Thick Neogene Research Articles

The Vienna Basin: Petroleum Systems, Storage and Geothermal Potential

The Vienna Basin is located in the transition zone between the Alps and the Carpathians. Its tectonostratigraphic structure is complex, and includes from base to top crystalline basement, autochthonous Mesozoic sediments, Cenozoic foreland basin deposits, the Alpine nappe system and a thick Neogene basin fill. The Vienna Basin area hosts one of the major hydrocarbon provinces of Central Europe, but also a significant geothermal potential. The petroleum system is mainly based on the Upper Jurassic Mikulov source rock in the authochthonous succession, which became mature during nappe stacking and Neogene basin subsidence. Migration along faults and via basement highs filled oil and gas reservoirs in all tectonostratigraphic units. Despite a 100-year-long exploration history, new discoveries are still being made. Two geothermal systems are known in the Vienna Basin. One system is based on hot fluids ascending along the major faults in the southern part of the basin and has been used for balenological purposes since Roman times. The other is based on deeply buried, highly permeable carbonate rocks in the Alpine wedge (e.g. Hauptdolomite Formation) and clastic reservoirs in the Neogene basin fill. Current projects aim to supply the city of Vienna with geothermal heat from Lower Miocene fluvial conglomerates.

Structural styles at the junction between the Eastern Atlas Fold and Thrust Belt and the northern Sahel-Gulf of Hammamet foreland basin (Tunisia)

The Tunisian Eastern Atlas Fold and Thrust Belt is characterized by NE-SW fold-thrust structures derived from intracontinental basin inversion during the convergence between Africa and Eurasia. In this paper, we study the transition from this folding-thrusting domain to its foreland basins by discussing a regional structural transect based on field investigations, well data correlations and, seismic lines interpretation. Well correlations highlight significant thickness and facies variations particularly affecting the Cretaceous units, deposited during the Tethyan rifting. Successive compressional events during the Eocene and the upper Miocene-Quaternary have controlled the inversion process, thus shaping the overall present-day structure. Along the Atlas, we observed several large anticlines controlled by deep NE-SW thrusts that root down into a regional-scale major décollement that corresponds to Triassic evaporites. In the field, contractional structures such as fault propagation folds and pop-ups are identified. Unlike the Atlas Fold and Thrust Belt, the Sahel Mesozoic and Paleogene series are buried under thick Neogene to Quaternary syntectonic deposits with sediments supplied from the erosion of the uplifted Atlas structures, recording a high subsidence rate. The Sahel foreland is characterized by buried fold and thrust structures, which define an outer deformation front. Besides contraction, the Sahel Foreland basin is also affected by extension related to the Strait of Sicily rift. Seismic profile interpretation highlights the presence of a large scale rollover developed over a listric fault controlled by a deeper extensional system affecting the Paleozoic series.

Subsurface Geological Characterization of the Late Neogene–Quaternary Argive Basin, Peloponnese, Greece Using Transient Electromagnetic Data and Vintage Stratigraphic Logs

The onshore and offshore clastic deposits of the Argive Basin and the Argolic Gulf, respectively, in Peloponnese, Greece, form a Late Neogene–Quaternary half-graben that connects with the Aegean Sea. The onshore Late Neogene–Quaternary sequence, comprised of chaotically intercalated cohesive and granular clastic deposits, is in angular unconformity with bedrock comprised of Triassic–Upper Cretaceous strongly-weathered, highly-fractured karstic limestones thrusted against Paleogene flysch deposits. While the surface geology of the Argive Basin is well-known, the subsurface geology remains both poorly mapped and understood. We utilized transient electromagnetic (TEM) soundings coupled with 185 vintage stratigraphic logs, current surface geology knowledge, and insights from available geophysical surveys to characterize the subsurface conditions of this sedimentary basin. We estimated the thickness of the young deposits (the depth to bedrock) and detected potential subsurface tectonic structures. The TEM-FAST 48HPC data acquisition system with integrated inversion and visualization software package was used with a single-loop dimension of 50 m × 50 m to collect a total of 329 TEM soundings at 151 stations scattered throughout the basin. The TEM station spacing varied from 200 to 750 m allowing the mapping of 80 km2. The total depth of investigation with the inverted TEM data and the lithology logs was 130 m and 183 m, respectively. The joint interpretation produced several quasi-two-dimensional electrical resistivity profiles that traverse the sedimentary basin in various azimuths and depth slices of average electrical resistivity covering the basin. The depth slices and the vintage stratigraphic logs revealed an uneven bedrock topography overlain by an irregularly thick (over 180 m) Late Neogene–Quaternary heterolithic sediment cover.

Tectonostratigraphy and evolution of the West Greenland continental margin

Large structural highs and sedimentary basins are identified from mapping of the West Greenland continental margin from the Labrador Sea to the Baffin Bay. We present a new tectonic elements map and a map of thickness from the seabed to the basement of the entire West Greenland margin. In addition, a new stratigraphic scheme of the main lithologies and tectonostratigraphy based on ties to all offshore exploration wells is presented together with seven interpreted seismic sections. The work is based on interpretation of more than 135 000 km of 2D seismic reflection data supported by other geophysical data, including gravity- and magnetic data and selected 3D seismic data, and is constrained by correlation to wells and seabed samples. Eight seismic mega-units (A–H) from the seabed to the basement, related to distinct tectonostratigraphic phases, were mapped. The oldest units include pre-rift basins that contain Proterozoic and Palaeozoic successions. Cretaceous syn-rift phases are characterised by development of large extensional fault blocks and basins with wedge-shaped units. The basin strata include Cretaceous and Palaeogene claystones, sandstones and conglomerates. During the latest Cretaceous, Paleocene and Eocene, crustal extension followed by oceanic crust formation took place, causing separation of the continental margins of Greenland and Canada with north-east to northward movement of Greenland. From Paleocene to Eocene, volcanic rocks dominated the central West Greenland continental margin and covered the Cretaceous basins. Development of the oceanic crust is associated with compressional tectonics and the development of strike-slip and thrust faults, pull-apart basins and inversion structures, most pronounced in the Davis Strait and Baffin Bay regions. During the late Cenozoic, tectonism diminished, though some intra-plate vertical adjustments occurred. The latest basin development was characterised by formation of thick Neogene to Quaternary marine successions including contourite drifts and glacial related shelf progradation towards the west and south-west.

Anthropic-induced salinization in a dolomite coastal aquifer. Hydrogeochemical processes

The dolomite coastal aquifer of Balanegra (SE Spain) is the only source of irrigation water in an area of flourishing intensive agriculture. This scenario has consequently led to a severe exploitation of the water resources. This aquifer extends from the edges of the Sierra de Gádor to the sea, deepening progressively under the thick Neogene sedimentary sequence that confines it. Based on the content of major ions, the groundwater was classified into four groups (G1 to G4). G1 samples were characterized by their high content of dominating ions HCO3− and Mg2+. Samples classified as G2 showed a similar composition, but with a higher SO42− ion content. As for samples corresponding to groups G3 and G4, they were characterized by their chloride content. However, Na+ is the major cation in G3 and Mg2+ in G4.Based on the study of ionic relationships, ionic deltas and mineral saturation indices, it was possible to elucidate the main processes that condition the chemical composition of each of these groups of samples. The most important among said processes is the dissolution of dolomite or dolomitic limestone, the primary component of the aquifer rock. Compositional changes are due to SO42− concentration resulting from gypsum dissolution (G2). In G3, salinity is the result of dissolution of salts, or contamination through saline waters of the upper detrital aquifer. Seawater intrusion, a consequence of the overexploitation of the aquifer, increases the salinity in the G4 samples. In this intrusion process, waters of marine origin rich in Mg, interact with the carbonated rock, favouring a cation exchange in which Mg is fixed in dolomite or dolomitic limestone and Ca ion is released. This process is supported by the ionic delta values of these two cations. The marine intrusion in the area is facilitated through groundwater overexploitation, and also by the structuring of the aquifer. Marine intrusion exists where tectonics has allowed interconnection between the carbonate rock and the sea. In other areas on this coast, marine intrusion has not been possible because of the presence of impermeable Neogene materials that prevent this interconnection.

Magma origins and geodynamic implications for the Makran-Chagai arc from geochronology and geochemistry of Bazman volcano, southeastern Iran

Bazman is an isolated voluminous (3497 m elevation; >300 km2) composite volcano that is located within the western segment of the Makran-Chagai arc. This arc obliquely tracks the modern EW-trending trench offshore the Oman sea coast of Iran and Pakistan, with several Quaternary stratovolcanoes (Bazman and Taftan in SE Iran, and Kuh-i-Sultan in western Pakistan) superimposed on a preexisting Neogene arc. High spatial resolution and high sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry (SIMS) yielded four clusters of zircon ages for Bazman volcanic rocks which range from ca. 8.6 Ma to 41 ka. Lava and pyroclastic materials erupted between 8.6 and 5.9 Ma forming a thick Neogene volcanic succession (Ngv). These deposits were intruded by hypabyssal andesite-dacite domes at ca. 4.1 Ma. The next younger group of the Quaternary Bazman center comprises several eruptive pulses at 1.37 Ma, 1.13 Ma, 0.943 Ma, 0.853 Ma and 0.635 Ma, which constructed the main edifice around a central vent through alternating eruptions of lava and pyroclastic rocks (Qtv and Qa2). Subsequently, ignimbrite flows also sourced from the main vent erupted at 0.59 and 0.47 Ma, and covered the earlier erupted deposits. The youngest dated eruptive phase (Qa3) comprises the emplacement of dacitic to andesitic lava around the top of volcano. Based on Uranium-series (238U-230Th) disequilibrium in Qa3 zircons, model ages of ca. 41 ka define a maximum eruption age of these lavas, which tapped a long-lived magma reservoir with zircon crystallization ages predating eruption by 10’s to 100’s of ka. Low- medium-K calc-alkaline basaltic-andesitic eruptions formed parasitic cones located on northeastern flanks of the edifice as the youngest unit (Qa4) of Bazman. Both Neogene and Quaternary rocks from Bazman volcano exhibit homogeneous trace element characteristics, including enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and heavy rare earth elements (HREE), respectively. High Th/Yb and negative anomalies of Ta, Nb and Ti are consistent with an enriched mantle source in a continental subduction zone. Trace element modeling for the most primitive basalts erupted at Bazman suggests an origin of parental magmas through ∼15% partial melting of an enriched spinell-lherzolite mantle, whereas the genesis of Bazman acidic volcanic rocks with SiO2 > 62% involves partial melting in the middle/lower continental crust which became subsequently overprinted through assimilation – fractional crystallization (AFC) processes. Compositional continuity of Quaternary Bazman volcanic rocks with their Neogene precursors implies a steady magma source above the subducted Oman slab. Consequently, subduction geometry in the Makran-Chagai arc has experienced little modification over the past few million years.

Across‐axis variations in petrophysical properties of the North Porcupine Basin, offshore Ireland: New insights from long‐streamer traveltime tomography

AbstractThe Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (Vp) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D Vp structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new Vp–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The Vp model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The Vp model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.

Late Cenozoic seismic stratigraphy of the Andaman Forearc Basin, Indian Ocean

Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin (AFB) in the northern Indian Ocean is presented here. The high-quality multichannel seismic data from the Andaman Forearc region enable us to examine the seismic characters and to demarcate seismic sequences bounded by distinct unconformities. Ages of marked seismic horizons have been calibrated with available litholog data from nearby industry boreholes. Seismic interpretation of new data shows that the AFB is filled with ~ 4.5-s-two way travel time (TWT) thick Neogene to Recent sediments. The entire basin assemblage exhibits two distinct major sequences pertaining to the Neogene and Quaternary times. A large part of the basin is filled with intermittent mass transport deposits (MTD). We infer that the episodic uplift of the Invisible Bank, protuberance of the outerarc and regular deformation through reactivation of preexisting normal faults since the Pleistocene could be attributed as causal mechanisms for the MTDs. Strong bottom simulating reflectors are identified in the Late Miocene and younger sediments of the outerarc and AFB at a depth of ~ 0.6 s TWT and correspond to the presence of gas hydrates in this region. Our interpretations have significant implications for geodynamic as well as resource exploration in the AFB.

Basement-involved deformation overprinting thin-skinned deformation in the Pampean flat-slab segment of the southern Central Andes, Argentina

AbstractIn the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27–33ºS Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28ºS, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. Atc. 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucumán oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Sincec. 6.8–6.1 Ma, basement block uplift linked to the Miranda–Chepes and Valle Fértil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle–ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.

Gravity constraints on the underground structural framework and associated volcanism of the Maghrebian allochthonous domain: The Sejnene Numidian flysch, Tunisian Tell

Tectonic framework of the Neogene Numidian flysch is one of the primary subjects in the Tell of northern Tunisia for understanding the late Miocene kinematics around the Mediterranean Basin. Belonging to the extreme North of Tunisia, this domain is marked by a confuse structuring. Much of the Sejnene area is represented by thick Neogene Numidian flysch, which is widely exhibited around the Mediterranean and directly deposited on Mesozoic and Cenozoic strata.Gravity measurements covering the Sejnene area coupled with geological outcropping data were used to constrain the deep structural setting of the Neogene Numidian flysch and its associated volcanic outcrops. Integration of different gravity processed maps, leads us to highlight relationship between outcropping and sealed deformed structures and proposed a tectonic framework model for raised volcanic rock within the thrust sheet system. Maps resulted from gravity processing reveal structural variations marked by several geometries and oriented lineaments. This structuring suggests that the Sejnene study area is marked by two tectonic framework domains: to the North, the areas is mainly characterized by NE- and E- oriented subparallel lineaments corresponding to thrusts, reverse faults and strike slip reverse fault system. Conversely to the South, the same master trends are crossed by others NW- and N- trending transversal and deep-seated lineaments corresponding to strike slip reverse faults.Neogene NW- then N- striking compressional phases have induced inversion of inherited structures that are fossilized by rejuvenation of ancient faults affecting the basement. The final obtained structural setting suggests South-verging evolution of the Numidian domain under low-angle decollement layer to the North and under deep-seated thrusting and strike slip movements to the South. Master strike slip movements induced rising of lithospheric volcanic rocks.Geophysical and geological study of the Sejnene area provides a new insight into tectonic framework of the Numidian flysch and the associated Mogods volcanic rocks. Structuring of this Tellian domain well correlated with orogenic episodes of the western Mediterranean and seems to be related to the Alpine orogeny.

Pleistocene ice as a factor in supposed Neogene exhumation of the southern British Isles

Until recently exhumation of much of the southern British Isles, outside of the area of well-documented Neogene folding in south-eastern parts of England has been ascribed to large-scale early Palaeogene erosion following igneous underplating in the Irish Sea and surrounding areas during the early Cenozoic. Recent publications have, however, identified Neogene structures over parts of this area and claim that a thick Neogene succession has been deposited and then eroded. Herein we suggest that such a claim has overlooked the thick ice-sheets that once covered the area that could largely explain the observed uplift figures. We conclude that the structures in the Irish Sea are of composite age and their Neogene component is minor.

Neogene terrestrial sedimentary environments of the Orava-Nowy Targ Basin: a case study of the Oravica River section near Čimhová, Slovakia

The Orava-Nowy Targ Basin is an intramontane depression located at the border of the Inner and Outer Carpathians. It is filled mostly with fine-grained terrestrial and fresh-water deposits of Neogene and Quaternary age, which record the structural evolution and denudation processes of the surrounding regions: Orava, Podhale, and the Tatra Mts. The Orava-Nowy Targ Basin fill was studied in the Oravica River section, which reveals 90 m thick Neogene deposits and allows observation of their lateral diversity. Fifteen lithofacies and seven facies associations distinguished here document the depositional evolution of the basin fill, starting from hill-slope debris flow through silt turbidite-bearing lake, to flood-dominated fluvial plain, swamp and alluvial fan deposits, as well as a pyroclastic event. The palaeorelief was low during deposition and the Orava-Nowy Targ Basin could have spread significantly more to the south.

Facies associations map of the Pliocene marine deposits of the central-southern Siena Basin (Tuscany, Italy)

The Siena Basin is a post-collisional basin of the inner Northern Apennines (Tuscany, Italy) characterized by a thick siliciclastic Neogene infill, mainly composed of marine sediments with subordinate alluvial deposits close to basin margins. The central-southern sector of the basin shows a more complex stratigraphy with the occurrence of sandy deposits also in distal areas, far from the basin margin. The aim of this paper is to provide a new 1:10,000-scale geological map of this key sector (about 45 km2) of the Siena Basin, helpful for a better reconstruction of its sedimentary evolution. The new fieldwork was based on the identification and mapping of different facies associations (expression of different sedimentary environments), whose shifts in time and space provide elements to understand the basin-fill history. The recognition of two main intra-Pliocene erosional surfaces allowed the subdivision of the succession into three alloformations. Therefore, a more complex depositional history, with respect to the previous knowledge for this key-sector of the Siena Basin, has been reconstructed, thus highlighting the importance of this kind of approach with respect to the classical lithostratigraphic criteria.

Lithofacies and depositional environments of Miocene deposits from tectonically-controlled basins (Red River Fault Zone, northern Vietnam)

The Lao Cai, Yen Bai and Bao Yen Basins connected with the Red River Fault Zone (RRFZ) are filled with over 1000 m thick Neogene (Miocene) clastic deposits of different type and origin. Coarse-grained alluvial deposits predominate along the south-western margin of the Lao Cai and Yen Bai Basins, while different types of conglomerates and sandstones, even siltstones occur in their central and along their north-eastern parts. Such asymmetry in the cross-section basin fill points to the south-western active margin of these basins. Three main facies associations composed of 12 lithofacies have been recognized in the Miocene succession filling the studied basins: (1) debris-flow dominated alluvial fan association, (2) sheetflood dominated alluvial fan association, and (3) lacustrine association. Both alluvial fan facies associations are broadly spread in the studied basins, especially along their south-western margins. The debris-flow dominated deposits are represented by decimetre to several meters thick, sand- and mud-supported disorganized pebble- to cobble-size, occasionally boulder breccia and conglomerate beds. The sheetflood dominated deposits are represented by alternating planar-bedded pebble and cobble gravel couplets. The occurrence of conglomerate bodies within and overlapping fine deposits of the lacustrine facies association may suggest deposition in a debris-flow and sheetflood dominated alluvial-fan setting associated with narrow, elongated basins that could be occupied by lakes and bordered by active fault scarps. The facies pattern of the particular basins is connected with the evolution of the RRFZ. The origin of these basins may be correlated with a sinistral transtensional regime which occurred in the Early to Middle Miocene. Moreover, the high-grade metamorphic core of the Day Nui Con Voi Massif, presently the main structural element of the RRFZ, was probably not exposed during sedimentation of most of the strata in the studied basins.

Magnetostratigraphy of the Orce region (Baza Basin), SE Spain: New chronologies for Early Pleistocene faunas and hominid occupation sites

The Orce fossil quarries, in the Baza Basin of southeastern Spain, are a rich source of Early Pleistocene Palaeolithic tools and vertebrate remains. Geologic fieldwork during the last decade has placed these fossiliferous strata within the context of a thick Neogene continental sequence. Detailed lithostratigraphic and magnetostratigraphic results indicate that at least the upper 60m of this sequence are of Early Pleistocene age. The quarried strata (Venta Micena, Barranco León and Fuentenueva-3) are from a narrow time span (<100ka) starting before 1.3Ma. A new, lower excavation at ∼1.5Ma (Fuentenueva-1 quarry) has a distinctly older fauna. These Orce strata provide a high resolution, Early Pleistocene record of grassland fauna that shows the end of Mammal Neogene fauna (MN17) in the Fuentenueva-1 site (with Gazella borbonica, Equus stenonis) and the beginning of more characteristic Pleistocene fauna in the Venta Micena site (with Hippopotamus antiquus, Equus granatensis, Homo sp.). Thus far, no evidence for human occupation has been found within the earlier Fuentenueva-1 quarry, although many of the same terminal MN17 species have been found with hominids on the other side of Europe at the Dmanisi site (earliest Pleistocene) in the Republic of Georgia.

Cretaceous to Holocene structural and stratigraphic development in south Lake Maracaibo, Venezuela, inferred from well and three-dimensional seismic data

The subsurface of the southern part of the Maracaibo Basin has been studied in much less detail than the central and northern basin because hydrocarbon-rich rocks of Cretaceous and Paleogene age are deeply buried to a depth of 4.5 km (2.7 mi) by a thick Neogene clastic section eroded off of surrounding mountain ranges. In this article, we describe the results of the interpretation of 1600-km2 (617-mi2) three-dimensional (3-D) seismic reflection data centered on this horizontal to monoclinally southwestward-dipping middle Cretaceous to Holocene section adjacent to a zone of northwestward overthrusting along the mountain front of the Mrida Andes. To establish the structural and stratigraphic history of the 3-D seismic study area, a series of time slices were constructed at key horizons through the 3-D seismic volume. The ages of units in these time slices were correlated to stratigraphic formations known from outcrops at the basin edges, well data from the 3-D seismic study area, and subsurface correlation with better studied areas in the northern basin. The type and orientations of structures along each time horizon were mapped to constrain the successive structural and stratigraphic events that affected the different tectonosequences in the southern Maracaibo Basin. The main tectonic events recorded both by structures and unconformity-bound tectonosequences include the following events from youngest to oldest: (1) Pliocene–Holocene shortening produced by the uplift of the Mrida Andes and overthrusting at the mountain front has produced an approximately 6-km (3.7-mi)-deep localized foredeep basin; the axial traces of these folds trend northeast-southwest and parallel the strike of a set of normal faults; all structural features are consistent with regional northwest-southeast shortening. A low-angle thrust fault responsible for the formation of the main northeast-southwest fold is inferred to continue downdip as a flat-lying thrust that may be continuous with low-angle thrust faults observed along the northern Mrida Andes mountain front. The level of thrust detachment occurs at the Cretaceous Coln shale and accommodates a negligible amount of shortening (500 m; 1640 ft). (2) Late Miocene–Pliocene shortening caused by uplift of the Sierra de Perij and the Mrida Andes produced an inversion of north-south–striking normal faults and folds in a parallel orientation. The axial traces of these folds and the trend of inverted structures coincide with the trends of the most prolific Eocene sandstone reservoirs in the central and northern Lake Maracaibo areas. (3) Late Paleocene–middle Eocene formation of a foreland basin is related to the southward overthrusting at the northeastern margin of the basin; normal faults with an average trend of S37E record flexure of the basin during this event; faults also include the southern splayed termination of the left-lateral Icotea strike-slip fault. (4) The termination of the Icotea fault in the south lake area is consistent with the small amount of left-lateral strike-slip displacement previously inferred from the geometry of the Eocene Icotea pull-apart basin in central Lake Maracaibo.

Buried Neogene volcanic structures in Hungary

Surface Neogene volcanics in Hungary are abundantly documented in the literature, but buried volcanic structures are little known. Early burial of the volcanic centers beneath latest Miocene to Pliocene sediments preserved much of their original relief, permitting their classification into genetic types. More than two-thirds of Hungary is covered by thick Neogene and Quaternary sediments, below which buried volcanic eruptive centers and the extent of their products may only be recognized by complex geologic-geophysical methods. Our study is based on the data of several thousand wells, more than 60,000 km of seismic sections, as well as airborne and surface geophysical (gravimetric, magnetic, electromagnetic, radiometric) data. Results of chemical, mineralogical studies and K/Ar dating of deep cores were also included. The data were evaluated in terms of the regional deep structure of the Carpathian-Balkan region, the Miocene evolution of which was determined by the position, movement and welding of indivi...

Crustal-scale structure of the South Caspian Basin revealed by deep seismic reflection profiling

Two deep seismic reflection profiles from the South Caspian basin provide the first normal-incidence image of possibly the thickest sedimentary basin in the world. Significant features imaged with these seismic data are: (1) a series of high-amplitude folds, developed within a thick (>10 km) Neogene sedimentary section, bounded discordantly below by (2) a gently N-dipping, intermittent reflection at 14–20 km (8.0+ s), (3) a prominent deep reflector at 26–28 km depth (12.8–13.0 s) with a gentle northward dip, (4) an underlying layered interval with discernibly lower-frequency reflections down to ∼36–40 km (16.0–16.5 s), and (5) a noticeable decrease in reflectivity below ∼36–40 km. Based on the downward termination of fault-cored folds and their discordance with subhorizontal underlying reflections, the reflection at 14–20 km is interpreted to be the basal detachment for structures associated with the Absheron Ridge. The bright reflection at ∼26–28 km depth is interpreted as the basement/cover contact, and the underlying ∼10 km portion of the section is interpreted as crystalline basement. Downward diminution of reflectivity at ∼36–40 km is thought to represent the Moho despite the absence of a clearly reflective horizon. These interpretations are consistent with previous velocity models from both seismic refraction and teleseismic studies in the South Caspian region that suggested a minimum sedimentary thickness of 20 km and a crustal thickness of 35–40 km. Both the apparent lack of local structural relief on the basement reflector, and seismic velocities ( V p≈6.8 km/s) obtained from earlier studies suggest this crust is not thinned continental material. Although thicker than typical oceanic crust (6–8 km), the apparent ∼10 km thickness of the South Caspian crystalline crust is interpreted to be of oceanic affinity for this part of the basin. Active seismicity down to mantle depths (80 km) and gentle deepening of the crust from south to north suggest northward subduction of the South Caspian oceanic lithosphere beneath the continental lithosphere of the Central Caspian basin.

Seismic stratigraphy of the deep Ulleung Basin in the East Sea (Japan Sea) back-arc basin

Analysis of multi-channel seismic reflection profiles has revealed that the Ulleung Basin in the East Sea (Sea of Japan) back-arc basin comprises the southern and northern depocenters containing thick (over 11 and 5km, respectively) Neogene–Quaternary sediments. Back-arc closure that began in the middle Miocene caused regional uplift along the southern and southeastern basin margins, bringing large volumes of sediment into the basin. Much of this sediment supply was deposited directly onto the base of the slope and basin floor, resulting in widespread mass-transport deposits especially in the southern basin. Since the latest late Miocene, mass-transport deposits have retreated rapidly in an updip direction as the tectonic activity along the basin margins waned significantly. The apparent dominance of mass-transport deposits in the Ulleung Basin suggests that the sedimentation pattern in this basin is quite different from those in the other basins (Japan and Yamato Basins) in the East Sea where basinal turbidite and hemipelagic sedimentation has prevailed throughout the Neogene and Quaternary. The maximum two-way travel-time sediment thickness in the Ulleung Basin, about twice that of the Japan and Yamato Basins, further suggests more massive sedimentation in the Ulleung Basin.

Geochemical characteristics of terrigenous- and marine-sourced oils in Hokkaido, Japan

Crude oils in Hokkaido, Japan, are divided into two genetic groups based on bulk properties, carbon-isotope compositions and biomarker distributions. One group is characterized by high carbon-isotope canonical variables ( Sofer, 1984), high wax and low sulfur content, high Pr/Ph ratios, low C 27/(C 27+C 29) sterane ratios, relatively low Ts/(Ts+Tm) ratios, and the absence of tricyclic terpanes and C 35 homohopanes. These characteristics suggest that the oils were generated mainly from terrigenous organic matter in source rocks deposited under highly oxic conditions. The other group is characterized by low canonical variables, low wax and high sulfur content, low Pr/Ph ratios, high C 27/(C 27+C 29) sterane ratios, relatively high Ts/(Ts+Tm) ratios, and the presence of tricyclic terpanes and C 35 homohopanes, properties that suggest that they were generated from marine shales. Based on the biomarker distributions, the terrigenous oils correlate to Paleogene coals and coaly shales, whereas the marine oils correlate to Miocene marine shales. The atomic H/C ratios of the coals indicate that they have oil-generating potential. The geographic distribution of the terrigenous and marine source rocks, and their thermal maturity control the distribution of the oils in Hokkaido. The geographic distribution of the Paleogene coal-bearing formations corresponds to the distribution of the terrigenous oils in central Hokkaido. The marine oils are restricted either to areas of high heat flow related to volcanic activity, or to the area with thick Neogene and Quaternary sediments, where the Miocene source rocks are buried deep enough for oil generation.