Eustatic Sea-level Variations Research Articles

Astronomical time scale of the lower Miocene depositional sequences (Aquitanian-Burdigalian) in the Jambour Field, northern Iraq

The Miocene succession is considered one of the major shallow hydrocarbon reservoirs in Iraq. It consists primarily of lagoonal evaporites and carbonate alternations, with many barren intervals and non-diagnostic fauna. An astronomical time scale (ATS) was established in two wells in Jambour Field, Kurdistan, using high-resolution cyclostratigraphic analysis of GR data for the first time in Iraq to determine the absolute age of the Miocene sequences and their boundaries. Sixteen unique carbonate and evaporite facies types were discovered and classified into five facies associations, from the mid-outer ramp to the peritidal carbonate and evaporite ramps. The most significant of these implies that, after astro-climatic frequencies, sea-level change was the dominant regulator of lower Miocene deposition. The difference in GR and facies type is mostly due to third eustatic sequences, which describe a rich series of long and short-period eccentricities in orbitally defined terms. The eustatic sea-level variations of the third-order sequences correlate with the sedimentary noise model applied over the tuned GR. A sharp sediment accumulation rate (SAR) was caused by an abrupt shift in the depositional environment, as demonstrated by the applied eCOCO within the lower Aquitanian part of the Dhiban Formation; consequently, each well was split into two datasets. The SAR ranges from 3.1 to 1.97 cm/kyr in the Ja46 well and 2.3 to 3.3 in the Ja49 well. The MTM spectral analysis of the four datasets revealed the eccentricity cycles' dominance with a high degree of confidence level. The present study employs high-resolution ATS using tuning of the GR with the new merged short and long eccentricity cycle filter output method, which is supported by previous strontium isotope studies and compared to the La 2004 astronomical solution. According to the ATS, the Aquitanian-Burdigalian sequences range from 5.08 myr at Ja46 Well to 4.66 myr at Ja49 Well. The tuning output indicated the presence of two hiatuses, the first in the Dhiban Formation with a time gap of about 0.21 and 0.3 myr in the Ja46 and Ja49 wells, respectively, and the second at the top of the Jeribe Formation with a time gap of about 0.34 myr in the Ja49 well. This pause corresponds to a significant sea-level drop in the Arabian Platform Basin at 21.4–21.0 and 18.5 Ma.

Facies analysis and depositional model for the Oxfordian Hanifa Formation, Central Saudi Arabia

The Jurassic was a time for major global changes in climate, tectonics, and oceanography, reflected in the southern Neo-Tethys Ocean by a distinct transition from carbonate-poor to carbonate-dominated sedimentation. To better constrain the effect of this turning point in the Late Jurassic, key outcrops of Oxfordian deposits of the Arabian Plate were investigated through detailed sedimentologic analysis. High-resolution (centimeter-scale) description and analysis of eight outcropping sections with a cumulative thickness of 461 m through the Oxfordian Hanifa Formation along a N–S 535 km-long transect in central Saudi Arabia are used to develop a comprehensive conceptual depositional evolution through time. Nine sedimentary lithofacies are interpreted and grouped into four facies associations. These associations include: I) outer-ramp association (FA1), II) mid-ramp association (FA2), III) inner-ramp grainy-facies association (FA3), and VI) inner-ramp reefal association (FA4). The lithofacies succession distinguishes two depositional phases, a middle Oxfordian homoclinal ramp (Phase 1) that corresponds to the Hawtah Member, and a late Oxfordian reef-bearing carbonate ramp (Phase 2) that corresponds to the Ulayyah Member. After being restricted to the high latitudes in the northern Neo-Tethys during early and middle Oxfordian ages, the carbonate ramp evolution within the Hanifa Formation reflects the expansion of scleractinian coral reefs over the southern Neo-Tethys passive margin in the late Oxfordian. However, the distribution of the corals in the Arabian Plate is mainly controlled by water salinity and siliciclastic input; they are absent northward due to hypersalinity and southward due to siliciclastic input. Furthermore, the differential subsidence and the resulting uneven topography, in addition to the eustatic sea level variation during the Oxfordian age, controls the distribution of different depositional styles over the Arabian Plate (homoclinal ramp, distally-steepened ramp, rimed shelf, and intrashelf basin).

Global Phanerozoic sea levels from paleogeographic flooding maps

The validity of sea level estimates based on stratigraphic correlations has been debated since the 1990 s as relative sea level curves differ between sites due to local tectonics, different deposition rates and changes in dynamic topography. Here, we offer a new eustatic (global) sea level curve for the past 520 million years (Myrs) based on observations of global flooding. We use paleogeographic reconstructions to measure the area of today’s exposed land that was flooded in the past (modern-land flooding). We then apply the modern global hypsometric slope to reconstruct the sea level history. We find minimum sea levels (comparable to today’s level) towards the end of Pangea (210 Ma) and peak levels (∼280 m higher than today) at 80 Ma when Pangea was widely dispersed. A first-order “supercontinent” cycle of 250 million years (Myrs) is recognized but we also document a second-order cycle of 37 Myrs that was previously thought to be undetectable using the hypsometric method. The hypsometric slope is critical for reconstructing past sea levels, and steepening the hypsometric slope during Pangea assembly implies dramatically larger sea level fluctuations. Our new sea level curve shares strong similarities with stratigraphic constraints and correlates with seafloor production proxies throughout the Phanerozoic. Measurements of global flooding represent averages across great continental extents, making them less sensitive than stratigraphic analyses to regional-scale vertical land motion, such as from dynamic topography and hence more reliable for estimating eustatic sea level. This method can also help to identify local deviations caused by regional uplift or subsidence and serves to constrain geodynamic mechanisms for sea level change. Our new sea level reconstruction usefully tracks global variations in Phanerozoic eustatic sea level, but also opens opportunities to estimate such variations in deeper time.

Late Quaternary Marine Terraces and Tectonic Uplift Rates of the Broader Neapolis Area (SE Peloponnese, Greece)

Marine terraces are geomorphic markers largely used to estimate past sea-level positions and surface deformation rates in studies focused on climate and tectonic processes worldwide. This paper aims to investigate the role of tectonic processes in the late Quaternary evolution of the coastal landscape of the broader Neapolis area by assessing long-term vertical deformation rates. To document and estimate coastal uplift, marine terraces are used in conjunction with Optically Stimulated Luminescence (OSL) dating and correlation to late Quaternary eustatic sea-level variations. The study area is located in SE Peloponnese in a tectonically active region. Geodynamic processes in the area are related to the active subduction of the African lithosphere beneath the Eurasian plate. A series of 10 well preserved uplifted marine terraces with inner edges ranging in elevation from 8 ± 2 m to 192 ± 2 m above m.s.l. have been documented, indicating a significant coastal uplift of the study area. Marine terraces have been identified and mapped using topographic maps (at a scale of 1:5000), aerial photographs, and a 2 m resolution Digital Elevation Model (DEM), supported by extensive field observations. OSL dating of selected samples from two of the terraces allowed us to correlate them with late Pleistocene Marine Isotope Stage (MIS) sea-level highstands and to estimate the long-term uplift rate. Based on the findings of the above approach, a long-term uplift rate of 0.36 ± 0.11 mm a−1 over the last 401 ± 10 ka has been suggested for the study area. The spatially uniform uplift of the broader Neapolis area is driven by the active subduction of the African lithosphere beneath the Eurasian plate since the study area is situated very close (~90 km) to the active margin of the Hellenic subduction zone.

Detecting detrital carbonate in shale successions - Relevance for evaluation of depositional setting and sequence stratigraphic interpretation

The Portwood Member of the New Albany Formation is a dolomitic black shale succession that was deposited in the western portion of the Appalachian Basin. Correlative of the Tully Limestone interval found to the northeast in the Appalachian Basin and slightly more than 5 m thick, it nonetheless consists of three erosion-bounded stratal packages that record Late Givetian eustatic sea-level variations. Detailed petrographic study shows that its carbonate interbeds, though readily interpreted as transgressive in nature, are largely detrital, and represent lowstand deposits eroded from the Cincinnati Arch in the west. Petrographical inspection showed abundant silt-sized detrital carbonate within the black shales, as well as a range of other key features, such as mud-dominated composite particles and agglutinated benthic foraminifera, that shed a new and much differentiated light on depositional setting and history of sediment accumulation. Whereas earlier recognized internal erosion surfaces still anchor the sequence stratigraphic framework, reinterpreting carbonate beds as lowstand deposits, and identifying condensed intervals and maximum flooding surfaces via distinct glauconite maxima, supports a reassessment of system tracts that is in agreement with petrographic constraints. Thin section based microfacies analysis identified multiple sedimentary settings, ranging from comparatively shallow (10's of meters, suboxic to dysoxic, tempestites) to very shallow (meters, dysoxic to oxic, wave and current action), and enabled subdivision of the Portwood into a series of upwards shallowing parasequences. This study demonstrates that petrographic due diligence is essential to correctly identify the origin of shale hosted carbonate intervals, and as an added benefit yields a sophisticated understanding of depositional conditions and long-term depositional trends that rivals what is possible for sandstone or carbonate successions.

Integration of geophysical and geomechanical data to understand the depletion of the Marlim field, Campos Basin

Located in the Campos Basin, Brazil, the Marlim field, consists of two turbidite systems deposited during eustatic sea-level variations in the Oligocene/Miocene. The reservoir was discovered in 1985, and its production started to decline in 2002. One of the techniques selected to assist in the recovery of oil from the reservoir was the 4D seismic. However, its interpretation can be complex. In order to help address this issue, the present study proposed an analysis of the depletion of a small field area from 1997 to 2010, combining geophysical (4D seismic) and geomechanical (pore pressure) data through the construction of pore pressure 3D models for both years, which can be subtracted and compared to seismic anomalies. The results obtained were: an average depletion of 0.42 ppg (50.33 kg/m3) of pore pressure gradient in the field; the identification of potential fluid-flow barriers, such as an NW-SE-oriented channel and sealing faults; and the detection of two areas with an expressive presence of 4D seismic anomalies, one of them showing a quite evident difference between pore pressure gradients, suggesting field depletion. The use of very old and noisy seismic data hindered the application of this methodology. Nevertheless, this research demonstrated the relevance of estimating pore pressure in the reservoir and how this geomechanical parameter can be useful in assessing the level of field depletion.

Facies analysis of a Bartonian–Aquitanian siliciclastic‑carbonate system, Costa Rica

Costa Rican Eocene–Oligocene carbonate deposits have been known for more than a hundred years, however, there are few scientific contributions detailing the relation between facies analysis and regional basin evolution. Based on a detailed study in southern Costa Rica, a chronologic history of a Bartonian–Aquitanian sedimentary succession is proposed. The carbonate facies analysis allows for eight microfacies (MF1 to MF8) to be distinguish. The siliciclastic facies analysis permits identification of two main periods of sandstone deposition and one of shale formation. The mixed sandstone/carbonate succession exhibits three main periods of deposition: (1) Bartonian–lower Priabonian, represented by distal, relatively deep slope-to-basin environments. Deposits comprise thick accumulation of sandstones, calcareous breccias, and calciturbidites. This period terminates with a stratigraphic erosion surface; (2) Upper Priabonian–Rupelian, represented by more proximal, shallow to mid depth inner to middle ramp environments. Deposits are composed of peloidal/cortoidal grainstone, foraminiferal-algal rudstones, and rhodolith rudstones. This period terminates with a second stratigraphic erosion surface; (3) Chattian–Aquitanian, represented by proximal to distal, shallow to coastal and offshore environments. Deposits are composed of a mixed siliciclastic carbonate thin complex overlain by dark shale.The vertical and lateral facies changes permit a basin evolution model that is closely controlled by subsidence. Local tectonic uplift may have formed the first subaerial exposure surface. The second subaerial exposure surface may have been controlled by eustatic sea-level variations over relatively shallow water deposits. These results, the first-of-their-kind in the southern Costa Rican Fila Costeña, could permit improvements of lateral regional correlations by bringing new lithostratigraphic layers of reference.

Astronomically forced climate change in the late Cambrian

We report evidence for Milankovitch cycles in two drill cores from the Cambro–Ordovician Alum Shale Formation of Scandinavia. The signal is preserved in elemental abundances recorded at high stratigraphic resolution by core scanning XRF analysis (0.2 mm resolution). The new data enable us to establish a floating timeline calibrated to the stable 405 kyr eccentricity cycle for a ∼8.7 Myr interval across the Miaolingian–Furongian boundary. This interval spans the Steptoean Positive Carbon Isotope Excursion (SPICE), which is recorded in the δ13Corg in the studied drill cores. We calculate the durations of the Olenus Superzone to 3.4 ± 0.2 Myr, the Parabolina Superzone to 1.9 ± 0.3 Myr, the Leptoplastus Superzone to 0.33 ± 0.18 Myr, the Protopeltura Superzone to 0.51 ± 0.20 Myr, and the SPICE event straddling the Paibian and lower main part of the Jiangshanian Stage to 3.0 ± 0.2 Myr. The sedimentation rate shows similar trends at both drilling locations and is inversely correlated to eustatic sea level changes in certain time intervals, opening tantalizing new prospects of using cyclostratigraphic analyses of shales to track eustatic sea level variations. The identification of obliquity cycles enables us to calculate the Cambrian Earth–Moon distance as well as the day length at ∼493 Ma to 368.9 ± 2.3 ⋅ 106 m and 21.78 ± 0.29 hr, respectively.

Carbonate/evaporitic sedimentation during the Messinian salinity crisis in active accretionary wedge basins of the northern Calabria, southern Italy

Abstract This work deals with Messinian deposits belonging to the Neogene infill of the Rossano and Belvedere Basins, respectively developed along the fore-arc and the back-arc areas of the north Calabria accretionary wedge. The main goal is to characterize the carbonate and evaporitic sedimentation during the Messinian Salinity Crisis, in the general framework of the basin architecture and the interplay between eustatic vs tectonic controlled sea-level variations. Fieldwork integrated with seismic lines and well logs interpretations led to the revision of the general stratigraphy of the basins and the proposal of a new sequential stratigraphic model driven by cyclic sea-level variations. Each cycle, repeated at least two times during the Messinian Salinity Crisis time frame, begins with a relative sea-level fall responsible for the emplacement of prograding wedges composed of terrigenous and evaporitic deposits that, subsequently, evolve in the deposition of primary basin-fill evaporites. This phase is followed by open marine transgression due to relative sea-level rise that terminates the evaporite formation and predates the development of microbial dominated carbonate platforms associated with shallow-water evaporites. Both basins experienced intense tectonic activity during the Messinian, which could be responsible for huge basinward sediments exportation and fast decreasing in the accommodation space. However, this did not substantially influence the development of the systems tracts that, considering the basinal architecture, have been mainly controlled by eustatic sea-level variations. In fact, two major sea-level drops associated with basin restriction and aridity (cold) conditions seem to have caused the origin of two main evaporitic units as basin-fill evaporites, while consequent sea-level rises and less stressed condition, account for two carbonate units with limited evaporites and terrigenous deposition.

Chronology of deposition and unconformity development across the Cretaceous–Paleogene boundary, Magallanes-Austral Basin, Patagonian Andes

The Magallanes-Austral foreland basin preserves an important record of orogenesis and landscape evolution in the Patagonian Andes of Chile and Argentina. Throughout the retroarc foreland basin, a regional disconformity with little to no angular discordance separates Upper Cretaceous–lower Paleocene strata from overlying deposits of diachronous Eocene to Miocene age. Here we report detrital zircon U-Pb geochronological results for 11 sandstone samples, and vitrinite reflectance data for 6 samples of organic matter from a fossiliferous dinosaur-bearing mixed nonmarine and marine clastic succession in the Río de las Chinas valley (50–51°S) of central-southern Patagonia to: (1) determine the timing and duration of the unconformity using U-Pb maximum depositional age constraints, (2) reconstruct sediment provenance and dispersal patterns, (3) assess possible temporal variations in arc magmatism, (4) evaluate the amount of sedimentary overburden removed during unconformity development, and (5) confirm the presence of a fossiliferous southern hemisphere Cretaceous–Paleogene (K/Pg) boundary site. Samples from the Dorotea Formation yield maximum depositional ages spanning Maastrichtian to Danian time (with ages as young as ∼65–63 Ma), confirming preservation of the K/Pg boundary in a section with recently discovered fossils of dinosaurs, other terrestrial vertebrates, and plants. Samples from directly above the unconformity in the Man Aike Formation, yield middle Eocene maximum depositional ages (with a prominent 45–40 Ma age cluster), indicating a long-lived ∼20 Myr hiatus representative of nondeposition or erosion. Analyses of organic matter preserved in multiple coal horizons of the uppermost Dorotea Formation show consistently low vitrinite reflectance values, requiring limited sedimentary burial, consistent with nondeposition or sediment bypass rather than deposition and later erosional removal of a previously proposed thick package of Paleocene to middle Eocene clastic material. On the basis of regional trends in the age and geometry of the unconformity, timing of arc magmatism, and temporal variations in sediment provenance, we consider a range of potential mechanisms for unconformity genesis, including (1) shortening-related uplift of the frontal fold-thrust belt, (2) cratonward advance of a flexural forebulge, (3) accommodation changes driven by regional or eustatic variations in sea level, (4) ridge collision and slab-window genesis, (5) isostatic rebound during tectonic quiescence (or minor extension), or (6) regional foreland uplift during flat-slab subduction.

Depositional and diagenetic constraints on the abundance and spatial variability of carbonate-associated sulfate

Marine carbonate rocks are composed, in varying abundance, of cement, micrite, abiotic grains and fossils, which can provide information about the physical and chemical environments in which they formed. Geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic (including petrographic) information available, resulting in potentially faulty reconstructions of biogeochemical and environmental conditions. These concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies. Here, we have combined X-ray Absorption Near Edge Structure (XANES) spectroscopy and μ-X-ray Fluorescence (μ-XRF) imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in Upper Ordovician carbonates from Anticosti Island, Canada and early Silurian carbonates from Gotland, Sweden, across multiple depositional facies. Lithofacies and fossil communities between Anticosti Island and Gotland are similar, which allows for comparison of changes in the dominant S species and their abundance in separate basins, associated with variations in (glacio)eustatic sea level. Sulfide abundance is greatest in mudstone, wackestone and packstone facies, where interstitial micrite hosts abundant pyrite. Sulfate abundance, as carbonate-associated sulfate (CAS), varies within individual fossil fragments, as well as within the same fossil phylum and is particularly high in unaltered brachiopods. In contrast, sulfate abundance is generally very low in micrite (near the detection limit) and generally arises in situ from sulfide that has been oxidized as opposed to true CAS. In different cement fabrics, sulfate abundance is greatest in drusy, pore-filling cements. Organic sulfur compounds are also detected and, although low in abundance, are mostly found within micrite. The detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization. This approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates, as well as the effect of depositional setting and diagenesis on CAS incorporation, (re)mobilization, and ultimate abundance in sedimentary carbonates. Additionally, this work has implications for the CAS isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous CAS sections throughout Earth history.

Evolutionary trend of the Gorgan Bay (southeastern Caspian Sea) during and post the last Caspian Sea level rise

The Gorgan Bay (GB) is a semi-confined sedimentary environment at the southeastern corner of the Caspian Sea (CS). High sensitivity to eustatic sea-level variations resulted in changes in the sedimentary facies and patterns of erosion/sedimentation for the GB in the last Caspian Sea-level cycle (1977–1995). Therefore, the evolutionary trend in coastal sedimentary environments was studied to determine the pattern of erosion and sedimentation related to sea-level fluctuation cycles. The sedimentology of the bay basin was studied in 500 km2 of the study area in 2001. In addition, coastal changes over the two past and post periods of sea-level rise were studied using satellite images (Landsat 7 with a resolution of 30 m) and ArcGIS 10.1 software. Waves, wave-induced currents and sediment transport were simulated using the MIKE-21 software package. Mixed sediments, particularly at the western and southern areas of the bay basin, were detected through reworking of coarser barrier island sediments into deeper areas. Well-sorted (0.35–0.5φ) sediments, with 98.31 ± 2.8% sand-size particles, were identified as the common texture along the wave-dominated coasts. Sea progradation, destruction of the beach profile and an increase in sea coverage of approximately 15,600 ha around the GB were other impacts of the CS-level rise. Additional impacts included severe erosion and submergence of a 9000 ha area of the flat coastal plains (slope~ 0.035%). The results demonstrate that water-level rise has strengthened the sediment accumulation in the bay inlet and basin at a mean rate of 0.430 MCM and 12.7 MCM, respectively. Both field measurements and simulation results revealed the notable shoaling of the bay entrance at a mean rate of 0.2 m. Simulations concluded that annual longshore transport toward the bay inlet has occurred at a net rate of 80,000 m3 during the last CS-level rise. Natural events after the eustatic cycle in the southeastern corner of the CS resulted in severe shoaling of the GB inlet and basin and restriction in the water exchange between the bay and open sea. Therefore, comprehensive management practices to restore the GB capacity and its natural habitats are critically needed.

Sequence stratigraphy and controls on sedimentation of the Upper Cretaceous in the Afikpo Sub-basin, southeastern Nigeria

Outcrop-based sequence stratigraphic analysis and palynological biofacies were used to define depositional sequences and their bounding surfaces, and build a sequence stratigraphic model for the Upper Cretaceous succession of the Afikpo Sub-basin. Four unconformity-bounded third-order depositional sequences were identified. Sequence 1 comprises the Nkporo Formation and is subdivided into lowstand system tract (LST) representing an incised valley fill and transgressive systems tract (TST) consisting of estuarine and marine shales and mudstones. The base of the sequence is an angular unconformity correlated to the 77.5 Ma sequence boundary (SB) and the maximum flooding surface (MFS) is dated at 76 Ma. Sequence 2 is diachronous and straddles the lithostratigraphic boundary of the Nkporo and Mamu formations. The upper SB is dated at 71 Ma while associated MFS is dated at 73.5 Ma. Sequence 3 consists of the upper Mamu Formation and the Ajali Formation. The upper SB of sequence 3 is at 68 Ma while the MFS is dated at 69.8 Ma. Sequence 4 is the topmost depositional sequence belonging to the Nsukka Formation. The upper SB is placed at 66.5 Ma. The MFS within this sequence is dated at 67.8 Ma. The sequences encompass from tidally influenced bay head delta and central estuarine environments to coastal and shallow marine shelf environments. Stratigraphic architecture and facies types show that sequence development was controlled to a great extent by eustatic sea level variations though differential subsidence rates encouraged differential rates of sediment supply and rates of sea level change along different segments of the shoreline.

Extent and dynamic evolution of the lost land aquaterra since the Last Glacial Maximum

We study the evolution of a recently recognised global geographical feature, named aquaterra, enclosing those lands that in previous glacial cycles have been repeatedly exposed and flooded. So far, the geography of aquaterra has been studied as a first approximation neglecting the isostatic effects and assuming globally uniform (i.e. eustatic) sea-level variations. Focussing on the last deglaciation and considering both global and regional aspects, we show that isostatic effects related with mantle dynamics have indeed played a significant role in the evolution of aquaterra. Our analysis is based upon paleogeographic reconstructions in the framework of well-established Glacial Isostatic Adjustment theories.

A Mid-Miocene erosional unconformity from the Durban Basin, SE African margin: A combination of global eustatic sea level change, epeirogenic uplift, and ocean current initiation

Erosional unconformity surfaces are key indicators for the variations in eustatic sea level, ocean dynamics and climatic conditions which significantly affect depositional environments of sedimentary successions. Using a dense grid of 2D seismic data, we present new evidence from a frontier basin, the offshore Durban Basin, of a mid-Miocene age erosional unconformity that can be correlated with analogous horizons around the entire southern African continental margin.In the Durban Basin, this unconformity is typified by the incision of a mixed carbonate-siliciclastic wedge and ramp margin by a series of submarine canyons. Epeirogenic uplift of southern Africa characterised this period, with erosion and sediment bypass offshore concomitant with increases in offshore sedimentation rates. Although epeirogenic uplift appears to be the dominant mechanism affecting formation of the identified sequence boundary, it is postulated that an interplay between global eustatic sea-level fall, expansion of the east Antarctic ice sheets, and changes in deep oceanic current circulation patterns may have substantially contributed to erosion during this period.

Identification and characterization of fluid escape structures (pockmarks) in the Estremadura Spur, West Iberian Margin

Located on the West Iberian margin, between Cabo Carvoeiro and Cabo da Roca, the Estremadura Spur is a trapezoidal promontory elongated in an east-west direction, extending until the Tore seamount. Recently a field with more than 70 pockmarks was discovered in the NW region of the Estremadura Spur outer shelf (Lourinhã Monocline). Pockmarks are the seabed culminations of fluid migration through the sedimentary column and their characteristic seabed morphologies correspond to cone-shaped circular or elliptical depressions. The characterization of these features and the understanding of the associated fluid escape process are the main objectives of this work. Here we characterize these structures to understand their structural and stratigraphic control based on: 1) Seismic processing and interpretation of the high resolution 2D single-channel sparker seismic dataset, 2) Bathymetric and Backscatter interpretation and 3) ROV direct observation of the seafloor.The analysis of the seismic profiles allowed the identification of six seismic units, disturbed by the migration and accumulation of fluids. The Estremadura Spur outer shelf has been affected by several episodes of fluid migration and fluid escape during the Pliocene-Quaternary that are expressed by a vast number of seabed and buried pockmarks. At present, the pockmarks are mainly inactive, as the seabed pockmarks are covered by recent sediments. It is concluded that the migration of fluids to the seabed occurred over the Pliocene-Quaternary, as indicated by the buried pockmarks at different depths below the seabed. The vertical stacking of various pockmarks suggests a cyclical fluid flow activity that can possibly be the result of the eustatic sea level variations and the subsequent changes of the hydrostatic pressure.

Seismic tectono-stratigraphy of fluvio-deltaic to deep marine Miocene silicoclastic hydrocarbon reservoir systems in the Gulf of Hammamet, northeastern Tunisia

Most of hydrocarbon accumulations within the Gulf of Hammamet foreland basins in eastern Tunisia are reservoired within the Upper Miocene Birsa and Saouaf sandstones. It is the case of Birsa, Tazarka, Oudna, Baraka, Maamoura, Cosmos and Yasmine fields. These sandstones constitute oil and gas fields located on folded and faulted horst anticline highs and described as varying from shoreface to shallow marine and typically exhibit excellent reservoir quality of 30 to 35% porosity and good permeability from 500 to 1100 md. In addition, the fracturing of faults enhanced their reservoir quality potential. However, due to the lack of seismic stratigraphic studies to highlight depositional environment reservoir characterization and distribution, petroleum exploration faces structural and stratigraphic trap types and remains on targeting only high fold closures with limited reserve volumes of hydrocarbons. As an example of the Birsa concession case, syn-sedimentary tectonic structuring and geodynamic evolution during Middle to Upper Miocene Birsa reservoir sequences have guided the distribution of depositional environment of sandstone channel systems around horst and grabens by E-W, NE-SW and N-S strike slip flower faults controlling the subsidence distribution combined with the eustatic sea level variations. Seismic sequence stratigraphy study of Miocene Birsa reservoir horizons, based on the analysis and interpretations of E-W and N-S 3D selected regional lines that were compared and correlated to outcrops and calibrated by well data, permitted to highlight the basin configuration and sequence deposit nature and distribution. Sedimentary infilling of the basin from Langhian Ain Ghrab carbonate to Serravallian Tortonian Birsa and Saouaf sandstone and shale formations is organized in four third-order seismic sequences, limited by regional erosional toplap, onlap and downlap unconformity surfaces and by remarkable chronostratigraphic horizons of forced and normal erosive lowstand and highstand system tracts separated by transgressive and maximum flooding surfaces. Reconstructed sedimentary paleo-environment distribution vary from deltaic fluvial proximal deposits in the northern part of the high central Birsa horst to a delta front and prodelta coastal and shelf shore face and shore line channelized deposits in the surrounding borders of grabens. Distal deposits seem to be distributed from upper to lower slope fans and probably to the basin floor on the flanks of the subsiding grabens. Synthetic predictive paleogeographic depositional reservoir fairway map distribution of Lower, Middle and Upper Birsa sandstone reservoirs highlights four main domains of channelized superposed and shifted reservoirs to explore.

Luminescence dating of interglacial coastal depositional systems: Recent developments and future avenues of research

Luminescence dating offers new opportunities to explore the evolution of Quaternary marine coastal facies and landforms. This review highlights the main advances in luminescence geochronology of interglacial coastal sediments through the analysis of 547 luminescence ages, most of which were published during the last decade. The majority of these reported luminescence investigations have been carried out along passive margin coasts. Since the turn of the century, the discovery of a normalization procedure known as Single Aliquot Regeneration (SAR) has drastically reduced data scatter and improved precision, with the consequence that quartz SAR optically-stimulated luminescence OSL has become the dating protocol of choice for the Last Interglacial (LIG) period. A more complex technique, known as thermally-transferred OSL (TT-OSL), is presumably proposed for dating older interglacials of the Mid-Pleistocene and beyond. Feldspar luminescence is increasingly being applied to dating Pleistocene sea level high stands due to a much higher dose saturation level than quartz OSL. The use of feldspar IRSL (Infrared-stimulated luminescence) is limited by the occurrence of variable, but ubiquitous anomalous fading (AF). Following the advent of AF-correction methods, several Middle Pleistocene sites have been amenable to dating, albeit with significant related uncertainties. Recently, new protocols involving the measurement of post-IR IRSL at elevated temperatures have yielded relatively coherent ages for interglacial sediments up to ca. 300 ka. Quartz OSL/TT-OSL, AF-corrected IRSL, and post-IR IRSL ages are generally correlated with periods of sea level high stands. A few ages are reported from the early and middle part of the Middle Pleistocene, as MIS11, 9 and more commonly MIS7 high stands are documented in strongly uplifting active margin coasts. However, by far the most obvious age peak corresponds to the end of the LIG. The MIS5e shoreline is probably the most studied and luminescence-dated coastal feature of all, as the chronology of the sea level markers is crucial to assess global eustatic sea level variations through the course of the last interglacial. Nevertheless, the observed abundance of young (100–120 ka) luminescence ages for presumed MIS5e sediments may underline methodological issues, and/or reflect the higher preservation potential of late regressive sequences. On the other hand, the occurrence of geographically distant reports of MIS5a high stand might reflect a true eustatic origin for this event. Age analysis supports the impression of general reliability of luminescence for the timing of former sea level high stands. There is a clear need to address issues in dose rate variability, in the phenomenology of fading in feldspar, and in the behaviour of luminescence growth with dose, both in laboratory and natural conditions. These could be addressed and properly evaluated by dating different minerals, as multiple or single grains, with consequent variable internal dose rates. More robust practices in the application of luminescence dating techniques could eventually constrain the age uncertainties to no better than 2–3%. Therefore, the strength of luminescence as a dating tool is more in terms of its extended age range and the ubiquity of datable material.

Sequence stratigraphy, tectonostratigraphy and basin analysis

Abstract The Dyfed Supergroup of SW Wales ranges from Terreneuvian to Furongian in age and represents a prolonged and nearly continuous phase of siliciclastic sedimentation on the southern margin of the Cambrian Welsh Basin. The lower 564 m of the Supergroup are included in an extended Caerfai Group (six formations), embracing strata previously assigned to the Solva Group. Predominantly arenaceous, the Caerfai Group has important units of conglomerate and pebbly sandstone at its base, middle and top. The Caerfai Group is overlain by 687 m of mainly argillaceous sedimentary deposits of the Porth-y-rhaw Group (five formations), a newly erected unit that includes strata previously assigned to the ‘Menevian Group’ and ‘Lingula Flags’. This two-fold division of the Dyfed Supergroup is comparable with the distinction of the laterally equivalent Harlech Grits and Mawddach groups exposed around the Harlech Dome in North Wales. High resolution sequence-stratigraphical techniques, constrained by biostratigraphical data wherever possible, are applied to the Dyfed Supergroup across southern Britain: the revised lithostratigraphy is thereby integrated with a slightly modified version of the Avalonian chronostratigraphy initially developed for the western Avalonian successions of maritime Canada. Sequences 1 and 2 are not recognized in the Dyfed supergroup of SW Wales, reflecting the inner platform setting of the area. Those sequences are represented in North Wales however, which was situated on the outer platform. The relative ease with which the Cambrian successions of southern Britain can be incorporated into the general Avalonian framework reflects the shared epeirogenic history, sediment sources and accumulation history of a microcontinent unified by early Cambrian times. The gross lithological similarities that exist between Cambrian successions across Avalonia, and between SW Wales and North Wales in particular, are better understood when basin chronostratigraphy is placed within a sequence stratigraphic framework defined by systems tracts. This is particularly evident during Unnamed Cambrian Series 3 (C3) and Furongian times, when clear systems links are demonstrated between paralic depositional environments in SW Wales and deeper basin turbidites in North Wales. Sequences 3–7 (Terreneuvian–C3) in the Welsh Basin and its hinterland were dominated by siliciclastic deposition on an epeirogenically active platform. An extensional rift-like tectonic regime is proposed, where elongate basins dominated by mudstone deposition have time equivalents formed on areas of apparent uplift that were probably tilt-block highs. Local accumulation history, the development of regional unconformities, the extent of marine onlap, and the secular succession of lithofacies were controlled principally by phases of fault-accommodated subsidence along the NE-trending lineaments that bounded these basins and their intervening horsts or platforms. In southern Britain, movements first along the Menai Straits Fault System then the Welsh Borderlands Fault System, both of which are terrane boundaries, were responsible for the dominantly coarse-grained arenaceous sedimentation of pre-Drumian times. In the Welsh Basin, this is highlighted by a change in sediment source from the Monian Superterrane to the Wrekin Terrane. A rapid switch from marginal to inner platform source areas accompanied a major phase of tectonic reorganization of the Avalonian Superterrane during the development of the boundary between sequences 3 and 4. Although the role of secular variations in eustatic sea level as a control on sequence development and architecture has been dismissed previously, prominent lowstands such as those recorded during the basal Ptychagnostus gibbus Biozone, ‘ Solenopleura ’ brachymetopa Biozone (mid- Paradoxides forchhammeri Biosuperzone), and the upper part of the forchhammeri Biosuperzone also influenced the depositional sequence stratigraphy. The broad subdivision of Cambrian Avalonian stratigraphy into tectonically active and passive phases of sedimentation allows two megasequences to be distinguished. Megasequence 1 (sequences 1–7) represents the transform termination of Avalonian subduction following oblique convergence, and the accretion of island arcs onto the northern periphery of Gondwana. Under transpressional regimes, late Precambrian arc-related basins were inverted and recycled into pull-apart successor basins. Initially, the margins of these were dominated by alluvial fans and coarse-grained fan-deltas built by flood-generated sediment dispersal processes. In the later transform stage of Cadomian–Avalonian orogenesis, there was a switch to sediment supplied from highlands to the east. Sequence 8, represented in Pembrokeshire by the Aber Llong and Ogof Velvet formations, lies at the base of Megasequence 2 (late C3–early Ordovician). Excellent sections occur through these formations and their equivalents in North Wales, and their interpretation significantly improves understanding of younger Cambrian sedimentary basins in Avalonia. These successions reflect passive margin sedimentation and the culmination of the Avalonian–Cadomian orogenic cycle. Deposition occurred in part of a subaqueous delta platform at the mouth of a huge river system that drained a substantial part of West Gondwana, with Avalonia acting as a sink for vast quantities of fine-grained sediment. Secular variations in eustatic sea level and/or sediment supply, rather than active tectonism, were the main mechanisms controlling sequence architecture and depositional environments at this time.

Eustatic control of late Quaternary sea-level change in the Arabian/Persian Gulf

Accurate sea-level reconstruction is critical in understanding the drivers of coastal evolution. Inliers of shallow marine limestone and aeolianite are exposed as zeugen (carbonate-capped erosional remnants) on the southern coast of the Arabian/Persian Gulf. These have generally been accepted as evidence of a eustatically driven, last-interglacial relative sea-level highstand preceded by a penultimate glacial-age lowstand. Instead, recent optically stimulated luminescence (OSL) dating suggests a last glacial age for these deposits, requiring >100m of uplift since the last glacial maximum in order to keep pace with eustatic sea-level rise and implying the need for a wholesale revision of tectonic, stratigraphic and sea-level histories of the Gulf. These two hypotheses have radically different implications for regional neotectonics and land–sea distribution histories. Here we test these hypotheses using OSL dating of the zeugen formations. These new ages are remarkably consistent with earlier interpretations of the formations being last interglacial or older in age, showing that tectonic movements are negligible and eustatic sea-level variations are responsible for local sea-level changes in the Gulf. The cause of the large age differences between recent studies is unclear, although it appears related to large differences in the measured accumulated dose in different OSL samples.