Sequence Stratigraphic Surfaces Research Articles

First record of Reticulinella? kaeveri in the Laffan and Ilam formations (Zagros fold and thrust belt, SW Iran): new constraints on the chronostratigraphic calibration of the Arabian Plate sequence stratigraphy

Over the past two decades, sequence stratigraphy has proven to be a valuable tool in addressing challenges arising from the inconsistent application of lithostratigraphic nomenclature across diverse geological provinces. It has been particularly successful in achieving regional correlations within the Cretaceous deposits of the Arabian Plate. The key sequence stratigraphic surface in the currently adopted standard sequence stratigraphic scheme is the K150 SB, marking the transition from a passive to an active margin along the eastern edge of the Arabian plate. Despite its importance, the chronostratigraphic age of this surface is poorly constrained. In the Zagros FTB, the uppermost part of the Sarvak Formation, just below the K150 SB, was recently assigned to the upper part of middle Turonian to upper Turonian, based on the presence of Reticulinella? kaeveri. Our research reveals that the range of R.? kaeveri extends above the K150 SB as this species is also present in the Laffan and base of Ilam formations. The Last Occurrence of this taxon provides a new chronostratigraphic marker of the upper Turonian within the Ilam Formation. This finding proves that more Turonian deposit are present in the study area than was previously recognized. We have studied the deposits enclosing the K150 SB in outcrop sections and wells along a transect from basin to platform margin to inner platform setting. Based on planktonic foraminifera and calcareous nannofossils biostratigraphy of basinal sections and sequence stratigraphic correlation to carbonate platform sections, the age of the uppermost part of the Sarvak Formation, immediately below the K150 SB, is firmly constrained within the interval lower Turonian–lower part of middle Turonian. Sequence stratigraphic correlation from basin to platform interior reveals also that the oldest deposits above the K150 SB are not younger than late Turonian. Our new data represents a significant improvement in the chronostratigraphic calibration of sequence stratigraphy in the Cretaceous of the Zagros fold-and-thrust belt.

Further Development of the Lower Cretaceous Clinoform Model of the North of West Siberia Based on the Sequence Stratigraphy Principles: New Possibilities of Stratification

The clinoform concept of the Lower Cretaceous of Western Siberia was and remains the scientific basis for predicting its oil and gas potential. At the same time, new challenges and new technological opportunities require its further development. The basis for this can be a modern model independent methodology of sequence stratigraphy, the essence of which is to divide the stratigraphic section into a succession of systems tracts by mapping of all sequence stratigraphic surfaces. At the same time, the choice of the type of surfaces limiting the sequence is carried out already at the second stage on the basis of the expression of the surfaces in the stratigraphic section or the preferences of the researcher. The wide development of unconformities in the Western Siberian Lower Cretaceous clinoform complex makes it possible to subdivide clinoforms into four systemic tracts: highstand HST, falling-stages FSST, lowstand LST, transgressive TST. The subdivision is carried out by mapping sequence stratigraphic surfaces based on the analysis of stratal stacking patterns. According to the authors, the boundaries of the sequences in this case should be the tops of the TST – the downlap surfaces (DLS), traditionally interpreted as maximum flooding surfaces (MFS). This ensures the consistency of the proposed approach with the clinoform concept. The named clinoforms of Western Siberia can be considered as genetic sequences of the III order. However, the position of facies in their systems tracts is far from always predictable. For further detailing of facies models, clinoforms – III order sequences should be subdivided into clinoforms – IV order sequences, and subdivide them into systems tracts. But the latter are not always mapped by modern seismic method. Special conditions are necessary for this. Therefore, it is proposed at first to subdivide the III order sequences into systems tracts. Then – to map their thicknesses, and to allocate of their depocenters. Further, within the depocenters (under suitable seismogeological conditions), to define IV-order clinoforms – sequences and their constituent system tracts as a basis for mapping reservoirs and hydrocarbons traps.

Facies analysis and sequence stratigraphy of shallow marine, coarse‐grained siliciclastic deposits in the southern Utsira High: The Late Jurassic intra‐Draupne Formation sandstones in the Johan Sverdrup Field (Norwegian North Sea)

AbstractThin, condensed coarse‐grained shallow marine successions can be difficult to describe and interpret, especially in the subsurface since the recognition of finer‐grained intervals, typically associated with sequence stratigraphic surfaces, is challenging. This lack of mudstones and siltstones means that they also typically make excellent reservoir intervals. The Oxfordian to Volgian intra‐Draupne Formation sandstones in the Johan Sverdrup Field, southern Utsira High, represent such a system. This study presents a new sequence stratigraphic model for the Johan Sverdrup Field that unravels the detailed depositional history of the succession and places its formation within a regional Late Jurassic tectonostratigraphic framework. The intra‐Draupne Formation sandstones comprise four parasequences deposited following a regional Kimmeridgian marine flooding event. Sediments were mainly supplied through West‐derived fan deltas from the Haugaland High and NW‐SE‐directed tidal currents reworking the Augvald Graben and the Avaldsnes High at the East. The oldest parasequence shows a distinctive suite of facies consisting of fine‐grained and mud‐rich bioturbated sandstones deposited in a semi‐restricted lagoon. Subsequent parasequences lack fine‐grained sediments and are dominated by bidirectional cross‐stratified, very coarse‐to coarse‐grained sandstones and gravels deposited in a tidal strait. A progressive reduction of fault‐related subsidence in the Middle Volgian along with Late Volgian‐Ryazanian sea‐level rise and inversion of pre‐existing structures promoted backstepping of the feeder systems, sediment starvation and the progressive deposition of the black and green‐red shales of the Draupne and Asgard formations. The results of this study account for features previously unidentified in the Johan Sverdrup Field and which have implications for understanding the deposition of coarse‐grained shallow marine successions around the Utsira High and other transgressed basement highs.

The potentials of accessory palynomorphs as sequence stratigraphic and basin evaluation tools in the shallow offshore Niger Delta

Freshwater algae – Botryococcus and Pediastrum, marine indices – dinoflagellates and microforaminiferal linings, and charred Poaceae cuticles are accessory palynomorphs commonly recovered from Niger Delta sediments. The abundant recovery of these non-pollen palynomorphs necessitates a test of their usefulness for sequence stratigraphic purposes. Bio-sequence stratigraphic data and stratigraphic distribution trends of Botryoccocus, Pediastrum and marine indices as well as charred Poaceae cuticles from three oil wells from the offshore shallow Niger Delta were collated and evaluated against systems tracts as well as sequence stratigraphic surfaces - transgressive surface, marine flooding surface and sequence boundary. Results show that the ratio of Botryococus:Pediatsrum:Marine index, and charred Poaceae cuticles have reliable sequence stratigraphic value in the studied area. This value will help in sequence stratigraphic study of the area particularly in its sand-dominated sequences characterized by poor faunal contents typical of the offshore shallow Niger Delta.

Did sea level change drive carbon isotopic trends in the Madison Shelf? Sequence stratigraphy and carbon isotopes in the Mississippian Lodgepole Formation of southwest Montana

The Lower Mississippian Lodgepole Formation of Montana and Wyoming records one of the largest positive carbon isotopic excursions of the Phanerozoic. This globally recognized up to 7‰ increase in δ13Ccarb values occurs across the North American Kinderhookian-Osagean boundary (referred to as the KO excursion). It has been argued to reflect significant organic carbon burial, possibly linked to the onset of the Late Paleozoic Ice Age. Previously proposed correlations between carbon isotopic patterns and the sequence stratigraphic framework within these strata suggests that changes in sea level could have played a significant role in the expression and/or magnitude of the KO excursion in the Madison Shelf. This study explores the relationship between carbon isotopic values and sea level change at multiple scales. To accomplish this, we provide a comprehensive overview of the sedimentological and stratigraphic framework and address uncertainty about the number of sequences in the Lodgepole Formation. Our results support a three-sequence model for the Lodgepole Formation. Based on the number of sequences and the placement of sequence stratigraphic surfaces, we see little evidence of statistically significant correlation between carbon isotopic trends and the sequence stratigraphic framework. We argue that sea level change was not the primary driving mechanism for carbon isotopic trends in the Madison Shelf, nor the KO excursion. Instead, we support models that invoke global ocean anoxia and/or destabilization of the global carbon cycle due to land plants.

Ichnology of tidal ravinement omission surfaces in siliciclastic transgressive deposits from the Puesto El Moro formation (Upper Cretaceous), Southern Patagonia, Argentina

Omission surfaces represent a gap in the sedimentary record on a particular environment, constituting an important tool for basin analyses. Nevertheless, not always these surfaces constitute key sequence-stratigraphic surfaces, and their correct interpretation is highly important for the recognition of genetically related strata. Documentation and interpretation of ichnological data from stratigraphic surface can refine the sedimentological model. The Puesto El Moro Formation (lower Upper Cretaceous) in the Austral-Magallanes Basin, southern Patagonia (Argentina), shows excellent examples of several omission surfaces stacked during a transgressive context associated with the beginning of the foreland stage. Each of these surfaces, interpreted as Tidal Ravinement Surfaces, contain Thalassinoides isp. and Sinusichnus isp., which represent opportunistic colonization by crustaceans into a predominantly firm substrate. In this case study, the vertical recurrence of substrate-controlled trace fossil suites is the result of high-frequency repeated coastal conditions during the transgression; the associated surfaces, which have limited lateral extension, have low potential for correlation at basin scale.

An integrated clinoform trajectory and sequence stratigraphic model for the Labrador margin, offshore eastern Canada

The passive margin offshore Labrador began forming during rifting in the Early Cretaceous, with seafloor spreading starting in the Maastrichtian and ending near the Eocene–Oligocene boundary. A foundational understanding of the associated stratigraphy of this margin has been developed through previous studies of the lithostratigraphy, biostratigraphy, and seismic stratigraphy, but the significance of clinoforms, stratal boundaries, and transgressive and regressive events has never been fully explored. In this study, we identify a series of clinoform successions from the Late Cretaceous through Pleistocene and apply a second-order sequence stratigraphic model to refine the geological framework for the margin. We focus on the Hopedale Basin where there is sufficient well and reflection seismic data, with substantial new seismic data collected over the last decade. Seismic interpretation is combined with paleoenvironmental and biostratigraphic data from the wells to map clinothem packages along the margin including: seven shoreline, five subaqueous delta, one shelf-edge, and three shelf-edge deltas, with illustration of these on seismic profiles and paleogeographic maps representing several time slices. Trajectory analysis of these clinothems permits the delineation of eight second-order sequences, with systems tracts and sequence stratigraphic surfaces identified. Broader implications of our results include a revised lithostratigraphic column for the Labrador margin. We further demonstrate regional trends during rifting and opening in term of controlling factors on clinoform development through comparison with adjacent and conjugate margins. Tectonism dominated as a controlling factor in the Cretaceous and dwindled near the end of the Paleocene. Eustasy and climate also influenced clinoform development in the Late Cretaceous, but became major factors during the Cenozoic, enhancing relative sea level fall and sediment supply, with similarities to other clinoform successions developed during Greenhouse and Icehouse conditions. Impacts to petroleum prospectivity in the region are also discussed, and our overall seismic and stratigraphic framework provide a basis for future study.

Chapter 11. North Sea Basin Jurassic lithostratigraphy

Abstract The most important North Sea Jurassic–lowermost Cretaceous lithostratigraphic units, as developed in the UK, Norway and Danish sectors, are summarized in this chapter (55 units from the UK, 25 from Norway and 10 from Denmark). Some significant issues remain with the use and application of lithostratigraphic terminology in the Jurassic of the North Sea Basin. In particular, there are inconsistencies in unit definition and nomenclature changes across country sector boundaries that obscure the recognition of regional stratigraphic patterns that exist across the region. To aid clarity and to overcome some issues of definition, some revisions are made to the existing lithostratigraphic schemes. Several informal lithostratigraphic units are described, a number of unit definitions are revised and various formerly informal units are formalized (Buzzard Sandstone Member, Ettrick Sandstone Member and Galley Sandstone Member). It is recommended that use of the Heno Formation in offshore Denmark is discontinued. In addition, four new lithostratigraphic member terms are introduced (Home Sandstone Member, North Ettrick Sandstone Member, Gyda Sandstone Member and Tambar Sandstone Member). All described units are placed into a sequence stratigraphic context. All significant lithostratigraphic boundaries conform with key sequence stratigraphic surfaces.

Chapter 14. Summary and conclusions

Abstract An updated sequence stratigraphic framework, comprising 39 third-order stratigraphic sequences, for the Jurassic–lowermost Cretaceous of the North Sea, is described by reference to key wells and seismic lines across the UK, Norway and Denmark sectors, and, where possible, to onshore UK outcrops. It appears evident that regional tectonics provided the main control on sequence development, particularly during the Late Jurassic. There is a close relationship between key sequence stratigraphic surfaces and many lithostratigraphic formation and member boundaries throughout the North Sea Jurassic. Four new sandstone members are defined. A biozonation scheme for the study interval is described that provides essential characterization of the defined sequences.

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.

Geochemical expression of sequence stratigraphic surfaces: A case from Upper Cretaceous shallow-water carbonates of southeastern Neo-Tethys margin, SW Iran

Providing a chronostratigraphic framework often is problematic in shallow-marine carbonates with poor biostratigraphic resolution. In such cases, an integration of geochemical, sedimentological, and paleontological data can be used to construct a reliable sequence stratigraphic framework. This study focuses on elemental concentrations and isotopic ratios of Cenomanian–Santonian (C–S) neritic carbonates in SW Iran used to construct a sequence stratigraphic framework, asses sequence ages and durations of hiatus. The diagenetic and geochemical expression of two paleoexposure surfaces, representing type-I sequence boundaries (SBs), are discussed. Increase in Fe and Rb concentrations and decrease in Ca content are recorded at and/or below distinct erosional surfaces. Mg content shows a facies-dependent response with former LMC (low magnesium calcite) or aragonitic facies showing increase while HMC (high magnesium calcite) indicates decrease in response to SBs. Mn shows considerable changes apart from sequence surfaces that are strongly facies dependent. Sr concentrations show an increase in HMC and decrease in LMC in samples which experienced semi-closed meteoric diagenesis, far below the SBs. However, in samples characterized by open system diagenesis, directly underlying the SBs, Sr tends to decrease in all components. The Sr/Rb ratio is a reliable tool in detecting erosional SBs, shown as conspicuous negative excursions. C (carbon) and O (oxygen) isotopes show distinct negative excursions at erosional SBs and a slight increase at or around the MFSs. The 87Sr/86Sr ratio shows clear positive excursions at SBs probably due to 87Rb decay in incorporated aluminosilicate detritus, which makes it as the most reliable proxy for the detection of SBs in the studied sections. Two main erosional SBs are marked by 87Sr/86Sr ratios including the CT-ES (Cenomanian–Turonian boundary exposure surface) and mT-ES (middle Turonian exposure surface) with hiatus durations of 0.53 and 2.7 Myr, respectively. These paleoexposure surfaces are correlated with other neritic sections of C–S sequences in SW Iran.

Concurrent 1. Presentation for: A regional chronostratigraphic framework for play-based resource assessments in the Eromanga Basin

Presented on Tuesday 17 May: Session 1 Geoscience Australia is undertaking a series of basin-scale assessments to identify the ‘yet-to-find’ resource potential for hydrocarbons, as well as for groundwater resources and carbon capture and storage (CCS) opportunities in central Australia under the Exploring for the Future (EFTF) Program. A play-based exploration approach is being used to systematically evaluate the key risk elements for each resource type through the analysis of drilling results and spatial data to map ‘sweet spots’ for exploration. These assessments aim to reduce the risks and uncertainties for explorers by providing spatially enabled assessments of energy resources and CCS potential. The work will also improve the understanding of existing groundwater resources which may be impacted by future energy resource developments or provide feedstock for future green hydrogen projects. A key requirement for undertaking such play-based resource assessments is to apply a common regional chronostratigraphic framework across all the resource types that link different geological unit nomenclatures through defining the assessed reservoir and seal intervals and their associated sequence stratigraphic surfaces (sequence boundaries, transgressive surfaces and maximum flooding surfaces). A Mesozoic chronostratigraphic framework has been developed for the Eromanga Basin, which defines nine regional play intervals that host the known hydrocarbon and groundwater resources, or represent potential CCS targets. The Mesozoic play framework is now being applied to undertake play-based low-carbon energy resource assessments in the western Eromanga Basin, with initial work focussing on the interpretation and correlation of the nine play intervals in wells for post-drill analysis. To access the presentation click the link on the right. To read the full paper click here

A regional chronostratigraphic framework for play-based resource assessments in the Eromanga Basin

Geoscience Australia is undertaking a series of basin-scale assessments to identify the ‘yet-to-find’ resource potential for hydrocarbons, as well as for groundwater resources and carbon capture and storage (CCS) opportunities in central Australia under the Exploring for the Future (EFTF) Program. A play-based exploration approach is being used to systematically evaluate the key risk elements for each resource type through the analysis of drilling results and spatial data to map ‘sweet spots’ for exploration. These assessments aim to reduce the risks and uncertainties for explorers by providing spatially enabled assessments of energy resources and CCS potential. The work will also improve the understanding of existing groundwater resources which may be impacted by future energy resource developments or provide feedstock for future green hydrogen projects. A key requirement for undertaking such play-based resource assessments is to apply a common regional chronostratigraphic framework across all the resource types that link different geological unit nomenclatures through defining the assessed reservoir and seal intervals and their associated sequence stratigraphic surfaces (sequence boundaries, transgressive surfaces and maximum flooding surfaces). A Mesozoic chronostratigraphic framework has been developed for the Eromanga Basin, which defines nine regional play intervals that host the known hydrocarbon and groundwater resources, or represent potential CCS targets. The Mesozoic play framework is now being applied to undertake play-based low-carbon energy resource assessments in the western Eromanga Basin, with initial work focussing on the interpretation and correlation of the nine play intervals in wells for post-drill analysis.

Electro-sequence analysis and natural resources potential of a transitional environment in central swamp depobelt, Niger delta, Nigeria

Sequence stratigraphy and Electrosequence of the Don-Field Central Swamp depobelt of the Niger Delta Basin, was carried out using well logs, biostratigraphic and core data. The aim of this study is to re-characterise and revalidate the reservoirs within the study area and provide an optimum framework for predicting reservoir character, development and reservoir quality distribution. Detailed petrophysical evaluation reveals proficient hydrocarbon reservoir units with variable parameters (i.e. porosity, permeability and shale volume) that can accommodate productive hydrocarbon reserve. Core data analyses revealed heavy minerals like pyrites and glauconites that can be of economic interest. The gamma wireline signatures of the wells show reservoir units that are predominantly blocky in shape indicative of fluvial channels. Fluvial channels are typically associated with coastal and/or marginal marine environments. More so, the aforementioned observations were validated using palaeoenvironmental interpretation of strata penetrated by Well-1, Well-2, and Well-3 via the sidewall core analyses. Sequence stratigraphic surfaces such as maximum flooding surfaces (MFS) and sequence boundary (SB) were observed to show good correlation with palynological and foraminiferal zones. The stacking patterns (i.e. progradation, retrogradation and aggradation) encountered gave acumen into systems tracts (i.e. lowstand system tract (LST), transgressıve system tract (TST) and highstand system tract (HST) interpretation. These system tracts constitute a depositional sequence set of middle—late Miocene age bounded chronologically by MFS of 9.5 Ma, 12.8 Ma, 15.0 Ma, 15.9 Ma and SB of 12.1 Ma, 13.1 Ma, 15.5 Ma, 15.9 Ma. The SB and MFS were delineated based on Shell Petroleum Development Company (SPDC) biozonation scheme: F-zone and P-zone were within P820-P680 and F9500-F9600/F9700 as contained in the biofacies interpreted data of index fossils (Globorotalia foshi S.I; Globorotalia praefohsi; Globorotalia peripheroacuta; Orbulina spp; Praeorbulina sicana; Paragloborotalia mayeri; Neogloboquadrina acostaensis) which corresponded to the alphanumeric code N8–N16 planktonic foraminiferal zones of Blow.

Sequence stratigraphy of the Andaman Basin, northern Indian Ocean

Andaman Basin, a poly history basin in the northeastern Indian Ocean, preserves the sedimentary record from Late Cretaceous to Recent with several stratigraphic breaks. Studies from outcropping sedimentary succession, integrated with subsurface data from drilled wells in the offshore, have enables to work out a sequence stratigraphic framework of the sedimentary succession in the basin.The succession comprises of three first-order sequences viz., Late Cretaceous – Eocene, Oligocene and Neogene. These have been related to basin forming tectonic events related to accretionary prism formation and subduction in Paleogene, Oligocene uplift and renewed subduction in Neogene, respectively. Excellent bio-stratigraphic control and field evidence have enabled identification of unconformities of various magnitudes, which in turn has enabled mapping second-order sequence. The 3 second-order sequences within first-order Late Cretaceous–Eocene sequence are: Late Cretaceous-Early Paleocene, Late Paleocene-Middle Eocene and Late Eocene sequence. The Oligocene first-order sequence related to the major uplift and emergence of the main Andaman Island chain, contributed >2000m of flysch sediments. Three second-order sequences within the first-order Neogene sequence are Early Miocene - early Middle Miocene, Middle Miocene - Middle Pliocene and Late Pliocene - Pleistocene sequences separated by unconformities of various magnitude. Each sequence is discussed with respect to its extent, nature of sequence boundaries, sedimentary fill and sequence stratigraphic surfaces to understand the sequence stratigraphic architecture of the basin.

Jurassic–Lower Cretaceous sequence stratigraphy and allogenic controls in proximal terrestrial environments (Southern Junggar Basin, NW China)

AbstractThe terrestrial sequence stratigraphy in proximal terrestrial environments is different from its counterpart in distal terrestrial and marine environments. The Southern Junggar Basin in NW China has preserved well‐exposed proximal terrestrial outcrops and its Jurassic–Cretaceous sequence boundary remains contentious. The goal of this paper is to (a) establish a model of allogenic‐controlling mechanisms of a proximal terrestrial sequence, (b) investigate the Jurassic–Cretaceous sequence boundary, (c) the Jurassic climate, and (d) tectonic movements in Central Asia. The sequence stratigraphic framework was established assisted by the determination of the accommodation space/sediment supply (A/S) ratio, including two second‐order sequences (SS) and six third‐order sequences (SQ) in the Jurassic, as well as a second‐order sequence in the Cretaceous. SS1 was mainly controlled by a warm and humid climate and lake level fluctuations in a weak extensional tectonic setting; SS2 was mainly controlled by an arid climate and compressional tectonic movements; SS3 was mainly controlled by a semi‐humid climate and lake transgression in a stable tectonic setting. Additionally, the Jurassic–Cretaceous sequence boundary was distinguished within the Kalazha Formation based on its sedimentary characteristics, sedimentary evolution, and different allogenic controls. It is concluded that the proximal terrestrial sequences exhibit rapid sedimentary facies change, thick coarse‐grained sediments, and multiple provenances. Climate, tectonic movements, and lake level fluctuation dominate in different stages of basin evolution. Moreover, the sediment supply is more emphasized, and the accommodation space/sediment supply ratio, which corresponds to different channel geometry types, plays a critical role in identifying system tracts and sequence stratigraphic surfaces.

The Adıyaman Group carbonate succession on the northern Arabian Plate (southeastern Turkey): A review of lithostratigraphy, foraminiferal biostratigraphy, depositional environments and sequence stratigraphic evolution

Abstract This study presents a new stratigraphic framework for the Adiyaman Group succession developed on the northern margin of the Arabian Platform. This Upper Cretaceous carbonate sequence, distributed widely in outcrop and subsurface rocks in the southeast Anatolia region, is investigated in terms of lithostratigraphy, biostratigraphy, microfacies analysis and sequence stratigraphy. The sequence is divided into six lithostratigraphic units, which in ascending order are the Karabogaz, Saytepe, Saydere, Sayindere, Beloka and Sanli formations. Based on examination of benthic and planktonic foraminifera, the Adiyaman Group ranges in age from late Santonian to early late Campanian. The Saytepe, Beloka and Sanli formations are characterized by benthic foraminifers, while the Karabogaz, Saydere and Sayindere formations are defined by planktonic foraminifers. Following a detailed microfacies analysis, the Adiyaman Group carbonates are interpreted to have been deposited in settings that range from deep shelf to lagoonal depositional environments. Sequence stratigraphic studies identified three successive third-order depositional sequences that are separated by sequence stratigraphic surfaces. Sequence boundaries are correlated to global cycle boundaries. The abundance and diversity of foraminifera are interpreted as a function of relative sea-level changes. This study provides revisions in stratigraphic nomenclature that rely on precise biostratigraphic data.

Out-of-phase cyclical sediment supply: A potential causal mechanism for generating stratigraphic asymmetry and explaining sequence stratigraphic spatial variability

ABSTRACTAlthough acknowledged to be a simplification, the rate of sediment supply is usually assumed to be constant in sequence stratigraphic interpretations of clastic shelf systems. The simplified assumption taken in this work is that sediment supply can be represented by sine curves linked to climate changes driven by Milankovitch cycles. Three orders of sediment supply sine curves (amplitude and frequency scaled to order) are convolved with three orders of Milankovitch-forced eustatic sea-level sine curves and a constant rate of subsidence to generate curves for the ratio of rate of accommodation development to rate of sediment supply (δ A /δ S ). The relative-sea-level curve is then held constant whilst sediment supply is systematically changed from being constant to being cyclical across the three orders of Milankovitch frequencies and being in-phase, and out-of-phase with the eustatic cycles by 90°, 180°, and 270°. For each scenario, stratal architecture is then represented for sixty consecutive parasequences (fifth-order, regressive–transgressive shelf transit cycles) by converting the δ A /δ S curves into pseudo thickness / sandstone fraction plots (TSF plots). Constant sediment supply, in-phase sediment supply, and 180°-out-of-phase sediment supply produce symmetrical stratal geometries with equal periods of progradation, aggradation, and retrogradation. When sediment-supply cycles are 90°-out-of-phase (supply peak occurs later than sea-level peak), stratal geometries are asymmetrical with progradational architectures being dominant. When sediment-supply cycles are 270°-out-of-phase (supply peak occurs earlier than sea-level peak), stratal geometries are also asymmetrical but retrogradational architectures are dominant. These patterns are reproduced at all three orders of stratigraphic hierarchy (parasequence, sequence, and composite sequence). Comparison of these synthetic stratal geometries to real-world stratal geometries from Triassic to Neogene rocks across both the fifth-order (parasequence) and fourth-order (sequence) of stratal hierarchies suggests a consistently occurring asymmetrical, progradation-dominant motif. This indicates that 90°-out-of-phase sediment supply (supply peak occurs later than sea-level peak) may be a common occurrence through geological time. The work also corroborates the findings of earlier workers and suggests that sequence stratigraphic surfaces can change nature along depositional strike due to out-of-phase sediment supply and can thus also be diachronous. This work conceptually illustrates that Milankovitch climate-change-induced sinusoidal-sediment-supply cycles, out-of-phase with sinusoidal eustatic-sea-level cycles, may produce commonly observed asymmetrical stratal architectures and should be considered when invoking causal mechanisms for stratal architectures on clastic shelves.

Sequence stratigraphy of the subsurface cambro-ordovician siliciclastic deposits in the Chotts basin, Southern Tunisia: Petroleum implications

Subsurface study of the Palaeozoic deposits in the Chotts basin, part of the southern Tunisian Atlassic foreland, revealed an incomplete Pre-Hercynian deformed sequence. The remaining truncated Cambro-Ordovician siliciclastic deposits appear to be unconformably overlain by transgressive Late Silurian – Early Devonian shales. Early Ordovician El Atchane sandstones and Hamra quartzite constitute currently the most oil and gas productive levels in the Chotts basin. The main issues encountered in the exploration of these reservoirs are related to their spatial distribution along with their thickness and facies variations. Sequence stratigraphic analysis and correlation of the entire Cambro-Ordovician section using sequence stratigraphic surfaces seems necessary to locate both proven and prospective reservoirs in order to predict their spatial distribution and trend directions in the region. Seismostratigraphic facies analysis of these deposits in subsurface highlights their sequence organizations. The integration of well data revealed in detail the sequence hierarchy of the Cambro-Ordovician deposits throughout the Chotts basin.

Low‐accommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvio‐deltaic settings (Cretaceous Mesa Rica Sandstone, Dakota Group, USA)

AbstractThe adequate documentation and interpretation of regional‐scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low‐accommodation settings and control of backwater hydraulics on fluvio‐deltaic stratigraphy. Exhumed examples of full‐transect depositional profiles across river‐to‐delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down‐dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio‐deltaic deposition in the Tucumcari sub‐basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down‐dip into single‐story channel sandstones and eventually into contemporaneous distributary channels and delta‐front strata. Down‐dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi‐storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised‐valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta‐front geometries. This opposes criteria offered to differentiate incised valleys from flood‐induced backwater scours. As the incised valleys evidence relative sea‐level fall and flood‐induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down‐dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea‐level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low‐accommodation settings to dominantly promote bypass, especially during forced regressions. Source‐to‐sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea‐level cycle.