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1.
The suitability of marsh sites for sea‐level studies was examined based on field experiments along a transect from low to high marsh. Bead distributions were determined both seasonally and after 7 years. Seasonal sediment mixing was greatest in the low marsh and in the late spring and early summer, when biological activity is greatest. However, after an initial interval of relatively intense reworking, the bead concentrations reached an approximate equilibrium profile characteristic of each marsh environment as reflected by the profiles obtained after 7 years. Mixed‐layer thickness is greatest (>10 cm) in the intermediate and low marsh, and burial rates are rapid (3.7–11.1 mm yr?1). Moreover, burial rates are comparable to or even surpass longer‐term (30 to >150 yr) radiotracer‐derived sediment accumulation rates and rates of local and regional sea‐level rise (～4 mm yr?1). Therefore, sediment accumulation rates appear to reflect primarily sediment resuspension/redeposition within the system due to bioturbation. Thus, bioturbation may be critical to the ability of marshes to keep pace with sea level, while seemingly precluding the use of low marsh for high‐resolution sea‐level studies. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

2.
东海陆架全新统高分辨率层序地层学研究   总被引：2，自引：0，他引：2       下载免费PDF全文
在高分辨率１４Ｃ测年、岩石、生物、化学、同位素、气候及磁性地层学研究成果基础上,通过不同沉积背景典型钻孔的沉积学分析,运用层序地层学理论,对东海陆架全新统进行了高分辨率层序划分及对比;建立了全新世层序地层格架及海平面变化过程;提出了相应的层序成因模式。研究结果表明,东海陆架全新统相当于一个发育中的六级（１～１０ｋａ）层序,可进一步划分为３个七级（０．１～１ｋａ）层序和若干个更次级层序。代表１个六级或３个七级周期相对海平面变化过程中叠加有若干更次级的海平面波动,它们与地球旋回谱系中的太阳带、历法带密切相关。七级层序具有与三级层序相近的内部构型和成因格架。在东海陆架全新世沉积演化过程中,长江三角州至少有３次不同程度地越过东海陆架进入冲绳海槽,并滞留有至少３层海侵改造“残积砂”沉积。在东海陆架全新世海平面变化期间,最大海平面时期为约距今６～５ｋａ,大致高于现今海平面２～４ｍ,最低海平面在距今１０ｋａ左右,大约低于现今海平面１３０ｍ。目前,由于温室效应的影响,海平面仍呈小幅度波浪式上升。事实证明,层序地层学不仅丰富了现代海洋沉积学的内容,而且解决了许多海洋沉积学未能解决的问题。  相似文献   

3.
Siliciclastic intervals in Lower Permian carbonate–siliciclastic cyclothems in western Kansas record climate control on facies progression, deposition and preservation. The 26 000 km2 study area comprises seven marine‐continental (carbonate–siliciclastic) cyclothems caused by glacioeustasy. Core data and a three‐dimensional geological model provide a detailed view of the sub‐surface on a gently sloping ramp. Siliciclastic intervals in the cyclothems are fine‐grained red beds with extensive pedogenic features, indicating a continental origin. Bed geometry (sheet‐like deposits that thin to the east), lateral grading, grain size (very fine‐grained sand to silt) and grain angularity (sub‐angular to angular) suggest that the sediment is loess sourced from the west, probably the Ancestral Rocky Mountains. There is a repeated record of glacial‐cycle‐scale, climate‐controlled cyclicity within siliciclastic intervals that has not been recognized previously. Aeolian silt grain size coarsens upward towards the middle, then fines upward in each siliciclastic interval. When sea‐level was high (interglacial) and carbonate production flourished, aeolian sedimentation nearly ceased, suggesting increased vegetation and rainfall at the source. As sea‐level fell, fine‐grained siliciclastic sediments were deposited under relatively dry, but seasonally wet conditions on an exposed ramp. Laterally graded coarser grained siliciclastic sediments with diagnostic fabrics indicate drier conditions with seasonal rainfall during a continued relative fall in sea‐level. The coarsest siliciclastic sediments were deposited during the lowest sea‐level and driest conditions, but still with sufficient seasonal moisture to allow vegetative cover and bioturbation. Subsequent upward fining is correlated with sedimentological indications of wetter conditions during relative sea‐level rise. Unlike common sequence stratigraphic models that relate siliciclastic sediment accumulation to base‐level rise, continental deposits were preserved because plants and pedogenesis stabilized aeolian sediment. The aggradational landscape formed by this process had several metres of positive relief that reduced accommodation for overlying marine carbonate strata. Thus, this mechanism for continental siliciclastic aggradation has a significant effect on sequence stratigraphic architecture.  相似文献   

4.
Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.  相似文献   

5.
Islands rimming Pacific atolls typically form narrow, low‐lying lands that are commonly perceived to be particularly vulnerable to global changes such as sea‐level rise. As these, low islands form the only habitable land for many island nations, understanding the character of shorelines, and the rates and controls that operate to bring about changes, is an issue of central importance. The purpose of this study is to unravel the characteristics of coastal change on atoll islands of the Gilbert Island chain of the equatorial Pacific nation of Kiribati, especially as they relate to autogenic shoreline processes and El Niño/Southern Oscillation variability. Integration of field observations, differential global positioning system data, historical aerial photographs and ultra‐high resolution remote sensing images demonstrates the nature, spatial patterns and rates of change from 17 islands on Maiana and Aranuka atolls. The results illustrate that, between 2005 and 2009, ca 50% of the shorelines on these islands displayed a discernable shift in position; some shorelines were accretionary (at net rates up to ca 8 m year?1) and others were erosional (up to ca 18 m year?1). Long‐term net rates of change on Maiana between 1969 and 2009 were lower than short‐term net rates measured between 2005 and 2009. Both short‐term and long‐term observations illustrate some of the greatest change occur near terminations of the largest, north–south oriented islands, associated with longshore movement of coarse sand and gravel. Direct hits by tropical depressions and marked seasonality, factors interpreted as being essential in island growth and shoreline dynamics elsewhere, do not directly impact these equatorial atolls and can be eliminated as fundamental controls on shoreline dynamics. Similarly, observations over four years suggested that shoreline variability probably is not influenced directly by marked sea‐level change, although a recent increase in the rates of shoreline change could reflect instability related to the cumulative effect of a long‐term increase in the rate of sea‐level rise. Within this framework of global change, local anthropogenic effects, autogenic shoreline processes and El Niño/Southern Oscillation‐influenced wind and wave variability control many aspects of these dynamic shorelines. These results provide quantitative insights into the character and variability of rates of shoreline change, information essential for evaluating and mitigating the vulnerability of island nations such as Kiribati.  相似文献   

6.
Hurricane Frances is shown to greatly alter the hydrodynamics within Tampa Bay, Florida, and the exchange of water with the Gulf of Mexico in both observational data and a realistic numerical circulation model of the Tampa Bay estuary. Hurricane Frances hit Tampa Bay on September 5, 2004 with surface winds peaking twice near 22 m s−1. There were three stages to the hydrodynamic effect of Frances on Tampa Bay. The first stage included the approach of Frances up to the first wind peak. The winds were to the south and southeast. During this stage sea level was maintained below mean sea level (MSL) and the residual current (demeaned, detided) was weak. The second stage began as the winds turned to the east and northeast, as the eye passed near the bay, and ended as the second wind peak appeared. During this stage the residual currents were strongly positive (into the bay), raising sea level to 1.2 m above MSL at St. Petersburg. The measured residual circulation peaked at over +0.7 m s−1 near the surface. The model shows this velocity peak yielded a maximum volume flux into the bay of +44,227 m3 s−1, displacing a total volume of 1.5 billion m3 in just a few hours, about 42% of the bay volume. In the third stage a strong negative flow developed as the wind and sea level relaxed to near normal levels. The ADCP measured a peak outflow of −0.8 m s−1 during this time. Model results indicate a maximum flux of −37,575 m3 s−1, and that it took about 50 h to drain the extra volume driven into the bay by Hurricane Frances.  相似文献   

7.
Long‐term relative sea‐level cycles (0·5 to 6 Myr) have yet to be fully understood for the Cretaceous. During the Aptian, in the northern Maestrat Basin (Eastern Iberian Peninsula), fault‐controlled subsidence created depositional space, but eustasy governed changes in depositional trends. Relative sea‐level history was reconstructed by sequence stratigraphic analysis. Two forced regressive stages of relative sea‐level were recognized within three depositional sequences. The first stage is late Early Aptian age (intra Dufrenoyia furcata Zone) and is characterized by foreshore to upper shoreface sedimentary wedges, which occur detached from a highstand carbonate platform, and were deposited above basin marls. The amplitude of relative sea‐level drop was in the order of tens of metres, with a duration of <1 Myr. The second stage of relative sea‐level fall occurred within the Late Aptian and is recorded by an incised valley that, when restored to its pre‐contractional attitude, was >2 km wide and cut ≥115 m down into the underlying Aptian succession. With the subsequent transgression, the incision was backfilled with peritidal to shallow subtidal deposits. The changes in depositional trends, lithofacies evolution and geometric relation of the stratigraphic units characterized are similar to those observed in coeval rocks within the Maestrat Basin, as well as in other correlative basins elsewhere. The pace and magnitude of the two relative sea‐level drops identified fall within the glacio‐eustatic domain. In the Maestrat Basin, terrestrial palynological studies provide evidence that the late Early and Late Aptian climate was cooler than the earliest part of the Early Aptian and the Albian Stage, which were characterized by warmer environmental conditions. The outcrops documented here are significant because they preserve the results of Aptian long‐term sea‐level trends that are often only recognizable on larger scales (i.e. seismic), such as for the Arabian Plate.  相似文献   

8.
Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.  相似文献   

9.
Two of the most important factors that control the accumulation rate of material in carbonate platform environments on geological time scales are climate and eustasy. Accurately assessing the importance of these inter‐related factors through the study of both modern and ancient carbonate facies, however, is problematic. These difficulties arise from both the complexities inherent in carbonate depositional systems and the demonstrable incompleteness of the stratigraphic record. Here, a new compilation of more than 19 000 global Phanerozoic shallow marine carbonate accumulation rates derived from nearly 300 individual stratigraphic sections is presented. These data provide the first global holistic view of changes in shallow marine carbonate production in response to climate and eustasy on geological time scales. Notably, a clear latitudinal dependence on carbonate accumulation rates is recognized in the data. Moreover, it can also be demonstrated that rates calculated across the last glacial maximum and Holocene track changes in sea‐level. In detail, the data show that globally averaged changes in carbonate accumulation rates lagged changes in sea‐level by ca 3 kyr, reflecting the commonly observed delay in the response of individual carbonate successions to sea‐level rise. Differences between the rates of carbonate accumulation and sea‐level change over the past 25 kyr ostensibly reflect changing accumulation mode, with platform drowning (give‐up mode) pervasive during peak Early Holocene sea‐level rise, followed by a switch to catch‐up mode accumulation from ca 9 ka to the present. Carbonate accumulation rates older than the Quaternary are typically calculated over time spans much greater than 100 kyr, and at these time spans, rates primarily reflect long‐term tectonically mediated accommodation space changes rather than shorter term changes in climate/eustasy. This finding, coupled with issues of stratigraphic incompleteness and data abundance, tempers the utility of this and other compilations for assessing accurately the role of climate and eustasy in mediating carbonate accumulation rates through geological time.  相似文献   

10.
The warm pool in the Indian Ocean   总被引：2，自引：0，他引：2  
The structure of the warm pool (region with temperature greater than 28°C) in the equatorial Indian Ocean is examined and compared with its counterpart in the Pacific Ocean using the climatology of Levitus. Though the Pacific warm pool is larger and warmer, a peculiarity of the pool in the Indian Ocean is its seasonal variation. The surface area of the pool changes from 24 × 106 km2 in April to 8 × 106 km2 in September due to interaction with the southwest monsoon. The annual cycles of sea surface temperature at locations covered by the pool during at least a part of the year show the following modes: (i) a cycle with no significant variation (observed in the western equatorial Pacific and central and eastern equatorial Indian Ocean), (ii) a single maximum/minimum (northern and southern part of the Pacific warm pool and the south Indian Ocean), (iii) two maxima/minima (Arabian Sea, western equatorial Indian Ocean and southern Bay of Bengal), and (iv) a rapid rise, a steady phase and a rapid fall (northern Bay of Bengal).  相似文献   

11.
《Sedimentology》2018,65(4):1170-1212
Barrier‐island system evolution is controlled by internal and external forcing mechanisms, and temporal changes in these mechanisms may be recorded in the sedimentary architecture. However, the precise role of individual forcing mechanisms is rarely well understood due to limited chronological control. This study investigates the relative role of forcing conditions, such as antecedent topography, sea‐level rise, sediment supply, storms and climate changes, on the evolution of a Holocene wave‐dominated barrier‐island system. This article presents temporal reconstruction of the depositional history of the barrier‐island system of Rømø in the Wadden Sea in unprecedented detail, based on ground‐penetrating radar profiles, sediment cores, high‐resolution dating and palynological investigations, and shows that ca 8000 years ago the barrier island formed on a Pleistocene topographic high. During the initial phase of barrier evolution, the long‐term sea‐level rise was relatively rapid (ca 9 mm year−1) and the barrier was narrow and frequently overwashed. Sediment supply kept pace with sea‐level rise, and the barrier‐island system mainly aggraded through the deposition of a ca 7 m thick stack of overwash fans. Aggradation continued for ca 1700 years until sea‐level rise had decreased to <2 mm year−1. In the last ca 6000 years, the barrier prograded 4 to 5 km through deposition of a 10 to 15 m thick beach and shoreface unit, despite a long‐term sea‐level rise of 1 to 2 mm year−1. The long‐term progradation was, however, interrupted by a transgression between 4000 years and 1700 years ago. These results demonstrate that the large‐scale morphology of the Danish Wadden Sea shoreline influences the longshore sediment transport flux and the millennial‐scale dispersal of sediment along the shoreline. On decadal to centennial timescales, major storms induced intense beach and shoreface erosion followed by rapid recovery and progradation which resulted in a highly punctuated beach and shoreface record. Major storms contributed towards a positive sediment budget, and the sustained surplus of sediment was, and still is, instrumental in maintaining the aggradational–progradational state of the barrier island.  相似文献   

12.
Along the macro-tidal (10.7 m tide range) SW Cumbria coast in situ rooted tree stumps are infrequently exposed below present-day low Spring Tide elevations (≤−4.5 m OD). To date these remnants, we conducted a salvage operation at two sites on in situ rooted tree stumps exposed below the lowest tide levels; these sites are no longer exposed. We obtained four radiocarbon dates on these tree remnants and reference an earlier site recovered in 1972. One wood sample was identified as Quercus robur or Q. petraea, while samples from Haverigg and Kirby-in-Furness are tree Salix spp. The median group age for the rising relative sea level (RSL) that killed the trees was 8602 ± 243 bp cal a bp with an estimated Mean Tide Level (MTL) of ~≤−9 m. Evidence indicates that RSL rose ~10 m along this section of coast at a rate of between of between 18.5 and 8.7 mm a–1 until ~6800 cal a bp , or earlier, before the rate of global sea level rise fell below that of the local glacial isostatic recovery. The transgression is coeval with rapid collapse and deglaciation of the Laurentide Ice Sheet in Hudson's Bay and slightly lags the global Meltwater Pulse-1C.  相似文献   

13.
Seven boulders measuring 100 to 1000 m3are scattered along the coastal ridge of north Eleuthera. Some are situated on ridge crests up to 20 m above present sea level. The boulders were probably deposited during oxygen-isotope substage 5e or 5d, as shown by their stratigraphic setting and by amino acid racemization ratios.d-alloisoleucine/l-isoleucine ratios were determined for land snails, and oolite of both marine and eolian origin was associated with the boulders. Like the boulders, the probable source rocks exposed in the adjacent cliffs are composed of marine and eolian limestone of oolitic and peloidal composition. The source beds are correlated with stage 9 or 11. The largest boulder is about 10 times the size of the largest Holocene ones moved by waves in the area. Tsunamis are a reasonable possibility as a transporting mechanism of the Pleistocene boulders. However, if deposited instead by storms during the last interglaciation, the storms were of much greater intensity than those occurring in the region during the late Holocene.  相似文献   

14.
Slightly inclined Holocene marine terraces cover parts of two circular salt diapirs (Hormoz and Namakdan) in the Persian Gulf. Their relative altitude above present sea level results from a combination of general marine transgression/regression affecting the whole area, and of local uplift related to salt diapirism. Differential uplift rate of the studied diapirs in centre‐to‐rim profiles was calculated from results based on: (i) radiocarbon ages of skeletal remains of benthic faunas (19 samples), which originally grew close to sea level; (ii) original altitude of samples, estimated from general sea‐level oscillation curves for the last 10 kyr, and (iii) present sample altitude measured in the field. Calculated uplift rates increase from rim to centre on both diapirs in the range from: 2 mm yr?1 at the rim to 5–6 mm yr?1 at the interior of Hormoz, and 1–3 mm yr?1 at the rim to 3–5 mm yr?1 at the interior of Namakdan. Such uplift rate distributions fit into the parabolic profile of Newtonian fluid rather than to profiles typical for pseudoplastic fluids. The increase in uplift rate with distance from rim to centre of diapirs is gradual as demonstrated also by generally smooth surface of marine terraces. No tectonic dissections were found. The depositional history on both salt diapirs is similar although they are situated more than 100 km apart. Marine sedimentation started at about 9.6k cal. yr BP on Hormoz and at 8.6k cal. yr BP on Namakdan. Owing to rapid transgression, the sea partially truncated both salt diapirs and rapidly deepened, and carbonate mud was deposited on the peripheries of both salt diapirs. Between 7 and 5k cal. yr BP beach deposition replaced carbonate mud. Soon after 5k cal. yr BP, the sea retreated from most of the marine terraces on both salt diapirs. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
《Quaternary Science Reviews》2003,22(10-13):1191-1199
Pleistocene and Holocene transgressions advancing over the shelf that was exposed during glacial maxima drowned any continental and shallow marine sediments deposited during low sea level stands. To complement records from sequences exposed on land, core material from the shallow continental shelf is needed to reconstruct climatic and sea level fluctuations. Two cores drilled offshore Ashqelon off the southern Mediterranean coast of Israel, in a water depth of 10 and 25 m, were analyzed. Sedimentary facies and faunal analyses indicate that most of the sediments were deposited in nearshore environments, with only short intervals of continental episodes. Luminescence dating of alkali feldspar and quartz, as well as 14C ages of mollusks, date the cores to marine oxygen isotopic stages 6–1, between ∼140 and 5 ka. Comparison between the dating methods shows that most alkali feldspar ages agree with independent sea level and sedimentological constraints while quartz ages are overestimated.  相似文献   

16.
This study establishes for the first time the chronology and limnological history of Lake Amora (Dead Sea basin, Israel), whose deposits (the Amora Formation) comprise one of the longest exposed lacustrine records of the Pleistocene time. The Amora Formation consists of sequences of laminated primary aragonite and silty-detritus, Ca-sulfate minerals, halite and clastic units. This sedimentary sequence was uplifted and tilted by the rising Sedom salt diapir, exposing ∼320 m of sediments on the eastern flanks of Mt. Sedom (the Arubotaim Cave (AC) section).The chronology of the AC section is based on U-disequilibrium dating (230Th-234U and 234U-238U ages) combined with floating δ18O stratigraphy and paleomagnetic constraints. The determination of the 230Th-234U ages required significant corrections to account for detrital Th and U. These corrections were performed on individual samples and on suites of samples from several stratigraphic horizons. The most reliable corrected ages were used to construct an age-elevation model that was further tuned to the oxygen isotope record of east Mediterranean foraminifers (based on the long-term similarity between the sea and lake oxygen isotope archives).The combined U-series-δ18O age-elevation model indicates that the (exposed) Amora sequence was deposited between ∼740 and 70 ka, covering seven glacial-interglacial cycles (Marine Isotope Stages (MIS) 18 to 5).Taking the last glacial Lake Lisan and the Holocene Dead Sea lacustrine systems as analogs of the depositional-limnological environment of Lake Amora, the latter oscillated between wet (glacial) and more arid (interglacial) conditions, represented by sequences of primary evaporites (aragonite and gypsum that require enhanced supply of freshwater to the lakes) and clastic sediments, respectively. The lake evolved from a stage of rapid shifts between high and low-stand conditions during ∼740 to 550 ka to a sabkha-like environment that existed (at the AC site) between 550 and 420 ka. This stage was terminated by a dry spell represented by massive halite deposition at 420 ka (MIS12-11). During MIS10-6 the lake fluctuated between lower and higher stands reaching its highest stand conditions at the late glacial MIS6, after which a significant lake level decline corresponds to the transition to the last interglacial (MIS5) low-stand lake, represented by the uppermost part of the Formation.δ18O values in the primary aragonite range between 6.0 and −1.3‰, shifting cyclically between glacial and interglacial intervals. The lowest δ18O values are observed during interglacial stages and may reflect short and intense humid episodes that intermittently interrupted the overall arid conditions. These humid episodes, expressed also by enhanced deposition of travertines and speleothems, seem to characterize the Negev Desert, and in contrast to the overall dominance of the Atlantic-Mediterranean system of rain patterns in the Dead Sea basin, some humid episodes during interglacials may be traced to southern sources.  相似文献   

17.
Abstract In mid‐Middle Cambrian time, shallow‐water sedimentation along the Cordilleran passive margin was abruptly interrupted by the development of the deep‐water House Range embayment across Nevada and Utah. The Marjum Formation (330 m) in the central House Range represents deposition in the deepest part of the embayment and is composed of five deep‐water facies: limestone–argillaceous limestone rhythmites; shale; thin carbonate mud mounds; bioturbated limestone; and cross‐bedded limestone. These facies are cyclically arranged into 1·5 to 30 m thick parasequences that include rhythmite–mound, rhythmite–shale, rhythmite–bioturbated limestone and rhythmite–cross‐bedded limestone parasequences. Using biostratigraphically constrained sediment accumulation rates, the parasequences range in duration from ≈14 to 270 kyr. The mud mounds are thin (<2 m), closely spaced, laterally linked, symmetrical domes composed of massive, fenestral, peloidal to clotted microspar with sparse unoriented, poorly sorted skeletal material, calcitized bacterial(?) filaments/tubes and abundant fenestrae and stroma‐ tactoid structures. These petrographic and sedimentological features suggest that the microspar, peloids/clots and syndepositional micritic cement were precipitated in situ from the activity of benthic microbial communities. Concentrated growth of the microbial communities occurred during periods of decreased input of fine detrital carbonate transported offshore from the adjacent shallow‐water carbonate platform. In the neighbouring Wah Wah Range and throughout the southern Great Basin, coeval mid‐Middle Cambrian shallow‐water carbonates are composed of abundant metre‐scale, upward‐shallowing parasequences that record high‐frequency (104?105 years) eustatic sea‐level changes. Given this regional stratigraphic relationship, the Marjum Formation parasequences probably formed in response to high‐frequency sea‐level fluctuations that controlled the amount of detrital carbonate input into the deeper water embayment. During high‐frequency sea‐level rise and early highstand, detrital carbonate input into the embayment decreased as a result of carbonate factory retrogradation, resulting in the deposition of shale (base of rhythmite–shale parasequences) or thin nodular rhythmites, followed by in situ precipitated mud mounds (lower portion of rhythmite–mound parasequences). During the ensuing high‐frequency sea‐level fall/lowstand, detrital carbonate influx into the embayment increased on account of carbonate factory pro‐ gradation towards the embayment, resulting in deposition of rhythmites (upper part of rhythmite–mound parasequences), reworking of rhythmites by a lowered storm wave base (cross‐bedded limestone deposition) or bioturbation of rhythmites by a weakened/lowered O2‐minimum zone (bioturbated lime‐ stone deposition). This interpreted sea‐level control on offshore carbonate sedimentation patterns is unique to Palaeozoic and earliest Mesozoic deep‐water sediments. After the evolution of calcareous plankton in the Jurassic, the presence or absence of deeper water carbonates was influenced by a variety of chemical and physical oceanographic factors, rather than just physical transport of carbonate muds.  相似文献   

18.
In the Qinling orogenic belt. oceanic crust originated in the Early Palaeozoic. while the product of conti-nental collision appeared as late as after the Triassic. The Late Palaeozoic records there are of major impor-tance for understanding the tectonic regime at that time. The Carboniferous and even Permian sequences andthe distribution of sedimentary facies in northern Huaiyang indicate that the rocks were formed in a large basinopening towards the south. Regional stratigraphic correlation shows that the interior of the Qinling orogenicbelt was a sea trough lying between the Yangtze and North China plates in the middle part of the LatePalaeozoic. With subsequent northward migration of the South China Sea, the two seas were connected witheach other. Both the melanges and the Dabie block ia the eastern sector of the Qinling belt were formed in theMesozoic ?.  相似文献   

19.
Abstract Physical stratigraphy within shoreface‐shelf parasequences contains a detailed, but virtually unstudied, record of shallow‐marine processes over a range of historical and geological timescales. Using high‐quality outcrop data sets, it is possible to reconstruct ancient shoreface‐shelf morphology from clinoform surfaces, and to track the evolving morphology of the ancient shoreface‐shelf. Our results suggest that shoreface‐shelf morphology varied considerably in response to processes that operate over a range of timescales. (1) Individual clinoform surfaces form as a result of enhanced wave scour and/or sediment starvation, which may be driven by minor fluctuations in relative sea level, sediment supply and/or wave climate over short timescales (101?103 years). These external controls cannot be distinguished in vertical facies successions, but may potentially be differentiated by the resulting clinoform geometries. (2) Clinoform geometry and distribution changes systematically within a single parasequence, reflecting the cycle in sea level and/or sediment supply that produced the parasequence (102?105 years). These changes record steepening of the shoreface‐shelf profile during early progradation and maintenance of a relatively uniform profile during late progradation. Modern shorefaces are not representative of this stratigraphic variability. (3) Clinoform geometries vary greatly between different parasequences as a result of variations in parasequence stacking pattern and relict shelf morphology during shoreface progradation (105?108 years). These controls determine the external dimensions of the parasequence.  相似文献   

20.
The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ13C and δ18O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO2 at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ13C and δ18O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ18O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ18O and highly variable δ13C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.  相似文献   

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