Shallow-marine Carbonate Research Articles

Formation of vertical columnar seismic structures and seafloor depressions by groundwater discharge in drowned Miami Terrace platform and overlying deep-water carbonates, southeastern Florida

The presence of vertical cross-formational fluid migration passageways within sedimentary basins can profoundly impact aquifer and reservoir fluid-flow and their identification is fundamental to informing management of subsurface fluid resources (groundwater, oil, gas). In an onshore and offshore southeastern part of Florida, 2D/3D seismic-reflection and bathymetry data document ~153 vertical columnar structures composed of reflection disruptions up to 760 m in the height and averaging 360 m in diameter, and ~ 219 subcircular to circular seafloor depressions up to 1010 m wide. Our study focuses on these features found within the offshore shallow-marine carbonate Miami Terrace platform, which drowned approximately at the end of the middle Miocene, and within overlying Plio-Quaternary deep-water carbonate slope and drift deposits. Most columnar structures are rooted in stratiform aquifers of the Miami Terrace platform and associated with faults or fault intersections produced by Eocene and circa late Miocene tectonics. The columns commonly terminate within the platform or as subcircular depressions along an amalgamated karstic and drowning unconformity at the platform top. The columns typically stretch upwards from a zone of deep karst cavity collapse through the Miami Terrace platform with upward decreasing sag on internal reflections. Following drowning and Plio-Quaternary partial burial of the Miami Terrace platform by deep-water deposits, the subcircular depressions and faults along the platform top were points of origin for a second phase of column growth upward into the deep-water deposits. The continuation of deep platform cavity collapse and column evolution produced pockmarks along paleo-seafloors within the deep-water deposits and at the present-day sea floor. The Plio-Quaternary pockmarks formed at water depths too deep to suggest an origin related to meteoric karst above or near sea level, but rather their formation is suggested to be related to cyclic sea level falls that drove increased groundwater head and density gradients, and seafloor discharge of offshore freshened groundwater sourced from the underlying platform. Mixing of freshened groundwater and seawater at the seafloor discharge sites plausibly drove dissolution of the host deep-water deposits that contributed to the formation of the pockmarks and erosion by groundwater venting and current scouring.Seaward of the Plio-Quaternary seafloor pockmarks, at the late-middle Miocene upper slope of the Miami Terrace platform and along the regional karst/drowning unconformity is a slope-parallel band of ~188 densely distributed subcircular seafloor depressions with diameters up to 1010 m at water depths up to 663 m. It is plausible that along the upper slope, faults and fractures produced by gravity-driven slope instability and possibly tectonics formed a dense network of fluid passageways that promoted upward artesian freshened groundwater flow to sites of discharge where mixing with seawater generated limestone dissolution and the depressions. But tectonic uplift may have forced emersion and initial meteoric sinkhole formation circa late Miocene with later enhancement by freshened groundwater discharge and bottom current erosion.

Strontium Isotope Composition of Triassic Coastal and Shallow Marine Carbonates in the Western Balkanides, NW Bulgaria

Whole-rock samples of Triassic coastal and shallow marine carbonates were analyzed for their Sr isotope signatures and trace element concentrations were measured in order to assess the possible impact of diagenetic alteration. The 87Sr/86Sr ratios of Anisian to lower Carnian calcite micrites reproduce the global Triassic seawater variations fairly well. The recorded higher values can be explained by an influence from detrital aluminosilicate phases and a minor postdepositional alteration. The samples of tidal-flat dolomicrites from the base of the Anisian show no significant diagenetic overprint but have considerably more radiogenic 87Sr/86Sr signatures compared to the global Sr isotope curve that corresponds to the Triassic (Mesozoic) maximum. These anomalously high values may have resulted from some interaction between the dolomitizing brines and the underlying fluvial siliciclastics (Buntsandstein facies), but more likely from the high 87Sr/86Sr river waters discharged from a silicate-dominated drainage basin to the coastal area at the beginning of the Middle Triassic. This study demonstrates that bulk micrite can be used as a proxy for seawater 87Sr/86Sr after proper sample selection and diagenetic screening. It also suggests that the 87Sr/86Sr ratios of ancient carbonates precipitated in coastal settings should be interpreted with caution, considering the possible influence of continentally-derived fluids.

Impact of the Jenkyns Event on shallow-marine carbonates and coeval emerged paleoenvironments: the Plitvice Lakes region, Croatia

Early Jurassic (late Pliensbachian–early Toarcian) Large Igneous Province (LIP) magmatism affected the entire ocean-atmosphere system culminating in a cascade of paleoenvironmental perturbations known as the Jenkyns Event which globally impacted marine, transitional and terrestrial paleoenvironments. Carbonate platforms at low latitudes in the Western Tethys realm drowned or shifted to non-skeletal production during the early Toarcian due to sea level rise, global warming, and ocean acidification. Unlike deep-marine deposits, shallow-marine carbonates present a challenge in defining global geochemical signals due to common diagenetic modifications. An integrated dataset including δ13Ccarb and δ18Ocarb, TOC, biomicrofacies, SEM, XRD, and palynological study of two stratigraphic successions in the Plitvice Lakes region, Bjelopolje (BJ) and Plitvice Spring (PS), in Croatia, provides an overview of the paleoenvironmental evolution and lateral facies changes in a peritidal setting during the Early Jurassic on the Adriatic Carbonate Platform (AdCP). The investigated stratigraphic succession starts with upper Pliensbachian, peloid-ooid-bioclastic grainstones alternating with fenestral and massive mudstones overlain by lower Toarcian, lagoonal, bioturbated, “spotted” limestones with horizons indicative of short-lived subaerial emergence during the early Toarcian. A negative excursion in δ13Ccarb, foraminifer assemblages, and the predominance of Classopollis pollen within the “spotted” limestones marks the stratigraphic position of the Jenkyns Event. Global-scale events (sea level variation, climate change, C-cycle perturbation, anoxia) operated simultaneously with local to regional synsedimentary tectonics and eustatic movements that preserved the AdCP carbonate factory from collapse or drowning but resulted in pronounced facies differentiation on the shallow-marine carbonate platforms.

Recognition of a cryptic maximum flooding surface in shallow marine carbonate sequences using geochemical (Y/Ho) proxy data

ABSTRACTSince diagnostic primary depositional sedimentary structures and depth‐dependent grain‐size trends are rarely preserved, building a comprehensive sequence stratigraphic framework for the vast majority of the Phanerozoic carbonate platform sequences is pending. Among the two most important sequence stratigraphic surfaces, while the subaerial unconformity can be reliably identified by either karst development or the appearance of siliciclastic materials, the demarcation of the maximum flooding surface remains difficult in lithologically uniform shallow marine carbonate sequences. The present study attempts to identify the globally documented maximum flooding surface within the body of the negative carbon isotope excursion of the Palaeocene–Eocene Thermal Maximum recorded in the shallow marine carbonate platform sequences. The results show that, along with the carbonate microfacies, the yttrium to holmium ratio (Y/Ho ratio) of the carbonate fraction reliably records the sea‐level changes. A Y/Ho ratio between 70 and 80 demarcates the stratigraphic position of the maximum sea‐level state (the most open marine condition in the studied interval) and maximum flooding surface in the studied sections. Since the Y/Ho ratios remain relatively stable throughout diagenesis, they can be used for maximum flooding surface identification in shallow marine carbonate platform sequences. The possibility exists that the same method can also be applied to the mixed siliciclastic–carbonate systems.

The Moscovian–Kasimovian boundary problem in the Northern Urals: A case study of the Moleben-Iz Section, Ilych River

The Kasimovian lower boundary is usually determined by the first appearance of Obsoletes and advanced Protriticites fusulinids; however, this criterion is not always met. In the northern part of the Urals and Pai-Khoi, the Moscovian–Kasimovian boundary is determined by the abrupt appearance of fusulinids of the Subfamily Pulchrellinae. In this paper we present results of our study of the Moleben-Iz section, in an attempt to define the Moscovian–Kasimovian boundary. In this section, the upper Moscovian and lower Kasimovian deposits are represented by shallow-marine carbonates, including a diverse fusulinid complex. The most important stratigraphic level here is the lower part of bed 6, characterized by the abrupt appearance of the fusulinids Kanmeraia, Eowaeringella and Dagmarella. This level is also associated with the diversification of Usvaella (species U. plicata (Shamov and Shcherbovich) and U. porifera Remizova appear) and Pulchrella (Pul. whitensis (Ross and Sabins), Pul. hayasakai (Watanabe), and Pul. pokojamensis (Lebedeva) appear). The later appearance of Obsoletes obsoletus may be related to the later migration of this species from the Russian platform to the Urals. Placement of the Kasimovian lower boundary in the Northern Urals and Pai-Khoi at the base of the Kanmeraia ozawai-Usvaella usvae Biozone allows us to correlate this boundary with the Missourian base of the North American continent.

Controls on the terrigenous fractions in Early Kimmeridgian shallow-water carbonate deposits in Southern Iberia

AbstractTwo Kimmeridgian shallow-marine carbonate successions (mid-shelf), sharing a similar paleoclimatic framework (climatic zone), were previously explored using carbonate chemostratigraphy. Before, the goal was to detect signals related to paleoplatform bottom physiography, degree of connection with oceanic waters and overall circulation patterns. In this contribution, complementary bulk mineralogical composition and clay mineral fractions are investigated to contrast and complement previous information, aiming for a more complete overview of continent-ocean dynamics along shallow-carbonate platforms of southern Iberia. The goal is now to explore complex patterns of continental influence along proximal settings and their relative spatial distribution across differentiated settings without relevant difference in paleolatitude. The choice of both stratigraphic sections is based on this distinctness: Rocha Poço (Algarve Basin, Portugal) represents a more restricted and relatively proximal setting, in contrast to Puerto Lorente (South Iberian Paleomargin, S Spain) placed at a relatively more open and probably distal shallow-water context. Accordingly, quartz content was higher at Rocha Poço, especially at the lower siliciclastic interval of this section. Quartz contribution fades out at Puerto Lorente, where it was mainly controlled by short-lived terrigenous pulses. Clay mineral assemblages also differed, being more varied (smectite, illite and traces of kaolinite) and abundant at Rocha Poço, and generally leaner at Puerto Lorente. At the latter site, terrigenous pulses do not contribute to clay mineral abundance, only showing abundant illite at the topmost horizons. New information retrieved from mineralogical data provided evidence on depositional contrasts resulting from local differences in platform configuration, allowing a better understanding of mechanisms controlling the terrigenous fraction in the shallow-water carbonates analyzed.

The rise of biogenic silicon cycling and microbial silicification promoted Mesoproterozoic chert deposition

The silicon (Si) cycle is intimately interlinked with other biogeochemical cycles (e.g., carbon and nitrogen) and plays an important role in regulating long-term global climate change and biotic evolution in Earth's history. The abundance of cherts in Mesoproterozoic shallow-marine carbonates implies unique environmental conditions for silica precipitation, but the driving mechanism behind the Si cycle in Mesoproterozoic oceans remains unclear. In this paper, we report an integrated study of the cherts from the ∼1.48 Ga Wumishan Formation of North China. The Wumishan cherts predominantly consist of microquartz (∼90%), with some silica-replaced carbonate (∼5%) and minor pyrite (∼1%) grains, indicating that the cherts were largely formed through primary silica precipitation. High germanium/silicon molar ratios (Ge/Si = 0.71–19.1 μmol/mol; mean = 8.83) and positive europium anomalies (Eu/Eu* = 0.41–2.42; mean = 1.41) of the cherts suggest a considerable contribution from hydrothermally derived Si. Diverse microbial components, including organic-rich filaments, EPS (extracellular polymeric substances) relics, mat fragments and picocyanobacteria fossils, are observed in the Wumishan cherts. These components, together with abundant organominerals resulted from the interactions of microbes and dSi in ambient environments, imply that microbial activities played critical roles in silica deposition. The Si liberated from degraded EPS or EPS-Si complex may have locally increased the dSi concentrations and changed chemical conditions in the substrate and pore-waters, promoting silica precipitation. Given that picocyanobacteria and some other prokaryotes can accumulate significant amounts of silica in their cells and EPS, biogenic silica from decomposed microbial biomass may have exerted an important influence on silica precipitation in shallow-marine environments of the Mesoproterozoic ocean.

Intra-Turonian stratigraphic reorganization on the Arabian Plate

Shallow-marine carbonate deposition was widespread across the Arabian Plate during much of the Late Permian–Mesozoic but was significantly interrupted by a major tectonically driven stratigraphic reorganization that took place within the Turonian. Published literature terms this the ‘Wasia–Aruma Break’, the ‘mid-Turonian Unconformity’, or the ‘K150 Sequence Boundary (SB)’. It is widely understood that this event relates to subduction in a closing Neotethys Ocean and obduction onto the continental margin, especially in the southeastern part of the plate. As well as a change in facies, a hiatus of variable duration is present, associated with erosion of the underlying stratigraphy. Despite the event being known for several decades, precision on its timing has been lacking due to the limitations of biostratigraphic data and because few other chronostratigraphic proxies have been employed. In order to rectify this, all relevant biostratigraphic data from across the Arabian Plate have been synthesized and evaluated in the context of a review of chronostratigraphic calibration of standard biozones from multiple fossil groups. This shows that the youngest preserved rocks beneath the K150 SB can be assigned to the middle Turonian Helvetoglobotruncana helvetica planktonic foraminifera zone or Collignoniceras woollgari ammonite zone. The oldest rocks above the K150 SB can be assigned to the same biozones, supporting the assertions that it is a very short duration event. Whilst obduction may have initiated earlier (as supported by the recognition of precursor events), crustal loading was sufficient by 91.5 Ma to develop a forebulge in the eastern part of the plate with associated uplift and erosion. A review of relative sea-level change during the Turonian has been carried out to investigate eustatic signals during this time. In multiple sections around the world, a sea-level fall in the upper part of the woollgari Zone of European usage can be detected. This appears to coincide with an episode of climate cooling. However, despite there being correspondence between the K150 SB and a significant eustatic event, this seems mostly coincidental as the tectonic drivers at this time across Arabia were much more dramatic. This is especially true in southeastern Arabia. In northwestern Arabia, tectonic quiescence may have allowed eustasy to dominate. In any case, a eustatic fall would have amplified the effects of tectonics across the entire region.

Sedimentary Facies, Depositional Environment, and High-resolution Sequence Stratigraphy of Dernah Formation, N Cyrenaica Promontory, NE Libya

The depositional environment and high-order stratigraphic architecture of the Eocene Dernah Formation in N Cyrenaica Promontory, NE Libya, were interpreted through detailed sedimentary facies and sequence stratigraphic analyses of four measured sections located several kilometers apart. Five distinct lithofacies were delineated based on variations in lithology, grain composition, fossil content, texture, color, and stacking pattern. Dernah Formation mainly contains nummulitic limestone with a noticeable amount of Discocyclina and rare gastropods. The extraordinary presence of Nummulites indicates shallow-marine carbonate settings, perhaps a Nummulite bank environment, where the reworking of sediments by wave- or tide-induced currents and storms is high. Two complete fifth-order depositional sequences were defined. Each sequence was subdivided into a transgressive and a regressive hemicycle based on the vertical lithological variation and stacking pattern of facies. We conclude that each sequence may have formed as a result of a fifth-order sea-level cycle. This study reconstructs a three-dimensional model for the Nummulite bank environment in the Mediterranean region, which can serve as an analog for similar depositional systems elsewhere.

Depositional and diagenetic facies of Late Pleistocene (MIS 5) shallow-marine carbonates of the Capo Colonna terrace, Crotone Basin, Southern Italy (preliminary data)

Depositional and diagenetic facies of Late Pleistocene (MIS 5) shallow-marine carbonates of the Capo Colonna terrace, Crotone Basin, Southern Italy (preliminary data)

Sedimentology and biostratigraphy of the Biancone Limestone Formation of the Tolmin Basin (Southern Alps, NW Slovenia)

The Tolmin Basin constitutes the western part of the Slovenian Basin, a large Mesozoic interplatform basin that was bounded to the north by the Julian Carbonate Platform and to the south by the Dinaric Carbonate Platform. Today, it is found along the structural boundary between the Southern Alps and the External Dinarides in northwestern Slovenia. After the drowning of the Julian Carbonate Platform, the Tolmin Basin was dominated by pelagic deposits, namely the Toarcian marlstones (Perbla Formation), the Aalenian to lower Tithonian siliceous limestones and radiolarites (Tolmin Formation), and the upper Tithonian to the Berriasian Biancone Limestone Formation. In this study, a basin-scale sedimentary evolution and calpionellid biostratigraphy of the latter is presented. The Biancone Limestone Formation is a calpionellid-bearing pelagic limestone typical of all deepened segments of the Adria microplate in this time interval. It is generally monotonous but shows considerable vertical and lateral variations. The lower boundary with radiolarites is sharp, revealing a pronounced and rapid overturn in pelagic sedimentation. The lower Berriasian slumps indicate a tectonic pulse. Rare beds of resedimented limestones, assigned to the middle Berriasian, are similar in composition to those intercalated in the underlying radiolarites. They thus show that the factors that led to a significant shift in pelagic sedimentation had little or no effect on shallow-marine carbonate production. In the upper Berriasian, an increase in clay content is evident in the formation, which is explained by the uplift and erosion of the propagating thrust belt in the Neotethys area, superimposed by humidification.

A process-based geochemical framework for carbonate sediments during marine diagenesis

A significant proportion of marine calcium carbonate sediments are comprised of metastable minerals that are susceptible to diagenetic alterations during burial. These reactions can reset the geochemical signature of sediments and pore fluids and influence elemental cycling in the ocean. However, the timing and mechanisms by which these reactions take place are poorly constrained. This study uses cores drilled on the slope of the Great Bahama Bank to provide quantitative constraints on important diagenetic reactions; namely respiration-driven dissolution, authigenic carbonate mineral formation, and conversion of aragonite to low Mg calcite (LMC). We perform detailed mineralogical characterization using newly acquired, high resolution X-ray diffraction (XRD) data and over 1000 reanalyzed XRD scans, characterize sediments texturally and elementally using electron microprobe data, calculate pore fluid saturation state with respect to aragonite using a Pitzer ion interaction approach, and use pore fluid chemistry and experimental distribution coefficients to predict authigenic carbonate compositions. These data suggest that aerobic organic matter oxidation, enabled by the advection of oxygenated seawater throughout the upper ∼ 30 m interval of sediment, causes undersaturation and thus dissolution of biogenic high Mg calcite (HMC) and aragonite. Deeper, anaerobic organic matter oxidation takes over and causes supersaturation, promoting authigenic precipitation in the form of HMC, which in turn decreases pore fluid Mg/Ca and promotes aragonite conversion to LMC. One novel aspect of this study is the identification of three types of calcites using the newly acquired XRD and electron microprobe data. Based on their unique crystallographic characteristics and chemical compositions, these types of calcites are interpreted to represent pelagic biogenic LMC, bank-derived biogenic HMC, and authigenic HMC precipitated from pore fluids. Notably, the composition of the authigenic calcite matches that predicted to precipitate from pore fluids using an empirical Mg partition coefficient in calcite. Calcites that form authigenically and from aragonite via replacement are suggested to recrystallize with burial as evidenced by the decrease in Mg content and micro-strain. This process-based geochemical framework assigns diagenetic processes to a specific depth window within the sediment column and paves the way for a mechanistic understanding of carbonate diagenesis, one that is rooted in thermodynamic and kinetic bases.

Success and demise of exceptionally preserved terebratulide brachiopod accumulations in a Jurassic (early Pliensbachian) tropical lagoonal setting (Southern Alps, Italy): brachiopod response to environmental changes

During the Early Jurassic, the shallow marine carbonate platforms of the western-Tethys margins were characterized by highly diverse benthos including larger foraminifera, sponges, bivalves, gastropods, brachiopods, echinoderms, and dasycladalean calcareous algae. In this paper, we document examples of such assemblages within the lower Pliensbachian part of the Rotzo Formation (upper Orbitopsella Zone) of the Southern Alps, Italy. This carbonate succession was deposited in a complex mosaic of marine and brackish habitats within a tropical lagoon of the Trento Platform area. Large terebratulide brachiopod shells form autochthonous accumulations comprising exceptionally well-preserved monospecific assemblages of Lychnothyris rotzoana. These brachiopod-bearing successions were analysed in terms of biotic components, microfacies analysis, shell biofabric (three-dimensional arrangement of skeletal elements), and taphonomic signatures to understand brachiopod response to changing conditions within a highly variable lagoonal palaeoecosystem. Findings show that terebratulide shell accumulations are dominated by adult specimens and juveniles are rare. The brachiopods thrived during low energy conditions that resulted in the accumulation of highly temporally-condensed shell beds. Stabilized by microbialite encrustations, the shells were not re-oriented during the subsequent rapid burial. The abrupt demise of these communities was possibly related to rapid environmental change, and causal factors are discussed. The medium-term response of brachiopods to the relatively instable ecosystem of the tropical lagoon shows that they were not able to adapt to continuous perturbations, and that continuing stress severely compromised the resilience of benthic taxa.

Facies architecture, geochemistry and petrogenesis of Middle Triassic volcaniclastic deposits of Mt. Ivanščica (NW Croatia): evidence of bimodal volcanism in the Alpine-Dinaridic transitional zone

During the Middle Triassic, intensive volcanic activity took place along the eastern margin of Pangea, including the Greater Adria promontory, due to the Neotethyan oceanization. This resulted in the formation of various volcanic and volcaniclastic rock types. The region of NW Croatia, acting as a transition zone between the Southern Alps and the Dinarides, showcases the outcrops of these rocks. The present study investigates the facies of volcaniclastic rocks, the distribution of those facies, formation processes, as well as the genesis of the primary magma to gain a better understanding of the complex geodynamics of this region during the Middle Triassic. Six profiles across the Vudelja quarry front were surveyed using drone imaging and samples were collected for detailed petrographic and geochemical analyses. Two groups of volcaniclastic rocks were identified—mafic and intermediate/felsic. The former is represented by (I) autoclastic effusive facies and (II) resedimented autoclastic facies, while the latter is represented by (III) secondary pyroclastic facies. Mafic volcaniclastics were generated through basaltic effusions in marine environments, fragmentation in contact with seawater, mixing with shallow marine carbonate clasts, and subsequent redeposition in deeper marine areas. The secondary pyroclastic facies (III) consists of a regionally distributed felsic Pietra Verde tuff whose deposits may be related to pyroclastic density currents and syn-eruptive resedimentation by turbidite-like currents. Geochemical data indicate that parental magmas responsible for generating the mafic volcaniclastics had a calc-alkaline composition and originated in ensialic and mature arc settings of an active continental margin. The observed chemical composition is likely inherited from older, arc-related lithologies, associated with the subduction of the Paleotethys Ocean. Parental magmas are thought to have formed during continental rifting of the passive Middle Triassic margins of the Greater Adria through (i) partial melting of the heterogeneous lithospheric mantle, which had been metasomatized during an earlier Hercynian subduction, and (ii) subordinate processes related to the melting of the upper continental crust and subsequent fractionation. Ar/Ar dating on plagioclase separates yielded an age of 244.5 ± 2.8 Ma for mafic volcaniclastics. This aligns well with biostratigraphic ages of felsic tuffs which crop out on a broader regional scale of the Dinarides, the Southern Alps, and the Transdanubian Range. The overlapping ages obtained from radiometric dating of mafic volcaniclastics and biostratigraphic ages of the felsic Pietra Verde tuffs strongly suggest that the Greater Adria region experienced concurrent bimodal volcanism during the Middle Triassic.

Tectonic trigger to the first major extinction of the Phanerozoic: The early Cambrian Sinsk event.

The Cambrian explosion, one of the most consequential biological revolutions in Earth history, occurred in two phases separated by the Sinsk event, the first major extinction of the Phanerozoic. Trilobite fossil data show that Series 2 strata in the Ross Orogen, Antarctica, and Delamerian Orogen, Australia, record nearly identical and synchronous tectono-sedimentary shifts marking the Sinsk event. These resulted from an abrupt pulse of contractional supracrustal deformation on both continents during the Pararaia janeae trilobite Zone. The Sinsk event extinction was triggered by initial Ross/Delamerian supracrustal contraction along the edge of Gondwana, which caused a cascading series of geodynamic, paleoenvironmental, and biotic changes, including (i) loss of shallow marine carbonate habitats along the Gondwanan margin; (ii) tectonic transformation to extensional tectonics within the Gondwanan interior; (iii) extrusion of the Kalkarindji large igneous province; (iv) release of large volumes of volcanic gasses; and (v) rapid climatic change, including incursions of marine anoxic waters and collapse of shallow marine ecosystems.

Signs of crustal extension in Lower Jurassic carbonates from central Slovenia

The Lower Jurassic Podbukovje Formation represents a succession of shallow marine carbonate rocks deposited on the former Southern Tethyan Megaplatform and one of its successors, the Adriatic Carbonate Platform. Several outcrops of the Podbukovje Formation from central Slovenia (southern margin of the Ljubljana Moor) are presented, bearing possible evidence of Early Jurassic extensional tectonics. Peritidal facies of the lowermost, Hettangian – Sinemurian, part of the Podbukovje Formation locally interfingers with bodies of matrix supported pervasively dolomitized polymictic breccia, several metres to tens of metres thick and is locally cut by neptunian dykes some few decimetres to metres wide. The same or slightly younger part of the formation locally contains grabens/half-grabens metres to tens of metres deep and filled with poorly sorted pervasively dolomitized matrix supported polymictic breccia. Small miliolid foraminifera are present within the clasts and in the matrix. Finally, partly dolomitized blocky breccia tens of metres thick locally overlies the Pliensbachian – lowermost Toarcian limestone with lithiotid bivalves. Besides completely and partly dolomitized clasts, the breccia contains a variety of limestone clasts and preserves common radial ooids and some bioclasts within the partially dolomitized matrix. The Hettangian-Sinemurian breccias and dykes are presumably related to the early, diffused rifting stage of the Penninic (Alpine Tethys) Ocean, whereas Toarcian breccias relate to the main, focused rifting stage. Together with evolving biota and changing paleo-oceanographic conditions, the extensional tectonics may have been an important factor behind the facies changes observed within the Podbukovje Formation.

The end of the Cretaceous: depositional palaeogeographical reconstruction of the Gulf of Mexico and adjacent areas just prior to the Chicxulub impact

Abstract Until recently, information about the end of the Cretaceous was based upon investigation of global outcrop sections. New subsurface drilling and characterization from well cores and logs in the Gulf of Mexico Basin have greatly illuminated the end Cretaceous event. However, the palaeogeography of the late Maastrichtian just prior to bolide impact is less well understood and is of great importance in terms of modelling the resulting distribution and composition of the Chicxulub impact material, as well as tsunami and seiche wave height. Here, we examine the Maastrichtian strata in the basin, synthesizing lithostratigraphy and chronostratigraphy, tectonic plate reconstructions, global and local sea level history, palaeoclimate and depositional systems. Our new Maastrichtian palaeogeographical reconstruction shows the basin prior to the Chicxulub impact at a time of globally high sea level, with widespread deposition of deepwater chalks and shallow marine carbonates and local siliciclastic shorelines fed by the nascent Cordilleran belt. Stratigraphic correlations of wells and outcrops illustrate the range of palaeoenvironments from coastal plain to deep marine. As much as 610 m (2000 ft) of Maastrichtian and Campanian section is mapped around the basin, reflecting accommodation provided by basin subsidence, salt deflation and palaeophysiography. A large thickness of carbonates accumulated in the basin centre, with steep shoreline to basin gradients particularly in Mexico. At the end of the Cretaceous, carbonate palaeoenvironments probably covered 96% of the Gulf of Mexico Basin, with less than 4% of the area likely occupied by siliciclastic systems, a distribution that evolved from the Early Cretaceous. Our maps thus explain dominance of carbonate breccia and chalks in K–Pg boundary units deposited over the basin sites proximal or distal to the Chicxulub impact crater. This also elucidates the large impedance contrast and high amplitude seismic response of the K–Pg boundary horizon, mappable over vast portions of the basin.

Albian sedimentation in western Atlas, Morocco

Detailed study of field sections and extensive ammonite collection allowed to specify the sedimentary evolution and age of the Albian series of the Western Atlas (Morocco). After a significant hiatus of earliest Albian age, the Albian transgression began in the late Douvilleiceras leightonense Zone with partly clastic deposits. A transgressive pulse gave way to clayey marl deposits (Douvilleiceras mammillatum Zone), which evolved upward to partly carbonate, laminated deposits of late early Albian to earliest middle Albian age (Lyelliceras pseudolyelli to early Lyelliceras lyelli zones). A major hiatus that encompassed most of the middle Albian, was followed by a new transgression marked first by sandy deposits (Dipoloceras cristatum and Pervinquiria pricei zones), and then by clayey marl and limestone nodules (Pervinquieria inflata Zone). A sharp sea level fall (late Pervinquieria inflata to early Pervinquieria fallax zones) led to the development of a shallow marine carbonate shelf (Kechoula Formation). A last transgressive pulse allowed deposition of outer shelf clayey marls (Pervinquieria rostrata to Arrhaphoceras briacensis (?) zones), interrupted near the Albian-Cenomanian boundary by a sea level fall and hiatus. Anoxic to disoxic conditions are recorded in the latest early Albian and the late Albian, which correlate with the Leenhardt and Breistroffer levels, respectively. Correlations with Albian successions in North Africa and southern Europe suggest a mainly eustatic origin for the most important sedimentary discontinuities, although the late Albian sea-level drop may be tectonically enhanced.

High-latitude platform carbonate deposition constitutes a climate conundrum at the terminal Mesoproterozoic

During the Mesoproterozoic Era, 1600 to 1000 million years ago, global climate was warm with very little evidence of glaciation. Substantial greenhouse warming would have been required to sustain this ice-free state given 5-18% lower solar luminosity. Paleomagnetic data reported here place voluminous ca. 1.2 Ga shallow marine carbonate deposits from India at an unexpectedly high latitude of around 70° from the equator. Previous studies noted high latitudes, but their implication was never considered. Here, we evaluate the temporal-latitudinal distribution of neritic carbonate deposits across the Proterozoic and identify similar deposits from North China that together with those from India are seemingly unique to the late Mesoproterozoic. A uniformitarian interpretation implies that this is cold-water carbonate deposition, but facies similarity with low-latitude neritic deposits rather suggests a hotter climate and elevated polar ocean temperatures of 15–20° or higher. This interpretation represents a climate conundrum that would require much greater greenhouse warming than documented for the Mesoproterozoic.

Rock Magnetic‐Based Cyclic Expression in Late Visean Ramp Carbonates and an Astrochronology for the Late Asbian From Northwest England

AbstractThe late Asbian appears to mark the initial, well‐documented, onset of far‐field glacio‐eustatic changes in equatorial Mississippian strata. This work unravels the nature of cyclicity in upper Asbian shallow marine carbonates, using a combination of petrographic study, rock magnetic proxies and astrochronological testing on samples from the Trowbarrow section, NW England. Rock magnetic data express the content of two types of siliciclastic sources; a marine‐delivered magnetite‐dominated source, and an eolian‐delivered, hematite‐dominated source. The eolian‐sourced material generally peaked during regressive and low‐stand parts of the carbonate rhythms. Astrochronologic testing methods based around the average spectral misfit and TimeOpt methodology show the magnetite abundance proxies are principally carrying the astronomically forced signal. Two likely sedimentation rate models are derived from the five better magnetic proxies using evolutive methods. In addition, a set of three likely major hiatus levels are inferred in the sedimentation rate models, based on testing possible major hiatus scenarios with TimeOpt methods, using eccentricity modulation. From these, using the three best proxies, an average astrochronologic duration for the Trowbarrow section suggests a late Asbian duration of 1976 ± 86 kyr (1σ), and a basal late Asbian age of 334.48 ± 0.35 Ma (2σ). Coupled atmosphere‐ocean models for the late Paleozoic, suggest that lows in short eccentricity correspond to glacials, when inferred delivery of siliciclastic sediment to the carbonate ramp is generally at a maximum. The glacial and lower sea‐level intervals also coincide with maximum delivery of eolian siliciclastics, likely linked to increased aridity and less vegetation cover on adjacent and distal parts of Laurentia.