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1.
New intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika’s central basin capture dramatic evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences (A–D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a major regression that shifted the paleo-lake shore ∼21 km towards the west prior to ∼106 ka. The topset-to-foreset transitions in these deltas suggest lake level was reduced by ∼435 m during the period of deposition. Mounded reflections in the overlying sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived desiccation event that reduced lake level by ∼260 m, interpreted as a product of Last Glacial Maximum (32–14 ka) aridity. Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged aridity.  相似文献   

2.
Sediments of Lake Van, Turkey, preserve one of the most complete records of continental climate change in the Near East since the Middle Pleistocene. We used seismic reflection profiles to infer past changes in lake level and discuss potential causes related to changes in climate, volcanism, and regional tectonics since the formation of the lake ca. 600 ka ago. Lake Van’s water level ranged by as much as 600 m during the past ~600 ka. Five major lowstands occurred, at ~600, ~365–340, ~290–230, ~150–130 and ~30–14 ka. During Stage A, between about 600 and 230 ka, lake level changed dramatically, by hundreds of meters, but phases of low and high stands were separated by long time intervals. Changes in the lake level were more frequent during the past ~230 ka, but less dramatic, on the order of a few tens of meters. We identified period B1 as a time of stepwise transgressions between ~230 and 150 ka, followed by a short regression between ca. 150 and 130 ka. Lake level rose stepwise during period B2, until ~30 ka. During the past ~30 ka, a regression and a final transgression occurred, each lasting about 15 ka. The major lowstand periods in Lake Van occurred during glacial periods, suggesting climatic control on water level changes (i.e. greatly reduced precipitation led to lower lake levels). Although climate forcing was the dominant cause for dramatic water level changes in Lake Van, volcanic and tectonic forcing factors may have contributed as well. For instance, the number of distinct tephra layers, some several meters thick, increases dramatically in the uppermost ~100 m of the sediment record (i.e. the past ~230 ka), an interval that coincides largely with low-magnitude lake level fluctuations. Tectonic activity, highlighted by extensional and/or compressional faults across the basin margins, probably also affected the lake level of Lake Van in the past.  相似文献   

3.
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.  相似文献   

4.
This paper describes the evolution of an extensional basin in regard to the nature and sequence stratigraphic arrangement of its carbonate deposits. The purpose of this study is to evaluate the respective effects of tectonism, eustasy, climate and oceanography on a carbonate sedimentary record. The case study is the early to mid‐Jurassic age carbonate succession of the Southern Provence Sub‐basin (SE France), located within the southern part of the extensional Western European Tethyan Margin. This work is based on sedimentologic, biostratigraphic (using ammonites and brachiopods) and sequence stratigraphic analysis of the carbonate facies of the Cherty Reddish Limestone Formation (late Sinemurian to earliest Bajocian). These strata were deposited in shoreface to lower offshore depositional environments. The succession of the various environments together with the recognition of key stratigraphic surfaces allow us to define four second‐order depositional sequences; of late Sinemurian to earliest Pliensbachian, early Pliensbachian to late Pliensbachian, earliest Toarcian to middle Aalenian and late Aalenian to early Bathonian ages. The architecture of the depositional sequences (thickness and facies variations within the systems tracts, wedge‐shaped geometries) reflects a strong tectonic control. The sub‐basin was structured by extensional faults (oriented approximately 070–090/250–270). Sea‐level variations, fluctuations in carbonate production and preservation, and environmental changes were also significant controlling factors of the carbonate deposition. The interplay of the tectonic control with the other factors resulted in five main phases in the sedimentary evolution of the sub‐basin: (1) dominant tectonic control during the initial rifting stage (late Sinemurian to early Pliensbachian); (2) increasing extensional tectonics (mid‐Pliensbachian); (3) global climato‐eustatic sea‐level fall (latest Pliensbachian) and global climato‐eustatic sea‐level rise plus hypoxia/anoxia (early Toarcian); (4) relative sea‐level fall linked to tectonic uplift related to the ‘Mid‐Cimmerian phase’ (mid‐Aalenian) and (5) oceanographic events (upwelling) and reduction in carbonate production (hypoxia/anoxia) plus tectonic downwarping (late Aalenian/earliest Bajocian).  相似文献   

5.
The North Sakhalin Basin in the western Sea of Okhotsk has been the main site of sedimentation from the Amur River since the Early Miocene. In this article, we present regional seismic reflection data and a Neogene–Recent sediment budget to constrain the evolution of the basin and its sedimentary fill, and consider the implications for sediment flux from the Amur River, in particular testing models of continental‐scale Neogene drainage capture. The Amur‐derived basin‐fill history can be divided into five distinct stages: the first Amur‐derived sediments (>21–16.5 Ma) were deposited during a period of transtension along the Sakhalin‐Hokkaido Shear Zone, with moderately high sediment flux to the basin (71 Mt year?1). The second stage sequence (16.5–10.4 Ma) was deposited following the cessation of transtension, and was characterised by a significant reduction in sediment flux (24 Mt year?1) and widespread retrogradation of deltaic sediments. The third (10.4–5.3 Ma) and fourth (5.3–2.5 Ma) stages were characterised by progradation of deltaic sediments and an associated increase in sediment flux (48–60 Mt year?1) to the basin. Significant uplift associated with regional transpression started during this time in southeastern Sakhalin, but the north‐eastward propagating strain did not reach the NE shelf of Sakhalin until the Pleistocene (<2.5 Ma). This uplift event, still ongoing today, resulted in recycling of older deltaic sediments from the island of Sakhalin, and contributed to a substantially increased total sediment flux to the adjacent basinal areas (165 Mt year?1). Adjusted rates to discount these local erosional products (117 Mt year?1) imply an Amur catchment‐wide increase in denudation rates during the Late Pliocene–Pleistocene; however, this was likely a result of global climatic and eustatic effects, combined with tectonic processes within the Amur catchment and possibly a smaller drainage capture event by the Sungari tributary, rather than continental‐scale drainage capture involving the entire upper Amur catchment.  相似文献   

6.
Palaeogeographic and lake-level reconstructions provide powerful tools for evaluating competing scenarios of biotic, climatic and geological evolution within a lake basin. Here we present new reconstructions for the northern Lake Tanganyika subbasins, based on reflection seismic, core and outcrop data. Reflection seismic radiocarbon method (RSRM) age estimates provide a chronological model for these reconstructions, against which yet to be obtained age dates based on core samples can be compared. A complex history of hydrological connections and changes in shoreline configuration in northern Lake Tanganyika has resulted from a combination of volcanic doming, border fault evolution and climatically induced lake-level fluctuations. The stratigraphic expression of lake-level highstands and lowstands in Lake Tanganyika is predictable and cyclic (referred to here as Capart Cycles), but in a pattern that differs profoundly from the classic Van Houten cycles of some Newark Supergroup rift basins. This difference results from the extraordinary topographic relief of the Western Rift lakes, coupled with the rapidity of large-scale lake-level fluctuations. Major unconformity surfaces associated with Lake Tanganyika lowstands may have corresponded with high-latitude glacial maxima throughout much of the mid- to late Pleistocene.
Rocky shorelines along the eastern side of the present-day Ubwari Peninsula (Zaire) appear to have had a much more continuous existence as littoral rock habitats than similar areas along the north-western coastline of the lake (adjacent to the Uvira Border Fault System), which in turn are older than the rocky shorelines of the north-east coast of Burundi. This model of palaeogeographic history will be of great help to biologists trying to clarify the evolution of endemic invertebrates and fish in the northern basin of Lake Tanganyika.
  相似文献   

7.
Swath bathymetry, single‐channel seismic profiling, gravity and box coring, 210Pb down‐core radiochemical analyses and sequence stratigraphic analysis in the Gulf of Alkyonides yielded new data on the evolution of the easternmost part of the Gulf of Corinth. Three fault segments, the South Strava, West Alkyonides and East Alkyonides faults, dipping 45, 30 and 45°, respectively, northwards, form the southern tectonic boundary of the Alkyonides Basin. Two 45° southwards dipping segments, the Domvrena and Germeno Faults, form the northern tectonic margin. The Alkyonides Basin architecture is the result of a complex interaction between fault dynamics and the effects of changes in climate and sea/lake level. Chrono‐stratigraphic interpretation of the seismic stratigraphy through correlation of the successive seismic packages with lowstands and highstands of the Late Quaternary indicates that the evolution of the basin started 0.40–0.45 Ma BP and can be divided in two stages. Subsidence of the basin floor during the early stage was uniform across the basin and the mean sedimentation rate was 1.0 m kyear?1. Vertical slip acceleration on the southern tectonic margin since 0.13 Ma BP resulted in the present asymmetric character of the basin. Subsidence concentrated close to the southern margin and sedimentation rate increased to 1.4 m kyear?1 in the newly formed depocentre of the basin. Actual (last 100 year) sedimentation rates were calculated to >2 mm year?1, but are significantly influenced by the presence of episodic gravity flow deposits. Total vertical displacement of 1.1 km is estimated between the subsiding Alkyonides Basin floor and the uplifting Megara Basin since the onset of basin subsidence at a mean rate of 2.4–2.75 m kyear?1, recorded on the East Alkyonides Fault. Gravity coring in the Strava Graben and in the lower northern margin of Alkyonides Basin proved the presence of whitish to olive grey laminated mud below thin marine sediments. Aragonite crystals and absence of the marine coccolithophora Emiliania huxleyi indicate sedimentation in lacustrine environment during the last lowstand glacial interval.  相似文献   

8.
The Permo‐Carboniferous to Eocene Sverdrup Basin in Canada's Arctic Archipelago is strongly influenced by evaporite diapirism. However, salt structures within the basin have not been extensively investigated recently due to their remote location. This study includes the interpretation of legacy seismic reflection and borehole data to characterize the geometry of selected evaporite domes, and 1D backstripping of wells to investigate tectonic and sedimentary influences on diapirism. Extensional rift‐structures appear to have played a significant role in the formation of evaporite domes by triggering and directing salt movement. Diapirism was initiated by at least the Middle Triassic and continued to develop during the Mesozoic. Differential loading of salt on opposing east–west dome margins led to their present day asymmetric geometries. Diapir growth rates in the Mesozoic were closely linked to the rate of sedimentation and influenced by regional tectonism.  相似文献   

9.
The Austral Basin (or Magallanes Basin) in southern Argentina is situated in a highly active tectonic zone. The openings of the South Atlantic and the Drake Passage to the east and south, active subduction in the west, and the related rise of the Andes have massively influenced the evolution of this area. To better understand the impacts of these tectonic events on basin formation to its present‐day structure we analysed 2D seismic reflection data covering about 95 000 km² on‐ and 115 000 km² offshore (Austral ‘Marina’ and Malvinas Basin). A total of 10 seismic horizons, representing nine syn‐ and post‐ rift sequences, were mapped and tied to well data to analyse the evolution of sedimentary supply and depocenter migration through time. 1D well backstripping across the study area confirms three main tectonic stages, containing (i) the break‐up phase forming basement graben systems and the evolution of the Late Jurassic – Early Cretaceous ancient backarc Austral/Rocas Verdes Basin (RVB), (ii) the inversion of the backarc marginal basin and the development of the foreland Austral Basin and (iii) the recent foreland Austral Basin. Synrift sedimentation did not exceed the creation of accommodation space, leading to a deepening of the basin. During the Early Cretaceous a first impulse of compression due to Andes uplift caused rise also of parts of the basin. Controlling factors for the subsequent tectonic development are subduction, balanced phases of sedimentation, accumulation and erosion as well as enhanced sediment supply from the rising Andes. Further phases of rock uplift might be triggered by cancelling deflection of the plate and slab window subduction, coupled with volcanic activity. Calculations of sediment accumulation rates reflect the different regional tectonic stages, and also show that the Malvinas Basin acted as a sediment catchment after the filling of the Austral Basin since the Late Miocene. However, although the Austral and Malvinas Basin are neighbouring basin systems that are sedimentary coupled in younger times, their earlier sedimentary and tectonic development was decoupled by the Rio Chico basement high. Thereby, the Austral Basin was affected by tectonic impacts of the Andes orogenesis, while the Malvinas Basin was rather affected by the opening of the South Atlantic.  相似文献   

10.
The Salar de Atacama forms one of a series of forearc basins developed along the western flank of the Central Andes. Exposed along the northwest margin of the basin, a salt‐cored range, the Cordillera de la Sal, records the Mid‐Miocene to recent sedimentological and structural development of this basin. Sediments of the Mid‐Miocene Vilama Formation record the complex interaction between regional/local climate change, halokinesis and compressional deformation. This study reveals how these factors have controlled the facies development and distribution within the Salar de Atacama. Detailed sedimentary logging, cross‐sections and present day geomorphology through the northern Cordillera de la Sal have been used to establish a lithostratigraphy, chronostratigraphy and the regional distribution of the Vilama Formation. The Vilama Formation documents an increase in aridity with a hiatus in sedimentation from Mid‐Miocene to 9 Ma with initial uplift of the Cordillera de la Sal. From 9 Ma to 8.5 Ma deposition of a meandering fluvial system is recorded followed by a rapid decrease in sedimentation till 6 Ma. From 6 to 2 Ma, the deposition of extensive palustrine carbonates and distal alluvial–mudflat–lacustrine demonstrates the existence of an extensive lake within the Salar de Atacama. Post 2 Ma, the lake decreased in size and braided alluvial gravels associated with alluvial fans were widespread through the region suggesting a final shift to hyperarid conditions. By comparing the Vilama Formation with similar age facies throughout northern Chile and southern Peru, several shifts in climate are recognized. Climate signatures within northern Chile appear to be largely diachronous with the last regional event in the Mid‐Miocene. Since that time, humid events have been restricted to either Precordillerian basins or the Central Atacama. Within the Central Atacama, the final switch to hyperarid conditions was not till the earliest Pleistocene, much later than previously estimated within the region.  相似文献   

11.
This paper discusses the Cenozoic interaction of regional tectonics and climate changes. These processes were responsible for mass flux from mountain belts to depositional basins in the eastern Alpine retro‐foreland basin (Venetian–Friulian Basin). Our discussion is based on the depositional architecture and basin‐scale depositional rate curves obtained from the decompacted thicknesses of stratigraphic units. We compare these data with the timing of tectonic deformation in the surrounding mountain ranges and the chronology of both long‐term trends and short‐term high‐magnitude (‘aberrant’) episodes of climate change. Our results confirm that climate forcing (and especially aberrant episodes) impacted the depositional evolution of the basin, but that tectonics was the main factor driving sediment flux in the basin up to the Late Miocene. The depositional rate remained below 0.1 mm year?1 on average from the Eocene to the Miocene, peaking at around 0.36 mm year?1, during periods of maximum tectonic activity in the eastern Southern Alps. This dynamic strongly changed during the Pliocene–Pleistocene, when the basin‐scale depositional rate increased to an average of 0.26 mm year?1 (Pliocene) and 0.73 mm year?1 (Pleistocene). This result fits nicely with the long‐term global cooling trend recorded during this time interval. Nevertheless, we note that the timing of the observed increase may be connected with the presumed onset of major glaciations in the southern flank of the Alps (0.7–0.9 Ma), the acceleration of the global cooling trend (since 3–4 Ma) and climate variability (in terms of magnitude and frequency). All these factors suggest that combined high‐frequency and high‐magnitude cooling–warming cycles are particularly powerful in promoting erosion in mid‐latitude mountain belts and therefore in increasing the sediment flux in foreland basins.  相似文献   

12.
The Dead Sea is an extensional basin developing along a transform fault plate boundary. It is also a terminal salt basin. Without knowledge of precise stratigraphy, it is difficult to differentiate between the role of plate and salt tectonics on sedimentary accumulation and deformation patterns. While the environmental conditions responsible for sediment supply are reasonably constrained by previous studies on the lake margins, the current study focuses on deciphering the detailed stratigraphy across the entire northern Dead Sea basin as well as syn and post-depositional processes. The sedimentary architecture of the late Quaternary lacustrine succession was examined by integrating 851 km of seismic reflection data from three surveys with gamma ray and velocity logs and the stratigraphic division from an ICDP borehole cored in 2010. This allowed seismic interpretation to be anchored in time across the entire basin. Key surfaces were mapped based on borehole lithology and a newly constructed synthetic seismogram. Average interval velocities were used to calculate isopach maps and spatial and temporal sedimentation rates. Results show that the Amora Formation was deposited in a pre-existing graben bounded by two N-S trending longitudinal faults. Both faults remained active during deposition of the late Pleistocene Samra and Lisan Formations—the eastern fault continued to bound the basin while the western fault remained blind. On-going plate motion introduced a third longitudinal fault, increasing accommodation space westwards from the onset of deposition of the Samra Formation. During accumulation of these two formations, sedimentation rates were uniform over the lake and similar. High lake levels caused an increase in hydrostatic pressure. This led to salt withdrawal, which flowed to the south and southwest causing increased uplift of the Lisan and En Gedi diapirs and the formation of localized salt rim synclines. This induced local seismicity and slumping, resulting in an increased thickness of the Lisan succession within the lake relative to its margins. Sedimentation rates of the Holocene Ze'elim Fm were 4–5 times higher than before. The analysis presented here resolves central questions of spatial extent and timing of lithology, deposition rates and their variability across the basin, timing of faulting at and below the lake floor, and timing and extent of salt and plate tectonic phases and their effect on syn and post-depositional processes. Plate tectonics dictated the structure of the basin, while salt tectonics and sediment accumulation were primarily responsible for its fill architecture during the timeframe examined here.  相似文献   

13.
The Cenozoic strata of the Xining Basin, NE Tibet, have provided crucial records for understanding the tectonic and palaeo-environmental evolution of the region. Yet, the age of the lower part of the sedimentary stratigraphy and, consequently, the early tectonic evolution of the basin remain debated. Here, we present the litho- and magnetostratigraphy of various early Eocene sections throughout the Xining Basin independently constrained by the U–Pb radiometric age of a carbonate bed. Our study extends the dated stratigraphy down to 53.0 Ma (C24n.1r) and reveals highly variable accumulation rates during the early Eocene ranging from 0.5 to 8 cm/ka. This is in stark contrast to the low but stable accumulation rates (2–3 cm/ka) observed throughout the overlying Palaeogene and Neogene strata. Such a pattern of basin infill is not characteristic of flexural subsidence as previously proposed, but rather supports an extensional origin of the Xining Basin with multiple depocentres, which subsequently coalesced into a more stable and slowly subsiding basin. Whether this extension was related to the far-field effects of the subducting Pacific Plate or the India–Asia collision remains to be confirmed by future studies.  相似文献   

14.
Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.  相似文献   

15.
The seismically and volcanically active Kivu Rift, in the western branch of the East African Rift System, is a type locale for studies of high‐elevation, humid‐climate rift basins, as well as magmatic basin development. Interpretations of offshore multi‐channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ca. 110 km‐long, N‐S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m‐high Lake Kivu to 770 m‐high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west‐dipping half graben with at least 1.5 km of sedimentary section; most of the ca. 2 km of extension in this sub‐basin is accommodated by the east‐dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault‐controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short‐term processes such as lake levels, inflow and outlets, as well as long term modification of classic half‐graben basin morphology.  相似文献   

16.
The North Sea Basin contains an almost complete record of Cenozoic sedimentation, separated by clear regional unconformities. The changes in sediment characteristics, rate and source, and expression of the unconformities reflect the tectonic, eustatic and climatic changes that the North Sea and its margins have undergone. While the North Sea has been mapped locally, we present the first regional mapping of the Cenozoic sedimentary strata. Our study provides a new regional sub‐division of the main seismic units in the North Sea together with maps of depocentres, influx direction and source areas. Our study provides a regional synthesis of sedimentation based on a comprehensive interpretation of a regionally covering reflection seismic data set. We relate observations of sediment characteristics and unconformities to the geological evolution. The timing, regional expression and stratigraphic characteristics of many unconformities indicate that they were generated by eustatic sea‐level fall, often in conjunction with other processes. Early Cenozoic unconformities, however, relate to tectonism associated with the opening of the North Atlantic. From observation on a regional scale, we infer that the sediment influx into the North Sea during the Cenozoic is more complex than previously suggested clockwise rotation from early northwestern to late southern sources. The Shetland Platform supplied sediment continuously, although at varying rates, until the latest Cenozoic. Sedimentation around Norway changed from early Cenozoic influx from the southwestern margin, to almost exclusively from the southern margin in the Oligocene and from all of southern Norway in the latest Cenozoic. Thick Eocene deposits in the Central Graben are sourced mainly from a western and a likely southern source, indicating that prominent influx from the south did not only occur from the mid‐Miocene onwards. We infer a new age for the increased progradational sediment influx in the Pleistocene of 2.5 Ma, coeval with Fennoscandian glaciation.  相似文献   

17.
Fluvial response to tectonic deformation is dependent on the amount and style of surface deformation and the relative size of the stream. Active folding in the New Madrid seismic zone (NMSZ) forms the Tiptonville dome, a 15-km long and 5-km wide surface fold with up to 11 m of late Holocene structural relief. The fold is crossed by streams of varying size, from the Mississippi River to small flood-plain streams. Fluvial response of these streams to repeated coseismic folding has only been preserved for the past 2.3 ka, since the Tiptonville meander of the Mississippi River migrated across the area forming the present flood plain. This surface comprises a sandy point-bar deposit locally overlain by clayey overbank and silty sand crevasse-splay deposits, an abandoned chute channel infilled with laminated sandy silt and silty clay, and an abandoned neck cutoff filled with a sandy cutoff bar and silty clay oxbow lake deposits.Dating various stream responses to coseismic folding has more tightly constrained the timing of earthquake events in the central NMSZ and provides a means of partitioning the deformation amount into individual seismic events. Three earthquakes have been dated in the Reelfoot Lake area, ca. A.D. 900, 1470, and 1812. The latter two earthquakes had large local coseismic deformation. Both of these events were responsible for numerous stream responses such as shifting depocenters, modification of Mississippi River channel geometry, and derangement of small streams. Overbank sedimentation ceased on the dome as it was uplifted above the normal flood stage, and sedimentation of crevasse-splay deposits from the Mississippi River, colluvium from the scarp, and lacustrine sediment accumulated in the adjacent Reelfoot basin. The much larger Mississippi River channel responded to uplift by increasing its sinuosity across the uplift relative to both upstream and downstream, increasing its width/depth ratio across and downstream of the uplift, and decreasing the width/depth ratio upstream of the uplift. Despite the size of the Mississippi River, it has not yet attained equilibrium since the latest uplift 190 years ago. Small channels that could not downcut through the uplift were filled, locally reversed flow direction, or formed a lake where they were dammed.Uplift and stream response to folding along the Tiptonville dome is less dramatic between 2.3 and 0.53 ka. During this interval, abandoned channel fill and overbank deposition across the dome suggests that it was not a high-relief feature. One earthquake event occurred during this interval (ca. A.D. 900), but coseismic stream response was probably limited to a slight aggradation of a small flood-plain stream.  相似文献   

18.
The computer simulation of a Lake Baikal seismic profile located in the Selenga River Delta area resulted in a lake level record of the last 600 kyr. This curve demonstrates several low-magnitude episodes and both a dramatic 300 m fall and a more than 150 m increase of the lake level relative to present situation. The greatest change in paleo-lake depth at 300 ka corresponds in time with the major glaciation in the Eastern Siberia and is probably the response of the lake to this climatic phenomenon. The results of this study conform with existing hypotheses on the regional tectonic history and climatic events.  相似文献   

19.
The Neoproterozoic basins of central Australia share many features of architecture and sedimentary fill, suggesting common large-scale extrinsic causal mechanisms. In an attempt to improve understanding of these mechanisms we have gathered and analysed new deep seismic reflection data and re-evaluated existing seismic and well-log data from the eastern Officer Basin, the largest and most poorly known of Australia's intracratonic basins. The Officer Basin is asymmetric and has a steep thrust-controlled northern margin paralleled by sub-basins as much as 10 km in depth. Further south the basin shallows gradually onto a broad platform. The basin rests on a thick crust (≈42 km) that is pervaded by a complex of northward-dipping surfaces most of which terminate erosionally against the sediments of the Officer Basin and are interpreted as prebasinal features, possibly faults. Some appear to have been zones of crustal weakness which were reactivated as thrust complexes and played a major role in basin evolution. The sedimentary succession can be subdivided into six megasequences separated by major tectonically and erosionally enhanced sequence boundaries. The megasequences have distinctive sequence stacking patterns suggesting that they were deposited in response to episodic subsidence induced by a major extrinsic tectonic event. The basin initially formed as part of a giant sag basin which incorporated all the present-day intracratonic basins (Amadeus, Georgina, Ngalia, Officer and Savory Basins) in a single large ‘superbasin’ perhaps as a response to mantle processes. Subsidence then ceased for ≈100 Myr producing a regional erosion surface. Beginning in the Torrensian or Sturtian five more major events of varying regional significance influenced the basin's evolution. Four were compressional events, the first of which activated major thrust complexes along the present basin margins, forming deep foreland sub-basins with elevated intervening basement blocks. Once activated, the thrust complexes and sub-basins persisted throughout the life of the intracratonic basins. From this epoch the intracratonic basins of central Australia were decoupled from the giant sag basin and became interrelated but independent features. Available information suggests that the Officer, Amadeus, Georgina, Ngalia and Savory Basins are related and are perhaps products of major tectonic events associated with the assembly and ultimate dispersal of the Proterozoic supercontinent. The closing phases of these basins were strongly influenced by events occurring along the newly created active eastern margin of the Australian continent in the Palaeozoic.  相似文献   

20.
The post-glacial history of the Great Lakes has involved changes in lake levels that are equivalent in vertical extent to the Pleistocene changes in global sea level and changes in sediment accumulation by at least two orders of magnitude. In the sediments of the northern Lake Michigan basin, these radical changes in base level and sediment supply are preserved in detailed records of changing depositional environment and the impact of these changes on depositional architecture. The seismic sequences of the sediment fill previously described in Lake Huron have been carried into northern Lake Michigan and used to map the history and architecture of basinal deposition. As the Laurentide Ice Sheet retreated northward in the early Holocene, it opened progressively deeper channels to the east that allowed the larger lakes to drain through the North Channel, Huron, and Georgian Bay basins. At the end of the Main Algonquin highstand, about 10,200 (radiocarbon) yrs ago, the eastern drainage passage deepened in a series of steps that defined four seismic sequences and lowered lake levels by over 100 m. Near the same time a new source of sediment and meltwaters poured across the Upper Peninsula of Michigan and into the northern Lake Michigan basin from the Superior basin ice lobe. A marked increase in deposition is seen first in the northern part of the study area, and slightly later in the Whitefish Fan area at the southern end of the study area. Accumulation rates in the area gradually decreased even as lake levels continued to fall. Drainage directly from the Superior basin ended before the beginning of the main Mattawa phase about 9,200 (radiocarbon) yrs ago.Although individual lowstand systems tracts are at the most a few hundred yrs in duration, their geometries and seismic character are comparable to marine systems tracts associated with sea level falls of similar magnitudes. In some of the thicker lowstand deposits a second order cyclicity in sedimentation can be detected in the high resolution seismic records.  相似文献   

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