Early Oligocene Time Research Articles

Ghosts of the Holobiont: Borings on a Miocene Turtle Carapace from the Pisco Formation (Peru) as Witnesses of Ancient Symbiosis

In spite of the widespread occurrence of epibiotic turtle barnacles (Coronuloidea: Chelonibiidae and Platylepadidae) on extant marine turtles (Chelonioidea: Cheloniidae and Dermochelyidae), and although the association between these cirripedes and their chelonian hosts has existed for more than 30 million years, only a few studies have investigated the deep past of this iconic symbiotic relationship on palaeontological grounds. We describe probable platylepadid attachment scars in the form of hemispherical/hemiellipsoidal borings on an Upper Miocene (Tortonian) fragmentary turtle carapace, identified herein as belonging to Cheloniidae, from the Pisco Lagerstätte (East Pisco Basin, southern Peru). When coupled with the available molecular data, this and other similar ichnofossils allow for hypothesising that platylepadid symbionts were hosted by sea turtles as early as in early Oligocene times and became relatively widespread during the subsequent Miocene epoch. Chelonian fossils that preserve evidence of colonisation by platylepadid epibionts in the form of pits on the turtle shell should be regarded as fossil holobionts, i.e., palaeontological witnesses of discrete communal ecological units formed by a basibiont and the associated symbionts (including the epibiota). A greater attention to the bone modifications that may be detected on fossil turtle bones is expected to contribute significantly to the emerging field of palaeosymbiology.

Diachronous initiation of Arabia–Eurasia collision from eastern Anatolia to the southeastern Zagros Mountains since middle Eocene time

ABSTRACT The persistent debate over the timing of initial Arabia–Eurasia collision has eluded consensus for decades, with most estimates ranging widely from ~50 to 10 Ma along both the Anatolian and Zagros segments of the orogen. Here, we independently evaluate the timing and location of initial collision using a quantitative plate tectonic reconstruction that is the first to include the entire Arabian microcontinent and regions to the north in eastern Anatolia, northwestern Iran, and the Caucasus. The reconstruction combines new kinematic constraints from recent global plate models with a synthesis of available constraints on crustal shortening and continental and back-arc subduction to restore the pre-collisional geometries of the Arabian and Eurasian continental margins and to track their interaction through time. Our preferred model suggests that initial continental underthrusting, or soft collision, of the relatively thin Arabian passive margin with Eurasia occurred by ca. 42 Ma along the eastern Bitlis suture zone and began transmitting strain across central and eastern Anatolia, the Caucasus, and northwestern Iran. Collision inception then propagated westward into central Anatolia and southeastward into the northwestern Zagros Mountains by late Eocene time (ca. 40–35 Ma), the central Zagros by latest Eocene to early Oligocene time, (ca. 35–25 Ma), and finally the southeastern Zagros by latest Oligocene to middle Miocene time (ca. 25–15 Ma). The diachroneity of Arabia collision led to widespread upper-plate (Eurasian) contraction and a similarly time-transgressive magmatic lull that lagged behind the onset of collision by ~2–12 Myr. By incorporating the entire Arabian microcontinent and regions to the north, this new tectonic reconstruction highlights the diachronous nature of Arabia–Eurasia collision that occurred first along the Bitlis suture zone in eastern Anatolia during late middle Eocene time and became progressively younger to the southeast along strike of the Zagros suture zone.

Formation of the Yangtze Three Gorges: Insights from detrital apatite fission‐track dating of sediments from the Jianghan Basin

AbstractThe establishment of the course of the modern Yangtze River is a topic of ongoing debate, with the formation of the Three Gorges, incised in the Huangling Massif, representing the key event under discussion. Downstream of the Three Gorges, the Cenozoic succession of the Jianghan Basin has been investigated to detect the moment of incision into the Three Gorges. Detrital apatite fission‐track data from Eocene to Miocene sediments drilled in the Jianghan Basin suggest that material was supplied by the Dabie, Qinling and Jiangnan Orogens during Eocene to early Oligocene times. At late Oligocene, the source of sediments switched to the Huangling Massif, implying incision of the Three Gorges. We therefore propose that the modern Yangtze River system formed at the late Oligocene.

Biotic community and landscape changes around the Eocene–Oligocene transition at Shapaja, Peruvian Amazonia: Regional or global drivers?

Since 2012, we have investigated a stratigraphic section encompassing the late Eocene–earliest Oligocene interval at Shapaja (Tarapoto area, Peruvian Amazonia, ca. 7°S), through paleontological and geological fieldwork. The measured sedimentary series (120 m-thick [West] plus 90 m-thick [East]), assigned to the upper member of the Pozo Formation, records fluvial micro-conglomeratic lenses intercalated with floodplain and evaporite-rich fine red deposits, estuarine/coastal-plain tidally-influenced fine sandstones, and oxbow lake nodule-rich blue clays. This sedimentary shift coincides locally with the demise of the large Eocene coastal-plain wetland known as Pozo System. The late Eocene–early Oligocene Shapaja section was extensively sampled for chemostratigraphy (δ 13 C on dispersed organic matter and pedogenic carbonate nodules), which in turn allowed for refining the location of the Eocene-Oligocene Transition (EOT) and other climatic events recognized at a global scale (i.e., Oi-1 and Oi-1a). The section has yielded nine fossil localities with plant remains (leaves, wood, charophytes, and palynomorphs), mollusks, decapods, and/or vertebrates (selachians, actinopterygians, lungfishes, amphibians, sauropsids, and mammals), documenting ~130 distinct taxa. Four localities of the upper member of the Pozo Formation at Shapaja predate the EOT, one is clearly within the EOT, while four are earliest/early Oligocene in age. The small leaf impressions found along the Shapaja section could be indicative of dry and/or seasonal conditions for this region throughout and after the EOT. Monkeys, indicative of tropical rainforest environments, are only recorded in a latest Eocene locality (TAR-21). Two biotic turnovers are perceptible in the selachian, metatherian, and rodent communities, well before the EOT [~35–36 Ma] and a few hundred thousand years after the EOT [~33 Ma]. The latter turnover seems to be primarily related to a global sea-level drop (ichthyofauna: marine-littoral elements replaced by obligate freshwater taxa) and/or the onset of a drier and more seasonal climate in early Oligocene times (terrestrial components). Changes in the structure of the Shapaja paleocommunities were mostly driven by the flexural subsidence during the late Eocene, and then globally driven by the earliest Oligocene climatic deterioration. • The Shapaja section spans the late Eocene–early Oligocene interval ( ca. 36–32.5 Ma). • Micro- and macroremains of 129 plant and animal fossil taxa were recognized. • Two faunal turnovers are recorded on both sides of the Eocene-Oligocene transition. • Shapaja records the earliest occurrence of South American primates and sloth. • Community changes are locally- then globally-driven (late Eocene/earliest Oligocene).

Continental palynomorphs from the Dabaa Formation, North-Western Desert, Egypt: a contribution to the reconstruction of the vegetation on the southern shores of the Tethys Ocean during the Early Oligocene

AbstractThe Eocene–Oligocene transition period was marked by one of the most abrupt and severe global environmental changes in the Cenozoic record, and this had a marked influence on the evolution of a number of animal and plant groups and entire ecosystems. This study documents continental palynomorphs recovered from the sedimentary rocks of the Dabaa Formation (Qattara area, North-Western Desert, Egypt) located on the southern shore of the Tethys Ocean and dated as Late Eocene–Early Oligocene. The botanical affinities, (phyto)ecology and distribution of the vegetation during the Eocene–Oligocene of the study area are discussed. The recorded assemblages are well preserved and comprise diverse lineages of algae, spores and pollen. They were identified, illustrated and assigned to 46 families encompassing chlorococcalean algae, lycopods, ferns, gymnosperms and angiosperms. The studied assemblages demonstrate the development of tropical vegetation, including tropical deciduous forest, grassland and (semi-)arid tropical shrubland, in which angiosperms were one of the main representatives; additionally open, drier habitats might have existed in the hinterland. Our data have been combined with previous megafossil and palynological evidence to assess and refine vegetation changes during the Early Oligocene time window in Egypt and across North Africa. Vegetation was a mosaic of different vegetation belts that ran more-or-less parallel to the coastline of the Tethys Ocean under the variable geographical influence of lagoons and streams. It is assumed that the belt of tropical forest along the coast of the Tethys Ocean narrowed during the Oligocene in parallel to climatic deterioration following the Eocene–Oligocene boundary, which may have also led to the fractionation of forest habitats.

Cenozoic Exhumation History of the Eastern Margin of the Northern Canadian Cordillera

AbstractNew low‐temperature thermochronology data from clastic sedimentary rocks in the northern Richardson Mountains, Canada, indicate significant exhumational cooling during late Eocene–early Oligocene time. Apatite (U‐Th‐Sm)/He (AHe) data were collected from 19 Proterozoic–Paleocene rocks across a 115 km transect. Eighty‐eight single‐grain AHe dates range from 16–300 Ma and are generally younger than stratigraphic ages, indicative of thermal resetting by burial. Additionally, zircon (U‐Th)/He (ZHe) dates from two Proterozoic–Cambrian rocks range from 49–123 Ma and suggest burial to >160°C. In contrast, ZHe dates from Jurassic sandstones are older than the stratigraphic age, which limits maximum burial to <160°C. Thermal history modeling reveals three phases of cooling, during the Paleocene–early Eocene (>65–50 Ma), late Eocene–early Oligocene (40–30 Ma), and late Oligocene–early Miocene (30–15 Ma). Most samples cooled during the first and second phases, whereas the third phase is less well constrained. In general, most rocks were below the sensitivity of AHe analysis since the early–middle Miocene. The results suggest a previously unrecognized phase of inferred deformation in the northern Richardson Mountains between 40–30 Ma. Our findings contribute to previous work that recognizes Late Cenozoic deformation along the eastern margin of the Northern Cordillera. We further investigated the potential mechanisms of this widespread deformation and suggest exhumation may relate to kinematic changes of the North American plate relative to structural trends along the margin of the Northern Cordillera.

Evolution of petroleum generation and expulsion of the oil-source rock in the north-western Say’un-Masilah Basin as inferred from 1D basin models of a single well (Al-Qarn-01)

The Madbi Shale Formation is the main oil-source rock for conventional petroleum extraction in the east-central Say’un-Masilah Basin. This study explores the generation and expulsion potential of oil from the Madbi source rock system in a deep well location (Al-Qarn-01) in the north-western Say’un-Masilah Basin using geological and geochemical modelling. One-dimensional (1D) basin models were constructed to the estimate thermal regime in the basin and the evolution of oil cracking and expulsion from the organic-rich intervals in the Madbi Formation over the geological time. In the studied Al-Qarn-01 well, the organic-rich intervals in the Madbi Formation are thermally mature, ranging from peak- to late-mature oil window stages; thus, half or more of the kerogen was converted to oil at these maturity stages. The basin modelling results of the studied well (Al-Qarn-01) show that oil was generated and expelled from Madbi source rock intervals in two main phases according to the conversion ratio (TR %) of kerogen. The main conversion of kerogen to oil (TR = 10–50%) occurred between Late Cretaceous and Early Oligocene (80–33 Ma) periods, corresponding to the peak-oil window (0.70–1.00 Easy %Ro). Furthermore, significant amounts of oil were expelled from the source rock, which subsequently migrated into the overlying reservoir rocks in the Early Oligocene time during the initial Tertiary rifting. The late-mature oil window (1.00–1.10 Easy %Ro) commenced during this time, with TRs of 50–77%.

The coupling of high-pressure oceanic and continental units in Alpine Corsica: Evidence for syn-exhumation tectonic erosion at the roof of the plate interface

The subduction of continental crust is now a matter of fact but which are the mechanisms and the factors controlling the exhumation of continental units and their coupling with oceanic units are still a matter of debate. We herein present the tectono-metamorphic study of selected continental units belonging to the Alpine Corsica (Corte area, Central Corsica, France). The tectonic pile in the study area features thin slices of oceanic units (i.e. Schistes Lustrés Complex) tectonically stacked between the continental units (i.e. the Lower Units), which record a pressure–temperature-deformation (P-T-d) evolution related to their burial, down to P-T-peak conditions in the blueschist facies and subsequent exhumation during the Late Cretaceous – Early Oligocene time span. The metamorphic conditions were calculated crossing the results of three different thermobarometers based on the HP-LT metapelites. The continental units only recorded the P-peak conditions of 1.2 GPa-250 °C, up to the T-peak conditions of 0.8 GPa-400 °C, and the retrograde path up to LP-LT conditions. The metamorphic record of the oceanic units includes part of the prograde path occurring before the peak conditions reached at 1.0 GPa-250 °C followed by the last metamorphic event related to LP-LT conditions. The results indicate that each unit experienced a multistage independent pressure–temperature-deformation (P-T-d) evolution and suggest that the oceanic and continental units were coupled during the rising of the last ones at about 10 km of depth, where the oceanic units were stored at the base of the wedge. Subsequently they were deformed together by the last ductile deformation event during exhumation. We propose a mechanism of tectonic erosion at the base of the wedge, by which slices of Schistes Lustrés Complex were removed at the roof of the plate interface during the exhumation of the Lower Units.

Postmagmatic Tectonic Evolution of the Outer Izu-Bonin Forearc Revealed by Sediment Basin Structure and Vein Microstructure Analysis: Implications for a 15 Ma Hiatus Between Pacific Plate Subduction Initiation and Forearc Extension.

International Ocean Discovery Program Expedition 352 recovered sedimentary‐volcaniclastic successions and extensional structures (faults and extensional veins) that allow the reconstruction of the Izu‐Bonin forearc tectonic evolution using a combination of shipboard core data, seismic reflection images, and calcite vein microstructure analysis. The oldest recorded biostratigraphic ages within fault‐bounded sedimentary basins (Late Eocene to Early Oligocene) imply a ~15 Ma hiatus between the formation of the igneous basement (52 to 50 Ma) and the onset of sedimentation. At the upslope sites (U1439 and U1442) extension led to the formation of asymmetric basins reflecting regional stretch of ~16–19% at strain rates of ~1.58 × 10−16 to 4.62 × 10−16 s−1. Downslope Site U1440 (closer to the trench) is characterized by a symmetric graben bounded by conjugate normal faults reflecting regional stretch of ~55% at strain rates of 4.40 × 10−16 to 1.43 × 10−15 s−1. Mean differential stresses are in the range of ~70–90 MPa. We infer that upper plate extension was triggered by incipient Pacific Plate rollback ~15 Ma after subduction initiation. Extension was accommodated by normal faulting with syntectonic sedimentation during Late Eocene to Early Oligocene times. Backarc extension was assisted by magmatism with related Shikoku and Parece‐Vela Basin spreading at ~25 Ma, so that parts of the arc and rear arc, and the West Philippine backarc Basin were dismembered from the forearc. This was followed by slow‐rift to postrift sedimentation during the transition from forearc to arc rifting to spreading within the Shikoku‐Parece‐Vela Basin system.

Cenozoic Sediment Provenance in the Northern Great Plains Corresponds to Four Episodes of Tectonic and Magmatic Events in the Central North American Cordillera

AbstractDetrital zircon U‐Pb provenance in the northern Great Plains reveals histories of drainage reorganization of the paleo‐Missouri River in response to Cenozoic tectonic and magmatic processes in the central North American Cordillera. During the latest Cretaceous, sediment provenance was confined to the Cordilleran hinterland in central Idaho and southwestern Montana, probably due to the existence of a subtle paleohydraulic divide in northwestern Montana. During the early to middle Paleocene, the paleodrainage was expanded to cover the Belt Supergroup in northwestern Montana in response to the eastward propagation of the Cordilleran thrust belt. During the late Paleocene to early Eocene, the final movement of the thrust belt and the initial extension of the Cordilleran hinterland shifted the drainage divide to the orogenic front and caused focused erosion of the upper Lower to Upper Cretaceous rocks to the east. During the middle Eocene to early Oligocene, a significant increase of Archean grains suggests renewed exhumation of the Laramide province and sediment delivery by the paleo‐Yellowstone River originated in the central Rocky Mountains. The presence of middle Eocene to early Oligocene zircons during this stage also suggests headwater erosion of the paleo‐Upper Missouri River into the Dillion volcanic field in southwestern Montana. These interpretations indicate that a drainage pattern similar to that of the modern Missouri River was established during middle Eocene to early Oligocene time. The detrital zircon maximum depositional ages also improve the chronostratigraphic framework of the Paleogene strata. Our study demonstrates that orogenic processes can be well archived in sedimentary records far away from orogenic systems.

Benthic foraminiferal fauna and sediment provenance of Eocene syn-rift sequences in Taiwan: implication for onset of Asian epi-continental marginal seas off China coast

Early Cenozoic rift basins developed commonly on the Mesozoic basement along the SE Asia Continent. However, Eocene–Oligocene sequences were only exposed widely onshore Taiwan. We study benthic foraminifera and determine U–Pb geochronology of detrital zircon grains in the Early Cenozoic sequence onshore Taiwan for understanding if there was any significant change in faunal content and sediment provenance between the Eocene syn-rift sequences and the overlying Oligocene post-rift strata across the breakup unconformity. The Eocene syn-rift sequences are mostly of thick, cross-bedded sandstones of fluvial-deltaic depositions without marine fauna. Intensive study on the Eocene stratigraphy in more than ten sections all over the Taiwan mountain belt in the last three decades, shallow marine benthic foraminifera of Gaudryina hayasakai-Hanzawaia asterizana-Planularia okinoshimaensis-Heterolepa praecinta assemblage were found only in very limited exposures in the Western Foothills, central Taiwan. This study shows that the Eocene benthic foraminiferal assemblage is similar to the Shihtsaoan fauna in the overlying Oligocene post-rift marine strata. This indicates no significant faunal break across the breakup unconformity. As a consequence, Gaudryina hayasakai Zone of the Shihtsaoan age can extend downward to the Eocene and the Shihtsaoan Stage would be equivalent to the Hsuehshanian Stage established previously from northern Taiwan. The Taiwan Eocene marine sequences could be deposited in a normal-fault bounded narrow seaway or gulf connecting southeastward to the open Huatung Basin. U–Pb geochronology of detrital zircons in the Taiwan Eocene syn-rift sandstones shows a prominent age peak centered in Yenshanian (100–155 Ma) similar to the Paleocene–Oligocene fluvial-deltaic deposits in the East China Sea and is also comparable to the U–Pb zircon age spectrum of modern river sands of the Fujian Province, SE China. Petrographic study also suggests a prevailing sediment provenance of the Cathaysia granites and volcanics. These Cathaysia detritals were transported to the Taiwan Eocene grabens through some SE-flowing rivers. Structural reconstructions in previous study indicated that the mountain belt with the Eocene sequences in NE Taiwan had been shortened by imbricated thrusting westward for 160–200 km wide during the arc-continent collision in the last 6.5 Ma. Integrating paleontology and sedimentology evidence reveal that the Eocene coast was located along the modern shelf break of the East China Sea ~ 350–400 km east off the modern Chinese coast line. This implies that in the Eocene time (~ 40 Ma) there was no wide Asian epi-continental marginal seas off Chinese coast until the onset sedimentation of the Oligocene post-rift strata during opening of the South China Sea in the Early Oligocene time (~ 33 Ma).

Micromorphological studies of the complex early Oligocene Himalayan foreland palaeosols in relation to Asian monsoon climate

The Cenozoic Himalayan orogeny not only modified Asia's climate but also affected global climate. One of its significant outcomes is the inception of Asian monsoon climate. Though researchers have shown that changes in Himalaya and Tibetan Plateau elevations control its evolution, however reached no consensus on pre-Miocene timing of monsoon climate inception, perhaps due only limited studies in pre-Miocene Himalayan sediments. Another possibility is that we have restricted our studies in deducing a relationship between Himalayan tectonic upheavals and monsoon climate. However, priority should be to first explore any palaeomonsoon signatures since deposition of Himalayan Foreland Basin (HFB) sediments because some researchers have reported palaeomonsoon signatures during Eocene-Oligocene time. Here we report another such study from early Oligocene Dagshai palaeosols of Subathu sub-basin, India. Detailed morphological and micromorphological studies revealed abundant presence of matrix and intrusive pedofeatures in mature palaeosol profiles. Most striking evidence is the presence of FeMn oxide and carbonate pedofeatures together. The concretions present in the thick Bk horizons having diffuse boundaries show intermixing and/or alternation of calcite and iron oxides. Also occurrence of pedogenic carbonate nodules is not restricted in Bk horizon as is observed in present monsoon dominant regions and nodules are impure, dense and largely micritic, similar to those documented in Indo-Gangetic Holocene soils. Further, most common FeMn oxide nodules are typical, impregnative and associated with granostriated fabric which is indicative of soils formed under seasonal wet and dry cycles. Furthermore, occurrence of illuvial coatings and carbonate together in the B-horizon indicate prevalence of sufficient wet and dry cycles respectively. All these features indicate presence of seasonal climatic conditions and therefore monsoon conditions prevailed during early Oligocene time which is contrary to the most common known Late Miocene age of Indian Monsoon inception.

The Thrace Basin and the Black Sea: the Eocene–Oligocene marine connection

AbstractThe Late Cretaceous – Recent West Black Sea Basin and the Eocene–Oligocene Thrace Basin are separated by the Strandja arch comprising metamorphic and magmatic rocks. Since Late Cretaceous time the Strandja arch formed a palaeo-high separating the two basins which accumulated clastic sediment of >9 km thickness. During late Eocene – early Oligocene time the marine connection between these basins existed through the Çatalca gap west of Istanbul. The Çatalca gap lies on the damage zone of a major Cretaceous strike-slip fault; it formed a 15 km wide marine gateway, where carbonate-rich sediments of thicknessc.350 m were deposited. The sequence consists of upper Eocene shallow marine limestones (SBZ18-20) overlain by upper Eocene – lower Oligocene (P16-P19 zones) pelagic marl with a rich fauna of planktonic foraminifera; the marls are intercalated with 31–32 Ma acidic tuff and calc-arenite beds. The Çatalca gap is bounded in the west by a major normal fault, which marks the eastern boundary of the Thrace Basin. Seismic reflection profiles, well data and zircon U–Pb ages indicate that the Thrace Basin sequence west of the fault is late Eocene – middle Oligocene (37–27 Ma) in age and that the fault has accommodated 2 km of subsidence. Although there was a marine connection between the West Black Sea and Thrace basins during late Eocene – early Oligocene time, no significant exchange of clastic sediment took place. Sedimentation in the Çatalca gap ended abruptly during early Oligocene time by uplift, and this eventually led to the paralic conditions in the Thrace Basin.

Oligocene and Lower Miocene source rocks in the Paratethys: palaeogeographical and stratigraphic controls

Abstract Oligocene and Lower Miocene deposits in the Paratethys are important source rocks, but reveal major stratigraphic and regional differences. As a consequence of the first Paratethys isolation, source rocks with very good oil potential accumulated during Early Oligocene time in the Central Paratethys. Coeval source rocks in the Eastern Paratethys are characterized by a lower source potential. With the exception of the Carpathian Basin and the eastern Kura Basin, the source potential of Upper Oligocene and Lower Miocene units is low. In general, this is also valid for rocks formed during the second (Kozakhurian) isolation of the Eastern Paratethys. However, upwelling along a shelf-break canyon caused deposition of prolific diatomaceous source rocks in the western Black Sea. Overall, Oligocene–Lower Miocene sediments in the Carpathian Basin (Menilite Formation) can generate up to 10 t HC m −2 . Its high petroleum potential is a consequence of the interplay of very high productivity of siliceous organisms and excellent preservation in a deep silled basin. In contrast, the petroleum potential of Oligocene–Lower Miocene (Maikopian) sediments in the Eastern Paratethys is surprisingly low (often <2 t HC m −2 ). It is, therefore, questionable whether these sediments are the only source rocks in the Eastern Paratethys.

Cenozoic North Atlantic deep circulation history recorded in contourite drifts, offshore Newfoundland, Canada

In the North Atlantic Ocean, contour-following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2km in thickness and extend for 100s of km. The stratigraphic records in the drifts have been used to reconstruct variations in North Atlantic deep-water circulation throughout the Cenozoic; however, uncertainties remain about certain aspects of the timing, intensity, depth distribution, and regional impact of these currents. Here, we use an integrated dataset of seismic-reflection profiles and IODP core data (lithology, biostratigraphy, and magnetostratigraphy) to document sedimentation history and the development of current effects in the Cretaceous to present sedimentary record on the J-Anomaly Ridge and Southeast Newfoundland Ridge, offshore Newfoundland, Canada. The Newfoundland ridges are in a key location, lying between well-studied areas in the northern and western North Atlantic and under the path of both the modern Deep Western Boundary Current and the Gulf Stream. Late Cretaceous through Early Eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation, but at ~47Ma, near the Early-Middle Eocene boundary, well developed contourite drifts began to accrete in paleo-water depths of ~4000–4500m, accompanied by an order-of-magnitude increase in terrigenous sediment mass accumulation rates. From this time forward, drift deposition, interrupted by brief episodes of erosion, continued unabated. This timing for the onset of persistent deep currents is coincident with reorganization of Atlantic circulation inferred from a change from biosiliceous to non-biosiliceous sedimentation in the western North Atlantic (Horizon AC) and with the current-eroded Intra-Eocene Unconformity (IEU) in the northern North Atlantic. A change in sedimentation style occurred within the Middle Eocene to upper Oligocene drift sequence, and it likely was related to a shift to deeper, more intense currents that eroded the widespread Horizon AU along the margin of eastern North America about Early Oligocene time. Beginning in the Late Oligocene (~25Ma) a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at ~3500–4500m paleodepth on the Southeast Newfoundland Ridge. This development correlates with widespread Late Oligocene through Miocene-Pliocene drift accumulation throughout the North Atlantic. The most recent phase of drift deposition, since Late Pliocene time (~3Ma), occurred after a shift to the ‘modern’ circulation system of deeper, swifter currents, and it includes mixed pelagic-hemipelagic sediments and ice-rafted debris that reflect glacial-interglacial influences on sedimentation.

The formation and inversion of the western Greater Caucasus Basin and the uplift of the western Greater Caucasus: Implications for the wider Black Sea region

The western Greater Caucasus formed by the tectonic inversion of the western strand of the Greater Caucasus Basin, a Mesozoic rift that opened at the southern margin of Laurasia. Subsidence analysis indicates that the main phase of rifting occurred during the Aalenian to Bajocian synchronous with that in the eastern Alborz and, possibly, the South Caspian Basin. Secondary episodes of subsidence during the late Tithonian to Berriasian and Hauterivian to early Aptian are tentatively linked to initial rifting within the western, and possibly eastern, Black Sea and during the late Campanian to Danian to the opening of the eastern Black Sea. Initial uplift, subaerial exposure, and sediment derivation from the western Greater Caucasus occurred at the Eocene-Oligocene transition. Oligocene and younger sediments on the southern margin of the former basin were derived from the inverting basin and uplifted parts of its northern margin, indicating that the western Greater Caucasus Basin had closed by this time. A predominance of pollen representing a montane forest environment (dominated by Pinacean pollen) within these sediments suggests that the uplifting Caucasian hinterland had a paleoaltitude of around 2 km from early Oligocene time. The closure of the western Greater Caucasus Basin and significant uplift of the range at approximately 34 Ma is earlier than stated in many studies and needs to be incorporated into geodynamic models for the Arabia-Eurasia region.

Cenozoic tectonic model of the Bohai Bay Basin in China

AbstractModelling results and seismic interpretation illustrate that the Cenozoic evolution of the Bohai Bay Basin (BBB) can be divided into different stages. A transtensional phase during Paleocene – early Oligocene time created NE-trending strike-slip faults and E–W-trending normal faults which were driven roughly by N–S–extension, making an angle of 25° with the strike-slip faults. Seismic data interpretation yields evidence that inversion phases occurred within the NE Xialiaohe Depression of the greater Bohai Bay Basin. This inversion phase is attributed to rotation and partial inversion that occurred during late Oligocene time, leading to formation of inversion structures along the NE part of Tanlu Fault. This episode is attributed to an anticlockwise rotation of the eastern part of the BBB driven by the convergence between the Pacific and Eurasian plates. The tectonic scenario described was simulated in scaled analogue models, which were extended by pulling two basement plates away from each other. Partial inversion was simulated by rotation of one of the plates relative to the other. Model results show many of the features observed in the BBB. Our model results are used to argue that, unlike the two-episode extension and whole-basin inversion models previously proposed for the BBB, a single N–S-aligned extension followed by anticlockwise rotation accounts for the Cenozoic evolution of the BBB and produces some of the structural complexities observed in the basin.

Transition from the Cretaceous ocean to Cenozoic circulation in the western South Atlantic — A twofold reconstruction

The Cretaceous oceanic circulation has been quite different from the modern with a different distribution of the continents on the globe. This has resulted in a much lower temperature gradient between poles and equator. We have studied seismic reflection data and used numerical simulations of atmosphere and ocean dynamics to identify important steps in modifications of the oceanic circulation in the South Atlantic from the Cretaceous to the Cenozoic and the major factors controlling them. Starting in the Albian we could not identify any traces of an overturning circulation for the South Atlantic although a weak proto-Antarctic Circumpolar Current (ACC) was simulated. No change in circulation was observed for the Paleocene/early Eocene South Atlantic, which indicated that this period has witnessed a circulation similar to the Albian and Cenomanian/Turonian circulation. The most drastic modifications were observed for the Eocene/Oligocene boundary and the Oligocene/early Miocene with the onset of an ACC and Atlantic meridional overturning circulation (AMOC) and hence southern sourced deep and bottom water masses in the western South Atlantic. A modern AMOC, which intensified in strength after closure of the Central American Seaway (CAS), and a strong ACC have resulted in current controlled sedimentary features and wide spread hiatusses in the South Atlantic since the middle Miocene. The opening of Drake Passage in early Oligocene times and the closure of the CAS at ~6Ma, i.e., tectonic processes, have been identified as the key triggers for the observed most severe changes in oceanic circulation in the South Atlantic.

Sedimentary record of regional deformation and dynamics of the thick-skinned southern Puna Plateau, central Andes (26–27°S)

The Puna Plateau, adjacent Eastern Cordillera and the Sierras Pampeanas of the central Andes are largely characterized by thick-skinned, basement-involved deformation. The Puna Plateau hosts ∼N–S trending bedrock ranges bounded by deep-seated reverse faults and sedimentary basins. We contribute to the understanding of thick-skinned dynamics in the Puna Plateau by constraining regional kinematics of the poorly understood southern Puna Plateau through a multidisciplinary approach. On the southeastern plateau, sandstone modal composition and detrital zircon U–Pb and apatite fission-track data from Cenozoic strata indicate basin accumulation during the late Eocene to early Oligocene (∼38–28 Ma). Provenance analysis reveals the existence of a regional-scale basin covering the southern Puna Plateau during late Eocene to early Oligocene time (∼38–28 Ma) that was sourced from both the western plateau and the eastern plateau margin and had a depocenter located to the west. Petrographic and detrital zircon U–Pb data reveal erosion of proximal western and eastern sources after ∼12 Ma, in mid-late Miocene time. This indicates that the regional basin was compartmentalized into small-scale depocenters by the growth of basement-cored ranges continuing into the late Miocene (∼12–8 Ma). We suggest that the Cenozoic history of the southern Puna Plateau records the formation of a regional basin that was possibly driven by lithospheric flexure during the late Eocene to early Oligocene, before the growth of distributed basement-cored ranges starting as early as the late Oligocene.

Thermal maturity of Carboniferous to Eocene Sediments of the Alpine–Dinaric Transition Zone (Slovenia)

The Alpine–Dinaric Transition Zone in Slovenia comprises the fold and thrust belt of the Southern Alps (South Karawanken Range, Julian Alps), Slovenian Basin and the Dinarides. The Slovenian Basin located between the Julian and Dinaric carbonate platforms evolved during Middle Triassic time and remained in a deep marine setting till the Late Cretaceous. The thermal history of Carboniferous to Eocene rocks in that area was investigated using vitrinite reflectance (VR) data, apatite fission track analysis and numeric 1D basin modeling. The study shows that maturity patterns are mainly controlled by the thickness of Upper Cretaceous to Eocene flysch deposits, filling the accommodation space. Therefore the thermal overprint reaches a maximum (>4%Rr) in Triassic to Cretaceous sediments of the Slovenian Basin and decreases towards the north and south. Minor sedimentary burial of the Adriatic Carbonate Platform and the Julian Alps results in a lower thermal overprint (<1.5%Rr). The thickness of flysch sediments was about 5km in the area of the Sava Folds, but significantly higher in the central part of the Slovenian Basin. Heat flow during maximum burial in Eocene time was in the order of the global average (60mW/m2). Cooling of Paleozoic and Mesozoic sediments below 110°C occurred between Late Eocene and Early Oligocene times in different parts of the study area. Nappe stacking due to Early (Dinaric) and Late Cenozoic (Alpine) compressional tectonics did not influence the thermal maturity of the sediments.