Climbing-ripple Cross-lamination Research Articles

Frond orientations with independent current indicators demonstrate the reclining rheotropic mode of life of several Ediacaran rangeomorph taxa

AbstractFossils from the deep-sea Ediacaran biotas of Newfoundland are among the oldest architecturally complex soft-bodied macroorganisms on Earth. Most organisms in the Mistaken Point–type biotas of Avalonia—particularly the fractal-branching frondose Rangeomorpha— have been traditionally interpreted as living erect within the water column during life. However, due to the scarcity of documented physical sedimentological proxies associated with fossiliferous beds, Ediacaran paleocurrents have been inferred in some instances from the preferential orientation of fronds. This calls into question the relationship between frond orientation and paleocurrents. In this study, we present an integrated approach from a newly described fossiliferous surface (the “Melrose Surface” in the Fermeuse Formation at Melrose, on the southern portion of the Catalina Dome in the Discovery UNESCO Global Geopark) combining: (1) physical sedimentological evidence for paleocurrent direction in the form of climbing ripple cross-lamination and (2) a series of statistical analyses based on modified polythetic and monothetic clustering techniques reflecting the circular nature of the recorded orientation of Fractofusus misrai specimens. This study demonstrates the reclining rheotropic mode of life of the Ediacaran rangeomorph taxon Fractofusus misrai and presents preliminary inferences suggesting a similar mode of life for Bradgatia sp. and Pectinifrons abyssalis based on qualitative evidence. These results advocate for the consideration of an alternative conceptual hypothesis for position of life of Ediacaran organisms in which they are interpreted as having lived reclined on the seafloor, in the position that they are preserved.

The interpretative significance of ripple-derived sedimentary structures within an upper Neogene fluvial succession of central Poland

Abstract Sedimentary structures discussed in the present study are genetically linked to ripples that consist of pure sand or alternating sand and mud layers. All types of ripple-related structures, such as climbing-ripple cross-lamination and heterolithic bedding, i.e., flaser, wavy and lenticular (nodular), have been identified for the first time in fluvial strata that have been characterised previously as commonly massive. These small-scale bedforms, produced by migrating ripples, have been documented in a fluvial channel of late Neogene age in central Poland. The abundance and co-occurrence of the structures discussed and their spatial distribution provide evidence of their formation under very low-energy conditions, when flow velocity changed markedly, but was often significantly less than 0.5 m/s. Therefore, these ripple-derived sedimentary structures are here recognised as typical of channel fills of an anastomosing river.

Carbonate-clastic sedimentation in the Parnaiba Basin, northern Brazil: Record of carboniferous epeiric sea in the Western Gondwana

The transgressive-regressive events due to changes in the eustatic sea level represent the occurrence of an epeiric sea in Western Gondwana. This event is recorded in the carbonate-siliciclastic succession exposed in the central portion of the Parnaiba Basin, Northern Brazil, mainly represented by densely fossiliferous deposits, with mollusks, brachiopods, corals and microfossils, related to the upper member of the Piauí Formation. Outcrop-based facies/microfacies and stratigraphic analysis of this succession allowed the individualization of two facies associations (FA), representative of a shallow carbonate platform system adjacent to a coastal dune field. The FA1– dune field/interdune, comprises well sorted, intensely biortubated, fine to medium-grained sandstone with even parallel and tabular cross stratification, as well as translatent climbing ripple cross-lamination. FA2-shallow sea deposits consist of fossiliferous pelletal carbonate and microspar carbonate layers, laterally continuous for hundreds of meters, interbedded with centimetric beds and lenses of organic shale with small pyrite crystals. Covariant values of positive δ18Ocarb and negative δ13Ccarb for the carbonate profiles, together with intense substitution of primary constituents by dolomite and an abundance of associated organic matter contributed towards determining an organogenic dolomitization model. Although the Pennsylvanian records a long icehouse event, glacioeustatic fluctuations were common during the Moscovian and Kasimovian. The icehouse periods contributed to the semi-arid to desert conditions that shaped the aeolic environments of the Piauí Formation. Later greenhouse events favored a rise in the sea level and development of an extensive epicontinental Pennsylvanian sea that extended throughout Western Gondwana called the Itaituba-Piauí sea in this paper, which connected the Andean, Solimões, Amazonas and Parnaíba basins.

Facies architecture of the fluvial Missão Velha Formation (Late Jurassic–Early Cretaceous), Araripe Basin, Northeast Brazil: paleogeographic and tectonic implications

Abstract Sedimentological analysis of the Missão Velha Formation (Araripe Basin, northeast Brazil) is the aim of this paper through detailed facies analysis, architectural elements, depositional systems and paleocurrent data. The main facies recognized were: (i) coarse-grained conglomeratic sandstones, locally pebbly conglomerates, with abundant silicified fossil trunks and several large-to-medium trough cross-stratifications and predominantly lenticular geometry; (ii) lenticular coarse-to-medium sandstones with some granules, abundant silicified fossil wood, and large-to-medium trough cross-stratifications, cut-and fill features and mud drapes on the foresets of cross-strata, (iii) poorly sorted medium-grained sandstones with sparse pebbles and with horizontal stratification, (iv) fine to very fine silty sandstones, laminated, interlayered with (v) decimetric muddy layers with horizontal lamination and climbing-ripple cross-lamination. Nine architectural elements were recognized: CH: Channels, GB: Gravel bars and bed forms, SB: Sand bars and bedforms, SB (p): sand bedform with planar cross-stratification, OF: Overbank flow, DA: Downstream-accretion macroforms, LS: Laminated sandsheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. The lithofacies types and facies associations were interpreted as having been generated by alluvial systems characterized by (i) high energy perennial braided river systems and (ii) ephemeral river systems. Aeolian sand dunes and sand sheets generated by the reworking of braided alluvial deposits can also occur. The paleocurrent measurements show a main dispersion pattern to S, SE and SW, and another to NE/E. These features imply a paleodrainage flowing into the basins of the Recôncavo-Tucano-Jatobá.

Crevasse splay processes and deposits in an ancient distributive fluvial system: The lower Beaufort Group, South Africa

Up to 12% of the mud-prone, ephemeral distributive fluvial system stratigraphy in the Permo-Triassic lower Beaufort Group, South Africa, comprises tabular fine-grained sandstone to coarse-grained siltstone bodies, which are interpreted as proximal to distal crevasse splay deposits. Crevasse splay sandstones predominantly exhibit ripple to climbing ripple cross-lamination, with some structureless and planar laminated beds. A hierarchical architectural scheme is adopted, in which ~1m thick crevasse splay elements extend for tens to several hundreds of meters laterally, and stack with other splay elements to form crevasse splay sets up to 4m thick and several kilometers in width and length. Paleosols and nodular horizons developed during periods, or in areas, of reduced overbank flooding are used to subdivide the stratigraphy, separating crevasse splay sets. Deposits from crevasse splays differ from frontal splays as their proximal deposits are much thinner and narrower, with paleocurrents oblique to the main paleochannel. In order for crevasse splay sets to develop, the parent channel belt and the location where crevasse splays form must stay relatively fixed during a period of multiple flood events. Beaufort Group splays have similar geometries to those of contemporary perennial rivers but exhibit more lateral variability in facies, which is interpreted to be the result of more extreme fluctuations in discharge regime. Sharp-based crevasse splay packages are associated with channel avulsion, but most are characterized by a gradual coarsening upward, interpreted to represent progradation. The dominance of progradational splays beneath channel belt deposits may be more characteristic of progradational stratigraphy in a distributive fluvial system rather than dominated by avulsion processes in a trunk river system. This stratigraphic motif may therefore be an additional criterion for recognition of distributive fluvial systems in the ancient record.

Shallow lacustrine system of the Permian Pedra de Fogo Formation, Western Gondwana, Parnaíba Basin, Brazil

The Permian Period of the Parnaíba Basin, northern Brazil, represented here by deposits from the Pedra de Fogo Formation, records important events that occurred in Western Gondwana near its boundary with the Mesozoic Era. The analysis of outcrop based facies from the Permian Pedra de Fogo Formation, which is 100 m thick, carried out along the eastern and western borders of the Parnaiba Basin, allowed the identification of eleven sedimentary facies, which were grouped into three distinct facies associations (FA), representative of a shallow lacustrine system associated with mudflats and ephemeral rivers. Bioturbation, desiccation cracks, silcretes and various siliceous concretions characterize the Pedra de Fogo deposits. The FA1 mudflat deposits occur predominantly at the base of the Pedra de Fogo Formation and consist of laminated claystone/mudstone, mudcrack-bearing sandstones/mudstones and sandstones exhibiting cross-lamination, massive and megaripple bedding. Popcorn-like silicified nodules and casts indicate evaporite deposits. Other common features are silica concretions, silicified tepees and silcretes. FA2 represents nearshore deposits and consists of fine-grained sandstones with evenly parallel lamination, climbing ripple cross-lamination, massive and megaripple bedding and mudstone/siltstone showing evenly parallel lamination. FA3 refers to wadi/inundite deposits, generally organized as fining-upward cycles of metric size, composed of conglomerates and medium-grained pebbly sandstones showing massive bedding and cross-stratification, as well as claystone/siltstone showing evenly parallel to undulate lamination. Scour-and-fill features are isolated in predominantly tabular deposits composed of mudstones interbedded with fine to medium-grained sandstones showing planar to slightly undulate lamination. Silicified plant remains previously classified as belonging to the Psaronius genus found in the uppermost levels of the Pedra de Fogo Formation, near the contact with the Motuca Formation, are considered here as excellent biostratigraphic markers. Fish remains, ostracods, bryozoans and scolecodonts represent other fossils that are present in the succession. Mudflat deposits developed in an arid and hot climate probably in the Early Permian. Semi-arid conditions prevailed in the Middle Permian allowing the proliferation of fauna and flora in adjacent humid regions and onto the lake margin. The climate variation was responsible for the contraction and expansion phases of the lake, fed by sporadic sheet floods carrying plant remains. The reestablishment of the arid climate, at the end of Permian, marks the final sedimentation of the Pedra de Fogo Formation, linked to the consolidation of the Pangaea Supercontinent. This last event was concomitant with the deposition of the Motuca Formation red beds and the development of extensive ergs related to the Triassic Sambaíba Formation in Western Gondwana.

Outcrop characterization of a submarine channel-lobe complex: The Lower Mount Messenger Formation, Taranaki Basin, New Zealand

The Upper Miocene Lower Mount Messenger Formation (LMMF) exposed in cliffs along the west coast of the North Island, New Zealand, includes 650 m of thick-bedded, fine-to very fine-grained sandstone, interbedded sandstone and mudstone, mudstone, and mass transport deposits. Deposition was from low- and high-density turbidity currents and small to very large, intraformational slumps, slides, and debris flows. The LMMF comprises five major facies associations (FA) representing the principal depositional settings within which the sediments accumulated: (FA1) distal to medial frontal lobes and lobe complexes, (FA2) more proximal frontal lobes and crevasse and/or overbank splays; (FA3) submarine channels; (FA4) levees; and (FA5) mass transport deposits (MTDs). The lower, northern part of the LMMF is dominated by FA1 and FA2 deposited as part of a large, amalgamated frontal lobe system. This section is overlain and, near the top, interbedded with FA3 levee deposits. The top of the LMMF includes a channel complex that interfingers with the levees and related lateral splay deposits. The submarine channel belt was nearly 3 km wide and delivered the sediments to the lobe complex more than 15 km downslope.The channels are filled largely with the deposits of low-density turbidity currents characterized by plane lamination and climbing-ripple cross-lamination. In contrast, lobes contain thick-bedded massive and cross-stratified sandstones deposited by high-density flows. This contrast of apparently higher energy, high-density turbidite divisions in the lobes and lower energy, low-density deposits in the upslope channel fill is thought to reflect diachronous deposition of these otherwise contiguous parts of the deposystem. When sand deposition was occurring on the lobe complex, large, high-density flows moved through and largely bypassed or actively eroded the upslope channels. As flow volumes and strength declined, deposition on the lobe complex waned and sand deposition from low-density currents occurred within the channels, with little sediment other than mud reaching the lobe complex.The overall northward paleoflow direction in the outcrop window is parallel to the inferred eastern paleoslope. The abundance of massive turbidites in the most downslope deposits and evidence for correlative deposits with south- and southwest-directed paleocurrents a short distance to the north suggest that the basin may have been ponded.

The wet desert system of the Upper Jurassic of the Parnaiba Basin, in the region between Formosa da Serra Negra and Montes Altos, Maranhão State, Brazil

O Mesozoico foi marcado por mudanças geológicas significativas decorrentes da Orogenia Gonduanide, que possibilitou a implantação de sistemas desérticos concomitantemente com expressivos eventos magmáticos. No Brasil, o registro desses eventos é observado na porção central da Bacia do Parnaíba, representados pelos depósitos siliciclásticos da Formação Corda, de idade jurássica, que sobrepõem derrames basálticos da Formação Mosquito. O estudo de fácies e estratigráfico de afloramentos dessa unidade, realizado na região entre Formosa da Serra Negra e Montes Altos, estado do Maranhão, possibilitou reconstituir o paleoambiente e inferir condições paleoclimáticas para essa região da Bacia Parnaíba durante o Jurássico. Foram identificadas quinze fácies sedimentares agrupadas em três associações de fácies (AF), representativas de um sistema desértico: 1) canal fluvial entrelaçado (AF1), caracterizado por conglomerados e arenitos finos a grossos, com grãos e seixos de rochas vulcânicas, com estratificações plano-paralela e cruzada acanalada; 2) lençóis arenosos (AF2), constituídos por arenitos finos a grossos e pelitos subordinados com laminação plano-paralela, laminação cruzada cavalgante, marcas de adesão e gutter casts; 3) campo de dunas (AF3), arenitos finos a médios, com grãos arredondados e estratificações cruzadas tabular e tangencial. Fluxos fluviais efêmeros (AF1), adjacentes ao campo de dunas (AF3), frequentemente retrabalhavam o topo das planícies vulcânicas e, esporadicamente, invadiam os lençóis arenosos (AF2). Comparado aos ergs do Permo-Triássico, o sistema desértico Corda foi mais úmido e menos extenso, precedendo os sistemas fluviais e costeiros de clima mais ameno do Cretáceo da Bacia do Parnaíba

Stratigraphic architecture and evolution of a deep-water slope channel-levee and overbank apron: The Upper Miocene Upper Mount Messenger Formation, Taranaki Basin

This paper re-examines the Upper Miocene Upper Mount Messenger Formation, Taranaki Basin, to characterize its architecture and interpret its environmental evolution. Analysis of stratal architecture, lithofacies distributions, and paleotransport directions over the 250 m thick formation shows the outcrops provide a nearly dip parallel section displaying the lateral relationships between contemporaneous channel-levee and overbank depositional environments. At least five 30–40 m thick upward fining units are recognized in the north-central parts of the outcrop and are interpreted as large-scale overbank avulsion cycles. Each unit consists of thick- to medium-bedded predominantly planar laminated sandstone turbidites at the base that fine upward into thin- to very thin-bedded, planar laminated and ripple cross-laminated mud-rich turbidites. The units are traceable laterally over a distance exceeding 3 km where they are cut by channels that show basal mudstone draped by medium- to thin-bedded sandstone, and onlapped by thick-bedded planar laminated sandstone at the margin. The channels are separated by tapered packages of medium- to thin-bedded turbidites containing climbing-ripple cross-lamination interpreted as levees. The individual channel-levee and overbank avulsion cycles formed through four stages: 1) a channel avulsion spread sand into the overbank as an unconfined splay, 2) preferential scouring in one area of the splay led to development of a channel with small levees that prograded across the splay, 3) a deep incision followed by abandonment of the channel deposited a mud lining. Alternatively, the mud lining was formed during the first stage as the downdip portion of the channel was abandoned. 4) The channel filled at first by thick-bedded planar laminated and then by climbing-ripple cross-laminated sand. At this time, the growth of constructional levees progressively limited sand into the overbank. Ratios of Bouma division thicknesses calculated over a stratigraphic interval present a new method to distinguish deep-water depositional environments.

Estratigrafia, arquitetura deposicional e faciologia da formação Missão Velha (Neojurássico-Eocretáceo) na área-tipo, bacia do Araripe, nordeste do Brasil: exemplo de sedimentação de estágio de início de rifte a clímax de rifte

The aim of this study was to carry out a stratigraphic revision of the Missão Velha Formation (Araripe Basin, northeastern Brazil) based on detailed facies analysis, architectural elements, depositional systems and palaeoflow data. The main facies are: 1. coarse-grained conglomeratic sandstones and locally pebbly conglomerates, with abundant silicified fossil trunks, several large-to medium-scale trough cross-stratifications and predominantly lenticular geometry; 2. medium-to coarse-grained lenticular sandstones with granules, abundant silicified fossil wood and trunks, and large-to medium-scale trough cross-stratifications, cut-and-fill features and mud drapes on the foreset cross-strata; 3. conglomerates and poorly sorted medium-grained sandstones with sparse pebbles and horizontal stratification; 4. fine-to very fine-laminated silty sandstone interlayered with 5. decimetric pelitic layers with parallel stratification and climbing-ripple cross-lamination. Ten architectural elements were identified: CH: Channels, GB: Gravel bars and bedforms, SG: sediment gravity flows, SB: Sand bars and bedforms, SB(p): sand bedform with planar cross-stratification, OFch: Overbank flow of channel (levee, crevasse and channel splay), DA: Downstream-accretion macroforms, LS: Laminated sand-sheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. Not all of these features were observed at each outcrop. These elements, which are defined by their geometry and bounding surfaces, form the basis for interpreting depositional environments. The Missão Velha Formation is interpreted as: a. high energy braided fluvial systems with fining-upward cycles, channelized features, truncated cross-strata (diastema), lenticular geometry, channeled sandstones, sparse pebbles and b. meandering river systems due to the presence of flood plain deposits, crevasse splays and point-bars deposits. Aeolian deposits can also occur.

Sedimentological and provenance response to Cambrian closure of the Clymene ocean: The upper Alto Paraguai Group, Paraguay belt, Brazil

Final Gondwana amalgamation was marked by the closure of the Neoproterozoic Clymene ocean between the Amazonia craton and central Gondwana. The events which occurred in the last stage of this closure were recorded in the upper Alto Paraguai Group in the foreland of the Paraguay orogen. Outcrop-based facies analysis of the siliciclastic rocks of upper Alto Paraguai Group, composed of the Sepotuba and Diamantino Formations, was carried out in the Diamantino region, within the eastern part of the Barra dos Bugres basin, Mato Grosso state, central-western Brazil. The Sepotuba Formation is composed of sandy shales with planar to wave lamination interbedded with fine-grained sandstone with climbing ripple cross-lamination, planar lamination, swaley cross-stratification and tangential to sigmoidal cross-bedding with mud drapes, related to marine offshore deposits. The lower Diamantino Formation is composed of a monotonous, laterally continuous for hundreds of metres, interbedded siltstone and fine-grained sandstone succession with regular parallel lamination, climbing ripple cross-lamination and ripple-bedding interpreted as distal turbidites. The upper part of this formation consists of fine to medium-grained sandstones with sigmoidal cross-bedding, planar lamination, climbing ripple cross-lamination, symmetrical to asymmetrical and linguoid ripple marks arranged in lobate sand bodies. These facies are interbedded with thick siltstone in coarsening upward large-scale cycles related to a delta system. The Sepotuba Formation characterises the last transgressive deposits of the Paraguay basin representing the final stage of a marine incursion of the Clymene ocean. The progression of orogenesis in the hinterland resulted in the confinement of the Sepotuba sea as a foredeep sub-basin against the edge of the Amazon craton. Turbidites were generated during the deepening of the basin. The successive filling of the basin was associated with progradation of deltaic lobes from the southeast, in a wide lake or a restricted sea that formed after 541±7Ma. Southeastern to east dominant Neoproterozoic source regions were confirmed by zircon grains that yielded ages around 600 to 540Ma, that are interpreted to be from granites in the Paraguay orogen. This overall regressive succession recorded in the Alto Paraguai Group represents the filling up of a foredeep basin after the final amalgamation of western Gondwana in the earliest Phanerozoic.

Rhythmic Climbing-Ripple Cross-Lamination in Inclined Heterolithic Stratification (IHS) of a Macrotidal Estuarine Channel, Gomso Bay, West Coast of Korea

Abstract Well-developed rhythmic climbing-ripple cross-lamination (RCRL) was described from estuarine tidal channels in Gomso Bay, west coast of Korea. Associated with upper intertidal point bars of closely spaced meandering channels, RCRL occurs between mean sea level and mean neap high-water level. RCRL is typically less than 40 cm thick, and constitutes the upper part of fining-upward channel-fill successions that are capped by intensely bioturbated mud. RCRL consists of mud-draped climbing-ripple cross-laminae that are continuous along strike direction of the channel for 10–20 m. Climbing patterns are dominantly supercritical and less commonly subcritical. Flood-oriented RCRL is developed in the landward part of the meander bend, while ebb-oriented RCRL is developed in the seaward part. RCRL demonstrates rhythmic change in cross-lamination thickness, which resembles various hierarchical tidal cycles, such as diurnal inequality, synodic neap–spring tidal cycle, and anomalistic tidal cycle. Truncated tidal rhythmicities registered in RCRL and fine-grained textures are consistent with an upper intertidal origin. Common links of RCRL with inclined heterolithic stratification (IHS) and tight meander bends as well as high sedimentation rate is suggestive that RCRL can serve as a diagnostic indicator of active channel migration in the fluvio-estuarine transition. This study highlights the significance of RCRL bearing well-preserved tidal rhythmicities in the reconstruction of paleodepositional environment, paleoelevation in terms of tidal frame, and paleogeography.

Biogenic Cross-Lamination Produced by the Pistol-Shrimp Alpheus bellulus in Microcosm Experiments

Abstract Study of aquaria containing the shrimp Alpheus bellulus and poorly sorted, coarse-grained sand has revealed the potential for macroorganisms to create structures closely resembling ripple cross-lamination, including climbing-ripple cross-lamination. A. bellulus excavates decimeter-scale craters in the sediment, carrying sediment grains up the steep inner (stoss-equivalent) slope of the crater and depositing them on the outside (lee-equivalent side). The fabrics produced resemble hydrodynamically produced ripple cross-lamination, particularly when observed in the limited cross sections available in core. This biogenic cross-lamination can be distinguished from hydrodynamic cross-lamination in that the biogenic cross-lamination is steeper than is typical for current-generated ripple cross-lamination, the tabular foresets are close to angle of repose, and the structure has a circular ripple crest.

宮崎県日南海岸沿いに分布する新第三系宮崎層群青島層の重力流堆積物とその層序パターンの特異性

The Neogene Aoshima Formation of the Miyazaki Group, well exposed along the Nichinan coast, has been considered to be a turbidite succession deposited at a forearc basin, although the thickness-frequency distributions and the sedimentary facies of the formation are in disagreement with those of typical turbidite successions. In this study, we investigated the depositional system and the sedimentary environments of the formation, on the basis of the facies of sediment-gravity flow deposits and stratigraphic variations of bed-thickness distributions. From the results, we can classify the sediment-gravity flow deposits of the formation into the following types; Type G: sedimentary structures containing grading beds, whose wavelength is in the range of 0.5-5m; Type I: inverse grading structures composed of a finer lower unit with climbing-ripple cross lamination or HCS-mimics, and a coarser upper unit with massive sandstones; and Type M: massive structureless sandstones. Individual sediment-gravity flow deposits can be correlated over 10 km using key-tuff markers along palaeocurrent directions. Stratigraphic fluctuations of the hemipelagic-mudstone thickness and sediment-gravity flow thickness show that the formation along the Aoshima Island consists of a lower progradational system indicated by a thinning-upward trend of hemipelagic mudstones and an upper system dominated by lobate sand bodies characterized by the cyclic sedimentation of thick-sediment-gravity flow deposits. These results indicate that the depositional setting of the formation is a deep-offshore basin of a fandelta along the south-east coast of the Kyushu Island.

Anatomy of a submarine channel–levee: An example from Upper Cretaceous slope sediments, Rosario Formation, Baja California, Mexico

To date, facies architecture models of submarine channel–levees have largely been derived from seismic data, isolated core data and limited field studies. We report field observations of an Upper Cretaceous submarine channel–levee complex within the Rosario Formation, Baja California, Mexico, which provide high-resolution data of lithofacies and ichnofacies distribution, and levee depositional thickness decay along transects perpendicular to the channel axis. Within the levee, both sandstone thickness and the overall proportion of sandstone decrease according to a power law away from the channel axis. Spatial variation in sedimentary structures away from the channel axis is predictable and provides an important link to the depositional flow regime. In channel-proximal locations, structureless sands, parallel lamination, overturned ripples, and ripple cross-lamination (including climbing ripple cross-lamination) are common; in channel-distal localities starved ripples are abundant. Sandstone bed thickness generally increases up stratigraphy within the levee succession, which is interpreted to indicate increasing turbidity current magnitude and/or contemporaneous channel floor aggradation reducing relative levee relief. However, in the most channel-proximal location sandstone bed thickness decreases with height; combined with evidence from both facies and palaeocurrent analysis this allows the position of the levee crest to be inferred. The thickest beds occur at higher levels with increasing distance from the channel axis, using this evidence we present a model for levee growth and migration of the crest. Quantitative analysis of ichnofacies distribution reveals that traces typical of the Cruziana and Skolithos ichnofacies are superimposed over the ‘normal’ background Nereites ichnofacies, forming a ‘bioturbation front’ which is indicative of proximity to the channel. By analogy with modern canyons and channels, the association of Cruziana and Skolithos ichnofacies with the channel may be attributed to oxygen and nutrient enrichment and possible turbidity current transport of organisms responsible for these ichnofacies.

The Benkovac Stone Member of the Promina Formation: A Late Eocene Succession of Storm-Dominated Shelf Deposits

The Upper Eocene Benkovac Stone Member of the Promina Formationof northern Dalmatia, Croatia, is a thinly bedded succession ofalternating carbonate sandstones and calcareous mudstones, ca. 40 mthick, exposed as a narrow, SE-trending outcrop belt near the town ofBenkovac. This unit occurs in the middle part of the Promina Formation,which is a spectacular calciclastic succession of deposits of lateMiddle Eocene to Early Oligocene age, about 2000 m thick, showingan upward trasition from deep-marine turbidites to shallow-marineand alluvial deposits.The sheet-like sandstone beds of the Benkovac Stone Memberare mainly 1–25 cm thick and have been classified into 6 facies and3 subfacies, differing in stratification or showing various internalsequences of stratification types. The thicker and most common bedsshow plane-parallel stratification passing upward into hummockycross-lamination and undulatory to flat parallel lamination (FaciesS1), or consist of only the latter two divisions (Facies S2). Subordinatebeds show convolute stratification (Facies S3), are amalgamated(Facies S4), or are homogenized and merely graded (Facies S6). The thinner beds have more uneven boundaries and show translatory ripplecross-lamination (Subfacies S5a), climbing ripple cross-lamination(Subfacies S5b) or pinch-and-swell lamination attributed to starvedand rolling-grain ripples (Subfacies S5c). The intervening mudstonebeds (Facies M) are silt-streaked and bioturbated. Trace fossils indicatea combination of Zoophycos and Cruziana ichnofacies.The sedimentary succession was deposited in a microtidal offshoretransition zone characterized by muddy “background” sedimentationpunctuated by discrete storm events. The observed spectrum oftempestite sandstone beds represents a wide range of storm events,varying in magnitude and in the mode of sand dispersal – from thepure action of oscillatory waves to pure geostrophic currents. Themajority of tempestites are attributed to a combination of these twoend-member factors, with the geostrophic currents often enhanced bya high load of sediment suspension (density-modified currents).The Benkovac Stone Member is underlain by muddy offshoredeposits (Debelo Brdo Member) and covered by sandy to gravellyshoreface deposits (Otavac Member), which in turn pass upwards intobraidplain deltaic and alluvial deposits. This regressive succession isconsidered to be a parasequence deposited as a highstand systems tractduring a gradual, stepwise rise of relative sea level. The thick parasequence consists of progradational and retrogradational sets of much smaller parasequences, the record of which differs markedly in theshoreface and offshore transitional part. The difference is attributed tothe underlying contrast in the physical factors controlling the supplyof sand to these shallow shelf zones.

Overbank and channelfill deposits of the modern Yellow River delta

The Huanghe is noted for its high transport rate of silt and clay, which may reach depth-averaged values of 200 kg m −3 during peak discharge. The sediment load transported through the river on entering the delta plain, amounts to 10 12 kg per year. In contrast to most other large deltas only one distributary channel is active at any one time. The high sediment load causes the rivermouth to prograde at a yearly rate of 1–4 km into the shallow (less than 20 m deep) Bohai gulf. The vertical aggradation of the channel belt and mouth bar complex is also rapid (decimetres per year on average), so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe. A series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front. A newly developed distributary passes from a multichannel to a single, straight channel system, and ends with the formation of meanders. The protruding mature delta lobe shows a radiating pattern of crevasse channels. Overbank/ crevasse deposits are made of vertically stacked dm-scale waning flow sequences, structurally characterized by (from bottom to top) small scour-and-fills, even (parallel) lamination, and climbing-ripple crosslamination. Accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge. It can be concluded that each sequence has been deposited within a few hours, and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence.

Sandstone and Conglomerate Shoreline Deposits in Triassic-Jurassic Newark and Hartford Basins of Newark Supergroup: ABSTRACT

Three types of sandstones and conglomerates in the Newark and Hartford basins of the lower Mesozoic Newark Supergroup are similar to Holocene and to presently forming deltaic and wave-sorted shoreline deposits in modern, block-faulted, closed basins in the western United States. (1) Sandstones and siltstones with internal, sequences dominated by climbing ripple cross-lamination that comprise 2 to 4-m thick, low-angle, inclined foresets. The foreset beds intertongue with silty shale at their toes. Stacks of these foresets define coarsening-upward sequences that are interpreted as small, Gilbert-like delta fronts stacked on the topset plain due to fluctuations in lake level. (2) Sheetlike, decimeter-scale, muddy sandstone beds. These are composed of g aded, deceleration-of-flow sequences similar to those described above but contain mudstone partings with large polygonal cracks and soft-sediment deformation structures. Fining-upward sequences of these beds are interpreted as broad, flat delta fronts formed by flash-flooding streams intersecting an expanding shallow lake. (3) Cobble and pebble conglomerates with well-sorted sand or granule matrix. These rocks are typically associated with well-sorted, medium to coarse-grained sandstones with planar, horizontal lamination or oscillatory ripple cross-lamination. These beds are interpreted as wave-sorted alluvial fan-toe deposits. The Gilbert-like deltaic sandstones and the sheetlike muddy sandstones in the Newark and Hartford basins generally have low porosity and, therefore, would be poor reservoir rocks. The wave-sorted conglomerates, which are limited to the faulted basin margins, are potentially excellent reservoir rocks, in many places occurring directly in contact with black shales. Thick shoreline sandstones and conglomerates deposited in deeper lakes may occur in the Newark and Hartford basins, but they were probably restricted to the basin margins and may have been removed by subsequent faulting or erosion, or possibly they have not yet been recognized due to the scarcity of large outcrop exposures. End_of_Article - Last_Page 1448------------

A classification of climbing-ripple cross-lamination

C limbing - ripple cross-lamination , Sorby’s (1859, 1908) ripple-drift bedding, is an important and significant sedimentary structure resulting from the action of unidirectional currents. The importance of the structure lies in its worldwide distribution and frequent occurrence in such diverse sediments as river channel and levee sands and in turbidity current deposits. Its significance is that, of all sedimentary structure, it stands alone in giving information on short-term rates of sediment deposition ( Sorby 1908 ; Kuenen 1967 ; Walker 1969 ; Allen 1970 , 1971 , 1972 ). Hence the study of this structure may lead to a deeper understanding of aspects of the character of past events—river floods and turbidity currents—of an intrinsically brief and catastrophic nature. This goal will not however be achieved until the nature of climbing-ripple cross-lamination is clearly perceived and defined within the framework of an unambiguous field classification. The most widely used existing classification of the structure is now recognized to be confused, because some of the criteria adopted for the definition of its categories relate to the individual cross-laminated set (and the set with its immediate neighbours) perceived at some station in a coset, whereas others properly describe the coset of which the individual cross-laminated set is generally a very subordinate part. Set and coset are here used in the well-known sense of McKee & Weir (I953). The consequence is that aspects of the structure at the level of the set and its immediate neighbours are not sharply enough distinguished from aspects of the structure at the level of the

Determination of Some Paleohydraulic Parameters: ABSTRACT

New river measurements combined with existing laboratory evidence show that, in a river reach where traction motion as well as net sediment deposition take place, the mean applied bed shear stress is equal to the critical shear stress required to move particles of median bed material size. Consequently, application of Shields' relation between shear stress and material size permits the determination of mean applied shear stress at an ancient depositional site from the grain-size distribution of the sediment. Furthermore, by applying laboratory values for the friction factor, determined for the appropriate bed configuration, we can calculate the paleoflow velocity by the Darcy-Weisbach relation. This method for the determination of paleoflow velocity is generally applicabl to any sedimentary deposit if a reasonable estimate of the friction factor can be made. For rock units containing climbing ripple cross-lamination, we can go one step farther and determine the rate at which the sediments were accumulated. The rate of bedload transport has a known functional relation to the mean velocity of the overlying column of water as determined in the first part of this study. Very high rates of sedimentation are found to exist in nature. Late Pleistocene deltaic sediments in the Truckee Canyon of Nevada, locally contain climbing ripple cross-laminated units indicating a local rate of aggradation of 0.1 mm/sec or 36 cm/hr. End_of_Article - Last_Page 797------------