Storm Beds Research Articles

Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold‐thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining‐upward succession of clastic, storm‐influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.

Simulation of sedimentary facies on the Northern California Shelf

Research Article| July 01, 1997 Simulation of sedimentary facies on the Northern California Shelf Yong Zhang; Yong Zhang 1Department of Oceanography, Old Dominion University, Norfolk, Virginia 23508 Search for other works by this author on: GSW Google Scholar Donald J. P. Swift; Donald J. P. Swift 1Department of Oceanography, Old Dominion University, Norfolk, Virginia 23508 Search for other works by this author on: GSW Google Scholar Alan W. Niedoroda; Alan W. Niedoroda 2Woodward-Clyde Consultants, 3676 Hartsfield Road, Tallahassee, Florida 32303 Search for other works by this author on: GSW Google Scholar Christopher W. Reed; Christopher W. Reed 2Woodward-Clyde Consultants, 3676 Hartsfield Road, Tallahassee, Florida 32303 Search for other works by this author on: GSW Google Scholar Julian A. Thorne Julian A. Thorne 3Chevron Petroleum Technology Company, P.O. Box 446, La Habra, California 90633 Search for other works by this author on: GSW Google Scholar Author and Article Information Yong Zhang 1Department of Oceanography, Old Dominion University, Norfolk, Virginia 23508 Donald J. P. Swift 1Department of Oceanography, Old Dominion University, Norfolk, Virginia 23508 Alan W. Niedoroda 2Woodward-Clyde Consultants, 3676 Hartsfield Road, Tallahassee, Florida 32303 Christopher W. Reed 2Woodward-Clyde Consultants, 3676 Hartsfield Road, Tallahassee, Florida 32303 Julian A. Thorne 3Chevron Petroleum Technology Company, P.O. Box 446, La Habra, California 90633 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (7): 635–638. https://doi.org/10.1130/0091-7613(1997)025<0635:SOSFOT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yong Zhang, Donald J. P. Swift, Alan W. Niedoroda, Christopher W. Reed, Julian A. Thorne; Simulation of sedimentary facies on the Northern California Shelf. Geology 1997;; 25 (7): 635–638. doi: https://doi.org/10.1130/0091-7613(1997)025<0635:SOSFOT>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract A dynamic model is developed to simulate the sedimentary facies on the Northern California Shelf. In the model, dispersal mechanisms of small time scale are described by deterministic sediment-transport equations. However, at large time scale, the shelf dispersal system is described in probabilistic terms owing to its stochastic behavior. The probability of storm-bed generation is described by the bed generation potential, Pg, which is determined by the probability density distribution function of the local storms. The bed preservation potential, Pp, valued between 0 and 1, is a parameter that describes the probable truncation a storm bed that will be buried in a bed succession. The truncation depends on the erosion and deposition caused by subsequent random storms. By estimating the preservation potentials of all generated storm beds, the most probable storm-bed succession is predicted. The computations support the hypothesis that sedimentary facies are produced by the coupled mechanisms of progressive sorting and stratal condensation on continental margins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The drowning of a carbonate platform: an example from the Devonian-Carboniferous of the southwestern Holy Cross Mountains, Poland

The broad Middle Devonian carbonate platform of the Holy Cross Mountains was fragmented during the Frasnian and afterwards submerged as isolated blocks and transformed into a topographic depression. The section exposed in the Ostrowka quarry illustrates a well-documented drowning of the last small fragment (seamount) of this carbonate platform which submerged during Late Devonian and Early Carboniferous times. The recognized facies succession reflects a progressive, stepwise lowering of the sediment surface through time from a peritidal, lagoonal environment to a deep-water, anoxic basin. The Frasnian shallow-water carbonate platform is represented by lagoonal facies with peritidal sediments. Small-scale deepening-upward cycles punctuated with emersion surfaces are a feature of this stage. They resulted from a combination and continuous balance between carbonate production, subsidence and low-order, eustatic sea-level fluctuations. The carbonate platform aggraded at rates around 100–125 m/Ma. The shallow-water peritidal carbonates are separated from the post-platform deposits by an unconformity surface developed during subaerial exposure. The flooding of the faulted block of Galezice in the pre-late marginifera time and the subsequent initial drowning resulted primarily from a rapid sea-level rise. Small dimension of the seamount and its cemented upper surface were the most important factors that facilitated the sediment removal and thus suppressed the sediment accumulation. A condensed section was deposited on the top of the seamount. Its stratigraphic succession and rare cephalopod storm beds account for a model of redistribution of sediment over the swell, disintegrated by small-scale faulting. During drowning, the seamount traversed down through the photic zone at an average rate not higher than 20–25 m/Ma. Transformation of the swell into a basin accelerated in the late Tournaisian anchoralis Zone. It is manifested by an increase of deposition rate and by a lithology of alternating limestones and clays, with participation of fine-grained calciturbidites. The associated tephra deposits and syndepositional ferruginous hydrothermal mineralization were the result of extensional tectonics on the facies evolution. The drowning was completed in the Visean with deposition of black siliceous shales in the starved basin. The ‘death’ of the Devonian carbonate platform in the Galezice area illustrates well the famed ‘paradox of drowned platforms'’as the subsidence rate was much slower during the drowning than during the phase of the platform aggradation.

Storm-generated, hummocky stratification on the outer-Scotian Shelf

Hummocky megaripples occur on Sable Island Bank, Scotian Shelf. Submersible observations show that the megaripples form during winter storms and are subsequently obliterated through bioturbation and fair-weather reworking. The megaripples of this study were underlain by a storm bed composed of: (A) a basal scoured and infilled gravel lag facies; (B) low-angle tangential crossbedding in gravel to coarse sand; (C) anisotropic hummocky stratification in medium sand; and (D) wave ripple cross-lamination in medium/fine sand. This sequence forms a tempestite bed created by a winter storm during our sampling program. Numerical simulation of bed conditions during the storm suggests that the hummocky megaripples and hummocky stratification formed together during late stages of storm decay from conditions of sheet flow. Near-bed wave motion during deposition exceeded steady currents by an order of magnitude.

Linking Sediment Transport and Stratigraphy on the Continental Shelf

T H E GOAL OF THE SHELF sediment dynamics component of STRATAFORM is to link sediment transport processes active on the continental shelf to the formation and preservation of beds in shelf sediment deposits. An approach combining shelf sediment-transport models with high-resolution measurements of water-column and properties over periods from several months to several years allows us to make quantitative estimates of modification caused by sediment resuspension during episodic transport events. These modifications include erosion and deposition of material, formation of graded storm beds, and changes in small-scale surface morphology. The characteristics of the resulting event bed (thickness, grading, physical structures) are a function of flow and properties, depending on both temporal and spatial variations in sediment transport. Sediment transport on the continental shelf depends on surface-wave conditions, bottom-boundary-layer currents, fluid stratification, and characteristics, including grain size, density, porosity, and surface roughness. In general, sediment transport rates and depths of reworking are greatest when large, long-period waves occur simultaneously with strong, persistent currents. Thus understanding the magnitude and frequency of transport requires knowledge of the wave and current conditions on the shelf over sufficiently long periods of time to develop statistical characterizations of the wave and current fields and to document the resulting resuspension and modification. This information also provides critical

Carbonate ramp depositional systems from a late Jurassic epeiric platform (Iberian Basin, Spain): a combined computer modelling and outcrop analysis

ABSTRACTNo counterparts to epeiric‐sea carbonate ramps are known in present‐day environments. This hinders the interpretation of the factors controlling the growth and evolution of these depositional settings. In this study we analyse the facies and geometries of two Jurassic examples both from outcrop study and through computer modelling. This analysis is constrained by two important features of these Oxfordian and Kimmeridgian ramps: firstly, they are very well exposed, allowing accurate reconstruction of a 200‐km section from proximal to distal ramp environments, and, secondly, a time framework for correlation, section reconstruction and modelling is provided by a well‐defined ammonite biostratigraphy. The modelling results in a synthetic stratigraphy which closely matches the reconstructed cross‐sections and, when integrated with the field study, constrains and provides additional quantitative data on the following aspects of carbonate ramp systems.Resedimentation by storms is an important process in maintaining the ramp profile through time. Down‐ramp transport distances of between 25 and 40 km are indicated from the distribution of storm beds and shallow‐water allochems and from model‐matching known stratigraphic thicknesses and geometries.Modelling sediment production within the time constraints from the ammonite biozones indicates that shallow‐water carbonate production was 1–2 orders of magnitude less than that predicted for present‐day open‐marine carbonate platforms. Deeper‐water production rates were reduced by lesser amounts. These proportionally higher, outer‐ramp production rates also help to maintain ramp geometries through time.The enigmatic slope crest of ramps is shown to result from a combination of higher, shallow‐water production and erosion rates, together with loss of accommodation during highstands and high‐stillstands in the modelled sea‐level curves.The most parsimonious modelling of the two ramp sequences comes from a relative sea‐level curve composed of a linear subsidence component superposed by 20‐ and 100‐kyr cycles on a third‐order cycle. The third‐order cycles and their timing do not correspond to those of the Exxon curve.

Relationships between trace-fossil communities and substrate characteristics in some Jurassic pelagic deposits in the Umbria-Marche basin, Central Italy

The study of trace fossil assemblages is a very useful tool for defining depositional palaeoenvironments, such asthose of some early Jurassic pelagic sequences in the Umbria-Marche basin (late Carixian to early Aalenian). These sequences - Valdorbia and Fiuminata Colle Corno- are represented by thick bioturbated marls and limestones which alternate locally with poorly-bioturbated shales. Clay and silty-sandy deposits representing turbidites and storm beds (HCS = hummocky cross-stratified deposits) are very common, especially in the Toarcian interval. A comparison between trace fossil assemblages and sedimentologic/stratigraphic features permits an accurate paleoecologic and palaeoenvironmental analysis of sediments deposited in regressive conditions such as, for instance, the sequences studied. Trace fossil communities provide evidence of periods of non sedimentation or erosion during the Jurassic, when firm- or hardgrounds were commonly developed on muddy or sandy substrates. Penetration depth, burrow diameter, trace fossil density and species diversity emphasize the relationships with substrate characteristics and oxygen variations on the seafloor or in the sediment column. Consequently, analysis of different types of substrate and of substrate-controlled ichnofacies permits an evaluation of the paleoecologic strategies of the endobenthos in low to high-stress environmental conditions.

Depositional Trends in Siliciclastic Deposits of the Stone City Transgressive Systems Tract, Middle Eocene, Texas: ABSTRACT

Strata exposed at Stone City Bluff, Texas, contain shelf sediments deposited during progressive marine deepening (from shallow water depths) and then capped by a maximum flooding horizon. They are part of the early Bartonian (NP16) Cook Mountain transgressive systems tract, exposed in the type section of the Stone City Member of the Crockett Formation. This locality lies on the inner portion of a shelf transect about 175 km (110 mi) shoreward of the Late Eocene shelf-slope margin and probably never farther than 80 km (50 mi) from the shoreline. Sedimentary structures in the sediments change upward, reflecting a shift from tidal current-dominated to storm-dominated deposition. In lower intervals, tidal control is dominant. The transition interval contains isolated storm sands, as sheets with lenses 40 cm (15 in) thick, in mudstones with tidal bedding. These storm beds contain teeth/bone concentrates and some gravel-size clasts at the bases of larger lenses. In higher intervals, storm deposition is dominant in all units, mostly as thin, distal layers of very fine, sand or silt. Amalgamated storm deposits form larger units (e.g., the Moseley bed). Local hardground surfaces are developed where storm washing exposed concretionary layers or on condensed, winnowed zones. Glauconite occurrence and siderite concretion formation are most common in the upper intervals. Storm deposits have dual sediment sources and contain 1) skeletal remains (shells/fish bone) and glauconite pellets concentrated by in-place winnowing, or 2) fine quartzose sand washed in from shallower water settings. Alternation of amalgamated storm deposits and mudstone units with thin storm deposits indicate a cyclic record of storm activity.

Shelf storm beds on the southern margin of the Welsh Basin: the Wenlock of the Tortworth Inlier, England

The Brinkmarsh Beds (Wenlock, Silurian) of the Tortworth Inlier, near Bristol, SW England were deposited in shallow marine water on the southern margin of the Welsh Basin. Occasional sandstone units are interpreted as storm-induced deposits that periodically overwhelmed the quiet-water deposition of siltstones and mudstones. Evidence for storm-bed deposition is the episodic nature of the sandstone units, their erosive bases, hummocky cross-stratification in some, and suspended coquinites in others. There are two types of shell bed within the sequence: in situ units within the siltstones containing shallow-water brachiopods, corals, and crinoids, and transported faunas in the sandstones exclusively of large brachiopods. Palaeonto-logical and sedimentological evidence suggests that deposition took place in a protected embay-ment on the shallow shelf.

Sedimentary substrate andtrace fossil assemblages in marine Pliocene deposits in Northeast Spain

The trace fossils in the marine Pliocene of the Baix Llobregat area display a strong relationship to their hostsediment. Clays include syndepositional traces belonging to Domichnia ( Ophiomorpha nodosa, O. annulata and O. irregulaire) and Fodinichnia ( Rosselia socialis). The trace fossils in sandstone layers are postdepositional and may be included in the groups Fodinichnia ( Thalassinoides suevicus), Pascichnia ( Neonereites uniserialis, N. biserialis, Laminites sp. and Planolites montanus) and Cubichnia ( Cardioichnus sp.). This relationship is the consequence of several characteristics of the sedimentary substrate, that are strongly influenced by grainsize (oxygenation, amount of organic matter). The way in wich the storm beds were colonised also played an important role.

Transgressive facies sequence of a high energy, wave-tide-storm-influenced shoreface: A case study of the east Frisian Barrier Islands (Southern North Sea)

Published information and recent observation of physical and biogenic structures as well as grain size patterns along the East Frisian barrier islands (southern North Sea) have been synthesized to develop a facies model for a high energy, wave-tide-storm-influenced transgressive shoreface. As exemplified by Spiekeroog Island, three major shoreface facies (Upper, Central and Lower) are distinguishable, each of which can be subdivided into a Proximal and a Distal subfacies. A transgressive shoreface sequence in the rock record, in ascending order, will show the Upper Shoreface facies (USF) at the base. This is characterized by very well-sorted fine sands exhibiting predominantly plane lamination, which is consistent with its wave-dominated flow regime. The Central Shoreface facies (CSF) overlying the USF shows a high proportion of shells, a broad range of sediment sizes (fine sand to pebbles), and marked diversity in sedimentary structures. Graded storm beds, tidal cross-strata, mud-sand couplets, and horizontal to subhorizontal lamination are common. Swaley/hummocky (?) strata may also occur in its Distal subfacies. The sequence is topped by the Lower Shoreface facies (LSF). It consists mostly of bioturbated fine sands, although graded beds do occur. Distal sabfacies sands are consistently finer, better sorted, and more positively skewed than their Proximal counterpart. Anomalies in both coastwise and cross-shore textural trends are noted. In contrast to most other coastal settings, both fair-weather (wave- and tidal-current) and storm deposits are likely to be represented in subequal proportion in the rock record.

EVOLUÇÃO GEOLÓGICA DOS GRUPOS CANASTRA E IBIÁ NA REGIÃO ENTRE COROMANDEL E GUARDA-MOR, MG

The stratigraphic column established for the Canastra Group, in the Alto Paranaíba region in the centralportion of the Brasília Fold Belt, represents a regressive megasequence of sediments deposited on an openseacontinental platform. The following units occur from base to top: a turbiditic sequence with arenaceousand argillaceous intercalations; an arenaceous sequence containing storm beds; and a clay-rich arenaceoussequence with structures reflecting shallow marine and tidal flat environments. The Ibiá Group overlies theCanastra Group on an erosional disconformity. Its base consists of a metadiamictite of glacial origin (CubatãoFormation) while the uppermost unit (Rio Verde Formation) comprises an extensive and monotonous packageof banded calcareous phyllites. The contact between these two formations is gradational and both units havesimilar mineralogic composition. The Canastra and Ibiá Groups were affected by a single event of progressivedeformation, E1, during the Brasiliano Cycle, manifested as two distinct compressional stages. The earliestand most compressive stage, D1, is represented by thrust tectonics, associated with greenschist faciesmetamorphism. The second stage of deformation, D2, is characterized by a dominant component of pureshear in a ductile-brittle regime. When compared with other similar units in the geochronological record, thecharacteristics of the studied units show that the Ibiá Group metadiamictites are contemporaneous with theJequitaí tillite, with a probable age of deposition between 900 and 1000 Ma and the Canastra Group istherefore older than 1000 Ma.

Modern Shoreface and Inner-Shelf Storm Deposits Off the East Texas Coast, Gulf of Mexico

ABSTRACT The modern shoreface and inner-shelf sediments off the east Texas coast on the Gulf of Mexico were examined using sediment cores and high-resolution seismic data. Low sediment input and low effective accommodation space resulted in the overall thinness of the modern marine sediment cover in the region. Erosion on the inner shelf during historic ( 100 yr) times produced patches of sand in a predominantly mud-dominated environment. Storm beds are scarce on the inner shelf. The shoreface succession off Galveston Island is characterized by offshore-directed crosscutting storm channels of varying sizes. The existence of these channels and the relatively high frequency of storms and hurricanes that have affected the region during historical imes are inconsistent with the paucity of storm deposits on the inner shelf. The offshore-directed channelized flows may have been prevented from going beyond the shoreface by strong along-shelf storm currents that impinged on the shoreface. In contrast to the storm record on the east Texas shelf, the central Texas shelf shows a greater frequency of and more laterally persistent storm sand beds due to greater effective accommodation space and higher sediment supply there.

Deposition and diagenesis of Clinton ironstones (Silurian) in the Appalachian Foreland Basin of Pennsylvania

Clinton ironstones (mid-Silurian) on the southeastern margin of the Appalachian Foreland Basin in central Pennsylvania consist largely of skeletal grainstone storm beds whose skeletal fragments are extensively impregnated and replaced by ferric oxide. Other forms of iron mineralization consist of ooids, superficial ooids, cavity fillings and cements composed of ferric oxide and/or chamositic clay, and two generations of iron-rich carbonate spar. True oolite is rare. Small proportions of ferruginized grains are present in coarse lags at tops of some small-scale, 1- to 5-m coarsening-upward sequences, which are the record of mid-shelf shoals from which the skeletal grainstone storm beds were derived. Ooids and superficial ooids are likely to have formed in dysaerobic conditions where the pycnocline of a highly stratified sea intersected shoal flanks. Ferrous iron from anaerobic deeper waters precipitated on quartz and skeletal fragment nuclei in weakly reducing conditions with episodic gentle agitation. Several stages of early diagenetic ferruginization affected skeletal grainstone storm beds and sequence-capping coarse lags: chamositic clay precipitated in pores and cavities of skeletal fragments, within quartz sand adjacent to crinoid ossicles, and as bed-parallel connectors of large skeletal fragments. As diagenesis progressed with further burial, many skeletal fragments were extensively replaced by ferric oxide, most chamositic clay altered to ferric oxide, other chamositic clay matured from berthierine to chamosite, and clusters of bladed specularite replaced parts of primary grains and secondary carbonate cements. Paleomagnetic and fluid-inclusion analysis indicates that many late diagenetic changes took place at elevated temperatures during deen burial, in an early phase of the Alleghanian orogeny.

Geographic gradients in the formation of shell concentrations: Plio-pleistocene marine deposits, Gulf of California

Shell bed (coarse concentrations of invertebrate bioclastic material) are rich sources of paleontologic and stratigraphic information. Studies often emphasize the relationship between shell bed formation and bathymetric gradients, because many sedimentologic and ecologic factors that influence shell-concentrating processes vary with water depth. However, shell bed formation is also influenced by factors that are not related to water depth. Shell beds in Plio-Pleistocene marine sediments of the Gulf of California (Mexico) formed in less than 10–15 m water depth, but under varying conditions of coastal topographic relief and tidal range. Sampling of shell beds focused on two regions: the northern mainland coast and the southern peninsular coast. Shell beds in the northern mainland region (13 total) formed along a macrotidal, low-relief coastline, whereas shell beds in the southern peninsular region (27 total) formed along a microtidal, high-relief coastline. Shell beds represent community beds (11); storm beds (3); beach berm beds (6); tidal channel bed (1); or current/wave-winnowed beds (19). The community beds represent biogenic concentrations; the other four types represent sedimentologic concentrations. Sedimentologic concentrations formed either through short-term concentrating events (storm beds), or long-term dynamic bypassing (beach berm beds, tidal channel beds, and current/wave-winnowed beds). Concentrations formed through dynamic bypassing are far more common (26 beds) than storm concentrations (3 beds). Of the 26 concentrations formed through dynamic bypassing, 10 cap major stratigraphic discontinuities, representing temporal breaks ranging from 10 3 to 10 8 years. The abundance and distribution of the five types of shell beds is significantly different between the two sampling areas. The northern mainland coast is dominated by allochthonous beach berm beds capping linear beach ridges. The formation of beach ridges in the north is favored by the wide shallow shelf (representing a large reservoir for beach shells), and sediment starved conditions resulting from the low topographic relief and periodic reductions in sediment supply from the Colorado River delta. Beach berm beds are absent along the southern peninsular coast, where coarse terrigenous sediment shed from adjacent highlands overwhelms shell input to the beach, and the narrow steep shelf supplies the beach with few shells. The southern peninsular coast is dominated by parautochthonous current/wave-winnowed beds and autochthonous community beds. Southern shell beds have undergone significantly less reworking, winnowing and transportation than northern shell beds. This is a function of both the relative weakness of tidal currents in the south, and the relatively higher rates of terrigenous sedimentation along the rugged southern coastline, which can bury shell concentrations relatively rapidly. Geographic variation in topographic relief and tidal range results in significant variation in the distribution and abundance of shell beds within these shallow marine sediments. These variables have root tectonic cause (relief reflects tectonic activity; tidal range reflects basin configuration), underscoring the potential utility of fossil concentrations for reconstructing earth history.

Depositional Gradient Analysis in Transgressive Systems Tracts and Highstand Systems Tracts, Eocene of the Brazos River Valley Section, Texas

Middle-late Eocene third-order sequences in the Brazos River section are composed of transgressive and highstand systems tracts of shelfal marine and marginal marine sediments. Diagenesis has destroyed most fossils (except palynomorphs, arenaceous forams, and traces), but sedimentary structures and fabric provide evidence for determination of depositional gradient. Tidalite deposits dominate shallower parts of the gradient and proximal and distal storm deposits dominate deeper parts of the gradient. As seen in the Weches, Crockett, Yegua, Caddell, and Manning formations, a characteristic shoaling gradient consists of a coarsening-upward section of deeper water mudstones with thin distal storm deposits overlain by sediment with proximal storm deposits (with upward-increasing storm bed content), by planar cross-bedded sands and by intensely bioturbated shore-zone sands. A characteristic deepening gradient is a fining-upward section of cross-bedded sands overlain by flaser bedded (and plant-fragment rich) tidalites, by proximal storm deposits (usually fossil and glauconite rich), and by shales with thin distal storm deposits. Maximum flooding surfaces (associated with sediment starvation) are indicated by submarine exposure surfaces and zones of amalgamated storm beds. The lithologic character of storm deposits varies with availability of grains (sand, shell, glauconite pellets, silt) and distance of transport. Gradient analysis was used to locate themore » maximum flood of the Jackson Stage in the Brazos River section. It lies in a parasequence within the lower lignite-bearing portion of the Manning Formation, as verified by the last appearance datum of Textularia hockleyensis.« less

Diagenetic Heterogeneities in Mid-Jurassic Sandstone Reservoirs in the North Sea

Carbonate-cemented intervals are common in the Mid-Jurassic deltaic and shoreline sandstone reservoirs of the North Sea and become a major problem when they are laterally extensive, as they affect vertical communication, create different fluid flow units, and reduce net pay. In a vertical section through a reservoir, these cemented zones commonly make up 110% of the total thickness. Cores and well logs from eight wells in three oil fields (Bruce and Lyell in the UK sector and Veslefrikk in the Norvegian sector) have been studied in order to understand the diagenetic processes responsible for these carbonate cements and to relate them to sedimentological and stratigraphic patterns within the deposit. The carbonates occur as nodules, concretions or stratiform layers 0.5 to 5 m thick. They comprise poikilotopic calcite formed during three successive phases of cementation, ranging from early to late in the burial history. Each phase of cementation can be characterized by oxygen and carbon isotopic signature, as well as by cathodolumiunescence patterns. By studying the occurrence of vertical variations of the 87Sr/87Sr ratio in the formation water above and below the carbonate-cemented horizons, it has been possible to identify those cemented horizons which are laterally extensive and form internal barriersmore » within the reservoir. These carbonate barriers generally coincide with sedimentological discontinuities such as flooding surfaces or the abrupt appearance of storm beds. This relationship between diagenetic barriers and sedimentological/stratigraphic events furnishes practical guidelines to predict their occurrence in this type of reservoir.« less

Rhythmic shelf storm beds: Proterozoic Kaimur Formation, India

The shallow-marine, turbidite-like storm beds of the Proterozoic Kaimur Formation exhibit intrabed rhythmic vertical variations in sedimentary structure and texture attributable to different patterns of velocity pulsations within an event of storm current deposition. The alternate repetition of parallel-laminated (S b) and ripple cross-laminated (S c) divisions (S bcbc…) within individual storm beds has been interpreted to represent periodic fluctuation in flow intensity beyond the stability limits of ripple and upper flat-bed during sedimentation. Sandstone beds characterised by successively occurring thin, internal mud partings indicate pulsing discontinuous flows with alternate periods of slack water within an event of storm current deposition. Successive mud drapes occurring within the ripple cross-laminated divisions (S c) of some of the storm beds also represent pulsing discontinuous flows. Palaeoccurrent analyses of these storm beds of the Kaimur Formation reveal a northwesterly current mode against an E—W trending shoreline. These storm beds thus may be interpreted to have deposited from geostrophic storm currents that were pulsating in nature with a periodicity of the order of tens of hours similar to that observed on present-day storm-dominated shelves.

Upper Cretaceous Stratigraphy of the Island of Brac within the Geodynamic Evolution of the Adriatic Carbonate Platform

Though news coming out of Yugoslavia these days is generally bad, this unusual book is a welcome exception. The work was stimulated by a project develop a new geologic map of Croatia; the result is a highly readable book, with the authors' senses of history, curiosity, and humor readily evident throughout. The purpose, as stated in the Introduction, is to rehabilitate the concept of (p. 128) from dull, descriptive cataloging of names of units an approach stratigraphy that is dynamic, interpretive and set in a global context. The authors recognized that their contribution could not be fully utilized by both local and international audiences unless their results were published in Croatian and in English. The book should be useful for geologists in at least four categories: (1) the work provides a synthesis of Upper Cretaceous stratigraphy of a sizeable portion of the Adriatic carbonate platform and will be a valuable resource for specialists; (2) the Croatian text will be an important educational tool in Yugoslavia, presenting both lithologic and paleontologic details and a philosophical approach stratigraphic research; (3) the English text provides the international audience with a demonstration of stratigraphy as an exciting, dynamic, and interpretive science; and therefore (4) will be useful as a thoughtprovoking reading in advanced stratigraphy courses. The book includes 160 pages of text (20 x 29 cm format) with 22 figures including three foldouts, and 20 plates. Figures, with both Croatian and English captions, are located in the 102 pages of Croatian text. The English text covers 32 pages. The first two sections, the Introduction and the Short Review of Previous Stratigraphic Research on the Cretaceous of Brac, are relatively brief and nearly comparable in length and content in both Croatian and English. Section 3, Description of the Lithostratigraphic Units, and the discussion sections that follow, are about 40 percent as long in English as in Croatian. Detailed arguments concerning history and choice of formation names are only available in Croatian. Furthermore, consistent with the educational and philosophical aim of the Croatian text, more extensive general discussions of sedimentological and paleoecological topics are included there than in English. In Section 3, lithofacies and biofacies of each of the six Upper Cretaceous formations outcropping on the island are described; brief descriptions of these units and the authors' interpretations of them are presented below. Biofacies descriptions and interpretations emphasize foraminifera, mollusks, algae, and cyanobacteria. Biostratigraphic interpretations are based upon foraminifera in cases where differences in age interpretations have arisen between diagnostic foraminifera and rudists. The authors utilize both modern and Cretaceous carbonate facies models, as well as ecological studies of modern foraminifera and mollusks, in developing paleoenvironmental interpretations of facies within formations. During the Upper Cretaceous, what is now the Island of Brac was located in the interior of the large Adriatic carbonate platform. The oldest of the Upper Cretaceous units on Brac is the Cenomanian age Milna Formation, which is on the order of 370 m in thickness. The unit is characterized by alternation of three predominant facies: grain-supported packstone/grainstone, peloidal-skeletal wackestone, and micritic and cryptalgal laminites. The paleoenviromental interpretation is restricted, muddy platform-interior setting as indicated by intertidal laminites and shallow subtidal Chondrodonta oyster beds and radiolitid biostromes, and by frequent storm beds. The predominantly mud-supported limestones of the Sveti Duh Formation, illustrated as 140 m in thickness, conformably overlie the Milna Fm. Where the transition is gradual, intercalations of micrite repeatedly alternate with cryptalgal laminites in the top 10 m of the Milna and the boundary is arbitrarily placed after the last occurrence of laminite. Sveti Duh mudstones contain calcispheres and small planktonic foraminifera, as well as other fossil debris in varying proportions. In some cases, concentrations of fossil debris form bioclastic calcarenites that appear be allochthonous gravity-displaced turbidites. The Sveti Duh Fm is interpreted represent hemipelagic deposition associated with the Latest Cenomanian-Early Turonian global eustatic sea level rise. Unfortunately the authors do not address two interesting aspects of this unit. They do not speculate on how hemipelagics might alternate with laminites, interpreted as intertidal. Neither do they suggest an origin for the gravitydisplaced shallow-water bioclastics. However, these are among the few such anomalies that they do not consider, which is partly why the book is so provocative. The third major unit, the Gornji

Coarse-Grained Storm Beds of the Upper Cretaceous Chungo Member (Wapiabi Formation), Southern Alberta, Canada

The Upper Cretaceous Chungo Member of the Wapiabi Formation at Mount Yamnuska, Alberta is composed of three cycles: two coarsening-upward progradational successions overlain by a fining-upward transgressive succession. The basal 3 m of the transgressive sequence is made up of interbedded sandstone and conglomerate, with minor shale that was deposited in a nearshore, shallow-marine setting. Discrete storm in this interval are characterized by: 1) a sharp-based conglomeratic division with local sole marks and onshore-imbricate intra- and extraformational clasts; 2) a cross-stratified conglomeratic division with cross-strata dipping predominantly onshore and capped by symmetrical coarse-grained ripples; 3) a horizontal to sub-horizontally laminated sandstone division containing pebble horizons near the contact with underlying conglomerate and commonly capped by symmetrical ripples; and 4) a capping shale division. Deposition of the coarse-grained storm beds took place in two stages: 1) transport from a beach by offshore-directed, storm-generated combined flows, followed by 2) reworking and selective sorting of bed material by asymmetrical oscillatory currents generated by shoaling swell waves propagating onshore. Stage 2 involved the onshore transport of selected coarse grain sizes, which deposited as the reworked portions of coarse-grained storms beds (divisions 2 and above), concurrent with offshore transport of fine sediment that may be deposited as the top portions of HCS storm beds.