Transgressive-regressive Sequences Research Articles

Devonian-Carboniferous boundary beds in the Kuz-Yama section (Northern Cis-Urals) - Conodonts, ostracods, and carbon isotope record

The Devonian-Carboniferous boundary interval is characterized by biotic crisis, global transgressive-regressive sequences and carbon cycle change, probably caused by climatic changes and volcanic activity. This interval attracts the attention of researchers worldwide in relation to selecting a new Devonian-Carboniferous boundary level. However the Northern Cis-Urals region remains poorly studied in this respect. This study focuses on the Kuz-Yama key section (Northern Cis-Urals) and contributes to our knowledge of the regional expression of the Devonian-Carboniferous boundary in northeastern Europe. The study of the Kuz-Yama section (Verkhnyaya Pechora Trough) involved an examination of the calciturbidites, hemipelagic shales and carbonates that constitute the poorly constrained uppermost Famennian to lower Tournaisian deep-water succession. The conodont and ostracod associations are poorly diversified. The presence of Polygnathus zikmundovae, in association with a positive δ13Ccarb excursion (up to 3.9 ‰), marks the base of the Late praesulcata conodont Zone (=base of the Protognathodus kockeli conodont Zone) in the lower part of the succession. The Devonian-Carboniferous boundary is indicated by the appearance of Siphonodella sulcata, that coincides with a positive shift in the decoupling of δ13Ccarb and δ13Ccon values, probably caused by a global decrease in CO2 associated with the regional crisis within pelagic ecosystems. The sulcata conodont Zone, bounded by the first occurrence of Siphonodella sulcata at the bottom and Siphonodella bransoni at the top of the section, contains Tournaisian ostracods. The ostracods compose mixed associations of the shallow-water Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris ostracod Zone and the deep-water Armilla uralica ostracode Zone. The co-occurrence of shallow-water and deep-water conodont and ostracod taxa in the Tournaisian part of the section enhances the correlation of the shallow-water and deep-water conodont and ostracod zonations. The base of the shallow-water Siphonodella ludmilae conodont Zone roughly correlates with the base of the deep-water bransoni Zone. The base of Armilla uralica deep-water ostracod Zone in the Kuz-Yama section lies within the Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris shallow-water ostracod Zone. The following sequence of events can be recognised in the Kuz-Yama section: first occurrence of Polygnathus zikmundovae Zhuravlev (approximately corresponding to the base of the Protognathodus kockeli Zone), positive shift in the inorganic δ13C (= Hangenberg carbon isotope excursion), latest Famennian regression, negative shift in the δ13C of conodont elements just below the base of the sulcata Zone, entry of the Tournaisian transgression, positive shift in decoupled inorganic and conodont δ13C near the base of sulcata Zone, positive shift in inorganic δ13C in the sulcata Zone, appearance of the Tournaisian ostracods, first occurrences of the conodonts Siphonodella bransoni Ji and S. ludmilae Zhuravlev & Plotitsyn (= base of the bransoni Zone). In general, this sequence corresponds to that identified around the Devonian-Carboniferous boundary in other regions.

Sedimentary evolution and distribution of benthic fauna of an Aptian protected bay (Oliete Subbasin, Spain)

Detailed facies and sequence stratigraphic analysis of an Aptian shallow-marine succession illustrates the evolution of sedimentary environments and their benthic faunal associations in a marginal bay developed in northeastern Iberia (Maestrazgo Basin, Oliete Subbasin). In this bay, marine circulation was hampered by wide stationary shoals located in the low-subsidence transitional areas between subbasins. The Aptian sedimentary record of this bay (Josa and Oliete formations) is composed of four transgressive–regressive (T–R) sequences bounded by major unconformities generated during relative sea-level falls. The Josa Fm (formally defined herein) encompasses an asymmetrical T–R cycle (sequence J). It comprises transgressive prodelta clays with episodic orbitolinid blooms, grading upwards to regressive freshwater-influenced littoral and delta-front sandstones and sandy limestones which yield diverse bivalve communities dominated by trigoniids. The Oliete Fm encompasses three T–R cycles (sequences O1–O3), mostly deposited in a protected lagoon. The transgressive hemicycle of each sequence is dominated by marls containing low-diversity faunal associations composed of ostracods and mm-sized gastropods and bivalves, indicating stressed (probably dysoxic) conditions in the deepest parts of the lagoon, related to a lack of water circulation. The regressive hemicycle of sequences O1 and O2 is characterised by low-energy intertidal to shallow subtidal deposits including sponge spiculites, mud-dominated accumulations of articulated bivalves and gastropods with low-diversity microfaunal associations composed mainly of Choffatella and few other textulariids, and marginal oyster-serpulid patches. The uppermost part of sequence O2 is characterised by the development of siliciclastic tide-influenced coastal plains. Sequence O3 consists of massive intertidal to shallow subtidal glauconite accumulations related to the transgressive reworking of siliciclastic deposits. Foraminiferal associations in sequence O3 show an increase in diversity (including miliolids, lageniids and diverse textulariids), which was probably fostered by a more stable connection with open seas, caused by tectonic activity in the bounding shoal areas. The analysis of these four sequences represents a comprehensive and detailed characterisation of shallow-to-marginal marine benthic communities integrated in a sequence stratigraphic framework.

ALLUVIAL-LACUSTRINE-MARINE COMPLEX OF MOUNT MEDVEDNICA: THE EARLY SYN-RIFT DEPOSITION AND PALAEOGEOGRAPHY (EARLY TO MIDDLE MIOCENE, NORTH CROATIAN BASIN)

Mt. Medvednica belongs to the western part of the Neogene rift-type North Croatian Basin that occupies the south-western Pannonian Basin System. The Lower to Middle Miocene continental to marine sedimentary complex was studied on Mt. Medvednica in order to interpret the early syn-rift depositional environment and reconstruct palaeogeography of the North Croatian Basin. Based on facies analysis, deposits are grouped into 16 facies based on their lithological characteristics. Six of the facies belong to the alluvial environment that is characterized by coarse-grained bedload siliciclastics. Nine facies are of lacustrine origin. They comprise: a) limestone of shallow lake carbonate bench with silty coal of vegetated marsh, b) deep lake and prodelta marl with sand and conglomerate intercalations deposited by gravity flows, and c) coarse-grained Gilbert-type delta conglomerate. The lacustrine deposits compose a transgressive-regressive sequence. The studied succession ends by facies of calcareous silt intercalated by conglomerates. These deposits belong to the marine offshore to prodelta as the consequence of establishment of the connection to the sea. The deposition was strongly controlled by allogenic factors, such as synsedimentary tectonics, climate, eustatic sea level changes and explosive volcanic activity. The alluvial deposits of Mt. Medvednica are the oldest syn-rift deposits and belong to the large alluvial plain that probably covered the entire North Croatian Basin in the Ottnangian and the Karpatian. These deposits are overlain by the lower Badenian lacustrine deposits, but the question of the existence of one large or several small lakes in the early Badenian North Croatian Basin remains open.

Geological Assessment of Turonian - Coniacian terrestrial sedimentation records during climatic recovery, Moreno Hill Formation, Zuni Basin

Understanding of Upper Cretaceous terrestrial sediments within the Western Interior Basin is advancing; however, the Turonian–Coniacian transition remains enigmatic. Recent chronometry of the Moreno Hill Formation indicates that sediment deposition took place during this interval of geodynamic upheaval and climatic recovery immediately following the peak of the Cretaceous Thermal Maximum (CTM). To decipher these effects, the sedimentary record of the Moreno Hill Formation was reassessed near the type section, and near Quemado, New Mexico (USA) using facies analysis and architectural reconstruction. Seven facies types (thirteen lithofacies codes), eleven architectural elements, and three facies associations were identified. Sedimentation within the floodplain of the lower Moreno Hill Formation was affected by the east-migrating forebulge of the Western Interior Basin. Furthermore, increasingly bedload-rich multi-story channel complexes and a transition from near-coastal to alluvial coals reflect gradual climatic cooling and overall regression of the Western Interior Seaway (with interruption by a regional [T2–R2] transgressive-regressive sequence). This is consistent with more subaerial conditions indicative of continued regression reflected within the floodplain sediments of the upper Moreno Hill Formation. Whilst diversion of westerly fluvial feeder systems by ongoing forebulge migration also affected sediment transport and deposition, a return to more suspended-load-rich single-story channels and thin coals are tied to an intervening (T3) transgression. Repetitive paleosol sequences throughout the Moreno Hill Formation indicate groundwater fluctuation in response to these base level changes. Together with detrital zircon-based geochronology, these slight sedimentary differences support a revised subdivision from three into two members: lower and upper. Beyond feeding the seaward Gallup Delta, the newly defined lower member correlates to the Toreva, Straight Cliffs (Smoky Hollow member), Ferron Sandstone, Funk Valley and Frontier formations (Dry Hollow Member) and the upper member to the Wepo and Straight Cliffs (John Henry Member) formations within the Kaiparowits, Notom, Last Chance and Vernal fluvio-deltaic systems. Landward sediments of the Cardium Formation (Canada) correlate with the lower and upper members of the Moreno Hill Formation.

Ostracod zonation of the Givetian-Frasnian boundary interval in the Timan-North Urals Region

Research subject. Ostracods and Givetian-Frasnian boundary deposits of the Timan-North Urals Region.Materials and methods. The work was based on the results of studying ostracods from the stratotype sections of the Southern Timan, which are considered referential for regional and global correlations. The analysis of the distribution intervals of the most significant taxa was based on the results of previous studies, both published and archival materials on the Timan-Urals Region. Ostracods were considered in association with conodonts, which is an important criterion for applying a reasonable biostratigraphic subdivision. The use of different-scale regional cyclicity of sedimentation and individual “marking” levels of manifestation of global events were shown.Results. The ostracod distribution in the Givetian-Frasnian boundary interval of the Timan-Northern Urals region was analyzed. The following most significant taxa were identified: Cavellina devoniana, Cavellina uchtensis, Cavellina batalinae, Cavellina chvorostanensis, Nodella faceta, Nodella ex gr. hamata. The correlation of established assemblages with ostracods from the Franco-Belgian basin was shown. The difference consists in the absence of a polyzygid assemblage with Polyzygia beckmanni beckmanni, which was established at the base of the Nismes Formation of Belgium near the historical boundary between the Givetian and Frasnian stages. The determined relationship of biostratigraphic data in combination with the transgressive-regressive sequence of sedimentation shows a more reasonable solution to the problem of the position of the lower boundary of the Frasnian stage of the Upper Devonian in the Timan-Northern Urals Region.Conclusions. An updated version of the scheme of zonal subdivision of the Givetian-Franian boundary interval in the Timan- Northern Urals region has been developed, taking into account the ostracod distribution of the Cavellinidae and Nodellidae families, as well as the discrepancies in the levels of the first appearance of index species and the boundaries of zones in the regional stratigraphic scheme. The correlation of ostracod zones relative to the conodont scale is shown.

Recurring Patterns of Sea Level Changes within a Palynological-Based Sequence Stratigraphy Framework of the Middle Jurassic Organic Matter-Rich Khatatba Formation, Egypt

The Middle Jurassic was a time of marginal to shallow shelf settings dominated by organic matter-rich deposition in northern Egypt, southern Tethys. Paleoenvironmental and sequence stratigraphic reconstructions are of paramount significance for a better understanding of basin history and related reservoirs and source rock units. For this purpose, a detailed palynomorph and palynofacies analysis of the Middle Jurassic Khatatba Formation in the Dahab-Mireir Basin, north Western Desert, was conducted. A moderately to well-preserved, moderately diverse palynomorph assemblage of spores, pollen, and dinoflagellate cysts is identified. Marker dinoflagellate cysts, including Dichadogonyaulax sellwoodii, Escharisphaeridia pocokii, Gonyaulacysta adecta, Korystocysta gochtii, Pareodinia ceratophora, and Wanaea acollaris, defined a Bajocian to Callovian age. Palynofacies analysis of the Khatatba Formation revealed two assemblages, whereby PFA-1 is the most abundant in the study well and is comprised mainly of terrestrial phytoclasts deposited nearby fluvio-deltaic sources. The infrequently reported PFA-2 consists of moderate abundances of phytoclasts and AOM, deposited in an inner shelf environment. A more reliable paleoenvironmental interpretation was indicated based on three palynomorph assemblages. The spore-dominated assemblage indicated deposition in a deltaic environment, while the mixed palynomorph assemblage revealed fluvio-deltaic to marginal shallow marine conditions. The microplankton-dominated assemblage is represented by minor samples and reflects an offshore inner shelf condition. Furthermore, the Middle Jurassic relative sea level was reconstructed based on the quantitative variations in the terrestrial/marine (T:M) ratios and abundances of spores, pollen, and dinoflagellate cysts. This led to the subdivide of the Khatatba Formation into thirteen third-order transgressive-regressive sequences, which can be correlated with the global Jurassic short-term sea level changes. Active tectonics and accelerated drifting of the Eurasian Plate during the Middle Jurassic are suggested to control basin uplift/subsidence and, therefore, recurring patterns of relative sea level change.

Reservoir characterization of the Oligocene–Miocene siliciclastic sequences (Ghar Member of the Asmari Formation) in the northwestern Persian Gulf

This paper presents an integrated facies, diagenesis, and sequence stratigraphic analysis, and reservoir quality evaluation of the siliciclastic hydrocarbon reservoirs (Ghar Member of the Asmari Formation) in the NW Persian Gulf. Results of core descriptions, thin sections petrography, X-ray computed tomography (CT) scanning, X-ray diffraction, sieve analysis, porosity–permeability measurements, and petrophysical logs are used. Four petrofacies are recognized indicating deposition in shore-line (beach) sand bodies and supratidal to intertidal sub-environments. Calcite and anhydrite cementation, dolomitization, recrystallization, compaction, and fracturing are diagenetic processes which occurred in marine, hypersaline, and shallow burial realms. A large-scale transgressive–regressive sequence and five deepening-upward cycles are defined. Hydraulic flow units and pore-size classes are defined. The best reservoir units (hydraulic units 5 to 8 and pore-size classes 4 and 5) are formed within the well-sorted, rounded, and mature sand/sandstone facies in which intergranular pores are predominant and diagenetic imprints are negligible. These facies are located in lower-half of depositional cycles. Carbonate-dominated facies show variable reservoir properties depending on their diagenetic alterations. In these facies, dolomitization and fracturing improved the permeability. This study shows that reservoir properties of the Ghar Member are strongly facies-controlled and, therefore, are easily predictable within a sequence stratigraphic framework.

The Ordovician record of North and West Africa: unravelling sea-level variations, Gondwana tectonics, and the glacial impact

Abstract The Ordovician of North and West Africa comprises three main transgressive–regressive sequences understood as ‘second-order’ cycles of 10–15 myr duration. Tide- to wave-dominated shallow-marine clastic successions, preserving incidental bryozoan carbonates to the north, include fluvial deposits over the most proximal southern stretches of the platform. The boundary with Cambrian strata remains unclear but the latter are progressively less represented to the south in the undifferentiated ‘Cambro-Ordovician’. To the north, graptolites, brachiopods and trilobites combined with palynomorphs provide a robust biostratigraphic frame. Maximum flooding intervals occurred in the early to middle Tremadocian, middle Darriwilian and middle to late Katian. Two events interfered with an overall long-term transgressive trend. The ‘intra-Arenig’ (late Floian?) tectonic event highlighted palaeohighs coinciding with Paleoproterozoic basements. Gondwanan drainage basins were reorganized, which had an impact on sediment sourcing and distribution of detrital material (e.g. zircons) feeding the pre-Variscan Europe. The second event is the end-Ordovician glaciation. The domain supported the greatest part of the Hirnantian glaciers and may also have preserved pre-Hirnantian glacial archives. It is not until the very latest Ordovician that offshore conditions developed far inland; it is however suspected that this inundation benefited from a transient postglacial isostatic flexure.

Middle Jurassic multi‐scale transgressive–regressive cycles: An example from the Lusitanian Basin

AbstractMulti‐scale transgressive–regressive cycles from the mid‐Jurassic were recognised in the Central Lusitanian Basin, Portugal. These cycles allow the depositional evolution of the basin to be better understood and aid in the construction of stratigraphic sequences composed of three hierarchies. The stacking pattern of high‐frequency transgressive–regressive sequences forms larger clusters that define medium‐frequency transgressive–regressive sequences. Likewise, the stacking pattern of medium‐frequency transgressive–regressive sequences generates two Bathonian–early Callovian low‐frequency transgressive–regressive sequences. Integration of several methods supported the interpretation of facies associations representing clastic deposition in offshore to shoreface environments and carbonate sediments in outer to inner ramp settings. New data from calcareous nannofossils and dinoflagellate assemblages constrained the interval's Bathonian–early Callovian age, thus unveiling the Middle–Upper Jurassic disconformity and filling the Middle Jurassic stratigraphic record gap in the Central Lusitanian Basin. This study may be helpful for similar successions in Tethyan domains and comparable depositional settings elsewhere.

Sea level changes and sequence stratigraphy of the Lower-Middle Jurassic Quse and Sewa formations in the Qiangtang Basin, central Tibet: Geochemical and palynological perspectives

ABSTRACT Sequence stratigraphy is a commonly used approach to understand basin-fill history and the occurrence of conventional reservoir and/or probable source rocks. Reconstructing a sequence stratigraphic framework in carbonates, marls, and organic-rich mudstones is challenging due to environmental, petrophysical, and sedimentological variations that can be subtle, whereas seismic data may be lacking or of limited use. For this reason, incorporation between palynological analysis and chemostratigraphic profiles is becoming increasingly common to facilitate sequence stratigraphic reconstruction. This study investigates the Quse (lower Toarcian) and Sewa formations (upper Bajocian-Bathonian) in the Bilong Co section from the Qiangtang Basin of central Tibet. Eighty-three rock samples were analysed for their whole-rock inorganic geochemical composition, of which 25 rock samples were tested for palynological and palynofacies analyses. The palynological composition defined two associations for the study section. The lower part of the Bilong Co section consisted is dominated by a moderate to high abundance of opaque phytoclasts versus low to moderate amorphous organic matter (AOM) content with a high concentration of sphaeromorphs pollen grains, revealing deposition of this interval at times of low relative sea level in fluvio-deltaic settings. The middle to upper parts of the section comprised of significantly high AOM content versus low phytoclasts and high content of sphaeromorphs pollen grains, but with increasing the relative abundance of dinoflagellate cysts, indicating a phase of high sea level. Chemostratigraphic variations relevant to changes in relative sea level delineated the subdivision of the Quse and Sewa formations into two and three third-order transgressive-regressive sequences, respectively. The collision between the Lhasa and Qiangtang blocks during the Early-Middle Jurassic is suggested to control consequent subduction and sea level highstand, especially during deposition of the Sewa Formation. Geochemical proxies that relate to shelf carbonate production are calcium, strontium, whereas proxies that relate to siliciclastics include titanium, zirconium, manganese, aluminium, and silicon, other redox-sensitive proxy like uranium is also implemented. This indicates that chemostratigraphic proxies are a promising tool for the interpretation of sea level cyclicity in terms of systems tracts and bounding surfaces.

Sedimentology, T-R sequence stratigraphy and geochemical characterization of sedimentary successions of the West and Central African Rift System, Koum Basin

The Koum Basin (KB) is among the numerous relatively unexplored sedimentary basins lining theWCARS(West and Central African Rift System), hence limiting the understanding of how sedimentation evolved along the full extent of the WCARS. The intensification of petroleum exploration within WCARS has further emphasized the need to study these greenfields. This study contributes in solving the above issue by establishing a sequence stratigraphic framework and expanding the sedimentological databank of the KB. Systematic fieldmapping and lithofacies analyses of theKB along three NNW-SSE traverse lines cross-cutting the basin exposed alternations of conglomerates, sandstones and shales, intercalated by siltstones and carbonate mudstones. Dinosaur, bivalve and plant fossils are common. X-Ray Fluorescence data of studied samples plot as non-marine, immature, low to moderately weathered shales and greywackes of intermediate to felsic igneous and quartzose sedimentary sources. Sedimentation occurred under oxic, through anoxic to dysoxic depositional conditions, in a semi-arid active continental margin. These deposits inferred to be fluvio-lacustrine, have been subjected to outcrop-based sequence stratigraphic analyses through section matching and correlation of its material-based sequence stratigraphic surfaces, following the Transgressive-Regressive sequence stratigraphic model. The deposits have been subdivided into five repetitive T-R sequence stratigraphic units, bounded by four first order unconformities (SB-1, SB-4, SB-5 and SB-6) and two correlative conformities (SB-2 and SB-3), representing deposition under repetitive base level rise and fall regimes. These results demonstrate that any potentialmarine incursion in the WCARS probably did not reach the Koum Basin.

Palynomorph composition and palynofacies analysis of the Cenomanian Bahariya Formation in the north Western Desert of Egypt: Depositional palaeoenvironment and sequence stratigraphic implications

Abstract This study investigates the Bahariya Formation using 39 rock-cutting samples from the Abu Gharadig Basin in the north Western Desert, Egypt. An approach combining palynomorph composition and palynofacies analysis with lithological composition and geophysical gamma ray log values is used to assess biostratigraphic age constraints, prevalent depositional palaeoenvironments, and relative sea level changes. A moderately diverse assemblage comprised mainly of terrestrial palynomorphs versus minor content of marine palynomorphs is identified. Marker spore and pollen taxa are used to name three interval zones of early Cenomanian, and early-middle Cenomanian ages. Quantitative distribution of particulate organic matter using cluster analysis reveals two palynofacies assemblages: an older assemblage dominated by phytoclasts (mainly wood particles); and a younger assemblage characterised by moderate abundances of amorphous organic matter and phytoclasts. Palynofacies analysis suggests deposition of the Bahariya Formation in fluvio-deltaic to inner neritic shelf environments. Based on the pronounced stratigraphic variations in the particulate organic matter composition, including terrestrial:marine palynomorph ratio, lithological and gamma-ray log data, three complete transgressive-regressive sequences and one incomplete sequence and their systems tracts are defined. The transgressive-regressive trends reflect 3rd order sequences. Additionally, successive changes in relative sea level indicate a continuous rise at the topmost part of the succession.

Sequence stratigraphy and the Ubarana-Upper Cretaceous exploratory play within the lower Albian – middle Eocene section of the Mundaú sub-basin, Ceará Basin

The Ceará Basin is located at the east portion of the Brazilian Equatorial Margin, and it is subdivided from west to east into four sub-basins: Piauí-Camocim, Acaraú, Icaraí and Mundaú, the latter being the focus of this study. The lower Albian to middle Eocene section of the Mundaú sub-basin is characterized by the Ubarana-Upper Cretaceous exploratory play, with turbidite reservoirs. That same chronostratigraphic interval shows important discoveries on the African Margin, such as the Jubilee field, and on the Equatorial Margin in offshore fields in Guyana and Suriname. For this study, it was used data from eight exploratory wells and ten 2D seismic lines, located on the shelf-edge portion, in shallow waters, to the deep-water region. In this study, six low resolution Transgressive-Regressive sequences were identified. These T-R sequences are bounded at the top and bottom by maximum regressive surfaces (MRS), that were recognized in the well data, and posteriorly in the seismic data, through seismic stratigraphy methods. In addition to the MRS, maximum flooding surfaces (MFS) are recognized in the exploratory wells, through low and high frequency T-R cycles. In addition, this work allowed the identification of turbidite plays in deep waters regions The genesis of those turbidites, defined as the low-density type, is associated with a base-level rise at the shoreline, when the normal regression phase occurs. Thus, this study contributes with stratigraphic description of the lower Albian-middle Eocene section, and identification of turbidite reservoirs within that interval, which are important to the exploration and discovery of hydrocarbons in context in the Brazilian Equatorial Margin.

Stratigraphic styles of the Upper Paleocene-Lower Eocene rocks along the escarpment of Jabal Waddan, NW Libya

Abstract
 Upper Paleocene-Lower Eocene carbonate rocks along the escarpment of Jabal Waddan of NW Libya are displaying two major sedimentary cycles based on stratigraphic and foraminiferal attributes. The first one is representing by a regressive-transgressive sequence and is attributed to the Shurfah Formation. It indicates that during the Thanetian (Upper Paleocene) the deposition of the lower part (the Bú Rá's Member) was in shallow water depths of the subtidal environment under low energy conditions and is represented primarily by Dasycladacean and Codiacean green algae. Their association up-levels with small shallow benthic foraminifera such as miliolids and nonionids, however, may indicate a broad nearshore environment of deposition. The decline of the green algae and the prevalence of small-sized (<1 cm) and thick operculinids witnesses in the middle and upper parts (i.e. the Qaltah and Ammúr members) are characteristic of an open-shelf setting down to a water depth of a few dozen meters and typify warmwater habitats. Evidence of further deepening is indicated up-section by the existence of common shallow shark-teeth in a water depth limited to the photic zone ((<120 m)
 The second sedimentary cycle is showing a transgressive-regressive sequence and is ascribed to the Bishimah Formation. It shows that during the Ypresian (Lower Eocene) the deposition of the lower, middle and upper parts (i.e. Khayir, Wadi Zakim and Rawaghah members) of the Bishimah Formation was in the shallow marine neritic environment. The association of alveolinids with common textulariids in the lower and middle members points to a shallow inner shelf with water depth up to 60 m whereas their association with miliolids, nonionids and elphidiids seen at the upper levels indicate innermost shelf or restricted platform conditions. Evidence of further shallowing, however, is indicated up-section (the Rawaghah Member) by the presence of cross-bedded dolomitic limestone and chert nodules which delineate an advanced episode of the second sedimentary cycle. The closing part for the second cycle, however, is attributed to the overlying Al Jir Formation, elsewhere from the study area, in which a thick sequence of evaporates and chalky dolomites is reported to include a rich Spirolina-miliolid assemblage with globular alveolinids similar to those recovered from the studied deposits. Consequently, we recommend considering the Al Jir Formation as the upper member of the Bishimah Formation as originally described in the local literature. The boundary between the Ammúr Member of Shurfah Formation and the overlying khayir Member of the Bishimah Formation, however, is erosional everywhere in the study area and sets the contact between the first and second sedimentary cycles. It has been considered here to represent the Upper Paleocene/Lower Eocene boundary. Based on planktonic foraminiferal evidence, described from the subsurface of the Sirt Basin, the lower part of the khayir Member has been considered in the current study to represent a Ypresian (Lower Eocene) rather than Thanetian (Upper Paleocene) in age as previously thought

Paleoenvironmental redox evolution of Ediacaran-Cambrian restricted seas in the core of West Gondwana: Insights from trace-metal geochemistry and stratigraphy of the Bambuí Group, east Brazil

The sedimentary evolution of the Bambuí foreland basin system in the interior of West Gondwana is marked by periods of connection and isolation from the global ocean during the late Neoproterozoic and early Paleozoic. To understand the link between these periods and seawater redox conditions, we present an integrated study of trace-metal geochemistry and stratigraphy of the carbonate-siliciclastic rocks from the two lowermost second order transgressive-regressive sequences of the Bambuí Group, east Brazil. The basal 2nd-order sequence trace-metal pattern shows a progressive decrease of mass fractions of Co, Cr, Ni, Cu, Mo, U, V, Zn, and Cd, concomitant with a progressive decrease of Al and Fe contents and Mo/TOC ratios. Among all these elements, only Cd, Mo and U mass fractions seem to be less or not affected by detrital influence, so they can be used as reliable redox proxies for the paleoenvironmental analysis of the studied Bambuí strata. Moreover, normalization to aluminum shows a progressive increase of trace-metal enrichments for Cd and Mo in the order of 0.1–10 times and for U in the order of 1–100 times, accompanied by a progressive increase of organic carbon content upward section. These changes in sedimentary trace-metal patterns provide evidence for the chemical evolution of basinal deep-waters, whose conditions changed progressively from suboxic-anoxic to anoxic-euxinic at the basal transgressive-regressive sequence. We suggest that the paleomarine system represented by the basal Bambuí Group has probably evolved as an intracontinental silled basin recording changes in seawater chemistry associated with redox variations and restricted hydrographic conditions. Sedimentary trace-metal patterns indicate that Bambuí epeiric sea was initially in communication with open ocean followed by the marine restriction during the transgressive and regressive cycles, respectively. This resulted in a long deepwater residence time and chemical evolution of deep watermass as a response to tectonic pulses and consequent sea-level variations during the restricted stage. Under anoxic-euxinic conditions of seawater, trace metals scarcity and micronutrient fixation limitation would have impacted nitrate bioavailability, preventing the evolution of early benthic metazoans in the Bambuí paleomarine system during the late Ediacaran and early Cambrian.

Lithological-genetic features of Starorusskaya, Vasileostrovskaya and Siverskaya formations of the Upper Vendian-Lower Cambrian Baltic-Ladoga Monocline

Research subject. Sections of the Upper Vendian-Lower Cambrian, opened at the beginning of the XXI century by wells Avloga-12R, Utkina Zavod - 1 and Thais-2 with a depth of 150-370 m, in the north and in the central part of the Leningrad region. Methods. The analysis of the structural and textural features of the rocks of the contact layers, the distribution of allotigenic (quartz, feldspar) and autigenic (pyrite, siderite, glauconite) minerals in them allowed us to supplement the characteristics of the Old Russian, Vasileostrovsky and Siverskaya formations previously isolated in these wells. The use of the structural-genetic method made it possible to clarify the volumes and boundaries of the formations, to prove the absence in the section of the Lomonosov formation, previously isolated in the well Thais-2. Results. Based on the reconstruction of sedimentation conditions and settings, the development of sedimentation processes over time, three transgressive-regressive sequences of layers - cyclothemes were established in sections. The first corresponds to the Old Russian, the second - Vasileostrovskaya, and the third - Siverskaya formation. It is shown that in the studied part of the paleobasin at the end of the Vendian-the beginning of the Cambrian, there were open shelf environments (an area of mobile shallow water with underwater shafts (zone Y), an area of relative deep water with background and storm sedimentation (zone X) and a transitional zone of patchwork sands between X and Y with a variable hydrodynamic regime. The maxima of transgressions corresponding to the development of the deepest water environments occurred in the middle of the Old Russian, Vasileostrovsky and Siversky time, and the maxima of regressions marked by shallow water deposits belong to the end of these time intervals. According to isochronous levels - maxima of transgressions and regressions, correlation of well sections was performed.

Progressive opening of a shallow-marine bay (Oliete Subbasin, Spain) and the record of possible eustatic fall events near the Barremian-Aptian boundary

In this paper, we analyse the upper Barremian-lowermost Aptian Alacón Formation (Oliete subbasin, Maestrazgo Basin) of Spain to understand the progressive opening of a shallow-marine bay found on the northeastern margin of Iberia. The succession consists of four transgressive-regressive sequences (S1S4) bounded by widespread unconformities caused by rapid base-level fall. The transgressive interval of each sequence is characterized by relatively high detrital input and fresh-water inflow that caused the demise of shallow-marine benthic communities. The regressive interval of each sequence triggered progressive regeneration of the benthic carbonate producers. Sequences 1 and 2 consist of lacustrine to restricted shallow-marine facies, followed by accumulations of the gryphaeid oyster Ceratostreon , with progressive colonization of the bay by marine benthic communities subjected to frequent salinity oscillations. Sequences 3 and 4 represent the stabilization of open-marine conditions in the bay with widespread development of high-energy bioclastic tidal dunes. The long-term transgression around the Barremian-Aptian boundary, which gave rise to the progressive flooding of the Oliete bay, was episodically interrupted by shorter-term episodes of abrupt base-level fall, with the eventual subaerial exposure of the entire bay at the end of the deposition of the Alacón Formation. Our data suggest that the observed sequence architecture was mostly controlled by climatic and eustatic sea-level oscillations, with local synsedimentary tectonics having a minor impact. Two major sea-level fall events responsible for the discontinuities found on top of sequences 3 and 4 are possibly correlated to the cooling episodes close to the Barremian-Aptian boundary and during the earliest Aptian (predating the OAE 1a) described in many other worldwide basins. • The progressive opening of a shallow marine bay in Iberia (W Tethys) is documented. • Four sequences register progressive paleogeographical and paleoecological changes. • Sequence boundaries are tentatively linked to major cooling events KBa5 and KAp1.

Global and local drivers of the Ediacaran Shuram carbon isotope excursion

The Ediacaran Shuram carbon isotope excursion (CIE) follows the regional Gaskiers glaciation and occurs before the appearance of macroscopic animal fossils. Previous interpretations for the Shuram CIE have proposed global perturbations to Earth's carbon cycle accompanied by significant climatic and environmental change. These studies assume that carbonate carbon isotopes through the Shuram CIE record the composition of dissolved inorganic carbon in seawater. Through a compilation of new geochemical, sedimentological, and stratigraphic data from five localities on four separate paleocontinents, we find that all of the analyzed Shuram CIE successions share general similarities including a common transgressive-regressive sequence that is coincident with similar shifts in a host of geochemical proxies (δ13C, δ18O, δ44/40Ca, Sr/Ca, Mn/Sr, U/Ca). Despite these broad similarities between sections, there are important differences in the proxies between water depths that may be linked to changes in carbonate diagenesis. Specifically, we find the excursion's magnitude is largest in slope environments where diagenesis is sediment-buffered and smaller in shallow-water settings dominated by fluid-buffered diagenesis. The transgression that accompanied the Shuram CIE is consistent in amplitude (∼10-100s m) and duration (>1 My) with eustatic fluctuations driven by plate reorganization. These coupled geochemical and stratigraphic observations argue against meteoric and burial diagenesis as drivers for the excursion; instead, they are best explained by global changes to the locus and intensity of photosynthetic primary productivity in shallow-water environments and do not necessarily require large changes in global climate or marine redox.

Tonian deltaic and storm-influenced marine sedimentation on the edge of Laurentia: The Veteranen Group of northeastern Spitsbergen, Svalbard

The Hecla Hoek succession of northeastern Svalbard, Norway, is an ~7 km thick Tonian–Ordovician sedimentary succession that overlies Stenian–Tonian felsic igneous and metasedimentary rocks. The carbonate-dominated upper Tonian–Ediacaran (ca. 820–600 Ma) Akademikerbreen and Polarisbreen groups have yielded important insights into Earth's Neoproterozoic climate, environment, and biological evolution. However, the underlying siliciclastic-dominated lower Tonian (ca. 950–820 Ma) Veteranen Group has garnered little attention despite the fact that it is remarkably well-preserved and hosts diverse microfossil assemblages. Here, we present the first detailed sedimentological analysis of the Veteranen Group from a continuous ~4.4 km thick stratigraphic section at Faksevågen, Ny Friesland, Spitsbergen. Integrated facies analysis, sequence stratigraphy, and carbonate δ13Ccarb and δ18Ocarb chemostratigraphy elucidate the early depositional history of the Hecla Hoek basin and provide fundamental paleoenvironmental constraints for future investigations of this succession as an archive of Tonian Earth History. The Veteranen Group records a long-lived deltaic and storm-influenced marine sedimentary system that reveals dynamics of Precambrian clastic sedimentation prior to the evolution of land plants. Five asymmetric transgressive-regressive (T-R) sequences within the Veteranen Group thin upwards, providing support for the hypothesis that the contact with the Akademikerbreen Group represents a rift-to-drift transition. This complex record of Tonian deltaic and storm-influenced marine sedimentation along the Laurentian margin strengthens correlation between the Veteranen Group and coeval strata from East Greenland and sets the stage to better understand the Proterozoic tectonic evolution of the North Atlantic–circum-Arctic region following the Grenville orogeny.

Palynostratigraphy, Palynofacies, T-R Cycles and Paleoenvironments in the Middle Jurassic–Early Cretaceous Ramså Basin, Andøya, Northern Norway

Palynostratigraphy and palynofacies analyses are applied to identify transgressive-regressive sequences and changes in paleoenvironment through the Middle Jurassic–Early Cretaceous succession of the Ramså Basin on Andøya. The conglomerate, the succeeding lacustrine-swamp deposits of the Hestberget and Kullgrøfta members (Ramså Formation) and the overlying terrestrial to marginal marine deposits of the lower Bonteigen Member (Ramså Formation), comprise the Bajocian T-R sequence. Bathonian–Oxfordian strata appear to be missing in the studied boreholes, and the second T-R cycle spans the Kimmeridgian to Berriasian open marine deposits of the upper Bonteigen Member and the Dragneset Formation (Breisanden, Taumhølet and Ratjønna members). The overlying Nybrua Formation comprises a condensed marine succession of Valanginian–Early Barremian calcareous sandstone and marl, followed by brownish-red siltstone. The upper T-R sequence (Skarstein Formation) consists of marine transgressive Barremian dark siltstones, silty shales and mudstones, followed by dark mudstone and shale. Marine palynomorphs recovered in these stacked marine slope turbidite sediments are of Late Barremian age, but possibly the youngest T-R cycle also includes Aptian deposits elsewhere in the basin.