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1.
The glaciomarine model for deglaciation of the Irish Sea basin suggests that the weight of ice at the last glacial maximum was sufficient to raise relative sea‐levels far above their present height, destabilising the ice margin and causing rapid deglaciation. Glacigenic deposits throughout the basin have been interpreted as glaciomarine. The six main lines of evidence on which the hypothesis rests (sedimentology, deformation structures, delta deposits, marine fauna, amino‐acid ratios and radiocarbon dates) are reviewed critically. The sedimentological interpretation of many sections has been challenged and it is argued that subglacial sediments are common rather than rare and that there is widespread evidence of glaciotectonism. Density‐driven deformation associated with waterlain sediments is rare and occurs where water was ponded locally. Sand and gravel deposits interpreted as Gilbert‐type deltas are similarly the result of local ponding or occur where glaciers from different source areas uncoupled. They do not record past sea‐levels and the ad hoc theory of ‘piano‐key tectonics’ is not required to explain the irregular pattern of altitudes. The cold‐water foraminifers interpreted as in situ are regarded as reworked from Irish Sea sediments that accumulated during much of the late Quaternary, when the basin was cold and shallow with reduced salinities. Amino‐acid age estimates used in support of the glaciomarine model are regarded as unreliable. Radiocarbon dates from distinctive foraminiferal assemblages in northeast Ireland show that glaciomarine sediments do occur above present sea‐level, but they are restricted to low altitudes in the north of the basin and record a rise rather than a fall in sea‐level. It is suggested here that the oldest dates, around 17 000 yr BP, record the first Late Devensian (Weichselian) marine inundation above present sea‐level. This accords with the pattern but not the detail of recent models of sea‐level change. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

2.
Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483. Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS 14C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 14C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 14C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 14C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 14C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.  相似文献   

3.
Core 2011804‐0010 from easternmost Lancaster Sound provides important insights into deglacial timing and style at the marine margin of the NE Laurentide Ice Sheet (LIS). Spanning 13.2–11.0 cal. ka BP and investigated for ice‐rafted debris (IRD), foraminifera, biogenic silica and total organic carbon, the stratigraphy comprises a lithofacies progression from proximal grounding line and sub‐ice shelf environments to open glaciomarine deposition; a sequence similar to deposits from Antarctic ice shelves. These results are the first marine evidence of a former ice shelf in the eastern Northwest Passage and are consistent with a preceding phase of ice streaming in eastern Lancaster Sound. Initial glacial float‐off and retreat occurred >13.2 cal. ka BP, followed by formation of an extensive deglacial ice shelf during the Younger Dryas, which acted to stabilize the retreating margin of the NE LIS until 12.5 cal. ka BP. IRD analyses of sub‐ice shelf facies indicate initial high input from source areas on northern Baffin Island delivered to Lancaster Sound by a tributary ice stream in Admiralty Inlet. After ice shelf break‐up, Bylot Island became the dominant source area. Foraminifera are dominated by characteristic ice‐proximal glaciomarine benthics (Cassidulina reniforme, Elphidium excavatum f. clavata), complemented by advected Atlantic water (Cassidulina neoteretis, Neogloboquadrina pachyderma) and enhanced current indicators (Lobatula lobatula). The biostratigraphy further supports the ice shelf model, with advection of sparse faunas beneath the ice shelf, followed by increased productivity under open water glaciomarine conditions. The absence of Holocene sediments in the core suggests that the uppermost deposits were removed, most likely due to mass transport resulting from the site's proximity to modern tidewater glacier margins. Collectively, this study presents important new constraints on the deglacial behaviour of the NE Laurentide Ice Sheet, with implications for past ice sheet stability, ice‐rafted sediment delivery, and ice−ocean interactions in this complex archipelago setting.  相似文献   

4.
In a sequence of glacigenic sediments at Aberdaron, Foraminifera were obtained from samples located specifically in order to differentiate between opposing models of depositional environment. All the diamict samples yielded remarkably uniform assemblages, with similar numbers of benthic specimens and benthic species per unit weight of sediment, similar planktic : benthic ratios, and similar ratios of clearly allochthonous to possibly autochthonous elements. This is precisely as predicted by the terrestrial model of sedimentation, where all of the sediments are interpreted as being derived from the melting of glacier ice rich in marine debris entrained during passage along the Irish Sea Basin. The results lend no support to a glacial marine model, since no faunal responses to increasingly distal sedimentary environments are observed. However, the fauna is dominated by the Foraminifera Elphidium excavatum (Terquem) forma clavata Cushman, which is commonly assumed to indicate glacial marine conditions. The modern distribution of similar assemblages suggests that it is just as likely to represent the cold, reduced salinity conditions that would have prevailed in the northern Irish Sea Basin for much of the Quaternary.  相似文献   

5.
Recent work on the last glaciation of the British Isles has led to an improved understanding of the nature and timing of the retreat of the British?Irish Ice Sheet (BIIS) from its southern maximum (Isles of Scilly), northwards into the Celtic and Irish seas. However, the nature of the deglacial environments across the Celtic Sea shelf, the extent of subaerial exposure and the existence (or otherwise) of a contiguous terrestrial linkage between Britain and Ireland following ice retreat remains ambiguous. Multiproxy research, based on analysis of 12 BGS vibrocores from the Celtic Deep Basin (CDB), seeks to address these issues. CDB cores exhibit a shell‐rich upward fining sequence of Holocene marine sand above an erosional contact cut in laminated muds with infrequent lonestones. Molluscs, in situ Foraminifera and marine diatoms are absent from the basal muds, but rare damaged freshwater diatoms and foraminiferal linings occur. Dinoflagellate cysts and other non‐pollen palynomorphs evidence diverse, environmentally incompatible floras with temperate, boreal and Arctic glaciomarine taxa co‐occurring. Such multiproxy records can be interpreted as representing a retreating ice margin, with reworking of marine sediments into a lacustrine basin. Equally, the same record may be interpreted as recording similar conditions within a semi‐enclosed marine embayment dominated by meltwater export and deposition of reworked microfossils. As assemblages from these cores contrast markedly with proven glaciomarine sequences from outside the CDB, a glaciolacustrine interpretation is favoured for the laminated sequence, truncated by a Late Weichselian transgressive sequence fining upwards into fully marine conditions. Reworked rare intertidal molluscs from immediately above the regional unconformity provide a minimum date c. 13.9 cal. ka BP for commencement of widespread marine erosion. Although suggestive of glaciolacustrine conditions, the exact nature and timing of laminated sediment deposition within the CDB, and the implications this has on (pen)insularity of Ireland following deglaciation, remain elusive.  相似文献   

6.
Coastal exposures of Late Pleistocene sediments deposited after 19 000 yr BP near Dublin, Ireland, provide a window into the infill of a subglacially-cut tunnel valley. Exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi-storey U-shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. Gravels are truncated by poorly sorted ice-proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin. The sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive, poorly sorted gravel facies interpreted as subaqueous debris flow deposits. Gravel beds show local inverse and normal coarse-tail graded facies recording the restricted development of turbulent flow. Sediment gravity flow deposits fill broad (<2 km) shallow (10 m) and overlapping channels. Penetrative deformation structures (e.g. dykes) are common at the base of channels. The same subglacially-eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km2 of the western Irish Sea. Tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio-isostatic downwarping below the last British Ice Sheet. The facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves.  相似文献   

7.
Southwestern Barents Sea sediments contain important information on Lateglacial and Holocene environmental development of the area, i.e. sediment provenance characteristics related to ice‐flow patterns and ice drifting from different regional sectors. In this study, we present investigations of clay, heavy minerals, and ice‐rafted debris from three sediment cores obtained from the SW Barents Sea. The sediments studied are subglacial/glaciomarine to marine in origin. The core sequences were divided into three lithostratigraphical units. The lowest, Unit 3, consists of laminated glaciomarine sediments related to regional deglaciation. The overlying Unit 2 is a diamicton, dominated by mud and oversized clasts. Unit 2 reflects a more ice‐proximal glaciomarine sedimentary environment or even a subglacial depositional environment; its deposition may indicate a glacial re‐advance or stillstand during an overall retreat. The uppermost Unit 1 consists of Holocene marine sediments and current‐reworked sedimentary material with a relatively high carbonate content. A significant proportion of the sedimentary material could be derived from Svalbard and transported by sea ice or icebergs to the Barents Sea during the late deglacial phase. The Fennoscandian sources and local Mesozoic strata from the bottom of the Barents Sea are the likely provenances of sediments deposited during the deglacial and ice re‐advance phases. Bottom currents and sea‐ice transport were the main mechanisms influencing sedimentation during the Holocene. Our results indicate that the provenance areas can be reliably related to certain ice‐flow sectors and transport mechanisms in the deglaciated Barents Sea.  相似文献   

8.
The Late Devensian (<20 ka BP) glacial geology of the Irish Sea Basin (4000 km2) is an event stratigraphy recording the entry of marine waters into a glacio-isostatically-depressed basin, and the rapid retreat of the Irish Sea Glacier as a tidewater ice margin. Marine limits occur up to 140 m O.D. Across much of the central basin, the ice margin was uncoupled from its bed exposing a subglacially-scoured topography to glaciomarine processes. The Irish Sea Glacier was a major drainage conduit of the last British Ice Sheet; calving of the marine ice margin resulted in fast flow (surging) of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys. Rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones (e.g. Cheshire/Shropshire Lowlands) and initiated the collapse of the ice sheet.Thick wedges of ice-contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the Irish Sea coast. Where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins, the development of fast flowing ice streams, drumlinization and the pumping of subglacial sediment to tidewater. Raised delta complexes are locally associated with marine limits along the high relief coastal margins of Wales, east central Ireland, and the Lake District. Associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat. Co-eval offshore deposits are represented by well-stratified glaciomarine complexes that infill a subglacially-scoured topography that shows networks of tunnel valleys. Glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin (e.g. North Devon, Scilly Islands, Southern Ireland). The age of these distal sediments, previously mapped as pre-Devensian tills, is constrained by amino acid ratios.Basin rebound following deglaciation was rapid, with over 100 m recovery in 3 ka, and was followed by a low marine still stand. Peat, accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level.The glacio-sedimentary model identified in this paper, involving rapid ice retreat and related sedimentation triggered by rising relative sea level, suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves.  相似文献   

9.
The Jæren area in southwestern Norway has experienced great changes in sea‐levels and sedimentary environments during the Weichselian, and some of these changes are recorded at Foss‐Eikeland. Four diamictons interbedded with glaciomarine and glaciofluvial sediments are exposed in a large gravel pit situated above the post‐glacial marine limit. The interpretation of these sediments has implications for the history of both the inland ice and the Norwegian Channel Ice Stream. During a Middle Weichselian interstadial, a large glaciofluvial delta prograded into a shallow marine environment along the coast of Jæren. A minor glacial advance deposited a gravelly diamicton, and a glaciomarine diamicton was deposited during a following marine transgression. This subsequently was reworked by grounded ice, forming a well‐defined boulder pavement. The boulder pavement is followed by glaciomarine clay with a lower, laminated part and an upper part of sandy clay. The laminated clay probably was deposited under sea‐ice, whereas more open glaciomarine conditions prevailed during deposition of the upper part. The clay is intersected by clastic dykes protruding from the overlying, late Weichselian till. Preconsolidation values from the marine clay suggest an ice thickness of at least 500 m during the last glacial phase. The large variations in sea‐level probably are a combined effect of eustasy and glacio‐isostatic changes caused by an inland ice sheet and an ice stream in the Norwegian Channel. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

10.
Throughout northern Canada, live‐collected, pre‐bomb, deposit‐feeding marine molluscs from calcareous sediments yield greater apparent radiocarbon ages than do suspension feeders. We explore the size of this effect in a set of 57 paired datings of deposit feeders, mainly Portlandia arctica, and suspension feeders, mainly Hiatella arctica and Mya truncata, collected from both calcareous and non‐calcareous Holocene sediments. Deposit feeders from calcareous sediments are older than their suspension‐feeding counterparts by as much as 2240±130 14C years. This is attributed to the uptake of ‘old’ bicarbonate derived from calcareous bedrock. The age discrepancy between suspension and deposit feeders in calcareous terrain is non‐systematic in space and time, thereby invalidating the application of a correction. In contrast, the age comparisons are concordant at sites located on the Precambrian Shield. In terrestrial environments underlain by carbonate, previous acceptance of dates on deposit feeders led to erroneous interpretations of deglaciation and relative sea‐level history, in both the North American and the Eurasian Arctic. This has prompted several researchers to exclude deposit feeders from their late Quaternary reconstructions. The same chronological problem of deposit‐feeding molluscs now needs to be more widely acknowledged by the marine community.  相似文献   

11.
Using extensive data sets from three separate areas in the German North Sea sector, consisting of seismic grids, cores and in‐situ cone penetration tests (CPT), we have established a revised stratigraphical framework for the mid to late Quaternary deposits of the German North Sea sector. This framework consists of four regional unconformities and 15 other local unconformities derived from seismic profiles. Using these unconformities, along with lithological and geotechnical data, it was possible to define and correlate 14 major units and 21 subunits within the framework. The Quaternary cover in the area is characterized by a variety of environmental settings ranging from glacial terrestrial and fluvial to lacustrine as well as brackish and marine environments with associated erosion, reworking and deposition. The complexity of Quaternary deposits within the area is explained by its history of repeated ice advances interrupted by marine transgressions and exposed periglacial landscapes. Within the framework, eight buried tunnel valleys and two shallow buried river valleys are identified from seismic profiles with four phases of tunnel valley generation inferred. These phases of tunnel valley generation are associated with the Elsterian (three) and Saalian (one) glacial stages. Infill of these tunnel valleys consists of glaciofluvial sands, thick sequences of marine and lacustrine fine‐grained sediments and some reworked till remnants. Elsewhere, extensive tabular units have formed consisting of marine and fluvial sediments. We compare this new stratigraphy with previous stratigraphies for the German North Sea sector, attribute informal stratigraphical names and offer preliminary correlations with established stratigraphies from other sectors of the North Sea.  相似文献   

12.
The nature and origin of glacial sediments at Wylfa Head are described, and their significance with regard to sedimentary environments during Late Devensian deglaciation of the Irish Sea Basin is discussed. Recent models of deglaciation under glaciomarine conditions are challenged. The Quaternary sequence at Wylfa consists of eroded and glaciotectonically deformed bedrock, locally derived lodgement till, calcareous silt-rich lodgement till containing northern erratics, discontinuous units of orange-brown silty sand of possible aeolian origin, and grey laminated freshwater silts filling a small kettle hole. The till units thicken to the south where the surface is drumlinised. It is concluded that the landforms and deposits result from a warm-based Irish Sea glacier, which moved towards the southwest. Spatial variation in basal water pressure resulted from localised drainage through zones of more heavily jointed bedrock. Rapid glacial erosion occurred in areas where subglacial water pressure was relatively high, while deposition of the resulting basal sediment took place where water pressures were reduced. The glacier also carried basal calcareous silty till onshore, which was deposited by lodgement processes. None of the deposits at Wylfa are interpreted as glaciomarine in origin, and there is no evidence at this site for an isostatically induced marine transgression prior to deglaciation.  相似文献   

13.
A sequence of Middle Pleistocene (approximately early 'Cromerian Complex') sediments has been subdivided into subglacial, proximal glaciomarine, distal glaciomarine and marine facies. The subglacial facies represents lodgement till deposited during the final stages of ice-sheet advance. At the onset of ice-sheet retreat, streams deposited their load into a shallow-water; glaciomarine environment; gravelly sediments immediately in front of the ice-grounding line and finer material, in suspension, to more distal areas. Ice-rafting, slumping and traction currents were also active within the glaciomarine environment. The lithofacies characteristics of this sequence are consistent with deposition from a grounded tidewater ice-sheet. The glacigenic succession is restricted to the Forth Approaches area, which implies that the ice-sheet had a limited offshore extent.  相似文献   

14.
Pebbly clays and diamictons containing marine shell fragments and peat lenses exposed beneath subglacially deposited Late Devensian till at the Burn of Benholm provide new insights into the glacial history of Quaternary sequences in eastern Scotland. The peat yielded pollen of interstadial affinity (including Bruckenthalia spiculifolia) and non‐finite radiocarbon dates. Comparisons with other pre‐Late Devensian pollen records in northern Scotland suggest that the peat lenses are remnants of an Early Devensian interstadial deposit, of Oxygen Isotope Substage 5c or 5a age. Reworked faunal assemblages in the shelly sediments include Quaternary marine molluscs of low boreal aspect, as well as Mesozoic and Palaeozoic microfossils. Amino acid ratios from fragments of Arctica islandica suggest that the shells are of Oxygen Isotope Stage 9 age or older. The fabric and composition of the shelly sediments are consistent with their emplacement as deformation till during the onshore movement of glacially transported rafts of marine sediment. Folded and sheared contacts between the shelly deposits, peat lenses and the overlying Late Devensian till indicate that the fossiliferous sediments were glacitectonised during the main Late Devensian glaciation, when ice moved from Strathmore and overrode the site from the southwest. British Geological Survey. © NERC 2000.  相似文献   

15.
Sedimentary records from the southwestern Kara Sea were investigated to better understand the extent of the last glaciation on the Eurasian Arctic shelf, sea-level change, and history of the Ob' and Yenisey river discharge. Sediment-core and seismic-reflection data indicate that the Quaternary depositional sequence in the southwestern Kara Sea consists of glacial, glaciomarine, and marine sedimentary units. Glaciogenic sediments in the deep Novaya Zemlya Trough are presumably related to the Last Glacial Maximum (LGM), whereas further east they may represent an earlier glaciation. Thus, it is inferred that the southeastern margin of the LGM Barents-Kara ice sheet was contained in the southwestern Kara Sea east of the Novaya Zemlya Trough. Changes in mineralogical, foraminiferal, and stable-isotopic composition of sediment cores indicate that riverine discharge strongly influenced sedimentary and biotic environments in the study area during the Late Weichselian and early Holocene until ca. 9 ka, consistent with lowered sea levels. Subsequent proxy records reflect minor changes in the Holocene hydrographic regime, generally characterized by reduced riverine inputs.  相似文献   

16.
A macrofossil-rich glaciomarine–marine–lacustrine sediment from a soft-water lake in southwestern Sweden has provided an opportunity to 14C date different components of its sediments. Bulk sediment dates are 100 to 500 yr older than fragile terrestrial macrofossils of corresponding levels, with a mean age difference of ca. 300 yr. This is explained by the presence of old and reworked organic material in the sediment. Five age comparisons between terrestrial macrofossils and periostraca of marine bivalves (probably Arctica islandica) of Allerød age give a mean age difference of 380 yr, i.e. slightly greater than the present reservoir age of 340 ± 30 yr for the Swedish west coast. This difference is roughly the same as for the marine macroalga Desmarestia aculeata. A date from shell carbonate fragments of Mytilus edulis yields an age that is more than 1000 yr older than corresponding periostraca and terrestrial macrofossils and 500–1000 yr older than the age of the supposed deglaciation of the site. Altogether this indicates a larger marine reservoir effect during the Allerød than at present. © 1998 John Wiley & Sons, Ltd.  相似文献   

17.
The deglaciation history of the Malangen‐Målselv fjord and valley area proximally to the Tromsø‐Lyngen (Younger Dryas) moraine at Bakkejord, Malangen, northern Norway, is reconstructed based on morphostratigraphic, lithostratigraphic and geophysical evidence, and 25 radiocarbon dates from marine shells and foraminifera. The results show that following the Skarpnes event c. 12 200 14Cyr BP, and prior to the Younger Dryas readvance, the area was deglaciated at least as far as Sandmo situated 22 km proximally to the Tromsø‐Lyngen moraine. Two moraine ridges crossing the fjord at Sandmo and buried beneath thick glaciomarine sediments are correlated with this period. The area was subsequently deglaciated between 10 300 and 9200 14Cyr BP, following the Tromsø‐Lyngen (Younger Dryas) readvance. Five ice‐front accumulations post‐dating the Tromsø‐Lyngen moraine and situated 19, 27, 42, 55 and 77 km behind it are identified and dated based on radiocarbon dates and correlation of marine limits: Målsnes (c. 10 050 14Cyr BP), Kjerresnes (c. 10 000 14Cyr BP), Solli (c. 9750 14Cyr BP), Bardufoss‐Brentmoen‐Storskogmoen (c. 9600–9700 14Cyr BP) and Alapmoen (c. 9200 Cyr BP). The largest of these, at Bardufoss‐Storskogmoen, possibly accumulated as a response to an ice advance. Fourteen dates of apparent late Allerød/Younger Dryas age (11 100–10 000 14Cyr BP), obtained from fossils in glaciomarine sediments in the Målselv valley up to 77 km proximally to the Tromsø‐Lyngen moraine, are interpreted as postdating rather than predating this moraine. Several of these are considered to be too old because of uncertain reservoir age, carbon‐dating plateaus and/or contamination. This highlights uncertainties associated with radiocarbon‐dating and the profound effect such uncertainties may have on interpreting geological events.  相似文献   

18.
Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio-chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS-1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore-filling blocky fabric throughout the core. Oxygen isotope compositions (δ18O = −30·0 to −8·6‰ Vienna Pee-Dee Belemnite) and clumped isotope temperatures (TΔ47 = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ18Ow) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND-2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS-1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS-1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.  相似文献   

19.
Miocene siliciclastic sediments of the Marañón Foreland Sub‐basin in Peru record the sedimentary response to regional marine incursions into Amazonia. Contrary to previous interpretations, the Late Miocene Nauta Formation provides evidence of the last known marine incursion before the current Amazonia river basin became established. Sedimentological, ichnological and palynological data from well‐exposed outcrops along a ca 100 km road transect suggest that the Nauta Formation represents a shallow, marginal‐marine channel complex dominated by tidal channels developed in the inactive, brackish‐water portions of a delta plain. The main facies associations are: FA1 – slightly bioturbated mud‐draped trough cross‐stratified sand; FA2 – locally, pervasively bioturbated inclined heterolithic stratification (IHS); and FA3 – moderately bioturbated horizontally bedded sand–mud couplets. These identify subtidal compound dunes, tidal point bars and shallow subtidal to intertidal flats, respectively. Bi‐seasonal depositional cycles are ascribed to the abundant metre‐ to decimetre‐scale sand–mud couplets that are found mainly in the IHS association: semi‐monthly to daily tidal rhythmicity is inferred from centimetre‐ and millimetre‐scale couplets in the mud‐dominated parts of the decimetre‐scale couplets. The ichnology of the deposits is consistent with brackish depositional conditions; the presence of Laminites, a variant of Scolicia, attests to episodic normal marine conditions. Trace fossil suites are assigned to the Skolithos, Cruziana and mixed Skolithos–Cruziana ichnofacies. Pollen assemblages related to mangrove environments (e.g. Retitricolporites sp., Zonocostites sp., Psilatricolporites maculosus, Retitricolpites simplex) support a brackish‐water setting. Uplift of the Mérida Andes to the North and the consequent closure of the Proto‐Caribbean connection, and the onset of the transcontinental Amazon drainage, constrain the deposition of the Nauta sediments with around 10 to 8 Ma, probably contemporaneous to similar marine incursions identified in the Cuenca (Ecuador), Acre (Brazil) and Madre de Dios (Southern Peru) (sub)basins, and along the Chaco‐Paranan corridor across Bolivia, Paraguay and Argentina.  相似文献   

20.
Sparker and shallow drilling data indicate that the Quaternary deposits in the Central Deep of the Barents Sea are mainly composed of glacigenic sediments. They comprise basal till and proximal and distal glaciomarine sediments deposited during the last glacial cycle. Apparent glaciotectonic features imply strong glacial erosion of Mesozoic bedrock. The general ice movement is assumed to have been from off Novaya Zemlya and it is concluded that the whole eastern Barents Sea was covered by the Late Weichselian ice-sheet.  相似文献   

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