Eustatic Sea-level Variations Research Articles

Foraminiferal proxies of dissolved oxygen and their changes across the Miocene/Pliocene boundary in the Japan Sea

This study discusses paleoceanographic changes in the Japan Sea across the Miocene/Pliocene boundary based on foraminiferal distributions and geochemical evidence obtained in the Akita Prefecture of northern Japan.Q-mode cluster analysis differentiates six foraminiferal associations from a total of 55 assemblages. Their paleoceanographic implications are based on previous studies on the ecology of benthic foraminifers Spirosigmoilinella compressa and Miliammina echigoensis, microhabitat preferences of constituent species inferred from morphotypes, and other ecologic information of extant species. The stratigraphic distribution of each association reveals a history of oxygen depletion in the bottomwaters of the Japan Sea during the Late Miocene, followed by the oxic bottomwaterwith conditions of higher organic matter flux in the Early Pliocene. This interpretation is reinforced by the geochemical data of organic facies such as total organic carbon and hydrogen index fromRock-Eval analysis. It is inferred that this paleoceanographic change was a result of the rise in the eustatic sea level during the earliest Pliocene. Throughout the stratigraphic interval organic matter flux, related to primary productivity, is inferred to be higher in the northern sections than in the southern sections. This geographic variation is in agreementwith previous studies on other Japan Sea regions, and suggests that eutrophic conditions were prevalent in the northern region, where the Tsugaru Strait links the Japan Sea with the Pacific Ocean. The organic matter fluxes and oxygen depletion controlled by eustatic sea level variation are closely related to the source rock deposition of the oil and gas fields in the Japan Sea region.

Rift-initiation development of normal fault blocks: insights from the Hammam Faraun fault block, Suez Rift, Egypt

An integrated structural and stratigraphic study of the Hammam Faraun fault block, Suez Rift, Egypt, provides insights into the rift-initiation tectonostratigraphic evolution of the crustal-scale normal fault blocks. The shallow marine to offshore Tayiba Formation (Lower Oligocene) represents the youngest preserved pre-rift unit, and key stratal surface development indicates that relative sea-level variations exerted a marked control on its stratigraphic evolution. A major sea-level fall, which may have been a regional (i.e. eustatic) event, occurred during the mid-Oligocene and was synchronous with the onset of rifting. A major erosional unconformity (the base synrift unconformity) formed in response to the sea-level fall and defines a series of NNE–SSW- to NE–SW-trending palaeovalleys up to 40 m deep by 500 m wide, which are infilled by continental deposits and volcanic rocks of the Abu Zenima Formation (Upper Oligocene–Lower Miocene). During the rift initiation, palaeovalleys controlled depositional patterns and the evolving fault-controlled topography was insufficient to modify drainage patterns. Through time, however, surface-breaking faults began to exert a marked control on deposition. This study indicates the complexity that can occur during the rift-initiation phase caused by extrabasinal factors such as eustatic sea-level variations and antecedent drainage.

Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.

Paleobathymetry in the backstripping procedure: Correction for oxygenation effects on depth estimates

This paper aims to provide a straightforward and easily applicable method for estimating the depositional depth evolution of marine basins. Vertical movements of the basin floor can be reconstructed from the sedimentary record, and more accurately constrained when information from the sedimentary history is combined with palaeodepth estimates derived from fauna. To this end we propose to extend an existing method based on the percentage of planktonic foraminifera with respect to the total (planktonic and benthic) foraminiferal association, which is expressed as the percentage planktonics (% P). The ratio between planktonic and benthic foraminifera is related to water depth, and the % P generally increases with increasing distance to shore. However, next to water depth the oxygen level of bottom waters has a profound effect on the abundance of benthic foraminifera, and as such influences the % P. Depending on basin configuration, the oxygen level at the sea floor can vary on Milankovitch time scales and is reflected by the fraction of benthic foraminiferal species that indicate an effect of oxygen stress on the biotic system. These species can be used as stress-markers and their percentage with respect to the total benthic population is here expressed as % S. To assess whether the effect of sea-floor oxygenation impairs depth reconstructions, we studied the percentage of planktonic foraminifera (% P) in five well-dated sedimentary successions from the Lower Pliocene of Crete, Corfu and Milos in Greece. Additionally, we assessed whether different foraminiferal size fractions and counting methods affect the determination of the percentage of planktonic foraminifera. The palaeobathymetric evolution calculated for each basin was confirmed for all successions by an independent check on depth-related occurrences of benthic foraminifera. After correction for bathymetry changes of the basin due to sedimentation, compaction and eustatic sea level variations, the vertical movement history of the basin floor was inferred. We propose a standard methodology for reconstructions of palaeobathymetry of marine sedimentary successions from foraminiferal associations.

Geologic evolution and geodynamic controls of the Tertiary intramontane piggyback Meso-Hellenic basin, Greece

Abstract The Meso-Hellenic Basin (MHB) is a large, narrow and elongated basin containing up to c. 5 km of Cenozoic sediments, which partially covers the tectonic boundary between the external, western zones (Pindos) and the internal, eastern zones (Pelagonian) of the Hellenide fold-and-thrust belt. New results, based on micropaleontologic, sedimentologic and tectonic field data from the southern half of the MHB, suggest that the MHB originated as a forearc basin during the first stages of a subduction (Pindos basin), and evolved into a true piggyback basin as a result of the collision of thicker crustal units (Gavrovo-Tripolitsa). The late Eocene forearc stage is marked by sharply transgressive, deep sea turbiditic deposition on the subsiding active margin. At this stage, large scale structures of the Pelagonian basement (i.e. the newly defined “Pelagonian Indentor”) control deposition and location of two main subsiding sub-basins located on both sides of the MHB. The Eocene-Oligocene boundary corresponds to a brief tectonic inversion of the basin, at the onset of collision (main compressive event). The true piggyback stage (Oligo-Miocene) is recorded by slope deposition and dominated by gravity processes (from slumped, fine grained turbidites to conglomeratic fan- or Gilbert-deltas). The new elongated geometry of the MHB is controlled by the underthrusted, NNW-SSE trending, thick external zones. During this stage, the locus of subsidence migrates in the same direction (eastward) as underthrusting. This subsidence, favoured by thick dense ophiolitic basement, is attributed to basal tectonic erosion of the upper Pelagonian unit while the tectonic structures of this upper unit control the stepped migration of subsidence. Growing duplexes in the Gavrovo underthrusted unit, which formed local uplifts, were mainly situated on the eastern side of the subsiding areas and associated with normal faulting (late Oligocene–early Miocene). They constituted new loads that could also have been responsible for minor but widespread lithospheric subsidence. The development of the local and regional uplifts explains the basin evolution toward shallow, dominantly conglomeratic deposits and its final emergence at the end of the middle Miocene. This trend toward emersion is emphasized by the late Miocene global sea-level fall. The MHB was subsequently overprinted by neotectonic deformation associated with the development of a continental basin (Ptolemais) and uplift attributed to the evolution of the Olympos structure that developed further east as the underthrusting moved in this direction. These results demonstrate that the Meso-Hellenic Basin evolves as a large scale piggyback Basin and that its sedimentary infill is largely controled by tectonic activity rather than only eustatic sea-level variations.

Tectonic control on coarse-grained foreland-basin sequences: An example from the Cordilleran foreland basin, Utah

Newly released reflection seismic and borehole data, combined with sedimentological, provenance, and biostratigraphic data from Upper Cretaceous–Paleocene strata in the proximal part of the Cordilleran foreland-basin system in Utah, establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin. During Campanian time, coarse-grained sand and gravel were derived from the internally shortening Charleston-Nebo salient of the Sevier thrust belt. A rapid, regional Campanian progradational event in the distal foreland basin (>200 km from the thrust belt in <8 m.y.) can be tied directly to active thrust-generated growth structures and an influx of quartzose detritus derived from the Charleston-Nebo salient. Eustatic sea-level variation exerted a minimal role in sequence progradation.

The interplay between true eustatic sea-level changes, tectonics, and climatic changes: what is the dominating factor in sequence formation of the Upper Oligocene–Miocene succession in the eastern North Sea Basin, Denmark?

The Upper Oligocene–Miocene succession of the eastern North Sea is subdivided into six sequences on the basis of an integrated study of outcrops, boreholes, and seismic data. All available data and methodologies/criteria for the definition of sequences have been used. Datings of the sequences are based on palynology, micropalaeontology, and macropalaeontolgy, which give them a high confidence. The six sequences defined in this study do not correlate to the eight marked changes in true eustatic sea-level variation as indicated from oxygen isotope curves nor to the nine glacial maxima that have been recognised within the Miocene. The development of the individual sequences and the stacking pattern of the sequences is dependent both on true eustatic sea-level changes, relative sea-level changes (tectonics), and sediment supply. For example, a true eustatic sea-level fall as well as uplift may result in major progradation of a siliciclastic wedge. In the Miocene succession studied here, a distinct progradation in the lower Aquitanian was the result of a true eustatic sea-level fall. This resulted in deposition of a clean and widespread sand-rich clastic wedge. Marked progradation associated with an uplift of the hinterland in the Langhian resulted in sand-rich deposits alternating with coals. The interbedded coal seams were the result of a rising ground water table within the basin due partly to stable or even rising sea level and partly to local subsidence around faults. Similarly, a major transgression may be a result of a true sea-level rise or accelerated subsidence of a basin. In the case of the eastern North Sea Basin, accelerated subsidence was responsible for the major transgression in the Serravallian and Tortonian times, which is a period with a general climatic cooling. The frequency of true eustatic sea-level changes and relative sea-level changes (tectonic pulses) in the Miocene is of the order of 2.9–2.0 to 4 m.y., respectively. Both these intervals are within the time range of the studied sequences, which span 2–4 m.y. Consequently, but not surprisingly, neither of the two processes responsible for changes in relative sea level can be solely attributed to glaciations nor to tectonics. The climatic changes during the Late Oligocene–Miocene, which changed from cool temperate to subtropical with several reversals, did not influence the sediment yield.

ABSTRACT: Three-Dimensional Numerical Modelling of Eustatic vs Local Controls on Depositional Sequences

A three-dimensional numerical model of sediment transport and deposition in coarsegrained deltas is used to investigate the controls on depositional sequence variability. We consider the stratigraphic response to eustatic sea-level (amplitude and rates) and local controls, such as physiography, sediment supply and local subsidence. The development of key stratal surfaces, stratal geometry and facies stacking patterns show a systematic development with respect to stages in a eustatic sea-level cycle. However, even with constant sediment supply and a simple sinusoidal eustatic sea-level variation many of the features developed in the models are, by their nature, three-dimensional and twodimensional analysis of the model results can lead to erroneous interpretations of the causes of along-strike variability. Local controls generate significant variability in depositional sequence development, including the timing and amount of fluvial incision during relative sea-level fall, and the timing and nature of transgression during rapid phases of relative sea-level rise. The model results suggest that different systems tracts may be coeval and that key stratal surfaces defining and subdividing depositional sequences may be of local extent. Furthermore, the results highlight pitfalls in sequence stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.

The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara)

Palynomorph assemblages, especially chitinozoans and acritarchs, from the Upper Ordovician of well Nl-2 (north-east of the Algerian Sahara) are studied in order to precisely date the ultimate effects of the Late Ordovician glaciation and to document the impact of this major climatic stress on the diversity of the palynoplankton. References are made to stable isotope excursions and to global eustatic sea level variations in order to improve the local age determination. The Hassi el Hadjar Formation, i.e. glacio-marine diamictites, is interpreted as a transgressive event resulting from the melting of the northern Gondwana ice cap. It yields poorly preserved and moderately diverse chitinozoans of late Hirnantian age. Acritarchs are more abundant in the lower part of these “microconglomeratic clays”, but display a low diversity and are badly preserved throughout the whole formation. Reworked individuals are recorded in both groups. The marine sediments of the M'Kratta Formation of latest Hirnantian age contain better preserved, more abundant and more diverse palynomorph assemblages, especially in the Upper Member. The composition of this palynoplankton indicates a fairly good faunal and phytoplankton recovery after the early Hirnantian climatic stress. The extinction of the Ordovician forms, and the appearance of Silurian type taxa occur only in the uppermost Hirnantian, i.e. following with a slight delay the glacial event. The overlying black shales of Wenlock age (lower part of the Oued Mehaiguène Formation) are indicative of marine anoxic environments. They yield a virtually exclusive, but enormous number of Tasmanacea. The very peculiar composition of this palynoplankton seems to be independent of the Late Ordovician glaciation and is most likely related to the factors that, later, generated and maintained anoxic conditions in this area. A new species of chitinozoan, Spinachitina oulebsiri sp. nov. from the latest Hirnantian M'Kratta Formation, is described and illustrated.

The post-Triassic evolution of the Sorgenfrei–Tornquist Zone — results from thermo-mechanical modelling

The Sorgenfrei–Tornquist Zone (STZ) is part of the Fennoscandian Border Zone separating the Danish Basin from the Fennoscandian Shield. The STZ as a structural element is of Palaeozoic origin and represents the north-westerly segment of the Tornquist–Teisseyre Zone, which separates the younger West European crust from the older East European Platform and extends from the Black Sea to the eastern North Sea area. The STZ was reactivated in Triassic–Jurassic extension and Late Cretaceous and Paleogene compression.This paper investigates the regional geological consequences of the reactivations by quantitative modelling along a profile across the STZ in the Danish area. The numerical model invokes elastic, viscous and plastic deformations of the lithosphere as well as surface processes governed by erosion, sedimentation and lateral transport under the influence of eustatic sea level variations and regional isostatic compensation. Surface processes and lithospheric mechanics are coupled through thermal blanketing effects and loading.The results, in general, address the regional geological consequences of the existence of intracontinental zones of structural weakness. More specifically the results show that the Late Cretaceous and Paleogene chalk depocentres in the Danish Basin are a direct consequence of the inversion of the STZ, and that the STZ inversion together with falling sea level in Cenozoic time are amongst the principal controlling factors in the geological evolution in the eastern North Sea area.

Crenistria event and Asbian-Brigantian transition in the South Portuguese zone-sea level control on a hemipelagic late dinantian platform

The basal Vale Figueira Member, upper Murracao Formation (Late Visean), is a shaly succession with intercalated limestone beds bearing a mostly pelagic fauna. It has been deposited on an open marine, deep platform in the most external part of the South Portuguese Zone. Striking lithofacies analogies to the time-equivalent Kieselige Ubergangsschichten from the starved basin realm of the Rhenohercynian Zone of Germany indicate eustatic sea level variations as the predominant common factor controlling lithofacies development. The obviously glacioeustatic controlled late Asbian third-order transgression and the succeeding basal Brigantian regression can be elucidated by a sequence stratigraphic approach. The impact of the transgression and associated paleoclimatical and paleooceanographical processes in the deeper water environments are seen by repeated oxygen starvation. Maximum flooding is recorded in the unique deposition of a thin, ammonoid-rich, auttchthonous deep-water limestone, thecrenistria horizon. Several interplaying factors of micrite formation and pelagic mass mortalities are discussed. This event, calledcrenistria event is recorded in different deeper water facies all across the Rhenohercynian Zone in Portugal, southwestern England and Germany, and in adjoining parts of the Belgian Carboniferous Limestone platform. Homologous limestones most probably mark further maximum flooding horizons in Devonian and Carboniferous deeper water settings.

The tectonosedimentary evolution of the northern margin of the Carboniferous foreland basin of NE Germany

The geometry and evolution of the northern margin of the Carboniferous foreland basin in NE Germany was examined in terms of depositional environments, petrographic and geochemical data. Lower Carboniferous units are carbonate-rich and predominantly marine. By Namurian times continental environments are established along the northern basin margin with turbiditic sedimentation persisting in the south. The Upper Carboniferous succession is continental and clastic. The sandstones were derived from a quartz-rich source, located to the north, with subordinate sedimentary input. Basin evolution and the development of a characteristic marine-to-continental sedimentary succession, while being predominantly controlled by flexure induced by the Variscan Orogen to the south, was also influenced by eustatic sea-level variations and more localised tectonic movements.

Effects of variations in fault slip rate on sequence stratigraphy in fan deltas: Insights from numerical modeling

Results from a newly developed numerical model incorporating tectonic subsidence, eustasy, and sedimentation are presented for rift-basin settings. In particular, we simulate contemporaneous fan-delta deposition, subsidence, and eustatic sea-level variations to investigate the three-dimensional nature of depositional sequences in the hanging wall of a normal fault. A random-walk algorithm is used for sediment delivery from source to shoreline, together with a nonlinear three-dimensional diffusion equation for sediment movement on steep delta foresets. Three sediment sources are modeled; they are located along strike in areas of low, medium, and high subsidence. Rates of hanging-wall subsidence, eustatic sea-level variation, and sediment supply are derived from studies of active extensional basins. The resulting stratigraphy contains three fan deltas with distinct morphologies and internal stacking patterns. Sequence boundaries and offlap (forced regression) are restricted to areas of low subsidence, toward the fault tip; aggradationally stacked deltaic wedges are along strike where subsidence rates are high, near the fault center. The simulated geometries compare well to natural examples and highlight along-strike sequence variability and the importance of local controls in such settings.

Glaciation-induced variations in the Earth's precession frequency, obliquity and insolation over the last 2.6 Ma

Summary We compute, using many-body orbital integration routines, perturbations in the Earth's orbital elements (e.g. precession frequency, obliquity) and solar insolation associated with predicted, glaciation-influenced, time-series of the precession constant H(t). The time-series are computed, in an a priori calculation, using spherically symmetric, self-gravitating, Maxwell viscoelastic earth models, gravitationally self-consistent ocean loads and two ice models constrained to yield eustatic sea-level variations that match the observed fluctuations in oxygen isostope records over the last 0.8 and 2.6 Ma. We also perform a series of orbital integrations that assume a time-independent perturbation to the precession constant, relative to the present-day value, for periods extending over the last 12 Ma. Recent analyses have suggested that the relationship between the precession constant and precession frequency may depart significantly from linearity in the event that glaciation-induced perturbations to H reduce it by more than 0.15 per cent from its present-day value. We confirm this suggestion; however, we also show that the glaciation-induced perturbations to the dynamic ellipticity of the planet must remain at this level for more than 6 Ma in order for the non-linearity, which is linked to an external forcing associated with the orbits of Jupiter and Saturn, to have a significant effect on the precession frequency. In the case of integrations extending over only the last 2.6 Ma we have parametrized our main results in terms of a simple linear scaling with the logarithm of lower-mantle viscosity (vLM). In the range 2 times 1021≤vLM≤ 1023 Pa s, and assuming the form a+b(log vLM - 21.3), the constants (a, b) for the mean relative perturbation to the precession constant, and the perturbations to the precession frequency and the frequency of the obliquity variation are (relative to the present-day values): (-0.0003, -0.86), (-0.01 ′′yr-1, -0,05′′ yr-1) and (-0.01′′ yr-1, -0.05′′ yr-1) The rate of change of the temporal shift between our prediction of the mean daily solar insolation at a high-latitude geographic site, relative to the nominal solution, maybe parametrized using the same form with (-0.1 ka Ma-1, -0.8 ka Ma-1). We also present preliminary results for the influence of ‘climatic friction’ on the secular trend in the obliquity over the last 2.6 Ma. We find that this rate may be as high as 0.06° Ma-1. However, plausibly small values of the lag time between insolation variations and ice-sheet growth reduce this trend significantly. As a final point, predictions of glaciation-induced perturbations to the precession constant described herein are consistent with those presented in our earlier, more limited, analyses (Mitrovica, Pan & Forte 1994; Mitrovica & Forte 1995), but they disagree with subsequent predictions by Peltier & Jiang (1994) and Jiang & Peltier (1996). We show that the latter set of predictions (and the conclusions based upon them) are in error.

Ambrus Beds - Important Key for Interpretation of Neocomian Paleogeography, Sea-Level Changes, depositional Setting and Tectonics in Suha Krajina Area (Slovenia)

The lower part of the Lower Cretaceous stratigraphie sequence at Ambrus, composed of limestones, dolomitic limestones, dolomites, intertidal breccias and overlying heterogeneous carbonate breccias is described in this paper On the basis of stratigraphie order, fossil contents as well as the well-determined fossiliferous underlying Upper Malm and overlying Barremian carbonate sediments, the for­ mation denominated the Ambrus beds was ascribed to the Neocomian. Fenestral limestones, laminated limestones and dolomites containing stromatolites, mudcracks and numerous erosion and oxidation surfaces are interpreted as tidal storm deposits. The shallow water sediments, the breccias and the subaerial exposures, evidenced by relatively common karstification phenomena, are considered to be connected with eustatic sea-level variations and epeirogenetic movements. The lithology and areal distribution of these deposits indicate a sedimentation in a rel­ atively symmetric, rather shallow lagoon. The breccias composed of various poorly sorted angular and subangular limestone and dolomite fragments, as well as of calcific and dolomitic cement, have been attributed to tidal and cliff breccias. The majority of the breccia fragments indicate a pretty shallow water and subaerial sedimentation conditions.

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

Silurian Tectonics of Western Avalonia: Strain-Corrected Subsidence History of the Arisaig Group, Nova Scotia

The Arisaig Group is a Silurian succession of predominantly shallow marine clastic sediments overlying volcanics, exposed in northern mainland Nova Scotia. Sediment accumulation provides a record of the subsidence of western Avalonia during the interval when terranes were being accreted within the Canadian Appalachians. To calculate the amount of subsidence, one must correct the measured thicknesses for the effects of tectonic strain. Deformed fossils on bedding surfaces indicate strain ratios mainly between 1.2 and 1.6. An empirical porosity-depth relationship is used to correct for compaction. The subsidence curves are then adjusted to allow for variations in water depth and eustatic sea level. The resulting curves show significant variations in subsidence rate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian time, with deeper-water sedimentation during an early Ludlovian eustatic high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidence rate occurred during deposition of the Pridolian Stonehouse Formation. This episode was accompanied by a rapid increase in sediment supply, and a change in paleocurrent flow from southwest to northwest. The rapid Pridolian subsidence probably resulted from oblique collision between Avalonia and the Meguma Terrane to the south.

Global and local controls influencing the deposition of the La Luna Formation (Cenomanian-Campanian), western Venezuela

Bulk and molecular geochemical, micropalaeontological, and carbon-isotopic data are used to address the different local and global factors influencing the environment of sedimentation of the La Luna Formation (Cenomanian-Campanian, approximate palaeolatitude 15°N) in a single section in western Venezuela. Based on the constructed chronostratigraphic framework, oxygen-depleted bottom-water conditions and black-shale deposition started in western Venezuela well before the widespread occurrence of organic-rich sediments in higher palaeolatitude regions such as the Tethys and the North Atlantic near or at the Cenomanian-Turonian boundary. In the La Luna Formation, palaeoenvironmental conditions that allowed the preservation of organic matter (mainly of marine origin), prevailed until Santonian times in a distal platform facies with very low siliciclastic input. Changes in lithology appear to reflect the local response to eustatic sea-level variations and the presence of a migrating upwelling belt affecting the bioproductivity of silica and carbonate. A marked δ 13C org isotopic excursion is recognised in the middle part of the section, and is apparently unrelated to local palaeoenvironmental changes in bioproductivity and oxygen depletion. Biological marker data show no variations in association with the isotopic excursion, being mainly controlled by local fluctuations in organic-matter input and preservation.

Phylogenetic evolution and biogeography of Southeast Asian shrews (genusCrocidura: Soricidae)

Genetic variation of 20 species of shrews from the Malay peninsula, Sumatra, Java, Borneo, the Philippines and Sulawesi was assessed by allozyme electrophoresis at 32 loci. According to Mantel's tests, the genetic differentiation of these species of shrews is not a function of the geographic distance separating them (r = 0·09, NS), but is correlated to the water depth surrounding the islands where they live (r = 0·49,P < 0·01). The results are just the reverse if the correlations are computed for the Sunda Shelf taxa only. In this case, the sampled populations show an isolation-by-distance relationship (r = 0·32;P < 0·01), while no significant correlation with water depth was detected (r = 0·20;P = 0·07). Qualitative predictions based on eustatic sea level variation and water depth were formulated as a model of historic connections between the islands. This palaeogeographic model was tested through Brooks Parsimony Analysis. The assumption of a simple vicariant evolution of the shrews was rejected, but several concordant patterns indicate that the phylogeny of these mammals was indeed shaped by these events. Homoplasies demonstrated that the SE Asian species ofCrocidurainclude composite zoogeographic histories. Sulawesi, for example, supports at least six species, five of which are closely related, while the last one,C. nigripes, is more closely related to a Bornean taxon. This pattern was interpreted as the result of a first wave of colonizers which subsequently radiated, followed by a more recent, second colonization event from Borneo. The overall small genetic distance found within the assemblage of the five old endemics (DN = 0·151 ± 0·041) suggests that the radiation was not accompanied by extensive differentiation, although from a karyological point of view, they exhibit unusual variations when compared to other IndomalayanCrocidura. By contrast, the four species found on Sumatra are more differentiated (DN = 0·221 ± 0·063) and never form sister-group relationships in any phylogenetic reconstruction; each one is more closely related to different taxa living outside Sumatra. This suggests that they are probably remnants of an important centre of dispersal for the entire Malay Archipelago. The standard genetic distance averaged among all Southeast Asian species (DN = 0·235 ± 0·094) is about half that measured within Palearctic or African taxa. Such an overall lower mean level of genetic variability is consistent with the hypothesis of a relatively recent colonization of the Malay Archipelago by shrews of the genusCrocidura.

Phylogenetic evolution and biogeography of Southeast Asian shrews (genus Crocidura: Soricidae)

Genetic variation of 20 species of shrews from the Malay peninsula, Sumatra, Java, Borneo, the Philippines and Sulawesi was assessed by allozyme electrophoresis at 32 loci. According to Mantel's tests, the genetic differentiation of these species of shrews is not a function of the geographic distance separating them (r = 0·09, NS), but is correlated to the water depth surrounding the islands where they live (r = 0·49,P < 0·01). The results are just the reverse if the correlations are computed for the Sunda Shelf taxa only. In this case, the sampled populations show an isolation-by-distance relationship (r = 0·32;P < 0·01), while no significant correlation with water depth was detected (r = 0·20;P = 0·07). Qualitative predictions based on eustatic sea level variation and water depth were formulated as a model of historic connections between the islands. This palaeogeographic model was tested through Brooks Parsimony Analysis. The assumption of a simple vicariant evolution of the shrews was rejected, but several concordant patterns indicate that the phylogeny of these mammals was indeed shaped by these events. Homoplasies demonstrated that the SE Asian species ofCrocidurainclude composite zoogeographic histories. Sulawesi, for example, supports at least six species, five of which are closely related, while the last one,C. nigripes, is more closely related to a Bornean taxon. This pattern was interpreted as the result of a first wave of colonizers which subsequently radiated, followed by a more recent, second colonization event from Borneo. The overall small genetic distance found within the assemblage of the five old endemics (DN = 0·151 ± 0·041) suggests that the radiation was not accompanied by extensive differentiation, although from a karyological point of view, they exhibit unusual variations when compared to other IndomalayanCrocidura. By contrast, the four species found on Sumatra are more differentiated (DN = 0·221 ± 0·063) and never form sister-group relationships in any phylogenetic reconstruction; each one is more closely related to different taxa living outside Sumatra. This suggests that they are probably remnants of an important centre of dispersal for the entire Malay Archipelago. The standard genetic distance averaged among all Southeast Asian species (DN = 0·235 ± 0·094) is about half that measured within Palearctic or African taxa. Such an overall lower mean level of genetic variability is consistent with the hypothesis of a relatively recent colonization of the Malay Archipelago by shrews of the genusCrocidura.