Middle Eocene Oligocene Research Articles

Geology of the Magura Nappe, south-western Gorce Mountains (Outer Carpathians, Poland)

ABSTRACT The studied area, located in the south-western part of the Gorce Mountains, is built of flysch deposits representing the Krynica and Bystrica Subunits of the Magura Nappe. Here, in the Late Cretaceous–Early Miocene part of the lithostratigraphic succession of the Krynica Subunit, four formations containing several members were distinguished, whereas in the Bystrica Subunit, only one Middle Eocene–Oligocene formation was identified. The Turbacz and Kudłoń Thrust Sheets in the tectonic structure of the Krynica Subunit and the southernmost fragment of the Tobołów Thrust Sheet in the Bystrica Subunit are prominent. Several folds occur in the studied area. Numerous transverse and oblique faults are also present. The result of the present geological investigations in the study area was the creation of a geological map of the Magura Nappe deposits in the Gorce Mountains. In general, a high-resolution digital elevation model contributed significantly to the progress of this geological research.

Paleomagnetic rotations in the circum-Marmara region, northwestern Turkey since the Late Cretaceous

The Marmara region (northwestern Turkey) is located along the Alpine-Himalayan orogenic belt, and consists of tectonic fragments of the İstanbul Zone, the Rhodope Strandja Massif, the Sakarya Zone and the Thrace basin. The Middle Eocene-Lower Miocene sedimentary rocks of the Thrace Basin covers a large area over the Rhodope Strandja Massif. It has been reported that the İstanbul Zone began to migrate southward, away from the Moesia Platform, along the dextral Western Black Sea Fault to the west and the sinistral Western Crimean Fault to the east, post the opening of the Western Black Sea in the Late Cretaceous. The İstanbul Zone and the Sakarya Zone collided after the closure of the Intra-Pontide Ocean in the Early Eocene. In this study, the Upper Cretaceous–Oligocene sedimentary and volcanic rocks, sampled from 90 sites in the Marmara region, were used to examine the deformation history of the Marmara region since Late Cretaceous. Our palaeomagnetic data showed counterclockwise rotation of R ± ΔR = 33.2° ± 9.8° and R ± ΔR = 28.5° ± 6.8° in the Sakarya Zone, for periods of Late Cretaceous and Middle Eocene-Oligocene, respectively. Similar rotations were detected from the Middle Eocene to Oligocene rocks in the Thrace Basin and the İstanbul Zone. Our results indicate that the tectonic entities in this region were underwent a post-Oligocene counterclockwise rotation of ca. 27° due to the westwards movement of the Anatolian plate, regardless of fault affected block deformation.

Forearc Crustal Structure of Ecuador Revealed by Gravity and Aeromagnetic Anomalies and Their Geodynamic Implications

AbstractAlong the Western Cordillera of Ecuador, fault-bounded ophiolites derived from the Late Cretaceous Caribbean Large Igneous Province (CLIP) have provided key petrotectonic indicators that outline the nature and the mechanism of continental growth in this region. However, most of the forearc basement across Western Ecuador is buried under sediments impairing its crustal structure understanding. Here, we propose a first crustal model throughout the spectral analysis of gravity and aeromagnetic data, constrained by observations made both at the surface and at the subsurface. Three main geophysical domains, within the North Andean Sliver in Western Ecuador, have been defined based on spectral analysis and augmented by 2D forward models. An outer domain, characterized by magnetic anomalies associated with mafic rocks, coincides with evidence of a split intraoceanic arc system. An inner domain is governed by long-wavelength mid to deep crust-sourced gravity and magnetic anomalies possibly evidencing the root of a paleoisland arc and the residuum of a partial melting event with subsequent associated serpentinization, the latest possibly associated with an obduction process during the middle Eocene-Oligocene. In addition, our model supports the presence of a lithospheric vertical tear fault, herein the southern suture domain, inherited from an oblique arc-continent interaction. Our interpretation also brings new insights and constraints on the early geodynamic evolution of the Ecuadorian forearc and provides evidence on the structural style and preservation potential of the forearc basement, most likely the roots of a mature island arc built within an oceanic plateau.

Ballıca (Elazığ Güneyi, Doğu Anadolu) Civarindaki Denizel Sedimanter Kayaçların Jeokimyasal Özellikleri

In this study, the geochemical characteristics of the samples collected from the fine grin sediments of the Middle Eocene-Oligocene Kirkgecit Formation which is deposited in marine environment were studied, In these way, it was aimed to investigate the degree of chemical weathering, and tectonic setting of the source rocks,arround Ballica Village which is located nearly 12 km south of the Elazig Province. The Kirkgecit Formation around Ballica consists of carbonate-rich gray-gray colored mudstones which are composed of very fine-grained sandstones alternation. In the log (SiO2/Al2O3) vs log (Fe2O3/K2O) diagram the samples are clustered in the Fe-shale. Chemical index of alteration (CIA) and chemical index of weathering (CIW) of the sediments suggested that the intensity of weathering in the source area was moderate weathering. Chondrite-normalized patterns show that the the compounds of Ballica samples are similar to Elazig Magmatites. The fact that the Ballica samples different from the Post Australian Arkeen Shales (PAAS) shows that they presents a neutral, basic character. According to the Eu / Eu * - Th / Sc ratios, the source rocks feeding the Kirkgecit sediments are of basic character. In the Th / Co - La / Sc diagram samples are between the basic and silicic rocks.The La-Th-Sc triangle diagram also showed that the samples are different from PAAS and granite, and were in the range of andesite and basalt in composition. According to the La-Sc-Th, Th-Sc-Zr/10 diagrams, composition of the samples fall into the oceanic island arc area.

Controls on deep-water slope channel complex fill, propagation, stacking, and orientation in the Middle Eocene-Oligocene Kırkgeçit Formation, Elazığ, eastern Turkey

Deep-water channels are well-exposed along the northern margin of the east-west oriented Late Paleocene to Oligocene Elazığ Basin of eastern Turkey. The deep-water slope channel complexes are characterised by their association with prominent normal faults, where they seem to have controlled their geometries at the northern margin. The channels were fed by coarse-grained sediment shed by subaqueous gravity flows such as high and low-density turbidity currents, and both cohesive and noncohesive debris flows, from elevated hinterland across narrow shelves and through canyons and gullies into the channels. The faulting and subsequent folding along the northern basin margin created a significantly irregular deep-water slope, and these gravity flows were deflected around, and ponded against this topography. The nature of the impact of seafloor topography, created both before and during channel activity, was examined to create a scheme that documents the range of such effects. Detailed field mapping in this semidesert area shows that these slope channel complexes were strongly controlled by block fault topography generated during early basin evolution in the Late Paleocene, and again during fold development from thrust nappe emplacement during basin filling in the Late Middle Eocene when the Hazar- Maden Basin closed in the south. Four styles of slope channel complex-fold interaction have been recognised: deflection, blocking, diversion, and confinement. In each style of interaction, folding and faulting controlled both channel complex planform geometry and architectural style of the channel fills. In addition to seafloor topography inherited from these earlier phases of compression, subsequent gravitational collapse on the northern basin margin also created normal fault-bound blocks that locally controlled turbidity current pathways and channel complex orientation. It has also been shown that the orientation of the slope channel complexes within mudprone slope sequences has later controlled the orientation of younger fold axis in the Middle Eocene. The study illustrates very well-exposed examples of complex seafloor topography generated by compressional and extensional tectonics, and the impact this has had on slope channel complex evolution.

Structural architecture and tectonic evolution of the Cenozoic Zhanhua Sag along the Tan–Lu Fault Zone in the eastern North China: Reconciliation of tectonic models on the origin of the Bohai Bay Basin

The Zhanhua Sag, a major depocenter within the larger Bohai Bay Basin (BBB) along the Tan–Lu Fault Zone (TLFZ) in the eastern North China, developed through two successive phases with different stress regimes during the Paleogene. The kinematics and the slip history of the NW–SE– and N–S–striking normal fault systems strongly controlled the internal structure and stratigraphy of the Paleocene–Lower Eocene strata in the Zhanhua Sag during Phase 1. Isopach maps of the Paleocene–Lower Eocene stratigraphy show ~4000–m–thick clastic rock sequences in these structurally controlled depocenters. The average dip–slip rates of the NW–SE– and N–S–striking faults were 78.96 and 50.82 m/m.y. (respectively) in the Paleocene–Early Eocene, but decreased to 45.87 and 35.81 m/m.y. in the Middle–Late Oligocene. The NE–SW–striking fault systems and their slip history controlled the development of the Middle Eocene–Oligocene depocenters and their strata during Phase 2. The average dip–slip rate of the NE–SW–striking faults was 10.47 m/m.y. in the Paleocene–Early Eocene but increased to 93.49 m/m.y. in the Middle–Late Oligocene. The NE–SW–oriented faults cut across and displace the NW–SE–striking faults, indicating their relatively younger ages. These fault patterns and kinematics recorded in the Zhanhua Sag reflect an abrupt change in extension directions from ENE–WSW to NW–SE before and after the Middle Eocene that was related to a reversal in the slip direction from sinistral to dextral along the TLFZ. This slip reversal was, in turn, a result of a major change in the subduction direction of the Pacific Plate beneath eastern Asia and in the mode of strain partitioning across the TLFZ in the continental upper plate.

Soft-sediment deformation: deep-water slope deposits of a back-arc basin (middle Eocene-Oligocene Kırkgeçit Formation, Elazığ Basin), Eastern Turkey

The Kirkgecit Formation (middle Eocene-Oligocene) is exposed in an approximately E-W direction around the city of Elazig, eastern Turkey. This unit represents marly and sandy deep-marine sediments deposited in a slope setting with channels and contain soft-sediment deformation structures (SSDSs). These SSDSs include slump folds, chaotic strata, load casts, flame structures, convolute laminations, clastic dykes, water-escape structures and syn-sedimentary faults. The main underlying deformation processes are liquefaction, fluidization and gravity-induced loading. Some of the SSDS could be interpreted as seismites, which originated due to earthquakes that also triggered high-density mass flows. This interpretation is consistent with the active tectonics in the Elazig back-arc basin. The SSDS in the study area are compared with SSDS in other regions, as well as with SSDS in other marine deposits. In this way, findings obtained from the study area are compared with SSDS at different locations in the marine Kirkgecit Formation around Elazig and with similar structures developed in marine settings worldwide, such as SE Crete and NW Africa.

Deformation within the Cycladic subduction–exhumation channel: new insights from the enigmatic Makrotantalo nappe (Andros, Aegean)

New detailed geological/structural mapping coupled with microstructural investigations and amphibole chemistry analyses are used to elucidate the enigmatic tectonic evolution of the Makrotantalo nappe on northern Andros (Aegean region). On the basis of the new geological map, we re-evaluate the structural position of samples with previously published metamorphic ages to show that the Makrotantalo nappe has been affected by the early Eocene high-pressure (HP) metamorphism. Therefore, the Makrotantalo nappe is an integral part of the Cycladic Blueschist Unit (CBU). The early, main ductile deformation phase recognized in the mapped area, is associated with the emplacement of the Makrotantalo nappe over the North Cyclades nappe at deep subduction levels during early Eocene. The subsequent deformation phase occurred at the middle Eocene–Oligocene synchronous with the decompression of blueschist rocks from the stability field of glaucophane to that of actinolite. During this phase, the nappe pile was folded by map-scale folds, which were locally transposed by ductile shear zones. These shear zones were related with NE-directed oblique thrusting and, locally, either restacked the nappe pile or reworked the early nappe contact. Our results in combination with previous studies in the northern Cyclades show that the top-to-the-NE shearing was dominant at a large part of the CBU during exhumation. Opposite, SW-directed shearing was restricted at the upper structural levels of the CBU. We suggest that the observed spatial variation in shear sense is compatible with NE- rather than SW-directed ductile extrusion of the CBU.

Facies and depositional settings of the Middle Eocene-Oligocene carbonates in Kutch

Middle Eocene Fulra Limestone and Oligocene Maniyara Fort Formation represent platform carbonate deposits of Kutch at the north-western margin of India. These carbonates contain larger benthic foraminifera, including Alveolina, Assilina, Discocyclina, Lepidocyclina, Miogypsina, Nummulites and Spiroclypeus. This study presents paleodepositional and paleobathymetric interpretations for both formations using benthic foraminifera in combination with lithological association, sedimentary structures and early diagenetic features. The six carbonate facies comprising the Fulra Limestone indicate a depositional spectrum ranging from bar-lagoon to mid-ramp depositional conditions. It records several shallowing upward cycles, leading to emergence and formation of paleokarst. The four carbonate facies of the Maniyara Fort Formation represents deposition within the inner ramp setting in bar-lagoon and patch-reef environment, while intervening fine siliciclastics correspond to episodes of relative sea level fall. Nummulitic accumulations form low-relief bars within the fair weather wave base in both the formations. The depositional setting of the Paleogene carbonate in Kutch broadly resembles Eocene platformal deposits in the circum-Tethys belt.

Evolution of a passive crustal-scale detachment (Syros, Aegean region): Insights from structural and petrofabric analyses in the hanging-wall

New detailed (micro-)structural investigations, quartz petrofabric analyses and geological/structural mapping in southeast Syros (Cycladic massif, Aegean region) allow us to place new constraints on the tectonic evolution of the Uppermost unit, which occupies the immediate hanging-wall of the crustal-scale Vari Detachment. We show that the Eocene ‒ Oligocene deformation history in the hanging-wall of this detachment is associated with SW-directed ductile shearing. This history includes an early distributed constrictional deformation expressed by transport-parallel upright folds, L-tectonites and cleft-girdles quartz c-axis fabrics that were formed at temperatures ∼500 °C. Ductile deformation progressively localized at the bottom of the Uppermost unit leading to the formation of a greenschist-facies mylonitic zone under plane strain conditions inferred from Type-I cross-girdles quartz c-axis fabrics. The ongoing mylonitization was also associated with temporally increasing pure shear component of deformation coupled with cooling from ∼500 °C to ∼400 °C. We suggest that the Vari Detachment represents a passive normal-sense roof fault resulted from the NE-directed ductile extrusion of the Blueschist unit (footwall) at middle Eocene ‒ Oligocene times. In the middle Miocene, the activation of the brittle SSW-directed Late Vari Detachment enhanced the brittle exhumation of both the Blueschist unit and the Vari Detachment.

The Paleogene vegetation and petroleum system in the tropics: A biomarker approach

The present study investigates the biomarker signatures of sediments and crude oils to infer the paleovegetation from the two petroleum systems of Assam Basin, eastern India. They are classified as the Paleocene-middle Eocene and the middle Eocene-Oligocene petroleum systems. The sediments are oil and gas prone with an early catagenetic stage of thermal evolution. Here we report tricyclic diterpanes such as rimuane, pimarane, rosane and isopimarane along with tetracyclic diterpanes: ent-beyerane, phyllocladane, kaurane from Paleocene-Eocene coal-bearing sediments and crude oils. These diterpane biomarkers attest dominant contribution from conifers, possibly from Podocarpaceae and Araucariaceae. However, diterpanes are not detected in Oligocene sediments and oils. Crude oils from both Paleocene-Eocene and Oligocene reservoirs and organic rich sediments contain angiosperm biomarkers such oleanane and related triterpanes along with bicadinane, a biomarker specific for the Dipterocarpaceae family of angiosperms. The equatorial position of India during early Paleogene along with the presence of angiosperm signatures including Dipterocarpaceae and gymnosperms reflect the emergence of tropical rainforest elements in eastern India. Absence of diterpenoids in Oligocene samples and difference in angiosperm parameters from Paleocene-Eocene samples suggests a change in source vegetation and organofacies.

A foraminiferal study on Middle Eocene-Oligocene break-up unconformity in northern Taiwan and its correlation with IODP Site U1435 to constrain the onset event of South China Sea opening

Taiwan is situated in the SE Asian continental margin where lower Cenozoic strata are well exposed. Its geological and tectonic setting provides a unique opportunity to study the time constraint of the break-up unconformity relevant to the opening of the South China Sea. A detailed foraminiferal study on four land sections and a deep core across the boundary between the Upper and Lower Wulai Groups in the Hsüehshan Range, northern Taiwan, was conducted. Results show remarkable changes of depositional environment and lithology from the amalgamated fluvial dominant sandstones (>2000m) of Middle Eocene in the Lower Wulai Group to the overlying inner neritic shales and siltstones (>2000m) of Oligocene in the Upper Wulai Group. Integrated sedimentological and foraminiferal data further suggest an occurrence of a depositional hiatus between 32 ∼33 and 40Ma across the boundary between the Kankou Formation of the Upper Wulai Group and the Szeleng Sandstone of the Lower Wulai Group in the northern Hsüehshan Range, as same as the scenario seen in the Western Foothills (25–39Ma) of central Taiwan and the Penghu Basin (25–40Ma) in the Taiwan Strait.This stratigraphic hiatus in the Taiwan mountain belt marks a regional break-up unconformity between the Middle Eocene syn-rift strata and the overlying Oligocene post-rift sequences in the NE South China Sea. The break-up unconformity can be correlated with those commonly recognized in the seismic profiles of the Pearl River Mouth Basin and drilling of the IODP site U1435 (Expedition 349) in the northern shelf-slope of the South China Sea. The upper time limit of the breakup unconformity 32Ma could mark the tectonic change from rifting to depression in the Paleogene Asian continental margin which eventually led to opening of the deep South China Sea in the Early Oligocene (∼33Ma).

Tracking the Adriatic-slab travel beneath the Tethyan margin of Corsica–Sardinia by low-temperature thermochronometry

A new multi-thermochronological dataset from Corsica–Sardinia is here employed to constrain the Meso–Cenozoic evolution of the Western Mediterranean area and the problematic transition in space and time between the opposite-dipping Alpine (European) and Apenninic (Adriatic) subductions.The dataset, including zircon and apatite fission track and apatite (U–Th)/He data, covers the whole Meso–Cenozoic time interval, and fits the theoretical age pattern that is expected in distal passive margins after continental break-up. This demonstrates that Corsica–Sardinia represents a fragment of the northern Tethyan margin still preserving the thermochronological fingerprint acquired during Middle Jurassic rifting. Mesozoic apatite (U–Th)/He ages from crustal sections located close to the Tethyan rift axis (i.e., central and eastern Sardinia) show that no European continental subduction took place south of Corsica since the Mesozoic. Along the Sardinia transect, post-Jurassic Adria–Europe convergence was possibly accommodated by Adriatic subduction, consistent with the onset of orogenic magmatism. In middle Eocene–Oligocene times, the northward translation of the Adriatic slab beneath the former Tethyan margin induced a coeval northward migration of erosional pulses at the surface, constrained by a trend of progressively decreasing fission track ages from southern Sardinia to NW Corsica. The Adriatic slab reached the Alpine wedge of Corsica by the end of the Oligocene without any breakoff of the European slab, and started retreating in Neogene times triggering the long-recognized basin opening in the backarc region.

Revised lithostratigraphy of the Mesozoic-Cenozoic succession of the onshore Rovuma Basin, northern coastal Mozambique

Research Article| March 01, 2008 Revised lithostratigraphy of the Mesozoic-Cenozoic succession of the onshore Rovuma Basin, northern coastal Mozambique R.M. Key; R.M. Key a British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom. e-mail: rmk@bgs.ac.uk; ras@bgs.ac.uk Search for other works by this author on: GSW Google Scholar R.A. Smith; R.A. Smith a British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom. e-mail: rmk@bgs.ac.uk; ras@bgs.ac.uk Search for other works by this author on: GSW Google Scholar M. Smelror; M. Smelror Geological Survey of Norway, Leiv Eirikssons vei 39, Trondheim NO-7491, Norway, e-mail: Morten.Smelror@ngu.no; Ola.Sather@ngu.no; Terje.Thorsnes@ngu.no Search for other works by this author on: GSW Google Scholar O.M. Sæther; O.M. Sæther Geological Survey of Norway, Leiv Eirikssons vei 39, Trondheim NO-7491, Norway, e-mail: Morten.Smelror@ngu.no; Ola.Sather@ngu.no; Terje.Thorsnes@ngu.no Search for other works by this author on: GSW Google Scholar T. Thorsnes; T. Thorsnes Geological Survey of Norway, Leiv Eirikssons vei 39, Trondheim NO-7491, Norway, e-mail: Morten.Smelror@ngu.no; Ola.Sather@ngu.no; Terje.Thorsnes@ngu.no Search for other works by this author on: GSW Google Scholar J.H. Powell; J.H. Powell British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom. e-mail: jhp@bgs.ac.uk Search for other works by this author on: GSW Google Scholar F. Njange; F. Njange Direcção National de Geologia, PO Box 217, Pemba, Mozambique, e-mail: c/o edaudi@teledata.mz Search for other works by this author on: GSW Google Scholar E.B. Zandamela E.B. Zandamela Direcção National de Geologia, PO Box 217, Pemba, Mozambique, e-mail: c/o edaudi@teledata.mz Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2008) 111 (1): 89–108. https://doi.org/10.2113/gssajg.111.1.89 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation R.M. Key, R.A. Smith, M. Smelror, O.M. Sæther, T. Thorsnes, J.H. Powell, F. Njange, E.B. Zandamela; Revised lithostratigraphy of the Mesozoic-Cenozoic succession of the onshore Rovuma Basin, northern coastal Mozambique. South African Journal of Geology 2008;; 111 (1): 89–108. doi: https://doi.org/10.2113/gssajg.111.1.89 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract A revised formal lithostratigraphy for the Mesozoic to Cenozoic succession of the onshore portion of the Rovuma Basin in northern Mozambique replaces a previous mixture of informal lithostratigraphical and biostratigraphical names. The following nine formations are formally described: Rio Mecole Formation (Jurassic? age), N’Gapa Formation (Jurassic? age), Pemba Formation (late Jurassic and early Cretaceous age), Macomia Formation (Aptian-Albian age), Mifume Formation (Albian (offshore)/Campanian (onshore)-Maastrichtian age), Alto Jingone Formation (Paleocene-Eocene age), Quissanga Formation (middle Eocene-Oligocene age), Chinda Formation (Neogene age) and Mikindani Formation (Neogene age). The basin’s geology records the temporal development of the coastline of northern Mozambique and southern Tanzania over the last c. 200 million years. Intracratonic rift-basins formed during the early Jurassic Period developed into a south-facing gulf prior to complete separation of eastern Africa from the rest of eastern Gondwana in early Cretaceous times. Concomitant with the lateral crustal movements, vertical isostatic and locally fault-controlled, uplift was a major influence on sedimentation in eastern Africa. This led to erosion with high-energy, fluvial sedimentation into the rift-basins and the establishment of a marine basin throughout the Cretaceous Period. Reduced sedimentation rates during the Paleocene and Eocene were followed by regenerated river-transported sedimentation due to doming and erosion of eastern Africa associated with the development of the East African Rift System since the Oligocene. The major onshore faults that control the shape of the Rovuma Basin and which have been intermittently active since the Jurassic cut across the east-northeast to west-southwest structural grain of the underlying Precambrian crystalline rocks of the East African Orogen. However, some of the east-northeast to west-southwest transfer faults identified in the offshore part of the Rovuma Basin may represent, in part, reactivated Pan African ductile shears You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic evolution of the northwestern Internal Dinarides as constrained by structures and rotation of Medvednica Mountains, North Croatia

Abstract This paper attempts to explain the tectonic history and possible reasons for the change of trend of the northwestern part of the Internal Dinarides in a transitional area between the Southeastern Alps, central Dinarides and Tisia, north of Zagreb. Structural and palaeomagnetic data collected in pre-Neogene rocks at Medvednica Mountains, combined with palaeomagnetic data available from Neogene rocks in the surrounding area, point to the following conclusions: (1) The reason for dramatic deflection in structural trend of the Internal Dinarides in the area north of Zagreb is a 130° clockwise rotation and eastward escape of a tectonic block comprising Medvednica Mountains and the surrounding inselbergs, bounded to the north by the easternmost tip of the Periadratic Lineament. In Medvednica Mountains, the main period of tectonic escape and associated clockwise rotation occurred in the Late Palaeogene, possibly in the Oligocene–earliest Miocene. (2) When rotated into the original position, the trend of observed pre-Neogene structures of Medvednica Mountains becomes parallel to the major structural trend of the central Dinarides. In view of their original orientation, these structures are interpreted in the following way: (a) The first D1 deformational event is attributed to the Aptian–Albian nappe stacking in the central–northern Dinarides that was accommodated by a top-to-the-north directed shearing and northward propagation of already obducted ophiolites of the Central Dinaridic ophiolite zone. This nappe stacking, which resulted in a weak regional metamorphism in tectonic units underlying the ophiolites, was orogen-parallel or at a very acute angle to known structural (and possibly palaeogeographic) trends. This implies a major left-lateral shear component along the former Adriatic margin and obducted Dinaridic ophiolite zone. (b) This was followed by Early Albian orogen-perpendicular shortening (D2) that was accommodated by folding and top-to-the-west thrusting. This deformation resulted in gradual cooling of the metamorphic stack and also in uplift and erosion of the higher structural units. (c) The D3 deformational event was driven by renewed E–W shortening that took place after the Paleocene, most probably during the Middle Eocene–Oligocene, i.e. synchronous with the main Dinaridic tectonic phase of the External Dinarides. This shortening was probably triggered by collision and thrusting of Tisia over the northern segment of the Internal Dinarides. (d) This was finally followed by D4 pervasive, right-lateral N–S shearing that is tentatively interpreted as being related to the right-lateral shearing of the Sava zone during the Eocene–Oligocene. (e) Following the main period of tectonic escape and induced clockwise rotation along the Periadriatic fault, possibly in the Oligocene–earliest Miocene, the Medvednica Mountains and the surrounding area were affected by repeated extensions and inversions since the Early Miocene to recent times. Palaeomagnetic data suggest that in the Early Miocene (but probably before the Karpatian) this area was part of a regional block that shifted northwards and rotated in a counter-clockwise sense. A second episode of counter-clockwise rotation occurred at the present latitude in post-Pontian times (since c . 5 Ma), driven by the counter-clockwise rotating Adriatic Plate.

Revised deformation history of the central Andes: Inferences from Cenozoic foredeep and intermontane basins of the Eastern Cordillera, Bolivia

Investigation of Cenozoic deformation, sediment accumulation, and provenance in the Eastern Cordillera of Bolivia at 17–21°S indicates major shortening (60–140 km) and foredeep development followed by limited internal shortening and intermontane basin development. Contrasting histories of shortening, deposition, sediment dispersal, and detrital composition distinguish a formerly extensive Paleogene foredeep exposed along an eastern belt of synclines from a zone of principally Neogene intermontane basins in the central Eastern Cordillera. New40Ar/39Ar ages of 25–17 Ma for interbedded tuffs reveal latest Oligocene–early Miocene accumulation in intermontane basins at rates <80 m/Myr, several times lower than Andean foredeeps. Although poorly dated, foredeep evolution probably coincided with middle Eocene–Oligocene deformation and denudation in adjoining regions to the west. A mid‐Cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement‐involved tectonic wedge beneath the Eastern Cordillera. In this interpretation, wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 Ma, whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas. Regardless of the regional subsurface structural geometry,40Ar/39Ar ages define a 25–21 Ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the Eastern Cordillera and possibly the age of initial shortening in the Interandean and Subandean zones to the east. In contrast to several popular models, the revised tectonic history presented here suggests significant pre‐Neogene shortening in the Eastern Cordillera and underscores the large uncertainties in estimates of long‐term shortening rates in the Andes.

Controls on architecture of the Late Cretaceous to Cenozoic southern Middle Magdalena Valley Basin, Colombia

The Maastrichtian-Cenozoic southern Middle Magdalena Valley Basin of Colombia contains a unique record of unconformities, strata, and structure, from which we extract the histories of exhumation of the Central Cordillera, to the west, and evolution of the Eastern Cordillera fold-and-thrust belt, to the east. This study integrates field-based analyses of stratigraphy, laboratory analyses of provenance, fission-track thermochronology, vitrinite-reflectance data, volcanic-ash geochronology, and studies of synorogenic geometries and structure displayed in seismic data. A major unconformity, the Middle Magdalena Valley unconformity, formed by eastward migration of Central Cordillera uplift during Late Cretaceous to early Eocene time, which transformed a Maastrichtian marine basin into a Paleocene depositional piedmont area This transformation is recorded by a coarsening-upward sequence of marine shales to alluvial-fan conglomerates, which was partly eroded during further early Eocene propagation of Central Cordillera deformation. Cessation of this phase of uplift led to formation of a pediment surface, the Middle Magdalena Valley unconformity, which was buried by westward-onlapping middle Eocene to Neogene alluvial deposits. Middle Eocene to Neogene facies, paleoflow, and unconformities were controlled by Eastern Cordillera deformation. In the Eastern Cordillera foot-hills, growth strata and thermal history reveal two phases of folding of middle Eocene-Oligocene and late Miocene ages, prior to intense Pliocene-Pleistocene uplift. Two unconformities of early late Miocene and Pliocene-Pleistocene ages occur to the west of the Eastern Cordillera and record flexural tilting related to episodes of Eastern Cordillera loading.

Cenozoic deep-sea ostracods from Maud Rise, Weddell Sea, Antarctica (ODP Site 689): a palaeoceanographical perspective

Cenozoic palaeoceanography of the Maude Rise, Weddell Sea, Antarctica, has been investigated using Palaeocene to Quaternary deep-sea ostracod faunas from 23 samples of ODP Site 689. The abundance of ostracods is high enough only during the Palaeogene (Palaeocene–Oligocene) to allow palaeoceanographical inferences based on changes in diversity, dominance, endemism and faunal turnover (first and last occurrences). The abundance is particularly high throughout the Palaeocene and Eocene, but declines irreversibly near the Eocene/Oligocene boundary. The diversity increases more or less continuously from the Early Palaeocene to the Middle Eocene, and then it generally decreases throughout the remaining part of the Palaeogene (Middle Eocene–Oligocene); an exception is a positive peak in the Shannon–Weaver index in a single sample in the Late Oligocene. No positive peaks in diversity and taxa originations (first occurrences) at c. 40–38 Ma, occurs at Site 689; so the site provides no evidence for the establishment of the psychrosphere at this time. This corroborates similar regional results from an earlier study of benthonic foraminifera. Explanations for this may be related to Late Eocene–Early Oligocene changes in sedimentology and clay-mineralogy (associated with the progressive cooling of the Antarctica) which could have negatively affected abundance and diversity locally at Site 689. Alternatively, by this time, the ostracod fauna could also have been subjected to selective removal (with possible local extinction) of taxa (due to increased ventilation) or to thanatocoenosis dissolution (due to a decrease in temperature and availability of CaCO 3). A further possibility may be related to the fact that Site 689 was at intermediate water depths and may have remained within older water masses near the Eocene/Oligocene boundary. Failing these explanations, the results could indicate that the Late Eocene–Early Oligocene palaeoenvironmental changes in the world oceans were more gradual and occurred over a longer time interval than the global ostracod data show, at least at southern high latitudes.

Macrofossils of Callicoma and Codia (Cunoniaceae) from Australian Cainozoic sediments

Leaves, leaf fragments and two infructescences from five widespread Australian Cainozoic fossil localities, Little Rapid River, Lemonthyme Creek, Cethana(Early Oligocene), Berwick Quarry (Oligocene–Early Miocene) and StuartCreek (Miocene–Pliocene) are assigned to, or validated as, the extantspecies Callicoma serratifolia Andrews. The extantspecies or, more likely, a very close relative, evolved prior to the EarlyOligocene and has since remained relatively unchanged. It has becomerestricted to the forests of eastern mainland Australia, probably in responseto climatic change during the Cainozoic. The first macrofossil ofCodia, C. australiensis R.W.Barnes & R.S.Hill sp. nov., is described from West Dale (MiddleEocene–Oligocene), Western Australia, and has affinities with thejuvenile foliage of at least one extant Codia species.Several structures of Callicoma and someCodia species (stomatal cutin frill/ledge and pairedhair bases) may be synapomorphies for these genera.Callicoma serratifolia and Codiamay share a unique common ancestor, or alternatively,C. serratifolia may represent a paedomorphic form ofCodia.

Structural and Stratigraphic Evolution of the Western Malvinas Basin, Argentina

The Malvinas Basin is one of the few basins on the Argentine continental shelf that contains a proven petroleum system; however, uneconomical oil discoveries keep the basin at a frontier exploration status. The Malvinas Basin evolved through three main tectonic phases: rift, sag, and foredeep. The sedimentary fill of the basin is closely related to its tectonic history. Middle Jurassic rifting resulted in north-northwest-oriented grabens that filled with volcanic and pyroclastic continental rocks. Diminished faulting and generalized subsidence during the Late Jurassic-Neocomian early sag phase were accompanied by deposition of a basal transgressive marine wedge. The Aptian-Maastrichtian interval was characterized by tectonic quiescence and deposition of offshore mud-prone sediments. Southerly localized early Paleogene transtensional tectonism accompanied the early development of a foredeep trough. Outer shelf glauconite-rich sandstones, basinal claystones, and localized carbonate buildups partially filled the basin. By the middle Eocene-Oligocene, a strong deepening event marked the initiation of the foredeep sensu stricto phase. This phase resulted in the full development of the Malvinas foredeep and the formation of compressional structures in the foreland. The foredeep basin was replenished by a westerly derived offlapping siliciclastic wedge of Oligocene-Miocene age. Noncommercial hydrocarbon discoveries in 5 of the 17 wells drilled in the basin suggest the presence of an undercharged Lower Inoceramus-Springhill petroleum system.