MABP Research Articles

Lead, sulphur, oxygen and carbon isotopes in the Zletovo ore field (Eastern Macedonia)

Results of isotopic studies of lead, sulphur, oxygen and carbon in the main hydrothermal ore veins and stockwork-disseminated polymetallic ore formations within the Tertiary volcanics in the Zletovo ore field are exposed and discussed in the present paper. Ore metals and sulphur are of endogenous origin, most probably from the deeper parts of the Earth’s crust or/and the upper mantle. On the basis of the isotopic composition of oxygen and carbon it can be supposed that mixed underground meteoric waters influenced to a great extent the ore-bearing fluids. The volcanic activity and the deposition of the polymetallic mineralizations took place within the interval between 27 and 16 Ma BP, according to the K-Ar data obtained.

Towards an orbital time scale for chinese loess deposits

In this study, we analyzed the grain size of the Baoji loess-soil sequence at 10 cm intervals. Results show that grain size variations are very sensitive to loess-soil alternations, with loess units being much coarser than soils. Such a change in loess-soil grains is interpreted as the result of the glacial-interglacial cyclic variations in the intensity of the winter monsoon winds out of Siberia. The grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by Berger and Loutre (1991) under the control of major magnetic reversals. The tuning is independent of any correlation with δ 18O signals in the deep-sea sediments. The resulting grain size time scale is tightly constrained, as suggested by the following facts: (1) the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records; (2) ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the K Ar -dated ages; (3) there is close coherence between the Baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0–2.5 Ma period; and (4) the grain size record on the orbital time scale shows a close similarity to the orbitally-tuned DSDP Site 607 and ODP Site 677 δ 18O records. Examination of time-dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long-term monsoonal cycles, one occurring at about 0.8 Ma BP and the other around 1.7 Ma BP.

Late cenozoic history of the eastern llanos of venezuela: Geomorphology and stratigraphy of the Mesa Formation

The study area corresponds to the Mapire River basin in the southern part of the State of Anzoátegui and constitutes a region of low relief covered by the Pleistocene Mesa Formation. This formation consists of horizontally bedded alluvial sediments, represented by light gray, coarse- to fine-grained quartz sand, with cross-bedding marked by ochre, red and pink alteration, and to a minor extent by marsh deposits, represented by laminated and spotted shale. The typical topography consists of mesas north of the left margin of the Orinoco River (the Musinacio and Mapire Mesas). To the north of the mesas there is a topography of low hills covered by blocks of ferruginous crusts. A typical tropical savanna climate exists in the region, with a strong dry season which lasts 4–6 months, followed by a rainy season with excess humidity. The dominant erosional process is scarp retreat, due to the action of the savanna climate on the lithology of the partly unconsolidated sediments; it has formed deep canyons in the headwaters, and is magnified by tectonic uplift and the lowering of the regional base level. However, some topographic forms have been preserved due to the hardening of the sands by iron oxides. Preliminary thermoluminescence dating of sand from two stratigraphic sections confirms the Pleistocene age of the Mesa Formation (0.5–1 Ma BP), previously suggested by its stratigraphic position and geomorphological interpretation.

Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences

Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences

Pliocene palaeotemperature reconstruction for the southern North Sea Based on Ostracoda

The development of a large northeastern Atlantic shelf ostracod database has confirmed that an intrinsic relationship exists between watermass temperature and the spatial (latitudinal) distribution of benthonic shelf Ostracoda. A second Pliocene ostracod database, using assemblages from four boreholes in The Netherlands and a number of localities from the East Anglian Neogene succession has also been constructed. A quantitative method of modern analogues and the Squared Chord Distance dissimilarity coefficient has been employed as a means of calculating the level of similarity between Recent and Neogene assemblages and, therefore, to aid in the reconstruction of palaeotemperatures in the southern North Sea Basin during the Pliocene. The results, in the form of contoured dissimilarity values, indicate the presence of at least seven palaeoceanographical climatic phases between ca. 5.2 and 1.6 Ma BP. An early Pliocene cooling phase (ecozone A), in which the fauna is dominated by the cryophilic species; Palmenella limicola (Norman), Elofsonella concinna (Jones), Robertsonites tuberculatus (Sars), Neomonoceratina tsurugasakensis (Tabuki) and Acanthocythereis dunelmensis (Norman), supervenes a period of stable subtropical conditions in the Miocene. A ‘mid’ Pliocene warming phase (ecozone B), which has been documented throughout the North Atlantic, can be recognized in the faunal composition of the sublittoral ostracod assemblages of both The Netherlands and East Anglia. A hiatus between the Coralline and Waltonian crags is thought to be associated with a deterioration in climatic conditions. Recent analogues to The Netherlands faunas of ecozone C (first phase late Pliocene cooling, ca. 2.9 Ma BP) indicate a decrease of approximately 5–6°C in mean summer surface temperatures. A return to elevated temperatures, of a Mauritanian aspect, can be seen in the fauna of the FA2 zone and Waltonian Crag. This amelioration of climate occurred prior to the Praetiglian cold phase (second phase late Pliocene cooling, ca. 2.4 Ma BP) which again saw the reintroduction of cryophilic species including Baffinicythere howei Hazel, Fimarchinella logani (Brady, Crosskey and Robertson) and Hemicythere emarginata (Sars), into the southern North Sea during the Newbournian and Butleyan stages. The final phase of the latest Pliocene (third phase late Pliocene cooling) is only documented in the ostracod fauna of the Norwich Crag. Evidence in the covariance of Loxoconchidae and Norwegian provincial cryophilic species, indicates that the Polar Front may have been as much as 2000 km further south during the deposition of the Chillesford Sand Member (Norwich Crag Fm., ca. 2.0-1.6 Ma BP).

Long-term terrestrial records of the Middle Danubian Basin

In eastern central Europe, in the Middle Danube Basin, loess and loess-like deposits cover various morphotectonic levels in a total area of ca. 150,000 km 2. Basin types of various elevations and sizes are predominant. Under different geomorphological or morphotectonic conditions — over an identical time interval — the variation in the rate of basin subsidence produces various litho- and chronostratigraphical sequences. The subaerial sequence of basins affected by prolonged subsidence in the Quaternary is subdivided by paleosols of larger number than the loess-paleosol sequence of basins, elevated plateaus or watersheds which underwent more moderate subsidence during the Quaternary. Loess-paleosol-sand sequences on pediments (100–150 m above sea level). In the loess-paleosol-sand sequences of 50–100 m thickness, ca. 12 loess, 10 sand and silt layers and 20–30 paleosols are present. The age of these profiles with hiatuses, is Pliocene-Pleistocene. Loess-paleosol-sand sequences on alluvial fans and terraces (10–100 m relative heights). The loess sequence is 40–60 m thick and subdivided by 10–12 loess horizons, 10–12 paleosols and 5 or 6 sand layers. LTR — Pliocene-Pleistocene. Subaerial basin sediment locally of 500–1500 m thickness. The surface of the basin is a flood plain at 90–100 m above sea level. In the basin sediments the number of paleosols may reach one hundred. In several boreholes there are 6–12 red soils or red clays between 600 and 1000 m. 12–16 intercalated sand layers are observed. The age of this almost complete sequence is 5.2–5.4 Ma BP. In the most intensively subsiding basins sedimentation was almost continuous; during the Pliocene ca. 50–60 and in the Pleistocene ca. 50 soils developed. In contrast on the non-subsiding foothills only 20–30 soils formed during the Pliocene and Pleistocene and several gaps are detected in the sequence.

‘Gradualistic’ evolution: Its interpretation in Quaternary large mammal species

Many fossil sequences display apparently directional morphological change. The interpretation of these sequences as ‘gradualistic’ depends on (i) demonstration that sequential samples belong to a single lineage and (ii) temporal and morphological spacing being fine enough to exclude unseen ‘punctuations’. Alternative models, such as allopatric speciation and replacement, should be borne in mind when interpreting such sequences and tests sought between these different modes. Directional trends have been observed in the moose lineage ( Alces), where relative antler length decreased across four levels spanning 2 Ma; and in mammoths ( Mammuthus), where a sequence of ten samples shows directional dental change from 2–0.4 Ma BP followed by 0.4 Ma of ‘dynamic stasis’. Cladogenetic speciation within these lineages cannot be ruled out, but is not clearly evidenced, so anagenetic microevolution remains a plausible mechanism, accounting for temporal transitions at least as great as those typically separating contemporary mammalian species.

Late Cenozoic Vegetational History and the Pliocene-Pleistocene Boundary in the Yushe Basin, S. E. Shanxi, China

Abstract A continental stratigraphy column covering the Pliocene and Early Pleistocene in SE Shanxi, China has been studied using pollen analysis, ten pollen zones are defined with their correlation to magnetic polarity and 10Be datings. The vegetational development and climatic changes during the period from 5.1 Ma to 1.8 Ma BP are discussed. Picea became a dominant component of the pollen assemblage in the Early Pliocene. Subtropical components such as Carya and Liquidambar occur in the Pliocene pollen spectra. The palaeoclimate of the Pliocene was moist and relatively warm, though a trend towards a colder and drier climate is reflected. Around 2.3 Ma BP the prevailing forest turned into a type of steppe and the subtropical components basically migrated out of the area. The Pliocene-Pleistocene boundary is also discussed using palynostratigraphy. It seems reasonable to place the Pliocene-Pleistocene boundary close to the Gauss-matuyama magnetic boundary.

The periodicity of geomagnetic reversals

Several periodicities have been claimed in the time series of geomagnetic reversals, but none has survived subsequent methodological objections. We apply the established tool of periodogram analysis to the most accurate geomagnetic scale available and obtain the following results: a trend, that is well fitted by an exponential curve, is present in the rate of reversals occurrence and there is a 15 Ma periodicity that is significant at approximately the 0.05 level; no significant period is present in the series of the intervals between reversals. These results indicate that geomagnetic reversals since 85 Ma BP occur according to a generalized Poisson process with a mean which increases exponentially, and which may have a superimposed periodic modulation. In physical terms, this would imply a mechanism of reversal generation which is stochastic and has an energy input increasing in time modulated by a 15 Ma periodicity. This is consistent with reversals originated by cyclones in the outer core, a long-term change in the lower mantle temperature distribution, and a pulsating 8–10 km thick layer at the base of the mantle.

Magmatism in the South China Basin: 1. Isotopic and trace-element evidence for an endogenous Dupal mantle component

A widespread episode of intraplate volcanism followed the cessation of sea-floor spreading in the South China Basin (≈32-17 Ma BP), affecting large parts of southern China and Indochina, and penetrating oceanic basement and stranded microcontinent fragments. Geochemical data for post-spreading seamount and island lavas define suites of quartz tholeiite, olivine tholeiite, alkali olivine basalt and nephelinite, characterised by OIB-type incompatible-element distributions. High-K alkalic lavas show extreme enrichment in large-ion lithophile and high-field-strength elements relative to N-MORB. 87 Sr 86 Sr and 143 Nd 144 Nd ratios are depleted relative to bulk Earth values and partially overlap with Central Indian Ridge MORB and associated OIB. In contrast, 208 Pb 204 Pb and 207 Pb 204 Pb ratios are variable and surprisingly radiogenic for given MORB-like 206 Pb 204 Pb . The isotopic and trace-element systematics confirm source heterogeneity but appear to be decoupled, implying complex mantle enrichment histories. At least two heterogeneous source components are required: a depleted but “contaminated” Indian Ocean MORB type, and an EM-2 reservoir whose isotopic composition corresponds to continent-derived sediment. Dupal-like Pb isotopic compositions ( Δ7 4 Pb = 2−13 , Δ8 4 Pb = 45−73 ) are shared by intraplate basalts from Hainan Island, the Penghu Islands, northern Taiwan and post-collision arc basalts from the Philippines. It is proposed these reflect endogenous mantle processes related to disaggregation of the south China margin rather than a northward extension of the southern hemisphere Dupal anomaly.

Thermal history modeling of the Georges Bank, U.S.A.: thermal inversion of transmittance color index (TCI) and vitrinite reflectance (VR) data

Using a one-dimensional basin analysis model for the COST G-2 well in the Georges Bank area, offshore East Coast, U.S.A., thermal inversion of measured Vitrinite Reflectance (VR) data and of the Transmittance Color Index (TCI) converted to an equivalent VR value yielded essentially identical paleoheat flux variations with time. The paleoheat flux showed two peaks at ∼175 Ma BP and at 125-75 Ma BP. The earlier event corresponds to continental breakup and the beginning of the rift phase. The latter event corresponds to continued drifting of the continents and formation of new oceanic basement. Beside answering some important tectonic questions, the observations corroborated the theoretical and empirical connections between TCI and VR found for the G-2 well and for other wells in different geological settings. This fact, together with the fact that TCI measurements on physically amorphous organic material can be made on rocks devoid of vitrinite or spores and pollen (carbonates, pre-Late Silurian rocks, and siliceous black shales), should promote the utility of TCI as an effective geothermal history indicator.

The beginning of the Ice Age in Europe and its major subdivisions

The time range of the Quaternary is discussed in terms of the view that this period is essentially the one in the Earth's history in which during certain intervals inland ice developed in the northern hemisphere. This process first became evident about 2.3 Ma BP. Since then, the Quaternary thus defined has witnessed many climatic variations. Although for practical reasons it is not feasible to assign the status of ‘stage’ to each distinclty warm or cold phase, climatic change is nevertheless the most appropriate basis for world-wide correlation. It seems possible to use both climatic change and other correlation parameters to arrive at a practicable subdivision of the Quaternary Period into stages. Before about 1 Ma, vegetational succession during interglacials in western Europe differed from that seen in later times. A possible cause for this divergence is discussed. In the later part of the Quaternary (after about 1 Ma) two distinct types of warm-temperate intervals occurred: one associated with high sea-levels and an oceanic climate, the other showing lower sea-levels and a cooler and more continental climate. The difference between these two types is shown by comparison of the distribution of Abies during these time intervals.

Further finds of the Derrick Peak meteorite, Transantarctic Mountains, and implications for terrestrial age

Abstract— Nine additional iron meteorite fragments weighing a total of 72 kg were recovered from the Derrick Peak area by a Canterbury Museum geological party in late 1988. One iron was located in the Onnum Valley, 6 km south of the previous finds. Geochemical analysis indicates that all irons belong to a single meteorite shower, greatly increasing the known extent of the fall zone. Kamp and Lowe (1982) have previously estimated the terrestrial age of the meteorite from glacial geological evidence. The location of the 1988 finds supports Kamp and Lowe's interpretation that the meteorites lie in situ, but recent revisions of the chronology of Cenozoic glacial history of the region reduce the interpreted terrestrial age. An age of between Oxygen Isotope stages 6 and 2 is probable (190–125 to 35–12 ka BP). This conflicts with a terrestrial age estimate of 1.0 ± 0.1 Ma BP from cosmogenic radionuclides.

Factors Influencing Biogeographic Patterns of Australasian Centropagid Copepods

Bayly & Morton (Verh. Verein. Limnol. 20, 2537) suggested that the peculiar distribution of the centropagid copepod Boeckella triarticulata (Thompson), found in Australia, New Zealand, and Mongolia, may have been determined by continental drift. In this report, we examine the distribution patterns of over forty centropagid copepods found in Australia and New Zealand to consider the extent to which historical events, dispersal, and current ecological requirements affect distribution. The widely disjunct distributions of several of the species and the high degree of endemism in Western Australia species are consistent with the hypothesis that major events of speciation occurred at least 50 Ma BP and possibly much earlier, at a time when Australia and New Zealand were joined but eastern and western Australia were separated by an epicontinental sea. Furthermore, the morphology of these species has remained virtually unchanged since the Eocene or even earlier. Thus, the hypothesis put forth by Bayly & Morton is supported by additional facts. The present-day distribution of most of these species cannot be explained readily by more recent dispersal or by ecological requirements, although incidence in local areas is determined, in part, by these factors. However, dispersal may well have played a significant role in present-day similarities between the centropagid fauna of Australia and New Zealand.

Problems of loess chronology

Loess as a typically subaerial, loess deposit has long been regarded a Pleistocene sediment and its date of formation has been put to ca 600 ka BP. Recently, the beginning of the Pleistocene is identified in the official concord reached — at 1.6 Ma BP, while not infrequently others propose 2.4 Ma BP. Until the fifties the whole loess (loess-paleosol-sand) sequence could be referred into the ‘shorter Pleistocene’ period, which —according to the climatic claendar by Milankovitch — consisted of nine cold and eight warm intervals. Some supporters of the ‘longer Pleistocene’ chronology hold that the beginning of loess formation can be put as far back as 1.6 – 2.4 Ma BP. Loess formation intervals are usually correlated with the cold stages (nos 2, 4, 6 etc.) of the Emiliani oxygen isotope timescale, while soil formation is believed to correspond to the warm stages (nos 1, 3, 5, 7 etc.). The typical loess, i. e. loess proper, are not older than stage no 23 (Jaramillo event — 0.96 Ma BP). Previous to this date climatic conditions had not generally favoured loess formation and paleosols formed one above the other with clay, loam or carbonate intercalations.

The environmental effects of the uplift of the Qinghai-Xizang Plateau

The intense upheaval of the Qinghai-Xizang Plateau took place 2 Ma BP. The terraces of the Yellow River in the Lanzhou Region and its upper sections provide a record of the uplift of the Plateau. The interaction of glaciation cycles resulting from climatic changes and pulsating tectonic upheavals are both involved in the record of environmental change across the Plateau. Three tectonic-climatic events which are termed respectively the Qinghai-Xizang Movement, the Huanghe (Yellow River) Movement and the Gonghe Movement occurred 2-1.5, 1.0 and 0.15 Ma BP. The upheaval of the Qinghai-Xizang Plateau was the trigger mechanism of the Asian Monsoon. The Loess Plateau was formed in the monsoon triangle of east Asia. Human origins and human evolution may have been stimulated by the uplift of the Qinghai-Xizang Plateau.

High precision radiometric ages from the northern Sydney Basin and their implication for the Permian time interval and sedimentation rates

Three pyroclastic samples that bracket the coal‐bearing Permian System of the northern Sydney Basin have been dated using the zircon U/Pb and hornblende K/Ar methods. The Matthews Gap Dacitic Tuff Member, situated 170 m below the base of the Permian System, gives a best estimate of 309 ± 3 Ma. Its age correlates well with the Paterson Volcanics which suggests that the immediately overlying clastic sediments are equivalent to the Seaham Formation. The Awaba Tuff, which at the sample locality is located 50 m below the top of the Permian System, gives a best estimate of 256 ± 4 Ma. An intervening horizon, the Thornton Claystone of the Tomago Coal Measures, gives a best estimate of 266 ± 0.4 Ma. The ages indicate an earlier beginning (∼ 299 Ma BP), an earlier termination (∼ 255 Ma BP) and slightly longer duration (44 ± 13 Ma) of the Permian System in the Hunter Valley than previously suggested. Sedimentation rates of ∼ 65 m/Ma, calculated from proximal sequence thicknesses, are only half the rate calculated f...

Dynamic hydrocarbon predictions for the northern part of the Danish Central Graben: an integrated basin analysis assessment

Geohistory, thermal history, source capacity and oil generation have been examined in the north-west part of the Danish Central Graben using a one-dimensional fluid flow-compaction model. The burial history suggests that this part of the Danish Central Trough developed through three stages of subsidence, a Late Jurassic differential stage, a Late Cretaceous—Early Tertiary uniform stage and a Late Miocene-Quaternary uniform stage. Pseudo-wells ‘drilled’ on seismic sections in areas without well data are used to improve the spacial distribution of wells. The palaeotemperature and palaeoheat flow have been modelled by inversion of vitrinite reflectance data. The inversion was carried out on wells with available vitrinite reflectance data and was based on known bottom hole temperature, some temperature measurements with depth and the surface temperature. The thermal history assessed by inversion of vitrinite reflectance data gives a consistently cooler past; the available data's resolution of the thermal history is discussed. The modelled maturation history of the Upper Jurassic shale in terms of vitrinite reflectance suggests that the shale reached maturity some 5–50 Ma ago. A geochemical study of the Upper Jurassic shales shows that these shales contain a mixture of type II and type III kerogens and have good to excellent source capacity. Modelling of the hydrocarbon generation data indicates that the peak generation took place some time between 10 Ma BP and the present day. Possible migration paths are determined from modelled excess fluid pressure, and four areas of possible accumulation of hydrocarbons are indicated. The hydrocarbon potential of the areas are evaluated and an area along the eastern boundary fault between the Tail-End Graben and the Ringkøbing-Fyn High is suggested as a new target area for further exploration.

Late Cenozoic sedimentary and tectonic history of south Buton, Indonesia

A description and interpretation are given of the Upper Cenozoic sedimentary record of south Buton. Various sections and outcrops were studied and sampled for their microfossil content, to provide age and paleobathymetrical data. Together with information from the literature, these data from the base for a geohistory analysis to evaluate the vertical motions. Deposition started some 11 Ma ago, after the main deformation of the island, which was related to the collision of a microplate carrying Buton, with the southeast arm of Sulawesi. Coarse and fine terrigenous debris accumulated in a rapidly subsiding foreland basin; subsidence may have exceeded 100 cm/ka. When the rate of subsidence decreased a late Miocene-early Pliocene period of quiet pelagic sedimentation followed. From the late Pliocene onwards (around 3.5 Ma bp) an overall uplift took place, with rates between 30–120 cm/ka. This drastic change is explained by the collision of Buton with a submerged microcontinent that presently forms the Tukang Besi platform, situated southeast of Buton, which interaction resulted in wrench type tectonics and a clockwise rotation of over 60° for south Buton.

Late pliocene and quaternary glaciations of New Zealand

Evidence of Late Pliocene glaciations is fragmentary. The Ross Glaciation in northwest South Island is correlated with the first marine faunal evidence of marked cooling in the Wanganui Basin in southern North Island; it is inferred to be in the 2.6-2.4 Ma age range. The Porika Glaciation, in a separate area of northwest South Island, is younger, judged by palynology, and is correlated with a cooling 2.2-2.1 Ma BP inferred from stratigraphic evidence of lowered sea level in the Wanganui Basin: A gap of >1.5 Ma in the record of glaciations is largely filled by Wanganui Basin marine sediments, although an unconformity cuts out the record between ca. 1.45 and 1.0 Ma BP; since then, the many sedimentary cycles caused by sea level changes are presumed to be glacio-eustatic in origin. Uplifted interglacial marine terraces, back to marine Oxygen Isotope Stage 17, and river terraces formed by cold-climate aggradation, are linked into the cyclic sequence through loess and tephra in their cover beds. East of the South Island, cores from the DSDP Site 594 in the southwest Pacific record the earliest glaciation and, higher in the core, nine periods of South Island glaciation in the last 0.7 Ma. Onshore, because of uplift and erosion, only the last four glaciations, assigned to Oxygen Isotope Stages 10, 8, 6, and 4+2, are identified, being named Nemona, Waimaunga, Waimea and Otira Glaciations. They are best known in north Westland (northwest South Island) where the relations of glacial outwash and interglacial shoreline deposits are displayed. The late Otira Glaciation (Stage 2) is radiocarbon dated as beginning somewhat before 22.3 ka BP, culminating ca. 18 ka BP, and ending at ca. 14 ka BP, when rapid deglaciation began. A relatively small advance at ca. 12 ka BP may have been followed by little change until after 8.6 ka BP. Recession was then followed by a period of advance from ca. 5 ka BP until recent centuries and a subsequent recession that still continues.