One of the most common and straightforward ways to explicitly represent spatial heterogeneity in simulations is with the use of some form of a lattice. Lattices are two-or three-dimensional grids in which entities are connected using various forms of local rules. They are thus ideal for representing systems with different levels of local interactions and, thus, for exploring the processes and impacts of self-organization. Models based on lattices have found wide usage in ecology and geology and often use the same basic formalism, despite the differences in the entities being studied. Groups of models, such as cellular automata, self-organized criticality, and diffusion limited aggregation, show how complex spatial structures and temporal behaviors can arise from local interactions only in the absence of external forcing. Other models that incorporate external processes, such as percolation-based models of fire and diseases, demonstrate that self-organization can strongly affect the signal produced by exogenous disturbances. Most lattice models are best used as tools for improving understanding of the dynamics of systems under various sets of assumptions of internal dynamics and external forcing, rather than as a means for accurate predictions of actual system behaviors. Lattice models that integrate ecology and sedimentology could be used to introduce an explicit spatial component into studies of Earth system history.
A phenomenon unique to fine-grained sediment is its ability to alter the physical characteristics of the overlying water column. Although the present state of research recognizes many aspects of fine-grained seabed and water column interactions, this study documents how an energetic sandy, shallow marine system can autogenically transition to a system capable of accumulating fine-grained bedforms to clinoforms. To understand these transitional processes this study examines the lithostratigraphy and depositional history of the Suriname portion of the Guiana Coast (French Guiana, Suriname, and Guyana). Four major lithologic facies (Pre-Holocene silty clay; peat-rich silty clay; sandy mud; silty clay with cheniers) were derived from the Late Holocene sea level rise and influx of sediments emitted from the Amazon River. Since approximately 6000 BP, similar to 10 to 20% of Amazon-derived sediments bypass the Amazon shelf and are transported northwestward toward the study area. Along the Suriname coast (similar to 900 km from the Amazon), however, significant mud accumulation did not commence until 3000 to 3500 BP. Suspended sediments can travel this distance in less than 1 month. A migrating (1.5 km/yr) mud bank could travel the 900 km from the Amazon mouth in 600 years or, after formation of a 400-km-wide subaqueous Amazon delta (by 1200 BP), migrate the remaining 500 km by 4500 BP, still 1000 to 1500 years prior to the time period during which radiocarbon dates indicate significant mud accumulation began. Consequently, either there was a major hiatus in sediment transport and accumulation between 6000 BP to 3000 BP or some other transport process other than suspension or mud-bank migration-controlled initial mud accumulation. Assuming steady-state conditions, lateral accretion rates from 6000 BP to 3000 BP equate to 0.3 to 0.4 km/yr. These rates, which are similar to migration rates cited by previous studies for the trailing edges of mud banks in French Guiana, may reflect postmigration erosion of the initial mud banks. Whether there is an erosional overprint or some other process, this lateral accretion rate is an indicator of the amount of fluid mud necessary for "mud to beget mud." More general prerequisites necessary for a shallow marine setting to autogenically form fine-grained clinoform-scale accumulations are, first, a single large source of muddy sediments (a major river) and, secondly, unidirectional transport processes to concentrate and continuously supply mud sediments to the system.
Determining how autogenic and allogenic processes and responses in deltas scale up from meter-scale laboratory experiments to actual field examples remains a challenge. This study was devised to bridge that scale gap using field data from small, hundreds of meter-scale natural deltas. Ground-penetrating radar and core data were collected from four different river-dominated delta morphotypes developing at the margins of freshwater coastal lagoons in southern Brazil. Since the sediment supplying these deltas is sourced from a nearby dune field and is similar between the deltas, it is hypothesized that major morphological differences in the four deltas are primarily the result of differences in sediment discharge rates (sediment-water ratio). As observed in published tank experiments, channel cross-section and distributary channel patterns in the deltas vary as a function of sediment discharge, from shallow sheet-like flow at high discharge to well-established, stable distributary channels (i.e., birdsfoot pattern). A contributing factor may be the development of vegetation on the slower growing deltas influencing sediment cohesion, a key control in laboratory-scale deltas. As in many tank experiments, these lagoon deltas are steep and sandy, with the Froude number modulated to just below Froude critical flow (i.e., they are Froude-scaled). Ground-penetrating radar sections were processed, interpreted, and integrated with cores, allowing the definition of radar units. Analysis of the radar units demonstrates the presence of both allogenic and autogenic signals. Allogenic control is identified in the stacking of clinoforms and is perceptible in both sides of a single delta (delta 4), as well as in two other deltas (deltas 1 and 2). An autogenic signal varies according to delta planform shape and was identified by the stacking of lobe elements, both in dip and strike. Base-level change (lake level) and autogenic avulsion cycles occur on similar timescales, and therefore it is a significant challenge to separate these different processes in the stratigraphy. The potential uses of these types of data include understanding the link between delta dynamics, channel patterns, and stratigraphy to develop improved genetic models of steep sandy deltas common in the stratigraphic record.
Spatial self-organization, the process where coherent spatial patterns emerge through internal interactions, is widely observed in modern natural systems. Compelling examples range from ripple and dune formation in aquatic and terrestrial systems to formation of patterned coral reefs and vegetation in arid regions. Despite this wide range of contemporary cases, the concept of self-organization and its potential effects on geological patterns have not yet been widely discussed by the geological community, especially in carbonate depositional systems. We present four case studies from modern bivalve beds, coral reefs, microbial carbonates, and tidal channels, and one from the rock record considering carbonate cyclicity, where spatial self-organization could explain regularity in preserved strata. Only two of these five case studies, bivalve beds and tidal channel systems, are accompanied by a firm understanding of the mechanisms that generate emergent patterning. Three types of ecosystem spatial self-organization-scale-dependent feedback creating regular patterns, criticality behavior causing scale-free patterns, and oscillating consumer resource interactions causing consumer waves-are well documented. The first two of those appear to hold most relevance for carbonate depositional environments. Considerable work remains to understand the processes and products of spatial self-organization in carbonate deposystems.
Autogenic dynamics and self-organization in sedimentary systems are increasingly viewed as significant and important processes that drive erosion, sediment transport, and sediment accumulation across the Earth’s surface. These internal dynamics can dramatically modulate the formation of the stratigraphic record, form biologically constructed depositional packages, affect ecological patterning in time and space, and impact aspects of geochemical sedimentation and diagenesis. The notion that autogenic processes are local phenomena of short duration and distance is now recognized as false. Understanding autogenic dynamics in sedimentary systems is thus essential for deciphering the morphodynamics of moderns sedimentary systems, accurately reconstructing Earth history, and predicting the spatial and temporal distribution of sedimentary and paleobiologic features in the stratigraphic record. The thirteen papers in this volume present exciting new ideas and research related to autogenic dynamics and self-organization in sedimentology, stratigraphy, ecology, paleobiology, sedimentary geochemistry, and diagenesis. Five papers summarize the current state of thinking about autogenic processes and products in fluvial-deltaic, eolian, and carbonate depositional systems, and in paleobiologic and geochemical contexts. A second group of papers provide perspectives derived from numerical modeling and laboratory experiments. The final section consists of field studies that explore autogenic processes and autogenically modulated stratigraphy in five case studies covering modern and ancient fluvial, deltaic, and shelf settings. This SP should stimulate further research as to how self-organization might promote a better understanding of the sedimentary record.
Geochemical self-organization is a process whereby a geochemical system acquires oscillatory behaviors or spatially repetitive patterns through its own internal autogenic dynamics. The concept of self-organization provides a new perspective for understanding the formation of repetitive patterns in sedimentary rocks, without invoking any unjustified external periodic allogenic force or template. Geochemical self-organization requires a system to be far from equilibrium and to have a positive feedback among the physical and chemical processes involved. Identification of a plausible mechanism for a specific self-organizational phenomenon usually boils down to finding a positive feedback consistent with laboratory and field observations. In this paper, we review the concepts and principles of geochemical self-organization, highlight some of the driving mechanisms and examples that typify sedimentary geochemical systems, and consider some of the basic techniques for model analysis. Two specific examples, formation of banded iron formations and development of periodic patterns in the distribution of porosity in dolomites, are presented in some detail to illustrate the insights generated from numerical modeling and linear stability analysis. Last, we offer some speculations on other chemical sedimentary and diagenetic features that might have self-organizational origins. Testing those hypotheses will require new types of sampling strategies and analyses not normally employed by sedimentologists and sedimentary petrologists.
The stratigraphic fill of incised valleys has traditionally been interpreted to be primarily modulated by allogenic controls. The use of this concept has been so dominant that the possibility of autogenic mechanisms controlling fluvial organization of incised valley fills (IVFs) is largely overlooked, particularly in rock-record interpretations. This has been mainly due to the fact that deconvolving autogenic from allogenic signals remains challenging, especially for IVF deposits. Using integrated light detection and ranging (LiDAR), outcrop, and core data, we investigated the fluvial architecture of two IVFs in the lower Blackhawk Formation (Upper Cretaceous) of the Western Interior Seaway, Utah. Contrary to conventional interpretation, our analyses demonstrate that an autogenic signal linked to differential compaction of coal-precursor peats underlying IVFs likely exerted substantial control in both the vertical and lateral organization of sand-body architecture in these two IVFs, which are up to similar to 15 to 20 m thick individually. Trends in vertical-amalgamation thickness, number of channel-story sand bodies stacked vertically, and width constraints of multilateral sand bodies (lateral amalgamation) of these two IVFs are correlated with thickness variation of underlying coal seams. Decompaction analysis of coal seams indicates that the magnitude of accommodation-creation by coal-precursor peat compaction was potentially much higher to overcome allogenic modulation. This is invoked as the principal reason for broad correlation between fluvial architecture of the IVFs and coal thickness in our data set. These findings contribute to isolating autogenic from allogenic signals in complex systems such as IVFs. They further provide insights on signal-shredding mechanisms in the depositional architecture of the Cretaceous Western Interior Seaway, and they supply evidence that paleovalley fluvial architecture should not be automatically attributed to allogenic processes.
Eolian dune fields self-organize through a hierarchy of autogenic processes that culminate at the dune-field pattern level. Interactions that occur between flow and grains, flow and dunes, and dunes and dunes define the levels of this hierarchy. These autogenic processes occur within sets of boundary conditions, which impart a uniqueness to each emergent dune-field pattern. The interpretation of allogenic forcing on dune-field patterns and their stratigraphic record requires an understanding of how these external environmental variables are manifested at the dune-field pattern level. The fundamental process in eolian systems is a wind event with basic boundary conditions of sediment supply, sediment availability, and the transport capacity of the wind. It is hypothesized that the basic high-frequency boundary conditions are remade at each level of the hierarchy of autogenic processes or have a cumulative effect over many wind events. The influence of these boundary conditions "trickles up" to and is manifested at the dune-field pattern level. Tectonic, climatic and hydrologic boundary conditions are low frequency and operate over much longer timescales than a wind event. It is hypothesized that these "trickle down" to be remade as high-frequency boundary conditions, which then trickle up. Analysis of the White Sands Dune Field in New Mexico supports these hypotheses by the manifestation of the influence of boundary conditions in the dune-field pattern. The dune field originated by wind deflation of a lacustrine sediment supply, which was made available episodically by climatic forcing that controlled the hydrodynamics of the tectonic basin. Although the dune-field pattern arose through autogenic dune interactions, the morphologies of which are ubiquitous throughout the field, the influence of boundary conditions is evident in the dune morphologies and field-scale pattern heterogeneity.
A series of alluvial fan experiments was compared to a series of submarine fan experiments in order to explore the similarities and differences of autogenic supercritical avulsion cycles in the two environments. Both systems have cycles of: distributive channel formation and basinward extension, deceleration and mouth bar deposition, flow interaction with the aggrading mouth bar, propagation of the channel-to-lobe transition in the upstream direction, and flow reorganization. The channel-to-lobe transition in both alluvial fan and submarine fan experiments was located at the supercritical-to-subcritical flow transition. Channel-to-lobe transitions were also the primary locus of deposition in each case, and their aggradation in turn forced upstream accretion. The commonalities between the two environments are striking and lend evidence toward the hypothesis that supercritical vs. subcritical flow in distributary channels is a more significant distinction than subaerial vs. subaqueous environment in terms of the hydraulic and sediment transport properties.
The Kayenta Formation, Warner Valley, Utah, shows lateral and vertical clustering of mud-delta propagating-channel sand bodies within a matrix of fine-grained open-lake deposits and further provides opportunity to develop a fluvio-deltaic depositional-process model. Clustering due to nonrandom stream avulsion is well documented for high-accommodation fluvial systems operating in alluvial plains but not well established for lacustrine systems with abundant fluvio-deltaic lobes. Kayenta Formation delta lobes have similar spatial clustering to those observed in fluvial channel belts and possibly extend this clustering concept to fluvio-lacustrine systems. Lithofacies were mapped on three large photo panoramas, and architectural-element analysis was used to identify bounding surfaces of fluvial channel-deltaic lobes. Fluvio-lacustrine delta lobes reflect linear channels that propagate across mud deltas with negligible bifurcation and generate fluvial channel belts incised into lake mudstone. Channels are associated with thin sand sheets or "blow-out wings" that extend multiple channel widths from the channel and cover levee and mud-delta deposits, but delta-front sand beds are absent. The stages of evolution for these propagating channels is preserved in the variation of channel-lobe architecture and reflects mud-delta propagation at the mouth of each channel in the absence of delta-front sand. Sand is outpaced by mud in the ever-lengthening channel, which reduces sand at the channel mouth and diminishes necessity for channel bifurcation. The resulting deposit is thus a frontal mud-delta lobe bisected by a later single sandy channel belt with lateral sand wings. Statistical analysis of these channel belts shows clustering. Clustering of fluvial bodies within shallow lakes is significant in predictive reservoir models because it improves connectivity and localization of delta-lobe reservoirs. The clustering of delta lobes in fluvio-lacustrine systems is theorized to reflect the preferential channel avulsion centered on the axis of the primary channel feeding into the lake and preferential avulsion fairways of feeder channels. Both the segregation of sand and mud though channel lengthening and the clustering are explainable in fully autocyclic terms. The needed allocyclic driver to trigger these fluvio-lacustrine processes is an accommodation rate sufficiently low compared with lake filling rate as to maintain shallow-water conditions across the lake system through multiple generations of channel propagation.
Bedforms that develop at the interface between a fluid flow and a loose sediment bed are among some of the most fundamental morphodynamic processes, perhaps among the greatest examples of canonical autogenic adjustments between flow and sediments. Because different types of bedforms develop under specific combinations of flow and sediment properties, these sedimentary features have commonly been used to aid interpretations of flow conditions and infer the nature of depositional environments. While subaerial (river) bedforms are relatively well understood, their counterparts in deep water (i.e., related to gravity underflows, namely, density or turbidity currents) remain somewhat elusive, largely due to the difficulty of direct observation in their natural setting, due to the limited number of experimental studies, and due to their inherent process complexity. Although widely practiced, extrapolation of equilibrium regime diagrams developed for subaerial bedforms to the deep-water realm remains questionable, particularly in light of recent experimental and field observations that suggest some departures from the subaerial counterpart. Here we present results from an experimental program aimed at investigating equilibrium bedforms resulting from saline density currents under bypass conditions. Saline density currents have been typically treated as the surrogate of muddy turbidity currents for which sediments never settle. More than 500 separate experiments were run, comprising currents that spanned a wide range of the densimetric Froude number including all flow regimes (supercritical, critical, subcritical: Fr-d = 0.6 to 2.8). Results confirm some similarities between subaerial and gravity flow bedforms both in process and product but also reveal some interesting differences. For example, ripples form under both subcritical and supercritical density currents, while supercritical currents yield dunes and both small-wavelength, downstream-migrating, and long-wavelength, upstream-migrating antidunes, where the latter may transition to cyclic steps. Supercriticality of the flow, the proportion of bedload to suspended load (when looking at the sediment composing the bed), and the bed characteristic sediment size are the major controls on the prevailing bedform observed. To investigate the flow and morphodynamic mechanisms related to some of the observed bedforms (e.g., supercritical dunes), detailed analyses of flow structure over the bed features were performed using particle image velocimetry techniques.Outcrop examples are presented to demonstrate that the gravity flow bedforms we observed experimentally might have counterparts at the field scale. Our findings underscore the rich spectrum of potential bed states produced by dense underflows and their deviation from bed behavior in open-channel flows. As a result, we argue that inversion of gravity flow bed features based on known subaerial bedform regimes might be potentially misleading.
The science of the internally generated behavior and spatial organization of depositional systems has come a long way since Beerbower first coined the term "autocycles" to refer to fining-upward sequences generated by river meander migration, cutoff, and eventual return. Ongoing research has broadened the scope and scale range of known autogenic dynamics, even as a unifying theme-sediment storage and release-has emerged. Many internally generated processes do not have a single characteristic length or time scale but rather occupy a broad scale range (hence, "autocyclic" has been gradually replaced by "autogenic"). But even where they are broad, the scale ranges for autogenic processes are bounded by limiting time and length scales. The central role of sediment storage and release provides a means of estimating these limiting length and time scales based on mass balance, geometry, and mean sediment flux. Recent research has also allowed us to expand the upper limits of autogenic behavior to time scales of 10(5) to 10(6) years. Finally, we recognize that autogenic dynamics is not simply superimposed on allogenic signals but interacts strongly with, modifies, and even destroys allogenic input. That the autogenic imprint on the stratigraphic record is stronger and more complex than once thought can be seen as an opportunity to focus on using the record to learn about intrinsic surface behavior under pre-human conditions, rather than simply as an archive of externally imposed signals.
Autogenic fluvial dynamics, including river avulsion, influence the distribution of channel sand bodies in alluvial deposits. Over long timescales, autogenically organized avulsions can generate stratigraphic patterns such as clusters of sand bodies when avulsions preferentially return to previous channel locations, or evenly spaced sand bodies when avulsions preferentially fill topographic lows. Consequently, quantifying stratigraphic patterns may provide an avenue for reconstructing paleoavulsion dynamics from ancient deposits. Several quantitative approaches have been used to quantify the degree to which channel-belt deposits are distributed randomly, evenly, or with clustered patterns; however, to date, there are only a few examples where these metrics have been applied in outcrop studies. Here we present a quantitative analysis of stratigraphic architecture in the lower Williams Fork Formation (Cretaceous, Colorado) to quantify the paleoavulsion pattern in this interval. A spatial-point-process statistic (the K function) and the compensation statistic are applied to stratigraphic data mapped from a terrestrial lidar digital outcrop model. Both analyses show random channel-body distributions and random basin filling at short (less than 200 m) spatiotemporal scales, which suggests that lower Williams Fork channels avulsed randomly. To evaluate the sensitivity of the K function to different degrees of stratigraphic organization, we use a two-dimensional (2D) geometric model to build synthetic stratigraphy with different degrees of sand-body clustering. Model results show that the lower Williams Fork data set should be of sufficient size and resolution to detect strong clustering signals, if they were present. This type of sensitivity analysis is helpful for comparing results of spatial-point-process analyses among outcrop examples with confidence. The random paleoavulsion pattern inferred from lower Williams Fork stratigraphy in this locality contrasts with previously published analyses that show qualitative clustering at larger scales; however, these results are not incompatible if avulsions remained clustered regionally over long timescales.
Self-organization refers to the emergence of large-scale ordered pattern starting from initially disordered conditions through small-scale interactions between components of a system. Although the influence of biological agents on sediments and landscapes is widely appreciated by sedimentary geologists, the role of life in sedimentary self-organization is less familiar. One of the oldest ecological concepts relevant to self-organization is ecological succession, which is the idea that species colonization of a virgin or a disturbed landscape follows a series of more or less orderly and predictable changes in taxonomic composition and habitat structure. Many early studies of ecological succession in the fossil record confounded allogenic facies succession reflecting environmental shifts with autogenic ecological changes driven by organism interactions. However, evidence for autogenic ecological succession can be found in a variety of depositional contexts and is particularly evident in environments that are influenced by ecosystem engineers (organisms that affect other species by physically modifying or building their habitats), like reefs and shell beds. In addition to influencing change in an ecosystem over time, biologically mediated feedbacks can also produce highly organized spatial patterns. The basic mechanism of spatial self-organization requires at least one negative feedback loop that acts at a distance. Although relatively few examples have been documented in the rock record, self-organized spatial dynamics have the potential to greatly influence the nature of depositional environments by altering the flow of matter and energy. Recognition of biotic feedbacks is necessary for a complete and predictive understanding of the architecture of the stratigraphic record, and it seems likely that with greater awareness of self-organization and autogenesis in the sedimentary record, the number of biologically mediated examples will increase.
Lower Permian (Wolfcampian) carbonate platform margin depositional profiles and stratigraphic architecture were controlled primarily by the type of reef community that characterized the time, peak icehouse glacioeustatic sea-level fluctuations, and regional tectonic pulses from the paleocontinental collision along the southern margin of the basin. During transgressions, mound-shaped reef cores began growth in sub-wavebase, low-light (oligophotic), upper slope settings. The reef mound communities quickly diversified, and the reef mounds aggraded and shallowed upward through the euphotic zone, eventually reaching wavebase during early highstand, and graded upward into progradational crestal shelf-margin shoals during late highstand. Reef mound growth seaward of and downdip from the crestal platform margin shoals resulted in distally-steepened platform margin profiles, which persisted throughout Permian time. Wolfcampian reef mound facies are composed of bafflestones-boundstones with frameworks of phylloid algae, calcisponges and heliosponges, and fenestrate and ramose bryozoans, which were encrusted by Tubiphytes, laminar encrusting red algae (Archaeolithoporella), fistuliporid bryozoans, and microbialite. Peloidal cement matrices are common. Syndepositional to early marine botryoidal radial fibrous cements are generally sparse but are more common in windward margin buildups. Reef mound cores are surrounded by shallowing-upward skeletal packstone to grainstone flank beds. Crestal shelf-margin shoals are composed of medium-to coarse-grained grainstones with fusulinids, Tubiphytes, and robust dasycladacean algae, and contain small Tubiphytes patch reefs.Early Permian icehouse glacioeustatic sea-level fluctuations and episodic tectonic pulses created erosional paleotopography on unconformities near the base of the Wolfcampian and at the mid-Wolfcampian. On the eastern margin of the Central Basin Platform, Lower Hueco Group (lower Wolfcampian) carbonate bank complexes composed of reef mounds and packstone-grainstone flank beds grew along an erosional escarpment on the underlying Bursum Formation (uppermost Pennsylvanian), as seen at South Cowden (8790 Canyon) Field, Ector County, Texas. The escarpment depositional slope was >10 degrees. Reservoir porosity in the Lower Hueco platform margin carbonate banks was greatly enhanced by subaerial exposure and meteoric dissolution at the overlying mid-Wolfcampian unconformity. Lithoclastic-skeletal debris flows are common in Wolfcampian forebank, deeper water, slope-to-basin facies.Outcrop analogs for the subsurface carbonate platform margin banks and forebank slope-to-basin facies are well exposed in the Hueco Group of the western outliers of the Hueco Mountains in far west Texas, which were paleogeographically located along the western (leeward) margin of the Diablo Platform and adjacent Orogrande Basin. Platform margin banks prograded over an irregular paleotopographic surface on the sub-Hueco Group erosional unconformity. The leeward upper slope to shelf-margin reef mounds have phylloid algal-dominated core facies with only sparse radial fibrous cements, which graded rapidly upward near wavebase into crestal shelf-margin grainstone shoals with Tubiphytes patch reefs. Proximal forereef, upper slope facies are composed of autochthonous upper slope crinoidal-fusulinid packstones, burrowed dark mudstones, and productid brachiopod wackestones and have interbedded allochthonous tongues of forereef grainflows, turbidites, and lithoclastic-skeletal packstone debris flows. More distal slope-to-basin facies are composed of autochthonous dark cherty nonburrowed mudstones-wackestones with interbedded allochthonous lithoclastic-skeletal packstone debris flow deposits and turbidites. Slope facies distributions reflect icehouse glacioeustatic sea-level fluctuations.
Tengiz Field is a steep-sided, isolated carbonate platform in the Precaspian Basin, Kazakhstan, with hydrocarbon production from Carboniferous platform and slope facies. Systematic differences in reservoir pressure decline during production indicate that this reservoir consists of three subcompartments or material balance regions: (1) a central "platform reservoir" made up of cyclic platform-top facies that acts like a single, stratified, multistory reservoir; (2) a "wedge reservoir" formed by a prograding margin containing upper slope microbial facies; and (3) an "apron reservoir" containing allochthonous facies deposited in deep water around the base of the buildup. The facies in the apron reservoir accumulated during an early depositional stage and were subsequently partly to fully buried by prograding microbial slope facies of the wedge reservoir. The wedge and apron reservoirs together form a succession 800 to 1000 m thick within the Tengiz oil column.The wedge reservoir shows uniform pressure decline with time and is well connected. Field data (cores and well logs) are insufficient to determine internal continuity of lithofacies and depositional environments or to quantify the pore network responsible for the high connectivity. An outcrop analog (Asturias, Spain) with facies matching those observed in Tengiz cores was used to predict that the microbial lithofacies form a distinct and continuous mechanical unit within the wedge reservoir. Tengiz microbial facies contain a high concentration of solution-enlarged, syndepositional and other early fractures oriented parallel and normal to depositional strike. Borehole image logs provide data on enlarged fracture apertures and local fracture density, but no data related to fracture height or length. An outcrop analog with early fractures in similar facies (Windjana Gorge, Australia) was used to obtain large-scale height and spacing data for solution-enlarged syndepositional fractures. Dissolution processes in the outcrop are different from those of Tengiz, but the fracture aperture and cavern sizes are comparable to their known counterparts in the Tengiz wedge reservoir, and application of the outcrop height data to geologic models of the Tengiz wedge subcompartment can account for its dynamic behavior. The apron reservoir shows a nonsystematic pressure decline with time and is less depleted than the wedge reservoir. The irregular decline indicates reduced internal connectivity within the apron reservoir, which is corroborated by core and borehole image data indicating high lithofacies heterogeneity and the absence of continuous microbial facies responsible for reservoir continuity in the wedge reservoir. A reservoir pressure increase of 1700 psi from the wedge reservoir to the apron reservoir observed in a single well penetration suggests reservoir communication between them may be reduced across a stratigraphic baffle.The wedge and apron reservoirs both contain a late burial matrix diagenetic overprint represented mainly by co-precipitated bitumen and calcite cement and local development of matrix microporosity. Enlargement of the early fractures in the wedge reservoir also occurred during burial diagenesis based on the presence of diagenetic halos containing the burial overprint around the fractures and based on the presence of co-precipitated bitumen and calcite in the fractures. Scenarios and mechanisms for fracture enlargement are evaluated against the observations from field data and the outcrop analogs.
A 630-km-wide continental shelf characterized by mixed carbonate and siliciclastic sedimentation formed during the Quaternary across the Bonaparte Basin, NW Australia. During this time interval (similar to 2.6 million years), shelf-margin and slope deposits were disconnected from the inner shelf and hinterland by the 200-km-wide, low-gradient Malita intrashelf basin. In this study, two-dimensional (2D) and three-dimensional (3D) seismic, well log, and core data were used to determine the relative importance of allogenic and autogenic controls on the stratigraphic architecture of shelf-edge and slope deposits at multiple timescales. This work has determined that Quaternary sea-level variations (glacio-eustasy) provided a primary control on the stratigraphic evolution of shelf-margin and slope deposits. The early Quaternary period was marked by the aggradation and progradation of a carbonate margin under global sea-level highstand conditions. The onset of high-amplitude sea-level fluctuations at the Mid-Pleistocene Transition (ca. 0.9 Ma BP) enhanced the development of a mixed clastic-carbonate margin and slope system. During the late Quaternary, long-duration sea-level falls and lowstands and high rates of terrigenous sediment supply resulted in stacked fourth-and fifth-order systems tracts in the form of prograded shelf-margin and slope wedges. Conversely, rapid, high-amplitude, fourth-and fifth-order transgressions between these time intervals enhanced the aggradation of carbonate buildups at the shelf edge. Hence, high-frequency sea-level changes resulted in reciprocal sedimentation similar to many other mixed depositional systems of the late Quaternary. However, the main locus of carbonate and mixed deposition across the Bonaparte Basin shelf margin and slope varied spatially at longer times scales. Indeed, conventional seismic data have revealed that the third-order systems tracts at two separate locations in the Bonaparte Basin (eastern and northwestern shelf-margin) show stratigraphic asymmetry (rimmed carbonate margin vs. shelf-margin and slope progradation), which reversed during the late Quaternary. Our results suggest that this reversal in the locus of carbonate vs. mixed sedimentation was related to the shift of the detrital feeder system (the Malita tidal valley) during a major sea-level fall of the late Quaternary (tentatively ascribed to the ca. 0.6 Ma BP lowstand). This study illustrates the importance of both allogenic and autogenic parameters in controlling the stratigraphic architecture of shelf-margin and slope deposits at multiple timescales, in a very wide, mixed carbonate and clastic depositional setting.
The Lisburne Group (Carboniferous-Permian) consists of a carbonate platform that extends for >1000 km across northern Alaska, and diverse margin, slope, and basin facies that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered from outcrop and subsurface studies during the past 20 years allow us to delineate the distribution, composition, and age of the off-platform facies, and to better understand the physical and chemical conditions under which they formed.The southern edge of the Lisburne platform changed from a gently sloping, homoclinal ramp in the east to a tectonically complex, distally steepened margin in the west that was partly bisected by the extensional Kuna Basin (similar to 200 by 600 km). Carbonate turbidites, black mudrocks, and radiolarian chert accumulated in this basin; turbidites were generated mainly during times of eustatic rise in the late Early and middle Late Mississippian. Interbedded black mudrocks (up to 20 wt% total organic carbon), granular and nodular phosphorite (up to 37 wt% P2O5), and fine-grained limestone rich in radiolarians and sponge spicules formed along basin margins during the middle Late Mississippian in response to a nutrient-rich, upwelling regime.Detrital zircons from a turbidite sample in the western Kuna Basin have mainly Neoproterozoic through early Paleozoic U-Pb ages (similar to 900-400 Ma), with subordinate populations of Mesoproterozoic and late Paleoproterozoic grains. This age distribution is similar to that found in slightly older rocks along the northern and western margins of the basin. It also resembles age distributions reported from Carboniferous and older strata elsewhere in northwestern Alaska and on Wrangel Island.Geochemical and isotopic data indicate that suboxic, denitrifying conditions prevailed in the Kuna Basin and along its margins. High V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions [MF]) and low MnO contents (<= 0.01 wt%) characterize Lisburne black mudrocks. Low Cr-MF/V-MF ratios (mostly 0.8-4.0) suggest moderately to strongly denitrifying conditions in suboxic bottom waters during siliciclastic and phosphorite sedimentation. Elevated to high Mo contents (31-135 ppm) in some samples are consistent with seasonal to intermittent sulfidic conditions in bottom waters, developed mainly along the basin margin. High delta N-15 values (6-12 parts per thousand) imply that the waters supplying nutrients to primary producers in the photic zone had a history of denitrification either in the water column or in underlying sediments.Demise of the Lisburne platform was diachronous and reflects tectonic, eustatic, and environmental drivers. Southwestern, south-central, and northwestern parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis within the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary (except in the southwest) and likely due to eutrophication associated with upwelling and sea-level rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the south-central area occurred in the Early Pennsylvanian and also may have been linked to regional extension. In the northwest, platform sedimentation persisted into the Permian; its demise there appears to have been due to increased siliciclastic input. Climatic cooling may have produced additional stress on parts of the Lisburne platform biota during Pennsylvanian and Permian times.
Stratigraphic analysis of the Yaxcopoil-1 core (Yax-1) and seismic analysis of offshore two-dimensional (2D) seismic data provide insight into the Paleogene history of the Chicxulub impact basin and Yucatan platform development. Ten facies were identified based on core and petrographic analysis. Slope sediments include redeposited and background facies. The former are carbonate supportstones and finer-grained facies with evidence of soft sediment deformation deposited as gravity flows. Background facies are shales and mud-wackestone interpreted as sub-storm wave base suspension deposits. Depositional setting ranged from a steep bathyal slope inside the crater rim to neritic outer carbonate platform environments of the seaward prograding Yucatan platform.Through sequence stratigraphic analysis of Yax-1, we documented five sequences based on identification of transgressive and maximum flooding surfaces and facies stacking patterns. Biostratigraphic ages are equivocal, but they imply that sequences 1 and 2 are Early Paleocene, sequences 3 and 4 are Early Eocene, and sequence 5 is Middle Eocene. Coarse-grained redeposited carbonates in lower sequences 1 to 4 indicate slope gravity flow processes. Upper sequence 3 records the first evidence of fine-grained turbidites, indicating progradation of the Yucatan platform. By the top of sequence 4, facies indicate that the platform margin had prograded over the position of Yax-1.Seismic analysis identified six units, the lower five of which appear to correlate with cored Yax-1 sequences. The geometry and distribution of seismic units A and B indicate deposition confined to the western and central parts of the basin. Unit C, with two sets of clinoforms, records a major progradational event in the eastern part of the basin likely related to Yax-1 sequence 3 turbidites. Mainly parallel reflectors in seismic units D and E indicate relatively level bottom conditions similar to the environments of facies in upper sequence 4 and 5. The tops of units D and E, in proximal settings, are erosionally truncated. This unconformity marks the base of unit F, which is characterized by discontinuous reflectors and is restricted to the northeastern portion of the basin.Stratal patterns in seismic units C to E are more controlled by relative sea-level change, as suggested by the development of clinoforms and regional unconformities. If Chicxulub and others like the Chesapeake Bay structure are representative, large marine impacts in tectonically quiescent regions may dominate local depositional environments for millions to tens of millions of years postimpact before returning control to eustasy.
Outcrop-based, reservoir-analog models are important tools for assessing reservoir potential and efficient schemes for exploitation in the subsurface. A new outcrop reservoir-analog model is documented herein for Upper Miocene deep-water sediment-gravity-flow and hemipelagic deposits within the Agua Amarga basin, southeast Spain. This study demonstrates that large volumes of resedimented deposits exhibiting high ratios of potential reservoir to baffle facies (net to gross) accumulate where funneling topographic features focus sediment-gravity flows from the long linear dimension of a carbonate platform into a confined channel (focused flow). Where topographic funneling features are absent, and where a short linear dimension of the carbonate platform margin is available as a source of sediment-gravity flows, deposits accumulate with lower volumes and high proportions of baffle facies (dispersed flow). Extensive outcrops in the Agua Amarga basin allow for characterization of facies and facies architecture using measured sections, photomosaics, and core-plug petrophysical data. Petra (TM) and Petrel (TM) were used for correlation, data integration, and static geomodeling to create a reservoir-analog model that synthesized geological observations noted in outcrop. Facies modeled as reservoir units consist of graded fine-to very coarse-grained skeletal packstones and fine-to very coarse-grained breccias. Graded skeletal packstone facies exhibit a mean porosity and corresponding permeability of 30.5% and 136 mD; breccia facies exhibit a mean porosity and corresponding permeability of 30% and 65 mD. Facies modeled as baffle units consist of foraminiferal, volcaniclastic foraminiferal, and skeletal foraminiferal wacke-packstones. These planktonic foraminifera-rich facies exhibit a mean porosity and corresponding permeability of 36% and 12 mD. Paleotopography, in conjunction with sea-level history, largely controls the geometry, lateral continuity, and volume of a given reservoir body. Paleotopographic differences that lead to focused flow versus dispersed flow result in markedly different reservoir properties. Static model volumetric results reveal that compared to the dispersed-flow system, deposits within the focused-flow system have greater reservoir to baffle facies volume ratios (0.70 compared to 0.09), and greater reservoir facies bulk volumes (46.5 million m(3) compared to 18.6 million m(3)). Further, the ratio of reservoir facies bulk volume to linear dimension of the shelf margin supplying both the focused-flow and dispersed-flow systems is similar, suggesting that deep-water reservoir volume may be predictable on the basis of the linear dimension of the shelf margin. Finally, interrogating modeled reservoir facies for different connected volume scenarios offers significant insight relevant to subsurface exploitation strategy and supports observations noted in the field.
