Early Cenozoic Research Articles

The French ambers: a general conspectus and the Lowermost Eocene amber deposit of Le Quesnoy in the Paris Basin

Aconspectus of the current knowledge on the French ambers is proposed. France is a very rich country with more than seventy localities known after the 'old' literature, especially from the Jurassic, Cretaceous and Paleogene. A more detailed new insectiferous amber locality is given for the amber from Le Quesnoy (Lowermost Eocene, Oise department, Paris Basin, France). After the preliminary survey of the flora and the vertebrate and arthropod faunas, we propose a reconstruction of a fluvio-lacustrine palaeoenvironment with a forest, under a warm and wet seasonal climate. This site is outstanding because of the richness, diversity and the state of preservation of the fossils. The present discovery opens a remarkable window on the terrestrial life during the earliest Eocene. It shows that future researches on the 'old' French amber localities shall be crucial for our knowledge on the evolution and diversity of the insects during the Mesozoic and Early Cenozoic.

The Grosmont: the world's largest unconventional oil reservoir hosted in carbonate rocks

Abstract The Upper Devonian Grosmont platform in Alberta, Canada, is the world's largest heavy oil reservoir hosted in carbonates, with 400–500 bbbl of initial oil in place at an average depth of about 250–400 m. Past pilot activity between 1975 and 1987, mainly with ‘huff and puff’ technology, had variable results and the reservoir was deemed uneconomical at the time. More advanced thermal recovery technologies, such as steam assisted gravity drive and electrical in-situ retorting, much higher world market prices for oil and certain political pressures have led to a flurry of activity in the Grosmont since 2006. To be successful, all these schemes require a solid geological, geophysical and petrophysical reservoir evaluation. Our study, which aims to aid in thermal recovery of this reservoir, shows that the current reservoir characteristics were created by a succession of five major factors and/or processes: sedimentary stratigraphy, dolomitization, polyphase and polygenetic fracturing, polyphase and polygenetic karstification, and biodegradation. Most of the present porosity and permeability is due to fracturing and karstification. The sedimentary stratigraphy of the Grosmont reservoir consists of six stacked carbonate units interbedded with marls and some evaporites. The latter two originally acted as aquitards during diagenesis but are breached or missing in parts of the area today. Dolomitization by density-driven reflux was the first pervasive diagenetic process. Most dolostones are fine-crystalline and tight, however, and the only notable porosity caused by and/or related to dolomitization is scattered molds and vugs. A dense fracture network was created in three or four phases. Most fractures probably originated from collapse following subsurface salt dissolution and/or from Laramide tectonics far to the west, whereby pulsed crustal loading in the fold-and-thrust belt created a dynamic forebulge in the Grosmont region via multiple pulses of basin-wide crustal flexing, each followed by relaxation. The fracture network probably was reactivated and/or expanded by glacial loading and post-glacial isostatic rebound in the Pleistocene and Holocene, respectively. The region experienced three or four prolonged periods of epigene (top-down) karstification, although there is tangible evidence for only two of them in the Grosmont platform. The first of these episodes was a ‘warm epigene karstification’ during the Jurassic–Cretaceous, and the second was/is a ‘cold epigene karstification’ that started sometime in the Cenozoic and is continuing to this day. In addition, there is circumstantial evidence for hypogene (bottom-up) ‘karstification’ (=dissolution) throughout much of the geological history of the Grosmont since the Late Devonian, with two possible maxima around the time of hydrocarbon emplacement, that is Early–Middle Cretaceous and Early Cenozoic, respectively. Epigene karstification was accompanied and/or followed by extensive biodegradation. Present bitumen in-situ viscosities are >1 million cP and API gravities range from 5 to 9°. Taken together, these conditions pose considerable challenges for any type of thermal recovery scheme.

Pelagic crinoids (Roveacrinida, Crinoidea) discovered in the Neogene of Poland

Until recently, it has been assumed that pelagic crinoids, the roveacrinids (Roveacrinida, Crinoidea), became extinct during the Cretaceous-Paleogene boundary event. Recent finds of well-preserved roveacrinidal remains (brachials and radials) in the Danian (Early Paleogene) of Poland showed that they survived into the earliest Cenozoic. This group was thus characterized as a "dead clade walking". Here, we present fossil evidence that these pelagic crinoids survived in Poland until at least the Middle Miocene (Badenian, ca. 14Myr ago)-more than 50Myr after their supposed extinction. These Miocene roveacrinids constitute the first documented evidence of Roveacrinida in strata of Neogene age, thus prolonging the stratigraphic range of pelagic crinoids. This find characterizes the order as a "Lazarus taxon" rather than a "dead clade walking" group.

Mesozoic-Cenozoic tectonothermal evolution of the eastern part of the Tibetan Plateau (Songpan-Garzê, Longmen Shan area): insights from thermochronological data and simple thermal modelling

Abstract We present a synthesis of the tectonic and thermochronological evolution of the Eastern Tibet since the Triassic. The long-term cooling histories obtained on magmatic and metamorphic rocks of the South Songpan-Garzê, Kunlun and Yidun blocks are similar showing a very slow and regular cooling during Late Jurassic and Cretaceous, confirming the suspected lack of major tectonic events between c . 150 and 30 Ma. The exhumation linked to the Tertiary growth of the Tibetan Plateau initiated around 30 Ma and concentrates at the vicinity of the major tectonic structures. Exhumation rates increased again from about 7 Ma in the Longmen Shan. To interpret this very slow cooling rate between Late Jurassic and Early Cenozoic from granites of this area, we use a simple 1D thermal model that takes into account the thermal properties of both sediments and crust. The results suggest that: (1) high temperature (500 °C) can be kept over a long period of time; (2) during Cretaceous, cooling is mostly controlled by the thermal properties of sediments of continental origin; and (3) the initial Late Triassic rapid cooling rate was caused by the large thermal contrast between the granite body and the sedimentary rocks rather than by a high exhumation rate.

Geology of the Caucasus: A Review

The structure and geological history of the Caucasus are largely determined by its position between the still-converging Eurasian and Africa-Arabian lithospheric plates, within a wide zone of continental collision. During the Late Proterozoic-Early Cenozoic, the region belonged to the Tethys Ocean and its Eurasian and Africa-Arabian margins where there existed a system of island arcs, intra-arc rifts, back-arc basins characteristic of the pre-collisional stage of its evolution of the region. The region, along with other fragments that are now exposed in the Upper Precambrian-Cambrian crystalline basement of the Alpine orogenic belt, was separated from western Gondwana during the Early Palaeozoic as a result of back-arc rifting above a south-dipping subduction zone. Continued rifting and seafloor spreading produced the Palaeotethys Ocean in the wake of northward migrating peri-Gondwanan terranes. The displacement of the Caucasian and other peri-Gondwanan terranes to the southern margin of Eurasia was completed by ~350 Ma. Widespread emplacement of microcline granite plutons along the active continental margin of southern Eurasia during 330-280 Ma occurred above a north-dipping Palaeotethyan subduction zone. However, Variscan and Eo-Cimmerian-Early Alpine events did not lead to the complete closing of the Palaeozoic Ocean. The Mesozoic Tethys in the Caucasus was inherited from the Palaeotethys. In the Mesozoic and Early Cenozoic, the Great Caucasus and Transcaucasus represented the Northtethyan realm - the southern active margin of the Eurasiatic lithospheric plate. The Oligocene-Neogene and Quaternary basins situated within the Transcaucasian intermontane depression mark the syn- and post-collisional evolution of the region; these basins represented a part of Paratethys and accumulated sediments of closed and semiclosed type. The final collision of the Africa-Arabian and Eurasian plates and formation of the present-day intracontinental mountainous edifice of the Caucasus occurred in the Neogene-Quaternary period. From the Late Miocene (c. 9-7 Ma) to the end of the Pleistocene, in the central part of the region, volcanic eruptions in subaerial conditions occurred simultaneously with the formation of molasse troughs. The geometry of tectonic deformations in the Transcaucasus is largely determined by the wedge-shaped rigid Arabian block intensively indenting into the Asia Minor-Caucasian region. All structural-morphological lines have a clearly-expressed arcuate northward-convex configuration reflecting the contours of the Arabian block. However, farther north, the geometry of the fold-thrust belts is somewhat different - the Achara-Trialeti fold-thrust belt is, on the whole, W-E-trending; the Greater Caucasian fold-thrust belt extends in a WNW-ESE direction.

Roveacrinids (Crinoidea, Echinodermata) survived the Cretaceous-Paleogene (K-Pg) extinction event

Research Article| October 01, 2010 Roveacrinids (Crinoidea, Echinodermata) survived the Cretaceous-Paleogene (K-Pg) extinction event Mariusz A. Salamon; Mariusz A. Salamon * 1Faculty of Earth Sciences, University of Silesia, PL-41-200, Sosnowiec, Poland *E-mail: paleo.crinoids@poczta.fm. Search for other works by this author on: GSW Google Scholar Przemysław Gorzelak; Przemysław Gorzelak 2Institute of Paleobiology, Polish Academy of Sciences, PL-00-818, Warsaw, Poland Search for other works by this author on: GSW Google Scholar Bruno Ferré; Bruno Ferré 32 rue Guy de Maupassant, F-76800 Saint Étienne du Rouvray, France Search for other works by this author on: GSW Google Scholar Rafał Lach Rafał Lach 1Faculty of Earth Sciences, University of Silesia, PL-41-200, Sosnowiec, Poland Search for other works by this author on: GSW Google Scholar Author and Article Information Mariusz A. Salamon * 1Faculty of Earth Sciences, University of Silesia, PL-41-200, Sosnowiec, Poland Przemysław Gorzelak 2Institute of Paleobiology, Polish Academy of Sciences, PL-00-818, Warsaw, Poland Bruno Ferré 32 rue Guy de Maupassant, F-76800 Saint Étienne du Rouvray, France Rafał Lach 1Faculty of Earth Sciences, University of Silesia, PL-41-200, Sosnowiec, Poland *E-mail: paleo.crinoids@poczta.fm. Publisher: Geological Society of America Received: 02 Mar 2010 Revision Received: 04 May 2010 Accepted: 06 May 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (10): 883–885. https://doi.org/10.1130/G31175.1 Article history Received: 02 Mar 2010 Revision Received: 04 May 2010 Accepted: 06 May 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mariusz A. Salamon, Przemysław Gorzelak, Bruno Ferré, Rafał Lach; Roveacrinids (Crinoidea, Echinodermata) survived the Cretaceous-Paleogene (K-Pg) extinction event. Geology 2010;; 38 (10): 883–885. doi: https://doi.org/10.1130/G31175.1 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 SocietyGeology Search Advanced Search Abstract Although crinoids appear not to have been involved in the great change in diversity at the Cretaceous-Paleogene (K-Pg) boundary extinction event, it has been assumed that representatives of order Roveacrinida became extinct during this time. Well-preserved fossils from the Danian (early Paleocene) of Poland demonstrate that these crinoids survived into the earliest Cenozoic. This find merits the qualification of this order as a “dead clade walking.” You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The impact of lithification on the diversity, size distribution, and recovery dynamics of marine invertebrate assemblages

Lithified marine sediments are not equitably distributed through time, raising the possibility that lithification masks biological signals when data from unlithified and lithified sediments are compared or combined. Using mollusk-dominated assemblages from the early Cenozoic of the Gulf Coastal Plain, we find that lithification conceals small taxa, decreases taxonomic resolution, and exacerbates the undersampling of rare taxa. Lithified assemblages appear less diverse and have less even abundance distributions than coeval unlithified samples. These limitations cannot be overcome by standardization procedures, nor are they likely to be circumvented by collecting larger samples. The effects of this bias, however, can be mitigated by restricting analyses to a single lithification state or to specific size classes. Since lithification selectively obscures small taxa, the magnitude of this bias will be most severe when organisms are particularly small, such as in the aftermath of mass extinctions. In the study area, lithification artificially protracts the recovery period following the Cretaceous-Paleogene mass extinction by ~7 m.y.

Evolution of Middle America and the in situ Caribbean Plate model

Abstract Regional geological data and global analogues suggest Caribbean Plate geology continues that seen along the margin of eastern North America in a more extensional setting, between the diverging Americas. From west to east there are continental masses with Triassic rifts, proximal continental blocks with kilometres-thick Mesozoic carbonates, more distal areas of Palaeozoic horsts flanked by Triassic–Jurassic dipping wedges of sediments, including salt and overlain by Cretaceous basalts, and most distal areas of serpentinized upper mantle. Plate history began along with the Late Triassic formation of the Central Atlantic Magmatic Province and involved Triassic–Jurassic rifting, Jurassic–Early Cenozoic extension and Oligocene–Recent strike–slip. Great extension promoted volcanism, foundering, eastward growth of the plate by backarc spreading and distribution of continental fragments on the plate interior and along its margins. Hydrocarbons probably are present. Caribbean geology has important implications for understanding of oceanic plateaus, intra-oceanic volcanic arcs, the ‘andesite problem’ and genesis of ‘subduction’ HP/LT metamorphic rocks. The model can be tested by re-examination of existing samples and seismic data and by deep sea drilling.

The deep crust beneath island arcs: Inherited zircons reveal a Gondwana continental fragment beneath East Java, Indonesia

Inherited zircons in Cenozoic sedimentary and igneous rocks of East Java range in age from Archean to Cenozoic. The distribution of zircons reveals two different basement types at depth. The igneous rocks of the Early Cenozoic arc, found along the southeast coast, contain only Archean to Cambrian zircons. In contrast, clastic rocks of north and west of East Java contain Cretaceous zircons, which are not found in the arc rocks to the south. The presence of Cretaceous zircons supports previous interpretations that much of East Java is underlain by arc and ophiolitic rocks, accreted to the Southeast Asian margin during Cretaceous subduction. However, such accreted material cannot account for the older zircons. The age populations of Archean to Cambrian zircons in the arc rocks are similar to Gondwana crust. We interpret the East Java Early Cenozoic arc to be underlain by a continental fragment of Gondwana origin and not Cretaceous material as previously suggested. Melts rising through the crust, feeding the Early Cenozoic arc, picked up the ancient zircons through assimilation or partial melting. We suggest a Western Australian origin for the fragment, which rifted from Australia during the Mesozoic and collided with Southeast Asia, resulting in the termination of Cretaceous subduction. Continental crust was therefore present at depth beneath the arc in south Java when Cenozoic subduction began in the Eocene.

The Mesozoic-Cenozoic tectonic evolution of the Greater Caucasus

Abstract The Greater Caucasus (GC) fold-and-thrust belt lies on the southern deformed edge of the Scythian Platform (SP) and results from the Cenozoic structural inversion of a deep marine Mesozoic basin in response to the northward displacement of the Transcaucasus (lying south of the GC) subsequent to the Arabia-Eurasia collision. A review of existing and newly acquired data has allowed a reconstruction of the GC history through the Mesozoic and Cenozoic eras. In Permo(?)-Triassic times, rifting developed along at least the northern part of the belt. Structural inversion of the basin occurred during the Late Triassic corresponding to the Eo-Cimmerian orogeny, documented SE of the GC and probably linked to the accretion of what are now Iranian terranes along the continental margin. Renewed development of extensional basin formation in the area of the present-day GC began in Sinemurian-Pliensbachian times with rift activity encompassing the Mid-Jurassic. Rifting led to extreme thinning of the underlying continental crust by the Aale-nian and concomitant extrusion of mid-ocean ridge basalt lavas. A Bathonian unconformity is observed on both sides of the basin and may either correspond to the end of active rifting and the onset of post-rift basin development or be the record of collision further south along the former Mesozoic active margin. The post-rift phase began with deposition of Late Jurassic platform-type sediments onto the margins and a flysch-like unit in its deeper part, which has transgressed the basin during the Cretaceous and Early Cenozoic. An initial phase of shortening occurred in the Late Eocene under a NE-SW compressional stress regime. A second shortening event that began in the Mid-Miocene (Sarmatian), accompanied by significant uplift of the belt, continues at present. It is related to the final collision of Arabia with Eurasia and led to the development of the present-day south-vergent GC fold-and-thrust belt. Some north-vergent retro-thrusts are present in the western GC and a few more in the eastern GC, where a fan-shaped belt can be observed. The mechanisms responsible for the large-scale structure of the belt remain a matter of debate because the deep crustal structure of the GC is not well known. Some (mainly Russian) geoscientists have argued that the GC is an inverted basin squeezed between deep (near)-vertical faults representing the boundaries between the GC and the SP to the north and the GC and the Transcaucasus to the south. Another model, supported in part by the distribution of earthquake hypocentres, proposes the existence of south-vergent thrusts flattening at depth, along which the Transcaucasus plunges beneath the GC and the SP. In this model, a thick-skinned mode of deformation prevailed in the central part of the GC whereas the western and eastern parts display the attributes of thin-skinned fold-and-thrust belts, although, in general, the two styles of deformation coexist along the belt. The present-day high elevation observed only in the central part of the belt would have resulted from the delamination of a lithospheric root.

Amphisbaenian paleobiogeography: Evidence of vicariance and geodispersal patterns

Paleobiogeographic patterns within the Amphisbaenia were evaluated using the modified Brooks Parsimony Analysis (BPA) and recently published morphological and molecular phylogenies. Extant amphisbaenians are present in Africa, South America, North America, Europe, and the Middle East. The modified BPA was used to determine the relative effects of Pangean breakup, sea-level change, and climate change on evolutionary and distributional patterns within the Amphisbaenia. The modified BPA also tested the biogeographic effect of the Rhineuridae's phylogenetic position as either most basal in the morphologic phylogeny or most derived in the molecular phylogeny. The morphological and molecular analyses resulted in two different biogeographic hypotheses. The morphological analysis indicated three major biogeographic regions for the Amphisbaenia: 1) Africa, South America, and the Caribbean, 2) western Asia, and 3) North America. The molecular analysis indicated two major biogeographic regions: 1) Africa, western Asia, and North America, and 2) South America. The morphological biogeographic pattern corresponds with the known timing of the breakup of Pangea and the resulting paleogeographic reconstructions of the Mesozoic and Early Cenozoic. While the molecular pattern is similar to patterns recovered from dinosaurian biogeographic studies, the closer connection of Africa with North America rather than South America does not match well-constrained geologic evidence for the sequence of Pangean breakup. Both paleobiogeographic analyses, however, resulted in congruent patterns of speciation through vicariance and geodispersal. This suggests that in addition to the breakup of Pangea, such cyclical Earth history processes as sea-level and climate changes played an important role in the biogeographic patterns of the Amphisbaenia.

The Southern Conifer Family Araucariaceae: History, Status, and Value for Paleoenvironmental Reconstruction

▪ Abstract The Araucariaceae are important to biogeography because they have an ancient origin and are a distinctive and sometimes dominant component of southern hemisphere forest communities. This paper examines recent information on ecology and phylogeny and on pollen and macrofossil assemblages to assess the history and present-day status of the family and its potential for refinement of past environmental, particularly climatic, conditions. From an origin in the Triassic, the family expanded and diversified in both hemispheres in the Jurassic and Early Cretaceous and remained a significant component of Gondwanan vegetation until the latter part of the Cenozoic. The development of angiosperms in the Middle Cretaceous probably assisted in the demise of some araucarian components but there was also evolution of new genera. Recorded diversity in the early Cenozoic of Australia is as high as it was in the Early Cretaceous. Continental separation and associated climatic drying, cooling, and increased variability progressively reduced the ranges of conifers to moist, predominantly mesothermal climates on continents. However, tectonic and volcanic activity, partially associated with Australia's collision with Southeast Asia, provided new opportunities for some araucarian components on Asia-Pacific islands. Araucarians provide information on climatic conditions suitable for rainforest vegetation throughout their recorded period, even prior to the recognition or even existence of these forests in the fossil record. High pollen abundance is also indicative of marginal rainforest environments where these canopy emergents can compete effectively with angiosperm forest taxa. Despite their apparent relictual status in many areas, they provide precise paleoclimatic estimates in late Quaternary pollen records and have particular value in providing evidence of climatic variability that has otherwise been difficult to detect.

Neotectonics of the Roer Valley Rift System, the Netherlands

The Roer Valley Rift System (RVRS) is located in the southern part of the Netherlands and adjacent parts of Gemany and Belgium. The last rifting episode of the RVRS started in the Late Oligocene and is still ongoing. The present-day seismic activity in the rift system is part of that last rifting episode. In this paper, the Quaternary tectonics of the RVRS are studied using the detailed stratigraphic record. Subsidence analyses show that three periods of subsidence can be discriminated during the Quaternary. A phase of rapid subsidence took place from the beginning of the Quaternary to the Upper Tiglian (∼1800 ka). This was followed by a phase of slow subsidence lasting until the Late Quaternary (∼500 ka). An acceleration in subsidence at the end of the Quaternary occurred in the central and northern parts of the RVRS (i.e. the Roer Valley Graben and the Peel Horst) during the last 500 ka. During the Quaternary, the most active fault zones in the RVRS are the Peel Boundary Fault zone and the Feldbiss Fault zone. Average displacements along these fault zones vary between 5 and 80 mm/ka. Periods of high and low displacement rates along faults can be discriminated. The magnitude of the subsidence rate in the central part of the RVRS, which in theory is caused by a combination of processes like faulting, cooling of the lithosphere and isostasy, is within the range of the rate of displacement along the major fault zones of the RVRS, which implies that the subsidence of the RVRS is to a large extent controlled by faulting. Along the wide and staggered Feldbiss Fault zone, the location of the largest displacement rate shifts during the Quaternary, whereas the Peel Boundary Fault zone, which is narrow and has a straight structure, is more stable in this respect. The present-day fault displacement rates inferred by geodetic measurements are two orders of magnitude larger than the rates inferred from the geological record. Such a large difference can be explained by a high variability of fault movements on a short time-scale due to fault–stress interactions. The stratigraphic record has preserved average displacement rates. Flexural analyses shows that the pattern of geodetically determined displacements is in accordance with the fault spacing in the fault zone. The NW–SE directed fault system active during the Quaternary and the Tertiary is inherited from the late stage of the Variscan orogeny. This fault system was also dominantly active during the Mesozoic and Early Cenozoic evolution of the RVRS. Lineament analysis of the topography indicates that apart from the dominant NW–SE-oriented faults, N–S and NE–SW directed faults are also prominent. These faults originate from the Caledonian tectonic phases. They have, however, no large displacements during the Mesozoic and Cenozoic. The fact that Paleozoic fault systems are reactivated during Quaternary and Tertiary indicates that these faults are fundamental weakness zones.

The Palaeozoic, Mesozoic and Early Cenozoic fishes of Africa

Africa has provided many fossil fishes from sediments laid down over a 400 million year period. The large array of fossil fishes come from diverse localities throughout the continent, representing a variety of environments. The marine fossil fishes of Africa have not been reviewed as a whole, while the freshwater Cenozoic fishes of Africa were last reviewed over 25 years ago. Since that time, many new finds have increased our knowledge of the history of African fishes. This paper summarizes the known fish fossils, excluding otoliths, from marine and freshwater deposits throughout Africa from the Palaeozoic, Mesozoic and first part of the Cenozoic (Palaeogene). Much new work is ongoing, in areas such as Mali and South Africa, from which more information on the ichthyofaunas should come to light. New information presented here includes the Eocene site of Mahenge, Tanzania, from which have come the oldest known members of the family Cichlidae.

Late Proterozoic extensional collapse in the Arabian–Nubian Shield

A structural and petrological study of the Late Proterozoic rocks in the Wadi Kid area, Sinai, Egypt indicates the presence of an extensional metamorphic core complex in the northern Arabian–Nubian Shield. Gneissic domes throughout the Arabian–Nubian Shield resemble the core complex of the Wadi Kid area and as a result, they are interpreted as extensional metamorphic core complexes. The presence of a widespread phase of extension at the end of the Pan‐African period in the Arabian–Nubian Shield requires a new interpretation of the tectonic history of this shield. Three main tectonic phases are recognized in the Late Proterozoic of the Arabian–Nubian Shield. Ophiolites and island‐arc remnants are relicts of an oceanic phase, the oldest one. This phase was followed by arc‐accretion, well established in the Arabian–Nubian Shield from the presence of individual terranes bordered by sutures, which was responsible for lithospheric thickening. The Late Proterozoic ended with widespread NW–SE extension. The metamorphic core complexes, late‐orogenic extensional basins and large strike slip zones were formed during this phase. Similarity of the tectonic evolution of the Arabian–Nubian Shield with the Mesozoic and Early Cenozoic evolution of western North America lead us to conclude that gravitational instability at the final stages of the arc‐accretion phase caused the collapse and resulted in extension at the latest stages of the Pan‐African orogeny in the Arabian–Nubian Shield.

THE ORIGIN AND DEVELOPMENT OF NEOGENE BASINS IN THE SE BETIC CORDILLERA (SE SPAIN): A CASE STUDY OF THE TABERNAS‐SORBAS AND HUERCAL OVERA BASINS

Neogene— Quaternary sedimentary basins in SE Spain contain a record of the geodynamic evolution of the Internal Zone of the Betic Cordillera. The basement of the Internal Zone is composed of Palaeozoic and Mesozoic metasediments which have undergone variable degrees of metamorphism. The External Zones consist of largely unmetamorphosed sedimentary rocks which were deposited on the SE margin of the Iberian Plate during the Mesozoic and Early Cenozoic. Westward tectonic emplacement of these terranes onto the Iberian Plate took place between the end of the Palaeogene and the middle Miocene. In this paper, we investigate the late Miocene (Tortonian‐Messinian) stratigraphy of two basins in the Internal Zone — the Tabernas‐Sorbas and Huercal Overa Basins. We also consider some recently‐acquired structural data.The Tabernas‐Sorbas and Huercal Overa Basins are east‐west trending depressions bounded to north and south by sierras in which basement rocks are exposed. The basins contain very similar sedimentary successions in which planktonic foraminifera have been preserved. However, the faunal composition is very variable, and the observed sporadic and abrupt changes in foraminiferal populations imply palaeo‐ecologic and palaeo‐oceanographic instabilities which may be associated with local tectonism. Stratigraphic markers were affected by these changes, making precise dating difficult near the Tortonian‐Messinian boundary.Our data indicate that Messinian rocks are more widely distributed than has hitherto been suspected. A Messinian age for the prominent coral limestones in the Tabernas‐Sorbas Basin has long been accepted; similar coral limestones in the Huercal Overa Basin have previously been dated as Tortonian. However, our data show that these carbonates are of Messinian age in both basins.The origin and development of the Tabernas‐Sorbas and Huercal Overa Basins have previously been interpreted in a number of ways. Many (but not all) models favour strike‐slip movement on NE‐SW or east‐west trending basin‐bounding faults. The formation and deformation of the basins occurred during the Tortonian and Messinian, at the same time as the uplift of the sierras. The sierras are here interpreted to represent structural culminations above westward‐verging, deep‐seated thrust faults, and the basins to be lateral folds (or lateral ramps, i.e. oriented parallel to the thrust transport direction). The east‐west trending strike‐slip faults at the margin of the Sorbas‐ Tabernas Basin may be compatible with such a model.Major NE‐SW trending, left‐lateral wrench faults have been described in this area. These faults cut through late Miocene and Pliocene deposits, and are still active at the present day. Although these faults were initiated during the late Miocene, they do not appear to have influenced the development of the Tabernas‐Sorbas and Huercal Overa Basins during the early Tortonian.

Could Early Cenozoic thermohaline circulation have warmed the poles?

The Paleocene and Eocene epochs provide examples of oceanographic conditions when the Earth had a stable climate much warmer than that of the modern world. Lower Cenozoic sediments thus provide a way to investigate how oceanographic processes might respond to extreme warming. Because high‐latitude sea surface temperatures were significantly warmer during the early Cenozoic while tropical sea surface temperatures were similar to or somewhat less than modern, there may have been a much higher poleward oceanic heat transport at that time [Shackleton and Boersma, 1981; Barron, 1987; Sloan et al. 1995]. Here I develop a simple ocean thermohaline model to demonstrate that poleward oceanic heat transport could strongly warm the polar regions only if the strength of vertical mixing (vertical eddy diffusion) in the oceans is sensitive to changes in oceanic density structure. If this is true, the early Cenozoic oceans, because they had significantly less density stratification, could have had a thermohaline circulation as much as 3 times as strong as that of the modern ocean despite reduced temperature gradients. The model also predicts that an optimum bottom water temperature exists for maximum poleward heat transport: continued warming of the Earth does not continue to increase oceanic heat transport to the poles. The hypothesized sensitivity of oceanic vertical mixing to the density gradient should provide a positive feedback to both the transition to ice ages and to a future transition out of them. A test of the sensitivity of vertical eddy diffusion to density gradients can be studied through map and depth transects. In the horizontal direction, high vertical diffusion would be marked by relatively strong bottom water temperature gradients away from bottom water source regions. In the vertical direction, high vertical diffusion should be indicated by very small temperature gradients.

Cretaceous and Early Cenozoic Nonmarine Sedimentation and Tectonic History of Kyongsang Basin, Southeastern Korea

Cretaceous and Early Cenozoic Nonmarine Sedimentation and Tectonic History of Kyongsang Basin, Southeastern Korea

Collision tectonics in the Cordillera of western N America: examples from Alaska

Summary The western edge of N America grew during the Mesozoic and Early Cenozoic by the addition of numerous accreted terranes consisting of fragments of island arcs, scraps of oceanic basins, seamounts, continental rise deposits and pieces of continents. The result of the accretionary process was production of new continental crust of extraordinary heterogeneity and structural complexity. This complexity is best portrayed in Alaska, where virtually 95% of the state was assembled during the Cretaceous, with attendant folding, large-scale thrust faulting, penetrative deformation and regional metamorphism. In this brief report, we discuss evidence for large-scale displacement of Wrangellia and for its time of incorporation into the Alaskan tectonic collage. The structures produced during accretion of this large coherent body are contrasted with those produced during coeval collapse of interval flysch basins that lay N of Wrangellia and with those associated with emplacement of enormous thrust sheets of oceanic rocks on top of continental platforms in Central and Northern Alaska. Despite compelling evidence for thousands of km of relative displacement between accreted terranes and between the terranes and the N American Craton, the resulting collage carries few readily discernible signals that allow application of currently popular simple plate tectonic models.

Flexural Analysis of Two Broken Foreland Basins: Late Cenozoic Bermejo Basin and Early Cenozoic Green River Basin: ABSTRACT

Lithospheric flexure that generates basin in a broke foreland setting (e.g., the Laramide foreland of Wyoming) is a three-dimensional system related to shortening along basin-bounding faults. The authors modeled the elastic flexure in three dimensions for two broken foreland basins: the early Cenozoic Green River basin and the analogous late Cenozoic Bermejo basin of Argentina. Each basin is located between a thrust belt and a reverse-fault-bounded basement uplift. Both basins are asymmetric toward the basement uplifts and have a central basement high: the Rock Springs uplift and the Pie de Palo uplift, respectively. The model applies loads generated by crustal thickening to an elastic lithosphere overlying a fluid mantle. Using the loading conditions of the Bermejo basin based on topography, limited drilling, and reflection and earthquake seismology, the model predicts the current Bermejo basin geometry. Similarly, flexure under the loading conditions in the Green River basin, which are constrained by stratigraphy, well logs, and seismic profiling and summed for Late Cretaceous (Lance Formation) through Eocene (Wasatch Formation), successfully models the observed geometry of the pre-Lance surface. Basin depocenters (> 4 km for the Green River basin; > 7 km for the Bermejo basin) and central uplifts are predicted to resultmore » from constructive interference of the nonparallel applied loads. Their Bermejo model implies that instantaneous basin geometry is successfully modeled by crustal loading, whereas the Green River basin analysis suggests that basin evolution can be modeled over large time steps (e.g., 20 Ma). This result links instantaneous basin geometry to overall basin evolution and is a first step in predicting stratigraphic development.« less