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Stratigraphic Record (stratigraphic + record)
Selected AbstractsThe microstratigraphic record of abrupt climate changes in cave sediments of the Western MediterraneanGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 5 2001Marie-Agnès Courty The purpose of this paper is to illustrate how calcareous sediments from Pleistocene and Holocene rockshelters and open caves of the Western Mediterranean can provide a stratigraphic record of abrupt climate change. The method proposed here is based on microstratigraphic examination of sedimentary sequences using microscopic techniques. The most important processes for characterizing the sensitivity of each cave to climate variables are: (1) the modes and rate of carbonate sediment production, (2) the nature and intensity of the pedogenic processes responsible for the synchronous alteration of carbonate materials (either those derived from the cave walls or those deposited on the ground surface), and (3) the supply of allogenic sediments, particularly by eolian activity. The cave sediment sequences presented record the marked coolings known as Dansgaard-Oeschger stadials and Heinrich events that occurred during the Pleistocene and the Holocene, as demonstrated by the high resolution records from ice and deep sea cores. At Abric Romanì in northeastern Spain, a series of sharp climatic deteriorations of increasing severity is shown to have occurred synchronously with the transition from the Middle to the Upper Paleolithic, with a period of seasonal frost and strong winds at ca. 37,000 yr B.P., tentatively correlated with Heinrich event 4. At Pigeon Cave, Taforalt (northern Morocco), the transition from the Aterian to Ibero-Maurusian/Epipalaeolithic cultures is dated to around 24,000,20,000 yr B.P. and is punctuated by a series of short cold pulses with evidence for seasonal freezing, soil erosion, and minimal evapotranspiration. In El Miron cave in north-central Spain, the exceptional nature of the Younger Dryas cooling produced a marked destabilization of the cave walls and roof. At El Miron, the stratigraphic evidence for sediment removal due to the rapid percolation of snow melt under a degraded soil cover allows us to reconstruct the nature of the negative excursion at ca. 8200 yr B.P. This example also illustrates how climate-controlled pedogenic processes can create a stratigraphic signature which has often been confused with a sedimentary hiatus. We conclude that cave sediments provide a valuable record of Pleistocene and Holocene climate changes. In appropriate contexts, these sequences allow us to examine the ecological stress generated by these unique global events at a local and regional level and improve our understanding of the complex anthropological processes that occurred at the same time. © 2001 John Wiley & Sons, Inc. [source] The sedimentary records in Mediterranean rockshelters and caves: Archives of environmental changeGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 4 2001Jamie C. Woodward It is important to develop rigorous methods and robust conceptual models for the interpretation of rockshelter and cave sediment records so that the cultural sequences they contain can be considered in their proper environmental context. Much of what we know about the prehistory of the Mediterranean region and adjacent areas has largely been pieced together from materials excavated from sedimentary sequences in these environments. The rockshelters and caves of the region form important environmental and sedimentary archives. Recent work has begun to consider if the remarkable climatic variability evident in the high resolution lacustrine and ice core records is manifest in the rockshelter and cave sediment records of the area. In this context, the two main characteristics of a rockshelter or cave site which control its usefulness as an archive of environmental change are the temporal resolution of the sedimentary record and the environmental sensitivity of the site. Many rockshelters and caves can be described as either Active Karst Settings (AKS) or Passive Karst Settings (PKS) and site type is an important influence on climatic sensitivity with a direct influence upon the usefulness of the sedimentary sequence as a proxy record of climate change. It is now clear that some sites may preserve detailed paleoclimatic records and the climatic signal may be represented by distinctive suites of micromorphological features, by variations in the input of allogenic sediment, or by fluctuations in the mineral magnetic properties of the fine sediment fraction. It can be argued that data derived from the analysis of bulk coarse-grained samples often lacks the stratigraphic resolution and environmental sensitivity that can be obtained from other approaches. The most favorable sites for detailed paleoclimatic reconstruction appear to be in active karst settings such as Theopetra Cave (Greece) and Pigeon Cave (Morocco) where micromorphological analyses offer insights into the stratigraphic record that are not otherwise obtainable. The temporal resolution of a site can only be established through a rigorous stratigraphic analysis and a comprehensive dating program. These are fundamental considerations in the study of rockshelter sediment records, especially when attempting to correlate between sites and draw comparisons with other proxy records of environmental change derived from sedimentary environments with rather different characteristics. Rockshelters and caves are part of a wider sediment system, and their investigation must be accompanied by detailed geomorphological, sedimentological, paleoecological, and geochronological studies of the off-site Quaternary record. © 2001 John Wiley & Sons, Inc. [source] Landscape and Coast Development of A Lowland Fjord Margin Following Deglaciation, East GreenlandGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3 2001Louise Hansen The landscapes of western Jameson Land bordering Hall Bredning fjord comprise upper river basins, glacial landscapes, lower river basins and a near-shore zone. The upper river basins are incised into bedrock and display no cover of young sediments whilst the glacial landscapes, located closer to the coast, are dominated by Pleistocene deposits and an irregular topography with hills and ridges. The lower river basins, dissecting the glacial landscapes, are connected to the upper river basins and contain well-defined Holocene delta terraces. The near-shore zone, which includes the present coast, displays a few raised shorelines. Geomorphological observations combined with stratigraphic work and 14C dates provide a chronological framework for the development of landscape and shoreline, as presented by a four-stage reconstruction. The first stage covers the deglaciation of western Jameson Land at the Weichselian-Holocene transition after a collapse of the main fjord glacier in Hall Bredning. The sea inundated the low-lying areas on Jameson Land forming small side-entry fjord basins that possibly follow the track of older valleys. This was followed by a second stage, the paraglacial period, when large meltwater production and sediment transport resulted in a fast infilling of the side-entry fjord basins by deltas. These are now exposed in terraces in the lower river basins at 70,80 m a.s.l. During a third stage, the relaxation period, fluvial activity decreased and the land surface was increasingly occupied by a cover of tundra vegetation. A glacio-isostatic rebound resulted in a relative sea level fall and fluvial incision. During stages two and three the coast was exposed to shallow marine processes that aided the alignment of the coast. Stages one to three presumably lasted for less than 2000 years. During stage four, the stable period, lasting for several thousand years till the present, there were minor adjustments of shoreline and landscape. The four-step reconstruction describes the sedimentary response of a lowland fjord margin to dramatic changes in climate and sea level. The distribution of erosion and sedimentation during this development was mainly controlled by topography. The reconstruction of the latest environmental development of Jameson Land puts new light on Jameson Land's long and complex Quaternary stratigraphic record. The reconstruction may also be used as a model for the interpretation of deposits in similar areas elsewhere. [source] Quantitative tests for stratigraphic cyclicityGEOLOGICAL JOURNAL, Issue 4 2008R. J. Bailey Abstract Periodic Milankovitch (M-) orbital forcing provides an explanation for subjectively recognized short-term repetition of lithofacies-,cycles'-in the stratigraphic record. Tests of this explanation often find no order in the lithofacies and/or no regularity in the recurrence of lithofacies. This does not disprove the influence of M-forcing, but a sedimentary response in terms of irregular M-forced ,cycles' is indistinguishable from one in which repetition of facies is not M-forced. Use of such cycles in time calibration is correspondingly suspect. Stricter, dimensional cyclicity invokes Sander's Rule, which suggests periodicity in sedimentation, for which M-forcing provides an obvious explanation. Time calibration on the basis of strict cyclicity thus appears more dependable. Objective tests for regular M-forced stratigraphic cyclicity commonly depend upon spectral analyses. Such tests are not unambiguous. Bilogarithmic thickness/frequency plots derived from objective layer thickness inventories (LTI) provide an alternative. Commonly, such plots show power-law relationships that preclude dimensional M-cyclicities. By contrast, a model data series that perfectly encodes the M-cyclic fluctuations in terrestrial insolation generates a strongly inflected, non-power-law LTI plot. Power-law plots result where the model data series is decimated by random hiatuses, with numbers and durations tuned to M-cycle frequencies. It seems improbable that natural data series record such tuning. The general absence of strict cyclicity in the M-frequency range is more likely to reflect the nonlinear response of sedimentary systems to cyclic M-forcing of insolation. Interestingly, when applied to the classically cyclic lacustrine Triassic sediments of the Newark Basin, USA, the LTI test suggests a decimated record, preserving some evidence of M-cyclicity. Copyright © 2008 John Wiley & Sons, Ltd. [source] Unravelling the multi-stage burial history of the Swiss Molasse Basin: integration of apatite fission track, vitrinite reflectance and biomarker isomerisation analysisBASIN RESEARCH, Issue 1 2006Martin Mazurek ABSTRACT A complex basin evolution was studied using various methods, including thermal constraints based on apatite fission-track (AFT) analysis, vitrinite reflectance (VR) and biomarker isomerisation, in addition to a detailed analysis of the regional stratigraphic record and of the lithological properties. The study indicates that (1) given the substantial amount of data, the distinction and characterisation of successive stages of heating and burial in the same area are feasible, and (2) the three thermal indicators (AFT, VR and biomarkers) yield internally consistent thermal histories, which supports the validity of the underlying kinetic algorithms and their applicability to natural basins. All data pertaining to burial and thermal evolution were integrated in a basin model, which provides constraints on the thickness of eroded sections and on heat flow over geologic time. Three stages of basin evolution occurred in northern Switzerland. The Permo-Carboniferous strike,slip basin was characterised by high geothermal gradients (80,100°C km,1) and maximum temperature up to 160°C. After the erosion of a few hundreds of metres in the Permian, the post-orogenic, epicontinental Mesozoic basin developed in Central Europe, with subsidence triggered by several stages of rifting. Geothermal gradients in northern Switzerland during Cretaceous burial were relatively high (35,40°C km,1), and maximum temperature typically reached 75°C (top middle Jurassic) to 100°C (base Mesozoic). At least in the early Cretaceous, a stage of increased heat flow is needed to explain the observed maturity level. After erosion of 600,700 m of Cretaceous and late Jurassic strata during the Paleocene, the wedge-shaped Molasse Foreland Basin developed. Geothermal gradients were low at this time (,20°C km,1). Maximum temperature of Miocene burial exceeded that of Cretaceous burial in proximal parts (<35 km from the Alpine front), but was lower in more distal parts (>45 km). Thus, maximum temperature as well as maximum burial depth ever reached in Mesozoic strata occurred at different times in different regions. Since the Miocene, 750,1050 m were eroded, a process that still continues in the proximal parts of the basin. Current average geothermal gradients in the uppermost 2500 m are elevated (32,47°C km,1). They are due to a Quaternary increase of heat flow, most probably triggered by limited advective heat transport along Paleozoic faults in the crystalline basement. [source] Assembling the stratigraphic record: depositional patterns and time-scales in an experimental alluvial basinBASIN RESEARCH, Issue 3 2002B. A. Sheets ABSTRACT Our understanding of sedimentation in alluvial basins is best for very short and very long time-scales (those of bedforms to bars and basinwide deposition, respectively). Between these end members, the intermediate time-scales of stratigraphic assembly are especially hard to constrain with field data. We address these ,mesoscale' fluvial dynamics with data from an experimental alluvial system in a basin with a subsiding floor. Observations of experimental deposition over a range of time-scales illustrate two important properties of alluvial systems. First, ephemeral flows are disproportionately important in basin filling. Lack of correlation between flow occupation and sedimentation indicates that channelized flows serve mainly as conduits for sediment, while most deposition occurs via short-lived unchannelized flow events. Second, there is a characteristic time required for individual depositional events to average to basin-scale stratal patterns. This time can be scaled in terms of the time required for a single channel-depth of aggradation, and in this form is constant through a four-fold variation of experimental subsidence rate. [source] Fluvial response to sea-level changes: a quantitative analogue, experimental approachBASIN RESEARCH, Issue 3 2001M. W. I. M. Van Heijst ABSTRACT Quantitative evaluation of fluvial response to allogenic controls is crucial for further progress in understanding the stratigraphic record in terms of processes that control landscape evolution. For instance, without quantitative insight into time lags that are known to exist between sea-level change and fluvial response, there is no way to relate fluvial stratigraphy to the sea-level curve. It is difficult to put firm constraints on these time-lag relationships on the basis of empirical studies. Therefore, we have started to quantify time-averaged erosion and deposition in the fluvial and offshore realms in response to sea-level change by means of analogue modelling in a 4 × 8-m flume tank. The rate of sea-level change was chosen as an independent variable, with other factors such as sediment supply, discharge and initial geometry kept constant over the course of 18 experiments. Our experimental results support the common view that neither fall nor rise in sea level affects the upstream fluvial system instantaneously. An important cause for the delayed fluvial response is that a certain amount of time is required to connect initial incisions on the newly emergent shelf (canyons) with the fluvial valley. Lowering of the fluvial longitudinal profile starts only after the connection of an active shelf canyon with the fluvial valley; until that moment the profile remains steady. We quantified the process of connection and introduced the quantity ,connection rate'. It controlled, in conjunction with the rate of sea-level fall: (1) the amount of fluvial degradation during sea-level fall; (2) the total sediment volume that bypasses the shelf edge; (3) the percentage of fluvial relative to shelf sediment in the lowstand delta; (4) the volume of the transgressive systems tract and (5) the amount of diachroneity along the sequence boundary. Our experiments demonstrate also that the sequence-stratigraphic concept is difficult to apply to continental successions, even when these successions have been deposited within the influence of sea level. [source] The Last Glacial Maximum in the North Sea Basin: micromorphological evidence of extensive glaciation,JOURNAL OF QUATERNARY SCIENCE, Issue 2 2006S. J. Carr Abstract Despite a long history of investigation, critical issues regarding the last glacial cycle in northwest Europe remain unresolved. One of these refers to the extent, timing and dynamics of Late Devensian/Weichselian glaciation of the North Sea Basin, and whether the British and Scandinavian ice sheets were confluent at any time during this period. This has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring. This study presents the results of seismic, sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the North Sea Basin. Thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process-based interpretations from sediment cores. On the basis of micromorphology, critical formations from the basin have been reinterpreted, with consequent stratigraphic implications. Within the current stratigraphic understanding of the North Sea Basin, a complex reconstruction is suggested, with a minimum of three major glacial episodes inferred. On at least two occasions during the Weichselian/Devensian, the British and Scandinavian ice sheets were confluent in the central North Sea. Whilst micromorphology can provide much greater confidence in the interpretation of Late Quaternary offshore stratigraphic sequences, it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records. Copyright © 2006 John Wiley & Sons, Ltd. [source] | |||||||||||||