Autocyclic Controls Research Articles

Allocyclic and autocyclic controls on facies distribution in interdistributary bayfill deposits of the Campanian Neslen Formation, Eastern Book Cliffs, Utah, U.S.A.

ABSTRACT The Campanian Neslen Formation, in the eastern part of the Book Cliffs, is divided into three stratigraphic intervals. The middle Neslen interval consists of paralic bayfill deposits that average 23 m in thickness. This article describes the lateral and vertical variability within the bayfills deposited in this middle Neslen interval. Comprehensive mapping along 27 km of outcrops, including high-resolution description of 62 sedimentary logs and incorporation of wireline log data from 86 wells, has led to a detailed understanding of the processes related to infilling of interdistributary bays. Six vertically stacked heterolithic to sand-prone bayfill units are identified. A distinction is made between bayfill units and bayfill successions, based on their bounding autocyclic or allocyclic flooding surfaces. Units are bounded by high-frequency flooding surfaces, and show large lateral variations in thickness, facies associations, and lithology. These bayfill units are laterally subdivided into a varying number of bayfill successions, separated by interdistributary channel systems or associated bayhead-delta facies associations. Lateral variation in facies architecture, salinity, and depositional energy levels are mapped in each bayfill succession. This variation is attributed to several controlling factors, including proximity to sediment entry points, water depth, the sequence stratigraphic position of the bayfill succession, and the compaction of underlying sandbodies. The detailed facies patterns and multitude of tributary causes described and discussed here are applicable to an overall understanding of facies architecture in paralic environments.

Paleohydraulic analysis of an ancient distributive fluvial system

ABSTRACTReconstructing the paleohydraulics of ancient fluvial systems has important implications when determining channel-body dimensions in the subsurface as well as aiding source-to-sink studies and quantitatively determining the impact of changing climatic conditions. We undertake a paleohydraulic analysis of the Upper Jurassic Salt Wash distributive fluvial system (DFS) of the Morrison Formation, SW USA, to determine if downstream trends such as decreasing channel size and discharge, inferred in studies of DFS, are present. Channel depth was estimated using cross-set height values and preserved bar thickness. Nine localities across the exposed part of the Salt Wash system were studied. In total, 49 bars were measured, full bar thickness was determined from 12 complete bars, and average cross-set height was calculated for 37 bars. Estimates of maximum bankfull channel depth were derived from measured bar thicknesses. Bar height was then obtained and converted to mean bankfull channel depth using a shape adjustment factor of 0.65. The bar-derived mean bankfull channel depths were then used to derive a factor for which dune cross-set heights could be converted to mean bankfull channel depth (4.6) and maximum bankfull channel depth (7.1). These factors were then applied to localities where only cross-set height data were available, thus allowing consistent comparison and extrapolation of mean bankfull channel depth over the preserved DFS area. The use of measured bar thicknesses to calibrate estimates of mean channel depth from reconstructed dune heights is considered a useful approach, with the factor of 4.6 estimated here being lower than that (6 to 10) commonly used in comparable studies.The datasets for the Salt Wash DFS record systematic downstream trends in cross-set height, bar thickness, calculated channel depth, estimated channel width, and estimated Q, with variability and overlap between the proximal to medial, and medial to distal parts. The variability superimposed on the regional downstream trends is attributed to a combination of autocyclic processes such as variations in discharge, depth of scour, and avulsion as well as more regional-scale channel-belt switching together with allocyclic controls. The wide spatial distribution of the dataset in this study allows distinction between local autocyclic controls and regional downstream trends. Formative discharge shows no downstream trend across the entire Salt Wash DFS, with a wide range in coefficient of variation of preserved cross set thickness (CV(dst) values of 0.1 to 1.1) indicative of flashy (variable) discharge.The spatial distribution of the Salt Wash dataset allows extrapolation of trends upstream to the unexposed part of the system that allows insights into the characteristics of the channel system in the apex area (∼ 150 km to the southwest and removed by post depositional erosion). The fluvial system would have a mean depth of 9 m, and a bankfull-depth discharge of around 1450 m3/s with mean cross-set heights of between 50 and 70 cm. These estimates are in line with those from present-day DFSs in the Himalayan and Andean foreland basins that have a scale similar to that estimated for the Salt Wash system.

Controls on deposition of shallow marine carbonates and evaporites – lower Permian Gipshuken Formation, central Spitsbergen, Arctic Norway

AbstractThe interbedded shallow marine carbonates and evaporites of the 95 to 105 m thick lower Permian Vengeberget member in central Spitsbergen form a lowstand to possibly early transgressive wedge sandwiched between open marine highstand carbonates along the northern margin of Pangea. Deposition took place on a wide (much greater than 40 000 km2) evaporitic platform separated from the open marine environments to the east by an inferred shelf margin barrier. Based on detailed description of facies in unique mountain‐scale outcrops, five CaSO4 and nine carbonate facies have been identified, all reflecting deposition in shallow evaporitic environments. The gypsum beds represent subaqueous deposition in broad hypersaline lagoons whereas the carbonates were deposited in protected shallow marine to peritidal environments and ooid‐dominated grainstone shoals. Cyclicity is expressed both as lithological shifts from carbonate to CaSO4 deposition and in the stacking of facies during CaSO4 and carbonate deposition. The number of carbonate–evaporite cycles varies from 12 to 28, but the overall thickness of the succession varies only slightly as a result of variations in cycle thickness, from 0·5 to 13·0 m. Two basic types of carbonate units can be recognized and reflect a combination of allocyclic and autocyclic controls. In addition, differential subsidence along older north–south orientated lineaments added to the complexity of cyclicity. The study provides insight into the complexity of the controls on deposition, architecture and dynamics of large‐scale mixed carbonate–evaporite successions formed in the shallow inner part of an epeiric sea or platform.

Quantitative comparisons of depositional architectures of unconfined and confined turbidite sheet systems

Degree of confinement refers to the degree to which sediment gravity flows are influenced by the surrounding basin relief and interact with the basin margins, and is a key controlling factor in determining the depositional architectures of turbidite sheet systems. Two well exposed late Palaeozoic sandy turbidite sheet systems in the Paganzo Basin at Cerro Bola, north-west Argentina, termed Stages II and IV (unconfined and highly confined respectively), provide three-dimensional constraints on facies associations, stacking patterns, depositional geometries and evolution of these turbidite sheet systems. Cluster analysis has been applied to define facies associations in these two systems at an appropriate hierarchical level depending on the overall nature of the system. The overlap index (OI), OI = A0/A1, where A0 is the overlap area and A1 is the area of underlying depositional element, is adopted to quantitatively characterise the stacking patterns. The two turbidite sheet systems show contrasts in that: (1) Stage II is built up by four hierarchical elements, each stacked in a shingled or compensational way with the majority of overlap index values of 0.7 and 0.8, to form higher hierarchical elements, whereas in Stage IV, individual beds stack vertically with OI = 1, together forming the stage and (2) Four facies associations have been identified in Stage II by cluster analysis and they occur in a spatial sequence from axis to fringe; one facies association has been identified in Stage IV and it shows little variation spatially across the basin. In unconfined systems, depositional elements are self-organized, and autocyclic controls are dominant, whereas in highly confined systems, confinement has suppressed the autocyclic signal and systems are allocyclically controlled.

Climatically Versus Eustatically Controlled, Sediment-Supply-Driven Cycles: Carbonate–Siliciclastic, High-Frequency Sequences In the Valanginian of the Neuquén Basin (Argentina)

Carbonate–siliciclastic high-frequency sequences are formed by reciprocal sedimentation. In siliciclastic-dominated successions, the carbonate-rich interval is typically associated with transgressive conditions and relatively low terrigenous influx, particularly in distal parts of the marine basin. However, key questions for each individual case are: What sort of depositional system developed during transgressions, and what controls sediment-supply fluctuations? This paper integrates outcrop and subsurface data to present a high-resolution sequence-stratigraphic study of small-scale (2–18 m), carbonate–siliciclastic cycles deposited during the Valanginian in a proximal–distal transect of the Neuquen Basin (western Argentina) and discusses those key issues. Carbonate hemicycles in this study are composed of cross-bedded oolitic–skeletal grainstones and packstones to massive skeletal floatstones, which accumulated in carbonate ramps with high biogenic production in their distal sector (bivalves, serpulids, echinoids), and significant contribution of non-biogenic (ooids) material in inner-ramp settings. Siliciclastic material is present only at the base of the hemicycles, but rapidly diminishes upwards. These carbonate hemicycles have retrogradational staking patterns in proximal sectors and less obvious trends in distal areas. The siliciclastic hemicycles comprise shallowing-upward successions that include mudstones, bioturbated muddy sandstones, sandstones with HCS and SCS, and bioturbated sandstones. These facies were collectively deposited in a storm- and wave-influenced offshore-shoreface depositional system during normal regressions. These shoreface systems were likely fed by littoral drift from riverine inputs located tens of kilometers to the south. The duration of the investigated small-scale, transgressive–regressive cycles can be estimated in 50 to 150 kyr, and they are bounded by marine transgressive surfaces placed at the bases of carbonate hemicycles. The bounding discontinuities together with the sequence architecture do not suggest periods of sea-level falls between cycles, nor high-amplitude relative sea-level changes during a cycle formation. Additionally, the low proportion of mixed sediments suggests rapid changes in the rate between terrigenous supply and carbonate productivity. The resulting sequence architecture cannot be reasonably explained by autocyclic controls or eustatic changes alone. It is argued here that high-frequency changes in sediment supply, driven by orbitally induced climate fluctuations, could be a better explanation for these sequences, particularly in the context of a long-term relative sea-level rise. In this scenario, periods of low siliciclastic influx during extreme arid conditions would develop a negative balance with accommodation and prompt carbonate systems and transgression. On the other hand, during more humid conditions relatively high sediment flux would temporarily overcome long-term accommodation, producing the regressive siliciclastic hemicycles. As this scenario does not require high-frequency sea-level falls, it would satisfactorily explain the observed sequence architecture. The distal Mulichinco carbonate hemicycles would be hard to distinguish from transgressive, fossiliferous-rich deposits that characterize transgressive–regressive cycles in clastic systems. In the Mulichinco case, however, these carbonate hemicycles are recording the onset of carbonate ramps, not the trapping of siliciclastic sediments in transgressive coasts (e.g., filling estuaries). This alternative scenario, ultimately controlled by reciprocal sedimentation, could be common in epicontinental basins developed under arid to semiarid conditions, and its identification could provide additional templates for basin-scale correlations and hydrocarbon exploration within carbonate–siliciclastic small-scale cycles.

Late Quaternary alluvial fans at the eastern end of the San Bernardino Mountains, Southern California

Alluvial fans at the eastern end of the San Bernardino Mountains in Southern California provide a record of climate modulated sediment transfer and erosion, and are deformed and displaced in places by active faults. Alluvial fans within two study areas, the Mission Creek and the Whitewater River drainages, were examined using geomorphic, sedimentological, and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure methods to define the timing of alluvial fan formation and erosion, and to examine the role of climatic, tectonic and autocyclic processes. These alluvial fan complexes were studied because they are amongst the best-preserved successions of alluvial fans in southern California and they are located at the mouths of two of the largest drainages, Whitewater River and Mission Creek, in the San Bernardino Mountains and traverse major faults, the Mission Creek and Banning. The alluvial fans comprise bouldery debris deposits that represent deposition dominated by flash flood and debris flow events. TCN surface exposure dating indicates that abandonment/incision of alluvial fan surfaces date to early in the Last Glacial or more likely the penultimate glacial cycle, to marine isotope stage (MIS) 4, and to the Holocene. The lack of alluvial fan ages during the latter part of the Last Glacial (MIS 2 and 3) suggests that there has been little alluvial fan lobe deposition/incision during that time. This is similar to findings for many other alluvial fans throughout the American Southwest, and supports the view that there is a strong climatic control on alluvial fan formation throughout this region. Furthermore, the oldest alluvial fan surfaces in the Mission Creek region are beheaded by the Whitewater River drainage, showing that the oldest alluvial fans in the Mission Creek region underwent significant capture by the Whitewater River drainage. This shows the autocyclic controls are also important on alluvial fan evolution in this region; but the importance of these processes to alluvial fan development in other regions of the American Southwest needs to be more fully assessed. The alluvial fans in the Mission Creek area traverse the Mission Creek fault, but are not deformed by it, which suggests that there may have been little if any movement along this fault since at least MIS 4. In contrast, alluvial fans in the Whitewater River study are displaced by active faults highlighting the influence of tectonism on alluvial fan development in this region. In addition to illustrating the importance of climatic controls on the development of alluvial fans in the American Southwest, a classic region for alluvial fan studies, this study illustrates the complex mixture of autocyclic and allocyclic factors that force alluvial fan development in tectonically active settings.

Precession‐scale cyclicity in the fluvial lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA)

AbstractLittle is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.

Basin-scale cyclostratigraphy of the Green River Formation, Wyoming

Research Article| January 01, 2013 Basin-scale cyclostratigraphy of the Green River Formation, Wyoming W. Aswasereelert; W. Aswasereelert † 1Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA2Department of Earth Sciences, Faculty of Science, Kasetsart University, 50 Phahon Yothin Road, Chatuchak, Bangkok 10900, Thailand †E-mail: wasinee@geology.wisc.edu Search for other works by this author on: GSW Google Scholar S.R. Meyers; S.R. Meyers 1Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar A.R. Carroll; A.R. Carroll 1Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar S.E. Peters; S.E. Peters 1Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar M.E. Smith; M.E. Smith 3Department of Geology, Sonoma State University, 1801 East Cotati Avenue, Rohnert Park, California 94928, USA Search for other works by this author on: GSW Google Scholar K.L. Feigl K.L. Feigl 1Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2013) 125 (1-2): 216–228. https://doi.org/10.1130/B30541.1 Article history received: 10 May 2011 rev-recd: 20 Apr 2012 accepted: 30 Apr 2012 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. Aswasereelert, S.R. Meyers, A.R. Carroll, S.E. Peters, M.E. Smith, K.L. Feigl; Basin-scale cyclostratigraphy of the Green River Formation, Wyoming. GSA Bulletin 2013;; 125 (1-2): 216–228. doi: https://doi.org/10.1130/B30541.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The fluviolacustrine Wilkins Peak Member of the Eocene Green River Formation preserves repetitive sedimentary facies that have been interpreted as an orbitally induced climate signal. However, previous quantitative studies of cyclicity in this member have used oil-yield data derived from single locations. Here, macrostratigraphy is used to quantitatively describe the spatiotemporal patterns of three different lithofacies associations from 8 to 12 localities that span much of the basin. Macrostratigraphic time series demonstrate that there is a reciprocal basin-scale relationship between carbonate-rich lacustrine facies and siliciclastic-rich alluvial facies. Spectral analyses identify statistically significant periods (≥90% confidence level) in basin-scale sedimentation that are consistent with Milankovitch-predicted orbital periodicities, with a particularly strong ∼100 k.y. cycle expressed in all lithofacies associations. Numerous non-Milankovitch periods are also recognized, indicating complex depositional responses to orbital forcing, autocyclic controls on sedimentation, or harmonic artifacts. Although fluctuations in Lake Gosiute water level did affect basin-scale patterns of sedimentation, they are not directly related to the 100 k.y. short-eccentricity cycle, as previously supposed. Instead, 100 k.y. cycles are principally recorded by the recurrence of alluvial environments, which exerted a dominant control on basin-scale patterns of sedimentation generally. Thus, the hydrologic controls on lake level that have been classically linked to short-eccentricity actually occurred at finer temporal scales (<100 k.y.). Understanding the complex links between orbital forcing and sedimentation in the Wilkins Peak Member is facilitated by analysis of time series that reflect spatial as well as temporal variability in stratigraphic data. Macrostratigraphy is, therefore, promising as an analytical tool for basin-scale cyclostratigraphy. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High frequency subtidal-peritidal cycles of the Callovian Calabozo Formation (Neuquén Basin, Western Argentina): Preliminary approach

The Calabozo Formation (Cuyo Mesosequence, Neuquen Basin) is made up by a thin, up to 30-m thick Middle Jurassic carbonate-dominated sequence. The minor, thin siliciclastic facies in this formation (i.e. lowermost granule-pebble shoreface conglomerate and fine-grained sandstones interbedded among the lower carbonates) record early transgressive shoreline and shallow lagoonal-subtidal sedimentation, respectively. The major carbonate deposits have been split into six facies and seven microfacies on the base of macroscopic field observation and petrological analysis. The carbonate facies have been grouped into three major assemblages: 1) Tidal flat facies assemblage (including the planar laminated algal boundstone facies; 2) Inner platform facies assemblage (including peloidal and bioclastic wackestone, coral floatstone and bioclastic and intraclastic packstone facies; and 3) Platform margin assemblage (including oolitic-peloidal packstone-grainstone facies). Afossil algal record made up by dasycladacean species (Salpingoporella annulata, Cylindroporella sp. and Heteroporella sp.) and blue-green algae (Cayeuxia (Rivularia) piae, and Cayeuxia (Rivularia) kurdistanensis) has been reported for the first time in this unit. The analysis of the intracycle facies and microfacies arrangements, as well as the nature of the cycle bounding surfaces, enabled one to identify two kinds of cycles. 1) Widespread subtidal cycles capped by shallow to intermediate subtidal facies, with no evidence of subaerial exposure at cycle tops; and 2) a minor subtidal cycle with similar shallowing-upward trend but capped by peritidal facies. The stacking pattern and thickness trend of these cycles was studied to determine possible allocyclic and autocyclic controls on their evolution. The Fischer plots reveal that the deposition of the Calabozo Formation subtidal-peritidal cycles took place under upward decreasing accommodation space conditions which may be related to eustatic lowering.

Sequence Stratigraphy and Depositional Facies of Lower Ordovician Cyclic Carbonate Rocks, Southern Missouri, U.S.A.

ABSTRACT Lower Ordovician cyclic carbonate strata of southern Missouri were deposited in a warm, shallow, epeiric sea on a fully aggraded carbonate platform. Sedimentological characteristics distinguish the Jefferson City and Cotter dolomites from the underlying Gasconade and Roubidoux formations. Mixed carbonate-siliciclastic sedimentation characterizes the Roubidoux Formation, with sandstones accounting for up to 60% of sedimentation. The Gasconade, Jefferson City, and Cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats. Casts of halite and ghosts of gypsum laths also exist in the Jefferson City and Cotter strata but are rarely found in the underlying units. Facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter-scale cycle types. Type I cycles consist of algal stromatolites, tidal-flat laminites (mechanical and algal), ooid grainstones, wavy peloidal wackestones, and quartz sandstones interpreted as peritidal facies. They are the dominant components of the Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Type II cycles consist mostly of subtidal facies such as strongly burrowed mudstone, thrombolite boundstone, and stromatolites. Type I cycles are thinner and represent highstand systems tracts, whereas the thicker type II cycles represent transgressive systems tracts and are dominant in the Gasconade Dolomite. The cycle stacking patterns, facies changes, and the intrabasinal correlatability of Fischer plots made from the widely spaced sections argue for a eustatic control on sea-level fluctuation on the platform. Interbasinal correlation with other North American basins is possible using biostratigraphic information and comparison of Fischer plots. Five Missouri sequences correlate with those described for other regions. The continent-wide uniformity in cycle stacking patterns indicates a primarily eustatic control on Lower Ordovician meter-scale cycle development. Regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins.

Clastic facies models, a personal perspective

Facies models evolved from classifications that were mainly descriptive, based on observable, measureable features such as the composition and texture of sedimentary rocks. As our understanding of sedimentary processes expanded, genetic facies models were developed based on the inferred process of formation. Since individual facies cannot be interpreted in isolation, they must be studied with reference to their neighbours, emphasizing the association of facies and sequences, in particular those that coarsen and fine upward. Environmental facies models are based on the interaction of studies on modern environments and ancient rock facies. Earlier facies models tended to invoke intrinsic, autocyclic controls. The advent of sequence stratigraphy led to greater emphasis on the surfaces that separate sequences and to external allocyclic controls. These were, initially, sea-level changes; later, changes in climate, tectonic movements and sediment supply were invoked. Over time, simple, all embracing models have given way to increasingly complex ones as our knowledge of the variability of nature has increased. Complex though these models are, they are only simplifications of reality. In nature there are no models and the majority of past environments differed in some respect from any modern environment. Each environment and rock sequence is unique.

Depositional controls in a bedload-dominated fluvial system: internal architecture of the Lee Formation, Kentucky

Sandstones of the early Pennsylvanian Lee Formation, central Appalachian foreland basin, were deposited in a low-sinuosity, bedload-dominated fluvial system. The internal architecture of the sandstones consists of a six-fold hierarchy of bounding surfaces, which are evaluated with respect to depositional controls. Minor (first-, second- and third-order) surfaces, generally less than a few hundred meters in extent, separate deposits of small- to intermediate-scale bedforms. Third-order surfaces, which dip gently in the downflow direction represent reactivation surfaces of downstream-accreting macroforms. Major surfaces (fourth-, fifth- and sixth-order) are hundreds of meters to kilometers in extent. Fourth-order surfaces separate deposits of individual macroforms. Fifth-order bounding surfaces are erosional boundaries of major (system-scale) channels. Individual sandstone members of the Lee Formation are bound by sixth-order surfaces, erosional at the base and gradational at the top.Deposits overlying major surfaces contain fining-upward sequences. The trends are more apparent with increased scale of the deposit. These sequences represent responses of the fluvial system to various mechanisms that controlled sedimentation. Both fourth- and fifth-order surfaces are interpreted as results of autocyclic controls. The nature of the Lee Formation fluvial system was such that a solitary major channel was relatively stable in one position within the alluvial plain for a significant period of time. During this time the channel aggraded with the passage of one or more downstream-accreting macroforms, creating a fourth-order erosional surface. The channel eventually diverted its flow to another portion of the alluvial plain by avulsion, creating a fifth-order surface. A gross fining-upward trend and upward change in fluvial style for individual sandstone members, both characteristic of paleovalley fills, indicate that sixth-order surfaces were controlled by allocyclic processes. These characteristics and the stair-step positioning of the sandstone members (younger away from the Alleghenian Mountains to the east), suggest that tectonic processes were the ultimate control on the system.

Cretaceous Sequence Stratigraphy of the Northern South American Passive Margin: Implications for Tectonic Evolution

The passive margin of northern South America, from Colombia to northeastern Venezuela, was relatively stable through the Cretaceous and only broadly affected by the entry of the Caribbean Plate into the Protocaribbean Basin. This region offers a unique opportunity to test the relative effects of global sealevel change, autocyclic sedimentologic processed, and regional tectonics in shaping the stratigraphic record of Cretaceous passive margins. High-resolution stratigraphic studies of Colombia and Venezuela have established a precise system of regional chronology and correlation with resolution <1 Ma (50-500 ka for the middle Cretaceous). This allows precise separation of allocyclic and autocyclic controls on facies development. This new chronology integrates assemblage zone biostratigraphy with event/cycle chronostratigraphy. Newly measured Cretaceous sections in Venezuela and throughout Colombia are calibrated to this new chronology, and sequence stratigraphic units independently defined to the third-order of resolution. Graphic correlation of all sections is used to identify sequences with regional stratigraphic expression, and those which correlate to sequence stratigraphic standards of North America, Europe and the global cycles of Hag et al. (1988). 50-60 percent of the stratigraphic sequences across the South American passive margin correlate to other continents and to the global sequence stratigraphic standard, reflecting strong eustatic influencemore » on Cretaceous sedimentation across northern South America. The remaining sequences in this region reflect tectonic modification of the passive margin and autocyclic sedimentary processes.« less

Stratigraphy, sedimentology and palaeontology of the Scarborough Formation (Middle Jurassic) at Hundale Point, North Yorkshire

SUMMARY The depositional environment of the type section of the Scarborough Formation (Ravenscar Group) has been investigated through detailed sedimentological and palaeontological analyses. The lithostratigraphical framework of the formation is revised and three new members, the Helwath Beck, Hundale Sandstone and Bogmire Gill members, are introduced. The marine transgression recorded in the lower part of the Scarborough Formation (Helwath Beck Member) is attributed to the abandonment and compactional subsidence of the underlying Gristhorpe Member (Cloughton Formation) delta lobe. The overlying deepening-upward package of siliciclastic and carbonate shelf deposits (Hundale Shale — Ravenscar Shale members) reflects the extent to which eustatic sea level rise amplified the effects of the dominant tectonic and autocyclic controls within the basin. The Bogmire Gill Member has been convincingly attributed to progradation of a wave-dominated sandy shoreface. Given that recent palynological analyses fail to support the presence of a significant hiatus at the contact with the overlying Scalby Formation, it is suggested that the Bogmire Gill Member may constitute the wave-dominated coastal fringe of a Scalby Formation delta lobe.