bluuebunny/crossref_metadata_2025_split huggingface dataset:
Genesis of evaporite‐associated platform dolomites: case study of the Main Dolomite (Zechstein, Upper Permian), Leba elevation, northern Poland
Authors: PERYT TADEUSZ MAREK , MAGARITZ MORDECKAI 
Published: June 2006

ABSTRACTDolomitization of the Zechstein (Late Permian) Main Dolomite carbonates of northern Poland was penecontemporaneous and/or very early diagenetic. Well‐ordered, stoichiometric dolomites are associated with the basinal facies. The platform dolomites are relatively poorly ordered and usually non‐stoichiometric. Most samples are highly enriched in 13C, as in other Zechstein carbonates. δ18O values show large variations from ‐5·1%0 to + 7·4%. There is an isotope zonation of the examined dolomites. The isotope signature indicates that dolomites formed from variable solutions of meteoric water, seawater, and evaporitic brines of possible marine or continental origin. Once initiated, dolomitization proceeded despite the evolution of dolomitizing brines. This evolution explains the occurrence of lagoonal dolomites with common evidence for dissolution in the lower part of sections compared with well‐developed rhombohedra in the upper part. Crystal zoning suggests the initiation of dolomite growth in hypersaline water and progressive dilution by fresh water. There is isotopic evidence for migration of continental waters into the basin, presumably following sea‐level fall at the end of the deposition of the Main Dolomite. Influence of fresh water on syndepositional dolomitization, well established in the Main Dolomite, strongly suggests that similar relationships may be characteristic for other evaporite‐associated dolomites as well.
Origin of fine‐grained carbonate and siliciclastic sediments in an Early Palaeozoic slope sequence, Cow Head Group, western Newfoundland
Authors: CONIGLIO M. , JAMES N. P. 
Published: June 2006

ABSTRACTThe Cow Head Group is an Early Palaeozoic base‐of‐slope sediment apron composed of carbonate and shale. Whereas coarse‐grained conglomerate and calcarenite are readily interpreted as debris‐flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre‐ to centimetre‐thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate‐shale sequences (GCSS) deposited from waning carbonate turbidites and fall‐out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin.The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments.These attributes characterize not only fine‐grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.
Glacial bedforms at the base of the Permo‐Carboniferous Dwyka Formation along the western margin of the Karoo Basin, South Africa
Authors: VISSER JOHANN. J. 
Published: June 2006

ABSTRACTWell to poorly preserved sandstone surfaces with glacial grooves, longitudinal ridges, bulbous bedforms and large lodged clasts occur sporadically at the base of the Dwyka Formation along the western margin of the Karoo Basin. The bedforms developed when ice overrode a thin (0·1–2·0m thick) subaqueous icemarginal apron formed primarily during periods of ice front retreat. Bergstone mud and rain‐out diamicton blanketed the glacial bedforms.The subglacial bedforms formed by (i) the lateral movement of water‐saturated sediment into low‐pressure zones, caused by crevasses and cavities at the base of the ice; (ii) the presence of areas of higher strength substrate, due to variations in bed lithology and porewater dissipation; and (iii) sediment flowage into low‐pressure zones on the leeside of obstacles formed in areas of higher strength substrate due to dissipation of pore‐water pressures and sediment compaction. The preservation of the bedforms, with their delicate slump fans, is attributed to separation of the glacier sole from the substrate during a sudden rise in sea‐level. A series of dynamic ice‐marginal events, including feedback relationships between sea‐level oscillations, isostatic responses, ice‐margin fluctuations, ice‐margin type and the type of substrate, controlled the deposition of the basal sedimentary sequence and the formation of the associated glacial bedforms. The presence of a complex combination of glacier‐related formative and depositional processes may have consequences for past interpretations of basal ‘tillites’.
Late Proterozoic aragonitic cement crusts, Bambuí Group, Minas Gerais, Brazil
Authors: PERYT T. M. , HOPPE A. , BECHSTÄDT T. , KÖSTER J. , PIERRE C. , RICHTER D. K. 
Published: June 2006

ABSTRACTThe Late Proterozoic Pedro Leopoldo facies (Bambuí Group) in the vicinity of Belo Horizonte, Brazil, comprises alternating laminated microsparitic limestones (10–35 mm thick beds) and fibrous limestones (10–55 mm thick). The latter are composed of a mosaic of sparry calcite crystals. These irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared‐off growth zones. Ghosts of an original fibrous mineral, hexagonal in cross‐section, are visible. The petrographic characteristics, very high strontium content and low magnesium content of the fibrous beds, as well as microspar beds, strongly argue for an original aragonitic mineralogy. The rays are interpreted as having formed by precipitation at the sediment‐water interface, whereas the micrite was precipitated from the water column prior to deposition on the sea floor. The lack of emergence features suggests widespread aragonite precipitation under persistently subtidal conditions.
Size segregation during aeolian saltation on sand dunes
Authors: SARRE R. D. , CHANCEY C. C. 
Published: June 2006

ABSTRACTRecent Monte Carlo computer simulations have modelled the local, geometric mechanism by which mixed populations of differently sized particles order themselves when shaken, such that larger particles rise at the expense of the smaller. This process, called size segregation, is distinct from the mechanism of sifting where smaller particles fall between the interstices of larger particles. Size segregation is applied here to the sand grain populations found on the surfaces of dunes, which are involved in a sorting process driven by the wind. We present data from four barchan dunes located in the Eastern Province of Saudi Arabia. A simple model is used to demonstrate that size segregation, driven by saltation impacts, is energetically plausible at the wind velocities typically encountered on dune surfaces. Laboratory tests which support this model are also discussed.
Carbonate platform flanks: slope angle and sediment fabric
Authors: KENTER JEROEN A. M. 
Published: June 2006

ABSTRACTMore than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric. Plots of slope angle versus sediment fabric show that grainy, non‐cohesive, mud‐free sediments build steeper slopes than muddy, cohesive, sediments. Examples near the end‐members of grainy and muddy carbonate platform flanks are found in the Triassic of the Dolomites in northern Italy and in the Bahamas, respectively. They document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded. The grainy flank sediments in the Dolomites, modified by shearing and avalanching, produce straight slope profiles with declivities up to 35°, whereas the muddy Bahamian flank sediments, modified by large‐scale creep and rotational to translational sliding and slumping, produce a concave‐upwards slope profile, inclined at less than 4°. The comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field, angle of shearing and mode of readjustment processes, is linked to the composition of the slope sediment. Among the variables such as sea‐level, subsidence, climate, plate motion and oceanographic setting (windward‐leeward), sediment fabric is suggested to be a major, if not the major control on slope angle and slope curvature of carbonate platform flanks. Besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics, this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems. Furthermore, it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling.
Petrography and geochemistry of the Belle Roche breccia (lower Visean, Belgium): evidence for brecciation by evaporite dissolution
Authors: SWENNEN R. , VIAENE W. , CORNELISSEN C. 
Published: June 2006

ABSTRACTThe lower Visean Belle Roche breccia (east Belgium) displays a number of features that indicate brecciation by evaporite dissolution collapse: the sharp lower contact of the breccia, the gradual transition into the overlying strata, the presence of semi‐continuous beds within the breccia giving it a crude ‘stratification’, and the existence of several types of (calcite, dolomite and silica) evaporite pseudomorphs. Furthermore, the majority of the breccia fragments indicates hypersaline to lagoonal sedimentation conditions. Most of these fragments display an interlocking fabric. The interpretation is also supported by the existence of continuous evaporite beds replaced by carbonates (calcite and dolomite) both under and overlying the breccia.The brecciation history is characterized by gradual subsidence. Multiple brecciation episodes are recognized, and are best seen in the lower breccia which underwent at least two major brecciation episodes. Here, brecciated and veined breccia fragments occur within a microsparite (neomorphosed mud) matrix. Brecciation of these strata was due to the dissolution of interlayered evaporites. The second brecciation event relates to infiltration of meteoric water and to the dissolution of the remaining evaporites. This infiltration was probably triggered by the orogenic event at the end of the Visean (Sudetic orogenic phase). The whole breccia was finally cemented by a blocky calcite.The different lithologies and cements were characterized by their trace element (Mg, Sr, Na, Fe, Mn, K), insoluble residue and organic matter content. Carbon/oxygen isotope data of the cements and replaced evaporite layers helped to place the multiple collapse episodes within a general diagenetic model. Solution‐reprecipitation processes within the original aragonite‐dominated mud fragments, as well as in the early diagenetic dolomite fragments, have been recognized. The geochemical data show that these transformation processes occurred in equilibrium with the same fluid. These processes may have occurred within a freshwater lens very early in the diagenetic evolution or under shallow burial conditions. Cementation of blocky calcite occurred in a meteoric realm under burial conditions.
The origin and significance of groundwater‐seepage gypsum from Bristol Dry Lake, California, USA
Authors: ROSEN MICHAEL R. , WARREN JOHN K. 
Published: June 2006

ABSTRACTGypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom‐nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater‐seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow.The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ18O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (˜– 6%0), whereas measured brine δ18O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ˜– 70%0, GWC are ˜– 60 to – 65%0 and brine values are ˜– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely.Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained.
Facies and sand‐body geometry of the Queen City (Eocene) tide‐dominated delta‐margin embayment, NW Gulf of Mexico Basin
Authors: RAMOS AMPARO , GALLOWAY WILLIAM E. 
Published: June 2006

ABSTRACTThe Queen City Formation (Eocene) displays an array of tide‐dominated coastal facies in the Tyler Basin of the northern Gulf of Mexico. This facies assemblage, which is atypical of the microtidal, wave‐dominated, coastal depositional complexes that characterize the Cenozoic Gulf basin, reflects tidal amplification in a generally protected embayment on the east flank of a strongly prograded delta system. The shallow embayment was confined to the east by contemporaneous uplift and shoaling across the Sabine Uplift. Fluvial, barrier (including ebb tidal delta), heterolithic tidal, estuary‐fill, and tidal point‐bar facies are all found at outcrop. These facies were projected into the three‐dimensional geometry of the tide‐dominated depositional complex. Inlet, estuary, and distributary‐fill sand bodies, which are linear and diporientated, dominate lithofacies maps.The Queen City facies assemblage in the Tyler Basin records a mixture of mesotidal to macrotidal environments that were interspersed in time and space with fluvial‐dominated lobes, which periodically prograded eastward from the deltaic depocentre into the flanking embayment. Queen City deposition terminated with regional marine flooding and deposition of glauconitic, fossiliferous shelf sands and muds of the Weches Formation. Transgression is marked by a prominent ravinement surface that truncates underlying facies of the tide‐dominated shore zone.
Fluvial, inertia‐dominated deltaic deposition in the Namurian (Carboniferous) of northern England
Authors: MARTINSEN OLE J. 
Published: June 2006

ABSTRACTThe Namurian (Upper Carboniferous) Scar House Beds of Yorkshire, northern England, are an example of a fluvial‐dominated deltaic sequence that cannot be adequately described using existing classification schemes for deltas. For substantial periods of the Scar House delta history, inertial processes and hyperpycnal mixing prevailed in the river mouth area due to repeated, frequent flooding in the distributary system. This generated voluminous density currents which deposited their sandy loads in successively stacked lobes beyond the river mouth bar in the prodelta area. The position of a lobe was directly controlled by the position of an active river mouth. Only during periods of low discharge in the distributary system did homo‐ and hypopycnal mixing take place. In these periods, frictional and buoyant forces operated, and sand was deposited from tractional sheet flow on the mouth bar while mudstone was laid down in the otherwise density‐current‐dominated prodelta.Because of the dominantly hyperpycnal mixing mode, the river effluent experienced a low lateral spread causing an elongate delta lobe to form that in geometry can be compared with some recent and ancient ‘bar finger’ sands. Important differences exist in terms of dominant depositional processes however. Most other ‘bar finger’ sands were controlled by a hypopycnal mixing mode and buoyant forces (e.g. South Pass, Mississippi), while the Scar House delta was controlled by hyperpycnal mixing and inertial forces.This study shows that similar sand‐body geometries can be generated from different river mouth processes. In the future, particularly in the field of hydrocarbon exploration, there may be a need to classify deltas both in terms of geometry and dominant river mouth processes. In that respect, the Scar House Beds represent a fluvial, inertia‐dominated elongate delta.
Facies descriptions and associations in ancient reworked (?transgressive) shelf sandstones: Cambrian and Cretaceous examples
Authors: HEIN FRANCES J. , ROBB GREGORY A. , WOLBERG ANDREW C. , LONGSTAFFE FREDERICK J. 
Published: June 2006

ABSTRACTPreferred facies trends in ancient shallow‐marine sediments have been determined by Markov‐chain and substitutability analyses of detailed sections measured in Lower Cambrian outcrops in the Southern Canadian Rocky Mountains (St Piran Formation, 600 m of vertical section; Fort Mountain Formation, 500 m of vertical section), and in a Cretaceous subsurface reservoir in south‐central Alberta (Viking Formation in the Garrington Field: 69 cores).Those sections with more defined cycles are interpreted to result from progradation of shelf‐ridge sands and gravels; examples with less defined order originated as inter‐ridge deposits, in zones with complex palaeoflows and in areas of subdued palaeotopographic relief. The highest degree of facies organization (> 80% significant facies transitions) occurs in the Cretaceous Viking Formation sandstone and conglomerate ridges, interpreted to have had abundant sediment supply, water depths between fair‐ and storm‐weather wave‐base, and a strong littoral current system. A moderate degree of facies organization (70% significant facies transitions) occurs in the inter‐ridge deposits of the Cambrian Fort Mountain Formation, interpreted to have had a variable sediment supply, water depths above storm‐weather wavebase, and a variable, oscillating tidal current system. A poor degree of facies organization (< 45% significant facies transitions) occur in the ridge deposits of the Cambrian Fort Mountain and St Piran formations and in the inter‐ridge deposits of the Cambrian St Piran and Cretaceous Viking formations. This facies disorganization occurs in sites where there was continuous and rapid subsidence (Cambrian) or in areas of subdued seafloor relief with complex current systems (Cretaceous). The pattern of facies relationships also varies. Relatively short linear patterns occur in all of the shelf‐ridge deposits. This contrasts with the inter‐ridge deposits in which there are long linear patterns or complex ring structures, interpreted to result from complex palaeoflow patterns in areas of subdued seafloor topography. Various statistical techniques (including Markov‐chain and substitutability analyses) can aid the interpretation of very complex lithological successions, as shown by the shelf sandstones and conglomerates examined in this study.
The sedimentary dynamics of active fluvial channels on the Okavango fan, Botswana
Authors: McCARTHY T. S. , STANISTREET I. G. , CAIRNCROSS B. 
Published: June 2006

ABSTRACTThe 1800‐km2 Okavango alluvial fan of northern Botswana represents an unusual depositional setting in which peat‐forming perennial swamps (6000 km2) occur in a region of aeolian and semi‐arid sedimentation within an incipient graben of the East African Rift. A channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water, which spreads laterally from the channels through permeable channel margins, sustaining the flanking swamps. All sediment introduced is deposited on the fan. A detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal. Bedload greatly exceeds suspended load (at least by a factor of four). Vegetation and peat form permeable levees which confine the channels. In the upper reaches, two‐way exchange of water occurs between channel and swamp depending on the season, but on the fan itself, channels lose water to the swamp. Bedload measurements reveal that the channel system is in a state of grade disequilibrium, with interspersed depositional and erosional reaches. Deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system, but on the midfan, deposition of bedload occurs by channel‐bed aggradation, at a rate of up to 5 cm yr–1. Further down slope, the channel enters a large lake where all remaining bedload is deposited. The presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system, either by avulsion or neotectonics.
Lateral migration of sand‐bed rivers: examples from the Devonian Glashabeg Formation, SW Ireland and the Cambrian Alderney Sandstone Formation, Channel Islands
Authors: TODD SIMON P. , WENT DAVID J. 
Published: June 2006

ABSTRACTOne mechanism of producing fluvial sheetsand bodies is by the amalgamation of individual channel belts by lateral migration. The lateral migration of sand‐bed rivers by periodic avulsion may be assisted by more steady channel combing. Combing occurs by the preferential deposition on accretionary bars on one bank of the river, concomitant with erosion on the other side. We present descriptions and interpretations of the deposits of two different, moderately large, sand‐bed rivers that produced extensive sandbodies at least in part by steady lateral migration or combing, which is evidenced by the geometry and directional properties of the preserved bedform and barform structure. Significantly, in both cases lateral migration occurred, or was preserved, in one direction only. Two basic types of model are possible to explain these phenomena. A consistent migration direction in many stacked fluvial cycles may be the product of either (1) the preferential preservation of the deposits of one migration direction in a steady‐state system, or (2) perpetual combing of the formative river in one direction during repeated aggradation cycles. Type (1) is the result of position on the floodplain, possibly with a tectonic control; type (2) requires a tectonic control on the palaeomorphology of the floodplain. We prefer a model for both our examples in which tectonic tilting of the floodplain causes combing of the river towards the zone of greatest subsidence. A rise in base level occurs with tilting, and therefore the rivers are also thought to have aggraded during lateral migration thus causing a preservational bias on the directional properties of the resulting fluvial succession.