bluuebunny/crossref_metadata_2025 huggingface dataset:
Avulsion mechanisms on the Okavango fan, Botswana: the control of a fluvial system by vegetation
Authors: McCARTHY T. S. , ELLERY W. N. , STANISTREET I. G.
Published: June 2006

A study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre‐existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.
Pliocene subaqueous fans and Gilbert‐type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA
Authors: WHITE JAMES D. L.
Published: June 2006

Maar craters of the Mio‐Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater‐fed lakes. Ephemeral streams crossing the playa entered some of the lake‐filled craters, and built coarse grained Gilbert‐type deltas and subaqueous fans along the margins of these craters. The small, coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits. However, the local production of coarse grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief and the small size of the receiving ‘basins’resulted in a unique combination of features. Cone‐shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density‐modified grain flows. Gravelly Gilbert‐type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream. They are concave lakeward, mimicking the underlying crater wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes. Bed‐specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate‐saturated lakewaters during between flood periods of high net evaporation.
Phosphatization of calcium carbonate in phosphorites: microstructure and importance
Authors: LAMBOY MICHEL
Published: June 2006

Fabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X‐ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrated.
Scanning electron microscope evidence for offshore‐onshore sand transport at Fire Island, New York, USA
Authors: WILLIAMS A. T. , MORGAN P.
Published: June 2006

Balancing the sediment budget of the Atlantic shoreline of Long Island has proved to be problematic because more sediment has been found to be moving westward in littoral drift than was estimated to be supplied by wave erosion of the Ronkonkoma terminal moraine at Montauk Point. There is a need for the existence of an additional, possibly offshore source. As part of a larger Atlantic shore study, 11 beach samples from Fire Island were compared with samples representing the known glacial source at Montauk Point, and also with four offshore samples to the south. Comparisons were based upon quartz sand grain surface texture patterns derived by scanning electron microscope checklist analysis. Initial evidence of an offshore‐onshore link was provided by qualitative results examining surface texture variability between samples, illustrated by photographic evidence and written notes. This evidence was confirmed consistently during quantitative analysis. In particular, canonical variate analysis grouped two offshore samples representing deposits of buried glacial to fluvioglacial lobes with beach samples from Fire Island in adjacent onshore locations or slightly downdrift. In addition, canonical variate analysis and factor analysis using the spssx package, together with cluster analysis using the ARTHUR package, depicted the main factors and texture variables largely responsible for sample discrimination. The significance of both the offshore‐onshore link and the associated factors influencing quartz grain surface texture was revealed by the fact that they are strongly related to each other and to the regional geomorphology.
The fluvial architecture of the upper Buntsandstein in the Iberian Basin, central Spain
Authors: GARCÍA‐GIL SOLEDAD
Published: June 2006

The Cercadillo Sandstone and Siltstone (CSS) Formation forms the upper part of the Buntsandstein fluvial facies in the western margin of the Iberian Basin. Sedimentation was controlled by two major pre‐Triassic tectonic systems: the Iberian Ranges System (NW‐SE) and the Central System (NE‐SW). Fluvial sedimentation during this period took place in coexisting braided and meandering channels, together with associated ephemeral episodes. This unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations.

The CSS Formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments. A widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence. These horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion‐sedimentation processes in the area, probably due to quiescent periods on the master faults that delineate the basin.

Minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled. Palaeocurrent data show a general flux of sediments to the south‐east, parallel to the axis of the Iberian Basin, but the final part of the upper sequence palaeocurrents to the north‐east are interpreted to indicate a local reactivation of secondary, transverse faults in the south‐east part of the study area.
Milankovitch cyclicity in Purbeck peritidal limestones of the Prebetic (Berriasian, southern Spain)
Authors: DE CISNEROS C. JIMENEZ , VERA J. A.
Published: June 2006

Peritidal carbonate rocks (Purbeck facies) of the uppermost Portlandian to Berriasian in the type section of the Sierra del Pozo Formation in the Prebetic Zone, southern Spain, are divisible into 141 shallowing upward cycles averaging 2 m in thickness. The subtidal facies in these cycles consist of micritic or marly limestones with dasycladacean algae and lituolids; the intertidal facies are micritic limestones containing birdseyes and miliolids; the supratidal facies comprise laminated algal limestones, the tops of which display desiccation cracks and rhizocretions, or more locally palaeosols, calcretes, or palaeokarst surfaces.

A statistical study, using power spectra of the Fast Fourier Transform, demonstrates that the periodicity of these cycles is in the Milankovitch frequency band. Most sedimentary cycles correspond to the obliquity cycles; eccentricity and precession cycles have also been recognized. Using a Fischer plot, third‐order tectono‐eustatic cycles are recognized, which can be correlated with the eustatic curve of the Exxon chart.

The shallowing upward sequences are characterized by a distinctive pattern of geochemical parameters. Carbon and oxygen isotopic (δ13C and δ18O) variations, calcium and magnesium carbonate contents and the abundance of organic matter and trace elements (Mn and Sr) all have predictable patterns of distribution within the sequences. The Sr content of the subtidal facies is relatively high whereas the δ13C and δ18O ratios are quite low; in the intertidal facies the Sr and Mn levels fall concomitantly with a rise in δ13C and δ18O. The highest δ13C and δ18O values occur in the lower part of the supratidal facies, whereas in the upper part of δ values and Sr contents drop sharply.

Cyclic variations in evaporation and in meteoric water influence, determined from oxygen isotopic composition, reveal that the cyclicity of the beds containing the most limestones (supratidal) and those with the most marls (subtidal) is related to climatic changes. The coldest periods are those represented by supratidal deposits, when the sea level was at its lowest. During the warmest periods, when the overall sea level was higher, subtidal deposits accumulated in the region.

A genetic model is proposed, according to which the asymmetrical sedimentary cycles occur in response to glacio‐eustatic changes with a periodicity similar to that of Plio‐Pleistocene sea‐level variations, but with a much lower range due to the smaller extent of polar ice caps during the Early Cretaceous. The glacio‐eustatic changes involved a rapid sea‐level rise and a slow sea‐level fall.
The surface texture of the Saharan Debris Flow deposit and some speculations on submarine debris flow processes
Authors: MASSON D. G. , HUGGETT Q. J. , BRUNSDEN D.
Published: June 2006

Deep towed 30 kHz sidescan sonar data from the Saharan Debris Flow deposit, west of the Canary Islands, show spectacular backscatter patterns which are interpreted in terms of flow banding, longitudinal shears, lateral ridges (levees) and transported blocks. Identification of these features is based on high resolution seismic profiles and on a comparison with similar structures seen in better known environments including other marine debris flows and slides, subaerial sediment failures (particularly rock fall avalanches), glaciers and lava flows. Flow banding in the debris flow, picked out by bands of differing backscatter intensity, is on a scale of tens to hundreds of metres. It is considered to result from flow streaming of clasts, with variation in clast size between bands. This primary fabric is cut by a series of distinct flow‐parallel longitudinal shears. Broad, high backscatter longitudinal bands along the edge of and within the debris flow are interpreted as lateral ridges associated with multiple flow pulses; the high backscatter possibly reflects either a concentration of coarse grained material or chaotic sediments deposited from a turbulent flow. Coherent, low backscatter patches are interpreted as rafted blocks, although streamlined haloes of high backscatter material around some blocks indicates differential movement between block and flow, possibly during the waning stages of the flow.

A non‐turbulent debris flow model is preferred, in which a raft of more or less coherent material is carried along by a base undergoing laminar flow. Speculatively, the lack of turbulent mixing preserves original sedimentological heterogeneity from the debris flow source area, possibly in the form of clast size distributions. These heterogeneous sediments are drawn out into a flow‐parallel banding which is imaged as the flow‐parallel backscatter intensity banding. The upper raft of material responds to cross‐flow velocity differences, and perhaps to variations in the timing of flow movement, primarily by longitudinal shearing. More complex deformation of the flow banding occurs at the flow margins and around obstacles in the flow, where lateral velocity shear would be expected to be highest.
Facies analysis and high‐resolution sequence stratigraphy of the Lower Eocene shallow marine Ametlla Formation, Spanish Pyrenees
Authors: DREYER TOM , FÄLT LARS‐M.
Published: June 2006

The Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn‐sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment‐starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway.

In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized.

(1) At the regional scale, elements of two 3rd‐order composite sequences (sensu Exxon) have been recognized. These include a 3rd‐order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd‐order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward‐stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment‐starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high‐frequency sea‐level fluctuations, and to downslope redistribution of sand.

(2) At the subregional scale, detailed mapping indicates the presence of five 4th‐order sequences. The 4th‐order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea‐level fall. Sharp‐based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea‐level. Where 4th‐order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone‐dominated transgressive systems tract and a siltstone‐dominated highstand systems tract.

(3) At the local scale, 2–9 5th‐order parasequences are present within the 4th‐order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonly stacked in a landward‐stepping manner.
Sedimentation and extensional basin evolution in a Miocene metamorphic core complex setting, Alvord Mountain, central Mojave Desert, California, USA
Authors: FILLMORE ROBERT P.
Published: June 2006

The Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre‐Tertiary basement complex and thickens eastward to c.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood‐dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood‐dominated fans. Igneous clasts and a south‐east palaeoflow direction suggest a proximal, low‐relief source in the western Alvord Mountain area. Pebbly sandstone of the southward‐prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood‐ and debris flow‐dominated alluvial fans that advanced across the axial braidplain and lacustrine system.

Basin development is interpreted to have been controlled by regional NE‐directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain ‐ Cronese Hills region was initially transported as a single hangingwall block on the E‐dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW‐striking, SW‐dipping normal fault antithetic to the low‐angle E‐dipping detachment fault.
Grain size, composition, porosity and permeability contrasts within cross‐bedded sandstones in Tertiary fluvial deposits, central Spain
Authors: HARTKAMP CHRISTEL A. , ARRIBAS JOSÉ , TORTOSA AMPARO
Published: June 2006

Permeability measured with a portable probe permeameter on outcrops of cross‐bedded sandstones ranges between 0·9 and 19 D. The highest permeability (2–19 D with an average of 8·5 D) occurs in the coarsest grained foreset laminae (CFL), intermediate values (2–12 D with an average of 5·3 D) occur in finer grained foreset laminae (FFL) and the lowest values (0·9–10 D with an average of 4·8 D) occur in bottomset layers (BL). In the cross‐beds the average grain size ranges from medium grained sand in the CFL to fine grained sand in the FFL and BL. In all three subfacies, the average size of the primary pores is approximately 1φ unit smaller than the average grain size. The abundance of unstable carbonate clasts correlates with increasing average grain size, micritic clasts being most abundant in the CFL. Conversely, quartz content increases with decreasing grain size and is highest in the FFL and BL. Diagenetic destruction of primary porosity by compaction and cementation, as well as generation of secondary porosity through dissolution, were controlled by the original mineralogical composition of the sand. Contrasts in grain size determine the primary pore size contrasts and differences in composition between CFL, FFL and BL. Permeability contrasts reflect variations in average primary pore size rather than differences in total porosity. Probe permeability contrasts between CFL, FFL and BL depend on contrasts in average pore size and contrasts in mineralogical composition between the subfacies.
Description and interpretation of fluvial deposits: a critical perspective
Authors: BRIDGE JOHN S.
Published: June 2006

A critique is given of recent methods proposed for the standardized description, classification and interpretation of fluvial deposits in terms of: (1) hierarchies of strata and their bounding surfaces; (2) lithofacies; (3) lithofacies associations (architectural elements); (4) geometry of sedimentary bodies. Much of the descriptive information can be conveyed using clear, explicit diagrams without recourse to complicated terminology. Any classification that is used should be based on easily measurable parameters which are used to define mutually exclusive classes. Terms used to refer to these classes should be explicit. Little agreement exists on the best terms to describe strata, mainly because existing terminology was not designed to account for all the different superimposed stratal scales. A simple terminology is suggested here whereby the relative scales of hierarchies of strata and cross‐strata are specified. Methods of describing hierarchies of strata by numerically ordering stratal bounding surfaces (rather than the strata themselves) are difficult to use in practice. The use of standardized lithofacies codes tends to discourage close observation and recognition of varieties and superpositions of lithofacies, has led to proliferation of acronyms, and to the suggestion that a particular lithofacies has a unique interpretation. In fact, many interpretations associated with these lithofacies codes are misleading or wrong. In a commonly used classification system of lithofacies associations (architectural elements) the different classes are not mutually exclusive, they are referred to using a mixture of descriptive and interpretive terms, and each element is represented graphically by only a single two‐dimensional section. Terms used to describe the three‐dimensional geometry of a sediment body should only be used to refer to two‐dimensional sections if three‐dimensional form can be reconstructed unambiguously. Fluvial ‘facies models’ are constructed to relate lithofacies associations and geometry to parameters such as channel and floodplain geometry, modes of channel migration and deposition rates. Most are of limited use or misleading because insufficient three‐dimensional information is shown. In addition, they commonly incorrectly associate channel planform (e.g. meandering, braided) with a characteristic lithofacies association and geometry, whereas other factors may play a dominant role.
The ecology of Lower Cambrian buildups from Zuune Arts, Mongolia: implications for early metazoan reef evolution
Authors: WOOD RACHEL , ZHURAVLEV ANDREY Yu. , ANAAZ CHIMED TSEREN
Published: June 2006

Reefal buildups in western Mongolia of mid‐Early Cambrian (late Atdabanian) age flourished during a period when shelf seas were globally widespread. The succession at Zuune Arts records the transition from shallow marine siliciclastic sediments (Bayan Gol Formation) to shallow marine, but still clastic‐influenced, carbonates (Salaany Gol Formation). The Salaany Gol Formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow, gently inclined shelf in a rapidly subsiding epicontinental sea. Cycles commenced with the growth of subtidal metazoan‐calcimicrobe aggregative communities on an open shelf. The resultant buildups were commonly engulfed by extensive, massive microbial stromatolites when they grew in agitated intertidal conditions. Latterly, they were smothered by ooid shoals in response to rapid sea level rise.

Four types of reefal buildup are distinguished: (1) green‐coloured calcimicrobe (Tarthinia, Epiphyton, Gordonophyton and Renalcis) boundstones; (2) red‐or green‐coloured Cambrocyathellus‐Tarthinia‐Epiphyton bafflestones; (3) red‐coloured Okulitchicyathus bindstones; and (4) red‐coloured radiocyath‐archaeocyath‐cribricyath bioherms. Each is interpreted as having grown at increasing depths and possibly sedimentation rates. The buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones, and are best developed towards the top of the formation, when sea level was high. Thickets of large, solitary archaeocyaths are also inferred in the deeper interbiohermal areas.

These buildups had synoptic relief and constructed porous structures with growth‐framework cavities housing diverse coelobiontic communities. Extensive synsedimentary cements are present, including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids. These early reefs thus have geological fabrics similar to later Phanerozoic representatives. It is proposed however, that this ecosystem was largely composed of generalist and opportunistic filter and suspension feeders which were dependent upon a far higher input of nutrients than modern day reefal developments. Bacteria were probably the main primary producers, from both planktic and benthic cyanobacterial communities. The diversity of each buildup assemblage appears to be controlled by primary cavity size, the richest fauna belonging to the highly tiered radiocyath‐dominated community inferred to have lived in the deepest waters. The communities at Zuune Arts can be compared with other buildups from the early Cambrian, and with Lower Ordovician receptaculid‐calcimicrobe‐solitary sponge bioherms known from the USA and Siberia.
Dust emission and transport in Mali, West Africa
Authors: NICKLING W. G. , GILLIES J. A.
Published: June 2006

Vertical dust fluxes were measured in the Inland Delta region of the Niger River, Mali, West Africa, during April‐June, 1989 and 1990. Measurements of dust flux represent, for the most part, non‐dust storm conditions or ‘dust haze’ periods. The observed concentration versus height relationships are similar to data presented by other investigators. The relationship between wind shear velocity (u*, m s−1) and vertical dust flux (F, μg m−2 s−1) can be described by a relationship in which F is proportional to u*4. However, there is considerable scatter within the data set which is attributable to textural controls and surface conditions. The vertical dust fluxes measured in Mali are compared to dust fluxes measured in Texas, USA, and Yukon Territories, Canada. The significantly different values for the constant of proportionality (a) in the F α a u*4 relationship for these geographically diverse areas is a function of surficial controls on the release of sediments to the air stream. Dust concentrations measured in Mali were found to be uniformly high and in general exceed WHO health standards for acceptable total suspended particulate loadings. As a result background dust may be considered a long term stress on health for the people of this region of Mali.
The effect of sampling scale on actualistic sandstone petrofacies
Authors: INGERSOLL RAYMOND V. , KRETCHMER ANDREA G. , VALLES PETER K.
Published: June 2006

Empirical correlations between plate tectonic setting and sand/sandstone composition have been the basis for large scale petrological models. These models do not explicitly treat sampling scale. Four areas from the western USA with diverse tectonic settings and rock types provide a natural laboratory for sampling sand at three different scales: talus piles to small drainages (first order), streams and rivers draining mountain ranges (second order), and large rivers and marine environments (third order). Existing plate tectonic petrofacies models should only be applied to third order settings because the data were derived from studies of such settings. This is especially true in tectonic settings with diverse source rocks (e.g. continental rifts and transform settings). On the other hand, some settings, such as active magmatic arcs and foreland fold‐thrust belts, provide uniform results at any sampling scale because of homogeneity of source rocks.

The Rio Grande drainage area is especially complex, with diverse igneous, metamorphic and sedimentary source areas. Some components (e.g. basalt) are destroyed with minimal transport, whereas others (e.g. quartz) are relatively enriched with greater transport. In this complex continental rift setting, first and second order sand is diverse and heterogeneous due to input from tributaries. The Santa Clara River of southern California also has heterogeneous sand due to diverse source rocks in this transform setting. It is only after considerable homogenization and stabilization due to weathering and mixing with more stable components, and/or considerable transport, that homogeneous compositions are produced in these two settings.

In contrast, the Cascade magmatic arc and the Canadian Rocky Mountain fold‐thrust belt have uniform source rocks (dominantly volcanic in the former and dominantly sedimentary in the latter). Uniform sand composition that is unique to each of these tectonic settings results at any sampling scale in these two cases.

Uniformity of data collection and analysis is essential for reproducible results. Use of the Gazzi‐Dickinson point counting method allows direct comparison among source rocks (zero order samples), modern sand of any order and ancient sandstone of unknown provenance.

Lack of recognition of the effect of sampling scale in the development of actualistic petrofacies models has led to incorrect rejection of many existing models. Third order sands are excellent predictors of plate tectonic setting, but first and second order sands can provide ambiguous plate tectonic interpretations in many settings. More complex actualistic petrofacies models based on diverse sampling scales are needed.
Sedimentation in a river dominated estuary
Authors: COOPER J. ANDREW G.
Published: June 2006

The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta.

Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 103 m3 s−1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level.

The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents.

During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change.

If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology.
Formation of fluidization pipes during liquefaction: examples from the Uratanna Formation (Lower Cambrian), South Australia
Authors: MOUNT JEFFREY F.
Published: June 2006

ABSTRACT Usually well preserved fluidization pillars and sand filled fluidization pipes occur within submarine channel sands of the basal Uratanna Formation (Lower Cambrian) in the Adelaide Geosyncline of South Australia. The morphology of these structures reflects complex lateral and vertical movement of fluids during liquefaction and dewatering. Fluidization pipes acted as conduits for highly concentrated, upward directed fluid flow. The formation and maintenance of these pipes was dependent upon the development of a pipe wall composed of clay plugged fine sand. Formed during initial fluidization, this lining acted as a permeability barrier, confining and concentrating fluidized flow within the pipe. Each of the pipes is surrounded by a cylindrical fluidization halo in which leakage through the pipe lining produced partial fluidization of the surrounding sediment. Fine scale structures within these haloes indicate that fluids flowed radially and upward out of the fluidization pipes at an acute angle. These fluids merged with and influenced the orientation and size of adjacent fluidization pillars. The fluidization pipes of the Uratanna Formation may represent unusual preservation of the unstable fluid flow conditions that occur during incipient fluidization of sand beds.
Formation of braided river floodplains, Waimakariri River, New Zealand
Authors: REINFELDS IVARS , NANSON GERALD
Published: June 2006

Floodplains along the braided gravel‐bed Waimakariri River are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines. In order of importance, three mechanisms lead to floodplain formation. River bed abandonment by lateral migration of the braid‐train initiates the formation of the largest floodpiains, which usually occur downstream of tributary fans and valley bedrock spurs. In the headwater reaches, localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit, or by its elevation over the river bed, or both. The least important mechanism initiating floodplain development is localized river or channel incision. A six stage model is proposed for the sequential development of floodplains on the Waimakariri River from: (i) active river bed; (ii) stabilizing bar; (iii) incipient floodplain; (iv) established floodplain; (v) mature floodplain; and to (vi) terrace. Two mechanisms, lateral migration by the braid‐train and reactivation of abandoned channels within floodplains, operating separately or in combination, are responsible for floodplain reworking and their relatively young age (<250 years). Clearly, braided rivers can construct substantial areas of well developed floodplain.
Drag reduction in turbulent muddy seawater flows and some sedimentary consequences
Authors: BEST J. L. , LEEDER M. R.
Published: June 2006

Turbulent saline flows which contain low concentrations of suspended clay (< 10 g 1−1) have previously been reported to possess dramatically different boundary layer characteristics from clearwater flows and to exhibit the phenomenon of drag reduction. Drag reduction causes significantly lower friction factors and higher erosion thresholds in muddy saline flows than predicted by experimental data gained from fresh‐water flows using the law of the wall to estimate bed shear stress. Confirmation is given by experimental results on drag reduction using non‐intrusive laser Doppler measurements in seawater flows with <2·2 g 1−1 maximum concentration of suspended clay. Increasing the concentration of clay caused progressively lower velocities near the wall due to a gradual thickening of the buffer region of the turbulent boundary layer. Several aspects of sediment transport, deposition and bedform development in natural marine environments are thus considered to be significantly affected by the drag reduction process.
Physico‐chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA
Authors: DRIESE STEVEN G. , MORA CLAUDIA I.
Published: June 2006

The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red‐brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non‐luminescent to dull luminescent, whereas later spar cements exhibit bright yellow‐orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow‐orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn2+ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe2+ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 13C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of PCO2 by as much as 30%.
Combined flow origin of edgewise intraclast conglomerates: Sellick Hill Formation (Lower Cambrian), South Australia
Authors: MOUNT JEFFREY F. , KIDDER DAVID
Published: June 2006

Unusually thick, coarse grained edgewise intraclast conglomerates occur at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the Lower Cambrian Sellick Hill Formation, South Australia. The intraclast beds are flat based and laterally discontinuous, forming bar‐like structures that must have exhibited bathymetric relief of as much as 1 m. The internal fabrics of these beds are variable. Thinner beds are dominated by flat‐lying intraclasts; thicker beds contain both chaotic, randomly oriented, steeply inclined intraclasts and clusters of fan‐shaped, vertically stacked edgewise intraclasts.

The Sellick Hill Formation intraclast conglomerates are inferred to have been formed by intense, storm‐generated combined flows on a broad, subtidal carbonate ramp. Superimposition of wave‐induced oscillatory motions on geostrophic bottom flows during large storms generates short‐lived, but exceptionally high instantaneous shear stresses in the bottom boundary layer. Entrainment of the relatively large intraclasts occurs through sliding, rather than pivoting. Edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclasts during passage of waves and the chaotic nature of collisions between intraclasts moving within the boundary layer. Collisions between intraclasts impart a rotating moment, causing intraclasts to tip up during maximum fluid shear stress. Lodgement or packing of clasts in vertical or steeply inclined positions occurs within scours, where intraclasts can wedge between other vertically inclined clasts, or where intraclasts are pinned in steep orientations by collisions with shallowly inclined intraclasts. Differential erosional resistance of the intraclast deposits probably led to the development of sharp lateral changes in thickness.

The Sellick Hill Formation intraclast conglomerates record erosion and reworking of subtidal, subfairweather wave base environments by exceptionally intense and presumably rare storm flows. The intraclast horizons represent a substantial loss in stratigraphic resolution due to widespread erosion of the ramp.
Depositional framework and controls on mixed carbonate‐siliciclastic gravity flows: Pennsylvanian‐Permian shelf to basin transect, south‐western Great Basin, USA
Authors: MILLER ROBERT P. , HELLER PAUL L.
Published: June 2006

Mixed carbonate‐siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley ‐ Owens Valley region of east‐central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate‐siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non‐channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles.

Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz‐rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point‐sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta‐fed submarine ramp facies model, which is a line‐sourced, shelf‐fed system that is not supply limited.

The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.
Biofilms, microbial mats and microbe‐particle interactions: electron microscope observations from diatomaceous sediments
Authors: WESTALL F. , RINCÉ Y.
Published: June 2006

Sediments and diatoms from the mudflats of the Bay of Bourgneuf in western France were examined in an electron microscope study of biofilms and microbial mats. The sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms. The sediment biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment‐water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. At shallow depths in the sediment column (0.5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benthic diatom culture consisted of EPS and bacteria and presented a ragged appearance.

Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh littoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS.

In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro‐organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation.