Offshore Mud Research Articles

A Neoproterozoic multi-phase rift sequence: the Grampian and Appin groups of the southwestern Monadhliath Mountains of Scotland

The Grampian and Appin groups of the southwestern Monadhliath Mountains form the earliest known syn-rift sequences of the Scottish central Highlands. They were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mountains was deposited during a period of initial basin rifting (NW–SE extension) followed by a phase of thermal subsidence. Syn-rift sediments comprise a 2.5–6 km thick turbidite system. Thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overlying Appin Group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition. This formed in response to renewed rifting and concomitant subsidence. Accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin widening and produced an onlapping marine sequence. There followed a period of progressive clastic deprivation when carbonates were precipitated, and at the onset of anoxic conditions, deposition of organic muds. The fundamental structural elements responsible for the formation of the Grampian and Appin group basins were also influential in the orogenic evolution of the basin-fill. Half-graben fills were deformed to produce regionally extensive folds such as the Stob Ban–Craig a’ Chail Synform.

Silurian calymenid trilobite Alcymene n. gen., with new species from the Ludlow of Gotland, Sweden

Alcymene n. gen. comprises a monophyletic group of calymenids known from the Ludlow Series of Sweden and the British Isles. The sister species Alcymene alveus and A. lindstroemi n. gen. and spp. (Hemse Beds, Gotland, Sweden) are most closely related to the type species A. neointermedia (Richter and Richter), which occurs in overlying and underlying strata at nearby localities. Structural modification of A. alveus and A. lindstroemi, especially the development of a greatly broadened and troughed librigenal border, can be interpreted as “snowshoe” adaptations for increased stability on an offshore mud substrate.

Ammonite shell shape covaries with facies and hydrodynamics: Iterative evolution as a response to changes in basinal environment

Shell shape varies within many ammonoid species, and some ammonoid lineages appear to have evolved in concert with changes in their environment. We report variation within an Upper Cretaceous ammonoid species that correlates with facies differences and is consistent with a hydrodynamic explanation. In the Turner Sandy Member of the Carlile Shale (Turonian) of South Dakota and Wyoming, more compressed morphs of Scaphites whitfieldi Cobban are found in nearshore sandy facies, whereas more depressed morphs occur in offshore muds. We measured drag forces on models of juvenile and adult shells that differed in lateral compression of the shell. Plots of drag coefficient as a function of Reynolds number indicate that thinner, more compressed morphs swam more efficiently at higher velocities and depressed morphs swam more efficiently at low velocities. Higher swimming velocities may be essential for life in nearshore sandy environments, which have higher ambient current velocities. Shelled cephalopods swim most efficiently at low swimming speeds; therefore, lower velocity, more energetically economical swimming should be preferred in more quiescent offshore settings. An analysis of power consumption supports this interpretation. Correlated changes in shell compression and environmental factors, here observed within a species, have been documented in numerous ammonite lineages. These iterative evolutionary changes within lineages may be similarly explained by selection for shell morphologies appropriate to environments that fluctuate cyclically with sea level.

Sediments and sedimentary processes in Mutsu Bay, Japan: Pelletization as the most important mode in depositing argillaceous sediments

Mutsu Bay is a large transgressive inland basin that originated in a set of structural depressions brought about by tectonic processes in early Quaternary time. The offshore beds of the bay consist of muds rich in organic material, while sands and gravels occur as a coastal sediment facies. Active sediment palletization of muds owing to feeding by polychaetes is recognized throughout the bay, and the bed surface is mostly covered with fecal pellets. Laboratory experiments in a seawater flume suggest that isolated fecal pellets are removed more easily than ambient muds. Continued pelletization dramatically reduces the critical entrainment velocity of the bed surface. Sedimentary processes in Mutsu Bay have been influenced not only by sea-level fluctuations but also by the oceanographic conditions of the Sea of Japan. Offshore mud deposition in the bay can be attributed to warm-water circulation which came into existence about 10,000 years ago. A wet and mild climate in consequence of temperatere seawater of post-glacial warming increased biochemical weathering of the land surface, leading to a voluminous discharge of fine-grained materials into the bay. Sediment pelletization by polychaetes facilitated easy transport of argillaceous grains in response to bottom currents, resulting in the development of offshore mud layers.

Biological and Environmental Control of Mercury Accumulation by Fish in Lakes and Reservoirs of Northern Manitoba, Canada

Enlargement of northern Manitoba lakes to form hydroelectric reservoirs caused a rise in the Hg content of walleye (Stizostedion vitreum), northern pike (Esox lucius), lake whitefish (Coregonus clupeaformis), yellow perch (Perca flavescens), and spottail shiner (Notropis hudsonius) owing to stimulation of Hg methylating microbes by submerged terrestrial organic matter. However, an increase in the supply of organic substrates beyond a critical amount mitigated this effect, apparently by promoting Hg demethylation and production of Hg-binding agents such as sulfides. Fish species, even at the same trophic level, differed markedly in their Hg content, Hg – fork length relationships, and regional and temporal variations in Hg content; this is ascribed to differences in habitat preference, metabolic rate, age, growth rate, size, biomass, diet, and excretory pathways, and hence different responses and degrees of exposure to spatially varying conditions influencing Hg speciation and availability. Whitefish had an anomalously weak tendency to accumulate CH3Hg+ and a high ratio of liver CH3Hg+ to muscle CH3Hg+, suggesting a special mechanism of excretion. Only walleye Hg correlated significantly with CH3Hg+ production in offshore mud, indicating the value of walleye for whole-lake Hg bioassays and the validity and utility of the methods used to quantify CH3Hg+ production.

Variations of the Sequence Stratigraphic Model: Past Concepts, Present Understandings, and Future Directions

The working hypothesis upon which the sequence concepts are based is that the relative sea level change results in changes in the capacity of a basin to accommodate sediment, which, in turn, results in a succession of sequences. The interplay between eustasy, tectonics, sediment flux, and physiography yields a predictable geologic response in carbonate, clastic, as well as mixed carbonate/clastic settings. The criteria for recognition of sequence boundaries can be varied within a given basin as well as between basins. They include but are not restricted to (1) a basinward shift of facies across a sharp bedding contact, (2) onlapping stratal geometry, and (3) truncation of strata. The key to the correct utilization of these concepts is to recognize sequence stratigraphy as an approach or a tool rather than a rigid template. Observations from the upper Albian, Cretaceous, Viking Formation of the western Canadian sedimentary basin are presented to illustrate the stratigraphic expression of clastic depositional sequences on a ramp margin. In this setting, forced regressions and lowstand shorelines commonly occur, incised valleys sometimes occur, and submarine fans rarely occur, in response to fluctuations of relative sea level. The base of the Viking Formation sometimes is characterized by relatively coarse-grainedmore » sediments sharply overlying fine-grained offshore muds and is interpreted as a third-order sequence boundary. Pebbles occasionally are observed at this contact. Subsequently, a number of higher-order sequences within the lower to middle Viking are observed and are characterized by the occurrence of forced regressions and lowstand shorelines without associated incised valleys.« less

The Placentian Series: appearance of the oldest skeletalized faunas in southeastern Newfoundland

The lowest Cambrian of Avalon, or Placentian Series, is a relatively thick sequence (1,400 m) in southeastern Newfoundland. A newly proposed body fossil zonation supplements an existing trace fossil zonation of the lower part of the Placentian Series and includes strata to the top of the sub-trilobitic Lower Cambrian.TheSabellidites cambriensisZone brackets the Precambrian–Cambrian boundary and comprises peritidal and wave-influenced subtidal facies deposited during deepening through the lower part of the Chapel Island Formation. Younger “Ladatheca”cylindricaZone strata include the deepest facies of the Chapel Island Formation. The base of the overlyingWatsonella crosbyiZone (a post-Nemakit Daldyn and pre-Tommotian equivalent) is significantly diachronous because the diagnostic mollusks were preferentially preserved in pyritiferous offshore muds rather than in coeval nearshore muds. High diversity, upperWatsonella crosbyiZone faunas (18 species) are limited to peritidal limestones of member 4 and are dominated by calcareous small shelly fossils. A thick interval (ca. 430 m) without body fossils and an important episode of block faulting that led to 750 m of differential erosion preceded deposition of the lower part of the Bonavista Group (=Sunnaginia imbricataZone, an interval considered to be largely older than the Tommotian). Although much Early Cambrian time may be lost as a result of erosion at Random Formation–Bonavista Group unconformities, manyWatsonella crosbyiZone species reappear in theSunnaginia imbricataZone. Shoaling accompanied the immigration event defining the base of theCamenella balticaZone, and an unconformity following regional offlap marks the top of the Placentian Series.Calcareous, and not phosphatic, composition is most common in earliest Cambrian shelly remains. Little evidence suggests that a global, Precambrian–Cambrian boundary interval “phosphogenic” event either resulted in deposition of local phosphate deposits in the Tethyan region or had a role in the appearance of mineralized skeletons.Twenty metazoans and problematica and an alga are illustrated from the Chapel Island Formation.Bemella?vonbitteriLanding n. sp. andHalkieria stoneiLanding n. sp. are described. The monoplacophoranArchaeospira?avalonensisLanding n. sp. has right-and left-handed conchs comparable to those ofArchaeospira(=Yangtzespira) from China.Anabaritesis the senior generic synonym ofTiksitheca.

Transgressive sand sheet covering the shelf and upper slope off Sendai, Northeast Japan

A thin sand sheet deposited during the last major transgression from 14,000 to 6000 yrs B.P. covers the shelf and upper slope off Sendai, Northeast Japan. The sand sheet overlies the sediments deposited during the last low sea-level stand, and is partly overlain by modern offshore muds. This sheet is regarded as palimpsest sediment, formed during the transgression in water depths of > 50 m. The sand sheet is 50–150 cm thick, and consists of alternations of poorly sorted thin sand layers and mud layers that are several millimeters to a few centimeters thick. It bears mixed-environment molluscan shells and small amounts of glauconite. Scour-and-fill structures are frequently found. Gravel with shell fragments is common near the base of the sheet.

Sedimentology of the cold-climate, coal-bearing, Lower Permian “Lower Freshwater Sequence” of Tasmania

The Lower Permian “Lower Freshwater Sequence” of Tasmania consists of fluvial and coastal sediments deposited in a cold-climate setting adjacent to ice-ridden, shallow seas. The sequence represents a regressive pulse during the filling of a marine basin, possibly a fjord, up to 150 km wide and more than 270 km long, occupied by glaciers in the late Carboniferous. The climatic conditions during the early Permian in Tasmania are revealed by (1) well-developed continental scree breccias, (2) the flora ( Glossopteris-Gangamopteris assemblage), (3) rarity of fossils in tidal deposits, (4) the structure of the trace fossil population (intense bioturbation in offshore muds), (5) the foramol nature of the fauna of the marine beds below and above the Lower Freshwater Sequence, a fauna dominated by brachiopods and bryozoa and containing Eurydesma, and (6) features of the sediments themselves, such as “lonestones” in muddy offshore marine deposits, and oxygen isotopes of associated marine carbonates. Although the sedimentation in the Lower Freshwater Sequence is qualitatively analogous to that of the Quaternary of Canada, the fluvial and coastal sediments do not show evidence of ice rafting nor of frozen ground. Lack of such evidence suggests a cold-temperate rather than a subpolar setting.

Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea

Radiochemical measurements of 234Th (t 1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t 1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month −1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y −1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current. 210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked ( D b = 26cm 2 y −1) relative to the zone between 5 and 25 cm ( D b = 2cm 2 y −1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y −1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is ⩾18.

Comparative toxicity of offshore and oil-added drilling muds to larvae of the grass shrimpPalaemonetes intermedius

Offshore drilling fluids (muds) varied widely in their toxicity to grass shrimp (Palaemonetes intermedius) larvae. The 96-hr LC50s for the eleven drilling muds tested ranged from 142 to >100,000 ppm (μl/L). There was a significant correlation between oil content of the drilling muds and their toxicity. Furthermore, addition of diesel oil (No. 2 fuel oil) or mineral oil to an offshore drilling mud having a low oil content or to an oil-free synthetic drilling mud led to a marked increase in the toxicity of these muds. Thus, much of the toxicity of the offshore drilling muds tested can be attributable to the oil content.

Basin Development and Hydrocarbon Occurrence Offshore Mid-Norway: ABSTRACT

The continental shelf offshore Mid-Norway was opened for exploration drilling in 1980. The area extends southward to the North Sea sedimentary basins and northward to the Barents Sea continental shelf. The central part is a proven petroleum province. By use of mainly seismic and well data, the tectonic evolution, sedimentary facies, reservoir potential, and hydrocarbon generation and distribution have been studied. After peneplanation of the Caldeonides, a large epicontinental basin connected the region between Greenland and Norway during the late Paleozoic and Mesozoic. Pre-Triassic rocks have not yet been recorded in the wells and nothing is known about their exact composition. The main targets for the exploration drilling have been the Triassic-Jurassic succession. The Triassic consists mainly of continental red shales with sandstone and salt intervals of poor to no source or reservoir potential. During the Triassic, regional extension initiated the formation of half grabens. A change in climate to more humid conditions toward the end of the Triassic led to coastal plain deposition that persisted into the Early Jurassic. These carbonaceous sediments are important source beds for gas and condensate. The reservoir potential is low because of expected lack of continuity of the channel sands and extensive kaolinite cementation. A major transgression took place during the Early Jurassic, leading to deposition of a sequence of shallow marine sands, tidal-flat sands, and offshore muds of medium to low reservoir quality. A Middle Jurassic regression resulted in deposition of shallow marine sandstones presently representing the main reservoir in the area. Diagenesis has not been detrimental to the reservoir properties, and a similar porosity-depth trend as seen in the North Sea is present. Normal and growth faulting during the Triassic to Early Jurassic culminated with the main Kimmerian (pre-Callovian) tectonic phase that resulted in extensive horst and graben development, with subsequent erosion of structural highs. The Upper Jurassic consists of marine shales, of which the upper part is an oil-prone shale of excellent source rock characteristics. The base of the Cretaceous is developed as a regional unconformity (late Kimmerian) onlapped by Cretaceous marine shales, marls, and minor limestone with no reservoir potential. Differential subsidence created the main platforms and basins. The resulting Cretaceous thickness ranges between 0 and 3 km (9,800 ft), with the main depocenter at the outer part of the shelf. The Tertiary represents a period of epeirogenic subsidence leading to rapid deposition of marine clastic sediments. The northward progress of the North Atlantic rift is seen in the sedimentary record as a series of tuffaceous layers within the upper Paleocene-lower Eocene. Along the margin, a volcanic high was formed. As the continental slope and shelf subsided, damming of glacial deposits landward of the outer high created the Voring Plateau. Reactivation of older fault zones took place, and there is evidence for strike-slip movements and folding, especially in the northern area. End_of_Article - Last_Page 1203------------

The food of juvenile trout, salmo trutta L., in loch leven, Kinross, Scotland

The food of juvenile brown trout in Loch Leven was studied by the examination of 662 stomachs of fish caught between October 1974 and Decenber 1976. The food consumption was estimated only on the basis of the analysis of the stomach contents. The main food items of juvenile trout in the loch are chironomids, crustaceans and nematodes: these are abundant in the offshore mud zone of the loch.

A subtidal bar model for the eze-aku sandstones, Nigeria

“Coarsening upward” successions typical of subtidal sand bars have been recognised in the NE-trending linear sandstone bodies which occur within marine shale in the Eze-Aku Formation (Upper Cretaceous) of southeastern Nigeria. The ideal succession, 15–20 m thick, consists of the following units from bottom to top: (1) bioturbated grey siltstone (offshore mud); (2) wave-ripple-laminated, fine-grained well-sorted sandstone (offshore sands); (3) trough and tabular, cross-bedded medium-grained sandstone with channelled base (subtidal channel complex); (4) trough cross-bedded, medium-grained sandstone with bimodal-bipolar paleocurrent pattern (subtidal bar); (5) coarse, pebbly trough cross-bedded sandstone with wave-rippled top, rare burrows and a bimodal-bipolar paleocurrent pattern (subtidal bar). A sixth facies, not a part of the normal sequence, consists of coarse, carbonate-cemented pebbly sandstone grading into pure shell-limestone (bar margin). The sand bars seem to have grown on a shallow mud-bottomed, wave-worked inland sea inhabited by burrowers. A model for the stages of the vertical growth of the bar is presented.

Holocene Mollusk Distribution Patterns in Northern Gulf of Alaska: ABSTRACT

Three recurring mollusk association types are defined in the northern Gulf of Alaska: (1) the shallow-water Yoldia-Siliqua-Lyonsia sand association; (2) the shallow to intermediate depth Cyclocardia-boreal turrid mud association (with a typical phase developing in offshore muds and a Clinocardium-Nitidella nearshore mud phase developing in Yakutat and Icy Bays); and (3) the deep-water Cadulus-thin shelled protobranch mud association. Associations are defined from 148 bottom samples containing 113 species. Substrate exerts a strong influence on shallow-water species composition. An unidentified depth-related factor, independent of substrate type, influences both species composition and taxonomic structure. Dramatic changes in taxonomic structure that occur with depth on fine-grained glacial marine sediments in the Gulf of Alaska provide a model for paleobathymetric interpretation of high latitude late Cenozoic fossil mollusk faunas. The major structural shifts include: decrease in the proportion of heterodont, suspension-feeding bivalves; increase in the proportion of thin-shelled, deposit-feeding protobranch bivalves; and increase in the proportion of carnivorous neogastropods, particularly small-shelled, toxoglossate turrids. The most abrupt and most readily recognized faunal break occurs at 200 m, separating the typical Cyclocardia-boread turrid mud association from the Cadulus-thin shelled protobranch mud association. Although the two associations have species in common, many species drop out as the 200-m isobath is approached and others appear at or not far below it. More precise definition of this faunal break should be explored because of its potential application in paleoecologic analysis. End_of_Article - Last_Page 444------------

Evolution, succession and replacement in the type upper caradoc (ordovician) benthic faunas of England

Measured sections through rocks of the type upper Caradoc Series show a succession of seven terrigenous shelf facies with decreasing sediment grain size, increasing bioturbation and sediment clay content and decrease in thickness of storm-deposited beds of laminated sand and silt with shelly coquinas. This indicates an environmental gradient of general deepening which is locally interrupted geographically, in the Actonian, with the formation of a thin proximal calcareous sandstone unit. The Laminated Shale Facies which is restricted to offshore areas may indicate upwelling centres, which result in bottom oxygen depletion and thus low benthic densities and diversities. Continuous faunal sampling reveals nine associations directly related to sedimentary facies and dominated by articulate brachiopods, bryozoans, bivalves, gastropods and trilobites. Substrate and sedimentation processes were important facets of the physical environment determining community organisation. Species diversity increases and number of opportunistic species declines into the more distal shelf environments. As far as can be ascertained from sedimentary information, this correlates with decreasing environmentally related stress. Local perturbations occur in the vicinity of the Laminated Shale Facies. Number of characteristic species peaks three times along the environmental gradient and relates directly to three association groupings based on the generalised adaptive modes of the constituent species. The highest concentration of characteristic species occurs in the most stable environment, containing the highest diversity Onniella reuschi — Chonetoidea radiatula Association. Greatest concentration of the ubiquitous and intergrading species is in the proximal environments. Community trophic structuring was relatively simple and dominated by low-level, suspension-feeding groups (mainly brachiopods) throughout. A more complex structure prevailed in offshore associations with infaunal deposit-feeding bivalves, epifaunal deposit-feeding or browsing gastropods and trilobites as well as the suspension feeders. Nearshore communities are viewed in terms of succession along a proximal to distal gradient of decreasing stress. It is suggested that community 1 ( Bancroftina robusta Association), composed of initial colonisers, is followed by community 2 ( Kjaerina typa Association), composed of later successional stage species in a less physically stressed setting. Severe downgrading of communities 2 and 3 should produce 1. Community compositions fluctuate and there appear to be some predictive successional patterns. The most important is the development of a trinucleiid trilobite, Onnia spp., and a palaeotaxodont bivalve mollusc, Nuculites planulatus as initial colonisers in offshore muds, a feature known from other Lower Palaeozoic sequences.

Sedimentary and Tectonic Controls of Uranium Mineralization in Morrison Formation (Upper Jurassic) of South-Central Utah: ABSTRACT

Sedimentologic studies in the Henry basin of south-central Utah indicate that uranium deposits in sandstones of the Morrison Formation are related to the depositional environment in which the host sandstone bed and nearby mudstones were deposited and to the tectonic setting at the time of deposition. Thus, an understanding of environments of deposition and contemporaneous tectonism may be helpful exploration guides for uranium deposits in sandstones. Discontinuous tabular uranium orebodies are present in fluvial and marginal lacustrine sandstone beds that lie just above, below, or a short lateral distance from offshore lacustrine gray mudstone beds. The mudstones are present only in the distal part of the lowermost alluvial-plain sequence of the Morrison where fluvial energy regimes and rates of sedimentation were low, allowing lakes to form while the Henry basin was slowly subsiding. The lakes were restricted to the east side of the basin where the least amount of clastic sedimentation by streams occurred. Palynomorphs, carbonized plant debris, and scarce pyrite in the mudstones indicate that the lakes were sufficiently persistent and deep during most of their lifetimes for reducing conditions to persist in the offshore muds. The also indicate that the mudstones were originally gray and are not bleached red mudstones. The gray mudstones are not present in other alluvial-plain sequences of the Morrison because little or no basin subsidence occurred after deposition of the lowermost sequence. Plant debris and palynomorphs in the gray lacustrine mudstones suggest deposition in humus-producing lakes in which humic and fulvic acids (degradation products of plant tissues) were generated in the lake sediments. The close spatial association of the ore-bearing sandstones and the gray mudstones suggests that pore fluids containing these organic acids were expelled by compaction or seepage from the mudstones into nearby sandstones where they were fixed as tabular humate deposits. Subsequently, uranium in groundwater passing through the sandstone was concentrated by the humate to form the ore deposits. Gray mudstones which cannot be used as exploration guides are bleached red mudstones and primary gray mudstones containing the alga Botryococcus. Sandstone beds adjacent to these gray mudstones are barren of uranium because (1) the gray color in bleached red mudstones is an alteration feature and does not indicate primary reducing conditions, and (2) Botryococcus thrives in humus-free lakes whereas humus-producing lakes are considered necessary to form ore deposits. Thus, the requisites for mineralization appear to have been active crustal downwarping to cause ponding of fluvial sediments, and formation of humus-producing lakes on the alluvial plain, in the most sediment-starved part of the area undergoing basinal downwarping. End_of_Article - Last_Page 837------------

Environmental setting of fossiliferous rocks from the uppermost proterozoic—lower Cambrian of central England

The Hartshill Formation of Warwickshire in England provides a section through the initial stages of the “Cambrian marine transgression”. The sea encroached an irregular land surface of weathered volcanic and plutonic rocks which was probably fringed by sand dunes. The first transgressive deposits reflect this provenance but later sedimentation can be interpreted as comprising a gradual retreat of the shoreline and an increase in water depth; superimposed on this trend are several short periods of regression. the transgressive coastal deposits contain meandering trails, Planolites and Arenicolites traces rather than the Skolithos and Diplocraterion assemblages found elsewhere in similar facies of Cambrian (Tommotian and younger) age. The episodes of deeper water, sediment-starved deposition correspond approximately to an early “ Cruziana facies” in laminated sands and muds, to a shelly and stromatolitic limestone facies with a Tommotian fauna and finally to monotonous offshore muds bearing the first local trilobite fragments.

Storm-Wave Genesis of Bioclastic Carbonates in Upper Jurassic Epicontinental Mudstones, East-central Wyoming

ABSTRACT The offshore mud facies of the Late Jurassic (Callovian-Oxfordian) North American epicontinental seaway is represented in east-central Wyoming by the Redwater Shale Member of the Sundance Formation. This member contains stratigraphically discontinuous concentrations of carbonate skeletal debris and fossiliferous mudstone within an otherwise siliciclastic lithologic association. Individual limestone beds thin and thicken laterally and range in thickness from a few centimeters to several tens of centimeters. Detailed petrographic analysis reveals several distinct carbonate depositional textures; grainstones (containing no mud-sized matrix grains other than remnant grain-sheltered patches), packstones and wackestones. These carbonate textures indicate multiple modes of formation. Fossil rains similar to those from carbonate lithologies occur, in much lower concentration, within surrounding siliciclastic mudstones. Muddy fabrics suggest that original skeletal accumulation took place under non-turbulent conditions. Subsequent winnowing during passage of greater-than-normal magnitude waves resulted in varying degrees of grain-support. Winnowing is attributed to turbulence generated near effective wave base, resulting in entrainment of grains and/or pressure gradients created by the passing wave form, which gently flush mud and interstitial waters from saturated bottom sediments. Wackestone and packstone fabrics are the result of incomplete winnowing of mud-sized matrix grains and form at greater distances from the effects of wave energy. Grainstone fabrics result from wave energies that either entrain bioclasts or cause a thorough flushing of mud from near-surface sediments. Progressively shoaling regressive conditions during upper Redwater Shale deposition facilitated the winnowing and concentrating effects of storm-wave interaction with the fossiliferous mud facies of topographically higher areas in the Late Jurassic sea. It is proposed that a variety of carbonate fabrics result from wave-caused winnowing effects, which vary with water depth and wave magnitude. Such fabrics are present in thin bioclastic carbonate units of the Redwater Shale Member of the Sundance Formation, and it is suggested that their origin is due to storm-wave winnowing of a slightly irregular muddy sea bottom. This mechanism and the depositional textures it produces are significant in the distinction between diastemic skeletal grain accumulations and wave-winnowed grainstones. The storm-wave process may also prove applicable in the re-evaluation of certain epicontinental marine deposits previously interpreted as classical turbidite lithologies. Given the vast expanse of geologic time, it is likely that catastrophic events, suc as intense storms and their related processes, have contributed significantly to the preserved sedimentary record.

THE SEDIMENTARY HISTORY OF A DELTA LOBE FROM A YOREDALE (CARBONIFEROUS) CYCLOTHEM

Summary A delta lobe has been recognised in the Great Limestone (Yoredale) cyclothem over an area of approximately 700 square kilometres in the northern Pennines (Weardale to Stainmore). The lobe occupies the entire clastic interval of the cyclothem and has a complex history of sedimentation involving four distinct phases. The coastal plain phase includes small-scale fluvial channels, and a mudstone-spiculite chert association deposited in a shallow, shoreline bay. This phase preceded lobe development in the Stainmore Trough and was terminated by a localised transgression. In the upstream and intermediate lobe area, the progradational phase consists of small-scale coarsening upwards sequences which represent the repeated infilling of an extensive interdistributary bay by levees, crevasse splays, sand spits and minor mouth bars. Small-scale (crevasse) channels and larger-scale (distributary) channels are associated with these sequences. Farther south the progradational phase comprises a single large-scale coarsening upwards sequence which represents progradation of a river-dominated (mouth bar) shoreline over offshore muds. During the abandonment phase, a thin marker horizon accumulated, comprising coal in the upstream area (widespread marsh development) and fossiliferous sandstone, mudstone and limestone towards the shoreline (reworking and normal sedimentation). This horizon reflects slow deposition rates related to conditions of minimal sediment supply and subsidence. In the post-abandonment phase, sediment supply recommenced and a barrier island—lagoon complex was established on the abandoned lobe. The lagoon facies include small-scale coarsening upwards sequences formed during infilling, in one case by washover fans. The barrier is made up of medium-scale, wave-dominated coarsening upwards sequences which demonstrate contrasted migration trends equivalent in time along the barrier.