Fine-grained Mud Research Articles

Effects of disaggregation on a fine-grained marine mud by two ultrasonic devices

Effects of disaggregation on a fine-grained marine mud by two ultrasonic devices

Intertidal habitats and benthic macrofauna of the Forth estuary, Scotland

SynopsisThe intertidal habitats of the Forth estuary are dominated by fine-grained muds, which experience a salinity range of 0–32%, j over the length of the estuary from Stirling to Queensferry. The current total area of the intertidal zone is 23.3 km2, the majority of which is found in the three central bays of Skinflats, Kinneil and Torry Bay. Over the centuries, agricultural and industrial reclamation has reduced by almost 50% the size of the Forth estuary intertidal habitat. The upper estuary has narrow intertidal shores, inhabited almost exclusively by oligochaete worms, which are very abundant due to organic enrichment of the area. The middle estuary is responsible for the greatest biomass and production of the intertidal benthic macrofauna. The lower estuary has a greater diversity of species than the upper and middle estuary, but has a lower biomass and abundance of macrofauna. It is shown that the production/biomass (P/B) ratio of several species in the Forth estuary is lower than that of the same species in other estuaries. This depression cannot be explained by natural environmental variables, and is suggested as being due to the levels of pollutants throughout the area. Despite this depressed productivity, and the threats from further reclamation, the Forth estuary retains abundant populations of intertidal animals, which are an important food source for birds and fish.

Sediment waves in the eastern equatorial Atlantic: sediment record during Late Glacial and Interglacial times

Summary Standing and migrating sediment waves along the West African continental rise (3000–4500 m water depth) are characterized by a regular spacing of wave length periods that are multiples of 280 m, and by long-term pelagic sediment composition. Interpretations of their origin must account for the following evidence: (1) The wave crests generally run parallel or oblique to the continental rise and also to prevailing (net) currents, as confirmed by SEA BEAM data; (2) Migrating waves along the West African continental margin are directed only upslope, i.e. towards the East. (3) The waves have been accreting for a long time, (>1 Ma). (4) Glacial to interglacial differences in the evolution of sediment accumulation rates on crests and troughs may indicate that the accreting regime was intermittent, and that the waves were more levelled prior to 7500 years bp due to increased deposition of siliciclastic matter in the troughs. (5) Absent patterns of grain-size sorting in the fine-grained muds contradict ‘conventional’ models of current-related sediment transport. A superposition of large tidal components and slow thermohaline net currents (Lonsdale 1978), secondary current processes (Unsöld, pers. comm.), or a superposition of internal waves and thermohaline currents (Kolla et al. 1980) are regarded as the most probable modes of origin.

Facies and sequence analysis of Nova Scotian Slope muds: turbidite vs ‘hemipelagic’ deposition

Summary Five fine-grained mud facies were identified in piston-cores obtained from the Nova Scotian Slope. The sedimentary structures of the muds indicated deposition under varying bottom-current conditions and rates of sediment supply. Two facies associations were recognized using Markov sequence analysis: (a) turbidite and (b) non-turbidite associations. The problems of recognizing turbidite, contourite and hemipelagic deposits in fine-grained sequences are discussed in the context of low-concentration, low-velocity flows. The dynamics of unconfined flows indicates that turbidity currents become unstable and are deflected by Coriolis to near contour-parallel flow. Dilution and deceleration of the flow may produce a transition to hemipelagic-type deposition. Transitions of this type within individual flows make recognition of discrete turbidite, contourite or hemipelagic processes very difficult in parts of the geological record.

Lithology and Structures of Quaternary Sediments of Indus Fan: ABSTRACT

The surface sediments of the Indus fan are primarily chalks, marls, and brown clays. Massive terrigenous sediment dilutions occur only in the areas within 100 km from the Indus River confluence. Throughout most of the fan, underlying the sediments, are gray-green muds and turbidites of Pleistocene age. However, the details of lithology and structures of Pleistocene sediments vary from region to region in the fan. The sediments of the upper fan region are primarily fine-grained muds (with several, small silt beds, Td-e), except on the valley floors where coarse-grained turbidites (Ta-e), are commonly present. Two main valley systems, one eastern and the other western, exist on the upper fan. Bioclastic turbidites are common in the eastern valley syste , and are derived from the sediment slumps of the Indian margin. Most of the terrigenous, coarse-grained sediments have bypassed the upper fan and reached the End_Page 734------------------------------ distal areas extending to the Carlsberg Ridge. The percent sand is higher and the number of thick sand beds is more common in the distal areas of the fan; Bouma turbidite sequences, Ta-e, are common. Two main sand lobes are distinguished in the distal fan--an extensive western lobe, derived through channels resulting from the branching of the western valley system, and a restricted, eastern lobe, derived from the eastern valley system. The distributions of these lobes coincide with areas of thick sediments as seen on seismic profile records, implying that sediment dispersal similar to that in the Pleistocene occurred in earlier times. End_of_Article - Last_Page 735------------

Major geochemical processes in the evolution of carbonate—Aquifer systems

As a result of recent advances by carbonate petrologists and geochemists, hydrologists are provided with new insights into the origin and explanation of many aquifer characteristics and hydrologic phenomena. Some major advances include the recognition that: (1) most carbonate sediments are of biological origin; (2) they have a strong bimodal size-distribution; and (3) they originate in warm shallow seas. Although near-surface ocean water is oversaturated with respect to calcite, aragonite, dolomite and magnesite, the magnesium-hydration barrier effectively prevents either the organic or inorganic formation of dolomite and magnesite. Therefore, calcareous plants and animals produce only calcite and aragonite in hard parts of their bodies. Most carbonate aquifers that are composed of sand-size material have a high initial porosity; the sand grains that formed these aquifers originated primarily as small shells, broken shell fragments of larger invertebrates, or as chemically precipitated oolites. Carbonate rocks that originated as fine-grained muds were initially composed primarily of aragonite needles precipitated by algae and have extremely low permeability that requires fracturing and dissolution to develop into aquifers. Upon first emergence, most sand beds and reefs are good aquifers; on the other hand, the clay-sized carbonate material initially has high porosity but low permeability, a poor aquifer property. Without early fracture development in response to influences of tectonic activity these calcilutites would not begin to develop into aquifers. As a result of selective dissolution, inversion of the metastable aragonite to calcite, and recrystallization, the porosity is collected into larger void spaces, which may not change the overall porosity, but greatly increases permeability. Another major process which redistributes porosity and permeability in carbonates is dolomitization, which occurs in a variety of environments. These environments include back-reefs, where reflux dolomites may form, highly alkaline, on-shore and continental lakes, and sabkha flats; these dolomites are typically associated with evaporite minerals. However, these processes cannot account for most of the regionally extensive dolomites in the geologic record. A major environment of regional dolomitization is in the mixing zone (zone of dispersion) where profound changes in mineralogy and redistribution of porosity and permeability occur from the time of early emergence and continuing through the time when the rocks are well-developed aquifers. The reactions and processes, in response to mixing waters of differing chemical composition, include dissolution and precipitation of carbonate minerals in addition to dolomitization. An important control on permeability distribution in a mature aquifer system is the solution of dolomite with concomitant precipitation of calcite in response to gypsum dissolution (dedolomitization). Predictive models developed by mass-transfer calculations demonstrate the controlling reactions in aquifer systems through the constraints of mass balance and chemical equilibrium. An understanding of the origin, chemistry, mineralogy and environments of deposition and accumulation of carbonate minerals together with a comprehension of diagenetic processes that convert the sediments to rocks and geochemical, tectonic and hydrologic phenomena that create voids are important to hydrologists. With this knowledge, hydrologists are better able to predict porosity and permeability distribution in order to manage efficiently a carbonate—aquifer system.

Tidally induced secondary circulations and their associated sedimentation processes

Fine grained (mud) deposits or coarse grained (sand) deposits, which occur in tidally dominated coastal environments (bays, tidal inlets and estuaries), are associated with topographical (small-scale) tidally induced eddies; these are considered to be caused by the phenomenon of flow separation, downstream from headlands and at abrupt coastal discontinuities. Extensive sand deposits, which have been described as bed-load convergence zones can, similarly, be associated with residual (large-scale) circulation eddies.

Fabric-Element-Based Classification for Low-Porosity-Shale Gas Reservoirs: ABSTRACT

Several hundred shale samples have been characterized by us with respect to mineralogic composition, chemical composition, and petrophysical properties. However, no study based on composition reveals the fabric or manner in which the components are put together to form the resultant rock. Classification by fabric elements, based on X-radiography and direct prints from thin sections, is practical and appears useful for interpretation of both depositional environments and reservoir properties of shale. Six lithotypes based on fabric elements are defined in Devonian shales of West Virginia: (1) sharply banded shale, (2) thinly laminated shale, (3) lenticularly laminated shale, (4) nonbanded shale, (5) siltstones, and (6) concretions. A gradational series exists from thinly laminated through lenticularly laminated to nonbedded. Thinly laminated and organic-rich lenticularly laminated shales appear the most favorable types for gas productivity because the laminated fabric permits the best lateral continuity of the low porosity (about 2%) present. The higher organic content of these lithotypes also probably acts as sites for significant sorption of gas, which is slowly released during production. No microfracture system appears to exist, and macroscopic fractures do not necessarily coincide with productive zones. Wells without natural fractures prove productive after stimulation, which suggests that outgassing of a favorable rock type is responsible for the bulk of productive gas in Devonian shale wells. Natural fracture porosity, while important in extending effective volume tapped by a given well, represents only a minor portion of reservoir storage volume. Thinly laminated shales are interpreted as being generally representative of shallow-marine deposits (dominantly shelf deposits) accumulating under anoxic conditions. Nonbanded and lenticularly laminated shales are deeper water accumulations deposited farther from shore. Sharply banded shales and some siltstones generally represent pulses of more rapid sedimentation in the prodelta environment. A single sample in itself is not diagnostic of depositional environment, but relative abundance of specific lithotypes in a given vertical section is useful for general environmental reconstruction. Better definition of the diagnostic worth of shale fabric-element lithotypes awaits results of more detailed fabric-element studies of modern fine-grained clastics and marine muds. End_of_Article - Last_Page 1585------------

Developing geochemical methods for marine exploration of oil and gas

Development of methods for geochemical petroleum-exploration and applications of the methods to the deposits on the Continental Shelf in the USSR and abroad are emphasized. Explorations were carried out in the Caspian and Black Seas using coring devices: gravitational, piston, and vibropiston types. The optimum distance between stations in a detailed survey was 0.5 to 1.0 km and was 4 to 5 km during reconnaissance surveys. In the Caspian Sea, chemical prospecting indicated large amounts of methane and its homologs concentrated above the well-known ''Neftyanyye Kamni'' oil deposit under conditions unfavorable to biogenic gas formation in sands and coquinas possessing minimum absorptive capacity as well as in the nonprospective regions of the Sumgait shelf where fine-grained muds possess great absorptive capacity. Results of geochemical investigations in the western deepwater part and within the southeast shelf of the Black Sea are reported. (BLM)

Late Devonian Current Directions in Western New York with Special Reference to Fucoides Graphica

The lower Upper Devonian sequence in the western half of New York consists largely of evenly stratified alternating layers of gray shale and mudrock, very dark gray to black shale, and gray siltstone. The sediments were derived from source areas to the east and southeast and were deposited in an arm of a shallow epicontinental sea that covered much of the eastern half of the United States. Vast numbers of linear current-oriented structures occur in this sequence. The current-oriented structures-both inorganic and organicare divided into two categories: those associated with the deposition of silt and sand and those associated with the deposition of fine-grained mud. Current-oriented structures such as groove casts, flute casts, bounce casts, brush casts, prod casts, and current ripple marks are associated with the deposition of silt and sand-presumably by turbidity currents. Most of these structures are preferentially oriented approximately east-west, parallel to the direction of current flow, and at a sh...

A New Deepkill Eurypterid

While engaged in a restudy of the stratigraphic distribution of graptolites in the type section of the Deepkill shale, Rensselaer County, N. Y., the writer discovered the small eurypterid described below. The form is of interest in that it is the first occurence in the Canadian strata of a eurypterid having the aspect of the Silurian genus Eusarcus. The generic position of this species is dubious. The prosoma is typical of Eusarcus, which is typically developed in the Silurian. Other members of the genus have been described from rocks of Trenton and Cincinnatian age. However, the present species occurs in beds which are certainly not younger than the Canadian, and are probably Middle Canadian, if not older, for graptolites homotaxial to those of the overlying graptolite beds 3-5 of the Deepkill Section occur in the Powell limestone of the Ozark region. With few exceptions, the body and appendages of Ordovician eurypterids are poorly known. A very large pterygotid, soon to be described by Dr. K. E. Caster from the Richmond of Manchester, Ohio, has a prosoma typical of Pterygotus, yet the remainder of the animal is so different in body pattern that a new genus must be erected for it. This indicates further the need of caution, in generic determination of Ordovician Eurypterida on the basis of prosomal characters alone. Three eurypterids have been previously described from the Deepkill. One of these, Pterygotus deepkillensis Ruedemann (1934) is from approximately the same horizon as the present species; that is, from the zone characterized by horizontal Didymograptus species, and represented in the Deepkill Section by beds 1 and 2. The other two, Dolichopterus antiquus Ruedemann and Pterygotus (?) priscus Ruedemann (1942) are from the beds exposed at the Ash Hill quarry at Mt. Merino, south of Hudson, N. Y. Ruedemann (1904, p. 499), on the basis of their fauna regarded the fossiliferous beds at this exposure as intermediate in position between beds 5 and 6 of the Deepkill Section. Some of the eurypterids found in graptolite shale occurred under such conditions as to suggest that they might have been only cast skins washed into the black shale facies. (Reudemann 1934, p. 40-41). The occurrence of eurypterids in a shale layer directly on top of Normanskill grits, with some specimens on the surface of the grit layer itself, suggested deposition of the materials during transition from strong to weaker currents. The specimen described in this paper was found under very different conditions. The type is from a layer of soft and very fine-grained black shale three-fourths of an inch in thickness. The material was evidently fine-grained mud and contains remarkably large and perfect specimens of Tetragraptus fruiticosus, Goniograptus thureaui var. postremus, and several species of Didymograptus. The underlying shales assume a slightly coarser texture for