Middle Eocene To Early Oligocene Research Articles

Paleostress analysis of Cenozoic faulting in the Kraishte area, SW Bulgaria

Paleostress calculations from fault planes in Paleogene sediments and in the underlying basement were used to determine the orientation and the chronology of the principal stresses during the Cenozoic tectonic evolution of the Kraishte area (southwest Bulgaria). The results show that middle Eocene–early Oligocene WSW–ENE extension led to the formation of grabens and half grabens filled with thick continental to marine deposits in the hanging wall of extensional detachments.The Paleogene sediments and their basement were then intruded by subvolcanic bodies and dykes during SW–NE extension between 32 and 29 Ma. These WSW–ENE and SW–NE extensional stages are related to the general rollback of the Hellenic slab.From the late Oligocene to the earliest Miocene, SSE–NNW transtension generated coal-bearing sedimentary basins. The anticlockwise rotation of the main tensile axis by almost 50° with respect to the previous tectonic stage was probably related to a switch from the Aegean back-arc extension to crustal stretching and extrusion of continental fragments around the Moesian platform.Since the middle Miocene extension in the Kraishte area, accommodated by faults with relatively small displacement, led to the formation of restricted basins filled with alluvial to lacustrine deposits.

Cenozoic record of elongate, cylindrical, deep-sea benthic foraminifera in the North Atlantic and equatorial Pacific Oceans

In the late Pliocene–middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal–abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20–70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction? In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene–early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene–Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle–late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene–early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene–middle Pleistocene. Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.

EVOLUTION AND PALEOGEOGRAPHIC DISTRIBUTION OF THE LEPIDOCYCLINIDS

The larger benthic foraminiferal family Lepidocyclinidae has been described extensively in the literature, with occurrences ranging from the middle Eocene in the Americas to the late Miocene (or possibly early Pliocene) in the Indo-Pacific Tethyan sub-province. Lepidocyclinids originated in the Americas but related forms are reported from West Africa from the middle Eocene-early Oligocene pointing to an eastward migration. This migration continued within the Tethys until the lepidocyclinids finally colonized the Indo-Pacific in the late early Oligocene. Various lepidocyclinid species have been used to zone and date the Neogene (particularly in the Indo-Pacific), and many, sometimes confusing, generic and suligeneric nomenclatures have been proposed to differentiate the often subtle morphological changes that have been observed to occur with time. Until now, it has not been possible to develop an effective global view of the evolution and migration of the lepidocyclinids because descriptions of specimens from the American province have been relatively rare. Here we present descriptions of new samples from 37 wells drilled off South America, and present a systematic comparison of the Lepidocyclinidae from the South America and the Tethyan sub-provinces of the Mediterranean-West Africa and the Indo-Pacific. Among the 87 specimens illustrated from the two provinces, 45 are recorded in America, including three new taxa, Lepidocyclina (Nephrolepidina) braziliana, Lepidocyclina (Nephrolepidina) americana and Eulepidina parkinsonia, and one new unnamed species of Eulepidina. By comparing the stratigraphic ranges of the South American specimens with previously described Tethyan forms, the phylogenetic evolution of the lepidocyclinids is reconstructed from their mid-Eocene origin in the Americas to their late Miocene (or possibly early Pliocene) extinction in the Indo-Pacific. Systematic comparisons enabled us to trace the timing and route of the paleogeographic migration of the lepidocyclinids from the Americas, through the Tethyan/Mediterranean corridor, and into the Indo-Pacific region. Our observations confirm that the series of global regressions in the middle Eocene and early Oligocene facilitated transoceanic migration of larger benthic foraminifera between the American and the Tethyan provinces. However, this migration stopped after rising sea-level in the early Oligocene separated the provinces. Subsequently, larger benthic foraminifera exhibited independent but similar lines of evolution. The resulting examples of parallel evolution led to some taxonomic confusion in the literature that we seek to resolve.

Allochthonous blocks misidentified as the basement: Implication for petroleum exploration in the SW Thrace Basin (Turkey)

The sedimentary sequence of the SW Thrace Basin (NW Turkey) comprises of three major sedimentary packages: a) Cretaceous (Meastrichtien)–Early Eocene package formed as pelagic carbonate, turbidite, shelf and nearshore-fluvial deposits, b) Middle Eocene–Early Oligocene package formed as shallow marine, slope apron and turbidite sediments, c) Middle Miocene–Early Pliocene package formed as alluvial fan-fluvial and nearshore deposits. This basin is structurally complex and its southern margin is represented by a fold-thrust zone. The Westward Propagation of the North Anatolian Fault (WPNAF) intersects the Thrace Basin along its southwestern flank. In the SW Thrace Basin, there are ophiolite and limestone blocks in the sedimentary succession, which previously were interpreted as suture of the Intra-Pontide Ocean, Cretaceous aged Yeniköy Mélange or Olistostromal Unit in the Eocene sediments. In this study these rocks have been defined as allochthonous blocks facies of the Gaziköy Formation aged Middle–Late Eocene. In previous studies the Karaağaç Formation (Early Eocene) was considered to be within oil window and have an average potential for oil and gas generation. According to our data the Karaağaç Formation has a moderate TOC content, is mature and overmature. In the study area, there are potential stratigraphic traps (submarine fans and channels of the Karaağaç and Keşan Formations, fluvial channels of the Fıçıtepe Formation and reefs of the Soğucak Formation), potential fractured tuffs (the Gaziköy Formation) and, potential structural traps related to the folds, thrusts and the WPNAF. Although 17 exploration wells have been drilled in the study area and vicinity, no discovery has yet been made. The cause of this may be that the wells were terminated at depths shallower than potential targets or possibly drilling may have been terminated at the allochthonous block facies of the Gaziköy Formation, which is especially derived from ophiolite mélange and was misidentified as basement.

Shallow-marine ostracode faunas around the Eocene/Oligocene boundary in the northwestern Kyushu, southwestern Japan

Middle Eocene–early Oligocene ostracode faunal changes in northwestern Kyushu of southwestern Japan are identified in this study. Ostracodes occur from shelf deposits of five formations: the middle Eocene Okinoshima, the upper Eocene Funazu, the uppermost Eocene–lowermost Oligocene Kishima, the lowermost Oligocene Itanoura, and the lower Oligocene Waita Formations. The middle Eocene–earliest Oligocene ostracodes were characterized by warm-water genera, indicating tropical, subtropical and Tethyan genera, whereas the late early Oligocene ostracodes do not include warm-water taxa, consisting of temperate realm genera. The middle Eocene–earliest Oligocene ostracodes do not include remarkable changes of species composition, in contract with equatorial Pacific radiolarians and West Coast USA molluscs. Ostracodes suggest that distinct climatic cooling did not occur in the southwestern Japan during the middle Eocene–earliest Oligocene.

Patagonian Vegetation Turnovers during the Paleogene-Early Neogene: Origin of Arid-Adapted Floras

The structure of the living Patagonian flora, dominated by the steppe, is a direct consequence of past climatic and tectonic events. These arid-adapted communities were widespread during the Late Neogene, but their origin in Patagonia can be traced back to the Paleogene. Vegetational trends throughout Paleocene-Miocene time are based on available paleobotanical and palynological information. Four major supported stages in vegetation turnovers are recognized: (1) Paleocene and Early Eocene floras were rainforest-dominated, including many angiosperms with warm-temperate affinities (e.g., palms, Juglandaceae, Casuarinaceae). However, mainly in the Early Eocene, some geographic areas influenced by warm but drier conditions are suggested by the occurrence of certain taxa (e.g., Anacardiaceae). These areas containing arid-adapted floras would have arisen in Patagonian inland regions, in a generally wet continent. (2) The Middle Eocene-Early Oligocene interval was distinguished by the invasion ofNothofagus forests. Progressive replacements of megathermal communities by meso- and microthermal rainforest are documented.Nothofagus forest expansion suggests a marked cooling trend at this time, although some megathermal elements (AquifoliaceaeIlex, Tiliaceae-Bombacaceae, Sapindaceae) were still present at the beginning of this period. Arid-loving taxa have not been recorded in abundance. (3) Late Oligocene-Early Miocene floras were characterized by the occurrence of shrubby-herbaceous elements belonging to Asteraceae, Chenopodiaceae, Ephedraceae, Convolvulaceae, Fabaceae, and Poaceae. They began to give a modern appearance to plant communities. Xerophytic formations would have occupied coastal salt marshes and pockets in inland areas. Megathermal angiosperms of the Rubiaceae, Combretaceae, Sapindaceae, Chloranthaceae, and Arecaceae occurred mainly during the Late Oligocene. Forests of Nothofagaceae, Podocarpaceae, and Araucariaceae are still documented in extra-Andean Patagonia; however, a contrast between coastal and inland environments may have developed, particularly in the Miocene. (4) Middle-Late Miocene records show an increasing diversity and abundance of xerophytic-adapted taxa, including Asteraceae, Chenopodiaceae, and ConvolvulaceaeCressa/Wilsonia. Expansion of these xerophytic taxa, coupled with extinctions of megathermal/nonseasonal elements, would have been associated with both tectonic and climatic forcing factors, led to the development of aridity and extreme seasonality. These arid-adapted Late Miocene floras are closely related to modern communities, with steppe widespread across extra-Andean Patagonia and forest restricted to the western humid upland regions.

Basin Evolution in Southern East Greenland: An Outcrop Analog for Cretaceous-Paleogene Basins on the North Atlantic Volcanic Margins

The Cretaceous-Paleogene Kangerlussuaq basin in southern East Greenland represents a unique outcrop analog for the frontier petroleum provinces along the deep-water volcanic margins of the northern North Atlantic. The 1-km-thick sedimentary succession comprises six facies associations: (1) alluvial plain and shallow marine (late Aptian?), (2) fluvio-estuarine (late Aptian-early Albian), (3) offshore marine (Late Cretaceous-early Paleocene), (4) submarine fan and channel-levee (early Paleocene), (5) fluvial (mid-Paleocene), and (6) volcanic (late Paleocene). Sedimentation was terminated in the late Paleocene by extrusion of flood basalts related to continental breakup. The basin fill is divided into two depositional megasequences related to regional tectonic events and sea level changes. The oldest megasequence (SQ1) spans the late Aptian to the earliest Paleocene with sea level rise in the late Aptian and maximum flooding in the late Albian-Cenomanian, followed by sea level highstand in the Late Cretaceous-early Paleocene. SQ1 is truncated by a basin-wide unconformity related to regional uplift and basin reorganization in the mid-Paleocene. The upper megasequence (SQ2) spans the mid- to late Paleocene and comprises sediments deposited during early sea level rise. Extensive volcanic deposits and continental flood basalts overlie it. Deep burial (> 6 km) and middle Eocene-early Oligocene(?) uplift excludes the Kangerlussuaq basin as a petroleum basin in itself; however, to evaluate the petroleum potential of similar volcanic influenced offshore basins, such as the West Shetland, Faeroe, More, and Voring basins, we discuss three conceptual play models based on the Kangerlussuaq basin: (1) early Albian stratigraphic trap, (2) early Paleocene structural trap, and (3) Paleocene stratigraphic trap.

Preliminary palynological data on floristic and climatic changes during the Middle Eocene-Early Oligocene of the eastern Ebro Basin, northeast Spain

Variations in pollen spectra observed in sediments from the middle Bartonian-Lower Oligocene of Igualada, Ebro Basin (northeast Spain), have permitted the identification of general trends in floristic and climate change. Detailed investigations of the biological affinities of certain fossil pollen have revealed a number of plant genera previously unidentified in the fossil record. The floral diversity and ecological characteristics of the pollen taxa indicates that the middle Bartonian vegetation of the eastern Ebro Basin was characterised by a complex mangrove swamp reflecting warm climatic conditions. During the Priabonian, the greater variety of herbaceous plants suggests the existence of a more open vegetation correlated with the change to a climate with a dry season. During the Priabonian the diversification of Caesalpiniaceae indicates African influence. During the Late Eocene taxa interpreted as indicators of a dry climate are identified, while those indicating humid climatic conditions disappear. Dry climatic conditions characterise the Lower Oligocene, as indicated by the abundance of cupressacean pollen interpreted as evidence of a sclerophyllous forest. A slight climatic cooling is also revealed by the presence of some microthermal taxa.

Paleogene retroarc flexural basin beneath the Neogene Pannonian Basin: A geodynamic model

The Neogene (Middle Miocene-Recent) Pannonian Basin is superimposed on an earlier Paleogene (Middle Eocene-Early Miocene) basin complex as a result of back-arc extension. Although the stratigraphy of the latter is very well known due to raw material exploration, its formation and structural evolution is much less understood. A transtensional origin was generally proposed for the Paleogene Basin of Hungary, assuming an origin analogous to that of the overlying Neogene Pannonian Basin. We suggest an alternative geodynamic scenario for the development of the Paleogene basin complex, as we think that the observations are better understood in terms of a retroarc foredeep basin model. According to this, a flexural basin developed south of the backthrust inner West Carpathian units. The thrust load resulted in a generally deep, underfilled “flysch” basin during Middle Eocene-Early Oligocene times. In the Late Oligocene-Early Miocene predominantly shallow marine and continental deposition occurred indicating the gradual cessation of thrusting in the adjacent thrust-fold belt and the beginning of “molasse” sedimentation. Throughout the whole Paleogene, a shift can be observed of the bathyal, dominantly siliciclastic (in the axis of the foredeep) and neritic, partly carbonate (in the foreland flank of the foredeep) facies belts. This migration of the Middle Eocene-Early Miocene depocenters of the HPB (Hungarian Paleogene Basin) to the east-northeast in present-day coordinates is the result of two processes: 1. (1) the advancement of backthrusting to the southeast; 2. (2) along-strike shift of thrusting to the northeast. While a thin-skinned thrust front can be reconstructed for the Late Eocene (Buda Anticline), the Diósjenö-Hurbanovo line is interpreted as the result of a major thick-skinned thrusting event at the base of the NP24 zone ( $ ̃ 31 Ma). The subsidence pattern, the distinct depositional environments and the structural observations can be more easily integrated into a flexural basin model than into a wrench basin model. Based on the flexural basin model of this work the early Tertiary Western Carpathians formed a compressional arc. While this arc probably developed above a B-subduction zone, during the Late Eocene with a corresponding trench (Magura), a forearc basin (Central Carpathian or Podhale) and a retroarc flexural basin (Hungarian Paleogene), from the Early Oligocene to the Early Miocene it was characterized by A-subduction with a corresponding peripheral foredeep (Dukla and Silesian) and a retroarc flexural basin (Hungarian Paleogene).

Metamorphic constraints on the thermal evolution of the central Himalayan Orogen

Recent studies that integrate conventional thermobarometry of pelitic mineral assemblages with thermodynamic modeling of garnet zoning reveal complex Tertiary P -T paths for the Greater Himalayan metamorphic sequence in the central Himalaya. Viewed in light of our current understanding of the structural evolution of the Himalaya, these data provide insights into the relations between tectonic and thermal processes during orogenesis. In this paper, we present an interpretive model for tectonothermal evolution of the Greater Himalaya in the central part of the range. This model involves: (1) middle Eocene—early Oligocene burial to depths of more than 30 km during the early stages of collision between India and Asia; (2) early—late Oligocene uplift and cooling; (3) late Oligocene heating and renewed burial synchronous with the early stages of anatectic melting and leucogranite plutonism; (4) latest Oligocene-middle Miocene rapid uplift and continued leucogranite production associated with ramping on the structurally lower Main Central Thrust and tectonic denudation on structurally higher low-angle detachment systems; and (5) middle Miocene—Recent rapid cooling during the final stages of uplift to the surface.

Eocene-Oligocene Benthonic Foraminifera: Implications for Deep-Water Circulation History: ABSTRACT

Quantitative analysis of middle Eocene-early Oligocene bathyal deep-sea benthonic foraminifera was carried out on samples from DSDP Sites 77, 292 (Pacific Ocean), 219 and 253 (Indian Ocean), 363 (Atlantic Ocean), and Eureka 67-128 (Gulf of Mexico) and compared with benthonic foraminiferal stable isotopic data to determine the effects of deep-water circulation changes on the faunas, Faunal changes (first and last occurrences) are found throughout the sequences, and a catastrophic turnover of the benthonic foraminiferal fauna at the Eocene-Oligocene boundary does not occur. A few distinct events do occur associated with inferred coolings at the middle/late Eocene and Eocene/Oligocene boundary. For example, Nuttallides truempyi, an important middle Eocene species, has an iso hronous last occurrence within the Globigerinatheka semiinvoluta zone in Sites 219, 253, 292, and 363 and coincides with a 3° deep-water cooling inferred from the O18 record. During the late Eocene and early Oligocene these bathyal sites are marked by a remarkably uniform assemblage dominated by Oridorsalis tener, Globocassidulina subglobosa, and Cibicidoides ungerianus. In Sites 292 and E67-128 additional species that are important are Bulimina alazanensis, Buliminella grata, and Bulimina tuxpamensis. This relatively uniform bathyal faunal assemblage in these Atlantic, Indian, and Pacific sites is similar to an assemblage found previously in North and South Atlantic bathyal sites. This faunal pattern, as well as the isochronous last appearance of N. truempyi, suggests that a relatively uniform and widespread bathyal water mass extended throughout the world ocean during middle Eocene-early Oligocene time. The faunal data show three responses to the sharp deep-water coolings at the middle/late Eocene and late Eocene/early Oligocene boundary: (1) a dominant species may have a last occurrence as a direct result of the cooling, (b) an increase in species abundance precedes the cooling followed by a sharp decrease associated with a decrease in deep-water temperatures, or (c) a species is largely unaffected by the temperature change. End_Page 443------------------------------ A comparison of the present data and previous work shows that the greatest occurrence of extinctions is found at the Paleocene/Eocene boundary with a lower number o extinctions at the middle/late Eocene and Eocene/Oligocene boundaries. The lack of a major catastrophic change at the Eocene/Oligocene boundary may be a result of previous faunal events at the Paleocene/Eocene and middle/late Eocene boundary eliminating stenothermal species, leaving environmentally tolerant species in the late Eocene that were largely unaffected by the 3°C (37°F) temperature decrease at the Eocene/Oligocene boundary. End_of_Article - Last_Page 444------------

Deep-sea benthonic foraminiferal faunal turnover near the Eocene/Oligocene boundary

Eocene-Oligocene deep-sea benthonic foraminifera in D.S.D.P. Site 277 in the southwest Pacific have been analyzed to determine the benthonic foraminiferal response to the development of the psychrosphere near the Eocene/Oligocene boundary. Biostratigraphic ranges of 41 taxa show that 23 taxa are found throughout the Late Eocene to Early Oligocene sequence, while 18 taxa exhibit first or last occurrences. Comparison of the faunal changes in Site 277 with a benthonic foraminiferal oxygen isotope record shows that the development of the psychrosphere did not have a profound effect upon the benthonic foraminifera, and the overall faunal change preceding and subsequent to the bottom-water circulation event occurred gradually. The inferred water-mass event affected the relative abundance of one species, Epistominella umbonifera. The lack of major faunal changes at the Eocene/Oligocene boundary in Site 277 probably reflects either wide environmental tolerances of the benthonic foraminifera, or a bottom-water temperature change less than 3°C.Examination of previously published benthonic foraminiferal biostratigraphic data from D.S.D.P. Sites 167, 171, 357, 360, 363, and 400A, and deep-sea ostracode data from D.S.D.P. Leg 3 show faunal changes occurred during discrete intervals in the Middle Eocene-Early Oligocene. The faunal patterns from these data and from Site 277 show that the Eocene/Oligocene cooling event did not cause rapid, catastrophic changes of the benthonic faunas of the open ocean, although significant faunal changes are associated with the water mass event in Sites 167, 171 and 400A.The benthonic faunal changes in Middle Eocene-Early Oligocene time are consistent with the gradual decrease of inferred bottom-water temperatures, based on previously published oxygen isotopic data. The δ 18O Eocene/Oligocene enrichment of 0.76‰ is a major event in the Southern Ocean oxygen isotopic record, but is considerably less in magnitude than the 1.75-2.00‰ change that occurred gradually from mid-Early Eocene to the Eocene/Oligocene boundary. The benthonic foraminiferal and isotopic data indicate that bottom-water circulation may have developed during the Middle Eocene to Early Oligocene interval, with the 3°C bottom-water cooling near the Eocene/Oligocene boundary representing part of this development.