Showing papers in "Journal of Sedimentary Research in 1981"

A New Geochemical Classification of Sedimentary Environments

Abstract: Because of the relative constancy of pH in most subaqueous sediments and the general lack of measurability of Eh, these parameters are not practically useful for classifying sedimentary environments. In their place a new classification is offered which, derived from studies of modern sediments, is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation. Sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen. Anoxic environments, in turn, are divided into sulfidic and non-sulfidic depending upon the presence of measurable dissolved sulfide. Anoxic-nonsulfidic environments are further divided into postoxic, or resulting from oxygen removal without sulfate reduction (weakly reducing), and methanic , or resulting from complete sulfate reduction with consequent methane formation (strongly reducing). The environments are shown to succeed one another during early diagenesis in the order: oxic, post-oxic, sulfidic, methanic. Iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record.

The lithobiontic ecological niche, with special reference to microorganisms

Abstract: Revised terminology is proposed that describes the ecological niches of microorganisms within hard, mineral substrates. Organisms attached to the external surfaces of the rock are termed epiliths, while those in the interior of the rock are all termed endoliths. The latter are called chasmoendoliths if they inhabit fissures in rocks, cryptoendoliths if they dwell within structural cavities, and euendoliths if they actively penetrate calcareous substrates.

Chemical Diagenesis of a Multicomponent Carbonate System -2: Stable Isotopes

Abstract: This study examines the oxygen and carbon isotopic distribution in the brachiopods, crinoids, rugose corals, and matrix/cement from the Mississippian Burlington Limestone (Iowa and Missouri) and the Silurian Read Bay Formation (Arctic Canada). The isotopic data can be reconciled with textural and trace element observations only if the 18O content of ancient oceans differed from that of the present oceans. This postulated secular variation in the 18O content of seawater requires a separate evaluation of the Mississippian and Silurian carbonate components. Examination of the 18O content of the least-altered low-Mg calcite fossil components of the two formations suggests that the 18O of Mississippian seawater was, on average, about 1.5 lighter, and that of Silurian seawater was about 5.5 lighter than the 18O of present day ocean water. The average diagenetic shift in 18O for the Read Bay Formation, which stabilized in a partly closed diagenetic phreatic meteoric system, is about - 1 for the original low-Mg calcite and about -2 for the original high-Mg calcite and aragonite components. Conversely, for the Burlington Limestone, which stabilized in an open diagenetic phreatic meteoric system, the average diagenetic shift in 18O is about -3 for the original low-Mg calcite and about -4 f r the original high-Mg calcite components. The 13C values of the originally low-Mg calcite (or high-Mg calcite with less than 7 mole % MgCO3) brachiopods and rugose corals, the originally high-Mg calcite crinoids, and the originally aragonite lime mud of the Burlington Limestone and Read Bay Formation are bimodally distributed. This bimodal 13C distribution is independent of geologic age, inferred original mineralogy of the component, degree of diagenetic alteration, and the type of diagenetic meteoric system, with components from both the Read Bay Formation and Burlington Limestone contributing to both 13C groups. The mode of the light -13C group is +0.25% and for the heavy -13C group it is +3.25% (PDB).Both 13C groups also show a decrease in 13C of about 1% with increasing diagenetic alteration (stabilization).

Storm-generated sedimentary structures in subtidal marine facies with examples from the Middle and Upper Ordovician of southwestern Virginia

Abstract: The role of storms is of major importance in forming stratification in open-shelf facies. Though storm processes on modern shelves are not fully understood, enough is known to see how sediments are influenced. Storms transport sand away from coastal areas, and suspend and transport sand and mud on the open shelf when wave base is lowered. Storms generate winnowed lag deposits of shells or gravel, but coarser sediment and shells of megafauna are little transported except in very near shore environments. The Martinsburg Formation (Middle and Upper Ordovician) of southwestern Virginia was deposited on a storm-influenced open-marine platform, in contrast to the classic turbidite flysch basin of the Martinsburg in the Great Valley of Virginia to the northeast. Deposition of most Martinsburg sediments was due to storms. Sediments deposited during single storm events occur in characteristic fining-upward sequences (1 to 80 cm thick) in both carbonate and terrigenous clastic dominated areas of the platform. Individual storm deposits consist of a sharp erosional base overlain by a couplet of whole-fossil packstone and laminated calcarenite/siltite or quartzose siltstone to very fine sandstone, which is gradational to shale. Packstones are typically intraclastic, have shells aligned parallel o bedding and convex up (often with adhering or contained mud), and have abundant infiltration textures such as cement filled shelter-voids. Packstones grade abruptly to a laminated bed with horizontal plane lamination, large- and small-scale in-phase undulatory lamination, and wave-ripple cross lamination, commonly occurring in ordered vertical sequences. Laminated beds contain escape burrows, are weekly graded, and have irregularly mounded upper bed surfaces. Laminated beds grade upward to shale containing horizontal branching burrows and marine fossils. The faunal content of the fining-upward sequences matches that of the interbedded shale. Storm-generated beds vary greatly in thickness laterally, and frequently pinch-out completely. Fining-upward sequences formed by scouring, winnowing, and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils. Storms preferentially preserve whole fossils rather than fragmenting them. Infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned. Laminated beds are molded into a distinctive suite of sedimentary structures by wave-generated currents. The shale represents deposition of finest storm-suspended material and continued hemipelagic sedimentation. Similar sequences are widely distributed in the geologic record. Variability of storm-generated sequences can be related to changing water depth. Thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events.

Climate and the origin of quartz arenites

Abstract: Holocene fluvial sands derived from the same type of crystalline parent rocks in humid-temperate and arid/semi-arid climates have distinct compositions. For a particular climate and parent rock, the similarity in composition of the fluvial sands and the sand-size fraction of soils collected from the associated interfluves indicates that climatically controlled weathering of a specific parent rock type is a primary control of sand composition. Published estimates of the magnitude of destruction of lithic fragments and feldspars associated with stream transport and deposition in a beach/coastal dune environment permit the simulation of quantitative approximations of compositional maturation of sand. For transport distances of <75 km the simulated maturation does not significantly modify the distinctive climatic imprints on composition. However, simulated maturation due to deposition in beach/coastal dune environment effectively destroys the climatic imprint. Although marine processes are highly capable of mechanically destroying lithic fragments and feldspar, our study suggests that large-scale production of first-cycle quartz arenites in such an environment is not probable. Only a rare, unique combination of extreme conditions of climate (tropical), relief (low), and sedimentation rate (slow) can give rise to first-cycle quartz arenites. Therefore we conclude that the bulk of ancient quartz arenite is multicycle in origin.

An interpretation of trends in grain size measures

Abstract: The mean grain size, sorting, and skewness of a sedimentary deposit are dependent on the sediment grain size distribution of its source and the sedimentary processes of i) winnowing (erosion), ii) selective deposition of the grain size distribution in transport, and iii) total deposition of the sediment in transport. If a source sediment undergoes erosion, and the resultant sediment in transport is deposited completely, the deposit must be finer, better sorted, and more negatively skewed than the source. This trend is referred to as Case I. The lag remaining after erosion, on the other hand, must be coarser, better sorted, and more positively skewed (Case II). If sediment in transport undergoes selective deposition, the resultant deposit can either be finer (Case IIIA) or coarser (Cas IIIB) than the source, but the sorting will be better and the skew more positive. Although exceptions to these trends may occur, they suggest that comparison of one sediment must be made with another for the proper identification of the sedimentary process, and therefore it is not possible for a single grain-size distribution to identify the depositional environment. The trends also suggest that the skewness of a grain-size distribution has been widely misinterpreted and implies neither the truncation of one of the tails nor the mixing of more than one mode. Rather, a skewed sediment is the natural result of the sedimentary process. In a system of related environments, these trends can be used to identify both the probable source and the probable deposit and, by inference, the net sediment transport paths among sedimentary deposits. Such an analysis provides a rapid understanding of the sedimentary processes, identifies patterns of erosion and accretion, and may suggest transport processes.

Shrinkage Cracks: Desiccation or Synaeresis?

Abstract: Casts of shrinkage cracks found within sedimentary sequence are frequently cited as being diagnostic of depositional environments periodically subjected to subaerial exposure The term shrinkage cracks, however, encompasses a broad suite of sedimentary structures having various origins Shrinkage cracks can form not only at the sediment-air interface by desiccation processes but also at the sediment-water interface or substratally by synaeresis processes Shrinkage cracks are induced in muddy sediments by variations in the salinity of the depositing medium, sediment compaction, and temperature in the case of some desiccation mudcracks Crack morphology is influenced by the interplay of numerous factors, such as sediment composition, bed thickness, and bed surface configuration Factors controlling desiccation mudcracks also include the rate of initial drying, total exposure time, depth of the groundwater table, and direction of surface drainage Unfortunately, because of the complex interplay of these factors, no single feature of any shrinkage crack is necessarily useful in differentiating between a desiccation or synaeresis origin Confidence in interpreting subaerial exposure from shrinkage cracks, therefore, depends on finding othe associated sedimentary structures diagnostic of exposure

Long-term Accretion Rates on Tidal Salt Marshes

Abstract: Long-term variations in salt marsh accretion rates were estimated using data collected on the marshes of the North Norfolk coast, England. Fourteen discrete marsh areas were sampled all lying within a 20 km stretch of coast. The age of each marsh had previously been determined using radiocarbon or archival evidence for the date of vegetation inception on each marsh surface. Marsh ages ranged from 10 years to +2000 years. The elevation of each marsh surface was determined in the field using levelling techniques. The relationship between age and surface elevation was assumed to be a function of the rate of marsh accretion within this area of coast. Statistical tests showed this to be a highly significant relationship while the accretion rates calculated from the age/height curve agree closely with field measurements on these marshes. Accretion rates were found to vary from 1.7 cm/yr on 10-year-old marshes to less than 0.002 cm on marshes older than 500 years. The age/height relationship describes an asymptotic curve with the asymptote lying 0.8 m below the level of the highest spring tides, and appears to be controlled by the frequency of tidal maxima which peaks at 0.8 m below the maximum value and declines sharply above that height.

Distinctions and Uses of Stratification Types in the Interpretation of Eolian Sand

Abstract: Eolian and subaqueous cross-strata cannot be distinguished reliably by many commonly cited criteria. They can, however, generally be distinguished by the characteristics of their component types of stratification, which represent processes of sorting and transport of the grain population on dunes. Eolian dune stratification types consist of grainflow cross-strata, grainfall laminae, and wind ripple-generated climbing translatent strata. Some of these types, especially translatent strata, have characteristics unique to the eolian realm. These same stratification types are found to compose some ancient cross-strata, and their occurrence confirms the eolian interpretations of pans of the Entrada (Jurassic), Navajo (Triassic-Jurassic), and Galesville (Cambrian) Formations, a well as revealing emergent islands in the Curtis Formation (Jurassic), previously considered to be totally marine in origin. Stratification types show a characteristic distribution on modern eolian dunes and differ in their relative abundances and structure on dunes of differing size and kind. These same relations allow some estimates of the type, shape, and original height of ancient dune deposits, as well as influencing the occurrences of surface features such as tracks and tipple forms. The geological record of stratification types and other dune features is greatly affected, however, by the extent of the post-depositional truncation of dunes.

Pore networks in Holocene carbonate sediments

Abstract: Permeabilities of Holocene carbonate sediments from Florida and the Bahamas, determined for 74 samples by water flow rates in a "variable-head" permeameter, range from 0.6 to 57,000 millidarcies. Porosities range from 40 to 78 percent. These parameters are related to depositional texture as follows: Table The muddiest sediments (i.e., the finest-grained) have the highest porosities but lowest permeabilities; this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks, even as young as Pleistocene. High porosity and low permeability in sediments show a strong correlation with percentage of fines (<62 µm). From capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 µm in diameter, at least after drying. Measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation; the calculated rates would require excessive time to produce the degree of cementation seen in Late Pleistocene rocks of Florida and the Bahamas. The amounts of rainfall and evaporation are the most important factors in the degree of cementation.

The formation of shadow dunes

Abstract: An investigation of the modification of airflow by a discrete, semi-circular roughness element (in this case, vegetation), the resulting flow structure, and the formation of pyramidal-shaped shadow dunes is presented. Two erect pioneer sand dune grasses, Festuca littoralis and Ammophila arenaria, were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities (4-20 m sec-1). The three-dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake. A vertical separation envelope is also formed over the vegetation. Pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs. Shadow dune height is dependent on element basal width and repose angle of the sand. Shadow dune length is dependent on element basal width and wind velocity. The results are extrapolated to include all semi-porous to non-porous, semicircular, discrete roughness elements. The role of shadow dune formation in the development of foredunes and related environments is examined.

Physical Processes and Fine-grained Sediment Dynamics, Coast of Surinam, South America

Abstract: The prograding Holocene mud wedge between the Amazon and Orinoco Rivers in the trade wind belt of northeastern South America provides a modern-day example of muds accumulating under moderate wave-energy conditions. Gigantic shore-attached mudbanks (10 km 20 km), composed partly of thixotropic fluid-mud gel, front this coast every 30-60 km to form a buffer to wave attack and a temporary storage for fine-grained sediments. This mesotidal coast (tide range 2.0 m) with gentle offshore slope (0.0006) allows the exposure twice a day of extensive tidal flat deposits, which are backed by mangrove swamps on a well-developed chenier-plain complex. Field experiments were conducted in Surinam du ing 1975 and 1977 to provide new information on process-form relationships in this interesting but unusual muddy environment. Simultaneous measurements of waves, currents, tide elevation, suspended-sediment concentration, and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency. Suspended-sediment concentrations typically exceed 1,000 mg/l at the surface as incoming solitary-like waves partially disperse fluid mud into overlying water on a falling or rising tide. Redeposition of mud may occur near time of high tide. The strong attenuation of shallow-water waves by these muds provides conditions that are favorable for further sedimentation. High concentrations of suspended fluid mud, together with solitary-like waves from the northeast throughout the year, can lead to extraordinarily high net sediment transport rates in the nearshore zone. Calculations based on solitary-wave theory and on data obtained from this study indicate that 15-65 106 m3 of mud can move along shore each year without involving breaking waves, the concept of radiation stress and a nearshore circulation cell, or bedload transport. Farther offshore, outside the zone of wave dominance, wind-driven currents and the Guiana Current combine to transport muds to the northwest, consistent with the observed direction of mudflat migration.

Sedimentation in an arid-zone anastomosing fluvial system; Cooper's Creek, central Australia

Abstract: Cooper's Creek is a large, ephemeral, multi-channel fluvial system in the Lake Eyre Basin of arid Central Australia. The large extent and aggradational regime of the system make it a suitable model for interpreting ancient successions formed under similar environmental conditions. The dominant channel pattern in the Cooper is a combination of relict braids and active anastomosing channels. The braids are the partly buried surface expression of a sand sheet attributed to a pluvial episode about 5,000 years ago, whereas the anastomosing pattern is interpreted as an adjustment to the present arid climate. Tectonism and eolian processes are minor, localized influences. The anastomosing channels accumulate sand but they occupy a small part of the floodplain, on which overbank mud is blanketing the relict braids. A preliminary depositional model for arid-zone anastomosing fluvial systems comprises a mud-dominated succession, with minor isolated channel sands. Other features are deep desiccation cracks, minor carbonaceous horizons, duricrusts, and evaporite horizons.

Production of Carbonate Sediments by Selected Large Benthic Foraminifera on Two Pacific Coral Reefs

Abstract: Carbonate production, deposition, and turnover rates by selected members of the rotaliine foraminiferal families Asterigerinidae, Calcarinidae, and Nummulitidae were calculated using growth and standing crop data. In Palau, Western Caroline Islands, carbonate production rates on seaward reef flats were up to 2.8 kg CaCO3 m-2 yr-1, which is equivalent to carbonate deposition of almost 1 mm yr-1. Productivity on lagoonal reef slopes was about 0.6 kg CaCO3 m-2 yr-1 or deposition of about 0.2 mm yr-1. Carbonate turnover rates in the living populations were 11-16 times per year. In Hawaii, production rates were much lower because of slower growth rates and the absence of the family Calcarinidae. Product on rates were up to 0.15 kg CaCO3 m-2yr-1 on seaward reef slopes, which is a depositional rate of about 0.05 mm yr-1. Carbonate turnover rates were 7-11 times per year. These production rates are comparable to many values reported in the literature for coral, coralline algae, and macrobenthos.

Clay Fabric of Selected Submarine Sediments: Fundamental Properties and Models

Abstract: Clay fabric of selected Mississippi Delta sediment and a red clay from the eastern equatorial Pacific is related to void ratio and depth of burial. Relatively high-void-ratio, naturally consolidated delta sediment is characterized by a fabric of randomly oriented domains and short linking chains with particles in edge-to-face and face-to-face contacts. Lower intermediate-void-ratio sediment showed greater particle-to-particle packing but a consistent predominance of randomly oriented domains with a few chains and a slight development of larger domains than observed in the high-void-ratio sediment. The deeply buried, very low-void-ratio sediment has highly oriented domains and thin, long voids. The red clay sample is characterized by flocs connected by long linking chains and a very hi h void ratio. Domains and "turbostratic-type" clay fabric are characteristic of high-porosity (high-void-ratio) Mississippi Delta sediments. Highly oriented domains and long chains are diagnostic of the deeply buried, very low-porosity delta muds. The clay fabric of natural (undisturbed) samples was compared with laboratory remolded samples for comparison with models suggested by earlier researchers. Observations using the transmission electron microscope (TEM) support the proposed fabric models for remolded sediment. Selected samples were studied using the scanning electron microscope (SEM). Tentative clay fabric models for undisturbed submarine sediment are proposed and related to void ratios.

Measurements and analysis of setting velocities of natural quartz sand grains

Abstract: Settling velocities in water were measured for natural quartz sand grains in the sieve-size range -0.75 to 1.50 (0.35-1.68 mm). Obtained from beach sand samples, the grains were selected so as to provide a wide range of roundness from very angular to very-well rounded (r = 0.6-5.1). In a first set of experiments only two axial diameters were measured under a microscope, the longest axial diameter, D1, and the intermediate diameter, Di. In a second data set the smallest axial diameter, Ds, was measured as well. Settling velocities were measured in a 6-meter long settling tube. It is found that the intermediate grain diameter is on average equal to the nominal diameter computed as Dn = (DsDiD1)1/3. This permits a simpler analysis of grain settling through microscopic measurements of Di alone but does not allow for corrections of grain shape effects. The measured settling velocity, wm, is then compared with the settling velocity of a sphere, ws, calculated using Di as the sphere diameter, and the relationship wm = 0.977w0.913s is found to be consistent with the data. An alternative approach is provided where ws is calculated directly from the sieve diameter, and then corrected to the actual settling velocity wm for that mean grain size. The results thus provide graphs and empirical relationships which permit the evaluation of actual grain-settling rates (rather than settling velocities of spheres) from the sieve diameter or from microscopic measurements of Di for individual grains. These procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands. A more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains. It is found that grain roundness has no measurable effect on the settling rate. The settling velocity is shown to be a function of grain sphericity. either expressed as the Corey Shape Factor or E Shape Factor. Grain asymmetries ust also be important, accounting for much of the scatter in the results.

Pressure solution features in a shallow buried limestone

Abstract: Three distinct types of pressure solution features are found in the Alpena limestone (Devonian, Michigan): stylolites, solution seams, and fitted fabric texture. The style of pressure solution is different in grainstones, packstones, and wackestones. The difference is interpreted to be the result of preferential cementation of the grainstones. Well-cemented grainstones typically have stylolites, whereas solution seams and fitted fabric texture are more common in poorly cemented grainstones, packstones, and wackestones. Pressure solution occurs at e80 percent of the lithologic transitions. This is probably due to competency contrasts between adjacent units. That material dissolved at pressure solution surfaces is not locally reprecipitated is indicated by porous allochems which abut against stylolites and units of abundant intergranular pressure solution (fitted fabric texture) which lack cement. The style of pressure solution in the Alpena limestone and its relationship to cementation is also observed in the Tuscarora quartz arenite (Silurian, Appalachian Basin).

Distributions and origins of clay minerals and quartz in surface sediments of the Arabian Sea

Abstract: Mineral distributions in surface sediments of the Arabian Sea (north of 5°N) reflect several sedimentary origins and processes. Smectite- and quartz-rich sediments occurring along the Indian margin have been derived from the western regions of peninsular India. In the northern- and western-most areas of the Arabian Sea, sediments probably have been derived from several sources: Indus River, Iran-Makran region, Arabian peninsula, and Somalia; these different sediment sources appear to have characteristically high amounts of quartz, illite, chlorite, or palygorskite. In addition, smectite becomes a significant component in the sediments off Somalia (Horn of Africa). The sediments derived from the peninsular India and Indus River may have been dispersed by prevailing surface oceanic irculation in the region. Sediments from other sources probably have been transported by wind. The sediments of the distal Indus Fan appear to be a mixture of sediments derived from the Indus River, Iran-Makran, and Arabia-Somalia regions. Palygorskite clays from the arid regions of Arabia-Somalia appear to have been transported by winds eastward to as far as the Indian continental margin. In the southernmost region of study, relatively high kaolinite, derived from the tropical soils of Africa, Madagascar, and southern India characterize the sediments.

Progradational Sequences in Miocene Shoreline Deposits, Southeastern Caliente Range, California

Abstract: An exceptionally well exposed marine-nonmarine transition in middle Miocene strata exists in the southeastern Caliente Range, California. About 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m (where predominantly nonmarine) to more than 2500 m (where predominantly marine). Paleogeographic evidence in basalt flows near the top of the succession and in overlying fluvial deposits indicates that these middle Miocene strata were deposited across a north-northwest trending shoreline. A complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units. Individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface. The gravel is overlain by structureless siltstone or fine-grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes. These rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation. Crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of fairly long-period surface waves. The bedded fine sandstone is sharply ove lain by a cross bedded coarse sandstone facies that is interpreted as a combined offshore bar-rip channel-surf zone assemblage. Cross-strata dip dominantly offshore, suggesting substantial deposition from rip currents. A secondary, shoreparallel mode of cross-strata direction suggests longshore currents produced by surface waves from the northwest. The crossbedded coarse-grained sandstone grades upward into planar-bedded medium-grained sandstone that is interpreted as a beach foreshore. This facies grades upward through structureless medium-grained sandstone into nonmarine or lagoonal red and green mudstone of the Caliente Formation. The middle Miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically; the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m.y. The average duration of the transgressive-regressive cycles, a few tens of thousands of years, together with their distribution in groups of three or four in the lower two-thirds of the succession, is consistent with the pattern of long-term climatic cycles produced by periodicity of the earth's solar orbit and may be related to eustatic sea level changes attendant to the development of the Antarctic ice cap. Changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may have been precursors to the onset of movement along th San Andreas fault in this area 12-14 m.y. ago.

Periodic mudflat progradation, northeastern coast of South America; a hypothesis

Abstract: The Guiana Coast of northeastern South America, backed by mangrove swamps and fronted by migrating mudflats and associated subtidal mudbanks, has developed during the Holocene as a prograding mud wedge from Amazon-derived sediment. A hypothesis is offered for the periodic stabilization of shifting mudflats, a process that has been recognized previously as the mechanism for initiation of new land growth. The hypothesis assumes that solar semiannual and 18.6-yr tidal components allow abnormal exposures of coastal mudflats and conditions that are favorable for establishment of Avicennia mangroves. Once colonized, mangrove roots displace muds upward, thus decreasing the frequency of tidal inundation and increasing the ability of vegetation to trap and hold muds. Based on estimated ates of root growth and forecasted tide records for the coast of Surinam, land elevation will increase by 25 cm in 10 yr and frequency of tidal inundation above 2.45 m (8.0 ft) will decrease from 106 to 2 per year during the same period.

The coral reef of Nijar, Messinian (uppermost Miocene), Almeria Province, S. E. Spain

Abstract: The Nijar Reef is a thoroughly exposed and representative example of the Messinian coral reefs outcropping along the periphery of the western Mediterranean. Facies patterns in the Nijar Reef Complex are presented in this paper as a generalized guide or model for the study of other Messinian reefs. Messinian paleogeography of this area was characterized by an archipelago whose islands are present-day "Sierras." At the beginning of the Messinian and before development of the reefs, marine terrigenous sediments (Marginal Terrigenous Complex) were deposited on an irregular erosion surface on the metamorphic basement. The Reef Complex developed as a belt, fringing the positive areas (islands) when terrigenous deposition decreased. The Reef Complex consists of: 1) reef-core, constructed by a framework of vertical, branching colonies of Porites coated by laminated micrite (probably submarine cements), which form pinnacles as well as thickets; and 2) fore-reef, made of sloping beds of broken skeletons and debris. The fore-reef is subdivided into: i) reef-talus slope, ii) proximal slope, and iii) distal slope. The reef talus forms massive strata with dips of 20-30° and abundant pebble-to-boulder size slumped blocks of reef-core and coral breccia. The proximal slope shows dips of 10-20° of well-bedded, thick strata with internal parallel layering of granule to sand-size skeletal calcarenites. The distal slope is made of predominant bioturbated skeletal calcisiltites and fine-grained calcarenites with ngles lower than 10°. The distal slope grades into the basinal pelagic marls of the Nijar plains. The original reef morphology is well-preserved in present-day topography, with relicts of pinnacles, reef valleys, and slopes. Messinian volcanic highlands located to seaward complicated the local paleogeography with atoll-like features. Reef growth was concomitant with a gradual fall of sea level and occurred before the Messinian gypsum was deposited on top of the basinal marls in what is now the plains area. The Reef Complex is partially truncated by a major erosion surface, and it is overlain by oolitic shoals and stromatolites of the so-called Terminal Carbonate Complex. The Terminal Complex is interpreted as an upslope equivalent (in age) to the Messinian Upper Evaporite, and it implies a new cycle of sea level rise.

Deep-sea fan deposits in the Macigno Formation (Middle-Upper Oligocene) of the Gordana Valley, Northern Apennines, Italy

Abstract: Facies characteristics are described for a 2300 m thick turbidite succession of the Macigno Formation (middle-upper Oligocene), which crops out as a continuously exposed section in the Gordana Valley (northern Apennines). In the study area the Macigno Formation consists of two sandy members separated by a shaly member. The sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes. The middle shaly member represents a period of basin starvation. Three main facies associations are recognized in the Macigno fan deposits, fan-fringe, outer-fan lobe, and braided midfan facies associations. The fan-fringe deposits consist of monotonous or small-scale cyclically arranged thin-bedded turbidites. The outer-fan lobe deposits are composed of medium-to very thick-bedded classical turbidites internally organized in thickening-upward cycles with minor intercalations of thin-bedded sandstone beds. Two distinct types of outer-fan lobe deposits, distal and proximal, are recognized on the basis of rock-type, internal organization, and vertical facies relationships. The midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning-upward cycles. The three main facies associations are superposed t define major thickening and coarsening-upward cycles reflecting distinct episodes of fan progradation. The relationships between the facies associations of the Macigno deposits and the existing models of submarine fan sedimentation are discussed. After the deposition of the fan complexes, fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes.

Modern Deep-Water Coral Mounds North of Little Bahama Bank: Criteria for Recognition of Deep-Water Coral Bioherms in the Rock Record

Abstract: Deep-water, ahermatypic coral mounds are present at water depths of 1,000-1,300 m on the lower slope north of Little Bahama Bank. The mounds are patchily distributed over a minimum area of 2,500 km2 and typically display 5-40 m of relief above the surrounding sea bottom. A diverse benthic community exists on these apparently unlithified mounds, including l 1 genera and 16 species of ahermatypic coral (Bathypsammia, Caryophyllia, Deltocyathus, Desmophyllum, Enallopsammia, Javania, Madrepora, Polymyces, Solenosmilia, Stephanocyathus, and a previously undescribed genus), alcyonaceans, gorgonians, antipatharians, hydroids, ophiuroids, crinoids, barnacles, galatheid crabs, polychaetes, gastropods, bivalves, and sponges. Conspicuously absent from the coral fauna are Lop elia and Dendrophyllia, common deep-water corals in other parts of the Atlantic. Radiocarbon dates on fresh coral and gorgonian fragments of 940 ± 40 and 860 ± 50 years indicate the mounds are at least in part Recent and are probably actively forming today. Bored and stained corals date at around 22 10314C years B. P., which establishes a minimum age for these mounds. We speculate that the mounds develop on sea-floor perturbations in areas where strong bottom currents provide needed oxygen and nutrients to the fauna. The mounds may have undergone multi-stage evolution from colony to thicket to coppice to bank. This evolution may be accomplished through the in situ contribution of skeletal material along with the baffling and trapping of fine-grained carbonate sediment winnowed from adjacent areas by bottom currents. If found in the rock record, the coral mounds north of Little Bahama Bank would probably be considered bioherms. Geologists should thus be aware that scleractinian bioherms are not uniquely shallow-water in origin. The distinction between ancient deep- and shallow-water coral buildups involves using multiple recognition criteria, including: 1) presence or absence of algae; 2) diversity of corals; 3) coral morphology and microstructure; 4) abundance of planktonic/benthonic components; 5) microborings; 6) surrounding litho- and biofacies; 7) stable isotopes of carbon and oxygen; and 8) trace element geochemistry, particularly Sr and U concentrations. The distinction between deep- and shallow-water bioherms is crucial to regional paleoenvironmental and stratigraphic interpretations.

Sedimentology of Muddy Fine-grained Channel Deposits in Meandering Streams of the American Middle West

Abstract: Many tributary valleys to major streams in Illinois, Indiana, and Wisconsin were sites of mud deposition in late Wisconsinan periglacial lakes. Today the flat lacustrine plains of these valleys contain freely meandering streams. Surveys during low-flow conditions have enabled documentation of the geomorphic and sedimentary features of the sinuous channels of several of these streams. The subaqueous bed material is a complex, variable mixture of quartzose sand (main component), mud, mud blocks, equant mud clasts, rock fragments, and plant debris. Point-bar topography is restricted to bends of low or moderate curvature (rc/w2 > 3); bends of greater curvature show dominantly erosional inner banks. An organic-rich ooze lies beneath the stagnation zone that develops near the inner bank of highly curved sectors. Subaerial sediment on depositional surfaces is primarily a muddy sand with variable content of plant detritus. The lateral-accretion deposits near the downstream end of bends were divided into a fine member and a coarse member, the former two to four times as thick as the latter. The fine member consists of three lithofacies: 1) upper interval of centimeter-thick beds of sandy mud and thinner beds of rooted sand with mud and some plant debris; 2) underlying interval of subequal thicknesses and numbers of beds of clean sand (mostly fine, likely with small-scale cross-stratification) and sandy mud; and 3) basal interval very similar to 2 but with distinctive beds of black, finely divided plant debris. The coarse member is dominantly a fine-to-medium quartzose sand, contains mud clasts towards the base, and is inferred to be large-scale trough cross-stratified. Logjam debris appears near the thalweg in places. Lateral-accretion bedding in the form of intercalated sand and mud-sand is inferred to extend throughout the fine member. Substantial local variations in cross-sectional geometry and channel lithofacies, especially near logjams, reduce the precision of existing paleohydrologic relations and the comprehensiveness of a single lithofacies model. This class of meandering stream produces lateral-accretion deposits which are closer analogues in lithofacies to the classical upward-fining model than are deposits from the graveliferous sand-bed streams that have received primary emphasis from sedimentologists.

Calcrete in Quaternary Coastal Dunes in Southwestern Australia: A Capillary-rise Phenomenon Associated with Plants

Abstract: Non-pedogenic calcrete is forming as a thin sheet (10-50 cm thick) just above the water table in the subsurface of a barrier dune terrain in South Western Australia. The calcrete is forming in response to the regional evapo-transpirative regime. The annual rainfall is high but of short duration, and it infiltrates rapidly into the sandy substrates. Relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain. The vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone, thereby precipitating CaCO3. Where plants are not drawing phreatic water, calcrete is developing by evaporation of water at the top of the zone of capillary rise. Various structure types of calcrete are developed in the study area, namely rhizoconcretionary, mottled, massive, and laminar forms; these occur in a definite stratigraphic sequence related to maturity of profile, vegetation cover, and thickness of the vadose zone. Rhizoconcretionary calcrete forms in the vadose zone; mottled and massive calcrete, frequently capped by laminar calcrete, develop in the zone of capillary rise. These latter calcrete types form a sheet that is ubiquitous and parallel to the water table. Since it is a feature of the water table, the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities. At the local level the various structural types of calcrete may he used as paleowater table indicators.

Geochemical Evidence for Modern Sediment Accumulation on the Continental Shelf off Southern New England

Abstract: An area of fine-grained sediment approximately 170 km 74 km in size, located in water depths between 60 m and 150 m, south of Martha's Vineyard, Mass., is a site of modern sediment deposition. The 14C ages systematically increase with sediment depth from about 1,300 years B.P. at the surface to 8,000-10,000 years B.P. at the depth of maximum core penetration. The old age for the surface sediments probably results from a combination of deposition of old carbon and faunal mixing. In the finest sediments, the sedimentation rates were approximately 130 cm/1,000 yrs when deposition began and have decreased to about 25 cm/1,000 yrs. The decreasing sedimentation rate reflects a diminishing source of fine sediments, which presuma ly came from the Georges Bank and Nantucket Shoals area. Inventories of excess 210Pb in undisturbed cores average 70 dpm/cm2 (disintegrations per minute per square centimeter), more than two times higher than the flux of 210Pb from the atmosphere and from 226Ra decay in the overlying water. This additional influx of 210Pb either must be with new fine-grained sediment material or from solutions that are stripped of their 210Pb by particulates in the bottom nepheloid layer. Stable Pb concentrations in surface sediments are about 28 ppm, as much as two times higher than concentrations at depth. The high accumulation rates, 210Pb inventories, and trace-metal profiles imply that this area is a modern sink for fine-grained sediments and for pollutants associated with particulate matter in the water column. To our knowledge, this is the only site of present-day natural deposition on the Continental Shelf off the eastern United States, exclusive of the Gulf of Maine. Because the net currents on the outer half of this Continental Shelf flow from northeast to southwest, this fine-grained deposit may receive its sediments and possible contaminants from the Nantucket Shoals and Georges Bank regions.

Flume Experiments on Lower-Flow-Regime Bed Forms in Coarse Sand

Abstract: Thirty-five runs were made in a recirculating channel 11.5 m long and 0.92 m wide with four sizes of very well sorted coarse sand to study geometry, migration, and hydraulics of bed configurations in equilibrium with steady uniform open-channel flows at flow depths of about 15 cm. Equilibrium energy slope and sediment transport rate were measured as well. Bed phases (kinds of bed configurations) studied were ripples, low-flow-velocity flat beds with sand transport, and dunes ( identical with megaripples). Current velocities in none of the runs were high enough for development of an upper-flow-regime flat bed. A plot of mean flow velocity versus sand size for all the runs shows that at low flow velocities the ripple field narrows with increasing sand size and pinches out at about 0.7 mm; the field for flatbed transport narrows with decreasing sand size and pinches out at about 0.5 mm, just above the ripple field. At high flow velocities, dunes are the stable phase for all sand sizes studied. Dune configurations are divided into two subphases. Two-dimensional dunes (2D dunes), with fairly straight, continuous, even crests and no strong localized scour in troughs, are formed at relatively low flow velocities; three-dimensional dunes (3D dunes), with strongly sinuous, discontinuous, uneven crests and strong, localized scour pits in troughs, are formed at relatively high flow velocities. 3D dunes tend to be higher than 2D dunes, have larger height/spacing ratios, and show less variability in height, spacing, and migration rate. Dunes can be viewed as kinematic shock waves; differences between 2D dunes and 3D dunes lie in the differing importance of shock-wave coupling and of sand transport in bed-form troughs. In both geometry and hydraulic ralationships, the 2D dunes and 3D dunes in our flume runs correspond to the relatively regular megaripples at low flow velocities and relatively irregular megaripples at high flow velocities described by other authors from intertidal environments.

Tidal Deposition in the Basal Upper Cambrian Mt. Simon Formation in Wisconsin

Abstract: The Upper Cambrian Mt. Simon Formation (0-65 m thick) is a basal quartz arenite exposed in westcentral Wisconsin. A detailed field investigation of the physical and biogenic sedimentary structures of the Mt. Simon has led to the recognition of three distinct lithofacies. The lower one unconformably overlies Precambrian basement rocks. It consists of medium- to very large-scale sets of tabular and trough cross-bedded, medium- to very coarse-grained sandstone and pebbly sandstone with minor intercalated horizontal beds of very fine- to medium-grained sandstone, siltstone, and shale. Sparse examples of Skolithos and Arenicolites are present. This facies consists of a very thin sequence of possible braided-fluvial and marine foreshore deposits, overlain by probable marine shoreface and tidal channel deposits. Much of the facies seems to represent shallow subtidal deposition in a relatively high-energy regime. The middle lithofacies consists of two distinctly different subfacies, which probably were deposited in a low tidal flat setting. The higher-energy subfacies consists of small- to medium-scale sets of tabular and trough crossbedded, fine- to coarse-grained sandstones containing distinct zones dominated by Skolithos and Arenicolites . This subfacies probably represents deposition in meandering tidal channels. The lower-energy subfacies consists of thinbedded, horizontally-laminated and ripple cross-laminated, very fine- to medium-grained sandstone, siltstone, and shale, with common specimens of Cruziana, Rusophycus, and Planolites . This subfacies probably represents deposition on lower-energy tidal flats adjacent to the tidal channels. The upper lithofacies consists predominantly of structureless, densely bioturbated, very fine- to coarse-grained sandstone containing abundant specimens of Skolithos . The upper few meters of the facies consists of small- to medium-scale sets of trough cross-bedded, very fine- to coarse-grained sandstone with layers of disarticulated valves of the brachiopod Obolus . The upper facies probably represents deposition on tidal flats, perhaps in a midtidal flat setting, characterized by slower sedimentation rates, a correspondingly higher degree of bioturbation, persistent reworking of shelled macrobenthos, and periodic subaerial exposure. The Mt. Simon Formation is interpreted as a largely progradational (regressive), shoaling- and fining-upward tidal sequence. A marine interpretation is supported by the widespread occurrence of marine trace fossils within this unit. Evidence for a tidal origin is seen in the presence of unimodal cross-strata associated with reactivation surfaces, compound cross-strata, numerous scour and truncation surfaces lined with intraformational conglomerates, common clay drape laminae separating sets of cross-strata, interference and flat-topped ripple marks, and desiccation cracks. Sedimentation continued without apparent interruption as the overlying Eau Claire Formation was deposited. also under tidal influence. Recent reinterpretations of other basal Cambrian cratonic quartz arenites, together with this new interpretation for the Mt. Simon Formation, suggest that the long-held concept of basal transgressive sandstones deposited as blankets across the craton may be too simplistic, for deposition in braided-fluvial, marginal marine (tidal flat-tidal channel), and marine foreshore and shoreface environments seems indicated.

Hydraulic interpretation of grain-size distributions using a physical model for bedload transport

Abstract: Grain-size distributions of bedload sediment are modelled by considering the time variation of sediment transport in a unidirectional turbulent flow. A frequency distribution of instantaneous bed shear stress is divided into a large number of shear stress ranges with mean values of oi. Each oi is capable of transporting a range of sediment sizes as bedload; the maximum size is just at its entrainment threshold, the minimum size is just too large to be suspended. Rates of bedload transport are calculated for each size range using a theoretical bedload function, and the proportion of time that these rates occur is obtained by integrating the frequency distribut on between the boundaries of each shear stress range. The relative weights of sediment transported in particular size ranges, determined for all oi values, constitutes the grain-size distribution obtained from bedload sampling at a fixed point on the bed. Cumulative grain-size curves have been simulated using the model in order to demonstrate the effects of the various model parameters (e.g., mean bed shear stress, viscosity). All curves show a distinct "break" in slope, which represents the maximum grain size transportable in suspension when the shear stress attains its maximum fluctuating value. Examination of many natural bedload distributions shows close qualitative and quantitative agreement with the model, and reasons for discrepancies can be fully explained. Typical grain-size distributions of deposited sediments can be obtained by mixing of different flow-related bedload distributions in various proportions. Thus although deposited sediments may primarily result from bedload transport during a steady flood flow, flow unsteadiness also plays a part. The effects of some deposition directly from suspended load can also be surmised.

Fining-upwards sedimentary sequences generated in seagrass beds

Abstract: Shallow-marine clastic and carbonate environments of southeast Florida contain fining-upwards sedimentary sequences being generated by beds of the seagrass, Thalassia testudinum . Lateral seaward migration of flute-shaped sand pockets (storm blowouts) in the grass platform produces a lenticular fining-upwards package 10-50 m in width, 20-100 m in length, and 0.3-2 m in thickness. Migration and vegetative restabilization of recurring "blowouts" on a grass platform can produce a composite sequence containing numerous complete or stacked incomplete fining-upwards packages. A complete fining-upwards sequence consists of 1) an erosional base, 2) a coarse basal shell layer representing the winnowed lag of the eroded, preexisting grass-stabilized platform, 3) a ripple-laminated, well-sorted, medium-grained sand zone representing the mobile sand on the leeward slope of the blowout, and 4) a shelly, bioturbated sand or silty sand having a strongly bimodal texture (modes at 100 and 500 mu m) in which the fine mode dominates upwards. This upper portion records increased trapping of storm suspensions and decreased bedload transport as seagrasses are reestablished. The mollusks Chione cancellata and Codakia orbicularis characterize the grass-stabilized zone. The distinctive textural sequence and molluscan assemblage serve to differentiate these biogenically induced fining-upwards sequences from others in the geological record.