Oligocene Succession Research Articles

Biofacies and sequence stratigraphy of the Oligocene succession, Central Basin, Iran

Biofacies and sequence stratigraphy of the Oligocene succession, Central Basin, Iran

Cenozoic basin development and stratigraphy of the Faroes area

The present paper reviews the geological development of the Faroes part of the NE Atlantic Margin in the Cenozoic Era. The Faroes area is located west of the post-Caledonian rift basin system formed in the Devonian after the collapse of the Caledonian mountains. Tectonic movements and plate reorganizations during several phases have strongly influenced sedimentation and erosion along the Caledonian front. During the Cenozoic four main tectonic phases with uplift, non-deposition or erosion, had a major influence on basin development and structural setting. First, the arrival of the Icelandic plume to the Faroes area caused a series of uplift phases, which are recorded in the Selandian–Early Ypressian sedimentary succession. Second, tectonic uplift in the Middle Eocene ended the period of almost continuous subsidence of the Judd Basin since the Danian. In the Middle and Late Eocene new depocentres formed in the northern part of the Faroe–Shetland Trough and in the Faroe Bank Basin. Third, due to uplift and sea-level fall in the Late Oligocene, widespread erosion of the Eocene and Oligocene successions resulted in a major unconformity on the shelf. Finally, in Neogene times, folding and uplift of the Fugloy Ridge occurred contemporaneously with renewed subsidence in the northern part of the Faroe–Shetland Trough and the Faroe Bank Basin. Maps of Cenozoic key horizons illustrate this geological development from the start of the Palaeogene volcanism to the present day. Four regional maps are presented, two maps showing depths to Top Basalt and Top Palaeogene, and two isochore maps of the corresponding post-Basalt sediment sequences, the Palaeogene and the Neogene. The petroleum system in the Judd Basin was strongly influenced by inversion in Middle Eocene to Recent times. Reservoirs and source rocks were uplifted up to about 600 m, with gas flushing, seal breakage and interruption in petroleum generation as consequences. Petroleum systems may also exist in the Faroe Bank Basin and in the northern part of the Faroe–Shetland Trough.

Palaeoclimate versus vegetation reconstruction – palynological investigations on the Oligocene sequence of the Sava Basin, Slovenia

An Oligocene succession from the Slovenian Sava Basin (Zasavje) was analysed to reconstruct climate and vegetation of the interval. Independent quantitative approaches were applied on a detailed palynological data set to reconstruct both parameters separately allowing for a synthesis and interpretation. The reconstruction of palaeoclimate is based on the coexistence approach and documents an equal, warm-temperate, humid climate with low annual variation. Quantitative results indicate values of mean annual temperature of 16–19°C, with winter temperatures of 6–9°C and summer temperatures of 25–28°C, and a mean annual precipitation ranging between 1100 and 1300 mm. In contrast to the very homogeneous climatic signal, the vegetational reconstruction based on multivariate statistical analyses shows distinct changes through time. During intervals of peat accumulation, the environment was a typical swamp vegetation dominated by Taxodiaceae and ferns, in which ephemeral small ponds developed due to changing groundwater levels. With the development of an expanded limnic environment, the Taxodiaceae retreated and a riparian swamp flora dominated. This vegetation, in turn, was displaced by elements of drier mesophytic habitats that increased in abundance during a tectonically forced lowering of the groundwater table. Subsequently, the entire study area was flooded by transgression and overlain by marine sediments. All changes in vegetation can be explained by influences of the basin development on the habitats of the plant communities. The results demonstrate a vegetational change in the Oligocene of the Sava Basin that is independent from climate. Vegetation change dynamics were controlled probably by the regional tectonic–palaeogeographic development within a very equable, warm-temperate, and humid climate.

THE HYDROCARBON PROSPECTIVITY OF LOWER OLIGOCENE DEPOSITS IN THE MARAGH TROUGH, SE SIRT BASIN, LIBYA

The main phase of rifting the Sirt Basin (Libya) had ceased by the mid‐Cretaceous but Alpine‐related tectonic pulses in the late Eocene resulted in northward tilting of the basin. In the Maragh Trough (SE Sirt basin), a regional unconformity consequently separates Eocene carbonates from the overlying Oligocene succession. The unconformity marks a change from Eocene carbonate sedimentation to more mixed shallow‐marine deposition in the Oligocene. A regional transgression re‐established fully marine conditions in the Miocene.Deeply‐buried (Triassic) source rocks in the Maragh Trough reached peak oil generation during the Oligocene. Two potential reservoir intervals have been identified: upper Eocene rudstones of the Augila Formation, and unconformably‐overlying sandstones of the Lower Oligocene Arida Formation. Mid‐Oligocene shales provide a regional seal.Facies distributions and reservoir properties are related to rift‐related structural highs. Despite the absence of a nearby source kitchen, Upper Eocene carbonates have been found to be oil‐bearing in the Maragh Trough at wells D1‐ and F1–96. This indicates that hydrocarbons have migrated along graben‐bounding faults from deeply‐buried source rocks to platform and sub‐platform areas. Traps are of combined structural and stratigraphic type.

Sequence stratigraphy, tectonics and depositional history in eastern Thrace Basin, NW Turkey

The eastern Thrace Basin forms the eastern portion of the Turkish Thrace Basin in NW Turkey. The pre-Tertiary crystalline basement shows platform and deep basin depositional morphology which results in facies pattern and thickness distribution localization during the early stages of basin development in Middle to Late Eocene time. Normal fault controlled early and lateral⧸reverse fault controlled later stage tectonic events characterize basin geometry and the present topography, respectively. The early stage normal faulting caused subsidence and sediment accumulation in the Late Eocene and Early Oligocene, while later stage tectonic events caused intense deformation and structuration in the Early to Middle Miocene. As a result, southeastern and northern regions were folded or uplifted and deeply eroded. Outcrop, well log, core and reflection seismic studies reveal that the sedimentary succession can be subdivided into five characteristic depositional sequences bounded at their base by unconformities. These unconformities were dated using paleontological data. The sequence boundaries, underlying consecutively Sequences-1, 2, 3, and 5 were dated at 39.5, 36, 30, and 25.5 Ma from older to young in order of succession, corresponding to the major erosional breaks between the Middle and Upper Eocene, the Upper Eocene and the Lower Oligocene, the Lower and Upper Oligocene, and the Upper Oligocene and the Lower Miocene. The sequence boundary underlying Sequence-4 was determined to lie within the Late Oligocene succession and, as an age designation could not be assigned, it was termed intra sequence boundary (ISB). The first two sequences correspond to relative sea level rises and a normal fault controlled tectonism coincides with them. Reefal carbonates, coarse grained clastics and marine shales, with abundant volcanic flows or layers represent the sedimentary facies of these two early sequences. The third and fourth sequences correspond to either a decreased relative rise of sea level or a relative still stand of sea level and a sediment-induced subsidence related to thermal relaxation provides accommodation. Prograding delta sediments of varying extent constitute the sedimentary packages of these two sequences. The Depositional Sequence-5, which makes up the uppermost part of the sedimentary succession in the basin, corresponds to a relative sea level fall and a tectonic style shaped by lateral and inversion faulting characterizes this period. Nonmarine alluvial fan, fluvial channel and various lagoonal deposits constitute the sedimentary facies of this last sequence. Porous reefal limestones and shallow marine beach sandstones of Sequence-1 and deltaic sandstones of Sequences-3 and 4 provide good reservoir rocks for the hydrocarbons produced in the study area. The marine shales and marls of the second and the third sequences provide good source rocks as they are rich in disseminated organic matter content. Although the non-marine alluvial and fluvial channel deposits of Sequence-5 are good reservoirs, they lack effective seals. The time interval in which Sequence-5 was deposited represents a period of major structural and trap development in the basin.

Factors controlling the Cenozoic sequence development in the eastern parts of the North Sea

The causal relationship between the Cenozoic sequence development in the southeastern North Sea Basin and sea-level changes, climatic fluctuations and tectonic events is unravelled by relating variations in the relative sea level and base level, based on interpretations of seismic surveys, to published δ 18 O variations and eustatic changes. The latter curve is based on the Earth's orbital forcing, and here informally termed as the GSI curve. The analysis shows that the Cenozoic sequence development in the southeastern North Sea was influenced by climatically and tectonically induced sea-level changes. The major Cenozoic sequence stratigraphic boundaries (lower order) are highly influenced by tectonic events, e.g. uplift of Fennoscandia and increased subsidence rates in the basin centre. Reactivation of Mesozoic fault zones controlled the deposition of minor sand bodies transported to the centre of the basin during the Late Palaeocene by mass flows. The location of an Oligocene mound structure, which constitutes part of a sequence, is controlled by the overall palaeotopography of the basin and local fault-related depressions. Correlation between (i) the ages of our sequences and the δ 18 O variations in the Oligocene succession, and (ii) the GSI curve and the base-level fluctuations of the late Miocene and younger sequences, show that the generation of the higher order sequence boundaries were driven by glacio-eustatic sea-level changes. A climatic control of the sequence formation due to glacio-eustatic sea level changes is therefore suggested for the Oligocene and Pliocene sequences, and probably also for the Upper Miocene sequences.

Cenozoic thermotectonic history of the Sunda–Asri basin, southeast Sumatra: new insights from apatite fission track thermochronology

The tectonic and thermal history of Cenozoic rocks from four wells (Asri-1, NE Ria-1, Hariet-1 and Yani-2) in the Sunda–Asri basin has been reconstructed using apatite fission track (AFT) analysis combined with modelling of previously determined vitrinite reflectance (VR) measurements. The wells all penetrate Recent to middle Eocene successions, in which only the Eocene to Oligocene successions have been subjected to temperatures suitable for partial annealing, due to rapid late Miocene to Recent burial. With respect to organic thermal maturities, the zone of minimum annealing coincides with undermature heating and records provenance AFT ages, whilst the zone of partial annealing agrees well with mature heating suitable for generation of hydrocarbons. The integrated AFT and VR data suggest that the Sunda–Asri basin continues subsiding, probably due to extension initiated in the late Eocene, and the successions near the base of the wells were recently exposed to the high present-day temperatures. The recent heating of rocks is consistent with up to 800 m of Plio–Pleistocene burial. Both the AFT and VR results indicate that geothermal gradients remained relatively constant through the Cenozoic.

The geometry of the Rupelian and Chattian depositional bodies in the Lower Rhine district and its border area: implications for Oligocene lithostratigraphy

Three profiles are constructed through the Oligocene succession of the borderland between Belgium, Germany and the Netherlands. The profiles illustrate the geometric relationships between Rupelian and Chattian successions and the lateral and vertical variations within the Rupelian and Chattian deposits. The end-Rupelian profile between Central Belgium and the Achterhoek area in the Netherlands shows a rather uniform thickness distribution. This is in sharp contrast to the fault controlled sedimentation during the Chattian as represented on an end-Chattian profile between the same areas. Within the Chattian succession several lithostratigraphic units can be identified and correlated using a dense net of geophysical well logs. Remarkably similar subdivisions are found in the different tectonic blocks, suggesting eustatic sea-level fluctuations. In the central part of the subsiding area the Chattian deposits are largely continental to deltaic. Laterally,these deposits developed into marine glauconitic sediments. The end-Chattian profile between Bonn and Asten in the Netherlands runs approximately parallel with the subsidence axis. The cyclicity expressed by facies alternations due to a periodically shifting coastline can be correlated over the different blocks again suggesting the involvement of eustatic controls. On this last profile, an anomalous geometry is due to SE-NW stress-induced movements along the Variscan Aachen overthrust.The three lithological profiles are meant to represent a frame for formal stratigraphic subdivisions and nomenclature.

Oligocene sequence stratigraphy and basin development in the Danish North Sea sector based on log interpretations

Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark. Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals. The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats.

Source Rock Potential in Pakistan

Pakistan contains two sedimentary basins: Indus in the east and Balochistan in the west. The Indus basin has received sediments from precambrian until Recent, albeit with breaks. It has been producing hydrocarbons since 1914 from three main producing regions, namely, the Potwar, Sulaisman, and Kirthar. In the Potwar, oil has been discovered in Cambrian, Permian, Jurassic, and Tertiary rocks. Potential source rocks are identified in Infra-Cambrian, Permian, Paleocene, and Eocene successions, but Paleocene/Eocene Patala Formation seems to be the main source of most of the oil. In the Sulaiman, gas has been found in Cretaceous and Tertiary; condensate in Cretaceous rocks. Potential source rocks are indicated in Cretaceous, Paleocene, and Eocene successions. The Sembar Formation of Early Cretaceous age appears to be the source of gas. In the Kirthar, oil and gas have been discovered in Cretaceous and gas has been discovered in paleocene and Eocene rocks. Potential source rocks are identified in Kirthar and Ghazij formations of Eocene age in the western part. However, in the easter oil- and gas-producing Badin platform area, Union Texas has recognized the Sembar Formation of Early Cretaceous age as the only source of Cretaceous oil and gas. The Balochistan basin is part ofmore » an Early Tertiary arc-trench system. The basin is inadequately explored, and there is no oil or gas discovery so far. However, potential source rocks have been identified in Eocene, Oligocene, Miocene, and Pliocene successions based on geochemical analysis of surface samples. Mud volcanoes are present.« less

The provenance of sediments in the Barrême thrust-top basin, Haute-Provence, France

Abstract This paper records the results of an integrated analysis of heavy minerals, sedimentary facies, palaeocurrents and structural data. The objective of the study was to evaluate the provenance of the sediments of the Barrême thrust-top basin. This basin contains a Late Eocene to Late Oligocene succession of shallow marine, alluvial and lacustrine sediments. Late Eocene marine transgression resulted in starvation of clastic supply to the basin hence the Calcaires Nummulitiques received only a limited influx of heavy minerals derived from local sedimentary sources, augmented by the first appearance of assemblages with andesitic affinities at c. 42–41 Ma. Clastic members associated with the overlying Marnes Bleues record a significant diversification of provenance and sediment transport paths. Clastic detritus supplied to the shallow marine Grès de Ville were derived from Permo-Triassic sediments to the south. Conglomeratic submarine channels of the La Poste member mark the introduction of Alpine ophiolite-blueschist-derived minerals at c. 37–35 Ma, eroded from the Embrun-Ubaya thrust complex to the NE. The deltaic St Lions member is dominated by volcaniclastic assemblages. Completing the marine fill, the shoreface Grès de Senez received detritus from the southerly Maures-Esterel crystalline massif. The succeeding Oligocene continental sediments record an increasing supply of minerals from Alpine lithologies. The alluvial Molasse Rouge contains reworked sedimentary detritus, including material cannibalized from the underlying marine Tertiary, and first-cycle Alpine minerals. The succeeding lacustrine Série Saumon-Série Grise show the increasing diversity of Alpine suites. The youngest sediments, the fluvial Grès Verts, signal a major influx of Alpine-derived assemblages related to reactivation of the Embrun-Ubaye thrusts between c. 28–27 and 25–24 Ma.

Storm-generated graded beds and debris flow deposits withOphiomorphain a shallow offshore Oligocene sequence at Nelson, South Island, New Zealand

Abstract An Oligocene (Whaingaroan) succession of sandstones with rare perigenic breccias and fossiliferous detrital conglomerates is well exposed for 1 km along strike and for a stratigraphic thickness of 140 m on a wave-cut platform at Magazine Point, Nelson. Regional tectonic and stratigraphic relationships are uncertain. All lithologies are lensoidal; virtually no bed extends for more than 200 m. Planar-laminated to hummocky cross-stratified, normally graded, mud-free sandstones 10-80 cm thick alternate with bioturbated muddy finer sandstones and sandy mudstones 10-35 cm thick. Ophiomorpha nodosa is the only common trace within the clean sandstones; it is locally abundant in the lower 60 m of the succession and exceedingly rare in the upper 80 m. Thallassinoides, formed by the same progenitor as Ophiomorpha, occurs with more common other indeterminate traces within the muddy units. Benthic microfauna from the muddy intervals indicate “shelf” settings ranging from inner, at the base of the sequence, to...