Herringbone Cross-stratification Research Articles

Tidal Modulated Shoreface Facies in Eocene Agbada Formation, Niger Delta Basin, Nigeria

Tidal influence shoreface are shoreface deposit generated by wave induced tidal action. The extent of the tidal impact from lower to upper shorefaces depend mostly on the tidal range thereby controlling the impact of different shoreline wave action on sediment distribution across shorefaces. The Tidally influenced shoreface sediment display wave generated structure, however the structure are strongly associated with tidal generated structures and trace fossil consistent with tidal depositional setting. Shoreface successions in Titun well display a wide range of variability, largely restricted to the lower and middle shoreface intervals. This variation was controlled by relative wave dominance and intermittent pulsating nature of associated tidal bed deposition. The successions were subdivided into three end members, although a complete gradation between them exists. High energy shoreface consist almost entirely of erosively based amalgamated tide and wave beds with minimal bioturbation. Intermediate energy shoreface consist of stacked laminated to burrowed intervals, recording repeated erosive storm bed emplacement, followed by opportunistic colonization, and commonly overprinted by resident fairweather burrowing. Weakly storm-affected low energy shorefaces typically show intense bioturbation with minimal preservation of high energy beds. Based on the association of the facies, facies vertical build-up patterns, sedimentary structures and trace fossil content, the T1 sands in Titun well are interpreted as a prograding tidally influence shoreface deposit. This interpretation is supported by presence of tide generated structures and marine trace fossil, due to the prevalence of several tidal imprint as suggested by several mud drapes and flasers, couplets of sands and shale’s in heterolithic sequences, and bidirectional current structures (herringbone cross stratification), a tidally influence shoreface geological model is suggested for deposit.

Mixed carbonate-siliciclastic contourite drift deposits associated with the entrance of an Atlantic-Mediterranean corridor (late Miocene, southwest Spain)

Carbonate contourite drifts are poorly documented in the onshore record because of the difficulty of implementing diagnostic criteria for their recognition. Accordingly, little is known about the relative position of carbonate drifts with respect to ancient carbonate platforms, seaways and shallow passages within the context of palaeoceanography. This study presents a fossil example of mixed carbonate-siliciclastic drift cropping out in a quarry in Osuna (Sevilla province, southern Spain) at the northern end of the Guadalhorce Corridor, a Miocene strait connecting the Mediterranean Sea and the Atlantic Ocean in the Betic Cordillera. Based on the facies and sedimentary structures, the studied succession is divided into three units: 1) the lower unit, Unit 1, is a 33-m thick succession of large carbonate bodies with mega cross-stratification pointing to the southeast and secondarily to the northwest interpreted as a contourite drift; 2) the intermediate Unit 2 is a 0.5–2-m thick terrigenous conglomerate body eroding the top of Unit 1; and 3) the uppermost Unit 3 consists of a 6-m thick siliciclastic-dominated succession with herringbone cross-stratification and a dominant direction of the structures to the northwest interpreted as tidal deposits. The large-scale sediment bodies with mega cross-beds, the presence of reactivation surfaces with grain-size changes, and the unidirectionality of the structures were diagnostic for the recognition of Unit 1 as drift deposits. The dominant sedimentary structures pointing to the southeast in the drift were generated by Atlantic inflow into the Mediterranean. This challenges the classical “siphon” model for the Atlantic-Mediterranean water-mass circulation pattern for this age. The conglomerates of Unit 2 evidence regional uplift of the southern margin of the Guadalquivir Basin that promoted a change in the depositional mode from a bottom-current dominated (Unit 1) to a tide-dominated environment (Unit 3) after the closure of the Guadalhorce Corridor in the Messinian.

Paleoarchean terrestrial to shallow marine sedimentation on Singhbhum Craton, eastern India (the Western Iron Ore Group)

Distinguishing shallow-marine and terrestrial (fluvial) sedimentary deposits is often difficult in the absence of fossils and challenging in deformed and metamorphosed early Precambrian terrains. Herein we report for the first time a > 3.4 Ga shallow marine to terrestrial deposit from the Western Iron Ore Group (WIOG), Singhbhum Craton, India. The WIOG sedimentary succession, lying below a ~ 3.4 Ga banded iron formation, has been divided into three facies associations: a lower alluvial fan-fluvial association, a middle shallow-marine association, and an upper fluvial association. The bottommost conglomerate–very coarse- to coarse-grained sandstone constitutes the alluvial fan-braided fluvial association. Sandstones of the facies association two bears mud drapes and herringbone cross-stratification and thus indicates tidal influence. These shallow-marine sandstones bear wave ripples, ladderback ripples, and adhesion structures and indicate an intertidal depositional environment. The overlying fluvial facies association is characterized by several fining-upward cycles of conglomerate–coarse- to medium-grained sandstones with large-scale trough cross beds, parallel lamination and penecontemporaneous deformation structures and minor shale, indicating the braided nature of the postulated river system. The WIOG succession indicates an overall shallowing-up trend because of sea level fall. The sedimentary structures and texture of the WIOG fluvial deposit are very similar to recent fluvial deposits and are likely to have formed by similar sediment transportation and depositional processes in a tectonically stable sedimentary basin.

Tidal estuarine deposits of the transgressive Naturita Formation (Dakota Sandstone): San Rafael Swell, Utah, U.S.A.

ABSTRACTThin tidal estuarine deposits of the Naturita Formation (0–23 m) of the San Rafael Swell record the initial flooding of the Cretaceous Western Interior Seaway, Utah, and capture the transition from inland fluvial systems to fully marine conditions over a time period of 5 My or less. A tide-dominated estuarine environment is favored due to the combined presence of mud and/or carbonaceous drapes on ripples and dunes, bidirectional flow indicators, sigmoidal cross-stratification, herring-bone cross-stratification, and bimodal paleocurrent measurements. Facies associations are arranged in a predictable manner. Locally at the base of the Naturita Formation, tidally influenced fluvial channel deposits are present. These are overlain by tidal bars, including subtidal bars and intertidal point bars. Overlying the tidal bars are sand-flat and mud-flat deposits as well as bedded coal and carbonaceous mudstone that represents a supratidal setting in the estuary. The Formation can be capped by a thin transgressive lag composed of shell debris, and/or pebbles, that marks the final transition into the fully marine Tununk Shale Member of the overlying Mancos Shale.Lateral relationships between estuaries and adjacent paleohighs shed light on the influence of foreland-basin tectonics on the location and preservation of tide-dominated estuaries. Estuarine and shoreface deposits are absent along the eastern flank of the San Rafael Swell and eastward for more than 80 km. This zone of nondeposition or erosion is coincident with the location of the forebulge in the developing foreland basin, implying that growth of the forebulge prohibited the development of, or enhanced the later erosion of, estuarine deposits. Conversely, enhanced accommodation in the transition into the foredeep depozone allow the preservation of tide-dominated estuarine deposits along the western flank of the San Rafael Swell. Additionally, the possibility of a pre-Laramide tectonic history for the San Rafael Swell is indicated by a distinct lack of Naturita Formation deposits in an area that is coincident with the modern-day axis of the anticline.Overall, the Naturita records the initial flooding of the Western Interior Seaway in the San Rafael Swell region and provides an excellent case study of the deposits that are laid down in a transgressive system that passes from coastal-plain to offshore deposits.

Ichnology and sedimentology of estuarina deposits, Mata Amarilla Formation, Austral Basin, Argentina

The Mata Amarilla Formation (Upper Cretaceous) is part of the infilling of the Austral Basin (Patagonia, Argentina). This unit is mainly composed of grey to black mudstones interbedded with fine-to coarse-grained sandstones, deposited in littoral and continental environments. Three informal sections have been defined; the littoral deposits of the lower and upper sections constitute the object of this study. The intervals contain Arenicolites, Chondrites, Cylindrichnus, Ophiomorpha, Palaeophycus, Skolithos, Thalassinoides, Teredolites and fugichnia. The sedimentological and ichnological analyses allowed the distinction of three facies associations (FA) developed in an estuarine palaeoenvironment The more proximal FA1 is the bay-head delta, comprising delta-front bars of course-grained sandstone, containing Arenicolites, Skolithos and allochthonous petrified wood with Teredolites. This trace fossils association indicates high-energy conditions. The FA2, interpreted as estuarine bars, is composed of fine-to medium-grained containing herringbone cross-stratification and mud drapes with Cylindrichnus, Ophiomorpha, Skolithos, Thalassinoides and fugichnia. This reflects a setting with a high sedimentation rates, moderate energy, unstable substrates, and normal salinity. Finally, the FA3 (fine-grained estuarine deposits) are characterized by heterolithic rocks with synaeresis cracks and Arenicolites, Chondrites, Cylindrichnus, Palaeophycus, Thalassinoides and Skolithos. These ichnogenera are generally characterized by small sizes, consistent with a Salinity stressed paleoenvironment. The distribution, size and abundance of the trace fossils were controlled by several environmental factors, e.g. energy, sedimentation rate and salinity contents. The palaeoenvironmental factors variability is directly related to the fresh water and sediment input from rivers flood, which took place during the wet season of a warm, temperate climate

Sedimentology and ichnology of the fluvial reach to inner estuary of the Ogeechee River estuary, Georgia, USA

Through the integration of sedimentological and ichnological observations, this paper explores the character of sediments deposited across the fluvio-tidal transition zone of the upper microtidal, mixed-energy, sand-dominated Ogeechee River estuary, Georgia, USA. A transect of tidally influenced to fluvial channel-bars and their facies variability is reported. Field and laboratory methods were employed, including observation of physical and biogenic sedimentary structures on the point-bar surfaces and in trenches, collection of grab samples, suction and box coring, grain size and total organic carbon analyses, optical microscopy, core logging, and daylight photography. The data presented in the paper can help in predicting facies changes across the fluvio-tidal transition of sand-dominated fluvio-tidal deposits in the rock record.The lower inner estuary is characterized by medium-fine and fine-medium sand with planar and trough cross-bedding, small-scale ripple lamination, tidal sedimentary structures (flaser and wavy bedding, herringbone cross-stratification), abundant organic debris, and mud rip-up clasts. Bioturbation of the intertidal point bars is low, but cryptobioturbation is locally observed. Upper inner estuary deposits comprise coarse-medium- and medium-coarse-grained sand, and are characterized by faint high-angle planar and trough cross-bedding. Organic debris, mud rip-up clasts, herringbone and current-ripple lamination are rarely observed. Bioturbation is absent to sparse. The fluvio-tidal transition is represented by very-coarse- to coarse-grained sand and granules. Physical sedimentary structures constitute massive, graded planar and trough cross-bedding with abundant plant detritus. Except for rare Siphonichnus- and Lockeia-like traces, bioturbation is absent. The fluvial setting is characterized by coarse-medium sand with unidirectional cross-bedding, current-ripple lamination, and rare organic-rich mud clasts. Bioturbation is absent. Inner estuary and fluvio-tidal transition zones are bounded by supratidal marsh deposits, and may develop a substrate-controlled Glossifungites ichnofacies, comprising Psilonichnus-like burrows. The fluvial reach is surrounded by forested floodplain with crayfish domes and chimneys, forming Camborygma-like traces.

Sedimentary Structures of Ajali Sandstone

The Ajali Sandstone is a Maastrichtian deposit in the Stratigraphic units that make up the Anambra basin, southeastern Nigeria. The approximate locations of the study areas; Okigwe and Uturu, are respectively Latitude 5˚45’N-6˚00’N &Longitude 7˚15’E – 7˚30’E and Latitude 5˚48’N-5˚55’N & Longitude 7˚25’E-7˚30’E. Samples were collected for further laboratory studies and the result showed that the Ajali Sandstone is dominated by quartz mineral grains which constitute about 67.23% modal composition with ironstone cementing material. This implies that Ajali Sandstone is basically quartz arenite and it is said to be mineralogically mature. The Ajali Sandstone as observed in the field is milky white in color and friable with intercalations of claystone, mudstone, and shale. Common sedimentary structures observed on the Ajali Sandstone are cross-stratification structures, lamination structures, wavy structures, biogenic structures, herringbone cross-stratification, clay drapes, and heterolytic structures, These are indicative of fluvial-marine and continental depositional environment.

Demise of Late Triassic carbonate platforms triggered the onset of a tide-dominated depositional system in the Dolomites, Northern Italy

Facies analysis of the mixed carbonate–siliciclastic Upper Triassic Heiligkreuz Formation in the Cortina–Tofane area (Dolomites, Northern Italy) shows a tide-dominated sedimentary succession developed in a marine strait environment connecting two small sub-basins on the western margin of the Tethys Ocean. Both ebb and flood paleocurrents are preserved and their directions are in agreement with the strait orientation, as inferred from the geographic position of high-relief carbonate platforms in the substrate. Facies and sedimentary structures including planar cross stratification, herringbone cross stratification, flaser to wavy to lenticular bedding and cyclical alternations of mainly oolitic–siliciclastic vs. mainly bioclastic–muddy laminae related to variations of the hydraulic regime have been described and compared with other tide-dominated systems developed in confined marine conditions, and in particular with the Plio-Pleistocene deposits of the central Mediterranean Sea. However, in contrast with all other known cases, this tidal system did not develop in tectonically confined submarine grabens or canyons, but rather in a narrow marine passage between two relict high-relief carbonate platforms.

Depositional Environment of Shallow-Marine Sandstones from Outcrop Gamma-Ray Logs, Belait Formation, Meragang Beach, Brunei Darussalam

The outcrop of shallow marine sandstone, Belait formation at Meragang Beach, Brunei Darussalam have long been regarded as tide-, wave- and storm-dominated environment that reveals the reservoir heterogeneities and variation in sand body geometry. Intensive outcrop study methods were available from previously conducted work on Belait Formation. But for this study the priority was to acquire gamma-ray profile at 5-10 cm scale by using Digital Total GR Count Detector and re-logged of Sedimentological log (in cm) of 311.8 m thick outcrop to describe the depositional environment and sedimentary facies architecture. The sediments deposited in a tide-dominated environment mostly interpreted in all cycles which were characterized by sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment with sedimentary facies of planner cross-bedding, trough cross-bedding, herringbone cross-stratification, wave ripples and thin parallel lamination with coal fragmented bed and carbonaceous material. The gamma-ray profile for tide dominated environment shows different shapes; block left-boxcar, bell shape, funnel shape and irregular trend. In wave- and storm-dominated are wave ripples, thin parallel lamination with mud drapes, large planner cross-bedding, trough cross-bedding, highly bioturbated (skalithos) sandstone and hummocky cross-stratification. Whereas, the gamma-ray profile shows different shapes; left-boxcar, right-boxcar, funnel shape and irregular trend and in storm-dominated it show only left-boxcar with no sharp edges on top and bottom. This approach combines excellent data quality to interpretate different depositional environment and its impact on sandstone quality which implies that the outcrop gamma-ray log can use to characterize the rock deposited in different depositional environment in different facies. It also reveals that the changes in gamma-ray profile with in the sandstone depend upon the environment in which the sandbody deposited that reveal in reservoir heterogeneities in subsurface well correlation.

Revision of Ursus and Marmota from Grotta del Cerè (Northeastern Italy)

Three sections which crop out near Tomačevica (2 km east of Komen), Slovenia, were measured and described. Thirty-seven rock samples were collected for thin section analysis. A database on all the fossils from the Komen area was compiled from collections of the Museo Civico di Storia Naturale in Trieste, Italy, the Museo Geologico Paleontologico G. Capellini in Bologna, Italy, and the Slovenian Museum of Natural History in Ljubljana, Slovenia. The data collected on the stratigraphy, sedimentology and palaeontology of the Komen platy limestones were used to build a new depositional model for the area. This new hypothesis is proposed in place of the “lagoonal model”, too often invoked in the past in order to explain the occurrence of dark platy limestones occurring worldwide in platform settings. Given the absence of any evidence in support of a true “lagoonal system” in the examined area (e.g. channel fills, herring bone cross stratification, sand bars or bioherms delimiting the area of interest) the latter has been examined in search of an alternative sedimentological model. New data regarding facies succession and faunal composition have been gathered. The typical facies succession of the Komen Limestone comprises: (a) flat pebble breccias that grade up-section into packstones and wackestones containing progressively rarer, and smaller, flat micritic intraclasts; (b) well bedded to platy limestones; and (c) stromatolitic limestones. All of these lithologies contain scattered chert nodules and sheets. The available evidence suggests that the depositional environment of the Komen Limestone was in an intraplatform basin, in the proximity of an exposed area (tidal flat plus probably other supratidal environments populated by conifers and where karstification could occur). The bottom water conditions were periodically dysoxic to anoxic, and the stacking of different facies was due to a combination of small scale sea-level fluctuations and local subsidence of the area.

Marine influence in the Barreiras Formation, State of Alagoas, northeastern Brazil

Tidal processes were important for deposition of the Barreiras Formation located in northern Brazil, while correlatable deposits in northeastern Brazil have been traditionally related to continental environments. Facies analysis in southern Alagoas revealed that the Barreiras Formation consists of cross-stratified conglomerates and sandstones (facies Sx and Cgx), compound cross-stratified sandstones (facies Cx), and heterolithic beddings (facies H). A significant portion of these deposits occurs within channel morphologies displaying fining and thinning upward successions. An abundance of sedimentary features is comparable to those from the northern Brazilian counterpart. These include: tidal bundles; herringbone cross-stratification; heterolithic beddings with sandstone and mudstone beds in sharp contacts; and ichnofossils mostly consisting of Ophiomorpha nodosa, Skolithos and Planolites. Altogether, these features point to a marginal marine depositional setting dominated by tidal processes, which are related to an estuarine system, an interpretation also provided for the Barreiras Formation in northern Brazil. The widespread occurrence of deposits with unambiguous evidence of tidal processes in the Barreiras Formation of northern Brazil, and now in the State of Alagoas, leads to argue that the early/middle Miocene worldwide marine transgression might have left a much more widespread sedimentary record along the Brazilian coast than currently regarded.

Depositional facies of the subsurface Neogene Surma Group in the Sylhet Trough of the Bengal Basin, Bangladesh: record of tidal sedimentation

The Bengal Basin, in the north-eastern part of the Indian subcontinent, contains a thick (± 22 km) early Cretaceous-Holocene sedimentary succession. The Neogene succession in the Sylhet Trough of the basin reaches a thickness of more than 6 km of which the Surma Group contains important sandstone reservoirs. Lithologically, the group consists of a succession of alternating shales, siltstones, sandy shales and sandstones, with minor conglomerates. This research work is a sedimentological analysis of the subsurface Neogene succession encountered in the petroleum exploration wells in the Sylhet Trough of the Bengal Basin. Detailed lithologic logs of the cores, based on considering texture and sedimentary structure, permit a subdivision into eight lithofacies, e.g., a shale-dominated facies, interbedded fine sandstones and mudstones, ripple-laminated sandstones, parallel-laminated sandstones, massive sandstones, cross-bedded sandstones, cross-bedded sandstones with pebble/granule lag and conglomerates. Characteristic sedimentary structures of the Surma Group, such as flaser-, wavy- and lenticular-bedding, bipolarity of ripple cross-stratification, evenly laminated sand/silt-streaked shales, reactivation surfaces within cross-bedded sandstone sets, mud-drapes on foreset laminae and herringbone cross-stratification as well as small-scale vertical sequences (several fining-upward cycles) are diagnostic for tidal influence. On the basis of the lithofacies associations and prograding character of the deposits revealed from the electrofacies associations, the Surma Group sediments have been interpreted as representing deposits of tide-dominated deltaic depositional setting.

New palaeoenvironmental model for the Komen (Slovenia) Cenomanian (Upper Cretaceous) fossil lagerstätte

Three sections which crop out near Tomačevica (2 km east of Komen), Slovenia, were measured and described. Thirty-seven rock samples were collected for thin section analysis. A database on all the fossils from the Komen area was compiled from collections of the Museo Civico di Storia Naturale in Trieste, Italy, the Museo Geologico Paleontologico G. Capellini in Bologna, Italy, and the Slovenian Museum of Natural History in Ljubljana, Slovenia. The data collected on the stratigraphy, sedimentology and palaeontology of the Komen platy limestones were used to build a new depositional model for the area. This new hypothesis is proposed in place of the “lagoonal model”, too often invoked in the past in order to explain the occurrence of dark platy limestones occurring worldwide in platform settings. Given the absence of any evidence in support of a true “lagoonal system” in the examined area (e.g. channel fills, herring bone cross stratification, sand bars or bioherms delimiting the area of interest) the latter has been examined in search of an alternative sedimentological model. New data regarding facies succession and faunal composition have been gathered. The typical facies succession of the Komen Limestone comprises: (a) flat pebble breccias that grade up-section into packstones and wackestones containing progressively rarer, and smaller, flat micritic intraclasts; (b) well bedded to platy limestones; and (c) stromatolitic limestones. All of these lithologies contain scattered chert nodules and sheets. The available evidence suggests that the depositional environment of the Komen Limestone was in an intraplatform basin, in the proximity of an exposed area (tidal flat plus probably other supratidal environments populated by conifers and where karstification could occur). The bottom water conditions were periodically dysoxic to anoxic, and the stacking of different facies was due to a combination of small scale sea-level fluctuations and local subsidence of the area.

PTB mass extinction and earliest Triassic recovery overlooked? New evidence for a marine origin of Lower Triassic mixed carbonate–siliciclastic sediments (Rogenstein Member), Germany

The Central European Permo-Triassic basin was land-locked with occasional connections to the North Pangaea shelf and Tethys. A thick sequence of siliciclastic fluvial and floodplain sediments separates the Late Permian marine carbonate–evaporite cycles of the Zechstein Basin from marine Middle Triassic marls and carbonates of the Germanic Basin. Within this Latest Permian–Middle Triassic siliciclastic interval, the location of the Permian–Triassic Boundary (PTB) is difficult to recognize. The first limestones after the PTB are Induan mixed carbonate–siliciclastic oolites and microbialites of the Bernburg and Calvörde Formations (Untere Buntsandstein Group). Consensus emerged after a long-lasting controversy that the carbonates of the so-called Rogensteine are the relics of an alkaline playa lake. The idea of the marine genesis was finally rejected, because of the obvious lack of calcified metazoans. The existing depositional playa lake-model will be tested using previously unconsidered data, such as currently discussed Early Triassic recovery scenarios of marine benthic communities after the Permian–Triassic mass extinction, global patterns of Early Triassic carbonate production, Permian–Triassic tectono-sedimentary evolution of northern Pangaea and stable oxygen and carbon isotope data. The Rogenstein is interpreted as marine carbonate, showing the characteristic features of marine carbonates of the Early Triassic recovery period: – Calcified invertebrates are rare in many marine settings during the aftermath of the Permian–Triassic mass extinction. Thin shells of the oolites resemble opportunistic bivalves flourishing in marine Early Triassic oceans. – The oolites and microbialites are the product of abiotic and biotically-induced carbonate precipitation following the Permian–Triassic mass extinction while biomineralisation is insignificant; Rogenstein microbialites look like marine Early Triassic microbial reefs. – The extraordinary size of the Rogenstein ooids resembles marine Neoproterozoic ooids. The term “anachronistic” has been used in the literature to describe the similarities in the fabric and composition of Early Triassic and Early Palaeozoic/Neoproterozoic marine carbonates. – Stable carbon isotopes of least altered Rogenstein ooids and stromatolites exhibit trends similar to the composition of Triassic seawater. – The presence of flat pebble conglomerates and sediment structures similar to herring-bone cross stratification confirm the marine genesis. I propose that cool seawater transgressed during a global rise of Late Permian–Early Triassic base-level from the Barents shelf southward along reactivated extensional graben structures and flooded the Central European Permo-Triassic basin. There, the extremely hot and arid climate of the megacontinent Pangaea caused increased evaporation of seawater and subsequent supersaturation with respect to calcium carbonate. Insignificant biomineralisation and high water energy in the shallow sea give rise to largely abiotic and biotically-induced carbonate precipitation which resembles lake sediments at first sight. Benthic tropical carbonate production started with the next transgression during the Röt Formation, leading to the sedimentation of marls and limestones during the Spathian (late Olenekian).

Discussion on tectonic evolution of the Proterozoic Itremo Group metasediments in Central Madagascar Special Publication 206, 2003, 381–399

Ronadh Cox writes: Fernandez & Schreurs (2003) confirm previous findings about the sedimentology of the Itremo Group (Moine 1967; Cox et al . 1998). They also report a generalized Itremo Group stratigraphy which agrees (except for some thickness differences) with that measured in the Iheninkenina synclinorium south of Ibity (Daso 1986), and also matches that described by Collins et al . (2003) for the same region. This is a more complete stratigraphy than that reported by Cox et al . (1998) and Moine (1967), and should therefore be accepted as the standard for the eastern Itremo region. The authors also make some highly controversial contributions to the ongoing debate about the tectonics of central Madagascar. I disagree with some of their interpretations, and therefore offer the following alternative points of view. ### There is no evidence for tidal currents or beach sedimentation in the Itremo Group. Fernandez & Schreurs (2003) show sedimentary structures (their fig. 4a) that they interpret as herringbone cross-bedding, based on which they infer tidal currents; but herringbone cross-stratification is characterized by planar set boundaries marking the tide-reversal surfaces, whereas the set boundaries in this photograph are concave up, and parallel to the overlying foresets. The photograph in fact shows trough cross-stratification, produced by the migration of crescentic or linguloid dunes. Such structures are very common in Itremo Group quartzites. The palaeocurrent direction is not reversing, but unidirectional and approximately normal to the plane of the photograph. Conglomerates are interpreted as possible beach deposits by Fernandez & Schreurs (2003), but the presence alone of conglomerate is insufficient for this interpretation. Thin lenticular pebble and cobble deposits like those found in the Itremo Group can accumulate in many current-swept shelf environments (e.g. Johnson & Baldwin 1996), and the association with fining-upward quartzite (their fig. 3) would be more consistent with a tempestite than with beach sedimentation. The abundant large-scale dune …

Large point bars in a monsoonal estuarine environment (East Coast of India): An atlas of nearsurface sedimentary and biogenic structures

Tidal point bars located 30–50 km inland from the present-day east coast of India have been extensively studied under varying seasonal conditions. The surface of these bars, when exposed during low tide, is sculptured by various physical sedimentary structures such as linguid ripples, interference ripples of different patterns, asymmetric ripples of cuspate and lunate forms, ladder-back ripples, double-crested ripples and wrinkle marks. The associated biogenic structures are tracks and trails of gastropods, hermit crabs, gobiid fish, birds and various surface designs made by bubbler crabs with their feeding pellets. Study of relief peels from 500 box cores taken from point bars developed in two tidal estuaries of contrasting character shows that horizontal lamination, small ripple lamination, flaser bedding, wavy and lenticular bedding, herringbone cross-stratification, mud/silt couplets, bubble sand in silt, etc. are the common features in these deposits. Flaser bedding and herringbone cross-stratification are more common in this transitional facies than in the open-sea tidal flats of this tropical coast. Similarly, biogenic structures related to the burrowing activity of polychaetes (e.g.Lumbrineris sp.,Nereis sp.) and bubbler crabs (Scopimera sp. andIlyoplax sp.) are the most common features found in the estuarine sediments. Although many features registered in the open-sea tidal flats of the region, such as resting and grazing trails of starfish, ghost crab burrows or agglutinated tubes ofDiapatra sp., are not found in the point bar facies, most of the physical and biogenic structures present in this facies bear a strong resemblance to those found in the open-sea tidal flat environment. This finding advises caution regarding the use of such structures for identifying near-coastal tidal flat environments in the rock record.

Geochemistry of a tidal transgressive heterolithic succession: The Ituzaingó Formation (Middle Miocene), Argentina

Geochemical analysis by means of XRF and ICP-AES was carried out on 35 selected samples. Major and trace elements were determined. They were: CaCO3, P2O5, Fe2O3, TiO2, SiO2, A12O3, CaO, K2O, MnO, Rb, Cs, Ga, Zr, Ta, Nb, Sr, Ba, Cr, Ni, Zn, Pb, Cu, V and B. The country rocks studied were of sandy-muddy lithofacies, belonging to a typical tide marine transgressive heterolithic succession of Middle Miocene age. They are called the Ituzaingo Formation. The formation outcrops in northeastern Argentina on the left of the Parana River Valley. The depositional setting largely depends on the outstanding internal structures and textures (i.e., herringbone cross stratification, tidal bundles, reactivation surfaces). In the study area, loessial friable sediment was used as geochemical background because it overlies shallow marine sediments. The results show significant differences. Boron is absent in loessial sediments. On the other hand, several trace elements such as vanadium and chromium are concentrated in the shallow marine sediments. These results may be useful to discern uncertain beds in the area studied.

Ituzaingó Formation. A key for the interpretation of upper tertiary stratigraphy, Mesopotamia-Chaco Paraná Basin, Argentina

Four years of sedimentological detailled study led me to reinterpretate the Upper Terriry Mesopotamia region stratigraphic squeme. Upon a detailed sedimentary environment analysis of the Mesopotamia – Chaco Paraná basin, I made a different interpretation about previous ideas of the several units present at this morphostructural domain. The Ituzaingo Formation was interpretated as continental (fluvial) in origin by several researchers based on an insecure fossil fauna of invertebrates, always present at the top of the layers. These fossil fauna his a broad and extend biochron over the Cenozoic time. The Paraná Formation infralies the Ituzaingó Formation. The idea of an erosion uncorformity between them was strongly proposed. The Paraná Formation carries marine fossil fauna of invertebrates dated as Middle Miocene. It generally is accepted that Toropí and Yupoí Formations rest over Ituzaingó Formation elsewhere. The structural contact was assumed as an erosional unconformity surface. Paraná Formation was dated by fossil invertebrate fauna as Middle Miocene. Topoí and Yupoí Formations were dated as of Midlle to Upper Pleistocene based on fossil vertebrate fauna. I assumed that the position of these fossils vertebrate fauna of is ofaloctonous for the Toropí and Yupoí muddy formations and I propose that the fossil vertebrate fauna is in Holocene sediments. I carried out detailled studies which include: architectural, granulometric and morphoscopic analysis, petrographic data, paleocurrent measurements, scanning electron microscopy, diffractometric x-ray measurements, detailled sedimentologic profiles, drill hole correlations and remote sensing digital processing image analysis. Architectural analysis me to distinguish the following internal sedimentary structures: hummocky cross stratification, tidal bundles, herringbone cross stratification, lenticular bedding, flaser bedding, bipolar cross stratification and several types of wavy stratifications. These studies led me to realize that the sedimentary environments and specially the relationship between these units are different from previously proposed studies. I propose a common shallow littoral marine origin (intertidal) for all these units in a Middle Miocene age, sincronous with the Paranense Sea ingression (12-14 MA) and an interdigitation between sand and mud lithofacies. A subsurface unit, near Paraná Formation, the Puelches Formatioin, is interpretated as belonging to this sandy-muddy typical shallow marine lithofacies, desoite of a previous work, that accepted a fluvial origin for this unit. An erosion unconformity was also proposed between Paraná and Puelches Formation. The Paraná and Ituzaingó Formations are composed by some 80% friable arenaceous facies and of about 20% of muddy facies. This is a typical heterolithic successioin. A neat but irregular ferricretization forehead is present, specially, in the sandy lithofacies. This physico-chemical phenomena led previous authors to a misinterpretation of the stratigraphic scheme. My results show that the contacts between forehead ferricretization in the sandy lithofacies was assumed as an erosion unconformity, which is not the case. In some place, the ferricretization formed ferrigenous sandstones, which I called ferrigenous duricrust. This is os a very recent age of about Upper Pleistocene-Holocene times. Calcretizatioin is also present in the outcrops. All these secondary processes hid the primary physical structures and chemical composition of the sandy-muddy lithofacies. Paraná, Ituzaingó, Toropí, Yupoí and Puelches Formations are suggested to have the same origin (intertidal) and the same age, Middle Miocene. They constitute a blanket body of about 1500 kilometers in extension at the northeastern portion of Argentina and have approximately 300 meters of thickness. They are composed only by medium, fine to very fine white sands and grey to beige mud lithofacies. When alternation is present (ferricretization), the sediments change its colours to reddish or yellowist, sometimes purple, brown or black. In a few cases the alternation (ferricretization) is very intense, then sands are converted into reddish to black sandstones. In this case, primary sedimentary tractive structures were always recognized. These units, Paraná and Ituzaingó Formations are sincronous and may be correlated with other similar outcrops at Uruguay, Brazil, Paraguay, Bolivia, Perú, Ecuador, Colombia, Venezuela and Guyanas. They were formed during Paranense-Amazon Seas.

Unusual Shoreface Sedimentology in the Upper Jurassic of the Boulonnais, Northern France

ABSTRACT Two Upper Jurassic sandstone formations (the Gres de Chatillon and Gres de la Creche) of the Boulonnais of northern France record shoreline conditions on the eastern margin of a large epicontinental sea--the Wessex Basin. The interpreted range of bedforms is more diverse than has been recorded from other shorelines, either ancient or modern. The lower shoreface to transition zone was characterized by small lunate dunes, oscillatory ripples, mudstone drapes, and discrete burrowed beds, indicating the alternation of moderate currents, oscillatory flow, and prolonged quiescent intervals. The shoreface itself was dominated by dunes that ranged in height from a few decimeters to more than a meter. The larger sizes are twice the height of any shoreface bedforms previously described. Paleocu rent indicators within the shoreface deposits are highly variable from bed to bed and within individual cosets, a feature that may indicate deposition on a barred shoreface. Evidence for tides is care (occasional herringbone cross-stratification). In contrast to many other ancient shoreface sandstone units, storms left little sedimentary record (rare, large, shell gravel bars in the upper offshore zone), although they generated numerous erosion surfaces in the lower shoreface. The unusual aspects of the Boulonnais sandstone units may relate in part to the particular paleogeographic location of the region (an open embayment). Regardless, this is a rare example of a fair-weather wave-dominated, medium sand shoreline from the geological record.

The architecture of precambrian high energy tidal channel deposits: an example from the lyell land group (eleonore bay supergroup), northeast Greenland

Within the 2.5 km thick Late Proterozoic Lyell Land Group of the Eleonore Bay Supergroup in East Greenland, numerous successions of stacked sheet sandstone bodies, 10–70 m thick occur. Each succession consists of several, 5–50 m thick multistorey sand sheets, separated from each other by 0.2–2 m thick heterolithic mudstone, or sandstone beds. Individual sand sheets are 0.2–3.0 m thick. They have highly erosive lower contacts, show a rough fining-upward trend and are capped by a thin mudstone veneer containing desiccation cracks. Dominant sedimentary structures within the sand sheets are sets of planar cross-bedding showing a range of characteristic tidal features, such as successive bundles of foresets, herringbone cross-stratification, ebb and flood caps and numerous reactivation surfaces. Horizontal lamination is abundant and may in some instances comprise more than half of the structures within a sand sheet. The sand sheets represent high energy tidal channels, with the multistorey sand sheets constituting larger tidal channel complexes. The heterolithic beds represent mixed tidal flat sediment, deposited during periods of tidal channel complex abandonment, probably caused by channel aggradation and changing drainage patterns. The tidal channels were wide, shallow and very sandy, with a very poorly developed channel morphology, caused by lack of fine-grained sediments and vegetation. Most likely they evolved within a meso- to microtidal, sandy back-barrier setting where the channels migrated swiftly across tidal flats and reworked most interchannel sediment. An architecture was developed consisting of stacked sand sheets interbedded with rare heterolithic mudstone beds. The model presented incorporates sedimentary parameters and processes characteristic of a pre-vegetative landscape and it has no direct modern analogue.