bluuebunny/crossref_metadata_2025 huggingface dataset:
Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man
Authors: DICKSON J. A. D. , BARBER C.
Published: June 2006

Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz.

The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore‐waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater.

The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods.

A four‐fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions.

Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.
Clastic wedges and patterned ground in the Late Ordovician—Early Silurian tillites of South Africa
Authors: DAILY B. , COOPER M. R.
Published: June 2006

The widespread Late Ordovician to Early Silurian glaciations are represented in South Africa by glaciogenic sediments of the Pakhuis Formation. Previously unrecorded clastic dykes within the basal tillite of this formation on Table Mountain, Cape Town, are interpreted as casts of frost‐fissures filled directly with clastics (sand‐wedges or ground‐wedges) rather than filled subsequent to the melting of ice‐wedges (ice‐wedge casts). This suggests sub‐aerial deposition for the tillite of the Sneeukop Member and the presence of a cold, dry climate with steep temperature gradients following deposition of that tillite in the Table Mountain area. A peculiar three‐dimensional joint‐pattern shown by the Pakhuis Formation tillites is interpreted as a relict of a reticulate ice‐vein network that formed in sandy till in permafrost. Such networks have been documented from the modern environment, but so far their relicts have not been recorded in ancient sediments.
Subaqueous liquefied and fluidized sediment flows and their deposits
Authors: LOWE DONALD R.
Published: June 2006

A clear distinction must be made between liquefied and fluidized systems. In liquefied beds and flows, the solids settle downward through the fluid, displacing it upward, whereas, in fluidized beds, the fluid moves upward through the solids, which are temporarily suspended without net downward movement. Many recent references to fluidized sediment gravity flows refer, in fact, to flows of liquefied debris.

Most uniformly liquefied beds of well‐sorted sand‐ or gravel‐sized sediment will resediment as simple two‐layer systems. Liquefied flows can originate either by liquefaction followed by failure, as in many retrogressive flow slides, or by failure followed by liquefaction, as in the case of some slumps. Empirical and theoretical estimates of flow velocity, thickness, and travel distance suggest that natural laminar liquefied flows of fine‐grained sand will generally resediment after moving a kilometre or less. Laminar flows of coarse‐grained sand will resediment after moving only a few metres. Grain dispersive pressure is thought to be of little significance in the development or maintenance of liquefied flows.

Many surficial submarine sand beds are apparently susceptible to liquefaction, including submarine canyon and continental rise deposits. Within submarine canyons and narrow fjords, steep slopes and channels promote the evolution of liquefied flows from slumps by liquefaction after failure and of high density turbidity currents from liquefied flows by the development of turbulence. Upon moving into the lower parts of submarine canyons or into proximal fan channels, liquefied flows will resediment and high density turbidity currents will tend to decline to flows transitional between liquefied flows and turbidity currents. The liquefied, coarser detritus within such transitional flows will be deposited while finer‐grained debris will remain in suspension and continue downslope as dilute turbidity currents. Resedimentation of the liquefied portions of such flows may be responsible for the deposition of the A‐subdivision of many turbidites and many thick, structureless ‘proximal turbidites’ or ‘fluxoturbidites’. Similar units can originate by liquefaction of the traction deposits of normal turbidity currents.

Fluidized flows are probably uncommon, thin, and, where formed, originate through fluidization of the fine‐grained tops of liquefied graded beds.
Authigenic pyrite and gypsum in South West African continental slope sediments
Authors: SIESSER WILLIAM G. , ROGERS JOHN
Published: June 2006

An unusual association of authigenic pyrite and authigenic gypsum has been found in silty clays recovered from the South West African continental slope. Nannofossil content suggests that the sediments are Upper Miocene‐Lower Pliocene in age.

Pyrite occurs as (1) granular masses, (2) ‘worm’tubes, (3) foraminiferal infillings, and (4) framboids. Gypsum occurs as euhedral single or twinned crystals of selenite. ‘Worm’tubes and foraminiferal infillings of pyrite are partially or completely enclosed in some gypsum crystals. Electron microprobe analyses show a relatively high concentration of manganese in both the granular masses and tubes.

Present‐day waters off this coast are dominated by the upwelling of cold, nutrient‐enriched waters (the Benguela Current). These rich waters support an enormous population of plankton. Death and decomposition of these plankton consume oxygen, thereby creating a belt of anaerobic sediments close to shore. An Upper Miocene‐Lower Pliocene regression (Dingle & Scrutton, 1974) lowered sea level and shifted an older analogue of this upwelling zone seaward, eventually establishing an anaerobic environment in places on the present continental slope.

Anaerobic bacteria thrived in these conditions. They reduced SO4 dissolved in sea water, initiating the formation of H2S. The H2S reacted with iron minerals present in the sediment to form FeS. Addition of elemental sulphur produced pyrite. This strongly reducing, low pH, environment became saturated with calcium obtained by the dissolution of planktonic calcareous organisms. Gypsum was precipitated once the product of the concentrations of dissolved calcium and SO4 exceeded the gypsum solubility product.
Relationships between sand input from rivers and the composition of sands from the beaches of Southern California*
Authors: RICE ROBERT M. , GORSLINE DONN S. , OSBORNE ROBERT H.
Published: June 2006

During the period from 1967 through 1972, a sampling programme was completed to determine the economic potential of heavy metals in the beach and river sands of the southern California coastal zone. These samples were employed to test the hypothesis that sand composition in a given beach cell is dominantly controlled by the provenance draining into that cell and is not strongly influenced by longshore leakage from upcurrent cells. Sample sets obtained before and after the exceptional flood of 1969 made it possible to compare the sediment supplied by normal river flow with that supplied by a major flood. Multivariate statistical analysis of the heavy mineral distribution of southern California beaches and rivers indicate that the sand composition of the two northern cells is controlled by the dominantly sedimentary Transverse Range provenance, whereas the composition of the three southern cells is controlled by the dioritic Peninsular Ranges. Some leakage occurs between the two northern cells around the Point Dume‐Hueneme‐Mugu Canyon Zone, whereas no important southward mixing occurs between cells around the Palos Verdes‐Redondo Canyon Zone. Even though the Santa Clara River flows mostly through sedimentary terrains, samples from this river strongly reflect the granodioritic source present in its headlands. Although the basic sand composition within each cell persists during major flooding, leakage between cells may increase following flooding and the compositional packages present may show evidence of greater mixing.
The fissility of some Carboniferous shales
Authors: SPEARS D. A.
Published: June 2006

Laminations control the fissility of a sequence of shales and mudstones from the Upper Carboniferous Coal Measures in Britain. This was established by measuring the thickness of pairs of laminations in unweathered borehole material and comparing with the thickness of shale fragments at the outcrop of the same sequence. The parallel orientation of the clay minerals is not responsible for the fissility and it could be that its importance has been overestimated. The processes responsible for the formation and preservation of the laminations will also produce a well orientated clay fabric and hence fissility and clay orientation may be indirectly related.
The influence of palaeosalinity, grain size distribution and clay minerals on the content of B, Li and Rb in Quaternary sediments from Eastern Jutland, Denmark
Authors: VILLUMSEN ARNE , NIELSEN OLE BJØRSLEV
Published: June 2006

Bulk sample analysis of Late‐ and Postglacial sediments from the Randers fjord area, Eastern Jutland, seem to indicate a relationship between palaeosalinity and B, Li and Rb contents determined on weak acid extracts of the sediment. Investigations of different size fractions, grain size distribution, and of clay minerals show, however, that variations in palaeosalinity as determined by palaeontological methods in this case have no direct influence on the chemistry.

The variation of Li and Rb is related to variation in grain size, while the B variation seems to be related to the content of montmorillonite in the clay fraction.
Chertification in the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico
Authors: MEYERS WILLIAM J.
Published: June 2006

Chert distribution in the Lake Valley rocks is selective to mud‐supported facies; it is not related to proximity to unconformities. The facies selectivity of the chertification is believed to be a function of the depositional distribution of indigenous silica as sponge spicules, an interpretation that is supported by high positive qualitative correlation of chert with spiculitic rocks. Petrography indicates that the spicules were all originally siliceous, and that they all went through a moldic stage during which many molds were compactively destroyed and distorted. Remaining molds were subsequently cemented by calcite or chalcedony. Chert distribution and spicule petrography argue for an intraformational source for much of the silica.

Chert micro‐fabrics are dominated by microquartz, a replacement of grains and lime mud; length‐fast chalcedony, a pore‐filling cement; and megaquartz, a post‐chalcedony pore‐filling cement.

Petrography of compaction features within chert masses indicates that chertification occurred after some burial. Based on stratigraphic reconstruction this burial depth was a maximum of about 215 m. and was most likely a few metres to a few tens of metres.

Petrography of chert‐calcite cement relationships indicates that chertification occurred before and during first generation, pre‐Pennsylvanian non‐ferroan calcite cementation, and was completed before late‐stage, post‐Mississippian ferroan calcite precipitation. Petrography of chert clasts in basal Rancheria (Meramecian) and basal Pennsylvanian conglomerates proves these clasts derived from the Lake Valley Formation and were chertified before redeposition. Thus, some cherts in the Lake Valley are pre‐Meramecian in age, but all are pre‐Pennsylvanian in age. Furthermore, association of the cherts with the non‐ferroan cements indicates the cherts were probably precipitated in meteoric phreatic lens established beneath the pre‐Meramecian and pre‐Pennsylvanian subaerial unconformities.
Distinctive thin‐bedded turbidite facies and related depositional environments in the Eocene Hecho Group (South‐central Pyrenees, Spain)
Authors: MUTTI EMILIANO
Published: June 2006

The vertical and lateral stratigraphic relations of facies and facies associations, palaeocurrent directions, and geometry and internal organization of associated thick‐bedded and coarse‐grained bodies of sandstone provide the framework for distinguishing five thin‐bedded turbidite facies in the Eocene Hecho Group, south‐central Pyrenees, Spain. Each facies is characterized by a number of primary features which are palaeoenvironmental indicators by themselves. These features and their palaeoenvironmental significance are summarized below.

The impressive regularity and lateral persistence of bedding and depositional structures, combined with the association of thin hemipelagic intercalations are typical characteristics of the basin plain thin‐bedded turbidites. Lateral variations in bed thickness, internal structures, grain size, sand: shale ratio, and amounts of hemipelagic intercalations are present in these sediments, but take place so gradually that they cannot generally be recognized at the scale of even very large exposures. The basin plain facies has a remarkable character of uniformity over great distances and considerable stratigraphic thicknesses.

Thickening‐upward and/or symmetric cycles with individual thicknesses ranging from a few metres to a few tens of metres are typical of lobe‐fringe thin‐bedded turbidites. The sediments that comprise the cycles contain small but recognizable variations in bed thickness and sand: shale ratio. The diagnostic cyclic pattern can be detected in relatively small exposures. It should be noted that in absence of coarse‐grained and thick‐bedded sandstone of the depositional lobes the above cyclic pattern is diagnostic of fan‐fringe areas.

An extremely irregular bedding pattern with lensing, wedding, and amalgamation of individual beds over very short distances, sharp rippled tops of many beds, and internal depositional structures indicative of mainly tractional processes without substantial fallout, are typical and exclusive characteristics of channelmouth thin‐bedded turbidites.

Bundles of interbedded thin‐bedded sandstone and mudstone as thick as a few metres that are separated in vertical sequences by mudstone units of roughly similar or greater thickness are typical of interchannel thin‐bedded turbidites. The most diagnostic feature of this depositional environment is the presence of beds of sandstone filling broad shallow channels as probable crevasse‐splays.

Thin, thoroughly rippled sandstone beds with marked divergence of the bedding attitude characterize the channel‐margin facies. The divergence or expansion in thickness, is consistently toward the channel axis. Small and shallow channels filled with thin‐bedded deposits, interpreted here as crevasses cut into channel edges or levees during period of severe overbanking are also characteristic.
Chalk deformation and large‐scale migration of calcium carbonate
Authors: MIMRAN Y.
Published: June 2006

Sixty Upper Cretaceous chalk exposures were examined and sampled in Dorset, the Isle of Wight and Guildford, South England, in order to investigate the effects of tectonic deformation on the chalk fabric. Light and scanning‐electron microscopes were used extensively, and chemical, mineralogical and isotopic analyses ware carried out.

Two types of fabric modification were distinguished. The first type involves more than 90% volume loss by mechanical compaction at the early stages of deformation followed by dissolution and removal of calcium carbonate in the advanced stages of deformation. Chalks which have undergone this type of fabric modification are dense and consist of well rounded 0.5–3 μ calcite crystals with well developed pressure‐solution contacts. These chalks contain a high proportion of calcispheres (many of which are plastically deformed) but very few well preserved planktonic foraminifera and coccoliths. They are relatively enriched in insoluble constituents and depleted in strontium.

The second type of fabric modification involves introduction of calcium carbonate into the pore spaces. Chalks which have undergone this type of modification are dense and contain a high proportion of 3–5 μ polygonal interlocking calcite crystals. Fossils are virtually never deformed, and delicate foraminifera and coccoliths are well preserved. Insoluble constituents similar to those found in the unmodified chalks occur in low concentrations.

Removal of calcium carbonate at crystal contacts has taken place in chalks which have been subjected to high effective tectonic stresses and at crystal peripheries in chalks which have been subjected to high pore fluid pressure. Dissolution at crystal peripheries is responsible for the extensive calcium carbonate losses, and it is termed herein ‘confining pressure solution’.

Introduction of calcium carbonate occurs in rocks which were under low tectonic stresses. During deformation calcium carbonate migrates from chalks under high stresses towards those under tow stresses, while some goes into solution in sea water. Well developed joints and high pore fluid pressure might increase the rate of calcium carbonate removal by three orders of magnitude. Petrographic and isotopic data suggest‐that all fabric modifications studied took place in meteoric water.
Sedimentation and water escape structures in some late Precambrian shallow marine sandstones from Finnmark, North Norway
Authors: JOHNSON HOWARD D.
Published: June 2006

Water escape structures are abundant in the Grønnes Formation, a tectonically undeformed, late Precambrian shallow marine sandstone deposit in North Norway.

Trough cross‐bedded sandstones of the current‐dominated shallow marine environment were frequently liquefied, presumably due to recurring seismic shocks. Subsequent dewatering resulted in deformation of the cross‐bedding and the formation of convolute lamination. A three‐fold upward vertical sequence developed where liquefaction occurred below the sediment‐water interface: convoluted bed → passively deformed bed → undeformed bed. The passively deformed bed resulted from differential subsidence of a relatively plastic bed above a liquefied bed. It is characterized by anticlinal ridges and sand volcanoes at the sites of vertical sediment extrusion, and synclinal troughs at the sites of lateral sediment movement.

Liquefaction may have been induced by either tectonic (earthquake shocks) or non‐tectonic (storm‐induced microseisms) trigger mechanisms, or a combination of both. The restriction of such a high frequency of water escape structures to deposits immediately above a gentle regional unconformity lends support for a tectonic trigger mechanism.
Shallow marine sand bar sequences: an example from the late Precambrian of North Norway
Authors: JOHNSON HOWARD D.
Published: June 2006

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop.

The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high‐ and low‐energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross‐bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low‐angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments.

One sand bar was dissected by channels and infilled by tabular cross‐bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels.

The inferred processes of these sand bars are compared with those associated with modern storm‐generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.
Uranium oxidation and probable subaerial weathering of phosphatized limestone from the Pourtales Terrace*
Authors: BURNETT WILLIAM C. , GOMBERG DAVID N.
Published: June 2006

Phosphatized limestones from the Pourtales Terrace, Straits of Florida, have undergone fresh‐water diagenesis, as shown by several selective petrologic features such as dissolution of aragonitic skeletal material and preservation and recrystallization of Mg‐calcite. This interpretation is supported by the fact that greater than 94% of the uranium in these samples is in the U(VI) oxidation state.

On the deeper Pourtales Escarpment, phosphatized limestones show no alteration features indicative of fresh‐water diagenesis. The uranium in these samples is a mixture of U(IV) and U(VI) with oxidation‐state ratios similar to other sea‐floor phosphorites.

Our interpretation of these data is that uranium in the terrace samples was oxidized when that area was emergent. The karsted surface of the terrace, therefore, is a result of subaerial exposure of this portion of the Floridan platform. The escarpment samples were never above sea level, and so more closely retain their original uranium oxidation‐state ratios.
Clay minerals as an index of the degree of metamorphism of the carbonate and terrigenous rocks in the South Wales coalfield
Authors: GILL W. D. , KHALAF F. I. , MASSOUD M. S.
Published: June 2006

Illite, illite‐montmorillonite (I‐M) mixed layer, kaolinite and chlorite are the principal clay minerals identified in the carbonate and terrigenous carboniferous rocks in the South Wales coalfield. Mineralogic changes and illite crystallinity are the main modifications in the studied rocks. Progressive illitization of I‐M mixed layers (decrease in their expandabilities) towards the western part of the coalfield (i.e. in the direction of increase in coal‐rank and grade of metamorphism), and the development of vermicular kaolinite, dickite, allevardite and pyrophyllite in the anthracite area are the main mineralogic changes. Illite crystallinity was determined by Weaver's sharpness ratio (S.R.) and isosharpness ratio maps of illite in the carbonate and terrigenous rocks are produced showing a gradual increase in illite crystallinity towards the west. Illites in the terrigenous rocks tend to be aluminous (evidenced by the increase in their intensity ratios, ‘I.R.’) with increase in coal‐rank and grade of metamorphism towards the west. These changes are controlled to some extent by lithology. Correlation of illite S.R. with either its I.R. or expandability of I‐M mixed layer, has outlined two diagenetic zones in the coalfield: zone I, the Diagenetic zone, in the eastern and southern regions; and zone II, the Metadi‐agenetic zone, covering the central and northern parts of the central region. The Anchimetamorphic zone (zone III), which corresponds to the anthracite area in the northwestern region, was differentiated by correlating the illite S.R. with coal‐rank. The occurrence of the above mentioned diagenetic zones and modifications of the clay minerals in the carboniferous rocks of the South Wales coalfield are mainly lateral without variation with increase in depth.
Environmental setting and microstructure of subfossil lithified stromatolites associated with evaporites, Marion Lake, South Australia
Authors: VON DER BORCH C. C. , BOLTON B. , WARREN J. K.
Published: June 2006

A variety of finely laminated, subfossil, aragonitic stromatolites and oncolites occur on a regressive marginal flat surrounding Marion Lake, South Australia. These algal forms overlie a substrate of coarse, highly porous, moldic aragonitic limestone which passes progressively towards the take centre through a zone of interstatified aragonite and gypsum and ultimately to pure crystalline gypsum. All of these facies overlie Holocene marine carbonate bank sediments which unconformably overlie at least one upper Pleistocene marine unit. Detailed petrographic and stratigraphic studies, combined with comparative studies of related nearby lakes containing a variety of living aragonitic cryptalgalaminates, provide a model for development of the Holocene sedimentary sequence. Marion Lake last became inundated by the sea around 6500 years ago during the Holocene transgression, when a protected marine environment was initiated. Lateral sediment accretion sealed marine passes into the resulting lagoon system soon after sealvel stabilized, and a variety of gypsum and gypsum‐carbonate‐algal facies evolved. Pure gypsum was deposited in waters 2–3 m deep in the central basin area concurrently with formation of seasonally alternating gypsum and aragonite layers towards basin margins. Blue‐green filamentous algae thrived in the shallower marginal areas and at least partly controlled carbonate deposition, which must have occurred during seasonal outflow of carbonate‐rich ground water from the calcareous dune aquifer over denser gypsum‐saturated waters. These systems eventually migrated towards the centre of the lake to produce the relationships preserved today. The fresher waters also leached the gypsum from the marginal gypsum‐carbonate facies. Collapse due to gypsum dissolution, along with aragonite crystallization, combined to form a lake‐marginal mega‐polygonal facies. Teepee structures formed around polygon margins, with optimum conditions for stromatolite development occurring on the teepee crests. The actual stromatolites which occur around Marion Lake are strongly indurated and involve a variety of morphologies, the most common of which are laterally linked hemispheroids. Stacked hemispheroids and oncolites are also relatively common, along with irregular forms, many of which encrust a variety of substrate irregularities. Vertical relief of the stromatolites varies from centimetres to tens of centimetres and all forms are characterized by extremely fine internal interlaminations of alternate light and dark grey laminae which typically occur several per millimetre. The microstructure comprises micritic aragonite crystals with fibrous habit associated with organic matter, and occasional zones of abundant algal filament molds which are generally oriented normal to the laminae.
Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan
Authors: HIRAYAMA JIRO , NAKAJIMA TERUMASA
Published: June 2006

On the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker‐beds.

Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow‐water fauna in contrast with mudstones with well‐preserved deep‐water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts.

In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee‐like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee‐like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee‐like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d‐division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope.

The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.
Sandstone mounds and associated facies sequences in some Late Precambrian and Cambro‐Ordovician inshore tidal flat/lagoonal deposits
Authors: BALDWIN CHRISTOPHER T. , JOHNSON HOWARD D.
Published: June 2006

Sandstone mounds occur in some shallow marine heterolithic deposits from the Late Precambrian Stangenes Formation (N. Norway) and the Cambro‐Ordovician Crozon Formation (N. W. France) and Cabos Series (N.W. Spain). The sediments displaying the mounds accumulated in partially protected tidal flat/ lagoonal environments immediately before and during major transgressions.

The mounds are erosional features typically occurring on the tops of sheet sandstones (ca. 50–500 mm thick) some of which may have a storm washover origin. Mound genesis related to periodic emergence and late stage run‐off is supported by their intimate association with mudcracks and other very shallow water features (e.g. bidirectional current‐formed structures, wave ripples, ladder and interference patterns, mudflakes, etc.). Variation in mound morphology suggests that post‐depositional dissection began as elongate ridge‐gully couplets with secondary erosion of the ridge flanks leading to the development of more characteristic hemispherical geometries. Emergence may have been a function of tidal fluctuations and/or subsidence of storm surge events.

Facies sequences point to the repeated filling of these inshore environments by storm washovers superimposed on ambient tidal conditions which possibly resulted from the progressive decay of beach barriers during transgression.
Genesis and diagenesis of hiatus‐ and breccia‐concretions from the mid‐Cretaceous of Texas and northern Mexico
Authors: KENNEDY W. J. , LINDHOLM R. C. , HELMOLD K. P. , HANCOCK J. M.
Published: June 2006

A study of septarian concretions from late Cretaceous shale sequences of Texas and northern Mexico reveals complex burial‐exhumation histories. First generation concretions and pre‐fossilized moulds formed in silty clays before significant compaction occurred. Syneresis cracks developed and were filled by length slow fibrous calcite with a decrease in iron content toward the centre of each vein; in some cases this was succeeded by growth of equant or bladed calcite. Subsequent erosion of enclosing sediment caused collapse and break up of concretions, leaving fragments and some unbroken concretions exposed to encrustation and boring on the sea floor. These were subsequently buried and enclosed in a second generation of concretions, which also developed syneresis cracks. The calcite that filled these latter cracks was generally bladed (sometimes followed by equant calcite) and contains more iron toward the centre of the vein. Differences between the calcite filling first and second generation septarian veins indicates changing diagenetic regimes during burial in the marine environment, without a major shift in diagenetic conditions, such as the introduction of meteoric waters. These conclusions have significant implications to cementation in ancient limestones, which show textural and chemical sequences similar to those present in the septarian concretions described.
The sedimentary history of a debris‐flow dominated, Devonian alluvial fan–a study of textural inversion
Authors: LARSEN V. , STEEL R. J.
Published: June 2006

Karlskaret fan, with a radius of less than 11/2 km and dominated by debris‐flow conglomerates, is one of numerous alluvial fans built out from the fault margins of Hornelen Basin (Devonian, Norway). The fan body is more than 170 m thick proximally, consists of four main coarsening‐upwards segments and thins distally by a rising of its base and by a vigorous interfingering with very fine‐grained sediments originating from an adjacent, impinging floodbasin system.

Within the entire fan body, and within individual lobes, is a proximal‐distal (and vertical) facies change from sheet‐like, polymodal debris‐flow conglomerates through matrix‐rich conglomerates that are commonly distorted by loading, slumping and faulting, to remarkably sheet‐like, matrix‐rich granule sandstone of subaqueous debris‐flow origin.

Because the alluvial fan prograded into an actively aggrading floodbasin the primary fanglomerates, themselves having been subject to some sorting on the fan surface, incorporated large quantitites of very fine sediments. This inclusion of fines, effectively a textural inversion on the lower fan reaches, frequently led to remobilization and resedimentation of material beyond the fan toe. Anomalous maximum particle size/bed thickness relationships and a variety of graded textures within these resedimented beds suggest deposition in lacustrine areas of the adjacent floodbasin.
Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico
Authors: MEYERS W. J. , JAMES A. T.
Published: June 2006

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co‐precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non‐ferroan calcite cements indicates that both are original precipitates that formed during pre‐Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ18Os of megaquartz (mean =+27.00/00 SMOW; range =+ 24.8 to + 28.90/00) and calcite (mean =+ 28.00/00 SMOW; range =+ 27.3 to + 28.40/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre‐Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre‐Pennsylvanian opal‐CT precursors, and therefore probably re‐equilibrated during recrystallization in late or post‐Mississippian time.

We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ18Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground‐water system, the seaward terminus of which moved southward during lowering of pre‐Pennsylvanian sea level. The calcite cements and some of the opal‐CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ18Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.
Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea*
Authors: HEIN JAMES R. , SCHOLL DAVID W. , BARRON JOHN A. , JONES MARJORIE G. , MILLER JACQUELYN
Published: June 2006

Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300‐725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300‐400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal‐A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal‐A between 600 and 700 m sub‐bottom depth. Inorganic opal‐A is rapidly transformed by crystal growth to opal‐CT. The result is formation of silica cemented mudstone and porcelanite beds.

A regional acoustic reflector (called the bottom‐simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal‐A is transformed to opal‐CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub‐bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal‐A to opal‐CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°‐50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary.

X‐ray diffractograms and SEM photographs show three silica phases, biogenic opal‐A, inorganic opal‐A’, and opal‐CT. These have crystallite sizes of 11‐16 A, 20‐27 A, and 40‐81 A, respectively, normal to 101. The d(101) reflection of opal‐CT decreases with depth of burial at DSDP Site 192. This occurs by solid‐state ordering and requires at least 700 m of burial.

Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.
Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania
Authors: HAY R. L. , REEDER R. J.
Published: June 2006

Pedogenic calcretes are closely associated with Pliocene to Holocene wind‐worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes.

Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite‐type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.
Low‐magnesian calcite limestones forming at the deep‐sea floor, Tongue of the Ocean, Bahamas
Authors: SCHLAGER W. , JAMES N. P.
Published: June 2006

Carbonate ooze in the deep troughs between the Bahama Banks is a mixture of pelagic and bank‐derived material. It consists of aragonite, calcite and magnesium calcite in a ratio of about 3:2:1. Where exposed in erosional cuts at the sea floor, this ooze lithifies within 100,000 years and is transformed into calcite micrite of only 3.5‐5 mol % MgCO3. Where buried, the ooze maintains its original composition for at least 200,000‐400,000 years and remains unlithified for tens of millions of years.

Quite unexpectedly, the path of sea‐floor diagenesis of peri‐platform ooze was found to be the same as that of freshwater diagenesis. Most of the aragonite is leached, pteropod shells often leaving cement‐lined moulds behind; magnesian calcite recrystallizes and loses magnesium; polyhedral calcite of 2‐4 μm size appears as cement. The setting and the carbon‐oxygen isotope ratios rule out any freshwater influence. Carbon isotope ratios remain heavy, oxygen ratios shift towards equilibrium with the cold bottom water.

The calcite cement has 3.5‐5 mol % MgCO3 and can be interpreted as the least soluble form of calcite emerging from alteration at the sea floor or, alternatively, as a direct precipitate from cold sea water. The change in the composition of calcite cements with water depth supports the second interpretation. In the Bahamas and elsewhere in the world ocean, magnesium in calcite cements decreases from the warm surface waters down to 700‐1200 m, i.e. the boundary between intermediate and cold deep‐water masses. Below this level, calcite prevails and magnesian calcite and aragonite cements are restricted to semi‐enclosed seas with exceptionally warm bottom waters.
Boulder shapes and grain‐size distributions of debris as indicators of transport paths through a glacier and till genesis
Authors: BOULTON G. S.
Published: June 2006

Debris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing‐plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like a roche moutonnée. Textural and boulder shape characteristics can be used to help distinguish different types of till.
Alluvial fan and lacustrine sediments from the Stephanian A and B (La Magdalena, Ciñera—Matallana and Sabero) coalfields, northern Spain
Authors: HEWARD ALAN P.
Published: June 2006

Vertical sequence analysis within 1500‐2500 m thick coarse‐grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine‐grained coal‐bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid‐fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1‐25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross‐bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine‐grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm‐1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt‐stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine‐grained, low‐energy, regressive, coarsening upward sequences.

Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid‐fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm‐20 m thick, occur in the distal fan and lacustrine environments.

These alluvial fan deposits infill‘California‐like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits.
Grain‐size distribution of suspended load in relation to bed materials and flow velocity
Authors: SENGUPTA SUPRIYA
Published: June 2006

Grain‐size frequency distributions of suspended loads at different flow velocities and over sand beds of four different grain‐size patterns were studied in a laboratory flume. The proportion of bed material which went into suspension increased with decrease of grain‐size in each case, but the modes of the suspended loads occurred in the size classes intermediate between the coarsest and the finest. With increase of flow velocity, as also with decrease of the bed's mean grain‐size, the total amount of material in suspension markedly increased, mainly due to addition of particles to the medium size classes.

The coarsest grains in the bed resisted erosion due to their weight, whereas the finest ones were either not available in sufficient quantities or resisted erosion due to their homogeneity. The finest of the erodible grains which were abundantly available in bed were therefore, lifted up in large quantities. This size sorting took place at or near the bed surface and was closely related to the process of bed form migration.

Large accumulation of medium sized particles in suspension at high velocities led to lognormal grain‐size distributions when the nature of the bed (source) material was suitable. At lower velocities, or over other types of bed materials, the phi (log)‐probability plots of cumulative grain‐size distributions of the suspended loads resolved into a number of straight lines. Mixtures of linear segments on phi‐probability graphs therefore, need not necessarily indicate different modes of sediment transportation, as is commonly believed, but might reflect the conditions of flow and the nature of the source material.