Primary Sedimentary Structures Research Articles

Sedimentology of the upper Karoo fluvial strata in the Tuli Basin, South Africa

The sedimentary rocks of the Karoo Supergroup in the Tuli Basin (South Africa) may be grouped in four stratigraphic units: the basal, middle and upper units, and the Clarens Formation. This paper presents the findings of the sedimentological investigation of the fluvial terrigenous clastic and chemical deposits of the upper unit. Evidence provided by primary sedimentary structures, palaeontological record, borehole data, palaeo-flow measurements and stratigraphic relations resulted in the palaeo-environmental reconstruction of the upper unit. The dominant facies assemblages are represented by sandstones and finer-grained sediments, which both can be interbedded with subordinate intraformational coarser facies. The facies assemblages of the upper unit are interpreted as deposits of a low-sinuosity, ephemeral stream system with calcretes and silcretes in the dinosaur-inhabited overbank area. During the deposition of the upper unit, the climate was semi-arid with sparse precipitation resulting in high-magnitude, low-frequency devastating flash floods. The current indicators of the palaeo-drainage system suggest flow direction from northwest to southeast, in a dominantly extensional tectonic setting. Based on sedimentologic and biostratigraphic evidence, the upper unit of the Tuli Basin correlates to the Elliot Formation in the main Karoo Basin to the south.

Permian Cyclothem Development within the Onshore Canning Basin, Western Australia

Earth's climate during the latter part of the Permian has long been the subject of speculation and debate. Some studies suggest that Permo-Carboniferous glaciation persisted on parts of Gondwana up to at least the Permo-Triassic boundary. Other studies argue that deglaciation occurred as early as the late Sakmarian (lower Early Permian). Many such investigations have been based upon the sometimes ambiguous occurrence of direct glacial deposits (e.g. striated clasts, tillites). In contrast, this study evaluates the likelihood of post-Sakmarian glaciation by testing high-frequency, sedimentary cycles for a glacio-eustatic driving mechanism. The focus of this thesis is the Artinskian (upper Early Permian) Tuckfield Member of the Poole Sandstone. The Tuckfield Member is exposed around the periphery of the St George, Poole, and Grant Ranges of the Fitzroy Trough (onshore Canning Basin, Western Australia). In the St George and Poole Ranges, the Tuckfield Member outcrops as a 50 to 100 m thick package of vertically stacked, laterally continuous, coarsening- and thickening-upward cycles. In contrast, the Tuckfield Member outcrops in the Grant Range as a less than 50 m thick package of vertically stacked, laterally discontinuous, fining- and thinning-upward cycles. Primary sedimentary structures, trace fossils, and the vertical succession of facies suggest that the Tuckfield Member records shallow-marine, shorezone deposition in the St. George and Poole Ranges and non-marine, coastal plain deposition in the Grant Range. Gamma-ray and grainsize data, collected from 17 measured outcrops (11 in the St George Ranges; 4 in the Poole Range; 2 in the Grant Range), were analyzed for cyclicity. Spectral analyses identified several orders of meter- to decameter-scale cycles in both the shallow-marine and non-marine facies. The average periodicities of the identified cycles occur in the ratio of ~36:10:4:2 m. This ratio correlates strongly with the known mid-Permian orbital periodicities of elongated eccentricity, eccentricity, obliquity, and precession (~410:100:40:20 thousand years). Additionally, the Tuckfield Member is herein traced throughout the northern Fitzroy Trough and onto the adjacent Lennard Shelf via sub-surface data. Similar to outcrop, subsurface gamma-ray logs display several orders of meter- to decameter-scale cycles that correlate strongly with the orbital periodicities. This cyclicity is manifested by the systematic bundling of smaller-scale cycles into larger-scale cycles that can be traced laterally for more than 200 km. For comparison, correlative shallow-marine cycles from the Artinskian to Ufimian Pebbley Beach Formation (Sydney Basin, New South Wales) were also studied. Four outcrop sections were measured and correlated, giving a total stratigraphic column of approximately 45 m. Spectral analysis of gamma-ray data identified several orders of meter- to decameter-scale cycles. These cycles correlate strongly with the mid-Permian orbital periodicities and the identified Tuckfield Member cycles. The favorable comparison between the Tuckfield Member cycles and the orbital parameters indicates that Milankovitch-forcing of climate influenced the formation of depositional cyclicity. Furthermore, it is highly probable that these cycles are glacioeustatic in origin due to their laterally extensive nature and positive correlation with identified cyclicity in New South Wales. This suggests that Permo-Carboniferous glaciation must have persisted into at least the Ufimian stage of the Late Permian.

Microbially Induced Sedimentary Structures--A New Category within the Classification of Primary Sedimentary Structures: PERSPECTIVES

Microbially Induced Sedimentary Structures--A New Category within the Classification of Primary Sedimentary Structures: PERSPECTIVES

Quartz thermoluminescence in a storm deposit and a welded beach ridge

We examined the natural residual thermoluminescence (NRTL) of quartz in samples collected from the surface and the subsurface of the beach on a sand spit in the Gulf of Mexico located at St. Joseph Peninsula State Park near Apalachicola Florida. The subsurface samples were collected within two berm structures that respectively, represented a hurricane deposit and a subsequently deposited smaller tidal berm (beach ridge) produced by wave action during a seven-week period following the storm. Higher NRTL was found in the underwater zone of the beach surface than in the areas exposed to full daylight on the beach and in the dunes (seawater is known to strongly absorb the ultraviolet component of daylight). In each of the berms, similar variations in NRTL could confidently be correlated to primary sedimentary structures. Strong transitions in NRTL occurred at boundaries between sedimentary units. These variations were used to interpret processes of berm formation and the identity of local sources of quartz incorporated into the berms.

Fluvial and aeolian interaction under permafrost conditions: Weichselian Late Pleniglacial, Twente, eastern Netherlands

Close interaction between fluvial and aeolian deposition led to the formation of a distinct ‘fluvio-aeolian’ depositional environment in river valleys in the Netherlands during the last glacial maximum. We describe three sections illustrating this sedimentary environment. The primary sedimentary structures and periglacial phenomena document the dynamic interaction between fluvial, aeolian and periglacial processes. The rivers were ephemeral in nature, with alternating fluvial and aeolian deposition in overbank environments. Within the sections, successive levels of frost cracks, ice-wedge casts, soil wedges and cryoturbations are found. The sections demonstrate that not all these levels of periglacial structures indicate climatic oscillations, but are caused by lateral migration of fluvial activity. However, one large cryoturbation level truncated by a gravel lag (the Beuningen gravel bed) is evidence of widespread permafrost degradation induced by climatic warming between ca. 16 and 22 ka. This climatic warming is followed by a significant increase of aeolian activity.

The sapropel record of the eastern Mediterranean Sea — results of Ocean Drilling Program Leg 160

Research on sediments recovered during Ocean Drilling Leg 160 has concentrated on two issues: the first concerned the stratigraphy of sapropel formation, the second was oriented to clarify specific processes that explain sapropel origin. Progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites. On the composites, initial work has resulted in the establishment of high-resolution and intermediate-resolution stratigraphies for three sites (963, 964, 967); correlation of sedimentary cycles to astronomical (insolation) cycles extends the stratigraphies to Sites 969 and 966. The sapropel occurrences in the marine and land sequences over the entire Eastern Mediterranean are correlated; with the resolution that can be obtained from isotope studies, groups of sapropels occurred simultaneously over the entire basin. In detail, however, the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins. The differences may be related to effects of water depth, diagenesis, and post-depositional tectonic attenuation of sequences. Studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels, caused the enrichment of trace metals associated with sapropels, and helped to preserve primary sedimentary structures. Besides, all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events.

Reservoir modelling sand-rich deep-water clastics: the necessity of down-scaling

Channels, lobes and levees are inadequate architectural elements to describe sand body geometry in sand-rich deep-water systems. Sand sheets are the most common architectural element. Elutriation modifies primary sedimentary structures by mobilizing and reorganizing grains. This changes pore size distribution and permeability characteristics. Bed amalgamation and offset stacking of sand sheets makes problematic the use of bed thickness in correlation. A down-scaling approach for reservoir modelling is recommended, as a hierarchy of architectural elements that define up-scaling relationships cannot be established.

Deep-water massive sands: nature, origin and hydrocarbon implications

Deep-water massive sands (DWMS) are here defined as very thick (>1 m) sand beds or units that are devoid of primary sedimentary structures and that occur in association with other deep-water sediments — the massive sand facies association. Following careful examination of some 70 examples of massive sands drawn from deep-water successions of all ages and lithologies, we are confident that the shroud of mystery surrounding these deposits can be lifted, their origin and nature can be explained and their importance as hydrocarbon reservoirs can be brought sharply into focus.Besides their very thick bedding and structureless aspect, key features of DWMSs are the common presence of water escape structures, subtle amalgamation surfaces and shale clasts. Typically they show poor to moderate sorting and compositional immaturity. The two key processes involved in their long-distance transport and emplacement are sandy debris flows (SDFs) and high-density turbidity currents (HDTs). Post-depositional liquefaction and sand injection can significantly affect either type. They generally occur as part of a thicker sand-dominated sequence or sand body (sand/shale ratios 7:1 to >9:1) fed from a clean sand and/or gravel-rich source. The variety of scales and geometries is dependent upon the depositional setting: chutes, scours, flow-slides and lobe sheets on smaller-scale fan-deltas; channel, ribbon and tongue-like bodies on open slopes and proximal fans; lobate and lensoid bodies more distally; and trough/basin sand bodies that are broadly lensoid to tabular. The process/facies type and depositional setting profoundly affect both the internal architecture and external geometry of DWMS bodies.

Applications of ground-penetrating radar (GPR) to sedimentological, geomorphological and geoarchaeological studies in coastal environments

Abstract Acquisition of high resolution data regarding the stratigraphy and internal structure of coastal sedimentary sequences is becoming increasingly important in many sedimentological, geomorphological and geoarchaeological studies. Such information is usually obtained from the logging of field exposures, shallow trenches and cores. However, ground-penetrating radar (GPR), a relatively new and rapidly developing non-invasive geophysical technique, may also aid such investigations in certain coastal settings, providing additional, complimentary and often unique data sets. The technique is based on the transmission, reflection and reception of high frequency electromagnetic (radar) waves, with reflections occurring in the subsurface due to the varying electrical properties of the sediments. Such reflections can occur due the presence of primary sedimentary structures, lithological/material changes or water content changes. Examples presented from the UK indicate that the technique can accurately delineate the stratigraphy and internal sedimentary structure of coastal barriers, spits and strandplains, both above and below a fresh groundwater table. Sand and/or gravel-dominated beach-dune systems on moderate to high wave energy, macrotidal coasts appear to provide optimum settings for GPR deployment. In addition, the technique also has the potential to locate, map and provide stratigraphic context for a wide range of archaeological features that are commonly found in such coastal environments.

Ituzaingó Formation. A key for the interpretation of upper tertiary stratigraphy, Mesopotamia-Chaco Paraná Basin, Argentina

Four years of sedimentological detailled study led me to reinterpretate the Upper Terriry Mesopotamia region stratigraphic squeme. Upon a detailed sedimentary environment analysis of the Mesopotamia – Chaco Paraná basin, I made a different interpretation about previous ideas of the several units present at this morphostructural domain. The Ituzaingo Formation was interpretated as continental (fluvial) in origin by several researchers based on an insecure fossil fauna of invertebrates, always present at the top of the layers. These fossil fauna his a broad and extend biochron over the Cenozoic time. The Paraná Formation infralies the Ituzaingó Formation. The idea of an erosion uncorformity between them was strongly proposed. The Paraná Formation carries marine fossil fauna of invertebrates dated as Middle Miocene. It generally is accepted that Toropí and Yupoí Formations rest over Ituzaingó Formation elsewhere. The structural contact was assumed as an erosional unconformity surface. Paraná Formation was dated by fossil invertebrate fauna as Middle Miocene. Topoí and Yupoí Formations were dated as of Midlle to Upper Pleistocene based on fossil vertebrate fauna. I assumed that the position of these fossils vertebrate fauna of is ofaloctonous for the Toropí and Yupoí muddy formations and I propose that the fossil vertebrate fauna is in Holocene sediments. I carried out detailled studies which include: architectural, granulometric and morphoscopic analysis, petrographic data, paleocurrent measurements, scanning electron microscopy, diffractometric x-ray measurements, detailled sedimentologic profiles, drill hole correlations and remote sensing digital processing image analysis. Architectural analysis me to distinguish the following internal sedimentary structures: hummocky cross stratification, tidal bundles, herringbone cross stratification, lenticular bedding, flaser bedding, bipolar cross stratification and several types of wavy stratifications. These studies led me to realize that the sedimentary environments and specially the relationship between these units are different from previously proposed studies. I propose a common shallow littoral marine origin (intertidal) for all these units in a Middle Miocene age, sincronous with the Paranense Sea ingression (12-14 MA) and an interdigitation between sand and mud lithofacies. A subsurface unit, near Paraná Formation, the Puelches Formatioin, is interpretated as belonging to this sandy-muddy typical shallow marine lithofacies, desoite of a previous work, that accepted a fluvial origin for this unit. An erosion unconformity was also proposed between Paraná and Puelches Formation. The Paraná and Ituzaingó Formations are composed by some 80% friable arenaceous facies and of about 20% of muddy facies. This is a typical heterolithic successioin. A neat but irregular ferricretization forehead is present, specially, in the sandy lithofacies. This physico-chemical phenomena led previous authors to a misinterpretation of the stratigraphic scheme. My results show that the contacts between forehead ferricretization in the sandy lithofacies was assumed as an erosion unconformity, which is not the case. In some place, the ferricretization formed ferrigenous sandstones, which I called ferrigenous duricrust. This is os a very recent age of about Upper Pleistocene-Holocene times. Calcretizatioin is also present in the outcrops. All these secondary processes hid the primary physical structures and chemical composition of the sandy-muddy lithofacies. Paraná, Ituzaingó, Toropí, Yupoí and Puelches Formations are suggested to have the same origin (intertidal) and the same age, Middle Miocene. They constitute a blanket body of about 1500 kilometers in extension at the northeastern portion of Argentina and have approximately 300 meters of thickness. They are composed only by medium, fine to very fine white sands and grey to beige mud lithofacies. When alternation is present (ferricretization), the sediments change its colours to reddish or yellowist, sometimes purple, brown or black. In a few cases the alternation (ferricretization) is very intense, then sands are converted into reddish to black sandstones. In this case, primary sedimentary tractive structures were always recognized. These units, Paraná and Ituzaingó Formations are sincronous and may be correlated with other similar outcrops at Uruguay, Brazil, Paraguay, Bolivia, Perú, Ecuador, Colombia, Venezuela and Guyanas. They were formed during Paranense-Amazon Seas.

Sedimentology and depositional history of the Claret Formation (‘carse clay’) at Bothkennar, near Grangemouth

Synopsis The estuarine sediments of Flandrian age which comprise the Claret Formation in the Bothkennar area can be divided into bedded, mottled and laminated facies on the basis of primary sedimentary structures, frequency of silty laminae and the nature and extent of bioturbation. The mottled facies can be further subdivided by the style of bioturbation. The facies are very similar in overall particle size distribution and mineralogy. Biogenic traces occur in all the facies and have been tentatively matched to organisms present in the modern infauna Equation ()of the Firth of Forth. The succession of facies has been controlled by the fall of relative sea-level since the Flandrian maximum and records an emergent sequence from sub-tidal (probable water depth around 20 m) to inter-tidal. In the Bothkennar area four local subdivisions can be recognized in the Claret Formation, which reflect changes in sedimentary environment consequent on the reduction in water depth.

Distribution and deposition of mudstone facies in the Upper Devonian Sonyea Group of New York

ABSTRACT Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to (1) improve our understanding of mudstone facies, (2) examine coeval mudstone facies within a broader depositional context, and (3) promote a simple methodology for the study of mudstones. Six mudstone facies are distinguished, each characterized by several component mudstone types or subfacies. Sedimentary conditions and environments are reconstructed from primary sedimentary structures and bioturbation characteristics. Mudstone facies are characterized by soils and flood deposits in the coastal-plain region, by rapid sediment deposition and frequent reworking in nearshore areas, by storm-dominated offshore transport on a wide, basin-margin platform, by turbidite slope deposits below storm wave base, and by bottom currents and slow settling in the distal, deep-basin black mudstones. The latter, although commonly thought of as deposits of a stratified anoxic basin, contain indications of benthic life, such as burrows, disrupted laminae, and clay/silt fecal pellets. These observations are incompatible with a stratified-basin model, and indicate the need to search for alternative models of black-shale accumulation.

Eolian sedimentation and soil development on a semiarid to subhumid grassland, Tertiary Ogallala and Quaternary Blackwater Draw formations, Texas and New Mexico High Plains

ABSTRACT Eolian sediments have accumulated as non-glacigenic loess and thin sand sheets on the Central and Southern High Plains grasslands of Texas and New Mexico since the late Miocene. Locally as much as 110 m of eolian sediments with numerous paleosols are preserved in the Quaternary Blackwater Draw Formation and the upper part of the Miocene-Pliocene Ogallala Formation. These sediments and paleosols, which cover more than 130,000 km2, are similar to recent surface sediments and soils and record a long period of episodic eolian transport and sedimentation, and pedogenesis on a stable low-relief grass-covered landscape. Eolian sections, which comprise the fine sand to coarse silt lithofacies of the Ogallala Formation, and the very fine to fine sand and sandy mud lithofacies of the Blackwater Draw Formation, generally lack primary sedimentary structures. Grain size of Ogallala sediments decreases from west to east, and grain size of Blackwater Draw sediments decreases from southwest to northeast. Soil horizonation is well developed in most sections, and buried calcic and argillic horizons are common. Calcic horizons are characterized by sharply increased CaCO3 content in the form of filaments, nodules, and petrocalcic horizons (calcretes). Argillic horizons are characterized by increased illuvial clay, pedogenic structure, and darker reddish hues. Rhizocretions are common locally. Open root tubules, which are typically less than 1 mm in diameter and characteristic of small plants like grasses, are present in all Ogallala and Blackwater Draw eolian sediments. Paleosols preserved in eolian sediments of the High Plains reflect periods of sedimentation followed by episodes of landscape stability and pedogenesis, and negligible sedimentation. Episodes of sedimentation and soil development likely resulted from cyclic decreases and increases in available moisture and vegetative cover. Eolian sediments were eroded and transported eastward during dry periods when vegetation was sparse in source areas, such as the western High Plains and the Pecos Valley. During humid periods more abundant vegetation probably protected source areas from deflation, and resulted in landscape stability across the High Plains.

Stratigraphy and Subaerial Exposure of Late Quaternary Tidal Deposits in Haenam Bay, Korea (South-eastern Yellow Sea)

Late Quaternary stratigraphy of the coastal deposits in Haenam Bay, south-western coast of Korea (south-eastern Yellow Sea) consists of two depositional units: a Holocene intertidal deposit (Unit I), and an underlying Late Pleistocene tidal deposit (Unit II), both of which are distinguished by distinct unconformity. The yellowish colour and more consolidated and oxidized nature are characteristics of the sediments in the upper part of Unit II. In spite of the lack of primary sedimentary structures, a conspicuous cryogenic structure was found in the upper part of Unit II, reflecting cold and dry conditions. The magnetic susceptibility enhancement in the upper part of Unit II added the possibility of pedogenesis during subaerial exposure. The upper part of Unit II is characterized by the deficiency of smectite and chlorite, in contrast to the clay mineral suites of Unit I and lower part of Unit II, which may be attributed to post-depositional and diagenetic alteration. The degree of weathering measured by the chemical index of alteration shows that the removal of labile minerals by leaching also occurred in the upper part of Unit II. Thus, the upper part of this unit appears to have undergone subaerial exposure and weathering during low sea-level stands. The supporting evidence of the sediment properties, including sediment colour and structure, water contents and shear strength, geochemical and clay mineralogical data, and magnetic susceptibility, provides the potential signature of the subaerial exposure and weathering of Late Pleistocene sediments in the tidal-dominated Haenam Bay sedimentary basin.

On the Significance of Small-scale and Short-lived Air Escape Structures for the Destruction of Primary Sedimentary Laminations in the Çolakı Beach Deposits, Gulf of Antalya, Turkey (Eastern Mediterranean)

Çolakı beach is a 4km long coastal depositional system on the Mediterranean coast of southern Turkey. It has been accumulating at the foot of Taurus tectonic belt since the late Pleistocene. The beach deposits are composed of fine-grained sands with no significant longshore changes in granulometry and mineralogy. In addition, the sequence of beach deposits is characterized by a homogenous internal appearance due to the lack of preserved primary sedimentary structures. On the foreshore, air-escape structures comprising small tubes, 5–30mm in length and 0·5–3mm in width are extensively formed along the upper swash zone during the backwash phase of wave action. When the next wave swashes across the beach, the former structures are destroyed, at the same time producing conditions for the creation of new ones. The escape structures destroy the original sedimentary lamination. It is thus proposed that the homogenous internal appearance of the older beach deposits has been produced by the same mechanism.

Formation and diagenesis of bedding cycles in uppermost Cretaceous chalks of the Dan Field, Danish North Sea

Metre‐scale lithologic cycles, visible in core and on logs from Maastrichtian chalks of the Dan Field, were examined to determine their mechanisms of deposition and relation to hydrocarbon production. The lower parts of cycles consist of porous, cream‐coloured, largely non‐stylolitic, commonly laminated chalk with limited bioturbation (mainly escape burrows). Cycles are capped by thinner intervals of white to grey, hard, stylolitic chalk with concentrations of bioclastic material, intense burrowing and few preserved primary sedimentary structures. The cycle caps contain nearly twice as much Mg as compared to the more porous parts of cycles and also have slightly larger δ18O values (−4·1‰ for the caps; −4·4‰ for porous zones). There is a significant reduction of average cycle thickness, as well as total thickness of the Maastrichtian chalk section, from SW to NE across the Dan Field. The cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the NE side of the field. These data indicate that episodic winnowing removed fine‐grained constituents from highstanding northeastern areas. Porous cycle bases were deposited at relatively high rates that precluded complete bioturbation; preserved laminae, coupled with escape burrows, reflect episodic sediment influx in areas that flank the seafloor highs. Cycle tops apparently accumulated more slowly (throughout the region, but especially on seafloor highs), perhaps because of reduced productivity of planktic organisms. Slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures, and also led to incipient high‐magnesium calcite seafloor cementation (sufficient to yield firmer sediment and enhanced burrow preservation, but not to form true hardgrounds). Thus, the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis.Rates of chalk deposition, as inferred from physical and geochemical evidence, appear to be a significant control on reservoir characteristics in North Sea chalks. The highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized. Topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics, and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties.

Sedimentological, taphonomic, and climatic aspects of Eocene swamp deposits (Willwood Formation, Bighorn Basin, Wyoming)

Alluvially deposited carbonaceous beds are a major source of leaf compression fossils, but the accumulation of both the sediments and the fossils is poorly understood. We have studied the sediments and fossil plant assemblages of three laterally extensive carbonaceous beds from the Eocene of the Bighorn Basin, Wyoming. Field, laboratory, and microscopic observations show that carbonaceous beds are highly heterogeneous, varying in the amount and type of organic matter, preservation of primary sedimentary structures, and the prevalence of paleosol features. Small-scale lateral and vertical variations indicate a mosaic of conditions in the wet floodplain backswamp. The variables controlling this mosaic were primarily rate of clastic input and degree of substrate flooding which, in turn, influenced the amount of pedogenesis. Fossil plant assemblages occur in sediments least modified by pedogenesis, and the best preservation is in the distal, fine-grained portions of overbank flood deposits. There is no evidence for transport of plant material from channel to backswamp. Fossil leaves in a single quarry sample (i.e., those collected from within 10-20 cm stratigraphically and from an area of 4-10 m 2 ) probably accumulated during one or several closely-spaced flood events. Quarry samples are thus spatially and temporally constrained snapshots of the vegetation that grew on the alluvial floodplain. Each carbonaceous bed is estimated to represent roughly 2,000 years, based on the length of fluvial avulsion cycles. Multiple quarry samples from the same carbonaceous bed can thus be considered coeval for most evolutionary or paleoclimatic analyses. In the central Bighorn Basin, laterally extensive carbonaceous beds are abundant in the lower and upper Willwood Formation, but essentially absent in the middle. We attribute this to tectonic and climatic change, especially to higher rates of sediment accumulation that caused greater dispersal of organic material within sediments and climatic drying that allowed degradation of organic material before it could be preserved.

Hemipelagites: processes, facies and model

Abstract Detailed sedimentological studies have been carried out on Pleistocene to Recent sediments from the Makran margin and on Miocene to Recent sediments from the Oman margin of the NW Indian Ocean. In both area, hemipelagites make up c. 50% of the succession, being closely interbedded with turbidites (Makran) and pelagites (Oman). Their sedimentary features (structure, texture, fabric and composition), bedding style and rates of sedimentation are reported and compared with previous detailed studies of both modern and ancient hemipelagites. A composite facies model is presented. Hemipelagites are finegrained sediments typically occurring in deep-water settings. They generally comprise an admixture of >10% biogenic pelagic material and >10% terrigenous or volcanigenic material, in which >40% of the terrigenous (volcanigenic) fraction is silt size or greater (i.e. >4 µm). They are deposited by a combination of vertical settling and slow lateral advection. Hemipelagites are mostly thoroughly bioturbated with ichnofossils including Zoophycos, Planolites, Chondrites, Phycosiphon and others. They are generally devoid of primary sedimentary structures, except for an organic-rich facies deposited in anoxic conditions which has a distinctive fissile lamination. The submicroscopic fabric is closely packed with subparallel alignment of clays. The grain size is strongly influenced by the composition, although in general the deposits are fine grained (mean size 5–35 µm), and poorly sorted, in some cases with bimodal, trimodal or polymodal grain-size distributions. The composition includes a biogenic fraction that is calcareous and/or siliceous, and a terrigenous fraction dependent on the nature of the supply pathway, i.e. river plumes, aeolian dust, glacigenic input or volcaniclastic fallout. Chemogenic components (phosphorites, glauconite, ferromanganese nodules) are common in some settings, whereas high organic carbon contents characterize others. Cyclic bedding, typical of climatic forcing, is common although bed boundaries are generally very gradational. Sedimentation rates typically vary from <5 cm/ka to >20 cm/ka and greatly influence development of the different characteristics outlined above. Extremely high rates (>100 cm/ka) occur in zones of hemipelagic sediment focusing, such as slope canyon systems and sediment pathways.

Benthic macrofauna in a shallow water carbonate sediment: major bioturbators at the Dry Tortugas

Functional group analysis was used to determine the major bioturbators in the Dry Tortugas, Florida Keys. The surface community is dominated by surface deposit feeding polychaetes and burrowing bivalves capable of mixing the top 0–4 cm of the sediment on time scales of days to weeks. Bioturbation by the Notomastus sp. and Callianassa sp. deep community effectively removes primary sedimentary structures. Their fossilized burrow structures may be geologically important in this system. Surface microtopography is controlled by stabilizing and destabilizing forces that determine the potential for surface resuspension and sediment transport in the Dry Tortugas.

SEDIMENTOLOGY OF TERRIGENOUS MUD FROM THE ORANGE RIVER DELTA AND THE INNER SHELF OFF NAMAQUALAND, SOUTH AFRICA

ABSTRACT Sediment cores, from the inner shelf off the Namaqualand continental shelf have been examined using radiographic and grain-size techniques. Size-frequency distributions reveal that the sediments are poly modal, with a dominant very fine-silt mode being between 8-4μm, and suggest that the weak, poleward-flowing De Decker Current causes southward dispersal of terrigenous mud off the Orange River mouth. It appears that this current is not strong enough to transport material coarser than very fine silt. Material >63μm in size is rare, but may be indicative of storm events. Quartz and mica in the sediments are related to the semi-arid climate of Namaqualand, where they are entrained by adiabatic Berg winds moving offshore from the interior. Sponge spicules indicate the presence of filter-feeding animals (epifauna). Planktonic foraminifera may be indicative of relatively high productivity associated with nutrient-rich upwelling waters, which support an enormous population of plankton. The presence of faecal pellets is associated with the presence of burrowing infauna. Authigenic gypsum is associated with the presence of anaerobic conditions in the sediments. Two of the lithofacies identified in these fine-grained deposits are: (a) laminated mud and (b) homogeneous mud. Primary sedimentary structures are present, particularly in the portion of the mudbelt closest to the Orange Delta, where the mud is well laminated. Laminations decrease in abundance southward from the delta, where the muddy sediment is increasingly bioturbated (burrowed) by infauna, probably by polychaete worms.