Eustatic Sea-level Oscillations Research Articles

Generating 3D lithology probability volumes using poststack inversion, probabilistic neural networks, and Bayesian classification — A case study from the mixed carbonate and siliciclastic deposits of the Cisco Group of the Eastern Shelf of the Permian Basin, north-central Texas

The deposition and mixing of carbonates and siliciclastics in the Cisco Group of the Eastern Shelf of the Permian Basin are complicated by the temporal overlap between icehouse eustatic sea-level oscillations and fluctuations in sediment influx due to the rejuvenation of the Ouachita fold belt. Previous investigators have used well-log correlation as the primary tool in their interpretations of the area’s reciprocal depositional model, but well-log correlation alone cannot explain the full range of spatial lithology variations in the system. To better understand the lithology variation in the area, we use an integrated technique that combines wireline log information from 17 wells with 625 km2 3D seismic data through poststack seismic inversion, probabilistic neural networks (PNNs), and Bayesian classification. We use deterministic matrix inversion to derive lithology classes from well logs. Crossplot analyses reveal that the acoustic impedance and neutron porosity log pair can be used to differentiate lithologies. We perform model-based poststack inversion to generate a P-impedance volume and use PNNs to generate a neutron porosity volume. We combine these volumes through supervised Bayesian classification to generate lithology probability volumes for each lithology and a most probable lithology volume throughout the seismic data. The lithology volumes highlight the dominant lithologies (carbonate, shale, sand, and mixed) that allowed the interpretation of major carbonate platforms, sand-to-shale ratio variations, carbonate buildups between wells, and channel fill lithologies. Our semiautomated lithology detection workflow applies to regional studies and is also valid for reservoir-scale studies to determine variations in lithologies.

Stratigraphy and evolution of the late Pleistocene (MIS 5) coastal Barrier III in southern Brazil

Abstract The structure and origin of the Pleistocene (Marine Isotope Stage [MIS] 5) coastal Barrier III in southern Brazil were investigated through analysis of lithofacies, numerical ages, and ground-penetrating radar (GPR) data obtained in outcrops and subsurface deposits. The stratigraphic succession is characteristic of transgressive barriers, with muddy lagoon bottom facies unconformably overlying an older unit (Barrier II) and overlain by landward-dipping lagoon margin and aeolian facies. The back-barrier lagoon was filled with sediments and shells transferred from the foreshore through overwash and/or inlets during the MIS 5e transgressive-high-stand phase, with a higher sea level that reached about +6 to +7 m relative to the present. Marine sediments and shells on the seaward side of the barrier dated to ~100–106 ka indicate another high stand at +4 to +5.1 m during MIS 5c. One shell dated to ~87 ka and aeolian deposits dated to ~82 and ~85 ka suggest a third high stand during MIS 5a that reached at least −2 m relative to the present. The two (possibly three) juxtaposed marine deposits show that Barrier III is a more complex unit than previously recognized, built by successive orbitally forced eustatic sea-level oscillations also recorded in other deposits along the Brazilian coast and worldwide.

The Amazon paleoenvironment resulted from geodynamic, climate, and sea-level interactions

The northern South American landscape evolved in response to complex interactions between geodynamic processes, climate, and sea-level oscillations, culminating in formation of the Amazon River. Although the history of this drainage basin was investigated in previous studies, the controlling factors on its depositional environment before Amazon River formation remain elusive. Based on existing sedimentologic and paleontologic observations and new numerical models, we show that Andean mountain building, evolving climate, and eustatic sea-level oscillations controlled the transition from intermittent marine to lacustrine environmental conditions in Amazonia before the onset of the transcontinental river. Our numerical simulations show that the incorporation of an evolving climate contributed to the shortening of the time interval between the end of marine influence in Western Amazonia and the onset of the transcontinental Amazon River during the late Miocene. Additionally, our numerical scenarios indicate that sea-level oscillations created intermittent marine incursions in central and western Amazonia only when negative dynamic topography induced by mantle convection was taken into account. After the establishment of the transcontinental river, the numerical scenarios produced no further marine incursions in central and western Amazonia, despite continued sea-level oscillation. We propose that this environmental evolution influenced regional biodiversity and the sedimentation history in northern South America, and explains the formation and disappearance of aquatic environments before the onset of the transcontinental Amazon River.

Late Pleistocene Geomorphic Evolution of Cephalonia Island, Western Greece, Inferred from Uplifted Marine Terraces

Combined with eustatic sea-level changes, uplifted Quaternary marine terraces provide insight into the tectonics of coastal areas. Cephalonia Island lies 35 km off the western coast of mainland Greece and 15 km northeast of the Hellenic subduction zone. Late Pleistocene eustatic sea-level oscillations and the long-term tectonic movements are imprinted on the landscape of the southern part of the island, in the form of seven uplifted marine terraces. In the present study we aim to identify and map in detail these terraces, applying Digital Elevation Model analysis, utilizing Geographic Information Systems techniques and extensive fieldwork. The GIS-based analysis combined with field geomorphological observations revealed a sequence of seven marine terraces at the southern part of the main island ranging in elevation between 4 m and 176 m asl. Microscope, petrological and microgeomorphological analyses on two caprock samples suggest strong marine influence during the deposition of the sediments covering the marine terraces. The age of the formation of the 32 m marine terrace was assigned to the MIS 3e, based on OSL dating of a caprock sample, and an average uplift rate of 1.4 ± 0.35 mm a−1 was calculated for the last 61 ± 5.5 ka. Assuming a uniform uplift rate for the Late Pleistocene allowed us to correlate the marine terrace with the sea-level highstands and constrain their ages.

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

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

Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin, SW China

The Middle Jurassic is characterized by major plate tectonic changes, variable atmospheric CO2 concentrations (pCO2), and climate oscillations in both marine and terrestrial realms. However, the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period. Well-exposed strata of the Shaximiao Formation in the Sichuan Basin, Southwest China, preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic (Bajocian to Callovian). A total of 151 paleosol profiles belonging to four paleosol types (Entisol-like, Inceptisol-like, Aridisol-like and argillic Inceptisol-like paleosols) were identified and characterized. The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid–subhumid climate alternating with arid–humid and cool/warm–temperate climates during this time interval. This dynamic climate terminated in a dry, subhumid, humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces. We suggest that these climatic fluctuations may have been related to global geodynamic (e.g., the fragmentation of Pangea, “megamonsoonal” circulation, and bolide impacts) and regional tectonic uplift (e.g., the uplift of the Longmenshan Mountains in the South China Block) during the Middle Jurassic. The pCO2 estimates suggest relatively low pCO2 levels within the range of ∼100 ppm–∼890 ppm during the Bajocian to Callovian. Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic. These pCO2 fluctuations, which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere, are possibly attributable to global geological events (e.g., wildfire events, eustatic sea-level oscillations, and the transient development of glaciations) in the Middle Jurassic.

Factors controlling sedimentation and sequence stratigraphy evolution in shallow marine (carbonates) platform: example of Middle Eocene deposits from Gafsa Basin

Based on their facies characteristics and stratal geometries, the carbonate deposits of the Middle Eocene outcropping in the Tamerza area (Western Gafsa Basin) led to the recognition of nine facies that were deposited in shallow marine environment summarized in a carbonate ramp model: homoclinal ramp under an arid climate. The depiction of facies features and the sequential analysis help to determine the depositional environments and establish the linkage between facies evolution, eustasy, climate and tectonics. The vertical evolution of facies shows a regressive trend that records a transition from infratidal to supratidal environments. The stacking pattern revealed several kinds of meter-scale, shallowing-upward cycles. Each cycle starts with relatively open-marine deposits, which are overlain by shallower and more restricted facies (tidal flat progradation). The varied facies, which are described within the Kef Eddour Formation, compose seven depositional sequences, mainly made of carbonates, marls and phosphates. Fauna associations and sedimentary structures imply a shallow subtidal to intertidal, moderately to high-energy platform. Seven sequences are interpreted as depositional sequences showing retrogradational (lowstand systems tract), aggradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The seven depositional sequences which show a hierarchical organization of many cycles, as described above, suggested that eustatic sea-level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Sedimentology, diagenesis, clay mineralogy and sequential analysis model of Upper Paleocene evaporite-carbonate ramp succession from Tamerza area (Gafsa Basin: Southern Tunisia)

Integrated sedimentological studies, diagenesis, sequential analysis and clay mineralogy on the Upper Paleocene rocks in Tamerza area provide important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic contexts. Facies analysis and petrographic studies have led to the recognition of nine facies that were deposited in three facies belts: Sebkha, inner ramp and outer ramp summarized in a carbonate ramp model: Homoclinal ramp under an arid climate.The upward and lateral changes in thickness and composition show a general regressive trend that records a transition from an outer ramp to Sebkha, creating different types of confinement. The facies stacking patterns constitute several kinds of meter-scale, shallowing-upward cycles. Nine different types of depositional cycles and several models of Sebkha sequences were defined. These different types of facies, characterized within the Thelja Formation, compose seven depositional sequences, mainly made of carbonates, marls and evaporates. Detailed multi approach analysis provides important information on evaporitic sequence stratigraphy.In carbonates beds, the diagenetic analysis provides an overview and chronology of diagenetic processes. A particular attention was paid to early stage cementation which enables us to characterize better the depositional environments. In addition to cementation, other features define the diagenetic history.X-ray diffraction reveals the presence of smectite suggesting an arid climate. Moreover, the clinoptilolite and the frequency of primary dolomite indicate different degrees of confinement.The seven depositional sequences showing a hierarchical organization of many cycles, as described above, suggested that eustatic sea level oscillations caused by cyclic perturbations of the Earth's orbit play a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Spatio-temporal evolution of sediment waves developed on the Gulf of Valencia margin (NW Mediterranean) during the Plio-Quaternary

Several fields of large-scale sediment waves have been observed along the Gulf of Valencia continental margin (NW Mediterranean). The largest sediment waves develop on the continental slope, extending from 250 to 850m water depth, with wavelengths ranging between 500m and 1000m and wave heights from ~2m to ~50m. On the lower part of the slope, sediment waves are quasi-stationary “vertically accreting”, becoming up-slope migrating towards the mid- and upper part of the slope. A second group of sediment waves have developed over the outer continental shelf, with wavelengths of 400 to 800m and heights of 2 to 4m, also displaying an up-slope migrating pattern. Multi-channel seismic lines crossing the continental margin show that the sediment waves over the continental slope region have been continuously developed on the foreset region of the prograding margin clinoform. Several units of sediment waves have been identified in the sedimentary record, evolving in accordance with the margin progradation. Detailed analysis of single-channel (sparker) seismic profiles revealed the presence of several sediment depositional subunits over the outer continental shelf, some of them with successive development of sediment waves being truncated by erosive surfaces, likely related to Quaternary eustatic sea-level oscillations. These erosional surfaces can be followed downslope into paraconformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. Based on geophysical data, the thickness of the sediment waves mapped units show that the largest sediment waves (in wave ratio, length and height) develop where sediment deposition rates are the highest, coinciding with the upper part of the continental slope (foreset clinoforms), confined by the presence of structural highs. The development of these sediment waves has been previously explained by the interaction of internal waves over the continental slope. Because sediment waves are preserved in the sedimentary record since the Lower/Pliocene, internal waves activity could have been present in this part of the margin shortly after the Zanclean reflooding of the Mediterranean Basin, following the Messinian desiccation event ~5.6 My ago. Deep water hydrodynamic conditions were re-established at that time, modulating sediment transport and deposition over the continental slope and outer continental shelf.

Sedimentology, sequential analysis and clay mineralogy of the lower Eocene sequence at Farafra Oasis area, Western Desert of Egypt

Integrated sedimentological studies, sequential analysis and clay mineralogy on the lower Eocene rocks in the Western Desert provided important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic conditions. Two formations are recognized; the Esna and Farafra formations, with a gradational contact in-between. The studied sequence exhibits lateral facies changes as revealed from field and microfacies investigations. Eight facies were recognized and summarized in a carbonate ramp model. It represents also a general regressive trend, which records a transition from an outer ramp into a peritidal zone. The facies stacking patterns constitute several kinds of meter-scale, shallowing-upward cycles. Two different types of depositional cycles are here defined. The stratigraphic sections show a hierarchical organization of many cycles defined by five depositional sequences. It is suggested that composite eustatic sea level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of the observed hierarchical cyclic organization. Summing up, it is believed that the paleotopography had resulted from the impact of the Syrian Arc Folding System. A confusing additional complication is introduced by syndepositional sedimentary structures, especially during the late Cretaceous/Eocene times, coupled by several tensional forces. Clay mineralogy has revealed the presence of smectite, kaolinite and illite. Their origin may be attributed to the gradual increase in the amount of erosion of the newly elevated crystalline source rocks to the south of Egypt, in areas of moderate rainfall and rapid weathering and/or to reworking processes of soils which presumably developed on basement rocks. Changes in source rocks or climatic influence during the early Eocene may account for the observed differences in clay mineral abundances.

CarboCAT: A cellular automata model of heterogeneous carbonate strata

CarboCAT is a new numerical model of carbonate deposystems that uses a cellular automata to calculate lithofacies spatial distributions and hence to calculate the accumulation of heterogeneous carbonate strata in three dimensions. CarboCAT includes various geological processes, including tectonic subsidence, eustatic sea-level oscillations, water depth-dependent carbonate production rates in multiple carbonate factories, lateral migration of carbonate lithofacies bodies, and a simple representation of sediment transport. Results from the model show stratigraphically interesting phenomena such as heterogeneous strata with complex stacking patterns, laterally discontinuous subaerial exposure surfaces, nonexponential lithofacies thickness distributions, and sensitive dependence on initial conditions whereby small changes in the model initial conditions have a large effect on the final model outcome. More work is required to fully assess CarboCAT, but these initial results suggest that a cellular automata approach to modeling carbonate strata is likely to be a useful tool for investigating the nature and origins of heterogeneity in carbonate strata.

Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: The influence of oscillating Pleistocene sea-levels

The submerged reefs on continental margins and surrounding oceanic islands are important archives of past climates, eustatic sea-level oscillations and oceanographic processes. Australia's north eastern submerged shelf edge reefs have previously been explored, but all studies have been limited by either low-resolution single beam profiles or by their limited spatial extent. The aim of this study is to improve the understanding of the origin and development of submerged shelf edge reefs, terraces and related features on the Great Barrier Reef (GBR), and the influence of Pleistocene sea-level oscillations on their distribution and geomorphology. We investigated the first large-scale (1400 km 2) 3D bathymetry and acoustic backscatter dataset of four widely-spaced sites located across ca. 650 km along the shelf edge at Ribbon Reefs 1–5, Noggin Pass, Viper Reef and Hydrographers Passage. We provide a comprehensive description and systematic classification of the shelf edge features, and constrain their spatial distribution and geomorphologic variability using a wide range of quantitative morphometrics. Fifteen features are defined, four of which are the focus of this study, including terraces, pinnacles, ridges and channels. Using modern reefs within the GBR for comparison, we interpret these as fringing reefs, patch and barrier reefs and tidal channels respectively. The influence of gross shelf geomorphology and oceanographic factors driving the observed variability within and between sites is discussed in conjunction with Mid-Late Pleistocene sea-level oscillations. We observed two distinct types of reef growth characterised by the degree of development, which we interpret as analogous to juvenile and mature modern reefs of the GBR. Juvenile reefs are very common and occur within each site at depths of less than 50 m to greater than 100 m. They are defined as discontinuous peaked barrier reefs, peaked patch reefs and fringing reefs. Mature reefs are less common and develop exclusively at a depth range of ca. 50–70 m. These reefs are defined as flat-topped continuous barrier and double barrier reefs. Through a comparison with published Mid-Late Pleistocene sea-level oscillations, we note that the depth range of mature reefs is consistent with the depth of repeated low-amplitude (< 20 m) eustatic sea-level oscillations, and independent of the relative time spent at sea-level. We conclude that repeated low-amplitude sea-level rises provide the much-needed accommodation space for vertical accretion that is not provided during stillstands, without inducing reef drowning, which can be caused by high-amplitude sea-level rise. Low-amplitude sea-level falls are also less detrimental than high-amplitude falls, in terms of habitat compression. Together, the effects of enhanced reef accretion and limited habitat compression at depths of 50–70 m have lead to a concentrated net gain in vertical relief that is not observed elsewhere on the shelf edge.

Yria (western Naxos island, Greece): Sea level changes in Upper Holocene and palaeogeographical reconstruction

The Saint Georgios coastal zone, located at the W coast of Naxos, the largest island of the Central Aegean Sea, was investigated in order to determine the palaeo-geography, sea level changes and their effect to the palaeo-environment of western Naxos island and to human activity. Detailed geomorphological mapping, study of micropaleontological and sedimentological characteristics and dating analyses of the Late Holocene of St. Georgios coastal zone were conducted. To obtain information about the Holocene stratigraphy under the recent alluvial cover, three boreholes followed the detailed geomorphological mapping. Microfaunal analysis took place and five samples of plants, shells, peat and charred material were also collected from several layers of the sedimentary sequence and were dated using AMS and conventional radiocarbon techniques providing temporal control of the sediments. The sea-land interactions during Upper Holocene, in relation to the eustatic sea level oscillations, as well as the geomorphologic observations and analysis on deposited sediments, aims to reveal the palaeo-geographic evolution of the landscape and its impact on the archaeological sites. Sea level rise along with sea-land interactions to the landscape evolution and the transgression of sea in 6144 BP have been verified.

Mn- and Fe-carbonate rich layers in Meso-Cenozoic shales as proxies of environmental conditions: A case study from the southern Apennine, Italy

Mn-rich layers and interbedded shales from a well exposed natural section of the Northern-Calabrian Unit (Late Jurassic‐ Early Oligocene) in the surroundings of the Terranova del Pollino village, southern Italy, have been mineralogically and chemically analyzed, in order to reveal the factors controlling their formation. Mn-rich layers are composed of micas/clay minerals, rhodochrosite, siderite, chlorite and quartz whereas shales are formed by micas, clay minerals, chlorite, quartz, and feldspars. The MnO abundances in the Mn-rich layers, which are depleted relatively to the UCC in SiO2, TiO2, Al2O3, Na2O, K2O, and P2O5, are in the range of 11.01 18.41 (wt. %). R-mode Factor analysis indicate that SiO2, Al2O3, TiO2, Na 2 O and K 2 O have high positive weights in the first factor (59.8% of the total variance) whereas high negative weights are observed for Fe2O3, MnO, and CaO. This factor accounts for the competition between the terrigeneous component, the authigenic carbonate phases accumulating Mn and Fe which likely formed during paucity of detrital supply. The negative weight of CaO and MnO in this factor, the higher Ca contents in the Mn-rich layers compared to shales, and the lack of calcite, suggest the presence of a mixed Mn‐Ca carbonate rather than pure rhodochrosite. It is generally retained that Ca‐ rhodochrosite precipitates within the pore waters of reducing sediments since neither rhodochrosite nor siderite can form in equilibrium with bottom seawater. Thus the resulting sediment should be a mixing between the detrital component and the authigenic one. Assuming Al2O3 as an index of the detrital component, it is clearly envisaged that in the Al2O3/MnO vs. Al2O3 diagram the carbonate-rich samples fall on the mixing curve having as end members the average shale and the richest MnO sediment. This supports the idea that carbonate-rich samples formed through precipitation of carbonate minerals in the pore waters of the terrigenous detritus accumulating at the sea bottom. Further the REE distribution of unaltered marine carbonates is expected to be representative of ambient seawater where carbonates precipitated. Carbonates normalized to fine-grained siliciclastic sediments, have typical HREE enrichment, negative Ce-anomaly, and lower total REE. In our case, the carbonate-rich samples normalized to the average composition of the interbedded freecarbonate shale, show HREE enrichment, lower total REE contents, and the lack of negative Ce-anomaly, due to the anoxic environment of formation for Mn‐ and Fe‐carbonate. Finally was observed that the mineralization is enhanced if the site of accumulation is protected from dilution by clastic sediment input. The alternation between Mn‐ and Fe‐carbonate silts and carbonate-free shales along the studied sedimentary succession, were likely controlled by eustatic sea-level oscillations which are well documented in the western Tethys during Middle and Late Triassic.

Paleoenvironmental evolution of Istron (N.E Crete), during the last 6000 years: depositional environment, climate and sea level changes

The Istron area is located in northeastern Crete, Gulf of Mirabello, on an alluvial fan of Holocene age. The archaeological importance of this area is suggested not only by its archaeological remains, but also by its significant location. Many important Minoan sites, like Gournia, Kavousi, Pseira, have been discovered near Istro. There are indications of human installations from the Neolithic to the Roman period, proving the continuous human activity in this area. The significant geological location- it represents an area with intense tectonic activity-, geomorphological regime and climatic changes, have influenced regional settlement and the overall cultural development of the area. The study of sea-land interactions during the last seven millennia in relation to the eustatic sea level oscillations and the regional neotectonic regime, as well as the geomorphologic observations and analyses on deposited sediments, aims to reveal the paleogeographic evolution of the landscape and its impact on prehistoric, classical and Roman establishments. A geomorphological mapping of the coastal area along with the drilling of five boreholes and the excavation of six trenches, have therefore been accomplished. Moreover, pollen and microfaunal (benthic foraminifera and ostracodes) analyses have been performed. Six samples were dated using AMS and Conventional radiocarbon techniques providing temporal control of the sediments. Sea level rise along with sea-land interactions to the landscape evolution and the transgression of sea in 5000BP have been verified. Additionally, several implications for the use of land and human impact civilization have been made.

Γεωμορφολογική εξέλιξη της παράκτιας περιοχής του Ίστρου-Καλού Χωριού του νομού Λασιθίου Κρήτης από το Μέσο Ολόκαινο μέχρι σήμερα

The Istron area is located in northeastern Crete, Gulf of Mirabello, on an alluvial fan of Holocene age. The archaeological importance of this area is pointed out not only by its archaeological remains, but also by its significant location. Many important minoan sites, like Gournia, Kavousi, Pseira, have been discovered near Istro,. There are indications of human installations from the Neolithic to the Roman period, proving the continuous human activity in this area. The significant geological location, as it represents an area with intense tectonic activity, the geomorphological regime of the area and the climatic changes , influenced human installations as well as the evolution of the civilization . The study of sea-land interactions during the last seven millennia in relation to the eustatic sea level oscillations and the regional neotectonic regime, as well as the géomorphologie observations and analyses on deposited sediments, aims to reveal the paleogeographic evolution of the landscape and its impact on prehistoric, classical and Roman establishments. Therefore, a geomorphological mapping of the coastal area along with the drilling of five boreholes and the excavation of six trenches, have been accomplished. Moreover, pollen and microfaunal (benthic foraminifera and ostracodes) analyses have been performed. Six samples were dated using AMS and Conventional radiocarbon techniques providing temporal control of the sediments. Sea level rise along with sea-land interactions to the landscape evolution and the transgression of sea in 5000BP have been verified. Additionally, several implications for the use of land and human impact civilization have been concluded.

Orbitally forced Lofer cycles in the Dachstein Limestone of the Julian Alps (northeastern Italy)

Lofer cyclothems of the Upper Triassic Dachstein Limestone of the Julian Alps (northeastern Italy) are the focus of a new sedimentological-cyclostratigraphic study. A 112-cycle-long field section reveals that the meter-scale basic cycle shallows upward, from subtidal megalodont-bearing subfacies to intertidal laminites, capped by a paleosol subfacies. Time-series analysis of a correlative grayscale scan of a high-resolution field photograph shows that a 166-m-thick succession of Lofer cycles has time-frequency characteristics consistent with Earth's precession index, suggesting that the Lofer cycles were generated by eustatic sea-level oscillations that were forced by Milankovitch cycles.

Vendian foreland basin of the Siberian cratonic margin: Paleopangean accretionary phases

Vendian, chiefly clastic sedimentation on the southwest of the Siberian craton is discussed under the tenet of a foreland basin that came into being in the Vendian epoch at the margin of the Siberian continent (craton) due to its collision with island arcs and microcontinents. The Vendian foreland basin of the Siberian cratonic margin, over 3000 km in length, was formed in the Early and Late Vendian. To use a paleotectonic framework, the terrestrial and marine deposition in point took place in the Yenisei-Baikal belt of foredeeps and in its coeval Baikit-Nepa belt of forebulges in the cratonic interior. Vendian age of molasse and the foreland basin is proved by new data: (1) tillites reported from the base of the Oselkovaya Group (Marnya Formation) of the Sayan foredeep and (2) the find of a Metazoa mollusk biota. The tillite-bearing horizon occurs at the base of the lower marine assemblage and, using cyclic sequences and carbonate sedimentary marker systems, it is traceable on the Yenisei Ridge, in Cisbaikalia, and in the cratonic interior. Sedimentologic data and the vector of clastic transport suggest that the ice cap was located on the Siberian craton, and the Siberian glacial horizon is correlatable to the global Laplandian horizon of the Vendian (Chumakov, 1978). The foreland basin passed three evolutionary phases: (1) initial accretionary, (2) glacial (pre-collisional), and (3) collisional. Based on sedimentologic data, each phase falls into several evolutionary stages, each stage reflecting an erosion pulse and redeposition of clastic sediments. During the accretionary and collisional phases, changes in sedimentary systems were associated with tectonic activity and, indirectly, with eustatic sea-level oscillations. During the glacial phase, deposition of third-order cyclic sequences was chiefly controlled by eustatic changes of the sea-level, forced by growth and melting of ice caps. The synchroneity and interplay of climatic and tectonic events in the Vendian on most continents and microcontinents can be explained by a model for the Paleopangean supercontinent accretion in the Vendian.

Lower Cretaceous turbidites of the Moray Firth: sequence stratigraphical framework and reservoir distribution

Lower Cretaceous depositional systems of the Moray Firth are influenced by eustatic sea-level oscillations which have been dramatically overprinted by two major phases of pulsed tectonism, the Late Cimmerian and Austrian. The biostratigraphical resolution obtained has allowed the timing and differentiation of distinct tectonic/sequence boundaries, some of which are utilized as important seismo-stratigraphic markers. The construction of detailed facies maps for individual sequences has, in parallel, allowed an insight into the tectonic history of the main source areas during the Early Cretaceous.

Temperature/Pressure Changes Impact Hydrates from Coast to Slope, Northern Gulf of Mexico

The northern Gulf of Mexico's passive margin is interrelated and synergistic with both regional and local inputs. Regional inputs may be thought of as deterministic and may include eustatic sea-level oscillations, sediment deposition from erosion of uplifted areas, climate above and below sea level, compression and dewatering of sediments once deposited, and related tectonics. The synergistic sum of these deterministic processes impacts the occurrence of natural gas and their hydrate phases. Along the foot of the continental slope, compression is transmitted to the sediment beneath and down-dip. Hydrates occur when suitable cold temperatures and high pressures are present. In water depths of ∼4 km, the hydrate stability zone (HSZ) may be up to 1 km thick. A prograding slope imparts regional pressures. Local pressures may be sufficient to catalyze the gas-to-hydrate phase change. Further compression may cause the zone to heat and/or to generate fractures, promoting hydrate disassociation and/or escape. The impact of a salt/shale lateral wedge or diapir (or both) entering the zone can be observed in several ways. Increased salinity raises the gaseous phase, thereby reducing hydrate existence. Increased heat also augments gaseousness. Note that shales dissipate heat and that the top of a salt diapir can be cool, even though overall salt is a heat conductor relative to sediments. Migrating salt/shale can promote fracturing, where the size and rate of migration determine the dimensions and frequency of fractures. As gases enterfractions, gas may expand and possibly block further gas entrance. Local sediment shattering and thermal anomalies could obviate the gas expansion effect. As the Louann Salt, which is located over the slope-sediment wedge, advances basinward, the amount of compression rises. Thus, temperatures and pressures change and the pattern of gases/hydrates are dynamic. Specific local predictions are subtle and may be regarded as probabilistic in model development. Equal subtlety is experienced along the ocean/land contact. Hydrate existence is determined by gas chemistry, whether methane or ethane is present. As the sea level oscillates, temperatures and pressures also oscillate. Landward, the HSZ limit during sea-level highs is disassociated, and a new, more basinward HSZ is established. Specific landward HSZ limit is determined by the gas chemistry.