Mixed Siliciclastic-carbonate Sediments Research Articles

Trace Fossil Assemblages in Mixed Siliciclastic-Carbonate Sediments of the Kaladongar Formation (Middle Jurassic), Patcham Island, Kachchh, Western India

Abstract The Middle Jurassic rocks of the Kaladongar Formation well exposed in the Kaladongar Hill range of the Patcham Island and Kuar Bet of the Northern Kachchh comprises of ∼450 m thick sequence of mixed siliciclastic-carbonate sediments intercalated with shales. These Mixed siliciclastic-carbonate sediments show wide variation in textural and mineralogical composition and represent genetically related six rock types: micritic sandstone, allochemic sandstone, sandy allochemic limestone, micrtic mudrock, sandy micrite and muddy micrite; which are highly bioturbated and show behaviourally diverse groups of trace fossils. Total 34 ichnogenera are identified, which includes, Arenicolites, Asterosoma, Beaconites, Bergaueria, Chondrites, Cochlichnus, Dactylophycus, Daedalus, Didymaulichnus, Diplocraterion, Gordia, Gyrochorte, Gyrolithes, Ichnocumulus, Laevicyclus, Lockeia, Margaritichnus, Monocraterion, Nereites, Ophiomorpha, Palaeophycus, Phoebichnus, Phycodes, Pilichnus, Planolites, Plug Shaped Form, Protovirgularia, Rhizocorallium, Scolicia, Skolithos, Taenidium, Teichichnus, Thalassinoides and Walcottia. These trace fossils are classified into six morphological groups namely, circular and elliptical structures; simple structures; branched structures; rosette structures; spreiten structures; and winding and meandering structures. These trace fossils are further group into eight assemblages which occurred together into mixed siliciclastic-carbonate sediments, include, Asterosoma assemblage, Gyrochorte assemblage, Rhizocorallium assemblage, Thalassinoides assemblage, Planolites-Palaeophycus assemblage, Phycodes assemblage, Ophiomorpha assemblage and Skolithos assemblage. The recurring pattern of these assemblages through the sequence displays the development of Skolithos and Cruziana ichnofacies and at places the mixed Skolithos-Cruziana ichnofacies which suggest a low wave and current energy conditions with intervening period of high wave and current energy conditions and an intermediate period of stressful environments, respectively. Sedimentological and ichnological data suggest that the deposition of the mixed siliciclastic-carbonate sediments of the Kaladongar Formation took place in the foreshore to offshore environment under fluctuating wave and current energy condition.

Paleoenvironmental framework of dinosaur tracksites and other ichnofossils in Early Cretaceous mixed siliciclastic-carbonate deposits in the Neuquén Basin, northern Patagonia (Argentina)

The study of the uppermost section of the Early Cretaceous Agrio Formation in northern Patagonia (Neuquén Basin) where dinosaur tracks assigned to cf. Therangospodus pandemicus are exposed (tracksites I and II) evidence mixed marginal marine siliciclastic-carbonate deposits. The succession was divided in two intervals. A lower one containing theropod tracks, recorded on top of subtidal oolithic limestones with tiny wave ripples suggesting shoreline fluctuations and subaerial exposure. Tidal influence is recognised by fining upward and prograding cycles starting with subtidal carbonates and ending with fine-grained siliciclastic deposits at the top, or rarely laminites. Dolomitization affects subtidal deposits generated in an alkaline media stressful for tracemakers. Intertidal facies include abundant heterolithic deposits, coquinas composed of gastropods encrusted by multilayered bryozoans and muddy levels with incipient mud cracking. Invertebrate ichnofossils recognized from tidally dominated deposits include Arenicolites, Kouphichnium, and Rhizocorallium. The upper interval is a transgressive–regressive cycle that starts with dark shales, deficiently oxygenated, and covered by prograding sandstones and finally sand flat deposits. This interval contains Gyrochorte, Hillichnus, and Ophiomorpha documented in wave-influenced sandstones. Dinosaur tracks as well as Hillichnus, attributed to tellinoid bivalves, and Kouphichnium assigned to xiphosurans, imply the activity of producers rarely recorded previously as body fossils in marginal marine deposits of southern South America. Previous paleogeographic schemes are questioned by our analysis, which shows evidence of extremely shallow and tide-controlled sedimentation, sometimes with subaerial exposure, with high cyclicity related to a marginal marine depositional setting and lack of significant erosion by the overlying unit, as traditionally was suggested.

Acoustic facies and seabed features of the mixed carbonate-siliciclastic deposits of the last eustatic cycle in the La Maddalena Archipelago (North Sardinia, Italy)

A high-resolution Chirp acoustic survey of 250 nautical miles performed in the La Maddalena Archipelago provided a scheme of the acoustic facies of the recentmost sediments and a frame of the large-scale bedforms linked to the last eustatic cycle that charac - terize its channels. Five main acoustic facies have been recognized and mapped. Due to the good coherence with the sediment texture map of the area, an interpretation of the acoustic facies in sedimentary terms is presented. The archipelago is characterized by a complex system of inlets, promontories, bays and channels ruled by the main tectonic trends of the region. A main, 20 km-long channel (Bucinara Channel) with numerous straits and some shoals divides the northern Sardinia coast from the main islands of Spargi, La Maddalena, Santo Stefano and Caprera. A shorter and larger channel occurs in the northern part of the Archipelago. Two minor sea branches connect these channels. The bathymetry of the area reveals that the channels generally have a flat bottom and steep flanks, with water depth mostly ranging between –35 and –60 m, with the exception of saddles and shoals. The sedimentary regime of the area is controlled by storm-induced currents and by a high carbonate production associated to the widespread presence of the Posidonia Oceanica seagrass, a marine Phanerogama that populates the flanks of the channels of the Archipelago. A multitude of morphologic forms can be attributed to the phases of falling, lowstanding, rising and high-standing of the last Pleistocene-Holocene eustatic cycle. Angular unconformities, gullies and sediment-incised palaeo channels along with abrasion platforms in the granitic basement witness the last sea level fall and low-stand. These forms now lie at water depths ranging between –33 m and –60 m and are covered by a thin veneer of sediments of the Holocene transgression. They have all been formed when the sea-level was about –110 m below the present datum and the Archipelago was a landscape of rocky hills separated by large, flat valleys scoured by torrent-like rivers. The Holocene rise of sea-level (Versilian transgression) caused the flooding of the landscape and the deposition of the thin sedimentary cover imaged by the acoustic profiles. The rising was punctuated by some sea-level still stands evidenced by terraced surfaces lying at depths between –33 m and –47 m which are coherent with the mapped and dated beach-rocks of the Bocche di Bonifacio strait. The decrease of sea level rise at 5000-6000 years B.P. and the following stabilization of sea level have triggered the present sedimentary and hydrodynamic conditions which, in turn, favoured the formation and maintenance of the bedforms. The Archipelago is dominated by an eastward directed sea current, and subordinately by a westward current. In the two main WNW-ESE and W-E trending channels of the Archipelago, the current flows have formed some longitudinal sand ridges up to 3 km long and 5 m high, while in the N-S oriented channels, transversal sand banks up to 10 m thick occur. Sea floor undulations, interpreted as sand waves, are present in some restricted areas close to the Sardinia coast in downflow position with respect to the headlands. Their wavelength (L) ranges between 10 to 40 m and the height (H) is up to 1,5 m, the ratio H/L

Recent foraminiferal assemblages from mixed carbonate-siliciclastic sediments of southern Somalia and eastern Kenya

A total of 256 recent benthic foraminiferal species belonging to 111 genera is identified from 73 carbonate-siliciclastic sedimen samples collected along the coast of southern Somalia (Burgao channel) and eastern Kenya (Lamu Archipelago). This represents the first survey of recent foraminifers in this region of East Africa. The sediment samples are representative of different environments (e.g., mangrove flats, tidal channels, restricted shelf, and open shelf), water depths (0-60 m), and salinities (strongly brackish to normal marine conditions). Q-mode Hierarchical Cluster Analysis reveals that nine assemblages can be distinguished; for each assemblage, faunal composition and distribution, diversity indices and dominance are assessed. As predicted by ecological models, there is an overall trend of increasing species diversity from the backwater zone within channels to open marine conditions, and from intertidal to subtidal settings. Salinity, suspended sediment, nutrient levels, and tidal exposure are the most influential factors in determining benthic foraminiferal distribution patterns. An interesting feature is the nearshore contraction of the depth gradient determining the shallower distribution of several larger foraminiferal species, as evidenced by the depth ranges of five species of Amphistegina

Palaeoenvironmental and palaeoecological change on the northern Tethyan carbonate platform during the Late Barremian to earliest Aptian

AbstractA major shift from Urgonian oligotrophic carbonate accumulation to orbitolinid‐rich mixed siliciclastic–carbonate deposition is observed near the Barremian–Aptian boundary in many sections both within and outside the shallow‐marine Tethyan Realm. This important facies change in the Swiss Helvetic Alps is documented here and interpreted in the context of general palaeoenvironmental change. To achieve this, a detailed micropalaeontological, sedimentological, mineralogical and geochemical study has been carried out on six sections across the upper part of the lower Schrattenkalk Member (Late Barremian), the Rawil Member (formerly ‘Lower Orbitolina Beds’, earliest Aptian) and the lowermost part of the upper Schrattenkalk Member (Early Aptian). The sediments of the Rawil Member exhibit inner‐platform facies with rudists, miliolids, orbitolinids and dasycladals to outer‐platform facies characterized by small benthic foraminifera, orbitolinids, crinoids and bryozoans. Stratigraphic trends in microfacies environments and the composition of microfossil assemblages, indicate that the Rawil Member includes a transgressive systems tract and the base of a highstand systems tract which are composed of an increasing number of parasequences in distal directions (five to nine in the sections studied here). The sea‐level rise discerned in the Rawil Member is coeval with increased detrital input and phosphorus burial, with maximum values up to 80 times and 21 times the background values in the subjacent part of the lower Schrattenkalk Member, respectively. Furthermore, the Rawil Member records the appearance of kaolinite, indicating a change towards tropical and more humid climate conditions. This change may have led to an increase in continental weathering rates and an associated increase in detrital and nutrient fluxes towards the ocean. The phase of climate change observed near the Barremian–Aptian boundary may have been triggered by a phase of intensified volcanic activity linked with the onset of the Ontong Java large igneous province and the Rawil Member may be the expression of a precursor episode to Oceanic Anoxic Event 1a in the shallow‐marine environment.

Late Neogene chronology and sequence stratigraphy of mixed carbonate-siliciclastic deposits of the Cibao Basin, Dominican Republic

A stratigraphic study of late Miocene–Pliocene mixed carbonate and siliciclastic sediment (upper part of the Yaque Group) was conducted in northern Hispaniola to refine the chronostratigraphy of uplifted, nearshore marine sequences in the Cibao Basin. Classic late Neogene sections in these sequences include a spectacularly preserved faunal record for the northern Caribbean. Refined ages of the Cercado Formation (6.6–6.0 Ma), the Gurabo Formation (ca. 5.8–4.0 Ma), and the Mao Formation (3.9–3.5 Ma) allow correlation to regional and global sea-level events. Each of the three formations corresponds to one depositional sequence, composed of mixtures, at different proportions, of skeletal debris, mostly coral, and mud-size (silt) siliciclastic sediment. Distinct stratal geometries and lithofacies individualize the three formations, each separated by pebble conglomerates. The Cercado Formation depositional sequence consists of prograding inner-shelf siliciclastic sands and patch reefs. The basal transgressive unit of the Gurabo Formation is made of seaward-prograding low-angle clinothems of bedded and burrowed siliciclastic sand with molluscs and free-living corals. The upper Gurabo unit consists of massive middle-shelf clinothems of silt with bedded reef debris and in situ branching coral thickets. These clinothems grade basinward into massive, deeper-water silt deposits at the shelf edge and basin slopes. The Mao Formation sequence records the transition from lowstand fluvial-deltaic facies to transgressive and highstand marine clinothems made mostly of coral debris in a silt matrix. In the Mao Formation outcrops, the coral-silt clinothems can be divided into at least 10 high-frequency cycles (40 k.y.?) based on lithofacies. Channelized and slumped coarse siliciclastic sand, gravel, and nonmarine alluvium cap the sequence, likely associated with lowered sea level starting at ca. 3.3 Ma and perhaps the onset of regional uplift. The major lithologic and sequence geometry changes in the Cibao Basin correlate with trends in the oxygen isotope ice-volume proxy and likely reflect deposition responding to major periods of marine transgression and highstand. An especially prominent feature is the major deepening and margin back step near the Miocene–Pliocene boundary, continuing in the early Pliocene, which resulted in the deposition of the Gurabo Formation. Transgression and sea-level highstand in the late–early Pliocene (3.9–3.5 Ma) produced distinctive margin progradation primarily by coral debris. This progradation event was synchronous with progradation in the nearby western Great Bahama Bank. The different stratal configurations of the three sequences can be attributed to a combination of factors, including: (1) shelf morphology and location on the shelf, (2) paleoceanographic conditions such as warm sea-surface temperatures and reduced upwelling, which controlled coral productivity and reef geometry, and (3) the influx of siliciclastic mud from the adjacent mountains.

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province, China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

AbstractTo understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform.

Mixed glauconitic-carbonate-siliciclastic surficial sediments on the north Kaipara continental margin, northwestern North Island, New Zealand

A mosaic of siliciclastic and mixed carbonate-siliciclastic sediments and authigenic minerals occurs at shelf and slope depths (30–1015 m water depth) on the open, wave-dominated north Kaipara continental margin (NKCM) off northern New Zealand. Texture and composition define five surficial sediment facies. Facies 1 (siliciclastic sand) comprises generally well-sorted fine sands that extend to outer shelf depths. Facies 2 (glauconitic sand) is composed of 30–95% authigenic glauconite grains at 150–400 m water depth in central to northern portions of NKCM. Facies 3 (mixed bryozoan-siliciclastic sand) occurs only in northernmost NKCM and involves a conspicuous (>40%) bryozoan carbonate content. Facies 4 (pelletal mud) occupies the mid shelf (100–150 m water depth) in northern NKCM and consists of muddy sediment with >30% mixed carbonate-siliciclastic pellets of probable fecal origin. Facies 5 (foraminiferal mud and sand) contains >30% foraminiferal tests at slope depths in southern NKCM and at both slope and mid-outer shelf depths towards the north. The siliciclastic mineralogy is consistent with mainly distant provenances to the south of the NKCM in central North Island and northern South Island, while much of the bryozoan material is likely reworked from the Three Kings carbonate platform to the north. No single shelf sedimentation model explains the complex facies distributions on the NKCM because the deposits record the interplay of several present and past hydrodynamic and sediment supply controls. While truly modern deposits may occur inshore, the bulk of NKCM surficial sediments are mainly palimpsest and/or relict deposits.

Role of tectonic-sedimentary melange and Permian–Triassic cover units, central southern Turkey in Tethyan continental margin evolution

Melanges play a key role in the interpretation of orogenic belts, including those that have experienced deformation and metamorphism during continental collision. This is exemplified by a Palaeozoic tectonic-sedimentary melange (part of the Konya complex) that is exposed beneath a regionally metamorphosed carbonate platform near the city of Konya in central Anatolia. The Konya complex as a whole comprises three units: a dismembered, latest Silurian–Early Carboniferous carbonate platform, a Carboniferous melange made up of sedimentary and igneous blocks in a sedimentary matrix (also known as the Halıcı Group or Sızma Group), and an overlying Volcanic-sedimentary Unit (earliest Permian?) . The Palaeozoic carbonates accumulated on a subsiding carbonate platform that bordered the northern margin of Gondwana, perhaps as an off-margin unit. The matrix of the melange was mainly deposited as turbidites, debris flows and background terrigenous muds. Petrographic evidence shows that the clastic sediments were mostly derived from granitic and psammitic/pelitic metamorphic rocks, typical of upper continental crust. Both extension- and contraction-related origins of the melange can be considered. However, we interpret the melange as a Carboniferous subduction complex that formed along the northern margin of Gondwana, related to partial closure of Palaeotethys. Blocks and slices of Upper Palaeozoic radiolarian chert, basic igneous rocks and shallow-water carbonates were accreted and locally reworked by gravity processes. Large (up to km-sized) blocks and slices of shallow-water limestone were emplaced in response to collision of the Palaeozoic Carbonate Platform with the subduction zone. The overlying Volcanic-sedimentary Unit (earliest Permian?) comprises alkaline lava flows, interbedded with volcaniclastic debris flows and turbidites, volcanogenic shales and tuff. The complex as a whole is overlain by shallow-water, mixed carbonate–siliciclastic sediments of mainly Late Permian age that accumulated on a regional-scale shelf adjacent to Gondwana. Successions pass transitionally into Lower Triassic rift-related shallow-water carbonates and terrigenous sandstones in the southwest of the area. In contrast, Triassic sediments in the southeast overlie the melange unconformably and pass upwards from non-marine clastic sediments into shallow-marine calcareous sediments of Mid-Triassic age, marking the base of a regional Mesozoic carbonate platform. During the latest Cretaceous–Early Cenozoic the entire assemblage subducted northwards and underwent high pressure/low temperature metamorphism and polyphase folding as a part of the regional Anatolide unit.

Petrographical and geochemical evidences for paragenetic sequence interpretation of diagenesis in mixed siliciclastic–carbonate sediments: Mozduran Formation (Upper Jurassic), south of Agh-Darband, NE Iran

The Upper Jurassic Mozduran Formation with a thickness of 420 m at the type locality is the most important gas-bearing reservoir in NE Iran. It is mainly composed of limestone, dolostone with shale and gypsum interbeds that grade into coarser siliciclastics in the easternmost part of the basin. Eight stratigraphic sections were studied in detail in south of the Agh-Darband area. These analyses suggest that four carbonate facies associations and three siliciclastic lithofacies were deposited in shallow marine to shoreline environments, respectively. Cementation, compaction, dissolution, micritization, neomorphism, hematitization, dolomitization and fracturing are diagenetic processes that affected these sediments. Stable isotope variations of δ18O and δ13C in carbonate rocks show two different trends. High depletion of δ18O and low variation of δ13C probably reflect increasing temperatures during burial diagenesis, while the higher depletion in carbon isotope values with low variations in oxygen isotopes are related to fresh water flushing during meteoric diagenesis. Negative values of carbon isotopes may have also resulted from organic matter alteration during penetration of meteoric water. Fe and Mn enrichment with depletion of δ18O also supports the contention that alteration associated with higher depletion in carbon isotope values with low variations in oxygen isotopes took place during meteoric diagenesis. The presence of bright luminescence indicates redox conditions during precipitation of calcite cement.

Components, facies and ramps: Redefining Upper Oligocene shallow water carbonates using coralline red algae and larger foraminifera (Venetian area, northeast Italy)

Chattian mixed carbonate-siliciclastic deposits from two palaeogeographic units of Southern Alps, the Lessini Shelf and Venetian foreland basin of northeast Italy, are studied with respect to facies distributions and controlling ecological factors on dominant biogenic components. These ramp depositional systems are located in the western Southern Alps on the Lessini Shelf, and in the eastern Southern Alps in the Venetian foreland basin. The successions show an extensive geographic distribution of at least 150 km × 40 km at the southern margin of the Southern Alps. Palaeoenvironments are inferred following microfacies characteristics and floral and faunal indicators, especially those of coralline red algae and larger foraminifera. An inner- to middle homoclinal ramp morphology is reconstructed based on facies distributions and taxonomic and coralline algal growth-form proxy data. The distribution along depositional strike shows variation from thinner, carbonate dominated sequence in the west (Monti Berici and Monti Lessini) to a more variable, mixed carbonate-siliciclastic dominated environment to the east (Vittorio Veneto, Alpago). Facies character and occurrence are controlled by local hydrodynamic conditions and varying water turbidity of the studied ramp depositional system. The general facies succession shows a transgressive sequence from the proximal inner- to proximal middle ramp. A shoal belt of trough cross-bedded sandstones occurs in a proximal inner ramp position in all of the studied areas. In a gentle-dipping homoclinal ramp, most benthic organisms inhabited the relatively quite environments off these shoals in a distal inner ramp setting. Basinward environments (proximal middle ramp) constitute areas of maximal carbonate production and consist primarily of larger foraminiferal facies and rhodolith pavements. Larger foraminifera, represented by two larger foraminiferal assemblages, are most diversified in the shallower settings (near the fair-weather wave base, distal inner/proximal middle ramp), whereas coralline algae predominate in deeper areas (proximal middle ramp) constructing extensive rhodolith pavements. Rhodolith pavements are distinguished with respect to rhodolith shape, size, growth-forms and coralline algal taxonomic composition. Larger foraminifera are interpreted with respect to shell structures, shell architectures and comparable Recent counterparts. The analysed benthic communities thrived in a mesotrophic regime within trophic gradients constrained by fluvial influence. Coralline algal dominance and a moderately diverse larger foraminiferal assemblage suggest a tropical/warm temperate transition. The considerable stability imposed on sediments by the rhodolith pavement must also have significantly contributed to the maintenance of a middle ramp profile bulge. This study will contribute to a better understanding of Oligocene carbonates which have received less attention than their Eocene or Miocene counterparts.

PLEISTOCENE ROLLING STONES OR LARGE BRYOZOAN NODULES IN A MIXED SILICICLASTIC-CARBONATE ENVIRONMENT (RHODES, GREECE)

Spectacular accumulations of abundant bryozoan nodules (bryoliths) occur in the early Pleistocene coarse-grained, mixed carbonate-siliciclastic deposits of the Cape Arkhangelos Calcarenite of Rhodes, Greece. These bryoliths are always associated with numerous bivalves (mostly ostreids and pectinids) and serpulid worms, together with other rarer skeletal organisms. The spheroidal bryoliths vary in diameter from 1 cm to 16 cm, and those with a more ellipsoidal shape range from 0.8 cm to 22 cm in length and from 0.7 cm to 16 cm in width; an elongated bryolith of exceptional size (40 cm) was even found. The bryoliths are composed almost exclusively of the anascan cheilostome Calpensia nobilis and are nucleated on gravels, pebbles, or bivalve shells. After settlement of bryozoan crusts, the bryoliths increased in size by repeated overturning of these mobile hard substrates by longshore currents. The bryoliths developed at depths of 20–40 m on a flat shoal between emerged areas where bottom currents episodically accelerated during storms. The currents also transported the bryoliths, together with other skeletal elements, into deeper settings (40–80 m) where they could continue their growth, upward or downward, or be colonized by other organisms.

High-Frequency Rhythmicity in a Mixed Siliciclastic-Carbonate Shelf (Late Miocene, Guadix Basin, Spain): A Model of Interplay Between Climatic Oscillations, Subsidence, and Sediment Dispersal

Abstract Rhythmites in pelagic sediments can include information on paleoclimate and with orbital cycles. This paper documents and interprets an unusual rhythmic succession from a cool-water shallow-marine shelf that includes features of Mediterranean-type ramps and open-ocean-type ramps. In the subsiding southwestern margin of the Guadix Basin (southern Spain, Betic Cordillera), during the late Tortonian, a 640-m-thick succession was deposited over a short period (7.8–7.2 Ma). This succession contains 30 sedimentary packages or rhythmites, each defined by two facies associations: a mixed cross-stratified siliciclastic–skeletal sandstone and a homogeneous marl rich in planktonic foraminifera. Sandstones ranging in thickness from 3 m to 20 m contain coralline algae, bivalves, bryozoans, benthic foraminifera, echinoids, and brachiopods (typical temperate-water shelf biofacies). These units exhibit cross-strata sets up to four meters high, interpreted to have been produced by strong unidirectional currents. The marly deposits range in thickness from 2 to 17 m and contain an association of cold-water planktonic foraminifera as well as thin sandstone layers that exhibit normal grading corresponding to typical incomplete Bouma sequences or a lower division with planar lamination and an upper division of very fine- to fine-grained sandstone with hummocky and swaly cross-stratification. On the basis of the sedimentary structures, the sandstone layers are interpreted as sporadic turbidity flows and storm ebb–surge deposits. On the basis of the sedimentary structures, the hydrodynamic conditions during deposition of the mixed siliciclastic–carbonate interval were high-energy. Based on sedimentary and biogenic features of the rhythmites and their temporal durations (ca. 20 ka), rhythmicity is interpreted to reflect alternating cool-wet and cold-dry climate episodes driven by precession orbital cycles. This occurrence of alternating mixed carbonate–siliciclastic sedimentation and pelagic sediments represents a unique occurrence in the geologic record.

Early Middle Frasnian platform reef strata in the Moravian Karst interpreted as recording the atmospheric dust changes: the key to understanding perturbations in the punctata conodont zone

Development of new stratigraphic techniques has led to better understanding of the compositional variability and quantity of dust-related impurity components in pure marine limestones, with a considerable impact on stratigraphic resolution and explanation of causality. The early Middle Frasnian (E-MF, punctata Zone) stratigraphic interval was selected for this study in order to assess the contribution of atmospheric dust and to distinguish between the robust climatically driven anomalies and the potential distant effect of the Alamo impact. The location of the study in the Moravian Karst area has two advantages: there are no mixed carbonate-siliciclastic sediments, only the pure limestone of a platform reef complex, and the authors have access to voluminous survey and drilling reports. The methodology employed for undertaking the most recent research was based on succession and combination of the following steps: biostratigraphy and facies analysis, magnetic susceptibility (MS), gamma-ray spectrometry (GRS), instrumental neutron activation analysis (INAA), and finally, separation and assessment of rare non-carbonate particles. The natural atmospheric dust burden, deposition, and embedding in pure carbonate traps were, most likely, threefold higher during the Frasnian in the Upper Devonian in comparison with Quaternary platform reef counterparts. Variation of 10-100 ka averages is medium in terms of the Frasnian ranges of MS-GRS values, and the base and top of the E-MF interval are manifested by robust elevations of these values. Forced anomalies in MS, GRS and geochemical signals that defy the normal sequence of rhythms were found: two anomalies disturb the broad middle part of the E-MF interval and one is superimposed on the upper part of the punctata-hassi zones strata. The most significant disturbance was found near the mid-punctata Zone level. In spite of the relatively low magnitude, it shows features that are usually related to major environmental crises which occured in the Devonian, such as the Chotec, Kacak or Kellwasser events. These include a large depression in MS and dust-particle concentration values coupled with a period of sea level lowstand and calm atmospheric conditions (stage A), and an abrupt shift to high MS that has a comb-like pattern that gradually fades upward, developed together with a forced flooding surface, increased detritism, and stormy conditions (stage B). It is a reverse of the normal cyclicity in pure limestone when the high impurity corresponds to lowstands. A very small amount of particulate material of an exotic nature was found at the A-B stage interface and assessed: it contained iron-rich silicate microspherules and drops, devitrified glasses, tiny mineral/rock clasts of hyperbasite compositions, as well as pellets and clumps of glasses and phlogopites. The silt-sized particles show ablated and striated surfaces, flow deformation of devitrified glasses, and layered onion-like structures. Iron-rich lamellae with meshwork crystal patterns occur among decrepitated crystalline mineral phases. This material is tentatively attributed to some vigorous ejection of deep Earth layers, or speculatively, to some previously undescribed olivine-phyric to basaltic and Ni-, Cr-depleted siderolite material of meteoritic origin.

PERMIAN CRINOIDS FROM THE SAIWAN AND KHUFF FORMATIONS,SOUTHEASTERN OMAN

Early Permian (Sakmarian) crinoids are described from six horizons in the Saiwan Formation of the Huqf area of southeastern Oman. Each horizon yielded only one to three taxa. All specimens lived in a shallow water environment characterized by mixed siliciclastic or siliciclastic-carbonate sedimentation. Disarticulated arm plates of an indeterminate articulate crinoid are described from a tempestite bed in the Khuff Formation (Wordian) of the Haushi area. The Saiwan Sakmarian crinoids are some of the earlier, although not the earliest Permian crinoids known. Representatives of both primitive (sycocrinids) and advanced (scytalocrinids, blothrocrinids, texacrinids, and cromyocrinids) cladids and articulates are present in the Saiwan crinoids that are dominated by advanced dendrocrinids and belong to families previously reported from the Carboniferous or Permian. They show greatest affinity to the late Sakmarian/early Artinskian fauna from the Callytharra Formation of Western Australia. New taxa described herein double the number of genera (three) previously reported from the same area. The diversity of the Saiwan crinoids is relatively low when compared to the large faunas reported from Timor, Western Australia, Russia, and North America. However, they show affinity with each of the larger faunas. New taxa described are Coeliocrinus arenaceous n. sp., Moapacrinus ? omanensis n. sp., Huqficrinus biserialis n. gen. and sp., and Campbellicrinus nodosus n. sp.

Carbonates within a Pleistocene glaciomarine succession, Yakataga Formation, Middleton Island, Alaska

AbstractUplifted during the 1964 Alaskan earthquake, extensive intertidal flats around Middleton Island expose 1300 m of late Cenozoic (Early Pleistocene) Yakataga Formation glaciomarine sediments. These outcrops provide a unique window into outer shelf and upper slope strata that are otherwise buried within the south‐east Alaska continental shelf prism. The rocks consist of five principal facies in descending order of thickness: (i) extensive pebbly mudstone diamictite containing sparse marine fossils; (ii) proglacial submarine channel conglomerates; (iii) burrowed mudstones with discrete dropstone layers; (iv) boulder pavements whose upper surfaces are truncated, faceted and striated by ice; and (v) carbonates rich in molluscs, bryozoans and brachiopods. The carbonates are decimetre scale in thickness, typically channellized conglomeratic event beds interpreted as resedimented deposits on the palaeoshelf edge and upper slope. Biogenic components originated in a moderately shallow (ca 80 m), relatively sediment‐free, mesotrophic, sub‐photic setting. These components are a mixture of parautochthonous large pectenids or smaller brachiopods with locally important serpulid worm tubes and robust gastropods augmented by sand‐size bryozoan and echinoderm fragments. Ice‐rafted debris is present throughout these cold‐water carbonates that are thought to have formed during glacial periods of lowered sea‐level that allowed coastal ice margins to advance near to the shelf edge. Such carbonates were then stranded during subsequent sea‐level rise. Productivity was enabled by attenuation of terrigenous mud deposition during these cold periods via reduced sedimentation together with active wave and tidal‐current winnowing near the ice front. Redeposition was the result of intense storms and possibly tsunamis. These sub‐arctic mixed siliciclastic‐carbonate sediments are an end‐member of the Phanerozoic global carbonate depositional realm whose skeletal attributes first appeared during late Palaeozoic southern hemisphere deglaciation.

Densely packed concentrations of sessile barnacles (Cirripedia: Sessilia) from the Early Pliocene of SE Spain

Sessile barnacle assemblages, dominated by Concavus concavus (Bronn) and Balanus perforatus Bruguiere, are very abundant in the Lower Pliocene deposits of the Almeria-Nijar and Carboneras basins (southeastern Spain). They occur in shallow-coastal siliciclastic and mixed siliciclastic-carbonate sediments, forming dense concentrations in two contexts: (1) sheltered shallow-marine depressions and (2) the mouth of distributary channels feeding a delta lobe. Extensive colonization took place during periods of quiescence with a high nutrient and food supply inducing the formation of hummocks. Crowding of high morphotypes was presumably triggered by a high larval supply and recruitment rate. The barnacles are exceptionally well preserved, often as in situ clusters, many with opercula, and include a range of ontogenetic stages with some specimens preserving original colouration. Good preservation is probably due to sudden burial without further reworking.

Depositional environment of Upper Carboniferous – Lower Permian beds in the Karavanke Mountains (Southern Alps, Slovenia)

Late Paleozoic rocks were studied in detail in the Dovžanova soteska section. The Upper Carboniferous sedimentary succession, correlated with upper part of Auernig and Schulterkofel Fm. in the Carnic Alps, indicates cyclic clastic-carbonate deposition in a coastal to shallow marine ramp setting with strong influence of coarse-grained fluvial-deltaic siliciclastics from the hinterland, storm dominated regime of nearshore sediments, and offshore algal buildups. The Lower Permian sequence is developed differently from its time equivalent Grenzland Fm. and is subdivided into Dovžanova soteska Fm., Born Fm., and Rigelj beds. It is marked by the formation of a reef mound on the platform margin. Open-marine inner platform close to the marginal shoals represented the depositional environment of the mixed carbonate-siliciclastic sediments. Thus, a platform evolution from a ramp into a rimmed shelf is suggested.

Análise estratigráfica das seqüências neoproterozóicas da Bacia do São Francisco

A stratigraphical analysis was performed under the principles of Sequence Stratigraphy on the neoproterozoic sedimentary sequences of the Sao Francisco Basin (Central Brazil). Three periods of deposition separated by unconformities were recognized in the Sao Francisco Megasequence: (1) Sequences 1 and 2, a cryogenian glaciogenic sequence, followed by a distal scarp carbonate ramp, developed during stable conditions, (2) Sequence 3, a Upper Cryogenian stack homoclinal ramps with mixed carbonate-siliciclastic sedimentation, deposited under a progressive influence of compressional stresses of the Brasiliano Cycle, (3) Sequence 4, a Lower Ediacaran shallow platform dominated by siliciclastic sedimentation of molassic nature, the erosion product of the nearby uplifted thrust sheets. Each of the carbonate-bearing sequences presents a distinct δ 13 C isotopic signature. The superposition to the global curve for carbon isotopic variation allowed the recognition of a major depositional hiatus between the Paranoa and Sao Francisco Megasequences, and suggested that the glacial diamictite deposition (Jequitai Formation) took place most probably around 800 Ma. This constrains the Sao Francisco Megasequence deposition to the interval between 800 and 600 Ma (the known ages of the Brasiliano Orogeny defines the upper limit). A minor depositional hiatus (700−680 Ma) was also identified separating sequences 2 and 3. Isotopic analyses suggest that from then on, more restricted environmental conditions were established in the basin, probably associated with a first order global event, which prevailed throughout deposition of the Sequence 3.

Análise estratigráfica das seqüências neoproterozóicas da Bacia do São Francisco

A stratigraphical analysis was performed under the principles of Sequence Stratigraphy on the neoproterozoic sedimentary sequences of the São Francisco Basin (Central Brazil). Three periods of deposition separated by unconformities were recognized in the São Francisco Megasequence: (1) Sequences 1 and 2, a cryogenian glaciogenic sequence, followed by a distal scarp carbonate ramp, developed during stable conditions, (2) Sequence 3, a Upper Cryogenian stack homoclinal ramps with mixed carbonate-siliciclastic sedimentation, deposited under a progressive influence of compressional stresses of the Brasiliano Cycle, (3) Sequence 4, a Lower Ediacaran shallow platform dominated by siliciclastic sedimentation of molassic nature, the erosion product of the nearby uplifted thrust sheets. Each of the carbonate-bearing sequences presents a distinct δ 13 C isotopic signature. The superposition to the global curve for carbon isotopic variation allowed the recognition of a major depositional hiatus between the Paranoá and São Francisco Megasequences, and suggested that the glacial diamictite deposition (Jequitaí Formation) took place most probably around 800 Ma. This constrains the São Francisco Megasequence deposition to the interval between 800 and 600 Ma (the known ages of the Brasiliano Orogeny defines the upper limit). A minor depositional hiatus (700−680 Ma) was also identified separating sequences 2 and 3. Isotopic analyses suggest that from then on, more restricted environmental conditions were established in the basin, probably associated with a first order global event, which prevailed throughout deposition of the Sequence 3.