Paleogeographic Changes Research Articles

Some pathways and factors of monogenean microevolution

This paper discusses probable causes for the origin and evolution of groups of monogenean congeneric sibling species infecting one fish host species, with special reference to freshwater fishes. These causes comprise strict specificity, topological differentiation of parasite micropopulations in microniches (microhabitats), ecological and geographic isolation of various parasite and host populations, paleogeographic or geomorphologic (but not paleogeologic) changes during the late Tertiary and Quaternary periods, with alternating marine transgressions and regressions acting upon continents which, after the Miocene, have been less affected by continental drift, orogenesis, and climatic fluctuations. Co-evolving with their hosts, Monogenea usually diverge faster than their hosts.

A time-slice approach to the history of Cenozoic sedimentation in the Indian Ocean

Results from drilling at over 100 sites have been combined with a new plate tectonic model to produce a series of palinspastic maps which represent “snap-shots” of the changing patterns of sediment accumulation in the Indian Ocean through the Cenozoic. The maps clearly delineate the main sedimentary depocenters and show how the sedimentary record has responded on an oceanwide scale to major changes in paleogeography (tectonics), paleocirculation and/or climate. Paleogene sediment accumulation rates were generally low, with significant accumulation in only limited areas, suggesting weak ocean circulation and stable, well-stratified conditions. By contrast, the vigorous thermohaline circulation of the Neogene resulted in substantial widespread sedimentation. Accumulation rates were generally high and displayed a wide range of values. Following rapid growth of the East Antarctic ice sheet in the Mid-Miocene, carbonate sedimentation has been progressively restricted to the shallow ridges and plateaus of the western Indian Ocean, but accumulates there with locally high rates. The link between tectonics and terrigenous sedimentation is exemplified by the major deep-sea fans off East Africa (Zambezi fan) and India (Indus and Bengal fans) which developed in response to uplift and erosion beginning in Late Oligocene time in the East African Rift System and the Himalayas, respectively. Although maps from only five widely spaced times are shown, the methodology is such that a continuous series of maps, at intervals as close as 1 m.y. can readily be generated. Ultimately the combination of such time-slice maps with the results of detailed time series studies will provide a powerful tool for understanding past oceanic environments.

Deglaciation, Earth crustal behaviour and sea-level changes in the determination of insularity: a perspective from Ireland

Abstract Insularity is defined here in terms of the presence or absence of land-bridge connections between adjacent landmasses. The development of a land-bridge depends upon the existence of a special combination of environmental conditions. These are created through the interaction of climate, sea-level and other physical environmental controls, together with changes in palaeogeography. Such factors operate together to produce conditions suitable for a land-bridge over only relatively short time-scales (10 to 100 years). New palaeoenvironmental data about such factors have emerged in recent years from the Irish Sea and from neighbouring regions. These data, derived from a variety of disciplines, have contributed to earlier discussions about the existence here of land-bridge links in the Late Quaternary. Some of this information, particularly that concerning patterns of Earth crustal behaviour, former ice distributions, deglacial history and sea-level changes, is reviewed. An assessment is given of their significance for possible land-bridge operation at this time.

The evolution of biodiversity among the Southwest European Neogene rodent (Mammalia, Rodentia) communities: pattern and process of diversification and extinction

The history of Southwest European Neogene rodent communities is analyzed in light of species diversity, immigration and extinction rates. The distribution in time of the 285 extinct species indicates that species-diversity changes occurred in parallel with the oxygen isotope marine curves and suggests a relationship between oceanic mean temperature and terrestrial mammals species diversity. Extinction rates also seem to coincide with environmental changes observed in the oceans. Three high extinction-rate periods can be recognized, at 15 Ma and between 3.3 and 2.4 Ma. Nested polycohort survival curves indicate an overall constant extinction rate interrupted by sudden bursts of higher extinction rates, which correspond to the periods of high extinction rates previously identified. Polycohort analysis indicates an increasing extinction rate through time, which corresponds also to a decreasing mean value of species duration, tentatively attributed to an increasing frequency of climatic changes. The survival curve for all species during the entire time interval is linear, demonstrating a constant extinction rate. Increased extinction rates during some short time intervals therefore do not effect the overall constant extinction rate and do not modify the prediction of the “Red Queen” hypothesis. This constancy of the extinction rate is used to recalibrate the biochronologic time scale. The results cannot be rejected on the basis of the available data, and represent a robust test for the hypothesis of constant extinction rate in mammalian communities. Bursts of increasing boodiversity correlated mainly with paleogeographic changes. The “proboscidean datum” at 18 Ma corresponds to a major faunal dispersion from the Middle East and Central Asia toward Western Europe, initiated by the closure of the Tethys sea. A second faunal dispersion from a Northest African centre across the Western Mediterrenean occurred between 7 and 6.5 Ma The third main event of increased diversity is contemporaneous with the Early Pliocene marine transgression. During Middle and Late Pliocene, the rodent faunas show a high turn-over rate, which is clearly related to the onset and strengthening of the glacial-interglacial cycles of the Northern Hemisphere. More detailed analysis and evaluation of the time-lag effects would need an increased precision in the calibration of the biochronological scales that are currently used.

Changing patterns of land use by Plio-Pleistocene hominids in the Lake Turkana Basin

The first step in a behavioural ecological study of stone-tool-using hominids involves the description of the character of lithic discard and the context within which the discard occurred. We examine and put into paleogeographic context the known archaeological traces in the Turkana Basin at three successive time intervals: 2·3 million years ago (Ma), 1·9-1·8 Ma, and 1·-7-1·5 Ma. At 2·3 Ma, hominid use of stone appears restricted to small areas on the landscape where many resources such as water, shade, and stone are juxtaposed. In contrast, archaeological traces at 1·6 Ma are found in a variety of settings, which may in part be explained by the paleogeographic changes taking place at that time. This change coincides with the emergence of Homo erectus. The hominid fossil and archaeological records are shown to complement each other in the generation of ecological hypotheses of H. erectus behaviour.

The Sedimentary Sequence and Palaeogeographic Changes of the South Yellow Sea Since the Olauvai Subchron1

Abstract According to the alternation of terrestrial beds and transgressive beds, the sedimentary sequence of the South Yellow Sea since the Olduvai subchron can be divided into 15 strdatigraphical intervals. According to the facies analysis, there are 7 transgressive beds an one bed with transgression marks, and classical transgressive‐regressive facies sequence also appears in the major transgressive beds.Palaeogeographic evolution in the shelf area involved 4 periods: (1) 1.70‐0.5 Ma B. P. was the stage for the development of the Palaeo‐Yangtze River alluvial plain and delta. Tide influenced‐river channel sediments formed during the Olduvai have been found in borehole QC2. The river mouth was located near 124° E and during the transgressive stage from 0.97‐0.73 Ma B.P. to the east of 122.2° E, a large‐scale undersea delta was formed. In the regressive stage, the shelf area became an alluvial plain. (2) 0.50‐0.75 Ma B.P. was a multi‐transgressive fluctuation stage during which 3 transgressive beds (HVI, HV, HIV)developed, being dated as 0.50‐0.30 Ma B.P., 0.27‐0.20 Ma B.P. and 127‐75 Ka B.P. respectively, with the extent of transgression increasing gradually. Influenced by southeast monsoons, the eastern China continent began to have a larger moisture source. (3) 75–14 Ka B.P. was the lower sea‐level stage during the last glacial period. The downcuttihg depth of rivers was up to ‐133 m. The palaeo‐Yangtze River system flowed northeastwards into the Sea of Japan through the Sea of Japan through the Tsushima and Korean Straits. The interfluvial area became an eroded, weathered high land. In the cores, two short‐lived, relatively high sea‐level stages were recorded, corresponding to two interstades in the last glacial period. (4) The period since 14 Ka B.P. is the stage for “Holocene transgression” and the development of the Yellow River‐Huaihe River delta. Before 14 Ka the sea level rose to ‐68 m, and before 11 Ka to ‐50 m, and the sea‐level was once at a standstill or slightly got down, which is coincident with the Younger Dryas event. The Yellow River‐Huaihe River delta developed on transgressive bed HI and consists of the superimpositions of 6 deltaic complexes.

Paleobiological and sedimentological evidence of Pleistocene/Holocene hiatuses and ironstone formation at the Pontian islands shelfbreak (Italy)

Iron-rich hardgrounds were grabbed near the shelfbreak NE Ventotene and SW Ponza, at water depths of 119 and 141 m, respectively. These 1 to 3 cm-thick ironstone layers formed at about the Pleistocene/Holocene boundary during rapid sea-level rise, which began about 16,000 years ago and within about 8000 years brought the bottom of the shelf from a water depth of less than 30–100 m or more. A rapid vertical transition from circalittoral deposits characterized by a coastal detritic community with calcareous algae to ironstones rich in iron-managanese pisoids or glaucony and containing planktonic foraminifera, siliceous sponge spicules and echinoderm remains, indicates sediment starvation during rapid drowning of the shelf. The crisis of the sedimentary system and the local ironstone formation at sites of favourable morphology were triggered by major paleogeographic changes during final deglaciation at the close of the Pleistocene, leading to modified circulation patterns and chemistry of sea-water, reduced carbonate productivity below the photic zone, and strong activity of northward-directed Tyrrhenian currents intruding onto the outer shelf. Abundance of organic matter transported from the drowned volcanic islands and diminished oxygenation, due to both increasing water-depth and sluggish vertical circulation, were responsible for local reducing micro-environments, as testified by the formation of pyrite, leaching of carbonate bioclasts, and growth of microbial mats. Offshore sedimentation did not resume until the last 2000–4000 years, when the deeper-water and finer-grained Ponza glauconitic ironstone was covered by a thin veneer of silty clay deposited on the uppermost slope, beneath the mudline. The shallower and coarser-grained Ventotene winnowed pisoidal hardground, instead, is probably still forming today under the action of strong northward directed marine currents sweeping the shelfbreak free of sediment.

The genus Czekanowskia Heer: principles of systematics, range in space and time

Study of a unique collection of leaf phytoleims (or compressions) of the genus Czekanowskia from more than 170 localities from the Triassic, Jurassic and Lower Cretaceous of northern Eurasia revelas that species of this genus cannot be identified by morphological characters of leaves. It is suggested that epidermal characteristics should be used for infrageneric systematics. Characteristics of the following taxonomical levels, namely class (order), genus, subgenus and finally species are distinguished. Within the genus Czekanowskia 3 subgenera ( Czekanowskia, Harrisella and Vachrameevia) and 72 species have been identified by epidermal structures by the year 1991. Each of the species existed during 1–2 geological stages. The distribution of the genus Czekanowskia was restricted to the Northern Hemisphere and its life history was very dynamic. During successive epochs its distribution was dependent on climatic and paleogeographic changes; these changes were accompanied by migration and speciation of plants. The first undoubted leaf remains of Czekanowskia have been found in Upper Triassic deposits (the Norian-Rhaetic). The systematic diversity of the genus increased gradually from the end of the Triassic up to the Middle Jurassic, then it decreased in a similar manner throughout the Late Jurassic and the Early Cretaceous; at the beginning of the Late Cretaceous, the genus vanished. In the history of the subgenus Czekanowskia two explosive speciations can be noted, in the Middle Jurassic and the Early Cretaceous, which coincide with climatic optima.

The Location of Relics and Paleogeographical Changes in the Northeastern Part of the Osaka Plain

In this paper, author studied the conditions of location of the Suna Site and deduced the geomorphological environment of those days by restoring the environment with geomorphic and geological methods and by tracing paleogeographical changes. Based on the results, the author concludes that the microforms in this area can be classified into 8 units. A geomorphological map (Fig. 2) was drawn based on the results.The Suna Site is located on the center part of an alluyial fan-like lowland. The surface of the lowland forms an incline of 8.5‰ toward the west. The height of the surface varies from 2 to 14 meters above sea level. This means that not the alluvial deposits of the Yodo and the Neya Rivers but those of the small rivers entrenching terraces and hills to the east formed the lowland. The extensive distribution of the Jori grid-system partition is a characteristic found on this surface. The surface rises to a relative height of 2 meters from the lowland of the Yodo and forms a terrace.According to the observation of archeologically excavated sections, the Suna Site and its vicinity were in a marshy environment with deposits of humus, silt and clay before the late Jomon Period (3, 000∼2, 300y.B.P.). The westerly inclination of layers of early and late Kofun Preriod indicates that deposits were supplied from the Ikoma Mountains, and the alluvial fan-like lowland was expanded to the westward at that time. But the surface of the coarse deposits from the Ikoma Mountains and that of the marshy deposits of silt interfinger each other. This indicates that the Suna Site was located near the end of the alluvial fan-like lowland of those days. The content of sand and gravel within the deposits increased remarkably in the late Kofun Period. This fact shows that the alluvial fan-like lowland expanded again in this period. However, from the Middle Ages onward, there was little accumulation of deposits.By pollen analyses, much pollen of broad-leaved trees such as Cyclobalanosis were detected from the black silt layer of the late Jomon Period. However, not so much pollen of pinus nor that of herbaceous plants were detected from the layer (Fig. 7). It seems that the forest around the Suna Site was not badly damaged at that time. And it seems that this area was situated in the Warm Temperate Zone, because vegetation in this area was dominated by Cyclobalanosis and Quercus. For lack of deposits, the author has not deduced the vegetational history of this area in the Yayoi Period (2, 300∼ 1, 700y.B.P.). Some pollen of Trapa and Typhaceae were detected from the dark gray sandy silt layer of early Kofun Period by pollen analysis. It seems that the ground water table rose. And abundant pollen of Cryptomeria and Chanaecyparis were detected from the dark grey silt layer with pebbles from the late Kofun Period. The author presumes that precipitation increased in this area. In addition, much pollen of Gramineae were detected from the deposits. This suggests the beginning of cultivation in this area.

Contourites in the Fram Strait

Multichannel seismic reflection profiles from the Fram Strait show many features that can be attributed to bottom-current influence on the sediments, by comparison with well-known contourite sequences elsewhere. A greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of Svalbard. The currents here have probably been controlled by local topography. We interpret sequences with abundant diffractions in the broader basins to be current-related, probably deposited during periods of intensified circulation. The continental slopes on the Svalbard side have fairly continuous sequences, whereas the younger sequences on the Greenland lower slope onlaps the lowermost one. This can be attributed to differences in current regime, sediment supply, or margin subsidence. The contourites of the lower slope in the northeastern part of the strait (about 1000 m water depth) can be explained by long-term influence of the West Spitsbergen Current, but current-influenced patterns down to 3000 m water depth are not compatible with the weak deep-water circulation observed at present. The oldest contourites west of Svalbard are estimated to be of Late Miocene to Pliocene age (between 10 and 3 Ma), based on correlation to the DSDP Site 344 and inferred basement ages near the present plate boundary. Hiatuses may be present beneath the contourites. The onset of currents may be related to opening of the gateway between Greenland and Svalbard, to climatic cooling, and to palaeogeographic changes outside the Fram Strait. Contourite deposition has continued through the late Neogene and Quaternary, but vertical changes in the pattern suggest that current intensities have decreased through time.

Global ocean—atmosphere change across the Precambrian—Cambrian transition

AbstractThe late Precambrian and Cambrian world experienced explosive evolution of the biosphere, including the development of biomineral skeletons, and notably of phosphate and siliceous skeletons in the initial stages of the adaptive radiation. Ongoing research indicates profound changes in climate and atmospheric carbon dioxide over this span of time. Glacial conditions of the Varangian epoch occur enigmatically at low latitudes, associated with carbonate rocks. Later changes in palaeogeography, sea level rise and salinity stratification encouraged prolonged ‘greenhouse’ conditions in both latest Precambrian and Cambrian times, with indications of relatively low primary production in the oceans. The Precambrian–Cambrian boundary interval punctuated this trend with evaporites, phosphogenic events and carbon isotope excursions; these suggest widespread eutrophication and conjectured removal of carbon dioxide from the atmosphere. Whatever the cause, nutrient–enriched conditions appear to have coincided with the development of phosphatic and siliceous skeletons among the earliest biomineralized invertebrates.

The faint young sun climatic paradox: A simulation with an interactive seasonal climate-sea ice model

A seasonal one-and-a-half-dimensional (1 1 2 -D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate the faint young sun climatic paradox. This model is shown to reproduce the present climate and sea ice observations. In spite of its greater complexity, its behavior is globally similar to simple energy-balance models with highly parameterized ice-albedo feedback used in previous studies of this question. It is found that when the solar luminosity drops by more than about 5% below its present value, the ice albedo feedback causes a global irreversible glaciation. Several sensitivity experiments show that the value of the critical solar constant and associated global surface temperature are only little sensitive to the set of model parameters describing the ice and snow albedo and meridional heat transport. In contrast, the absence or polar location of the continental mass introduce a nearly 10% decrease of the critical luminosity. The minimum level of atmospheric CO 2 needed to prevent a global glaciation through enhanced greenhouse warming is calculated as a function of the solar luminosity. A 30% drop in solar output requires a 2 × 10 4-fold increase in atmospheric CO 2, an unacceptably large value. However, in the absence of continents, a carbon dioxide partial pressure of 2000 times the present level is found to be sufficient to stabilize the climate. The effects of a reduced continental area, paleogeographic changes and higher CO 2 greenhouse effect combine to ensure a larger stability of the non-frozen configuration. Their cumulated and interactive effects may be able to solve the young sun paradox.

Paleobiogeography of Devonian vertebrates

Five distinctive vertebrate faunas characterised by endemic taxa can be recognised for the Early Devonian (Euramerica, Siberia, Tuva, China, and East Gondwana). By Late Devonian time these faunal provinces are obscured by widespread taxa which also inhabited nonmarine aquatic environments, but indicate faunal communication between Gondwana, Euramerica and China. This marked change in pattern between the Early and Late Devonian may be attributed to intrinsic (evolutionary) or extrinsic causal factors. Dispersal capabilities of aquatic vertebrates may have increased during the initial gnathostome radiation of the Devonian, but a predominantly extrinsic cause (e.g. global change in geography or climate) is suggested by the similar pattern for marine invertebrate faunas of Early Devonian endemism and Late Devonian cosmopolitanism. Outstanding problems of Devonian vertebrate biogeography include faunal differentiation on the largest landmass of the time (Gondwana), and the nature of barriers and connections between East and West Gondwana, East Gondwana and South and North China, and West Gondwana and Euramerica. A vertebrate equivalent of the cool-water Malvinokaffric invertebrate faunal realm of the Siluro-Devonian is not clearly identified, but vertebrate data from southern Africa and south America are sparse.Wide latitudinal distributions for some Late Devonian vertebrate taxa appear anomalous, and could indicate either reduced global climatic gradients, or erroneous paleogeographic base maps. There are difficulties in formulating a hypothesis of global warming and/or major paleogeographic change in a way which clearly distinguishes basic from interpreted data. Three major subdisciplines (paleomagnetism, paleoclimatology, paleobiogeography) contribute to Paleozoic paleogeographic reconstructions. Their data tend to be organised and represented in different ways, but each relies on the same principle of concordance with a general pattern (Young 1990). Degree of consilience of a hypothesis based on one data set (the extent to which it explains patterns within an unrelated set of data) is a primary criterion for accepting or rejecting the hypothesis. Apparent polar wander path representation facilitates testing of paleomagnetic data against those paleoclimatic or paleobiogeographic data which provide evidence of paleolatitude. However, as well as the simple indication of paleolatitude, biogeographic and some other qualitative data sets provide more complex evidence concerning connections or barriers between regions, for which APWP representation is not appropriate. Cladistic analysis of hierarchically organised data sets (Young, 1986, 1987) provides a means of integrating qualitative paleobiogeographic, paleoclimatic, and paleogeographic data such that inconsistencies in the evidence are emphasised, and the hypothesis is exposed to falsification. These ideas are illustrated using Devonian examples.

Processes affecting paleogeography, with examples from the Tethys

As global change becomes a greater concern to society, a more realistic procedure for interpretation of paleogeography is needed for a balanced view of Earth history. Paleogeography is extrinsically affected by the changes in heat regime of the Earth's interior, manifested by a chain reaction of tectonic processes on the Earth's surface. Tectonics largely controls oceanography, climate and hence glacio-eustasy, which are the other major forcing factors of paleogeographic change. Since the heat budget of the Earth has steadily declined for the last 4.5 Ga, rates of tectonic processes must have also changed and as a result, paleogeography of the geological past cannot be a mirror image of the present. Thus actualism and uniformitarianism are not applicable to paleogeographic interpretation of geological record when time scale exceeds tens or hundreds of millions years. Major paleogeographic changes can be correlated to tectonic changes of the Wilson cycle, as documented for the late Phanerozoic dispersion of Pangea. Seaway opening in response to shifts in plate position has left variable signals in deposited sediments, such as the termination of evaporite deposition and changes in ocean circulation, bottom water chemistry, sediment composition, and faunal proviciality. Other processes related to plate tectonics, like erratic latitudinal shift in the position of continental plates can lead to erroneous interpretations of climatic or oceanographic responses derived from sediments. Plate rotation under special conditions may result in crustal attenuation along one margin and compression along the other, giving rise to synchronous sea-level changes of opposite polarity. First-order eustatic sea-level cycles operating on a scale of 45–90 Ma in the Atlantic have a long-term controlling effect on paleogeography. These cycles are probably the result of cyclic changes in rate of mantle upwelling and conductive heat transmission through the lithosphere. Because of this dependancy on the Earth's heat budget, the length of such cycles should have progressively increased through time. If much more rapid paleogeographic changes resulting from third order Exxon sea-level cycles are caused by rapid stress-induced vertical motions of the lithosphere, then the influence of crustal and mantle inhomogeneities could explain different local and regional sea-level responses to plate tectonic. Application of the generalized, Mesozoic-Cenozoic, third order eustatic sea-level cycles of Vail and his colleagues on solutions of global geological problems can only be considered valid for divergent continental margins in areas under similar intraplate stress regime. Decreasing rates of fluctuation in intraplate stress on a cooling Earth have direct consequences for paleogeographic trends in the gelogical past.

The sedimentology of the Middle and Upper Border groups (Viséan) in the Stonehaugh Borehole, Northumberland

SUMMARY A sedimentological study has been made of the core from the Stonehaugh Borehole, which proved strata from the Viséan Middle and Upper Border groups of central Northumberland. Much of the Middle Border Group was deposited in a fluvially dominated delta, in interdistributary bays, minor mouth bars, crevasse splays and backswamps. However, a significant part of the group was deposited under more arid conditions in temporary lakes and on muddy floodplains. Sediments of the Upper Border Group were more tidally influenced, being deposited on tidal flats and as bay in-fills. All of these associations are locally cut by major fluvial and delta distributary channels. The transition from fluvial to tidally influenced sedimentation close to the Middle/Upper Border Group boundary is broadly coincident with a period of volcanism, footwall uplift and changing palaeogeography associated with the transition from extensional to thermal relaxation subsidence in the Northumberland Trough.

The mid-Cretaceous super plume, carbon dioxide, and global warming.

Carbon-dioxide releases associated with a mid-Cretaceous super plume and the emplacement of the Ontong-Java Plateau have been suggested as a principal cause of the mid-Cretaceous global warming. We developed a carbonate-silicate cycle model to quantify the possible climatic effects of these CO2 releases, utilizing four different formulations for the rate of silicate-rock weathering as a function of atmospheric CO2. We find that CO2 emissions resulting from super-plume tectonics could have produced atmospheric CO2 levels from 3.7 to 14.7 times the modern pre-industrial value of 285 ppm. Based on the temperature sensitivity to CO2 increases used in the weathering-rate formulations, this would cause a global warming of from 2.8 to 7.7 degrees C over today's global mean temperature. Altered continental positions and higher sea level may have been contributed about 4.8 degrees C to mid-Cretaceous warming. Thus, the combined effects of paleogeographic changes and super-plume related CO2 emissions could be in the range of 7.6 to 12.5 degrees C, within the 6 to 14 degrees C range previously estimated for mid-Cretaceous warming. CO2 releases from oceanic plateaus alone are unlikely to have been directly responsible for more than 20% of the mid-Cretaceous increase in atmospheric CO2.

Geochemical signature of the Cenomanian to Eocene rocks in Israel ‐a palaeoenvironmental indicator

AbstractDistinctive trace‐metal concentrations characterize Cenomanian to Eocene marine carbonates from Israel. The Cenomanian‐Turonian platform carbonates, including clayey formations, exhibit low average values ranging between 2 and 29 ppm for Zn, Cr, V, Ni, Cu, U and Co. The Santonian‐Campanian and Early to Middle Eocene marine chalks show higher average concentrations of these trace‐metals ranging between 3 and 56 ppm. The highest average concentrations of these metals (5–118 ppm) are found in the Maastrichtian and in the Palaeocene marine chalks and marls. The possible relationship between these metal background levels and the lithology, the biogenic productivity, the organic matter content, the iron oxide concentration, the rate of sedimentation of the studied time‐rock units as well as the palaeogeographical changes are discussed. The extent of the exposed palaeo‐landmasses due to tectonics, the intensity of weathering conditions and the detritus supply into the basin, control primarily the iron and trace‐metal content in the studied sediments.

Structure and seismic stratigraphy of the western Challenger Plateau

Abstract The Challenger Basin is a newly discovered sedimentary basin beneath the western margin of the Challenger Plateau. Seismic and gravity data near the site of two Deep Sea Drilling Project holes (DSDP 284 and 593) show more than 2 km of sediment in the basin, probably formed in the early stages of Late Cretaceous rifting in the Tasman Sea. Thinning of about 5 km occurred before separation took place to the southwest and stranded this portion of the rift system. Regional subsidence ensued in the early Cenozoic, followed by a major phase of submarine basaltic volcanism about 38 Ma ago. This volcanism may have been related to the onset of spreading southwest of New Zealand. Subsequent sedimentation has been virtually entirely pelagic. The Oligocene unconformity commonly observed in the New Zealand region was not found in the wells, presumably because of either oceanographic changes induced by tectonic events, or redeposition from adjacent highs. Changes in ocean current patterns and biogenic productivity caused by paleogeographic changes such as the separation of Australia and Antarctica and uplift of the Southern Alps produced unconformities identifiable on seismic reflection data.

The development of vegetation and climate during the miocene in the south-eastern sector of the Duero basin (Spain)

In recent years, the interest in micropaleontological studies of the Duero basin has increased. The present work offers a palynological analysis of Tertiary sediments of the south-eastern sector of this region. These sediments lie within the lithostratigraphic units defined by Armenteros (1986a) for the Miocene of the zone. Study of the pollen and spores found shows that the vegetation must have developed from an arid steppe landscape under a warm climate to one of open woodland and meadows under a seasonal Mediterranean climate. The transition from one stage to the other occurred more or less gradually, with an intermediate stage in which the flora almost disappeared caused by the paleogeographical changes taking place in the zone.

Development of a regional model for the Devensian Glaciation of County Durham for use in the assessment of sand and gravel resources

Abstract The sand and gravel resources of County Durham are mainly of Quaternary age, predominantly dating from the latter part of the Devensian stage of the Pleistocene and are the result of a complex pattern of glacial recession. A recent study predicted the distribution of drift deposits from a model based on published information, limited field observations and the analysis of approximately 6000 items of subsurface information; predominantly borehole records. This model, which describes the sequence of events during the Devensian glaciation of the area and infers palaeogeographical changes was an essential element in the assessment of the nature and distribution of granular Sediments and the prediction of mineral quality. The assessment identified 178 km 2 of potential resource, in both outcrop and subcrop, 53 km 2 of which had not been previously shown on published maps and plans.