bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluvial‐aeolian interactions: Part I, modern systems
Authors: LANGFORD R. P.
Published: June 2006

Two modern fluvial‐aeolian depositional systems (Great Sand Dunes National Monument, Colorado and the Mojave River Wash, California) are remarkably similar in spite of different climates, sizes, fluvial sediment textures, and relative directions of aeolian and fluvial transport. Dune growth and migration, and deflation of blowouts create 8–10 m of local relief in unflooded aeolian landscapes.

There are six prominent fluvial‐aeolian interactions. (1) Fluvial flow extends into the aeolian system until it is dammed by aeolian landforms; (2) interdune areas (overbank‐interdunes) upstream of aeolian dams, and alongside channels are flooded; (3) water erodes dunes alongside channels and interdunes; (4) flood waters deposit sediment in interdune areas; (5) fluvially derived groundwater floods interdunes (interdune‐playas); (6) wind erodes fluvial sediment and redeposits it in the aeolian system.

Unique and characteristic sediments are deposited in overbank‐interdunes and in interdune‐playas, reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions. These fluvial‐aeolian interdune deposits are characterized by irregular, concave‐up bases and flat upper surfaces containing mudcracks or evaporite cement.

Characteristic low‐relief surfaces form in aeolian systems as an effect of flooding. Fluvial deposits are resistant to aeolian deflation. Aeolian sand is preserved when flood sediments are deposited around the bases of the dunes. Thus repetitive fluvial and aeolian aggradation tends to be ‘stepwise’ as interdune floors are suddenly raised during floods. The effects of flooding should be easy to recognize in ancient aeolianites, even beyond the area covered with overbank muds.
Detrital modes and provenance of the Stilo‐Capo d'Orlando Formation (Miocene), southern Italy
Authors: CAVAZZA WILLIAM
Published: June 2006

The Stilo‐Capo d'Orlando Formation (SCO Fm) is a Miocene clastic unit nonconformably covering the basement of the southern portion of the Calabria‐Peloritani Arc (CPA), a small‐scale terrane which was accreted onto the Apennines‐Maghrebides orogenic system during Neogene time. The SCO Fm is composed of conglomerate, sandstone and mudrock deposited in a marine environment through the action of gravity currents.

Detrital modes of sandstones and conglomerates of the SCO Fm indicate a local provenance. In fact, petrological parameters can be matched directly with nearby basement lithologies. Two petrofacies with distinctive parameters are present. A northern petrofacies is predominantly composed of plutonic and low‐grade metamorphic detritus, and was derived from the Serre massif, which is mostly made of late Hercynian plutons intruded into metasedimentary rocks. A southern petrofacies is mostly composed of metamorphic detritus and was derived from the Aspromonte and the Peloritani Mountains, which are predominantly composed of middle‐to high‐grade metamorphic rocks.

Petrological results place important constraints on the geodynamic evolution of the CPA. According to several authors, the arc is composed of two sectors (microterranes) with different geological histories and partly composed of contrasting rock types, whose timing of juxtaposition is debated (Early Cretaceous against middle‐late Miocene; that is, much before or soon after deposition of the SCO Fin). The SCO Fm crops out only in the southern sector of the CPA and does not contain any detritus derived from the northern sector, in spite of a favourable sediment palaeodispersal system. This fact indicates that the amalgamation of the two sectors probably took place after deposition of the SCO Fm.
Near‐surface shrinkage and carbonate replacement processes, Arran Cornstone Formation, Scotland
Authors: TANDON S. K. , FRIEND PETER F.
Published: June 2006

A 47 m thick succession of conglomerates, sandstones and mudstones, of Late Devonian or Early Carboniferous age, outcrops at Fallen Rocks in northeast Arran (western Scotland). It is defined here as the type section of the Arran Cornstone Formation. At numerous levels in the succession, varieties of fissures and carbonate concretions formed during the accumulation of the Formation. The fissures opened as a result of drying‐shrinkage, and were closed again either by filling with different sediment, or by wetting and expansion of the fissure wall sediment. Carbonate concretions form complete beds, discontinuous, bedding‐concordant sheets, or bedding‐discordant nodules or rods (the rod cornstones). These concretions formed close enough to the surface to be incorporated, after erosion and redeposition, as clasts into overlying beds. The concretions were formed by growth of micrite, mainly by replacement, but shrinkage displacement played an important role in subsequently fracturing and reworking the micrite. The micrite was also locally replaced by microspar and spar, and this involved dissolution and precipitation. No independent evidence of biological influence in any of these processes has been found.
Palaeochannel reconstructions from point bar deposits: a three‐dimensional perspective
Authors: WILLIS BRIAN J.
Published: June 2006

Quantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel‐bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three‐dimensional (3‐D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down‐valley bend migration and/or increase in channel‐bend sinuosity. Two‐dimensional (2‐D) cross‐sections in varying orientations across these simulated deposits display lateral‐accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation.

Most cross‐sections show point bar deposits thickening away from the meander‐belt axis due to a lateral progression from thinner bend‐exit deposits to thicker bend‐apex deposits (caused by down‐valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down‐valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross‐section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3‐D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedsets of laterally accreted sediment.
Petrography and provenance of silicified early Archaean volcaniclastic sandstones, eastern Pilbara Block, Western Australia
Authors: DiMARCO MICHAEL J. , LOWE DONALD R.
Published: June 2006

Intense post‐depositional alteration has profoundly affected sandstones in the volcanic portions of Early Archaean (3·5–3·3 Ga) greenstone belts. The mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism, but grain textures are commonly well preserved. Consequently, microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes.

Silicified Early Archaean volcaniclastic sandstones assigned to the Panorama Formation and Duffer Formation, Warrawoona Group, eastern Pilbara Block, Western Australia, were originally composed of volcanic (VRF) and sedimentary (SRF) rock fragments, volcanic quartz, feldspar, traces of ferromagnesian minerals and pumice. Only volcanic megaquartz remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) and sericite. In most areas, the sandstones were composed of dacitic to rhyolitic VRFs, now totally replaced by sericite‐poor GMC and recognized by preserved microporphyritic textures. In a few areas, quartz‐poor dacitic to andesitic(?) VRFs dominated the detrital assemblage. Minor SRFs and mafic VRFs, now replaced by GMC, are recognized on the basis of colour, internal structures, and internal textures, including skeletal, possible spinifex textures. Detrital feldspar is represented by blocky, sericite‐rich grain pseudomorphs.

A semi‐quantitative point‐count scheme, developed for the analysis of heavily altered sandstones, indicates the following primary detrital‐mode ranges for Panorama arenites: quartz, 0–28%; feldspar, 0–28%, VRFs, 58–86%, and SRFs 0–25%. In about half the point‐counted samples, feldspar could not be distinguished from rock fragments. In such cases, both were counted as one grain type, Lv', which makes up from 84 to 100% of the framework modes of these rocks.

These sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments, but locally including minor mafic, ultramafic, and sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from plutonic or metamorphic sources. The Panorama modal assemblage represents a provenance that is lithologically more restricted than that of Archaean greywackes and other siliciclastic units common in the sedimentary portions of these same Early Archaean greenstone belts and younger greenstone belts (3·0–2·7 Ga).
Analcime in lake and lake‐margin sediments of the Carboniferous Rocky Brook Formation, Western Newfoundland, Canada
Authors: GALL Q. , HYDE R.
Published: June 2006

In the Deer Lake Basin of western Newfoundland, Canada, analcime has been found within fine‐grained, siliciclastic and carbonate, lake and lake‐margin sediments of the Carboniferous Rocky Brook Formation. Analcime is the only zeolite observed in these unmetamorphosed and non‐volcanogenic rocks. Microscopic analcime occurs as vug‐filling, limpid, isotropic to anisotropic crystals. Microprobe analyses show this type of analcime to be lower in silica than most analcime in sedimentary rocks. A second textural type of finer‐grained submicroscopic analcime can be detected from X‐ray diffraction spectra of Rocky Brook Formation mudstones, where it is associated with phyllosilicates (illite, interstratified chlorite/smectite, smectite, chlorite), other silicates (quartz, feldspar) and carbonates (dolomite, calcite). Results of this study suggest that the analcime in the Rocky Brook Formation formed either by direct lakewater/porewater precipitation or by reaction of these waters with one or more clay mineral types or plagioclase.
Nodular silcretes of the Cypress Hills Formation (upper Eocene to middle Miocene) of southern Saskatchewan, Canada
Authors: LECKIE D. A. , CHEEL R. J.
Published: June 2006

Siliceous nodules in the upper Eocene to middle Miocene Cypress Hills Formation in southern Saskatchewan formed on the distal portions of an extensive braidplain. The nodules are similar to silcretes described elsewhere in the literature but their morphology and occurrence suggests that they are of a rare variety. The silcretes are discrete equant to disc‐shaped nodules, 1–3 cm thick, and up to 15 cm long. The silicretes form horizontal, discontinous layers parallel to bedding within an unweathered profile. Void spaces and fractures within the nodules are lined or filled with drusy quartz. The silcretes contain less than 0.07% TiO2. The TiO2/SiO2/Al2O3 and TiO2/SiO2/Fe2O3 ratios are similar to values obtained from nodules formed in an arid to semi‐arid environment based on comparison with modern silcretes. This interpretation is confirmed by independent sedimentological and palaeontological climatic evidence for an arid to semi‐arid climate throughout Oligocene time in the western Canadian plains.
Lower Cambrian shelf and shelf margin buildups, Flinders Ranges, South Australia1
Authors: JAMES NOEL P. , GRAVESTOCK DAVID I.
Published: June 2006

Carbonate buildups in the Flinders Ranges of mid‐Early Cambrian age grew during a period of high archaeocyath diversity and are of two types: (1) low‐energy, archaeocyath‐sponge‐spicule mud mounds, and (2) high‐energy, archaeocyath‐calcimicrobe (calcified microbial microfossil) bioherms. Mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone, have a fenestral fabric, display distinct stromatactis, contain abundant sponge spicules and form structures up to 150m wide and 80 m thick. Bioherms are either red or dark grey limestone and occur as isolated small structures 2–20 m in size surrounded by cross‐bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across. Red bioherms comprise masses of white Epiphyton with scattered archaeocyaths and intervening areas of archaeocyath‐rich lime mudstone. Grey bioherms are complex intergrowths of archaeocyaths, encrusting dark grey Renalcis and thick rinds of fibrous calcite cement. The bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities, up to 1.5 m across, lined with fibrous calcite.

The buildups are isolated or in contiguous vertical succession. Mud mounds occur alone in low‐energy, frequently nodular, limestone facies. Individual bioherms and bioherm complexes occur in high‐energy on‐shelf and shelf‐margin facies. The two types also form large‐scale, shallowing‐upward sequences composed of basal (deep water) mud mounds grading upward into archaeocyath‐calcimicrobe bioherm complexes and bioherms in cross‐bedded carbonate sands. The uppermost sequence is capped by ooid grainstone and/ or fenestral to stromatolitic mudstone.

The calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout much of subsequent Phanerozoic time.
Large‐scale current‐induced erosion and deposition in the path of the 1929 Grand Banks turbidity current
Authors: CLARKE JOHN E. HUGHES , SHOR ALEXANDER N. , PIPER DAVID J. W. , MAYER LARRY A.
Published: June 2006

New observations concerning the degree of current‐induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley‐floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments.

The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current‐induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turbidite.
Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows
Authors: RUBIN DAVID M. , IKEDA HIROSHI
Published: June 2006

For more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6‐m‐diameter sand‐covered turntable on the floor of a 4‐m‐wide flume.

In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time‐averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s‐1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°.

One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction.

These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform‐normal transport (the orientation at which the sum of the bedform‐normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.
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

Dolomitization 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

The 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

Well 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

The 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

Recent 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

More 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

The 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

Gypsum 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

The 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

The 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

Preferred 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

The 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

One 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.
Calciturbidite composition related to exposure and flooding of a carbonate platform (Triassic, Eastern Alps)
Authors: REIJMER J. J. G. , TEN KATE W. G. H. Z. , SPRENGER A. , SCHLAGER W.
Published: June 2006

The compositional variation of calciturbidites (Pedata/Pötschen Formation), deposited adjacent to the Dachstein Formation carbonate platform in the Triassic Hallstatt Basin, was analysed using detailed field measurements and point‐counting of thin sections. The 35 m long section is located in the Northern Calcareous Alps of Austria. Six point‐count groups were distinguished separating basinal from platform‐derived input. Summary statistics, cluster and correspondence analysis of the point‐count data reveals a close relationship between the biota present on the Dachstein carbonate platform and the calciturbidite composition. The variations in turbidite composition are attributed to fluctuations in sea level and resulting flooding and exposure of the platform, which alternately created and destroyed shallow‐water habitats on the platform top.
Stratal geometries, facies and sea‐floor erosion in Upper Cretaceous Chalk, Normandy, France
Authors: QUINE MARK , BOSENCE DAN
Published: June 2006

Unusual lenticular stratal geometries and facies of the Upper Cretaceous Chalk of coastal Haute Normandie, France, are described and interpreted. Thirteen facies within these chalks are described and illustrated on the basis of field, thin‐section and SEM investigations: nannofossil mudstones, nannofossil hardgrounds, echinoderm wackestones and packstones, echinoderm hardgrounds, bryozoan mudstones and wackestones, bryozoan hardgrounds, bryozoan packstones and wackestones, inoceramid wackestones, inoceramid hardgrounds, sponge hardgrounds, marly chalks, conglomeratic chalks and debris flow chalks.

These facies occur within lenticular bedded structures with both concave‐up and concave‐down geometries which have been previously interpreted as megaripples, mud mounds or tectonic structures. Detailed examination of the structures and the associated facies indicates that the concave‐up geometries were formed from submarine erosion, and redeposition in NW‐SE longitudinal channels. The concave‐down geometries developed between adjacent channels. Assessment of the regional and temporal setting indicates that the erosion occurred in the Armorican‐Cornubian straits of the Anglo‐Paris Basin during sea‐level lowstands. Within these straits channelling is preferentially developed on the positive, south‐western block to the Lillebonne‐Fécamp‐Cotentin Fault.
Geochemistry of meteoric calcite cements in some Pleistocene limestones
Authors: SALLER ARTHUR H. , MOORE CLYDE H.
Published: June 2006

In this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ18O values and a broad range of δ13C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ18O composition of −6.5%0 (PDB) and δ13C values ranging from −9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ18O composition of −4.1%0 and δ13C values ranging from −6.3 to + 1.1%0. Sparry cements from Yucatan have a mean δ18O composition of −5.7%0 and δ13C values of −8.0 to −2.7%0. The mean δ18O values of these Pleistocene meteoric calcite cements vary by 2.4%0 due to climatic variations not related directly to latitude. The δ13C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ13C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ13C compositions of less than —4.0%0, which suggests that soil‐derived CO2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements.

Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO3; σ= 0.60 mol% MgCO3) and Cat Island (mean = 0.84 mol% MgCO3; σ= 0.52mol% MgCO3) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO3: σ= 0.84mol% MgCO3). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.