bluuebunny/crossref_metadata_2025 huggingface dataset:
A half-ridge transform model for the Hellenic-Dinaric ophiolites
Authors: Smith A. G. , Spray J. G.
Published: April 2008

We speculate that the similarity of the E and W ophiolite belts in Greece, Albania and Yugoslavia can be plausibly explained by the disruption of a once continuous subduction-linked marginal basin and adjacent continent by a Middle to Late Jurassic sinistral transcurrent fault along the E edge of the Pelagonian Drina-Ivanjica Zones. The fault may itself have been linked to the Jurassic opening of the central Atlantic. This model is supported by the age relations between igneous crystallization of the ophiolites and the timing of metamorphic sole formation. Present isotopic dating, together with simple thermal calculations, suggests that the ages of these events differed by at most 10 Ma. Together with lateral variations in the ophiolites themselves and their soles, this apparent similarity of ages is most readily accounted for by the overthrusting at the ridge crest of one half of a narrow marginal basin by the other half.
Palaeomagnetic evidence of Miocene and Pliocene rotational deformations of the Aegean Area
Authors: Kissel Catherine , Laj Carlo , Jamet Marc
Published: April 2008

Palaeomagnetic measurements have been made on the Mio-Pliocene sedimentary series along the external Hellenic Arc as well as in the Oligocene to Middle Miocene Ionian flysch of northwestern Greece.

The results show that the central and eastern parts of the arc (Crete and Rhodes) have not undergone significant rotation since the Tortonian-Messinian stage. On the other hand, the northwestern part has undergone a clockwise rotation which, in the Ionian islands, has begun 5 Ma ago and has proceeded since then at roughly 5 degrees/Ma. We suggest that the beginning of this rotation is related to the Lower Pliocene compressive phase which most probably initiated the tectonic activity of the present day Aegean Arc.

No rotational deformation occurred between 5 and 12 Ma, but the results from 16 sites in Epirus and Akarnania sampled in Ionian flysch sections indicate that another clockwise rotation, of the order of 30 degrees, occurred between 30 and 12 Ma ago.

These Oligo-Miocene and Plio-Quaternary episodes of rotational deformation may be related to the Oligo-Miocene and Plio-Quaternary arc-related volcanic activity in the Aegean Sea.
Subsidence history of the North Aegean Trough
Authors: Pichon Xavier Le , Lybéris Nicolas , Alvarez Francis
Published: April 2008

The subsidence of the North-Aegean trough is examined quantitatively using available geophysical data, the emphasis being on thermal and gravity computations. The two dimensional thermal calculations incorporate conduction as well as convection with lateral variations, the effect of the sedimentary cover being treated as a perturbation. The data can be explained within the framework of the homogeneous stretching model with a maximum stretching factor of about 3.5. The stretched lithosphere has a very small flexural parameter so that local compensation prevails. The effect of conduction is sufficiently important to prevent large-scale melting of the mantle, thus making the transition to oceanic accretion rather improbable. The narrowness of the zone of extreme stretching, compared to the widths now observed on most continental margins where comparable stretching values have been observed, may be due to the presence of an earlier deep lithospheric fault which enabled the strain to concentrate along its path.
Deep-sea clastics: where are we and where are we going?
Authors: Stow D. A. V.
Published: April 2008

The transition from our belief in a deep calm ocean to a recognition that deep-sea clastics other than pelagic clays exist in the oceans, spanned nearly a century. In the last three decades enormous strides have been made in understanding these sediments and their deposition. There is a continuum of processes that transfer material from shallow to deep water and rework sediments within the deep sea. These include: (1) resedimentation processes, ranging from giant rockfalls and slumps to low-density turbidity currents; (2) normal bottom currents; and (3) pelagic settling through the water column. More than fifty distinct facies have been described from the deep sea and these can be interpreted in terms of depositional process via ten standard facies models for resedimented, normal bottom current and pelagic sediments. Environmental models can be constructed for: (1) normal, faulted, carbonate and ridge-flank slope-aprons; (2) radial, elongate and fan-delta submarine fans; and (3) under- and oversupplied basin-plains. These show the generalized horizontal and vertical distribution of facies and the chief morphological elements in each of the three major deep-sea settings. Sedimentary, tectonic and sea-level changes are the main groups of factors that control deep-sea sedimentation within these separate environments. Part of the interest in deep-sea clastics stems from their demonstrable economic importance for the generation and entrapment of hydrocarbons. Many areas of deep-sea sedimentology remain to be investigated and earlier models to be refined; these advances will depend significantly on improvements in our methodology.
Diagenesis of shallow-marine carbonates
Authors: Dickson J. A. D.
Published: April 2008

The utility of carbonate petrography in solving diagenetic problems is often underrated. Staining and cathodoluminescence are complementary to petrographic studies; their application is compared. Interpretation of carbonate trace element concentration through the distribution coefficient, as illustrated by Sr
2+
, was oversimplified in the 1970s and often tied to repeated recrystallization episodes in meteoric waters. Kinetic factors complicate the use of the distribution coefficient in calcite. δ
13
C values from carbonates help in identifying the source of carbon but δ
18
O values are difficult to interpret due to control by temperature as well as the isotopic composition of the precipitation fluids. Data on the composition of sedimentary waters, geothermometry and geobarometry of buried sedimentary rocks can be provided by fluid inclusion studies but interpretation has often been over-optimistic, because insufficient account has been taken of the difficulties which exist with fluid inclusion work.

Near-surface, carbonate diagenesis is well understood through field studies of Recent and Pleistocene sediments. Research effort in the future, it is predicted, will concentrate on burial diagenesis because it is the least understood diagenetic realm. The large variations in temperature, pressure and time which distinguish burial from near-surface environments are exemplified by the burial history of the Carboniferous Limestone of South Wales.
Carbonate facies analysis in the exploration for hydrocarbons: a case-study from the Cretaceous of the Middle East
Authors: Burchette T. P. , Britton S. R.
Published: April 2008

This paper presents a case-study of an investigation into a Middle Eastern carbonate reservoir, the Mishrif Formation of eastern offshore Abu Dhabi, and its lateral equivalent, the Shilaif Member of the Salabikh Formation. The work represents a major part of a sedimentological/seismic stratigraphic study, the aims of which were to model reservoir facies and geometry, and to predict areas of best reservoir potential. The study is based on interpretation of cores, cuttings and wireline logs from 30 wells distributed over some 200 km
2
.

The Mishrif of the study area forms part of an extensive Cenomanian carbonate platform. It represents deposition on a local leeward (i.e. shelfward) margin which prograded westwards into an intrashelf (the Shilaif) basin. Six lithofacies associations were recognized in the Shilaif and Mishrif, representing lateral and vertical transitions from silt-grade basinal pelagic-foraminiferal wackestones (I), through a platform margin sequence (II) which coarsens upwards to coarse rudist/ostreiid shoal (III) and biostromal (IV) packstones and grainstones. Shoal and biostromal facies are overlain in the east by bedded back-shoal packstones (V) and nodular micritic platform-lagoonal (VI) sequences. A significant unconformity exists at the top of the Mishrif.

Three stages of diagenesis have been recognized in Mishrif limestones: early (micritization, submarine cementation); unconformity-related (extensive leaching, freshwater cementation); and burial diagenesis (stylolitization, fracturing, burial cementation, neomorphism). Leaching and burial cementation were the most important diagenetic events and significantly overprint the generally simple variations in reservoir character due to original depositional facies distribution. Most favourable porosity and permeability values occur in coarse-grained shoal, upper-slope, and biostromal sediments (upper II, III, IV) where exposed and leached at the final platform margin, whereas to the east, in the platform interior, these facies have suffered severe burial cementation. The slight regional eastwards dip of the Mishrif reservoir, its westwards pinch-out into basinal facies, and the presence of a proved top-seal fulfil the requirements for stratigraphic trapping of hydrocarbons, with the best reservoir facies advantageously situated. A well drilled to test this play encountered the predicted sequence but found only residual oil. This may be due to imperfect lateral or bottom reservoir sealing.
Intracontinental subduction, crust-mantle décollement and crustal-stacking wedge in the Himalayas and other collision belts
Authors: Mattauer M.
Published: April 2008

The Himalayas exhibit the large-scale geometric characteristics of deformation in a classic collision belt. After the continental collision, which occurred 40 Ma ago, the continuous northward motion of India resulted in an important northward subduction of continental lithosphere. The two main consequences of this subduction were (i) building of a
crustal-stacking wedge
with an average thickness of 70 km; the continental crust was thickened mainly by extensive low-angle (20–30°) overthrusting (crustal duplex) with the width of the wedge increasing with time by southward migration; and (ii)
crust-mantle décollement
along which the relatively rigid lithospheric mantle slid below the more deformable continental crust during crustal thickening. At the same time, farther N, large-scale strike-slip faulting gave way to lateral E-W crustal extension. Such a model may be applied to other collision belts such as the Alps, the Canadian Cordillera and the Variscan Belt of Europe.
Some thermal and tectonic models for crustal melting in continental collision zones
Authors: England Philip C. , Thompson Alan
Published: April 2008

Calculated geotherms and the pressure-temperature-time (
PTt
) paths followed by rocks during continental thickening episodes are interpreted with respect to the volumes of crustal melt that may be formed during orogenesis in the absence of heat transfer by mantle-derived melts. Particular attention is paid to a tectonic history that may characterize wider orogenic belts, such as are represented most obviously at present by Tibet. This comprises a period of crustal thickening, followed by an interval during which the crust is thinned by extensional strain, rather than by erosion. The amount of crustal melt produced depends strongly on the amount of water (free, and in hydrated minerals) contained in the lower crust. However, we may expect several (1–5) km
3
of crustal melt per km
2
of orogen if a crust of around average continental surface heat flux (60–70 mW m
−2
) is thickened by a factor of two. For the lower surface heat flux, partial melting of a sedimentary source would produce predominantly S-type granites and, with slightly higher geotherms, doubling of crustal thickness can lead to partial melting of amphibolites to give I-type granitic activity and calc-alkaline volcanism.
On the mechanics of the collision between India and Asia
Authors: Tapponnier P. , Peltzer G. , Armijo R.
Published: April 2008

Field studies of active faulting in S Tibet indicate that Quaternary extension has been taking place at a rate of ≃1 cm yr−1in a direction of ≃ 100°. This implies that underthrusting in the Himalayas now absorbs less than half of the total convergence between rigid India and Asia, the rest being taken up primarily by strike-slip faulting N of the collision belt.En échelonright-lateral, strike-slip faults in S Tibet now allow this corresponding eastward displacement of the plateau with respect to India. The reproducible pattern of faulting obtained from plane-strain indentation experiments on unilaterally confined blocks of plasticine suggests that this extrusion process has occurred during most of the collision history. The Tertiary geological record in SE Asia corroborates a polyphase extrusion model, with displacements in excess of 1000–1500 km, in which India has successively pushed Sundaland, then Tibet and S China towards the ESE. Most of the Middle Tertiary movements may have occurred along the then left-lateral Red River-Ailao Shan Fault Zone, together with the opening of most of the eastern S China Sea. Regional geology, stratigraphy and deformation observed in Yunnan are consistent with this inference, as well as the timing, geometry and rates of sea-floor spreading in the S China Sea. Fast spreading (5 cm yr−1) in that sea implies that the Tibetan highlands formed mostly after 17 Ma BP. Sideways movements can also account for the existence of large, conjugate but asymmetric, Tertiary strike-slip faults within Sundaland and the formation of Middle Tertiary pull-apart and rift basins on the Sunda Shelf. Changing directions of opening are predicted in the Mergui and Andaman Basins and the lowlands of Burma, as well as large right-lateral displacements along the Shan Scarp. Most of Sundaland probably lay initially in a frontal position with respect to impinging India and the Shan Plateau may have been a Middle Tertiary analogue of the present Tibetan Plateau. In contrast with dominant overthrusting in the Himalayas, Tertiary strike-slip faulting, with more subordinate folding and thrusting, appears to have been important along and N of the Zangbo Suture. This difference must be accounted for in all models of formation of the Tibet Plateau. The surface of the indentation mark, left by the impaction of India onto the presumably simpler Early Tertiary margin of Asia (> 6 million km2), implies that mountain building and strike-slip faulting have absorbed, perhaps alternately, roughly equal amounts of collisional shortening. Since analogous interplays of extrusion and thickening probably govern the evolution of most collision zones, the Tertiary tectonics of Asia may be the best guide to unravel the interactions between Palaeozoic and Precambrian plates, for which sea-floor spreading constraints are unattainable.
Some aspects of collision tectonics in the Indus Suture Zone, Ladakh
Authors: Colchen M. , Mascle G. , Van Haver T.
Published: April 2008

In the Indus Suture Zone, five tectonic zones can be recognized, which are from N to S: (i) the Ladakh or Trans-Himalayan Batholith (103−25 Ma) and the Dras Volcanoplutonic Group; (ii) the Indus Detrital Group with marine formations of Aptian to Ilerdian age (103−45 Ma) and continental formations of Oligocene- ?Miocene age; (iii) the Nindam (Cretaceous) and Lamayuru (Ladinian-Dogger) Flysch Nappes; (iv) the Zanskar Structural Nappe Complex (Upper Palaeozoic-Ilerdian) resting on a metamorphic ‘basement’; and (v) the Spongtang-Photaksar Ophiolitic Nappe. Several folding phases affect these tectonic units; analyses of mesoscopic and microscopic structures show a polyphase tectonic history with a syn-metamorphic cleavage (S1or S1–2, striking 090–120°) which is axial planar to recumbent folds associated with a N-S mineral lineation, a fracture cleavage (striking 120°) which varies in dip and this is cut by a still later steeply dipping fracture cleavage striking 030–040°. The tectonic contacts, the kinematics, relative chronology and structural evolution are discussed. We propose that the structural sequence results from continuous N-S compression occurring during collision.
Collision tectonics in the NW Himalayas
Authors: Coward Michael P. , Rex David C. , Asif Khan M. , Windley Brian F. , Broughton Roger D. , Luff Ian W. , Petterson Michael G. , Pudsey Carol J.
Published: April 2008

West Himalayan tectonics involve the collision of microplates between the Indian and Asian Plates. The Kohistan Complex consists largely of tightly folded basic volcanics and sediments generated as Late Jurassic to Late Cretaceous island arcs. These were intruded by post-folding Mid-Cretaceous — Eocene plutonics produced from continued subduction of the Indian Plate after closure of a suture between Kohistan and the Karakorum. The Himalayan structures show major thrust sheets and the Kohistan Arc is essentially a crustal ‘pop-up’ with southward-upright and northward-verging structures developed above a thick ductile decoupling zone (the Indus Suture), which can be traced for >100 km beneath Kohistan on large reentrants. This pop-up formed by a two stage process, closure of the Northern Suture followed by closure of the southern Indus Suture. Granitic rocks of the Kohistan-Ladakh Batholith (dated at ≅ 100-40 Ma) post-date most of the structures related to the Northern Suture but were deformed and carried southwards on shear structures related to the Indus Suture. Post-collisional deformation carried this Kohistan Complex on deep decoupling zones over the Indian Plate on a series of imbricated gneiss sheets, the thrusts climbing up section in the movement direction so that in the far S some override their own molasse debris. Folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms—i.e. the Nanga Parbat and Hazara Syntaxes. The Nanga Parbat Syntaxis probably formed due to a shear couple near a branch line where one of the main Himalayan thrusts joined the Indus Suture beneath Kohistan. Crustal delamination, to produce the imbricated gneiss sheets, could not account for all the displacement of India into Asia, suggested by palaeomagnetic data. There must also have been lateral displacement as demonstrated by the large oblique-slip shear zone in the Hunza Valley, N of Kohistan.
The intracratonic Irumide Belt of Zambia and its bearing on collision orogeny during the Proterozoic of Africa
Authors: Daly M. C.
Published: April 2008

The Irumide Belt of Zambia is often quoted as being typical of African intracratonic mobile belts which involve no crustal shortening or major displacements. This belt is shown here to include the NW-facing foreland fold and thrust zone of the Southern Moçambique Belt and to have involved considerable crustal shortening. A crustal scale ‘pop-up’ structure separates this foreland from the internal higher-grade granulite-facies rocks of Malawi and Moçambique where major SE-facing nappe structures of Irumide age have been described. Several possible suture zones are identified and the Irumide-Moçambique Belt is interpreted as resulting from collisional processes during the Mid-Proterozoic. The adjacent Ubendian Belt represents a continental transform fault zone associated with crustal shortening across the Irumide Belt. The implications of these interpretations regarding the onset of plate tectonics in the Proterozoic and Archaean are briefly discussed.
Precambrian collision tectonics in Africa
Authors: Shackleton R. M.
Published: April 2008

Stages of the collisional process are separated. Criteria are proposed by which A- and B-subduction may be distinguished. Late Precambrian structures in NE Africa are attributed to successive gentle collisions of island arcs; the sutures dip E or SE and relative plate motion was NW-SE. In the polycyclic Proterozoic Mozambique Belt of E Africa there are probably three sutures (Sekerr, Baragoi, Moyale-Shakiso) with ophiolites; they appear to dip E. It is argued that despite the lack of geochronological evidence, Archaean basement probably underlies the metasedimentary sequences, right across the Mozambique Belt of E Africa, from the Archaean Tanzania Craton on the W to the Archaean Craton, represented by Madagascar and S India, on the E. The belt is therefore interpreted as having been built by collision of continental plates. Widespread recumbent structures, complex and intense deformation and high-grade metamorphism imply crustal thickening and a multistage collisional history. Relative NW-SE motion was probably followed by N-S post-collisional ductile shear. The Archaean Limpopo Belt in southern Africa shows evidence of drastic crustal thickening and intense deformation in contrast to the much weaker deformation and little crustal thickening in the Kaapvaal and Rhodesian Cratons to the S and N. Collision is implied but in the absence of convincing ophiolites, subduction-type calc-alkaline magmatism and abyssal sediments, B-subduction remains unproven.
Post-collision isostatic readjustment in the Southern Banda Arc
Authors: Milsom J. , Audley-Charles M. G.
Published: April 2008

The collision between the Australian continent and island arcs marginal to SE Asia is currently taking place in the Banda Arc region, with some of the most intense deformation on, and around, the island of Timor. From Late Miocene to Mid-Pliocene times the area was dominated by a compressional tectonic regime which resulted in the emplacement from the N of large thrust sheets on the deformed Australian continental margin. During the last 3 Ma, however, compression seems to have been unimportant in the islands and vertical movements, both positive and negative; have been commonplace and rapid.

Considerable departures from isostatic equilibrium are suggested by gravity surveys in the region and in some cases the sign of the isostatic anomalies accords well with the observed sense of vertical movement. In other areas, such as Northern Timor and the inner (volcanic) arc, uplift is occurring where the the gravity data suggest that there should be subsidence. A possible explanation is that a contribution to the high gravity values is made by a cold, and therefore dense, subducted slab which is now sinking independently after rupture near the continental margin. Because of the rupture, the sinking slab no longer exerts a downward pull on the overlying lithosphere which has thus been freed to rebound isostatically.
Collision tectonics in the Cordillera of western N America: examples from Alaska
Authors: Jones D. L. , Silberling N. J. , Coney P. J.
Published: April 2008

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.
A critique of graptolite classification, and a revision of the suborders Diplograptina and Monograptina
Authors: Rigby J.
Published: April 2008

Three broad styles of modern graptolite classifications are identified, each being based on a framework laid down in the nineteenth century. Each style is critically reviewed with regard to its potential for the expression of presently know phyletic relationships. Following the style of classification adopted by Bulman (1970), family and subfamily taxa within the suborders Diplograptina and Monograptina are revised.
Heterochrony, punctuated equilibrium, and graptolite zonal boundaries
Authors: Finney Stanley C.
Published: April 2008

Many graptolite taxa are important biostratigraphically because their first appearances in stratigraphic sequences represent unique events that are recognizable and correlative worldwide. However, because the direct ancestry of many of these taxa are cryptic, the biostratigraphical value of their first appearances may be questioned.

Astogenetic and ontogenetic studies of well-preserved, isolated specimens of
Nemagraptus gracilis, Dicellograptus
(four species),
Leptograptus trentonenesis, Glossograptus ciliatus
and
Glyptograptus euglyphus
from the middle Ordovician Athens Shale of Alabama show that considerable graptolite evolution occurs through heterochony. And, what are here considered to be small heterochronic changes, often produce large morphological discontinuities between ancestors and descendants. In turn the presence of these morphological discontinuities in graptolite sequences suggest that graptolite evolution follows the model of punctuated equilibrium, rather that that of phyletic gradualism. Whether nor not this is the case is here considered to be a matter of semantics. More importantly, heterochronic evolution is here considered to explain the cryptic ancestry of many graptolite taxa, and with this mode of evolution in mind, the graptolite biostratigrapher may find it easier to determine ancestor-descendent relations and thus recognize those first appearances that are valid for biostratigraphical correlation.
Morphometric studies of
Climacograptus
(Hall) and the phylogenetic significance of astogeny
Authors: Mitchell Charles E.
Published: April 2008

Climacograptus typicals
and its varieties, along with the closely related
C. pygmaeus
, are abundant at many horizons in North American Caradoc strata. Morphometric analysis of colony form in
C. typicalis
and
C. pygmaeus
, from large collections of isolated, three-dimensionally preserved material, reveals low levels of variation for a colony. The analysis of astrogeny and its relationship to colony form suggests that many aspects of overall form are highly correlated with astogeny and that differences in the astogenetic pattern of
C. typicalis
and
C. pygmaeus
are correlated with some of the differences in colony form that distinguish the two species.

A theoretical framework is needed for the incorporation of astogenetic data into systematic and phylogenetic practice. The graptoloid colony was an integrated, functional entity and a unit of selection. Astogeny was a colonial character that displayed a morphometric and evolutionary pattern of change that suggested an analogy with von Baer’s Law and its evolutionary interpretation: morphogenesis during astogeny proceeds from generalized states to specific stages and shared astogenetic pattern is indicative of shared descent. This is proposed as a working hypothesis in the formulation of systematic relationships and phylogenies.
The lower Silurian graptolites of Oued In Djerane: a study of populations at the Ordovician-Silurian boundary
Authors: Legrand P.
Published: April 2008

Three zones can be distinguished in the Oued in Djerane region, Algerian Sahara, from the bottom to the top:
Climacograptus (Climacograptus)
tilokensis sp. nov. Zone;
Diplograptus(?) kiliani
Zone;
Glyptograptus (Glyptograptus) tariti
Zone. An example of a detailed study of the different forms of the
Climacograptus (Climacograptus) normalis
group is given. In addition the problem of the Ordovician-Silurian boundary and its application to the Algerian Sahara sequence is discussed.
Silurian graptolites from N Greenland
Authors: Bjerreskov Merete
Published: April 2008

Recent collections of Silurian graptolites from N Greenland have yielded much new information on the biostratigraphy of the area. The graptolites occur mainly in slope and trough sediments, some of which are now incorporated in the N Greenland foldbelt. The full sequence of graptolite zones established in Britain cannot be recognized. The oldest Silurian graptolites indicate a lower LlandoveryatavusorcyphusZone age and above this an intermittent succession of graptolite faunas are found throughout the Llandovery, Wenlock, Ludlow and into the Pridoli (transgrediensZone only). There are differences between the zones recognized in the western and eastern parts of N Greenland. ThespiralisZone appears to be the most extensive, occurring all over N Greenland. The graptolite sequences in N Greenland correspond most closely with those reported from the nearby Canadian Arctic Archipelago, but are easily correlated with the graptolite faunas from the Cordillera region, especially from the northern part. Differences of faunal assemblages are tentatively correlated with changes in sedimentary environments, possibly indicating a difference in the spatial distribution of species in a living graptolite community.
A mathematical study of the
Cyrtograptus sakmaricus
lineage with discussions of the evolutionary trends in this lineage
Authors: Huo Shi-cheng , Shu De-gan , Li-pu Fu
Published: April 2008

On the basis of a mathematical study of the
Cyrtograptus sakmaricus
lineage, we have found that all members of this lineage have the shape of a logarithmic spiral, as seen in plan view, which may be expressed in terms of logarithmic equation
ρ
= Ae
aθ
. Tightness of coiling and the shape of the volution are determined by the value of
a
. The principle and methods concerned are presented. This value which controls the form and function of cyrtograptids, increases in stratigraphically higher members and may be used as a basis for dividing the genus
Cyrtograptus
into various species. The cyrtograptids of the
Cyrtograptus sakmaricus
lineage from the Silurian deposits in Ziyang, Shaanxi, China show trends toward increase in number of thecae in number of volutions in tightness of coiling.
Unconformities in the Cenozoic of the North-East Atlantic
Authors: Pearson I. , Jenkins D. Graham
Published: April 2008

Unconformities in the North-East Atlantic and Norwegian Greenland Sea have been examined from 60 DSDP boreholes, and correlated to a single biostratigraphic time-scale of the Cenozoic. The unconformities can be divided into two types: (a) widespread events with equivalent episodes outside this study area, and (b) locally restricted events.

Unconformities equivalent to those recognized by Vail & Hardenbol (1979) and Keller & Barron (1983) are present in the late Pliocene, Miocene, mid-Oligocene and late to mid-Eocene. Hiatuses corresponding to other unconformities noted by Vail & Hardenbol are difficult to recognize, being encompassed within large local unconformities.

Hiatuses have been formed either by (a) a change in bottom water activity or (b) tectonic events, diapirism and slope instability. Recognition of these types through a series of filters allows greater accuracy in defining circulatory events.
Sedimentation on mid-ocean sediment drifts
Authors: Kidd Robert B. , Hill Philip R.
Published: April 2008

The paper reports results from DSDP Leg 94, where holes were drilled in the Feni and Gardar sediment drifts, with particular emphasis placed on studying sediment waves that ornament the surfaces of the drifts. The Feni Drift is a 1500–1700 m thick accumulation of sediment constructed on an early Eocene to late Oligocene seismic reflector. The Gardar Drift is 1300–1600 m thick, and is probably built directly on late Eocene oceanic basement. At Site 610 on Feni Drift, the deepest hole penetrated to upper Miocene sediments at 720 m, and at Gardar Drift, to upper Miocene sediment at 500 m. Pre-site surveys showed that the sediment wave fields on the drift surfaces were more irregular than Gloria sidescan records suggest, consisting of interfingered
en echelon
mounds. The waves did not show any well-defined sense of migration. The sediments recovered at Sites 610 and 611 show that the drifts consist of pelagic ooze and calcareous mud, with very few primary sedimentary structures preserved. No significant differences were observed between lithologies on sediment wave crests and those in through locations. On the other hand, systematic differences in sedimentation rates between crest and trough holes at Gardar Drift support the hypothesis that the waves migrated during the Pliocene but were evenly draped during the Pleistocene.
Turbidite deposition and the origin of the Madeira Abyssal Plain
Authors: Weaver P. P. E. , Searle R. C. , Kuijpers A.
Published: April 2008

Extensive investigations in the Madeira Abyssal Plain have revealed thick sequences of turbidites. Individual turbidites can be correlated over an area of 2° × 2°, and their emplacement can be shown to coincide with glacial onsets and terminations. The turbidites originate on the NW African margin and contain up to 2% organic carbon. They are virtually ungraded massive silts and clays, some with coarser basal layers.

Seismic records through the abyssal plain show distinct changes in sediment character at a few hundred metres depth. Rates of accumulation are calculated from piston cores to be about 7.5 cm/1000 years, suggesting that the turbidites are a recent phenomenon. If the link with glacial onsets and terminations has held throughout, the first major turbidites may have been emplaced at 2.4 Ma, at the onset of Northern Hemisphere glaciation. Some turbidites may also have been emplaced during the supposed late Miocene glaciations. The Madeira Abyssal Plain is therefore a recent feature, and before its initiation the area would have had a ridge-flank type topography.
North-east Atlantic Neogene benthic foraminiferal faunas: tracers of deep-water palaeoceanography
Authors: Schnitker Detmar
Published: April 2008

The qualitative and quantitative composition of deep-sea benthic foraminiferal faunas from DSDP Sites 400 (4399 m), 552 (2301 m), 553 (2329 m), 554 (2576 m), and 555 (1659 m) in the north-eastern North Atlantic have undergone strong changes during the past 27 Ma, very probably reflecting changes in the deep oceanic environment. Throughout this time interval the faunas of the deepest site (400) were distinctly different from the faunas of the shallower sites, indicating a persistent depth stratification of the deep water masses. The strongest qualitative faunal change occurred during the middle Miocene, about 15–13 Ma ago, when at all depths several species that had persisted since the Oligocene became extinct, several new species appeared for the first time, and the quantitative faunal composition of all sites changed drastically. This change is thought to be indicative of the (re-)initiation of North Atlantic Deep Water formation, and occurred at the same time as the rapid growth of the Antarctic ice sheet and a strong change in the carbon and oxygen isotopic composition of deep sea foraminifers worldwide. At the shallower and intermediate sites, the composition of benthic faunas changed again about 6.5–7 Ma ago, a change that also affected several key species at the abyssal site as well. This change was probably caused by the closure of the Mediterranean, the Messinian Crisis. The introduction of a new ‘shallow water’ species into the bathyal and upper abyssal sites, as well as a change in dominance in the deep abyss at 2.5 Ma, signifies the onset of modern glacial conditions.
Early to Middle Miocene benthic foraminiferal faunas from DSDP Sites 608 and 610, North Atlantic
Authors: Thomas Ellen
Published: April 2008

Benthic foraminifera were studied in lower and middle Miocene cores from DSDP Site 608 (3534 m, 42°50′N., 23°05′W.) and Site 610 (2427 m, 53°13′N., 18°53′W.), north-eastern North Atlantic Ocean. There were extremely high relative abundances of
Bolivina spp.
between about 19.5 and 17 Ma at both sites. These high abundances were probably caused by oxygen deficiency in the bottom waters, which is suggestive of sluggish circulation.

Some changes in the faunal composition occurred at the beginning of the period of sluggish circulation (20–19 Ma), and between about 15 and 14 Ma, at which time there was a global increase in oxygen isotopic ratios in benthic foraminiferal tests. The changes in faunal composition involved less than 20% of the total fauna. Most first and last appearances of benthic species were not coeval at the two sites. The first appearance of
Cibicidoides wuellerstorfi
is an exception and occurred at the same time as the middle Miocene increase in oxygen isotopic ratios.
Miocene to Recent bottom water masses of the north-east Atlantic: an analysis of benthic foraminifera
Authors: Murray J. W. , Haddon C. A. , Powell A. D. J. , Weston J. F.
Published: April 2008

The modern benthic foraminifera of the north-east Atlantic Ocean show a distribution pattern related to bottom water masses. Varimax factor analysis of data from depths > 2000 m gives factors which correlate with North-East Atlantic Deep Water (NEADW), North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW); a fourth factor corresponds with the basal part of the Mediterranean Water.

Miocene to Pleistocene benthic foraminiferal assemblages have been compared with the modern assemblages. On the assumption that species have not changed their environmental preferences through time, the fossil assemblages can be used to infer the past existence of water masses similar to those of today. The results presented here show that in the late Miocene, AABW penetrated to the southern flank of Rockall Plateau at the time of build-up of the East Antarctic ice sheet. During the early Pliocene the rate of production of NADW increased, perhaps as a result of a decrease in size of the Antarctic ice sheet, which in turn led to the southward retreat of the limit of penetration of AABW. Throughout much of the Pleistocene, as now, AABW has not penetrated north of the Azores Ridge. NEADW was the most extensive bottom water mass in the late Pliocene and Pleistocene.
Palaeoclimatic and palaeoceanographic development in the Pliocene North Atlantic:
Discoaster
accumulation and coarse fraction data
Authors: Backman Jan , Hermelin Otto , Pestiaux Pierre , Zimmerman Herman
Published: April 2008

Accumulation variations of the calcareous nannofossil genus
Discoaster
have been determined from a high latitude North Atlantic sediment sequence (DSDP Hole 552A) of late Pliocene age. Spectral analysis for the preglacial Pliocene reveals a dominant quasiperiodicity associated with obliquity-induced temperature variations in surface water. Spectral peaks corresponding to the 100 Kyr eccentricity and the 21 Kyr precession periodicities are also detected.

Coarse fraction analysis of core V26-145 (Blake Plateau) illustrates the Pliocene development of an increasingly intense subtropical circulation in the North Atlantic. A trend towards increased current velocity occurs through the early Pliocene and probably reflects the progressive emergence of the Panamanian Isthmus, and concomitant intensification of surface water circulation of the North Atlantic subtropical gyre system. In the Rockall Plateau area (Hole 552A), however, no evidence is seen for a related surface water warming.
Discoaster
accumulation data indicate that the preglacial Pliocene development of surface water temperature in the high latitude North Atlantic reflects a progressive climatic deterioration.
Palaeoceanographic setting of the Callovian North Atlantic
Authors: Robertson A. H. F. , Ogg J. G.
Published: April 2008

DSDP/IPOD Site 534 located in the Blake Bahama Basin adjacent to the Florida coast off eastern USA exemplifies the early opening stages of the North Atlantic in Callovian (mid-Jurassic) time.

The earliest sediments accumulated on a rugged ocean floor located near the edge of a WNW-ESE-trending fracture zone, not far from the spreading axis. At that time the ocean was
c.
500 km wide with a borderland experiencing a tropical climate. Input of both inorganic and organic material was mostly of marine origin, but also included quartz, illite, chlorite and large quantities of fine plant material. Carbonate banks flourished during a time of raised sea-level, shedding large quantities of sediment, mostly peri-platform ooze, which was redeposited by turbidity currents to Site 534 after a marginal ridge (Blake Spur ridge) was finally breached during later Callovian time. The site subsided
c.
300 m during the Callovian and for a time lay below the CCD which was still shallower than 3000 m. Distinctive hummocky seismic reflections and sedimentary structures in the cores suggest that sluggish bottom water circulation existed, giving rise to sediment-drifts (‘muddy contourites’). Hydrothermal precipitates drifted westwards from the ridge. Inferred trade wind patterns favoured extensive coastal upwelling along the African borderland, the closest margin to Site 534 during the Callovian. The favoured origin of numerous, but volumetrically minor, black shale intervals is that marine organic matter was concentrated where the oxygen-minimum-zone intersected the shelf-continental rise and that this material was subsequently redeposited by currents and/or gravity flows to form layers which readily became black and de-oxygenated during diagenesis.
Mesozoic palaeoceanography of the North Atlantic and Tethys Oceans
Authors: Roth Peter H.
Published: April 2008

The biogeography and preservation of calcareous nannofossil assemblages allow mapping of surface water fertility patterns and put constraints on carbonate dissolution in the global oceans of the middle Cretaceous. Cyclic changes in carbonate dissolution and surface water biologic productivity in the Cretaceous oceans were linked to oceanographic and climatic changes that had frequencies close to the Milankovitch cycles (tens of thousands to hundreds of thousand years). Evolution of calcareous nannoplankton during the Jurassic and Cretaceous periods occurred in two primary cycles of about 75 Ma duration that were punctuated by major diversification events, occurring every 15–30 Ma, that marked the beginning of periods of increased marine organic matter production and preservation (stagnation events). Calcareous nannoplankton evolved largely in shallow seas during the Rhaetian to Kimmeridgian. In the latest Jurassic, calcareous nannoplankton conquered the oceanic realm, became major producers of pelagic carbonates, and are thus responsible for the shift of carbonate deposition from shallow seas to the deep ocean. In the late Cretaceous chalk seas, nannofossils replaced shallow water benthos as major carbonate producers. Warm climates, due to high atmospheric CO
2
concentrations and increased albedo during high sea-level stands caused by global tectonic processes, made these tiny creatures flourish during the late Mesozoic.
Changes in the organic carbon burial during the Early Cretaceous
Authors: de Boer P. L.
Published: April 2008

The early and middle Cretaceous exhibit a paradoxical coincidence between low primary production in the oceans and entrapment of large amounts of organic matter in pelagic sediments. The high average temperatures and the decreased intensity of the (haline) circulation led to a strong decrease of the supply of oxygen to deep water, encouraging the accumulation of organic matter in black shales.

The nutritious elements, involved in the excess burial of organic matter, were available because nutrients of the terrestrial cycle of sedimentation and erosion were removed into the marine domain during the global rise of sea-level.

With respect to the warm climate, the sluggish circulation, and the widespread deposition of black shales, it is suggested that an initial rise of CO
2
, related to increased spreading and emissions of juvenile carbon, and to increased carbonate production, led to a first ‘greenhouse warming’. This was reinforced by a transfer of CO
2
from the warming oceans into the atmosphere. During subsequent cooling, CO
2
re-dissolved within the oceans, and conditions returned to ‘normal’.

Model calculations, based on the effect of changes of burial rate of organic matter upon δ
13
C of carbon in the ocean, show that the excess storage involves an increase of burial of organic carbon of the order of 20% only.
Geolipids of black shales and claystones in Cretaceous and Jurassic sediment sequences from the North American Basin
Authors: Farrimond P. , Eglinton G. , Brassell S. C.
Published: April 2008

Aliphatic hydrocarbon distributions in DSDP black shales and green claystones from the North American Basin have been examined by gas chromatography—mass spectrometry (GC-MS). The sediment sequences studied were: (i) black shale and claystone horizons (Callovian-Cenomanian) from the Blake-Bahama Basin (Leg 76, Site 534), and (ii) black shales and associated green claystones (Cenomanian-Turonian) from the US East Coast Continental Rise (Leg 93, Site 603). In both series of black shales, specific biological marker compounds provide evidence for contributions of organic matter from terrestrial, marine and bacterial sources. In addition, several of these geolipid distributions (e.g. hopanoids) are markedly similar for both sequences, indicating some common inputs of organic matter, and comparable diagenetic histories. For the Leg 93 samples, the relatively organic-lean green claystones are found to contain mainly terrigenous marker compounds, in constrast to their interbedded black shales which show a larger contribution from marine sources.

All the sediments are immature, with a trend of increasing diagenesis evident from changes in the geolipid distributions down the Leg 76 sequence. One sample, however, is anomalous; its enhanced maturity may be due to an input of reworked, older organic material at the time of deposition.
Turbidites, the principal mechanism yielding black shales in the early deep Atlantic Ocean
Authors: Degens E. T. , Emeis K.-C. , Mycke B. , Wiesner M. G.
Published: April 2008

A series of organic and inorganic geochemical parameters and facies indicators were studied in Late Jurassic and Cretaceous black shales and organic rich sediment sections from basins offshore Angola, Ghana and North Carolina, and from the western Wealden Basin in NW-Germany. Based on these data the sediments can be grouped into: (i) intracratonic (Bueckeberg Formation, NW Germany), (ii) shelf (Cretaceous basins offshore Ghana), (iii) continental rise turbidite (Hatteras Formation of Leg 93, Hole 603B, Section IV), and (iv) deep basin (Leg 75, Hole 530B, Section VIII) black shales. Trends of geochemical evolution are interpreted as the effect of multiple reworking and equilibration, when organicrich sediments from shelfbound basins are transferred into oceanic depocentres as turbidites. It is suggested: (i) that during turbidite phases, organic-rich suspensates originating from these primary basins on continental margins are advected along density boundaries within the water column overlying oceanic basins, and (ii) that these substrates are introduced into zones where responding microbial populations create expanded zones of oxygen depletion. Microbial degradation affects predominantly marine organic compounds contained in these recycled sediments, leaving a residual kerogen composed of structureless organic matter with pronounced terrigenous signals.

During slow sinking of these suspensions, chemical equilibration, possibly with the ambient water, leads to an association of certain meals, i.e. vanadium, zinc, and nickel, with the organic fraction. Pyrite placers form as the result of gravitational separation from oxygen depleted suspensates at the bottom of black shale beds. It appears that oceanic anoxia is not a prerequisite for black shale sedimentation in the Cretaceous basins of the Atlantic Ocean.