Storm Wave Base Research Articles

Microfacies of the Silurian/Devonian boundary interval in the Gorna Vrabcha 2 section (Morava Unit, SW Bulgaria) – preliminary results

This study is focused on the sedimentary strata of the Silurian/Devonian boundary interval in the Gorna Vrabcha section 2 from the Morava Unit (SW Bulgaria). Four microfacies types have been distinguished and described: Bioclastic wackestone/packstone (MFT 1), Bioclastic packstone with clayey mudstone laminae (MFT 2), Bioclastic mudstone (MFT 3) and Homogeneous mudstone (MFT 4). They are respectively interpreted as formed in various high- and low-energy open-marine environments. Thus, during the Přídolí and at the begging of the Lochkovian sedimentation occurred in an open-marine setting located between the fair-weather wave base and the storm wave base (MFT 1 and MFT 2), that was often affected by storms and storm-related currents. Later, during the Lochkovian, a calm and relatively deep-water environment predominated (MFT 3 and MFT 4) and was most probably located below the storm wave base. Based on the obtained microfacies data and depositional setting interpretations a deepening trend of sedimentary basin during the Přídoli–early Lochkovian time have been suggested.

Mudstone diagenesis in the Cenomanian–Turonian Eagle Ford Group in the San Marcos Arch area. PART I: Chemostratigraphy, early diagenesis, bitumen expulsion and migration pathways

The Cenomanian–Turonian Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited below storm-wave base on the drowned south Comanche Shelf. Also, eolian deposited volcanic ash layers are common being preserved as individual ash beds or reworked into the sediment. An investigation of diagenesis relative to burial delineated a complex diagenetic history as a result of changes in burial history and associated changes of water chemistry and redox conditions. The first part of this comprehensive study sheds light on initial depositional redox conditions, early mineral diagenesis, and expulsion and migration pathways of generated bitumen and oil from kerogen of the EFG in the study area (maximum burial depth <6000 ft [1828.8 m]). The EFG followed a diagenetic pathway from the deeper marine directly into the burial environment (not affected by meteoric diagenesis). Original depositional porosity of the EFG marl could be as high as 70–80%, whereas present-day porosity values have been decreased to around 10% because of burial related diagenesis. At shallow burial depths, compaction was the mechanism responsible for most porosity loss followed by cementation. The texture and fabric of these cements increase mechanical strength of the EFG mudrocks significantly. The shallow, immature EFG limestones and marls have abundant interparticle micropores, high porosity, permeability, and water saturation with relatively low resistivity. With thermal maturation, bitumen and hydrocarbon generation increases oil saturation and alters wetness of the rock. The migration and filling of the bitumen and possibly oil would limit further cementation of calcite and quartz. The interaction between diagenesis of mineral matrix and organic matter (OM) evolution is significant and notably impacts cementation in mudstones at relatively shallow burial depth. The impact of OM is also more remarkable in mudrocks compared to that in sandstone and carbonate counterparts.

Cambrian-Ordovician trace fossils of the Basissletta region, northeast Spitsbergen, Svalbard

Trace fossils of the Cambrian–Ordovician sedimentary succession of the Basissletta region, northeast Spitsbergen, Svalbard, belong to three ichnoassemblages. The first one, composed of Monocraterion and Diplocraterion, belongs to the Skolithos ichnofacies and characterises sandstones of the Tokammane Formation (lower Cambrian: Terreneuvian) deposited in a shallow subtidal setting. The overlying carbonates with pseudomorphs after evaporites (uppermost part of Terreneuvian and the Cambrian Series 2) display abundant Balanoglossites. These beds were deposited in an oxygenated but hypersaline environment. The younger Ordovician carbonates of the Kirtonryggen Formation and the mixed carbonate-clastic deposits of the Valhallfonna Formation display a more diverse but poorly preserved trace fossil suite (Phycodes, Curvolithus, Planolites, Palaeophycus) typical of the archetypal Cruziana ichnofacies. Some parts of the succession are not bioturbated, and these are characterised by dark coloured fine-grained sediments with primary lamination. They were mostly deposited between the normal and storm wave base or below the storm wave base in oxic to anoxic environments.

Mudstone diagenesis with depth and thermal maturity in the Cenomanian–Turonian Eagle Ford group. PART II: Diagenetic processes and paragenetic sequence

The Cenomanian–Turonian (C–T) Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited on the drowned south Texas shelf below storm-wave base. A burial diagenesis investigation using seven cored wells from shallow to deep burial (depth ranges from 7684 to 13,670 ft [2342 to 4166.6 m]; BHT from 183 to 285 °F [83.9–140.6 °C]) and from east (San Marcos Arch) to west (Maverick Basin) delineated a complex diagenetic history as a result of changes in water chemistry, redox condition, and burial history. The second part of this comprehensive study shows the variety of diagenesis and paragenesis. This study focuses especially in the organic-rich Lower Eagle Ford (LEF) member deposited largely under anoxic/euxinic conditions. Processes such as near seafloor sulfate-reduction, Fe-reduction, and methanogenesis drive various chemical reactions during lithification. Feldspar diagenesis included albitization and transformation of K-feldspar to kaolinite and albite, and from kaolinite to Mg-chlorite with depth. Clay mineral diagenesis is complex with authigenic kaolinite, mixed kaolinite/Mg-chlorite, and Mg-chlorite form in biota tests and matrix. Additionally, calcite dissolution, burial dolomitization, late Fe-chlorite replacement, and oxidation of pyrite to marcasite are also observed. Detritus-derived smectite starts to transform to mixed layered clay, illite-smectite (I/S) in the oil window; however, illitization trends were not observed in depth. This is because most smectite forms from alteration of volcanic ashes and the volcanic origin of the disorientated smectite did not follow regular illitization kinetics. The increase in Mg content from illitization transforms not only the kaolinite to chlorite, but precipitates authigenic dolomites. The clay minerals precipitated in the original interparticle pores, further reducing porosity, and impacted permeability, fluid saturation, wettability, and capillary pressure. With thermal maturation, bitumen and oil conversion increases oil-wetness of the rock and the resistivity measurement from wireline logs.

First description of Tithonian corals from the Picún Leufú Depocentre, Neuquén Basin, west-central Argentina

A scleractinian coral fauna from the Upper Jurassic (Tithonian) of the Neuquén Basin (west-central Argentina) is described for the first time. It comprises only two species: Eocolumastrea octaviae (Prever, 1909) and Stelidioseris columbaris (Scott and Aleman, 1984). The first was reported previously from the Lower Cretaceous of this region, and the second is a newly reported register for the basin. The new record of Eocolumastrea octaviae reported here extends the stratigraphic range of the genus to the late Tithonian. Both corals are colonial and, when favourable conditions arose, thrived in a mixed carbonate-siliciclastic environment above or slightly below the storm wave base level. The coral fauna of the Picún Leufú Formation presents, regarding its taxonomy and type of growth form, a great similarity to that of the Valanginian-Hauterivian Agrio Formation of the same basin. It differs notably from that of the Oxfordian La Manga Formation, suggesting a change in palaeoenvironmental conditions probably due to a general increase in the sedimentation rate in the studied basin between the Oxfordian and the Hauterivian, among other possibilities.

Depositional model of the Holocene coquinas - Albardão platform, southern Brazil

For the past decade, giant deepwater oil discoveries in the pre-salt section of the Campos and Santos basins of Brazil, have brought significant attention to offshore exploration activities along the South Atlantic margins. The prolific Cretaceous coquina deposits in these basins are part of the pre-salt rock record and constitute an effective but complex and heterogeneous hydrocarbon reservoir difficult to predict and model. Parting from this context, an evaluation of the sedimentological, structural and taphonomic criteria for coquinas are essential to better understand and predict the facies distribution and depositional models of the pre-salt coquinas strata. Based on this premise, the present work aims to genetically interpret 133 mixed carbonate-siliciclastic bottom sediments of the Albardão shelf – a modern marine coquina analogue, using facies description, investigating the relationship with hydrodynamic forces, and accessing the influence of morphology and structural framework on their deposition. From these analyses, we recognized a hybrid facies, three modern carbonate facies in analogy to the carbonate rock classification and four siliciclastic facies. These eight facies were then grouped into three facies associations representing high, moderate, and low energy facies. The high energy facies association comprises rudstones (Rf) and grainstones (Gf) with highly fragmented bivalve shells and barnacles abundantly present in the beach system, above the fair-weather wave base limit (FWWB). These facies also occur offshore on bathymetric highs above the storm wave base limit (SWB) but display less reworking than the coastal high energy facies above the FWWB due to wave shoaling. The moderate energy facies association consists of hybrid sand (Hs), sand (S) and muddy sand (mS) occurring between the FWWB and SWB limits in the offshore transition zone with extensive winnowing action and low rate of reworking. The low energy facies association includes sandy mud (sM), mud (M) and micritic mud (Mc), characterized by the decantation of the fine sediments below the offshore SWB limit. The results confirm a bottom facies distribution controlled by depth, shelf profile morphology and energy from incident waves. The fragmented rudstone and fragmented grainstone facies are the best-recognized reservoirs with both having high porosity and high permeability.

Does water transparency control the banding in shallow water iron formations?

Abstract Iron formations (IFs) are marine chemical sediments that are conined to the Precambrian rock record and provide unique insights into the co-evolution of the atmosphere-hydrosphere and biosphere through almost three billion years of Earth’s history. IFs commonly appear throughout the Archaean until the Palaeoproterozoic ca. 1.8 billion years ago and re-appear during the “Snowball Earth” epoch in the Neoproterozoic. The formation and deposition mechanism(s) of IFs are, however, still incompletely understood, hindering unique interpretations of palaeoenvironments. Many IFs are banded iron formations (BIFs) with layer thickness of alternating Fe- and Si-rich layers ranging over several orders of magnitude from the nanometre to the metre scale. A second textural type, so called granular iron formations (GIFs) that form above storm wave base become widespread in the Palaeoproterozoic. Understanding the fundamental mechanisms that are responsible for the textural types and the periodicity of banding in BIFs is crucial to link these features to the environmental and geochemical evolution of the Earth. We here provide a conceptual model that highlights the role of changing light conditions and water transparency for Iron Formation (IF) precipitation. We show that the model is particularly feasible for IFs deposited in shallow waters but may also be applicable for some IFs deposited in deeper water settings. The model builds on other primary Banded Iron Formation (BIF) precipitation models postulating that Fe(III)-(oxyhydr)oxide production can be dominated by anoxygenic photoautotrophic Fe2+-oxidising bacteria. These so called photoferrotrophs are adapted to very low light levels corresponding to about 1% of the light level required by oxygen-producing phototrophs allowing them to thrive deep down in the water column. The depth of Fe(III)-(oxyhydr)oxide production is mainly controlled by water turbidity which controls how deep photosynthetically available radiation (PAR) penetrates the water column. Eutrophic conditions result in relatively shallow Fe(III)-(oxyhydr)oxide production depth due to turbidity either induced by the biomass itself and/or by particles that are actively or passively produced by microorganisms (e.g., Fe(III) and Mn(IV)-(oxyhydr)oxides, sulphides), triggering the formation of Fe-rich bands. During oligotrophic stages, Fe(III)-(oxyhydr)oxides are only produced relatively deep down in the water column, so that only silica-rich bands form in the Fe(III)-(oxyhydr)oxide free upper water column. Reactive transport modelling adopted from Ozaki et al., (2019) shows that besides upwelling and nutrient supply, alternating Fe(III) production depth is mainly associated with changing light conditions as a result of water transparency. Periodicities reflected by alternating Fe- and Si-rich bands in IFs in our model can thus be associated with: (1) nutrient supply patterns; (2) additional sources of turbidity in the water column such as Fe(III) and Mn(IV) oxide particles, sulphides, and wind-blown silicate particles; or (3) formation and clearing of organic haze in the atmosphere. One or all of these reasons for low light conditions seem to become more important in the Palaeoproterozoic (&amp;lt;2.4 Ga) and could be partly responsible for the more widespread occurrences of shallow marine granular IFs relative to former epochs, which is often assigned to the gradual oxidation of the ocean. Our model shows that variable water transparency should be considered as additional factor for IF deposition especially for shallow marine settings. This model also reasonably explains the prominent layering in BIFs as syn-depositional feature.

Late Mesoproterozoic iron and manganese deposition on a deep-water carbonate platform (Penganga Group), Adilabad, India: Constraints from C, O and Sr isotopes

AbstractThe Penganga Group in the Pranhita-Godavari Rift Valley of central India includes a rather unique example of Late Mesoproterozoic iron and manganese formations deposited in a deep-water below storm wave base distal carbonate platform environment. The carbonate rocks of the Penganga Group are mostly lime-mudstones that have been classified into a number of sedimentary cycles with intervals mostly distinguished by color, ranging from brown/pink and siliceous gray to steel-gray and black. At least two about one-meter-thick Mn-Fe formations occur in siliceous gray limestone of the distally steepened part of the Penganga carbonate platform. A number of northwest-southeast striking thrust faults repeat the succession in down dip sections bringing up distal profiles of the Penganga Group against proximal counterparts at the present exposure level. This contribution provides the first comprehensive record of stable C and O and radiogenic Sr isotopes for proximal and distal profiles, allowing for direct comparison and thus, providing context for the environmental conditions of Fe and Mn deposition in a Mesoproterozoic platform environment.Carbon and oxygen isotope compositions determined from the proximal and distal profiles of the platform yield trends that are attributed to variations in circulation pattern and cycles of warm and cold climatic stages across the platform. Secular trends of Sr-isotope compositions suggest a variation in continental input between cold (0.7065) and warm climatic stages (0.7149), similar to that observed for modern marine sediments. The sedimentation of siliceous deposits and ferromanganese deposits in the distal platform environment was likely favored by the onset of upwelling circulation during cold climatic stages. The moderately negative δ13CPDB values (-5.34‰ to -6.34‰) of the Mn-carbonates and variation in δ13Corg values 31.7‰ to -21.7‰ obtained for early diagenetic Mn-carbonate ovoids indicate oxidation of organic matter in the ferromanganese deposits by Mn-oxides during early diagenesis. We conclude that upwelling and platform-wide deposition of ferromanganese deposits on the Penganga carbonate platform may mark the transition from the stagnant Mid-Proterozoic ‘sulphidic’ ocean to a ‘respiring’ open ocean during the Late Mesoproterozoic Era.

Climate cyclicity-controlled recurrent bottom-water ventilation events in the aftermath of the Toarcian Oceanic Anoxic Event: the Jenkyns Event

Environmental perturbations of the Toarcian Anoxic Event and its associated carbon isotope excursion (CIE) occurred in a cyclic fashion indicating an orbital control mechanism. Sedimentary strata of the E. elegantulum ammonite subzone in the Lorraine Sub-basin, Luxembourg, exhibit eight sedimentary cycles, most of which postdate the CIE, implying that its termination did not coincide with a full recovery from environmental stress. Sea-level and temperature fluctuations of the Toarcian crisis were linked to a cryosphere demise in the Northern Hemisphere, which modulated stadial versus interstadial phases on the orbital 100 ka eccentricity frequency band. Upon stadial phases, enhanced wind strength in combination with lowered sea level disrupted stratified shelf waters and shifted the storm wave base close to the sea floor. Ventilation of bottom waters interrupted accumulation of laminated and organic-rich black shales, but formed organic-lean and non-laminated gray claystones enriched in terrigenous wax lipids and spores provided via aeolian transport from the hinterland due to the enhanced wind strength. Wind systems are assumed to have been driven by high pressure differences between the cryosphere in the North of the Tethyan shelf and the warmer costal lowland. This distinguishes the atmospheric turbulences after the CIE from the postulated intensification of the cyclones that accompanied the rapid warming at the beginning of the CIE. The deep-water ventilation event following the CIE in the Lorraine Sub-basin was accompanied by a parallel evolution in the SW German Basin, indicating a supraregional driving mechanism in paleobathymetric sub-basins susceptible to lowering of storm wave base. In sub-basins with greater water depth on the NW-Tethyan shelf, post-CIE orbitally driven atmospheric turbulences had a minor or no effect on water column stratification and deposition of organic-rich laminated black shales persisted.Graphical abstract

Clypeina? pamelareidae n. sp., a new dasycladalean alga from the Upper Triassic of Lime Peak (Yukon, Canada)

A new calcareous alga is described from the Norian limestones of Lime Peak, Yukon, Canada. The algae-bearing limestones were formed in a low-energy environment below the storm wave base, as indicated by the abundance of carbonate mud. Clypeina? pamelareidae n. sp. usually appears in the form of separate laterals scattered in the sediment. The laterals are pyriform, with the tip slightly curved upwards. Sometimes they are arranged around a non-calcified axial cavity. The lack of calcification of the connection between the main axis and the laterals led to the detachment and scattering of the latter in the sediment. The shape of the laterals and their arrangement are reminiscent of the genus Clypeina (Michelin). Still, the lack of completely preserved specimens leads us to attribute it with uncertainty to this genus. Almost certainly, the alga in question is individualised as a new distinct species, basically different from other Mesozoic or Cenozoic species of the genus, as well as from the only species of Clypeina known from the Triassic (Clypeina besici Pantic).

Unravelling the genetic mechanisms and paleoenvironmental controls of early Cretaceous Oyster Mass occurrences (OMOs), Neuquén Basin, west-central Argentina

Lower Valanginian oyster mass occurrences (OMOs) from the Neuquén Basin of Argentina are analyzed using a multidisciplinary approach, including the description of their sedimentological signature and stratigraphic contacts, assessment of taphonomical attributes, and paleontological and paleoecological characteristics. These OMOs present a wide distribution in the study area, with lateral continuity for at least 2.5 km and up to 12 m thick. They occur within a single stratigraphic interval, constrained in terms of sequence stratigraphy and biostratigraphy. Three stacked tabular OMOs separated by mudstone levels were recorded in all the studied localities. The associated lithofacies point to a mainly outer ramp paleoenvironment, below storm wave base and occasionally disturbed by exceptional, distal storm flows. Internally, the OMOs share a common vertical trend characterized, from base to top, by a gradual increase in oyster abundance and a transition from mainly reclining, disarticulated oysters to articulated, cementing oysters conforming build-ups. Hence, a mainly biogenic origin is proposed, with autobiostromes grading vertically to bioherms. This vertical trend was interpreted in terms of development stages, namely, colonization, expansion, climax and extinction, which were in turn related to specific paleoenvironmental controls. Particularly, the OMOs establishment and development were associated to low sedimentation rates, salinity fluctuations and high nutrient input as a result of high primary productivity. At a larger scale, the overall paleoenvironmental conditions and subtropical geographical position of the basin were detrimental for most reef builders typical of the Cretaceous period (e.g., corals, sponges, rudists), and could have favored oyster proliferation and OMOs development instead.

Holocene Foraminifera, Climate, and Decelerating Rise in Sea Level on the Mud Patch, Southern New England Continental Shelf

Abstract We examined Holocene benthic foraminiferal biofacies, % planktonic foraminifera, and lithofacies changes from New England mud patch cores and present a relative sea-level (RSL) record to evaluate evolution of these rapidly deposited (30–79 cm/kyr) muds. Sandy lower Holocene sections are dominated by Bulimina marginata. The mud patch developed from 11–9 ka as RSL rise slowed from 10 to 7 mm/yr; mud deposition began when the cores (69 to 91 m modern) were inundated below storm wave base. An Elphidium-B. marginata fauna developed at ca. 7–6 ka as RSL rise slowed from approximately 7 to 2 mm/yr. A Globobulimina fauna developed at 3 ka as RSL rise slowed to 1 mm/yr, reflecting lower O2 conditions. Single specimen δ18O analyses of Globobulimina show ∼1‰ variations over the past 3 kyr, reflecting a shelf bottom water seasonal cycle of 4–5°C, and a temperature minimum during the Little Ice Age with warming since.

The deep-water, high-diversity Edgewood-Cathay brachiopod Fauna and its Hirnantian counterpart

The last phase of the end-Ordovician extinction event involved substantial sea-level changes. The Oslo/Asker District in Norway is a rare place where the deeper-water early Hirnantian fauna is succeeded by the equivalent deeper-water Edgewood-Cathay Fauna. Both faunas are highly diverse, with the same small-shelled brachiopods, Onniella, Leangella, and Eoplectodonta, dominating. They also share a large number of long-ranging and eurytopic genera. Taxa from contemporary shallower-water environments are rare. Near a third of the brachiopod genera in the Norwegian deeper-water early Hirnantian Fauna went extinct, including seven genera that survived into the Ordovician/Silurian boundary strata. Deeper-water early Hirnatian and Edgewood-Cathay collections are not well-known worldwide. Global quantitative samples from low latitudes, including the Norwegian samples, were compared against each other using NMDS (Non-metric Multidimensional Scaling) with the Bray-Curtis index. Qualitative samples used NMDS with the Raup-Crick index and Network Analysis. The Raup-Crick index sharply differentiated the early Hirnantian and Edgewood-Cathay faunas, possibly due to sensitivity to extinction and origination data. In contrast, the paleogeographic affinity between collections of the two faunas is pronounced using the Bray-Curtis index. Network Analysis also demonstrates regionality; the Edgewood-Cathay Fauna is especially heterogeneous. This contrasts with the shallower-water Hirnantia Fauna, which is more cosmopolitan. Quiet waters below the storm-wave base possibly hindered the spread of larvae, and anoxic plumes of water may have caused further barriers to the lateral spreading of the Edgewood-Cathay Fauna. As in Norway, long-ranging, eurytopic taxa were shared between the two faunas with few typical shallow-water taxa.

Upper Visean (Mississippian) coral biostromes during onset of the late Palaeozoic Ice Age: A record from the equatorial eastern Palaeotethys

The late Visean-Serpukhovian is a critical time in reef evolution and climate history, characterized by the gradual collapse of coral reef systems during the onset of the main glaciation phase of the late Palaeozoic Ice Age (LPIA). However, detailed palaeoecological variations of coral reefs during the collapse phase are not well understood. In this study, about one-meter thick coral biostromes are reported from the upper Visean Shangsi Formation in the Yashui section, central Guizhou Province, South China. Coral colonies are primarily in growth position resulting in the classification of autobiostrome and autoparabiostrome. Dominant colonial rugose coral species are Stylostrotion petalaxoidea and Siphonodendron pauciradiale, respectively. The biostromal framework is simple and primarily constructed by bafflestones with slightly positive relief. Diversified dwellers are found between coral skeletons, including abundant foraminifers, crinoids, calcareous algae and brachiopods with less bryozoans, tabulate and solitary rugose corals, gastropods and ostracods. Relative sea-level fluctuations control the growth and demise of the Yashui biostromes, evidenced from changes in both microfacies and biotic assemblages from their underlying to overlying strata. The petalaxoidea biostrome developed in relative deeper water depth than that of the pauciradiale biostrome, between storm wave-base and fair-weather wave-base on a shallow photic open-marine shelf. The pauciradiale biostrome in South China (eastern Palaeotethys) has similar biotic composition, colony preservation and depositional environments to coeval coral biostromes in Ireland (western Palaeotethys). The Yashui coral biostromes formed shortly after the LPIA onset, evidenced in small-scaled eustatic fluctuations and a positive shift in brachiopod oxygen isotopic signatures.

Nitrogen isotope gradient on continental margins during the late Paleoproterozoic

The late Paleoproterozoic was a key period in the evolution of Earth’s surface environments and life. It is generally accepted that during the 2.43–2.06 Ga Great Oxidation Episode, the atmospheric oxygen level rose to an intermediate state (0.1% to 10% of the present atmospheric level) and the chemical composition of the oceans changed significantly. The paleontological record indicates that eukaryotes appeared during the late Paleoproterozoic. Nitrogen, an essential nutrient for life, is thought to have been one of the key factors in eukaryote evolution. However, nitrogen bioavailability and spatial heterogeneity in nitrogen cycle in the late Paleoproterozoic oceans remain controversial. Here, we examine carbon and nitrogen cycling in the late Paleoproterozoic ocean, via analysis of redox-sensitive element contents and organic carbon and nitrogen isotope compositions of black shales of the ca. 1.9 Ga Nuvilic Formation of the Povungnituk Group, Cape Smith Belt, Canada. Two diamond drill-holes (DDHs) 718–3333 (∼50 m-long) and 4G8069 (∼90 m-long) investigated in this study contain turbiditic greywackes and black shales deposited on the northern margin of the Archean Superior craton below the storm wave-base. Sedimentary rocks of DDH 4G8069 might have been deposited closer to the continental source area, as indicated by sandstone abundance, than those of DDH 718–3333. Redox-sensitive element (V, U, and Mo) contents in black shales of DDH 4G8069 are low and show positive correlation with Al contents, whereas those of DDH 718–3333 are irregularly elevated. Fe/Al ratios of DDH 718–3333 are low, which likely reflect Fe loss from sediments under anoxic water-column conditions. Depositional environment is inferred to be above and below the redoxcline for DDHs 4G8069 and 718–3333, respectively. Organic carbon isotope values of DDH 718–3333 are approximately 1.7‰ lighter than those of DDH 4G8069, which might reflect methane cycling at the redoxcline. Nitrogen isotope values are positive (> +3‰) for all the black shale samples, corresponding to aerobic nitrogen cycling and presence of bioavailable nitrate on the northern margin of the Superior craton during the late Paleoproterozoic. Furthermore, nitrogen isotope values for DDH 718–3333 are approximately 2‰ heavier than those for DDH 4G8069. The observed isotopic heterogeneity is similar to that in modern oxygen-minimum zones and could represent intense denitrification at the redoxcline. Although no ca. 1.9 Ga microfossils reported to date are considered unambiguously eukaryotic, the aerobic nitrogen cycling inferred for the Nuvilic Formation, as well as for the previously studied Rove, Virginia, and Menihek formations, suggests that nitrogen limitation was unlikely to have inhibited biological evolution during the late Paleoproterozoic, a finding that is consistent with molecular clock studies suggesting that the emergence of eukaryotes occurred during the Paleoproterozoic.

Machine learning of the chemical elements for enhanced interpretation of depositional environments: Upper Jurassic strata case study in central Saudi Arabia

To explore how machine learning (ML) techniques can enhance the interpretation of depositional environments using chemical elemental data, this study analyzes data from 156 shallow marine strata samples. Several advanced machine learning (ML) techniques were integrated to predict lithofacies and stratigraphic units of shallow marine carbonate strata, focusing on the Arab-D reservoir equivalent outcrops in Central Saudi Arabia. Utilizing a detailed geochemical dataset comprising major, trace, and rare earth elements, we aimed to determine the feasibility of using chemical elemental data in sedimentary geology for predictive modeling. The studied strata are categorized into two lithological units based on depositional settings: one unit comprises rocks that dominantly deposited below the storm wave base with a single lithofacies association, whereas the other includes rocks dominantly deposited above the storm wave base, and consists of two lithofacies associations. The study demonstrates the efficacy of ML algorithms in distinguishing these two lithological units, with a minimum prediction accuracy of 76%. Four ML techniques, including K-nearest neighbors (KNN); random forest classifier (RFC); gradient boosting classifier (GBC); and multi-layer perceptron neural network (MLP-NN) were used, two of them (RFC and GBC) achieved 89% and 92% prediction accuracy. However, a slightly poorer performance was observed in predicting fine-scale lithofacies associations of these lithological units, especially for the lithofacies association representing a transitional interval between the two lithofacies units. Expanding the scope of the study, we tested the ML models against geochemically similar strata from the Oligocene shallow marine reef carbonates. These strata are globally comparable and provide an external validation for the predictive models developed from the Saudi Arabian dataset. GBC and RFC maintained a higher accuracy level than KNN and MLP, demonstrating their robustness and transferability across diverse geological settings. Based on these results, the study underscores the significance of specific oxides and elements as distinctive geochemical markers, elucidating the environmental conditions of the depositional environments and their deposits. This research advances the application of ML in interpreting the depositional environments of shallow marine strata using elemental data, as the majority of the existing researches focus on employing unsupervised machine learning for cluster analysis.

Shallow‐marine calciclastic mass‐transport deposits in an evolving thrust‐top basin: A case study from the North Dalmatian foreland basin, Croatia

AbstractMass‐transport deposits are products of resedimentation phenomena involving a broad spectrum of gravity‐driven processes, and commonly have a high preservation potential in deep‐marine environments. This study documents various types of mass‐transport deposits that are interbedded with intensely bioturbated shallow‐marine calciclastic sediments deposited along a reflective coast during the middle and late Eocene. The sedimentary succession, located in the vicinity of Novigrad in northern Dalmatia, Croatia, comprises sediments deposited in a range of nearshore and carbonate ramp environments, and represents the infill of a thrust‐top (piggyback) basin of the North Dalmatian foreland basin. Five types of mass‐transport deposits, ranging in thickness from 13 cm to 6 m, have been identified: (i) calcilutite and calcarenite slumps; (ii) conglomeratic slump‐debrites with a ‘dough‐like’ appearance; (iii) blocky‐flow deposits bearing large blocks of beachface and/or shoreface deposits; (iv) rockfall deposits comprising scattered blocks of beachface conglomerates and shoreface calcarenites; and (v) ‘classical’ matrix‐supported debrites. Calciturbidites are rare and mainly comprise Ta and Tb divisions. Conglomeratic slump‐debrites are mostly found in association with offshore‐transition deposits, suggesting that mass flows were triggered above the storm‐wave base likely due to a combined effect of: (i) strong earthquakes related to the tectonic development of the basin; (ii) sediment destabilization due to pore‐water overpressure during forced regressions; and (iii) storm‐wave loading affecting the shallow seabed. Progressive deepening likely favoured mass‐flow transformations, although the overall paucity of turbidites suggests relatively short mass‐flow transport distance and turbidity current bypass to deeper realms. Multiple erosion phases and resedimentation processes from the Cretaceous to the late Eocene contributed to the diverse suite of extraformational clasts in the mass‐transport deposits studied. The mass‐transport deposits may be triggered and emplaced in shallow‐marine settings mainly during regressive stages of basin development, as the diverse gravel clast composition suggests significant tectonic influence. Although the mass‐transport deposits reported herein are relatively small, some of their peculiar sedimentary features and occurrence within shallow‐marine calciclastic deposits render them rather unique and suitable for a re‐assessment of the nature and evolutionary continuum of processes involved in subaqueous sediment mass transport, as well as the preservation potential of sedimentary features in high‐energy wave‐reworked environments.

Early Cretaceous shifting of Zoophycos in the Ouarsenis Mountains (northwestern Algeria)

Early Cretaceous succession of the Oued Fodda Formation in the Ouarsenis Mountains (northwestern Algeria) is mainly composed of marl–limestone alternations, which are subdivided into four informal units (Units 1 to 4), based on distinct lithological, stratonomical, and ichnological features. The ichnological analysis reveals a low diversity of the trace-fossil assemblage, which is exclusively reported from Units 2 and 3. The ichnoassemblage contains six ichnotaxa (Chondrites intricatus, Ophiomorpha isp., Planolites isp., Thalassinoides isp., Zoophycos brianteus, and Zoophycos cauda-galli), among which Zoophycos and Chondrites are the most common elements of the assemblage and occur in distinct mud-rich substrates showing different bioturbation intensities. The development of Zoophycos in the middle part of Unit 2 shows a high degree of bioturbation (bioturbation index (BI) = 4). Zoophycos specimens are of large size, between 45 cm and 75 cm in width, which were interpreted to have formed in a lower offshore environment where the oxygenation amount was optimal, the sedimentation rate was low, and the benthic food was abundant on the seafloor. Toward the upper part of Unit 2, Zoophycos-bearing levels exhibit a less intense degree of bioturbation (BI between 1 and 2) in contrast to Planolites- and Chondrites-bearing levels which have a bioturbation index (BI) between 3 and 4. At these levels, Zoophycos displays relatively small, coiled to U-shaped spreiten, probably in response to stressful and dysoxic conditions prevailing in the water bottom. With improved oxygenation in a quiet lower offshore to shelf margin environment in Unit 3, the benthic organisms recovered, as represented by medium to large size Zoophycos in association with Ophiomorpha and scarce Chondrites burrows, even if the overall bioturbation intensity is very low. The combination of trace-fossil assemblage and lithofacies of the Oued Fodda Formation indicates relatively stable outer shelf environments below the storm wave base, which corresponds classically to the lower offshore to shelf edge environments, and the prevailing palaeoecological conditions are optimal and stressful for the benthic organisms.

Lower Jurassic trace fossil Zoophycos from the Ksour Mountains of northwestern Algeria and their palaeoecological and palaeoenvironmental significance

The Zoophycos ichnofacies is reported for the first time from the Lower Jurassic (Toarcian) deposits of Algeria. The trace fossil-bearing strata in the Ksour Mountains, northwestern Algeria, are an alternation of fine-grained limestones and marls. The assemblage is characterised by its low diversity, consisting of eight ichnospecies (Chondrites intricatus, C. targionii, Chondrites isp., Palaeophycus isp., Thalassinoides isp., Zoophycos brianteus, Z. cauda-galli, and Zoophycos isp.), representing the Zoophycos ichnofacies ethologically dominated by deposit-feeding behaviours of different producers. The abundance of Zoophycos and Chondrites within a low diversity assemblage suggest stressful conditions most likely indicating oxygen depletion within the sediment. Moreover, the presence of reworked burrows by Chondrites could indicate ecological succession. This is probably due to environmental conditions getting more stressful, and causing the need for exploiting the nutrient stored in the abandoned burrows after the death of the trace maker. This process is primarily associated with an oxygen-depleted environment, compelling the animal to engage in it as a means of obtaining energy, rather than actively searching for food within the sediment. On the other hand, the occurrence of Thalassinoides may indicate an improvement of palaeoecological conditions and short periods with partial increase of oxygenation. The ichnological, palaeontological and palaeoecological data of the studied succession indicate a relatively deep-water environment located well below the storm wave base at a deeper offshore setting.

Depositional and tectonic influences on preservation of Milankovitch record during long-term global cooling: Middle and Upper Ordovician convergent foreland, eastern USA

Influences of depositional setting and tectonics on recording Milankovitch (orbital) signals within evolving Ordovician foreland successions during arc-continent collision are poorly documented. To address this, we analyzed downhole gamma ray vs depth time series from three wells on the Central Appalachian foreland, eastern USA for evidence of Milankovitch forcing during the 25 Myr duration Middle and Upper Ordovician Darriwilian to Katian interval. Relatively slowly accumulating Darriwillian sabkha anhydritic dolomites (up to 450 m thick), are dominated by eccentricity and obliquity forcing whereas Darriwillian to Sandbian restricted inner ramp carbonates (350 m thick) show eccentricity forcing only as do the overlying basal Katian (80 m thick) mid-ramp units. Rapidly accumulating lower Katian deeper ramp, shale-prone sediments (400 to 500 m thick) and upper Katian peritidal siliciclastic units (∼300–550 m thick) show a full suite of short eccentricity, obliquity and precession forcing. Best recording of the astronomical signal occurred during Katian rapid tectonic subsidence whereas a much poorer record was preserved during the Darriwilian-Sandbian. Milankovitch forcing of climate influenced input of siliciclastics as well as controlling sea-level fluctuations and position of storm wave-base. Initially sea level changes may have been driven by greenhouse aquifer eustasy but following later Darriwilian cooling, glacio-eustasy was dominant.