rich strata, on the other hand, relate to non-aggradational, high-sinuosity channel conditions that developed during gradual abandonment of the pathways for coarse-sediment transport. In contrast, sudden deactivation of these pathways, most probably related to abrupt updip channel avulsion, led to accumulation of structureless sandstone passively filling the deactivated thalweg. In terms of hydrocarbon reservoir analogues, each of the five different channel-fill elements have unique reservoir attributes (connectivity and continuity), amalgamated and poorly stratified elements having the best (excellent to good) reservoir attributes. Due to high amalgamation at channel-fill scale, channel complexes would represent individual fluid-flow units where only laterally discontinuous permeability barriers (< 500 m long) are present. On the other hand, extensive thin-bedded elements and muddy debrites constitute kilometer-scale barrier-type facies that would effectively compartmentalize channel complexes within the channel-complex set. Strata of Isaac Unit 5 document the detailed stratigraphic complexity, evolution, and reservoir characterization that can be expected in turbidite-dominated slope channel systems developed on passive margins. Further, it is a potential analogue for similar systems developed in continent-margin basins that until now were known mostly from subsurface core and seismic data.

A Proposed Integrated Multi-Signature Model for Peritidal Cycles in Carbonates

Abstract: A new integrated multi-signature model for meter-scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record. Meter-scale shallowing-upward (micro)facies cycles are the most common type of peritidal parasequence. Peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models. However, the existence of meter-scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on (micro)facies and stacking pattern taken from previous studies. Nevertheless, the shallowing-upwards (micro)facies interpretive template remains widely used to identify these cycles and parasequences. We propose that the sedimentary signatures of meter-scale peritidal cycles vary systematically during the various phases of an underlying 3rd-order relative sea-level cycle. Although during specific phases of a 3rd-order relative sea-level cycle meter-scale peritidal cycles may be characterized by shallowing-upward units (e.g., early transgressive systems tract), during other periods the stratigraphic signal of cyclicity may be more cryptic and complex, causing them to be overlooked (e.g., late transgressive systems tract and highstand systems tract). We also propose that there is a half-cycle phase shift in periods of deposition and the formation of drowning surfaces, relative to 4th- to 5th-order relative sea-level cycles, over the course of a 3rd-order relative sea-level cycle. These points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity. Specifically, parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th- to 5th-order relative sea-level cycles at different stages of a 3rd-order relative sea-level cycle. We propose testable, new physical stratigraphic signatures for identifying meter-scale cycles that have consistent genetic significance, each cycle recording a relative sea-level cycle characterized by a 4th- to 5th-order relative sea-level rise and fall. Meter-scale peritidal cycles are recorded by an ordered pattern of systematically varying (micro)facies signatures related to their position within a 3rd-order relative sea-level cycle. The integrated multi-signature model of meter-scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record. As a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd-order relative sea-level changes, a redefinition of parasequence is required and one is presented here.

Seaward-Branching Coastal-Plain and Piedmont Incised-Valley Systems Through Multiple Sea-Level Cycles: Late Quaternary Examples from Mobile Bay and Mississippi Sound, U.S.A.

Abstract: Most incised valleys become more organized seaward through tributaries merging with the main trunk valley. Late Quaternary incised valleys on the Mississippi and Alabama inner continental shelf branch seaward, although they do coalesce towards the shelf break where they feed shelf-edge deltas. To link fluvial systems with their associated previously mapped incised valleys on the shelf, and evaluate the contribution of small coastal-plain valley systems to the lowstand systems tract, high-resolution seismic data and cores were collected from Mobile Bay, eastern Mississippi Sound, and the Mobile bay-head delta. These data show four unconformity-bounded stacked units, and this study focuses on the upper two regionally mappable units. The upper two unconformities were sampled in core as exposure surfaces, and, on the basis of stratigraphic position, depth of incision, and 14C dates, are interpreted as sequence boundaries. The shallowest sequence boundary (A) formed in response to the Oxygen Isotope Stage 2 sea-level lowstand, while the underlying sequence boundary (B) formed in response to an earlier lowstand (Stage 6?). A map of Sequence Boundary B shows a network of seaward-branching valleys, 20–35 m deep and 0.6–7.7 km wide, that extend across Mobile Bay and Mississippi Sound. These valleys, which are bounded by well-developed terraces, are extensions of the piedmont Mobile fluvial system and the coastal-plain Fowl and La Batre fluvial systems. A map of the Stage 2 Sequence Boundary shows that all systems generally reoccupied their previous valley positions and are separated by interfluve plateaus. Valley fill between Sequence Boundary B and the Stage 2 Sequence Boundary is generally composed of a basal unit of alluvial sediments overlain by bay-head delta deposits; however, Stage 2 to 1 valley fill is composed entirely of central-basin sediments. The presence of bay-head delta deposits on the inner shelf indicates this depositional environment backstepped across the estuaries to modern positions. Backstepping likely occurred as the low-gradient paleovalleys were inundated, resulting in rapid rates of transgression. Fluvial gradients measured on Sequence Boundary B and the Stage 2 Sequence Boundary, below the modern Mobile bay-head delta plain and the estuaries, are very low (1.3–0.3 m/km). The Mobile bay-head delta and upper Mobile Bay are confined by steep topography, which opens basinward into the low-gradient shorelines surrounding lower Mobile Bay and Mississippi Sound. During falling sea level, channel branching likely occurred as the low-gradient fluvial systems extended basinward beyond the confined topography. These channels incised as sea level fell below the steeper shelf break during the lowstand. Stage 2 valley morphology is partially influenced by the position of the underlying Stage 6 valleys. Both coastal-plain and piedmont valley systems exhibit compound fill, indicating that differentiation between these types of incised valleys cannot be based on valley-fill architecture alone.

Recognition of the Sedimentary Architecture of Dryland Anabranching (Anastomosing) Rivers

Abstract: The application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable. From modern observations, it appears that anabranching (anastomosing) rivers can be divided into separate humid and dryland classes. Each class has dissimilar characteristics to the other, necessitating different criteria for their recognition in the rock record. The criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions, heavily biased to a few specific cases in Canada. These criteria are dominated by the concept of ribbon sandstone bodies encased in fine-grained floodplain sediment, with an overall low sand-to-mud ratio in the succession. There has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected. Avulsion or obtrusion (the gradual formation of an alternative channel) are the characteristic dynamic of anabranching rivers, forming and maintaining coexisting multiple anabranch channels. Avulsion or obtrusion require stream banks to have a high resistance to erosion, usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats. In some drylands, riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content. Here, channel fills are mud dominated and floodplains have a uniform fine-grained character which is usually structureless because of intense pedoturbation and some bioturbation. The evidence from such present-day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud-rich beds with weak or enigmatic fluvial indicators. Elsewhere, dryland anabranching systems are sand-dominated, their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table. Though the stratigraphy of these has barely been investigated, they are clearly different from humid-region anabranching systems, for their channels and islands are predominantly sandy. Anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands. The interpretation of a dryland origin for ancient stratigraphic successions with this form is made, therefore, without evidence that they can actually exist. In the light of this, such interpretations should be reevaluated.

Climatic Conditions and Hydrological Change Recorded in a High-Resolution Stable-Isotope Profile of a Recent Laminated Tufa on a Subtropical Island, Southern Japan

Abstract: The stable-isotope composition of a tufa collected on subtropical Miyako Island, southern Japan, records climatic and hydrologic changes over a 15-year period (1989–2003). A total of 450 subsamples taken at increments of 0.2 mm define the high-resolution isotopic profile of a 9-cm-long sample. The oxygen isotope values clearly exhibit cyclical changes that generally correlate with the annual lamination pattern of the tufa. The observed stability of water δ18O values confirms that the seasonal records of water temperature are reflected in the cyclic change of tufa δ18O values. The oxygen isotope profile also exhibits a long-term decreasing trend that is equivalent to a 2.5°C temperature increase over the 15-year period. The range of the long-term change in δ18O values is at least partly compensated for by a warming trend of 0.95°C seen in metrological data for Miyako Island. However, the carbon isotope profile does not vary cyclically, and the values are clearly higher than for previously reported tufas from the temperate Japanese mainland. Vegetation on Miyako Island, which is dominated by C4-type sugar cane, is largely responsible for the high δ13C values of the tufa. The high-resolution isotopic profile provides data on the annual deposition rate, which clearly increased from 1996 to 1998, when the groundwater table was raised by the construction of two underground dams near the tufa locality. Increased water flow from the spring activated calcite precipitation and increased the depositional rate in the upper part of the core up to > 1 cm/year. This rate is much larger than those recorded for tufas from temperate settings in Japan.

Laurentian Sources for Detrital Zircon Grains in Turbidite and Deltaic Sandstones of the Pennsylvanian Haymond Formation, Marathon Assemblage, West Texas, U.S.A.

Abstract: U–Pb ages of individual detrital zircon grains from deltaic and turbidite sandstones of the lower to middle Pennsylvanian Haymond Formation (Marathon basin, west Texas) range from 320 Ma to 3 Ga, indicating a mixed provenance broadly similar to that of Ouachita Carboniferous turbidites and related fluvio-deltaic facies of Arkansas–Oklahoma. Differences with the Ouachita assemblage are attributed to the unique depositional setting of the Haymond Formation, which records final closure of a Hercynian-age remnant ocean basin at the western termination of the Ouachita–Marathon suture zone. Tectonic and sedimentary recycling along subduction zones separating Gondwanan (upper plate) and Laurentian (lower plate) blocks likely contributed to the Laurentia-dominated distribution of age provinces observed in the zircon population. Detrital-zircon age spectra are nearly identical for both facies of the Haymond Formation, reinforcing the hypothesis that subaerial fan or braid delta sandstones, and foredelta submarine-ramp turbidites, were part of an integrated depositional system fed by the same sources.

Degradative Calcification of a Modern Siliceous Sponge from the Great Bahama Bank, The Bahamas: A Guide for Interpretation of Ancient Sponge-Bearing Limestones

Abstract: Organic colloids are involved in the early calcification of the modern siliceous sponge Spheciospongia vesparium (Lamarck, 1815) from the Great Bahama Bank. Electron microscopy and in situ fluorescence microspectometry studies indicate that colloids attached within or onto a collagen network promote the precipitation of aragonite crystals in these sponges. Calcification occurs within those portions of the sponge that are buried in the sediment, preferentially in regions of agglutinated sediment particles, with sponge connective tissue being subjected to necrosis and significant degradation of the extracellular collagen matrix. The dismantling of collagen bundles leads to collagen scaffolds, which act as a sorbent and have significant adhesive effects for ions and/or organic colloids. Bacteria and other microorganisms support tissue degradation, but neither act as a substrate for aragonite precipitation or are present in significant numbers at calcification sites. This process of early calcification may explain the origin of fossil calcified siliceous sponges ("sponge mummies" and "tuberoids") as well as the occurrence of patchy calcified sponge materials, thereby calling into question the commonly accepted idea that pelletoidal texture associated with these fossil sponges indicates that bacteria are directly responsible for the calcification.

Process-Like Modeling of Flank-Margin Caves: From Genesis to Burial Evolution

Abstract: A major challenge in groundwater and petroleum modeling is how to represent the diagenesis effect "sensu lato" and its associated features (e.g., karst cavities, porosity and permeability characteristics). Both geometrical descriptions and genesis are necessary for modeling diagenetic features, because they allow the restitution of phenomenon in subsurface settings where they cannot be fully observed. This study focuses on flank-margin caves developed on carbonate islands (as in the case of the Bahamian islands). Over the last twenty years numerous measurements and observations have been carried out on Quaternary carbonate islands. These, together with reasonable hypotheses of genesis (fresh-water lens dissolution), make island karst an appropriate subject for developing concepts and modeling methods which can be then applied to other diagenetic features, such as reflux dolomitization and hydrothermal fluid effects. The speleogenesis of flank-margin caves was modeled following deterministic and stochastic methods to achieve a description of dissolution genesis. A fresh-water lens surface was built based on the topography of Long Island, Bahamas, and was then used to simulate virtual development of flank-margin caves at several formation phases, including uncertainty coefficients applied to both fresh-water-lens location and stability time by using 3D probability cubes. The cave simulations were compared with over sixty Bahamian cave surveys, in order to substantiate our modeling processes and results. Finally, the model was exposed to a cave collapse situation. The likelihood of preservation and the extent of the area affected were measured to provide an evaluation of the spatial distribution of associated petrophysical characteristics (e.g., porosity and permeability). The results reveal close resemblance and coherence between modeled and field data (including collapse features). Therefore the optimum method for restituting diagenetic features is by stochastic modeling actively driven by process concepts and most up-to-date methods.

Turbidite systems in the inner forearc domain of the Hikurangi convergent margin (New zealand): new constraints on the development of trench-slope basins

Abstract: The development of trench-slope basins on subduction wedges is strongly related to subduction processes. Theseconfined basins form on the lower trench slope, and their edges consist of structurally controlled linear bathymetric highs. In this particular setting, sedimentary processes may be controlled both by tectonism and by sea-level changes. Thus, the study of the sedimentary record may permit evaluation of the spatial and temporal contribution of tectonic activity on the development and stratigraphic evolution of trench-slope basins. The recent evolution of the Hikurangi subduction wedge (North Island of New Zealand) is characterized by the uplift and subaerial emergence of the trench­slope break, which has provided good exposure of the Miocene Akitio trench-slope basin. Identification of the main sedimentary discontinuities, reconstruction of the geometry of the sedimentary units, and paleocurrent studies were undertaken in the Akitio basin in order to propose a stratigraphic and structural scheme for the evolution of this emerged trench­slope break. Deep-sea facies associations are representative of three main fine-grained gravity-driven systems that may develop during various stages of development of mature trench-slope basins. These are, from base to top, large submarine slides (olistostrome deposits), fine-grained sand-rich submarine fans, and low-gradient submarine ramps (sheet-like turbidites). This basin-fill megasequence constitutes a lowstand systems tract, which can be subdivided into basin-floor fan, slope fan, and progradingwedge complex. Slope facies associations are common and highlight the important contribution of slope processes to the basin sediment budget. Rapid shifts of facies and important paleoenvironmental changes, reflecting slope creation or modification, characterize five major discontinuities that are related to regional tectonic events (i.e., onset of subduction at 25 Ma, beginning of regional subsidence at 15 Ma) or local tectonic events (i.e., local uplift of structural edges of the Akitio trench-slope basin at 17.5 Ma and 16.5 Ma, acceleration of subsidence at 13.2 Ma). Moreover, we show that the timing of development of some lowstand system tracts (i.e., of slope fans and of prograding-wedge complexes) may also be controlled largely by changes in style and/or amplitude of tectonic activity.

Siliciclastic Sediment Across the North Queensland Margin (Australia): A Holocene Perspective on Reciprocal Versus Coeval Deposition in Tropical Mixed Siliciclastic–Carbonate Systems

Abstract: According to conventional models of reciprocal sedimentation for tropical mixed siliciclastic–carbonate systems, shedding of carbonate material dominates slope and basin sedimentation during transgression and highstand while siliciclastic deposition dominates during lowstand. This understanding permeates the stratigraphic literature and is the accepted depositional model on the Great Barrier Reef (GBR) margin. The results of this study, however, document coeval carbonate and siliciclastic highstand deposition on the GBR slope and basin. Seafloor sediment from Queensland Trough slopes and basin floor east of the GBR contains 20% to 50% terrigenous siliciclastic material. One hundred and twenty-six sediment samples were obtained from core tops and sediments grabs previously retrieved from shelf, slope, and basin environments of the North Queensland Margin. These samples were analyzed for their carbonate content, mineralogy, and major-element composition. The amount of terrigenous siliciclastic material in sediment from the North Queensland Margin can be approximated by three independent methods: (1) the noncarbonate residual, (2) the sum of quartz, clays, and feldspars, and (3) the sum of SiO2, Al2O3, Fe2O3, and TiO2. All three tracers show a distinct distribution of siliciclastic sediment across the margin. Shelf siliciclastic content is highest on the inner shelf, decreasing eastward toward the GBR, with the exception of relatively high siliciclastic content in several interreef passages. Queensland Trough siliciclastic content is highest on the slope and basin between 15° and 17° S latitude, decreasing to the south and east. Although the relative abundances of quartz and clay minerals vary across the margin, the chemistry suggests a similar origin for the siliciclastic material. Moreover, the zone of high siliciclastic abundance in Queensland Trough reflects enhanced siliciclastic accumulation. Siliciclastic material escapes the outer shelf to Queensland Trough through interreef passages between 15° and 17° S latitude. It is then focused to the south by ocean currents. Siliciclastic sediment is likely sourced from a combination of rivers transporting Holocene sediment to the GBR shelf and late Pleistocene sediment eroded from the outer shelf and reworked to Queensland Trough. Siliciclastic material crosses the shelf, although the mechanisms (e.g., river plumes, nepheloid layers, tidal currents, cyclones, mushroom jets) remain poorly constrained. However, shelf width is clearly an important factor in allowing cross-shelf transport to occur. Mass-balance calculations indicate that up to 13% of the late Holocene annual riverine sediment output may be delivered to Queensland Trough. Modern and ancient mixed system depositional models must be reevaluated to allow for the possibility of coeval siliciclastic and carbonate deposition.