Paleogeographic Changes Research Articles

Paleogeographic evolution of the Lower Yangtze region and the break of the “platform-slope-basin” pattern during the Late Ordovician

Wide distribution of the black shales and diversification of the graptolite fauna in South China during the Late Ordovician resulted from its unique paleogeographic pattern, which was significantly affected by the paleogeographic evolution of the Lower Yangtze region. In the study, 120 Upper Ordovician sections from the Lower Yangtze region were collected, and a unified biostratigraphic framework has been applied to these sections to establish a reliable stratigraphic subdivision and correlation. Under the unified time framework, we delineate the distribution area of each lithostratigraphic unit, outline the boundary between the sea and land, and reconstruct the paleogeographic pattern for each graptolite zone. The result indicates that, with the uplift and expansion of the ‘Jiangnan Oldland’ in the beginning of the late Katian, the oldland extended into the Yangtze Sea gradually from south to north, which finally separate the Jiangnan Slope and the Yangtze Platform. Consequently, the longstanding paleogeographic pattern of “platform-slope-basin” in South China was broken. The paleogeographic change led to sedimentary differentiation among the two sides of the ‘Jiangnan Oldland’ during the Late Ordovician. This event also led to the closure of the eastern exit of the Upper Yangtze Sea, and formed a semi-closed, limited and stagnant environment for the development of the organic-rich black shales during the Late Ordovician. The major controlling factors of these paleogeographic changes in the Lower Yangtze region were not consistent from the Katian to the Hirnantian. In the late Katian, the sedimentary differentiation between the east and west sides mostly resulted from regional tectonic movement - the Kwangsian Orogeny. However, during the Hirnantian, the whole Yangtze region became shallower, which was mostly influenced by the concentration of the Gondwana ice sheet and the consequent global sea level drop.

Peldošais ezerrrieksts (Trapa natans) dabas un cilvēku vēstures liecinieks Latvijā

Peldošais ezerrrieksts (Trapa natans) dabas un cilvēku vēstures liecinieks Latvijā

Geochemistry, petrogenesis and tectonic significance of the volcanic rocks of the Las Tortolas Formation, Coastal Cordillera, northern Chile

The Las Tórtolas Formation extends from 29° to 26° S along the northern chilean coast and has been interpreted as a Carboniferous accretionary prism. This essentially metasedimentary turbiditic formation constituted the late Paleozoic basement of the Upper Triassic-Jurassic magmatism during the inception of the Andean cycle in northern Chile. The geochemical characteristics and field relations studied in the volcanic rocks of the Las Tórtolas Formation, described with detail in this work, mean a key fact to assess the notable paleogeographic changes occurred during the pre-Andean cycle in the western margin of South America.The volcanic domain included in the Las Tórtolas Formation is mainly comprised by basaltic rocks, with minor tuffs and rhyolites, and presents thrust-fault contacts with the mélange facies to the west and the metasedimentary Eastern Series to the east. Together, the Las Tórtolas Formation was conformed by the stacking of the basal highly deformed units of this accretionary complex, represented by the mélange facies, and the volcanic domain that became overlain by the turbiditic sequences at their top.The geochemical characteristics of the Las Tórtolas basaltic rocks involve an enriched mantle source, as a mantle plume. In spite of the N-MORB signature observed in the basal hyaloclastitic basalts, the lava flows, as the metabasites sampled in the mélange facies, yield E-MORB characteristics according to the multi-element and tectonic discrimination diagrams and several immobile element ratios as the Th/Yb, TiO2/Yb and Nb/Yb, among others. Therefore, the origin of the volcanic domain of the Las Tórtolas Formation may be related to a hot mantle plume impinged on the base of the oceanic lithosphere under the tip of the overriding plate, which firstly promoted the partial melting of the depleted surrounding lithosphere (N-MORB hyaloclastites) and continued with the extrusion of lava flows and pyroclastic rocks derived from the enriched source, interlayered with the distal turbiditic succession. The development of the magmatic system along the oceanic crust enabled the formation of magma chambers where small volumes of rhyolitic melts were fractionated. The geochemical evolution of this basaltic magmatism was correlated with the building of volcanic edifices during a first submarine stage (hyaloclastites, pillows and lava flows) and a subsequent sub-aerial explosive phase depicted by sedimentary and rhyolitic breccias.Finally, we explore a tectonic setting defined by a plume overridden by the lithosphere during the relative convergence with the continental margin to explain the formation of the volcanic domain and its accretion to the sedimentary units. Besides, this tectonic configuration may be taken into account as the driving force of the tectonic and magmatic events that characterize the pre-Andean cycle.

Monsoonal precipitation in the Paleo-Tethys warm pool during the latest Permian

Simulations of the late Permian (251Ma) are analyzed with respect to the northern hemispheric Pangean monsoon. We find that the presence and spatial distribution of the warm pool, and not land-sea temperature differences, are the primary forcing agents for the monsoon. The land-sea temperature gradient, as a monsoonal mechanism, is tested by eliminating the Cathyasian peninsula and is found to have little impact on the spatial character of the monsoon. Furthermore, the response of the monsoon to the warm pool was tested by removing all Paleo-Tethys equatorial islands, which allows the warm pool to expand and migrate westward thus shifting the pattern of monsoonal precipitation. Given that paleogeography changes through time, these results have important implications for the migration and strength of the tropical monsoon over geologic time. Additionally, different CO2 regimes are presented, in which a 10-fold change in forcing produces diverging climates and therefore different warm pool and monsoon locales. Tropical and equatorial ocean currents impact the seasonal progression and location of the warm pool and atmospheric mass flux for the monsoonal regime is characterized and shown to change with warm pool movement. Experiments were conducted using the low-resolution version of Community Climate System Model, Version 3 (CCSM3) in both coupled and fixed sea surface temperature (SST) context. For validation purposes, a modern control is briefly presented with observations and is shown to represent both the spatial and seasonal progression of the Indian monsoon.

Facies arrangement and cyclostratigraphic architecture of the Templet Member and the Kapp Starostin Formation (Permian) on Spitsbergen, Svalbard

Because of outstanding outcrops, Spitsbergen (Svalbard archipelago) provides unique opportunities to investigate the whole Upper Palaeozoic succession in great detail. This study can help to interpret the stratigraphic history and depositional evolution at other locations exposing coeval shelf strata along the northern margin of Pangea, e.g., the southern Barents Sea and Arctic Canada. Bed-scale outcrop observations are combined with microfacies studies to interpret the sedimentary settings and depositional environment of the Upper Palaeozoic strata. A sequencestratigraphic analysis has been carried out to evaluate the relative timing of sediment facies deposition in response to sea-level changes. The Early Artinskian to Kazanian successions of the Templet Member and the Kapp Starostin Formation were divided into five parasequences that are superimposed on a long-term, second-order, sea-level fall. These parasequences record a fundamental change of the sedimentary setting, from a restricted-marine, warm-water carbonate platform to an open-marine, cold-water, biosiliceous-carbonate ramp system. A cross-section across Svalbard comprising nine onshore sections shows that during deposition of the Kapp Starostin Formation a major depocentre marked by thick parasequences and a higher proportion of deep-water facies (bedded cherts) is located in the southwest of Spitsbergen (at Akseloya), whereas northeastern Svalbard records shallow-water microfacies. Svalbard was tectonically passive during the Permian; the local differences in accommodation space and facies were most likely linked to the rejuvenation of pre-existing structural elements, inherited from the Carboniferous. A deepening of the depositional environment combined with cold-water climatic conditions as recorded in our study area has also been documented in other Upper Palaeozoic successions around the Arctic, such as the Finnmark Platform (Norwegian Barents Sea) and the Sverdrup Basin (Arctic Canada). This transition in the depositional environment along the northern margin of Pangea is the result of large-scale changes in oceanic circulation patterns and local palaeogeographic changes during the northward movement of Pangea.

Late Cenozoic palaeogeography of Sulawesi, Indonesia

Sulawesi has a remarkable biodiversity, an unusually rich endemic fauna, and is the largest island in Wallacea, just west of the Wallace Line. Alfred Russel Wallace himself suggested it could perhaps be the most remarkable island on the globe because of its peculiar fauna. It was home to extinct fossil fauna such as dwarf proboscideans, records significant Pleistocene faunal turnover, and evidence of early human occupation suggests an important role in hominid migration through the Sunda-Sahul region. Information on Neogene palaeogeography is essential for understanding biogeographic patterns, biodiversity and faunal changes. New palaeogeographic maps reflecting recent work on Sulawesi's complex geology and changes in tectonic interpretations are presented for intervals from the Early Miocene to Pleistocene, at 20, 15, 10, 8, 6, 5, 4, 3, 2, and 1Ma. Additional maps illustrate the effects of glacially-driven sea level change in the last 1Myr. They are based on a field-based investigation of sedimentary rocks in Sulawesi, accompanied by palaeontological, petrological and heavy mineral studies and U-Pb dating of detrital zircons, to date and determine depositional environments. The new results have been supplemented by re-evaluation of previous studies, including reports from oil company wells and seismic lines. Igneous rocks provided ages, indications of surface environment of eruptions, and location of magmatic activity. For most of the Neogene from the Early Miocene Sulawesi was a shallow marine area with a number of small islands, surrounded by relatively deep marine areas. Deep inter-arm bays began to form in the Late Miocene and the islands became larger. The most significant palaeogeographic change began in the Pliocene with an increase in the area and elevation of land accompanied by major subsidence of the inter-arm bays. The separate islands gradually coalesced in the Pleistocene to form the distinctive K-shaped island known today.

The Cenomanian-Turonian Boundary Event (CTBE) in northern Lebanon as compared to regional data – Another set of evidences supporting a short-lived tectonic pulse coincidental with the event?

Constrained by both foraminifera and isotope data, the Zeitoun and Nahr Ibrahim sections in northern Lebanon record a steady flooding of the Cenomanian carbonate platform during the uppermost Cenomanian-lower Turonian interval. The flooding began just before the inception of the CTBE δ13C positive shift. It is marked by the deposition of finely bedded to laminated, fine-grained limestone overlying platformal carbonates. This scenario is entirely different to what has been published in northern Israel, where repeated emergences are instead recorded during the event. A survey of published data from nearby Middle East countries to North Africa suggests that the CTBE is accompanied on a larger scale, especially along the Syrian Arc, by tectonic disturbances responsible for sequence stratigraphic heterogeneity and palaeogeographic changes. Detailed analysis of available data makes it possible to extract from the sedimentary record four sequence stratigraphic scenarios implying two phases of heterogeneity bounding the event, and responsible for the outphasing of relative sea level changes in some sensitive areas. These disturb a stepped early Cenomanian to early Turonian overall transgressive trend recorded in some large stable areas, often used to support the concept of Mesozoic eustasy. The discussion focuses on the global processes that are possibly involved.The hypothesized tectonic pulse does not explain the CTBE. It is just another set of data to be taken into account in the search for causes.

Paleogeographic evolution of eastern Hispaniola

The geological record of Hispaniola documents a continuous increase in island surface since the Early Pleistocene due to the emergence of marine deposits. In eastern Hispaniola, the Llanura Costera del Caribe and the Los Haitises regions formed by the emergence in the Early-Middle Pleistocene of limestone of the Los Haitises Formation, and marls and marly limestone of the Yanigua Formation (Pliocene-Early Pleistocene). These formations were both deposited on the platforms surrounding the precursor relief of the Cordillera Oriental. In the Coastal Belt of the Llanura Costera del Caribe, continued uplift and Pleistocene global sea-level changes are reflected by two main stepped depositional terraces comprising reef limestone of the La Isabela Formation. Radiometric dating indicates the lowest terrace of the La Isabela Formation (Lower Surface of the Llanura Costera del Caribe) formed during MIS 5e in the Late Pleistocene. The age of the Upper Surface of the Llanura Costera del Caribe estimated by extrapolation of uplift rates deduced from the age of the lowest terrace is consistent with the available biostratigraphic age (Early Pleistocene) for the last deposition phases of the Los Haitises Formation. Correlation of the Upper Surface with erosional surfaces in the Cordillera Oriental and the Los Haitises region allows a reconstruction of the palaeogeographic changes in eastern Hispaniola during the Quaternary.

Tracking shoreline evolution in central Cyclades (Greece) using beachrocks

Beachrocks represent a significant paleo-environmental proxy because they can record both the vertical and the horizontal evolution of the shoreline. They have often been used to assess Holocene shoreline evolution and crustally induced Relative Sea-Level (RSL) changes in the Mediterranean Sea. In this paper, we used submerged beachrocks from Paros and Naxos Islands (Aegean Sea, Greece) to reconstruct the shoreline modification and provide new insights on the RSL evolution in central Cyclades. Paros and Naxos Islands are of great importance in terms of archaeological evidence of coastal occupation. However, the evolution of their coastlines was seldom explored. In this study, we coupled detailed underwater surveys, analysis of aerial photogrammetry, microstratigraphic analysis and luminescence dating to study beachrock outcrops found down to about 6m below the present sea-level. We, then, spatially and chronologically constrained some major palaeogeographical changes of a number of coastal sectors of the two islands. Furthermore, the multiple analyses of beachrocks, sediment coring and archaeological data suggested that RSL rose by at least 3.8m in the last 4.0ka and that RSL variation in the last 2.0ka did not exceed 2m with respect to the present mean sea level.

Porosity and reservoir potentiality of the Cherahil Formation limestone (middle-upper Eocene) in the Gulf of Gabes (Tunisia)

During the middle and upper Eocene, the deposits in the Gulf of Gabes correspond to the Cherahil Formation, which is sub-divided into three units, which are as follows from base to top: the Lower Cherahil A, the Siouf and the Upper Cherahil B members. The Siouf member has a lateral equivalent in the Souar Formation named Reineche member.The Cherahil Formation has never been considered by oil companies as a particular drilling target in the Gulf of Gabes (offshore east Tunisia) despite the presence of hydrocarbon at the bottom of Cherahil Formation in Sidi Behara and Sidi Litayem oil fields in Sfax Area (onshore east Tunisia) and in its equivalent carbonate beds in Jebel Trozza (Central Tunisia). Therefore, the evaluation of porosity in the carbonate levels of Cherahil Formation in 20 drilling wells were performed on well logging by applying Wyllie method. The obtained results show that the studied carbonates are characterized by an economically important total porosity average ranging between 5% and 55%, and both vertical and lateral variations. The vertical porosity variation was controlled by the sea-level fluctuation that, in turn, controlled the evolution of carbonate sedimentary environments and relative facies. The lateral porosity variation followed the Tunisian middle-upper Eocene paleogeography changes controlled by NW-SE synsedimentary tectonic trends. Considering the important features of the Cherahil Formation and the coexistence of components of an oil system in the Gulf of Gabes, this formation can be an important potential reservoir and subsequently a new petroleum exploration target in the Gulf of Gabes.

A Middle Miocene (13.5–12 Ma) deformational event constrained by volcanism along the Puna-Eastern Cordillera border, NW Argentina

The features of Middle Miocene deposits in the Puna-Eastern Cordillera transition (Valles Calchaquíes) indicate that Cenozoic deformation, sedimentation and volcanism follow a complex spatiotemporal relationship. The intense volcanic activity recorded in the eastern Puna border between 14 and 11.5Ma coincides with the occurrence of one of the most important deformation events of the Neogene tectonic evolution in the region.Studies performed across the Puna-Eastern Cordillera transition show different relationships between volcanic deposits of ca. 13.5–12.1Ma and the Oligocene-Miocene Angastaco Formation. In this paper we describe the ash-flow tuff deposits which are the first of this type found concordant in the sedimentary fill of Valles Calchaquíes. Several analyses performed on these pyroclastic deposits allow a correlation to be made with the Alto de Las Lagunas Ignimbrite (ca. 13.5Ma) of the Pucarilla-Cerro Tipillas Volcanic Complex located in the Puna. Outcrops of the ca. 13.5Ma pyroclastic deposits are recognised within the Puna and the Valle Calchaquí. However, in the southern prolongation of the Valle de Hualfín (Tiopampa-Pucarilla depression) that separates the Puna from the Valle Calchaquí at these latitudes, these deposits are partially eroded and buried, and thus their occurrence is recorded only by abundant volcanic clasts included in conglomerates of the Angastaco Formation. The sedimentation of the Angastaco Formation was aborted at ca. 12Ma in the Tiopampa-Pucarilla depression by the Pucarilla Ignimbrite, which unconformably covers the synorogenic units. On the contrary, in the Valle Calchaquí the sedimentation of the Angastaco Formation continued until the Late Miocene.The different relationships between the Miocene Angastaco Formation and the ignimbrites with ages of ca. 13.5 and ca. 12Ma reveal that in this short period (~1.5m.y.) a significant deformation event took place and resulted in marked palaeogeographic changes, as evidenced by stratigraphic-sedimentological and chronological records in the Angastaco Formation.

Evolution of the Bystrej Valley caves (Tatra Mts, Poland) based on corrosive forms, clastic deposits and U-series speleothem dating

The origin and age of cave deposits, as well as palaeogeographical changes in the Bystrej catchment during the last ca. 250 ka, were reconstructed in Magurska, Kasprowa Niznia, Goryczkowa, Kalacka and Bystrej caves (the Bystrej Valley). The reconstruction is based on the study of corrosive forms, heavy mineral analyses and U-series dating of speleothems. Two generations of palaeoflows were distinguished by observations of scallops and heavy mineral analyses. In the older stage, now abandoned caves drained massifs surrounding the Bystrej Valley and part of an adjacent valley. The direction of palaeoflow changed as a result of the water capture after Kasprowa Niznia Cave came into being. In the later stages, the evolution of cave systems was controlled by glaciation-deglaciation cycles. Probably at this time, some caves located in the lowest parts of the massifs also started to be formed. U-series speleothem dating allows the determination of five phases of speleothem deposition: ca. 220–150 ka, ca. 135–105 ka, ca. 95–70 ka, ca. 40–23 ka and during the Holocene.

DEEP WATER AGGLUTINATED FORAMINIFERA FROM THE JURASSIC/CRETACEOUS BOUNDARY AND PALEOENVIRONMENTAL SETTINGS OF THE MAIOLICA TYPE FACIES FROM THE CZORSTYN RIDGE (PIENINY KLIPPEN BELT, WESTERN CARPATHIANS)

Deep-water carbonates deposited on the Czorstyn elevated ridge of the Pieniny Klippen Belt paleogeographic domain record changes in agglutinated foraminifera morphogroups through the J/K boundary interval at the Erdutsky kostol section. Microfacies, abundance, taxonomic and morphogroup analyses of the agglutinated foraminifera show the response of the assemblages to the flux of organic matter. In the uppermost Tithonian C. brevis Zone a distinct high abundance assemblage is represented mostly by sessile epifaunal tubular agglutinated foraminifera such as Rhizammina and Tolypammina , pointing to an eutrophic setting. In the lowermost Berriasian C. alpina Zone the mobile epifaunal tubulothalamous taxa Ammodiscus and Glomospira and multilocular taxa Cribrostomoides and Trochammina increase while the epifaunal tubular agglutinated foraminifera decrease. The assemblage composition and the slender tubular foraminifera suggest oligotrophic settings. The infaunal foraminifera decrease and point to increasing oxygenation of the bottom and pore waters.

In search of Pollentia's southern harbour: Geoarchaeological evidence from the Bay of Alcúdia (Mallorca, Spain)

The Roman city of Pollentia was founded on the north-eastern coast of Mallorca (Balearic archipelago) after the Roman conquest of the island in 123BCE. There is evidence that the city had two harbours, a small harbour located to the north, whereas the main harbour was located to the south of Pollentia. Our study focuses on the southern harbour, on a coastal plain in proximity to the ancient city. Four sedimentary cores were taken and bio-sedimentological analysis was performed. Here, we describe the palaeogeographical changes in this area, evolving from an open lagoon environment between ca. 3200 and ca. 500cal.yearsBCE to a semi-enclosed lagoon during Roman times. Progressively, the lagoon became silted and was isolated from the sea by the development of a large sand spit, probably during the second half of the first millennium CE. We also discuss the water depth, linked to the palaeosea level, which is important in defining the possible ancient harbour location.

The cause of Late Cretaceous cooling: A multimodel-proxy comparison

Abstract Proxy temperature reconstructions indicate a dramatic cooling from the Cenomanian to Maastrichtian. However, the spatial extent of and mechanisms responsible for this cooling remain uncertain, given simultaneous climatic influences of tectonic and greenhouse gas changes through the Late Cretaceous. Here we compare several climate simulations of the Cretaceous using two different Earth system models with a compilation of sea surface temperature proxies from the Cenomanian and Maastrichtian to better understand Late Cretaceous climate change. In general, surface temperature responses are consistent between models, lending confidence to our findings. Our comparison of proxies and models confirms that Late Cretaceous cooling was a widespread phenomenon and likely due to a reduction in greenhouse gas concentrations in excess of a halving of CO2, not changes in paleogeography.

Epigenetic Relief Transformation in the West Curonian Upland and West Žemaičiai Plateau

The study was conducted in Highlands of Western Curland and Western Žemaičiai plateau, Skuodo district – Ylakių, Aleksandrijos, Šačių ir Skuodo parich, where it was tested Apšės, Kalties, Sarties, Spigino, Gypelkio, Luobos, Šatos ir Pragulbos rivers. Studied river valleys on flood plains, terraces and river beds at the bottom sediment samples were collected. In the laboratory they are dried, weighed and beams. Sifting have identified four groups: > 2, 2–1 1 to 0,5 and < 0,5 mm. In addition, sediment particles studied roundnees degree. Glacial melt waters have a direct impact on the surface of the formation relief. Thanks to them, creating a negative linear shape. Glacial melt waters forms necessary for the formation of two conditions: a sufficient amount of water flowing and significant surface slope. West Curland highland river sediments observed uneven distribution of the groups, which allowed the reconstruction process took place glacial melt water intensity. Apšė the upper basin have high fractions of sediment dispersion greater diversity than middle reaches of the basin. This indicates that the upper was intense glacial melt water erosive processes change. Meanwhile, the middle reaches of the long-term was glacial lake. Western Žemaičiu plateau river sediments even noticeable fraction distribution, which allowed the reconstruction process took place glacial melt waters intensity. Luoba basin upstream or downstream sediment fractions of the same dispersion. This indicates that the river carried evenly glacial melt water erosive processes. Meanwhile, the lower reaches of the basin are largely dominated by fine-grained and fine sand, which indicates that there was limnoglace lake, which has affected not only Luoba river basin lowland, but Apšė river basin lowland and middle reaches. West Curland upland furrow bottom sediment granulometry scatter in all workings are pretty even. Most sediment particles in the upper basin polished Apšė and its tributaries (Kaltis, Sartis, Vereta), while at least - Apšė down the river. There is significant correlation between the paleogeographic changes and sediment roundness degree. Western Žemaičiu plateau sediment granulometry spread is even. All workings found in the medium category roundness particles. This reflects the conditions paleogeographic uniformity.

Reply to “Comment on the Badenian–Sarmatian extinction event in the Carpathian foredeep basin of Romania: Paleogeographic changes in the Paratethys (Palcu et al., 2015)” by Silye and Filipescu (2016)

Reply to “Comment on the Badenian–Sarmatian extinction event in the Carpathian foredeep basin of Romania: Paleogeographic changes in the Paratethys (Palcu et al., 2015)” by Silye and Filipescu (2016)

Neogene changes in palaeogeography, palaeoenvironment and the provenance of sediment in the Northern Danube Basin

Neogene changes in palaeogeography, palaeoenvironment and the provenance of sediment in the Northern Danube Basin

Permian paleogeography of west-central Pangea: Reconstruction using sabkha-type gypsum-bearing deposits of Parnaíba Basin, Northern Brazil

Extreme aridity during Late Permian – Early Triassic period was the main factor for resetting the entire paleoclimate of the planet. Permian evaporite basins and lacustrine red beds were widely distributed along the supercontinent of Pangea. Sulphate deposits in Western Pangea, particularly in Northern Brazil, accumulated in an extensive playa lake system. Outcrop-based facies and stratigraphic analysis of up to 20m thick evaporite-siliciclastic deposits reveal the predominance of laminated reddish mudstone with subordinate limestone, marl and lenses of gypsum. The succession was deposited in shallow lacustrine and inland sabkha environments associated with saline pans and mudflats. Gypsum deposits comprise six lithofacies: 1) bottom-growth gypsum, 2) nodular/micronodular gypsum, 3) mosaic gypsum, 4) fibrous/prismatic gypsum, 5) alabastrine gypsum, and 6) rosettes of gypsum. Gypsum types 1 and 2 are interpreted as primary deposition in saline pans. Bottom-growth gypsum forms grass-like crusts while nodular/micronodular gypsum indicates displacive precipitation of the crust in shallow water and the groundwater capillary zone. Types 3 and 4 are early diagenetic precipitates. Abundant inclusions of tiny lath-like anhydrite crystals suggest a primary origin of anhydrite. Alabastrine gypsum, fibrous gypsum (satinspar) and rosettes of gypsum probably derived from near-surface hydration of anhydrite. The gypsum-bearing deposits in the Parnaíba Basin contribute towards understanding paleogeographic changes in Western Pangea. A progressive uplift of East Pangea, culminated in the forced regression and retreat of epicontinental seas to the West. Restricted seas or large lakes were formed before the definitive onset of desert conditions in Pangea, leading to the development of extensive ergs.

Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.