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1.
《Sedimentology》2018,65(4):1331-1353
The Faxe Quarry in south‐east Denmark offers excellent exposures of Early Palaeocene, Danian deep‐water intercalated coral and bryozoan mounds that form complexes at least 40 m thick and a few kilometres wide along and over submarine highs. The coexisting coral and bryozoan mounds represent two different biogenic carbonate factories with a highly dynamic interplay during growth. The sedimentary facies, mound geometries and the density, diversity and palaeoecology of the associated benthic invertebrates and nannofossils allow recognition of six successive growth units. Unit 1 represents an outer shelf bryozoan mound belt characterized by an oligotrophic cool‐water nannofossil assemblage. Unit 2 comprises a mixed faunal assemblage of bryozoans and octocorals with an initial sparse colonization of hexacorals. The nannofossil assemblage records a decrease in diversity and an increase in warm water forms. Unit 3 marks the onset of dense colonization of the scleractinian coral Dendrophyllia candelabrum with associated low‐diversity macrofauna and nannofossil assemblages. Unit 4 represents the main coral build‐up phase with frame‐building hexacorals of Dendrophyllia and Faxephyllia associated with a high‐diversity invertebrate fauna, and relatively low‐diversity nannofossil assemblages. Unit 5 represents the late coral mound phase showing extensive lateral distribution and finally death and erosion of the coral mounds. This event was contemporaneous with a warming trend in the pelagic environment. The succeeding Unit 6 marks the burial and overgrowth of the coral mound complex by bryozoan‐rich sediments. The coral mound complex in the Faxe Quarry initiated and terminated in global nannofossil zone NP 3 and regional nannofossil zones NNT p2G–3 suggesting a mound growth duration of ca 300 kyr and a mean vertical accretion of the coral mound of 13 cm kyr−1. The mound complex probably serves as the best‐exposed analogue to modern deep and cold‐water coral mounds in the North Atlantic.  相似文献   

2.
ERIK THOMSEN 《Sedimentology》1976,23(4):485-509
The lower Danian bryozoan mounds exposed in the cliff at Karlby are described. Analyses of some sixty samples taken from a single mound reveal that the limestone is composed of about 30% of bryozoan fragments in a fine grained matrix. The texture and the grain-size distribution show that two distinct sedimentary facies can be recognized: (1) the northwestern flank and the basins with fine bryozoan fragments, a relatively small amount of matrix, and grain-supported texture; (2) the southeastern flank and the summit with coarse bryozoans, a larger amount of matrix and partly mud-supported texture. It is shown that the differences in the size of the bryozoan fragments are due to adaptations to changes in the water-movement rather than a result of significant transport. Thus, currents were roughly from the southeast producing more agitated water on the southeast flank and the summit and more gentle water on the northwest flank and in the basins. The unusual relationship between the velocity of the water-movement and the sedimentary facies is caused by an influence of the bryozoans on the depositional environment. On the southeast flank the higher current velocity favoured the growth of bryozoans, resulting in a relatively dense cover that was able to trap and bind the matrix. On the northwest flank the currents were slower and less favourable for growth, and the ability of the bryozoans to bind the sediment was correspondingly smaller.  相似文献   

3.
A ground‐penetrating radar (GPR) survey of 101 mounds at the Sny Magill Unit of Effigy Mounds National Monument, Iowa, demonstrates that GPR can be an effective tool to evaluate the structure and condition of mounds without damaging them. Ideal survey conditions and improved processing technology allow for the identification of strata within the mounds, as well as areas of post‐construction disturbance and possible archaeological features within the mounds. Provisional interpretations indicate that 60 are intact conical mounds with minimal post‐construction disturbance, and two show very strong evidence of containing interior burial platforms; 29 are badly damaged by non‐cultural or cultural activity; two are probable non‐cultural mounds; nine are reasonably intact linear and effigy mounds; one is an excavated effigy mound. GPR and other remote‐sensing techniques are highly recommended for mound investigation, but wherever possible such techniques need to be coordinated with mound excavation so as to test the remote‐sensing results. © 2008 Wiley Periodicals, Inc.  相似文献   

4.
Based on integration of seismic reflection and well data analysis this study examines two major contourite systems that developed during the late Cretaceous in the southern Baltic Sea. The evolution of these Chalk Sea contourite systems between the Kattegat and the southern Baltic Sea started when Turonian to Campanian inversion tectonics overprinted the rather flat sea floor of the epeiric Chalk Sea. The Tornquist Zone and adjacent smaller blocks were uplifted and formed elongated obstacles that influenced the bottom currents. As a consequence of the inversion, the sea floor west of the Tornquist Zone tilted towards the north‐east, creating an asymmetrical sub‐basin with a steep marginal slope in the north‐east and a gentle dipping slope in the south‐west. A south‐east directed contour current emerged in the Coniacian or Santonian along the south‐western basin margin, creating contourite channels and drifts. The previously studied contourite system offshore Stevns Klint is part of this system. A second, deeper and north‐west directed counter‐flow emerged along and parallel to the Tornquist Zone in the later Campanian, but was strongest in the Maastrichtian. This bottom current moderated the evolution of a drift‐moat system adjacent to the elevated Tornquist Zone. The near surface Alnarp Valley in Scania represents the Danian palaeo‐moat that linked the Pomeranian Bay with the Kattegat. The previously studied contourite system in the Kattegat represents the north‐western prolongation of this system. This study links previous observations from the Kattegat and offshore Stevns Klint to the here inferred two currents, a more shallow, south‐east directed and a deeper, north‐west directed flow.  相似文献   

5.
Located to the north of the Stevns Klint Peninsula (Denmark), Kulstirenden shows the transition from Cretaceous chalks to Danian carbonates across the Cretaceous - Paleogene (K/Pg) boundary. The K/Pg boundary at Kulstirenden is represented by the Fiskeler Member (Fish Clay), a streaked marl interval with a smectite rich, red oxidised layer at its base marking the boundary. The Fiskeler Member is important as it includes the iridium concentration linked to the bolide impact which may have caused the end-Cretaceous mass extinction and is at its maximum thickness at Kulstirenden (c.45 cm). Calcareous dinoflagellate cysts (Calciodinelloideae) have been investigated from within the fine fraction (45-125 μm) at Kulstirenden in order to understand the ecological implications of these enigmatic microfossils. Several species were found within the Fiskeler Member, including Orthopithonella collarisWendler et al. (2001, Rev. Palaeobot. Palynol. 115, 69-77). This species is of particular interest as it demonstrates reduced paratabulation and may indicate a transgressive period immediately after the K/Pg boundary event. O. collaris has been described as a morphotype formed under post-K/Pg environmental conditions and is, therefore, described as a “disaster” taxon that marks the K/Pg boundary at Stevns Klint. The distribution of this taxon is more extensive than previously documented, disappearing as carbonate sedimentation returned in the Cerithium Limestone Member of the Danian. Several calcareous microfossil specimens that were found cannot be associated with the dinoflagellates and display Bolboforma-like features. While the origins of Bolboforma are enigmatic, the current finding expands their stratigraphic range and suggests an appearance of this group of organisms found at Stevns Klint may give an insight into the inception of this group into the earliest Danian.  相似文献   

6.
The Darwin Mounds are small (up to 70 m in diameter), discrete cold‐water coral banks found at c. 950 m water depth in the northern Rockall Trough, north‐east Atlantic. Formerly described in terms of their genesis, the Darwin Mounds are re‐evaluated here in terms of mound growth processes based on 100 and 410 kHz side‐scan sonar data. The side‐scan sonar coverage is divided into a series of acoustic facies representing increasing current speed and sediment transport/erosion from south to north: pockmark facies, ‘mounds within depressions’ facies, Darwin Mound facies, stippled seabed facies and sand wave facies. Mound morphometric changes are quantified and show a south‐to‐north divergence from an inherited morphology, reflecting the outline of coral‐colonized fluid escape structures, to developed, downstream elongated, elevated mound forms. It is postulated that increasing current speeds and bedload sand transport favour mound growth and development by a process of enhanced sand sedimentation within mounds due to current deceleration by frictional drag around coral colonies. Comparisons are made with similar growth processes attributed to comparably sized cold‐water coral mounds in the Porcupine Seabight, offshore Ireland.  相似文献   

7.
Ground‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.  相似文献   

8.
Cold‐water coral mound morphology and development are thought to be controlled primarily by current regime. This study, however, reveals a general lack of correlation between prevailing bottom current direction and mound morphology (i.e. footprint shape and orientation), as well as current strength and mound size (i.e. footprint area and height). These findings are based on quantitative analyses of a high‐resolution geophysical dataset collected with an Autonomous Underwater Vehicle from three cold‐water coral mound sites at the toe of slope of Great Bahama Bank. The three sites (80 km2 total) have an average of 14 mounds km?2, indicating that the Great Bahama Bank slope is a major coral mound region. At all three sites living coral colonies are observed on the surface of the mounds, documenting active mound growth. Morphometric analysis shows that mounds at these sites vary significantly in height (1 to 83 m), area (81 to 6 00 000 m2), shape (mound aspect ratio 0·1 to 1) and orientation (mound longest axis 0 to 180°). The Autonomous Underwater Vehicle measured bottom current data depict a north–south flowing current that reverses approximately every six hours. The tidal nature of this current and its intermittent deviations during reversals are interpreted to contribute to the observed mound complexity. An additional factor contributing to the variability in mound morphometrics is the sediment deposition rate that varies among and within sites. At most locations sedimentation rate lags slightly behind mound growth rate, causing mounds to develop into large structures. Where sedimentation rates are higher than mound growth rates, sediment partially or completely buries mounds. The spatial distribution and alignment of mounds can also be related to gravity mass deposits, as indicated by geomorphological features (for example, slope failure and linear topographic highs) in the three‐dimensional bathymetry. In summary, variability in sedimentation rates, current regime and underlying topography produce extraordinarily high variability in the distribution, development and morphology of coral mounds on the Great Bahama Bank slope.  相似文献   

9.
Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud‐mounds in northern Belize. They encompass areas of 20 km2 and 35 km2 in distal and proximal positions, respectively, on a wide and shallow‐water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening‐upward transgressive systems tract, condensed section and shallowing‐upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud‐mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea‐level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound‐core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound‐cores represents highstand systems tract sediments deposited in the last ≈ 4500 years during slow and decelerating sea‐level rise. Within these sections, there is an early phase of progradationally offlapping catch‐up deposition and a later (and current) phase of aggradational keep‐up deposition. The mound‐cores comprise stacked storm‐deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia‐colonized surfaces. Vertical stacking of these units imparts a quasi‐cyclic architecture to the section that superficially mimics metre‐scale parasequences in ancient rocks. The locations of the mud‐mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud‐mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand‐size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.  相似文献   

10.
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   

11.
Here we present research on previously uninvestigated frost peat mounds occurring on a peat bog in the southern part of Hermansenøya, NW Svalbard. Detailed characteristics are given of the environmental conditions of the peat bog and of the morphological features and surface structure of the frost peat mounds, as well as an analysis of the internal structure of one mound. Three types of frost peat mounds have been distinguished: disc‐shaped mounds (low), mid‐sized mounds with gentle sides, and high mounds with steep sides. Radiocarbon dating of the peat within the frost peat mound performed for the first time on Svalbard and a detailed analysis of the deposits demonstrated that in the high mound (1.3 m) there is an ice‐peat core and peat cover without ice. There are three layers of peat of different ages separated by at least two hiatuses. A generalized history of the development of the peat bog from about 8 ka BP is established. The studied mound displays two development cycles unknown elsewhere. The older relict part of the peat mound was formed during a climatic cooling about 3.0–2.5 ka BP, while the younger part originated during the Little Ice Age (c. AD 1550–1850). Despite certain similarities of these mounds to some palsas, this term should not be applied to the mounds because they are smaller and their cores consist mostly of layers of massive injection ice, the presence of which indicates a pressurized system in their genesis.  相似文献   

12.
Cold water coral covered carbonate mounds at the south‐west margin of the Rockall Trough form ridges several kilometres long and up to 380 m high. Piston cores obtained at three mound crests reveal the complex internal structure of the mound build up, with alternating unlithified coral‐dominated intervals and lithified intervals. The most recent lithified interval is covered by corals embedded in a fine‐grained matrix, comprising ca 11 000 years of continuous mound evolution. Before this time 230Th/U dating shows the presence of several hiatuses in mound build‐up. Aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth. Downcore X‐ray fluorescence scanning, computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration. The upper boundary of the most recent lithified interval shows some erosional features, but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long‐term non‐sedimentation, but to earlier sub‐surface diagenesis. Organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons. Depending on the openness of the system, this can lead to precipitation of a more stable low‐magnesium carbonate. A model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals.  相似文献   

13.
《Sedimentology》2018,65(5):1413-1446
Turbidites within Holocene lacustrine sediment cores occur worldwide and are valued deposits that record a history of earthquakes or storms. Without sedimentary architecture, however, interpretation of the cause, provenance and behaviour of their parent turbidity currents are speculative. Here, these interpretations are made from two‐dimensional ground‐penetrating radar images of ‘shore to shore’ architecture beneath three, previously cored lakes within the low seismicity New England (USA ) region. Shallow depths, low water and sediment conductivities, and signal sensitivity to density contrasts uniquely provided up to 30 m of sediment signal penetration. Core comparisons and signal analysis reveal that most horizons represent multidecimetre‐thick clusters of Holocene turbidites, which are denser than their organic‐rich silt matrix. Some horizons also represent erosional unconformities and sediment bypass interfaces. The key, common, architectural consequences of turbidity current activity include limited foreset progradation, conformably pinched or unconformable layers of organic‐rich sediment onlapped against slopes beneath 5 to 6 m of water, and mounded stratified sediments beneath rises. These features indicate that turbidity currents repeatedly bypassed the same slope without deposition and regardless of dip, and then simultaneously armoured and bypassed inter‐turbidite sediment along rises and basins to provide basinward, generally age‐conformable accumulation. The mounding precludes significant basinward focusing. Variable horizon amplitude suggests metre‐scale changes in armouring density. Unconformities localized near breaks in dip beneath slopes suggest erosive hydraulic jumps. One lake shows evidence of historically maintained channels associated with specific deltas. Shelf strata indicating inland current generation, similar key architecture in other, uncored lakes, countable, lake‐wide horizons, and absent slumps, slides and faults are consistent with storm‐driven turbidity currents, and with previous, core‐based conclusions that severe, Holocene storms were episodic throughout this region. The results generalize marine bypass and armouring to lacustrine settings, and so probably occur worldwide in lakes subject only to storms, including lakes where ground‐penetrating radar may locate core sites.  相似文献   

14.
Stromatactis‐bearing mud‐mounds remain an enigmatic reef type despite being common in Palaeozoic ramp settings. Two well preserved Upper Devonian (Frasnian) mud‐mounds in the Mount Hawk Formation crop out side by side in the southern Rocky Mountains of west‐central Alberta and provide an opportunity to develop a new case study that can be compared with the other coeval examples, such as those well‐known ones in southern Belgium, as well as evaluate competing hypotheses for mud‐mound formation. The southern mud‐mound is 46·2 m thick and 38·6 m wide at the base, whilst the northern one is 53·3 m thick and 72·2 m wide at the base, and they exhibit three or four growth stages indicated by interfingering and onlapping geometries with flanking strata. The biota is diverse, but fossils only occupy 10·7% by volume, among which sponge spicules, echinoderms, ostracods, brachiopods and calcimicrobes belonging to Girvanella and Rothpletzella are the most common. Five microfacies are discriminated in the mud‐mounds: biomicrite, clotted micrite, spiculite, stromatolite and laminite, with clotted micrite comprising the largest proportion. There is no internal vertical or lateral palaeoecological zonation, and the presence of calcimicrobes and calcareous algae throughout indicates accretion entirely within the photic zone, in a deeper ramp setting seaward of a large carbonate platform to the east. Stromatactis is abundant and the cavities were mostly due to excavation by currents rather than physical collapse of spiculate siliceous sponges. Formation of lime mud involved a combination of multiple organisms, mechanisms and processes. Cyanobacteria were integral to mud‐mound frame‐building and accretion because they stabilized the surface, often permineralized to form Girvanella and provided organic matter that was decomposed by bacteria. This induced precipitation of micrite, forming early indurated rigid masses, evidenced by the presence of intraclasts, stromatactis cavities, isopachous marine cements, absence of bioturbation and rare synsedimentary brittle deformation. The same microbial components, invertebrate biota and clotted micrite occur in underlying strata, suggesting that there was a protracted period of potential mud‐mound initiation before the exact conditions arose to trigger it. The ramp setting, antecedent sea floor topography and relative sea‐level likely contributed together to control this. This study indicates that mud‐mound formation was controlled by a combination of processes, but they are essentially a microbial buildup.  相似文献   

15.
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16.
Cape Henlopen, Delaware is a coastal spit complex located at the confluence of Delaware Bay and the Atlantic Ocean. This region was occupied by prehistoric peoples throughout the evolution of ancestral Cape Henlopen. A ground‐penetrating radar (GPR) survey was conducted at one of the prehistoric archaeological sites (7S‐D‐30B) located within the Cape Henlopen Archaeological District. The site was in a remote location in the center of a tide dominated back‐barrier marsh. Ground‐penetrating radar waves penetrated to depths of 7 m, and four major sets of reflections were observed. Three sets were interpreted to be GPR images of geomorphic units associated with the spit complex, and the fourth was identified as the GPR image of a shell midden deposit. The GPR survey was used to determine the approximate dimensions of the shell midden, including its depth below ground surface (up to 2.1 m) and horzontal extent (∼250 m2), and to establish the paleoenvironmental setting and antecedent topography of the site prior to occupation. The GPR data suggests that the shell midden was initially deposited upon an aeolian dune surface and the antecedent topography at the site included an up to 1 m deep trough located 5 m to the north of, and trending parallel to, the axis of a present‐day topographic high. This survey illustrates that GPR is a useful, noninvasive, tool that may be implemented at archaeological sites in coastal areas. It provides constraints on the environmental setting and topography of the terrain which prehistoric peoples inhabited, and it can be used in planning excavations at sites in coastal geomorphic settings. © 2000 John Wiley & Sons, Inc.  相似文献   

17.
The architectural framework and Holocene evolution of the Zeballos fjord‐head delta on west‐central Vancouver Island was established through a multidisciplinary field‐based study. The Zeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and an inset Holocene delta graded to present sea level. Both deltas have a classic Gilbert‐type tripartite architecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domain electromagnetic (TDEM) and ground‐penetrating radar (GPR) surveys, borehole data, and gravel pit exposures provided information on the internal form, lithologies and substrate of both deltas. Both sets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. The TDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m in depth, and it delineated saltwater intrusion into the deltaic sediments. Late Quaternary sea‐level change at Zeballos was inferred from delta morphology and the GPR survey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higher relative to the land than it is today. It was dissected when sea‐level fell rapidly as a result of glacio‐isostatic rebound. Relative sea‐level reached a position about 20 m below the present datum during the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction and topset beds that thicken to 26 m landward imply that the delta aggraded and prograded into Zeballos Inlet during the middle and late Holocene transgression. Sea‐level may have risen above the present datum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surface are preserved along the valley margins. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

18.
Flakket on the island of Anholt in Denmark is a cuspate foreland facing the microtidal Kattegat sea. It is composed of a number of beach ridges typically covered by dune sand and separated by swales and wetlands. OSL dating indicates that the evolution of Flakket began c. AD 1000. Foreland growth was punctuated by a major episode of coastal reorganization leading to coastal retreat c. AD 1800. Coastal retreat led to the formation of an erosion surface that separates older and higher‐lying beach‐ridge and swale deposits from younger and lower‐lying deposits. The palaeo‐sea level is deduced from the architecture of the deposits, and interpretation of ground‐penetrating radar data and geomophological observations indicates that relative sea level was about 1.90±0.25 m above present sea level c. AD 1000, but about 0.00±0.25 m relative to present sea level c. AD 1830 and c. AD 1870. Anholt is situated at the margin of the uplifted Fennoscandian area; assuming uplift to be about 1.2 mm a?1 it follows that absolute sea level was about +0.70±0.25 m at AD 1000, but around ?0.22±0.25 m at AD 1830 and around ?0.17±0.25 m at AD 1870. Within the uncertainties of the age control, the sea‐level indicators mapped by ground‐penetrating radar reflections and the variability of estimates of uplift found in the literature, the result obtained for AD 1000 is consistent with findings from the Stockholm area in Sweden and with a recently published global sea‐level curve.  相似文献   

19.
Small mounds of peat rise several metres above the level of the water‐table at Melaleuca Inlet and Louisa Plains on the buttongrass plains in southwest Tasmania. Possible origins of the peat mounds have been explored by pollen analysis and radiocarbon dating of a set of samples taken from a vertical section of one peat mound at Melaleuca. The peat accumulation is entirely of Holocene age although the mound is underlain by sapric peats preserving a cold climate palynoflora of probable Late Pleistocene age. Peats at and near the base of the mound accumulated under a heath sedgeland during the earliest Holocene while after about 7630 a BP the peat‐forming vegetation was shrub‐dominated. The radiocarbon data indicate two main phases of overall peat accumulation, between 7630 and 5340 a BP (Middle Holocene) and between 4450 and 450 a BP (Late Holocene), that were interrupted by a wildfire which burnt into the surface peats. The maintenance of high surface and internal levels of moisture almost certainly was the critical factor behind the low incidence of in situ fires burning into the surface peats on the mound. The perennial influx of groundwater below the mound is a possible origin that fits well with our observations, although the expansion and contraction of soils cannot be discounted as an initiating factor. Enhanced nutrient input from birds may have helped promote growth in the peat‐forming communities. The data do not support the mounds being eroded remnants of a former blanket peat cover or being due to periglacial activity. The peat mounds of southwest Tasmania deserve maximum protection because of their rarity in the Australian landscape and, it seems, elsewhere.  相似文献   

20.
Ground penetrating radar and single‐aliquot regenerative‐dose optically stimulated luminescence were used to determine the depositional environments and age of unconsolidated siliciclastic sediments near Apalachicola, Florida. Five direct‐push cores, five vibracores and 28 optically stimulated luminescence samples were collected, as well as 7 km of ground penetrating radar data. A new model of cosmic dose rate calculation, which removes the effect of a much younger aeolian cap, was utilized to calculate more representative optically stimulated luminescence ages. Five radar facies were identified based on reflector amplitude and orientation. The resulting data indicate that the Tertiary/Quaternary Shelly Sediments were deposited before marine isotope stage 6, the Quaternary Alluvium was deposited during marine isotope stage 6 and the Quaternary Beach Ridge and Dune was deposited during the marine isotope stage 5e sea‐level highstand, which peaked at approximately 2·5 m above present sea‐level in this area.  相似文献   

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