Continental Paleoenvironments Research Articles

Colonisation of disturbed deltaic paleoenvironments from the Early Cretaceous (Albian): Inferences from an exceptional record of the fern Ruffordia goeppertii (Dunker) Seward from northeastern Spain

Exquisitely well-preserved records of a nearly monospecific assemblage of in situ specimens in growth position of the fern Ruffordia goeppertii from the Albian deposits of northeastern Spain, clarify Early Cretaceous continental paleoenvironments during a key epoch of floral evolution during the Cretaceous Terrestrial Revolution. These macro and microfossils of vegetative and fertile organs are preserved in connexion with their rhizomes, and spores are retained within sporangia. This assemblage contains the first records of this iconic Cretaceous fern from the Albian of Spain. The fossils also fill a temporal and geographical gap in the distribution of this taxon in southwestern Eurasia during an interval of rapid floristic and faunal changes, globally. The biostratinomy of these fossils aid reconstruction of the paleoenvironments in which this type of ferns developed, and indicates that disturbed deltaic floodplains were colonised by Ruffordia in southwestern Eurasia during the Albian. Moreover, the presence of plant-insect interaction traces on this fern and the relationship of this plan fossil assemblage with evidence of herbivorous dinosaur tracks provides insights into the possible feeding strategies of these animals in deltaic-estuarine environments during the late Early Cretaceous in southwestern Eurasia.

Hierarchical Milankovitch and sub-Milankovitch cycles in the environmental magnetism of the lower Shahezi Formation, Lower Cretaceous, Songliao Basin, northeastern China

SK-2 borehole in Songliao Basin provides unprecedented geological materials for investigating the Early Cretaceous continental paleoenvironment and paleoclimate in northeastern China. The lacustrine successions of the lower Shahezi (K1sh) Formation at the depth from 4,542 to 5,695 m was systematically studied using environmental magnetism and cyclostratigraphy in this study. Magnetic analysis reveals an inverse correlation between magnetic susceptibility (MS) and lithological ranks in fine clastic sediments, with the highest values in mudstones and the lowest in sandstones. The main magnetism carriers in the lower K1sh are pseudo-single-domain (PSD) and/or multi-domain (MD) magnetite with minor presence of hematite. MS was used to further explore the genesis of the environmental and climatic variations through cyclostratigraphic analysis. Sedimentary cycles of 113 m, 34 m, 13 m and 6 m can be identified in the power spectrum, which were interpreted as long and short eccentricity, obliquity, and precession cycles, demonstrating the impact of astronomical cyclicity on sedimentary rhythmicity. Floating astronomical time scale (FATS) of 4,090 kyr and 4,148 kyr were established by tuning the inferred long and short eccentricity cycles to the artificial 405-kyr and 105-kyr orbital eccentricity curves respectively. The estimated sediment accumulation rate around 28 cm/kyr confirms the rapid deposition process within the faulted lacustrine basin. Based on this study, the lake level oscillations in Songliao Basin are assumed to be shaped by long and short eccentricity, precession and semi-precession cycles during Early Cretaceous. This study also indicates that the sand-mudstone alternations deposition in K1sh is most likely driven by the seasonal discrepancies of summer insolation during semi-precession periods.

Reconsidering Carboniferous–Permian continental paleoenvironments in eastern equatorial Pangea: facies and sequence stratigraphy investigations in the Autun Basin (France)

The late Carboniferous–early Permian represents a key period in the Phanerozoic history, given the major global geodynamic and climate modifications. The aim of this work is to better understand the context and characteristics of the sedimentation recorded in the continental environments of eastern equatorial Pangea at this time, through the example of the Autun Basin (northeastern Massif Central, France). The Autun Basin contains the historical stratotype of the Autunian continental stage, and its stratigraphy was recently improved by accurate numerical ages. This basin formed in an extensional tectonic context during the latest stages of the Variscan orogeny, and it is essential to study its paleoenvironmental evolution to provide new insights into the sedimentary evolution of contemporaneous surrounding basins. Using field and subsurface data, we propose a refined sedimentological model for the Autun Basin, relying on updated facies interpretations, organic matter content fluctuations, and sequence stratigraphy concepts and correlations. The continental environments of the lower sedimentary succession of the Autun Basin, previously considered to be fluvial and lacustrine, are herein re-interpreted as mainly lacustrine, comprising fine-grained organic matter-rich deposits, and supplied by coarser-grained deltaic siliciclastic sediments, without preservation of strict fluvial sedimentation. The determination of the sequence stratigraphy cycles, strengthened by the quantification of the organic matter content, and reflected by the temporal succession of progradational and retrogradational trends, is used to determine new correlations between several sections, as well as to reconstruct the paleoenvironment evolution at the Carboniferous–Permian transition. This study provides evidence that the sedimentation area of the Autun Basin at the time of its filling was much larger than the preserved basin area, and suggests connections with contemporaneous neighboring French basins, pointing to a large sedimentary system in the northeastern Massif Central area rather than narrow and isolated basins.

New ornithopod tracks from the Lower Cretaceous El Castellar Formation (Spain): Implications for track preservation and evolution of ornithopod footprints

Dinosaur tracks have been identified in several Upper Jurassic-Lower Cretaceous sedimentary units across the Maestrazgo Basin in eastern Iberia, which are preserved in a variety of transitional and continental paleoenvironments. Here, we described the lower Barremian San Benón tracksite with the first occurrence of clear dinosaur tracks in the El Castellar Formation within the Galve subbasin. This palustrine-lacustrine formation has yielded a rich osteological record although dinosaur tracks are notably scarce. The new footprints represent an uncommon case of dinosaur track preservation since the site has yielded several non contemporary tracks (part of different ichnoassemblages) preserved as carbonate casts at the base of a limestone bed forming a composite ichnofabric. The tracksite shows a complex history of sedimentation, track production and preservation linked to the lake level variations. The ornithopod tracks are identified to belong to the ornithopod ichnogenus Caririchnium , concretely to the ichnospecies C. magnificum. The studied tracks represent the oldest occurrence of this ichnotaxa in the Maestrazgo Basin and are coherent with other coeval (or almost coeval) occurrences in the Iberian Peninsula. The presence of C. magnificum in the El Castellar Formation fills a gap between the oldest (Tithonian-early Valanginian?) and younger (Barremian) occurrences of ornithopod tracks within the Maestrazgo Basin, being one of the most complete successions of ornithopod tracks in Europe. Interestingly, the underlaying formations have ichnoassemblages with Dinehichnus -like and Iguanodontipus -like tracks, whereas Caririchnium is mainly found in El Castellar Formation and other Barremian units. These changes in the ichnoassemblages reflect the ornithopod faunal changes shown by osteological data in the Iberian Peninsula recording a Late Jurassic-earliest Cretaceous stage dominated by basal iguanodontians followed by an Early Cretaceous stage with abundance of more derived and large-sized iguanodontians. • First occurrence of clear dinosaur tracks in the lacustrine El Castellar Fm in the Galve subbasin. • Uncommon case of dinosaur footprint preservation as casts in carbonates. • Complex history of sedimentation and track production linked to the lake level. • First occurrence of Caririchnium magnificum in the Maestrazgo Basin (Iberian Peninsula). • Ornithopod faunal changes during the Late Jurassic/Early Cretaceous.

Badenian (middle Miocene) continental paleoenvironment in the Novohrad–Nógrád Basin (Central Paratethys): a volcano-sedimentary record from the Páris-patak Valley in Hungary

Two levels of volcaniclastics, comprising conglomerates, sandstones and mudstones, are interbedded with upper middle Miocene (upper Badenian) andesite pyroclastics near the Hungarian-Slovakian border in the distal region of the Central Slovakian Neogene Volcanic Field. Based on the field sedimentological investigations, the facies of the volcaniclastics (e.g., lateral and vertical grain size changes, sedimentary structures, textures, clast composition), their geometry and field relationships are documented herein with the aim of reconstructing the depositional environment. The silica-cemented volcaniclastics are mostly andesite clasts with only ~ 5% being granitoid, quarzitic, and tuff clasts as well as charred fossil wood fragments. The coarse-grained facies association includes crudely stratified, tabular or lenticular, clast-supported pebble-cobble conglomerates with erosive basal surfaces, b-axis imbrication, alternating with sets of cross-bedding. The fine-grained facies association comprises cross-bedded pebbly to medium-grained sandstone and lenses of tuffaceous clayey siltstone with rare horizontal lamination and water-escape structures. Rip-up mudstone clasts, with diametre up to 1 m, are present in both facies associations, revealing the co-existence of abandoned silty palaeo-channel plugs. Facies associations are arranged in several 0.5-4-m-thick, fining-upwards successions that likely formed in shallow channels as downstream- to laterally accreting longitudinal bars, extensive gravel sheets and bars that migrated in peak flow during floods. Palaeocurrent indicators (i.e., clast imbrication, direction of planar cross-bedding, orientation of petrified wood logs) show bedload transport by traction currents, initially towards ~S, and later towards ~W. Intermittently debris flows also occurred. Cross-bedded sandstones formed as in-channel transverse bars during medium/low discharge. Variation of grain size shows frequent discharge fluctuations during permanently wet conditions in the late Badenian. The 4-5-m-deep, low-sinuosity channels were part of a high-energy, gravel-bed braided-river system on the south-eastern foothills of the Lysec palaeo-volcano. Here, pyroclastics were reworked and redeposited as volcaniclastics during inter-eruption, high-discharge episodes.

Paleoenvironmental contexts of South American Cretaceous sauropod tracks

The ichnological Cretaceous sauropod record of South America is analyzed for the first time in relation with skeletal and paleoenvironmental data. The updated database includes 39 tracksites and 71 valid species (53 titanosaurs and 18 non-titanosaurs) from Colombia, Ecuador, Peru, Chile, Brazil and Argentina. The track and bone records analyzed evidence a relationship with continental environments, specifically with fluvial ones. This is observed in all Sauropoda records, indicating an ecological association of the Cretaceous sauropods for these environments. In addition, the paleogeographic reconstruction integrating these records does not evidence any correlation between the distribution of sauropods and the latitudinal range. During the late Campanian–Maastrichtian interval, when the Atlantic transgression event was established, the titanosaur record started to show a singular panorama. The tracksites are preserved not only in continental paleoenvironments but also in marginal-marine ones, being the only last records of titanosaurs associated with that environment in South America. Both the paleoecological aspects based on sauropod Cretaceous record and the paleoenvironmental data collected in this work support the hypothesis that these tracksites were used by titanosaurs as ‘transit areas’ to move among the environments they inhabited.

Bioerosive traces in a Pleistocene Anatid bone from Uruguay

We report the first avian bone collected in the Late Pleistocene Dolores Formation, which crops out near Colonia city. The fossil was identified as a femur belonging to an Anatidae that represents the second Anseriform from Uruguay. Two bioerosive traces found on this fossil consist of borings of different sizes and depths were probably occasioned by larvae insect. These traces might have been produced during a brief period of subaerial exposure, in a continental paleoenvironment with dry episodes.

Depositional Environments and Geochemical Assessments of the Bende Ameki Formation Potential as Petroleum Source Rocks in the Ogbunike Quarry, South-Eastern Nigeria

This paper focuses on investigating the paleoenvironments and hydrocarbon generation potentials of the outcropping Eocene Bende-Ameki Formation at Ogbunike quarry, Anambra Basin southeastern Nigeria, which is the Niger Delta Agbada Formation subsurface equivalent. The fine to coarse sandstones interbedded with parallel laminated grey, coaly shales, and bioturbated claystones were the dominant rock facies. The shales contain Ammobaculities, Ammontium, lenticulina, and Reophax benthic foraminifera of brackish to outer shelf environments. The rock sequence and biofacies associations indicate a fluvial, shoreface to delta environments. The marine and continental paleoenvironments are supported by the concentration and association of redox-sensitive trace elements such as vanadium and nickel of oxic to dysoxic paleoconditions. The twenty shales have a range of TOC from 0.39 - 8.81 wt% (mean 2.2 2 wt%), suggesting a good to very good source rocks. The organic richness is highest within the depth of 2 – 6 m across the quarry. Their genetic potential (S1+S2) ranges from 0.22 - 27.35 (mean 2.8 kgHC/ton) of rock, and hydrogen index from 26 to 292 mgHC/gTOC with a mean of 67.3 mgHC/gTOC. This, however, indicates dominance of Type III gas prone kerogen of terrestrial origin. The oxygenated water column characterized by the presence of benthonic scavengers may not preserve lipidenriched organic constituents of anoxic paleoenvironments which could account for the rare Type II oil and gas prone kerogen in the source rock. The thermal history inferred from the Tmax between 401°C - 424°C suggests that the source rocks are immature at the present stratigraphic level.

Geomorphological and paleoenvironmental evolution in the prehistoric framework of the coastland of Mondragone, southern Italy

Using morphological, stratigraphic, paleoecological and geoarcheological data, as well as radiocarbon datings, we reconstructed the evolution of the coastal plain of Mondragone, in the northern sector of the Campania Plain, during the last 40 kyr. The Late Pleistocene-Holocene morphodynamics of this coastland were mainly dictated by mutual interaction between tectonics, sea-level fluctuations, Quaternary volcanic eruptions, and subsidence. These processes also influenced the dynamics of prehistoric and proto-historic human populations. Actually, the discovery over the last 25 years of several archaeological sites referable to Upper Paleolithic-Early Iron Age as well as the recent finding of artifacts, fauna and, for the third time in Campania, of Neanderthal human remains in the Roccia San Sebastiano cave, demonstrates that the coastal plain of Mondragone had always hosted human settlements. This constant frequentation is confirmed by, both emerged and submerged, ruins of Roman age and Middle Ages, and the high level of urbanization of the modern town. The interpretation of four borehole stratigraphic sequences down to 22 m bgl, of microfossils analysis and sediment facies highlighted the succession of transition, from marine to freshwater, and continental paleoenvironments in this coastal plain. These wetlands developed in climatic conditions that varied from glacial (Würm) to postglacial phases. Some deposits are interpreted as marshy sediments accumulated in shallow, elongated ponds behind sandy beach or dunes, which existed almost up to the present. The reconstruction of landscape morphodynamic evolution shows that after the “super eruption” of the Campanian Ignimbrite (∼39 kyr BP) the physiography abruptly changed. A wide gulf characterized by gray tuff cliffs and facing northwest formed, along the littoral between the Garigliano and the Volturno river mouths during the volcanic stasis of the Phlegrean Fields, which lasted about ten thousand years after the violent ignimbrite eruption. In this period, the presence of Neanderthal and of a settlement in the Roccia San Sebastiano cave, at the foot of Mt. Massico, is proven by the findings of an excavation. Later (∼20 kyr BP - Holocene), subsidence and sea-level rise activated strong erosion processes due to the postglacial marine ingression, with a consequent rapid shoreline recession and the genesis of transition environments. Finally, according to the results of previous multidisciplinary research carried out on other Campania coastal plains, adjacent or not to the studied area, distinct generations of post-Campanian Ignimbrite - Holocene coastal lakes (lagoons, ponds) and waterlogged environments (marshes, quagmires) were recognized, slightly below and at the current sea-level.

Orbitally-driven evolution of Lake Turkana (Turkana Depression, Kenya, EARS) between 1.95 and 1.72 Ma: A sequence stratigraphy perspective

Lakes act as major archives for continental paleoenvironments, particularly when the evolution of lake levels over time serves as a guide for understanding regional paleohydrology and paleoclimate. In this paper, two sections from the Nachukui Formation (Turkana Depression, East African Rift System) provide a complete record of lake level variability and then paleohydrology for Lake Turkana between 1.95 and 1.72 Ma. This period corresponds to a key time during which important human evolutionary and technological innovations have occurred in East Africa and in the Turkana area. Based on sedimentary facies and sequence analyses on coastal deposits, one long-term regressive-transgressive cycle is identified between 1.95 and 1.72 Ma. Superimposed on this trend, five higher-frequency cycles of lake level change are identified between 1.87 and 1.76 Ma. Origins of these periodicities are attributed to orbital forcings. The extents of bathymetry change and shoreline migration during these periods are explored, suggesting that the period between 1.87 and 1.76 Ma was relatively dry and that climate experienced a relatively low variability. This finding differs strongly from most of the previous paleoenvironmental investigations in the region that argue high climate variability during a relatively wet period. This work emphasizes the importance of using sequence stratigraphy for analyzing lacustrine deposits.

Paleoenvironmental Reconstruction of A Late Cretaceous, Muddy, River-Dominated Polar Deltaic System: Schrader Bluff–Prince Creek Formation Transition, Shivugak Bluffs, North Slope of Alaska, U.S.A.

Abstract Upper Cretaceous (Santonian to late Campanian) strata from the upper 196 m at Shivugak Bluffs along the Colville River of northern Alaska preserve a paleopolar, fluvial–deltaic succession deposited along a prograding paleo–Arctic Ocean coastline. The uppermost lower Schrader Bluff Formation is best classified as a river-dominated delta based on muddy prodelta and delta-front deposits that contain abundant hyperpycnites, subaqueous terminal distributary channels, interdistributary bays, and mouth bars. This deltaic succession is overlain by distal distributaries and a braided fluvial system of the Prince Creek Formation. Channel measurements and paleo-discharge calculations from Prince Creek Formation distributaries indicate that rivers were the appropriate size and had the appropriate discharge to regularly produce hyperpycnites. Delta front and interdistributary bays transitioned into lakes and floodbasins on the lower delta plain, alongside distributary channels during delta lobe reorganization and abandonment. These floodbasins were subsequently transgressed and are overlain by proximal shelf and prodelta muds containing Phycosiphon. Overlying strata containing delta front, interdistributary bay, terminal distributary channel, distributary mouth bar, and distributary channel deposits signaled the reactivation of the delta lobe. Ultimately, these deltaic environments were incised into and became overlain by braided channel deposits during a marked drop in relative sea level and progradation of the fluvial system. Combining ichnologic and sedimentologic observations was critical to identify eleven distinct deltaic and continental paleoenvironments. This study documents the architecture and evolution of an ancient, high-latitude, muddy, river-dominated deltaic system and highlights the autogenic and allogenic processes preserved in the system.

Seismic Stratigraphy of Pleistocene Deltaic Deposits in Bahía Blanca Estuary, Argentina

The Bahía Blanca estuary (Argentina) has a morphological configuration resulting from hydrological and sedimentary processes related to Late Quaternary sea level changes. This estuarine system occupies a large coastal plain with a dense net of tidal channels, low-altitude islands and large intertidal flats. Little is known about the sedimentary units of the marine subbottom. Therefore, a stratigraphical analysis of the northern coast of Bahía Blanca estuary was carried out using high resolution seismic (3.5 kHz) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. The seismic stratigraphic data collected and their correlation with drilling lithological data show five seismic sequences (S1, S2, S3, S4 and S5), of which S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age. Sequences S3 and S4, whose lithology and seismic facies (paleochannel structures and prograding reflection configurations), were defined on these materials, to evidence the development of an ancient deltaic environment which was part of a large Pleistocene drainage system. The S5 sequence was formed during the Holocene transgressive-regressive process and complete the seismostratigraphic column defined in the present study.

Permian continental paleoenvironments in Southeastern Asia: New insights from the Luang Prabang Basin (Laos)

In Laos (Southeastern Asia), Late Paleozoic sediments were identified by early French explorations across Indochina during the late 19th century (Pavie missions), but little work was undertaken to characterize the sedimentological and stratigraphical context until now. From detailed sedimentological and paleontological studies, we propose an interpretation of the depositional environment and of the stratigraphic context of series located on the right bank of the Mekong River in the Luang Prabang Basin where three main formations were described. The silicoclastic Red Claystone Formation, attributed to alluvial plain environment, contains large fragments of unidentified dicynodonts. The Limestones and Sandstones Formation preserves a new macrofloral assemblage displaying affinities with Middle to Late Permian Cathaysian floras of South China. This assemblage occurs as an intercalation within marine calcareous sandstones that have yielded a marine fauna, including the ammonoid Pseudotirolites sp. which indicates a Late Permian (Changhsingian) age. The well-developed Purple Claystones Formation yielded an abundant and well preserved Late Permian fauna composed of a carnivorous amphibian and numerous Dicynodon cranial and postcranial elements. This formation shows a vertical evolution from braided river to alluvial plain with sheet-flood sand bed and bed-load rivers, with a constant supply of volcanic clasts. Results from the analysis of the paleontological associations in the Luang Prabang Basin suggest that a continental communication between Laurussia and the Indochina Block existed during the Permian, allowing for migration of the terrestrial Dicynodon fauna.

13C 18O clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ 18O. Interpreting speleothem δ 18O records in terms of absolute paleotemperatures and δ 18O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ 47 (a geochemical variable reflecting the statistical overabundance of 13C 18O bonds in CO 2 evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ 47 value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation. Here we report Δ 47 values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ 47 and heavier δ 18O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ 47 disequilibrium and that of δ 18O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ 18O and Δ 47 values, probably inherited from prior degassing within the cave system. In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ 47 in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ 47 in inorganic carbonates.

Paleoproterozoic lateritic paleosol–ultra-mature/mature quartzite–meta-arkose successions in southern Fennoscandia—intra-orogenic stage during the Svecofennian orogeny

A Paleoproterozoic lateritic paleosol overlain by ultra-mature/mature quartzite is reported for the first time from three localities in the Svecofennian province in southern Finland. These are important indicators for warm paleoclimate and continental paleoenvironment. Ultra-mature quartzites above the lateritic paleosols are overlain by matrix-supported conglomerates followed by mature quartzites. These latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta-arkose. We present a tectonic model in which exhumation, after 1.88–1.87 Ga contraction, unroofed plutonic rocks. Volcanism and exhumation-related erosion produced deposition of both volcanic and sedimentary material. Weathering of these rocks and deposition of ultra-mature/mature quartzites took place between 1.87 and 1.85/1.84 Ga followed by rifting at 1.84 Ga. Tectonic slicing during basin inversion thrust the pre-rift quartzites and rift-related volcanic and sedimentary rocks over older mid-crustal rocks and produced a low-angle stacking system. To accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface, we propose that exhumation due to faulting followed by stabilization of the upper crust (0–15/20 km) occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion. The ca. 1.85 Ga paleosol-quartzite successions in southern Fennoscandia have many similarities with the widespread ca. 1.7 Ga paleosol-quartzite successions in the United States. Both are intra-orogenic, formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface. Accretion-type models presented for southern Fennoscandia at 1.87–1.80 Ga are not compatible with the available data and either far-field setting during outboard subduction or a newly formed continental interior setting are proposed. Anyhow tectonic models proposed for southern Fennoscandia between 1.87 and 1.80 Ga should take into account the occurrence of lateritic paleosols and ultra-mature quartzites.

Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes

Organic matter constitutes a minor fraction of marine and freshwater sediments, yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments. The organic matter content of sediments is the residue of past biota. The amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times. General sources of the organic matter are inferred from bulk properties such as elemental compositions, carbon and nitrogen stable isotope ratios, Rock-Eval pyrolysis data, and organic petrography. Details of organic matter origins are refined by analyses of biomarker molecular compositions. Source changes are proxies for fluctuations in sea-level, oceanic surface currents, and continental climates. Algal paleoproductivity rates are indicated by organic-carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions. These parameters record past availability of nutrients and, therefore, are proxies of surface mixing in the oceans and amount of land runoff to lakes. Sea-surface paleotemperatures are recorded by the number of carbon–carbon double bonds in lipid biomarkers produced by marine algae. Larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling. The δ 13C and δD values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture, respectively. Diagenesis, which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials, can bias organic geochemical paleoenvironmental records. The magnitude of this potential source of misinformation must always be considered and evaluated. Comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change.

Paleoclimatic and structural environment at the end of the Cretaceous along the western flank of the Congo basin, with application of underground microdiamonds around Brazzaville

In Brazzaville (R.P. Congo), the study of outcrops and well sections permitted an investigation of the sedimentary evolution of the Continental Paleoenvironments during the Cretaceous and Tertiary, along a vertical thickness of 70 m (so-called Stanley Pool Formations). The base of the deposits is made of reworked red Precambrian sandstone of Inkisi which is associated with an argillaceous phase with dominant montmorillonite.The late-Cretaceous involved a renewal of erosion which was manifested by the deposition of coarse sands of alluvial-flats where general metamorphic minerals are associated with relatively important evidence of microdiamonds. Climatic changes indicate a more ferralitic pedogenesis of the drainage basins as evident by an increase in kaolin content. Towards the top of the section and the beginning of the Tertiary, kaolin tends to dominate the clay mineral fraction. The metakimberlites of Gabon and the diamond concentrations of Bouenzien at the flank of the granitic massif of Chaillu are considered as possible origins of the microdiamonds.