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1.
Cockburn Island, Ontario (45°55′ N, 83°20′ W), holds at least six sets of elevated lake bluffs, scarps and bar deposits that mark distinctive water planes above the Nipissing Great Lakes water plane (∼198 m). These relict shoreline features occur at elevations that correspond closely with the elevations of others at nearby St. Joseph Island and in eastern upper Michigan. Together, the elevations and relative locations of steep relict bluffs suggest a proto-Cockburn Island once interrupted the surface of proglacial Lake Algonquin. The islet appears to have emerged and grown through a period of uplift and a sequence of lowering water levels. The highest relict shoreline (280.2 m) is correlated with the Main phase of Lake Algonquin. Lower shorelines at Cockburn Island cannot be correlated consistently, so additional work is required. This is the seventh in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.  相似文献   

2.
Geomorphology of a beach-ridge complex and adjacent lake basins along the northern shore of Lake Michigan records fluctuations in the level of Lake Michigan for the last 8000 to 10 000 14C yr B.P. (radiocarbon years Before Present). A storm berm at 204.7–206 m (671.6–675.9 ft) exposed in a sandpit provides evidence of a pre-Chippewa Low lake level that is correlated with dropping water levels of Glacial Lake Algonquin (c. 10 300–10 100 14C yr B.P.). Radiocarbon dates from organic material exposed in a river cutbank and basal sediments from Elbow Lake, Mackinac Co., Michigan, indicate a maximum age of a highstand of Lake Michigan at 6900 14C yr B.P., which reached as high as 196.7 m (645 ft), during the early-Nipissing transgression of Lake Michigan. Basal radiocarbon dates from beach swales and a second lake site (Beaverhouse Lake, Mackinac Co.) provide geomorphic evidence for a subsequent highstand which reached 192.6 m (632 ft) at 5390±70 14C yr B.P.Basal radiocarbon dates from a transect of sediment cores, along with tree-ring data, and General Land Office Surveyor notes of a shipwreck, c. A.D. 1846, reveal a late-Holocene rate for isostatic rebound of 22.6 cm/100 radiocarbon years (0.74 ft/100 radiocarbon years) for the northern shore of Lake Michigan, relative to the Lake Michigan-Lake Huron outlet at Port Huron, Michigan. Changes in sediment stratigraphy, inter-ridge distance, and sediment accumulation rates document a mid- to late-Holocene retreat of the shoreline due to isostatic rebound. This regression sequence was punctuated by brief, periodic highstands, resulting in progressive development over the past 5400 14C yr of 75 pairs of dune ridges and swales each formed over an interval of approximately 72 years. Times of lake-level fluctuation were identified at 3900, 3200, and 1000 14C yr B.P. based on changes in inter-ridge spacing, shifts in the course of Millecoquins River, and reorientation of beach-ridge lineation. Soil type, dune development, and selected pollen data provide supporting evidence for this chronology. Late-Holocene beach-ridge development and lake-level fluctuations are related to a retreat of the dominant Pacific airmass and the convergence of the Arctic and Tropical airmasses resulting in predominantly meridional rather than zonal air flow across the Great Lakes region.This is the 13th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.  相似文献   

3.
This study reviews Glacial Lake Algonquin, examines the Main and two “Upper Group” phases in northern Michigan and nearby Ontario, reports their spatial extents, and reassesses the lake history in light of isostatic rebound. Our paper presents the most accurate and detailed maps of Glacial Lake Algonquin in this region that have yet been published. Fieldwork was conducted at 243 ancient shoreline sites, which yielded position data that support geostatistical models that represent differentially upwarped water planes. Model parameters that describe water plane tilt are reported for the Main, Ardtrea and Upper Orillia phases. Geostatistical water plane models were used to adjust a digital contemporary elevation model, thereby creating a digital proglacial elevation model for each phase. Maps of these phases and the data that support them suggest (1) proto-Cockburn Island, Ontario existed as an islet in the lake that was deglaciated before the outlet at North Bay, Ontario was opened, (2) the Main and Ardtrea phases of the lake extended into the northern Lake Michigan basin, and (3) the Main and Ardtrea water planes intersect at places near Little Traverse Bay (by Lake Michigan) and Thunder Bay (by Lake Huron). Mapped isobases generally conform to those published in other works and suggest the regional pattern of isostatic adjustment has not changed substantially during the last 13,000 years.  相似文献   

4.
The post-glacial history of the Great Lakes has involved changes in lake levels that are equivalent in vertical extent to the Pleistocene changes in global sea level and changes in sediment accumulation by at least two orders of magnitude. In the sediments of the northern Lake Michigan basin, these radical changes in base level and sediment supply are preserved in detailed records of changing depositional environment and the impact of these changes on depositional architecture. The seismic sequences of the sediment fill previously described in Lake Huron have been carried into northern Lake Michigan and used to map the history and architecture of basinal deposition. As the Laurentide Ice Sheet retreated northward in the early Holocene, it opened progressively deeper channels to the east that allowed the larger lakes to drain through the North Channel, Huron, and Georgian Bay basins. At the end of the Main Algonquin highstand, about 10,200 (radiocarbon) yrs ago, the eastern drainage passage deepened in a series of steps that defined four seismic sequences and lowered lake levels by over 100 m. Near the same time a new source of sediment and meltwaters poured across the Upper Peninsula of Michigan and into the northern Lake Michigan basin from the Superior basin ice lobe. A marked increase in deposition is seen first in the northern part of the study area, and slightly later in the Whitefish Fan area at the southern end of the study area. Accumulation rates in the area gradually decreased even as lake levels continued to fall. Drainage directly from the Superior basin ended before the beginning of the main Mattawa phase about 9,200 (radiocarbon) yrs ago.Although individual lowstand systems tracts are at the most a few hundred yrs in duration, their geometries and seismic character are comparable to marine systems tracts associated with sea level falls of similar magnitudes. In some of the thicker lowstand deposits a second order cyclicity in sedimentation can be detected in the high resolution seismic records.  相似文献   

5.
《自然地理学》2013,34(3):252-268
We examined a large, Late Pleistocene delta in northern Lower Michigan, formed by the Black River in Glacial Lake Algonquin. Today, this sandy, arcuate, wave-influenced delta stands several meters above the lake floor. The Black River transported mainly well-sorted, medium, and fine sands to the delta—at remarkably rapid rates. Our subsurface data, taken at 153 sites across the delta, show subtle and consistent trends in sediment texture across the delta surface. Although found in low amounts, gravel and very coarse sands are concentrated near the shoreline, presumably eroded by waves from the till and bedrock that crop out there. Sediments of very fine sand size (and finer) exist in higher concentrations near the eastern shore, sourced from eroded tills and also carried there on longshore currents. A clear sediment plume of medium and finer sands also traverses the delta SE to NW, which we interpret as evidence of sand transport by longshore currents flowing east to west, driven by easterly winds. High, perched spits on the head of the delta also suggest westerly longshore drift. These paleoclimate proxy data support previous interpretations of strong easterly winds here during the Late Pleistocene, probably in association with a glacial anticyclone.  相似文献   

6.
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.  相似文献   

7.
Piston cores from deep-water bottom deposits in Lake Ontario contain shallow-water sediments such as, shell-rich sand and silt, marl, gyttja, and formerly exposed shore deposits including woody detritus, peat, sand and gravel, that are indicative of past periods of significantly lower water levels. These and other water-level indicators such as changes in rates of sedimentation, mollusc shells, pollen, and plant macrofossils were integrated to derive a new water-level history for Lake Ontario basin using an empirical model of isostatic adjustment for the Great Lakes basin to restore dated remnants of former lake levels to their original elevations. The earliest dated low-level feature is the Grimsby-Oakville bar which was constructed in the western end of the lake during a near stillstand at 11–10.4 (12.9–12.3 cal) ka BP when Early Lake Ontario was confluent with the Champlain Sea. Rising Lake Ontario basin outlet sills, a consequence of differential isostatic rebound, severed the connection with Champlain Sea and, in combination with the switch of inflowing Lake Algonquin drainage northward to Ottawa River valley via outlets near North Bay and an early Holocene dry climate with enhanced evaporation, forced Lake Ontario into a basin-wide lowstand between 10.4 and 7.5 (12.3 and 8.3 cal) ka BP. During this time, Lake Ontario operated as a closed basin with no outlets, and sites such as Hamilton Harbour, Bay of Quinte, Henderson Harbor, and a site near Amherst Island existed as small isolated basins above the main lake characterized by shallow-water, lagoonal or marsh deposits and fossils indicative of littoral habitats and newly exposed mudflats. Rising lake levels resulting from increased atmospheric water supply brought Lake Ontario above the outlet sills into an open, overflowing state ending the closed phase of the lake by ~7.5 (8.3 cal) ka BP. Lake levels continued to rise steadily above the Thousand Islands sill through mid-to-late Holocene time culminating at the level of modern Lake Ontario. The early and middle Holocene lake-level changes are supported by temperature and precipitation trends derived from pollen-climate transfer functions applied to Roblin Lake on the north side of Lake Ontario.  相似文献   

8.
Lake Simcoe is a large lake 45 km across and in places over 30 m deep, located between Lake Huron and Lake Ontario, in the glaciated terrain of southern Ontario, Canada. Seismostratigraphic analysis of high-resolution seismic reflection profiles, together with lakebed sediment sampling and pollen study, revealed distinctive sequences in the sediments beneath Lake Simcoe, Ontario. A surface unit (Blue Sequence) of soft Holocene mud (low-amplitude surface reflection, discontinuous parallel internal reflections) lies in the deeper basins of the lake. The underlying unit (Green Sequence) is characterized by high-amplitude parallel internal reflections; basal sediments of this sequence consist of clay rhythmites with dropstones. The Green Sequence was deposited by lacustrine sedimentation in proglacial Lake Algonquin; sedimentation persisted until the basin was isolated from other glacial lakes at about 10 14C ka at the Penetang post-Algonquin phase. Subsequent erosion of the uppermost portion of the Green Sequence is attributed to wave action in a low-level early Holocene lake, possibly closed hydrologically and coeval with closed lowstands in the Huron and Georgian Bay basins. Two sequences with high-amplitude surface reflections and chaotic internal reflections (Purple and Red Sequences) lie below the Green Sequence. Northeast-southwest trending ridges, tens of metres in height, on the Red Sequence (the lowermost of these two units) are interpreted to be drumlins. An erosion surface descends into narrow valleys 50–80 m deep beneath the lake in bays to the west and south of the main lake basin. These depressions are interpreted as subglacial tunnel channels cut by rapid flows of meltwater. The sediments of Purple Sequence are interpreted as channel-fill sediments rapidly deposited during waning stages of the meltwater drainage. The Red Sequence is correlated with the Newmarket Till of the last glacial maximum identified beneath the Oak Ridges Moraine to the south.  相似文献   

9.
《自然地理学》2013,34(3):233-244
Relatively low (<25 m) parabolic dunes and dune ridges occur inland of massive parabolic dunes in many dune complexes along the southeastern shore of Lake Michigan. The major study of these backdunes (Tagues, 1946) concluded, based on field criteria, that they were older than the massive parabolic dunes and originate at the Calumet and Algonquin stages of ancestral Lake Michigan (~14-10 ka). Younger ages are indicated by this study in which Optically Stimulated Luminescense (OSL) ages were obtained from the crest of three backdunes southwest of Holland, Michigan. All ages are within statistical error of each other and indicate dune stabilization at ~4 ka. Similarities in surface soil development throughout the backdunes support the conclusion that they all stabilized at about the same time. Radiocarbon ages from paleosols indicate that the massive parabolic dunes were active at 4 ka and that this activity persisted after the back dunes had stabilized. In the Holland area, dune growth and migration occurred in a broad zone, including both back and massive parabolic dunes, immediately after the rise to and drop from Nipissing II high lake levels but became confined to a narrower zone closer to shore after ~4 ka.  相似文献   

10.
The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 14C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline. As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to 30 m of infill sediment below the modern floodplain. About 3 × 108 m3 of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan. The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m) of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend. It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic feature rather than a classic unsorted moraine that would drape over pre-glacial topography. This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.  相似文献   

11.
New intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika’s central basin capture dramatic evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences (A–D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a major regression that shifted the paleo-lake shore ∼21 km towards the west prior to ∼106 ka. The topset-to-foreset transitions in these deltas suggest lake level was reduced by ∼435 m during the period of deposition. Mounded reflections in the overlying sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived desiccation event that reduced lake level by ∼260 m, interpreted as a product of Last Glacial Maximum (32–14 ka) aridity. Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged aridity.  相似文献   

12.
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity.  相似文献   

13.
The Dongting Lake is located in the south beach of the middle reaches of the Yangtze River. Its catchment, with an area of 262,823 km2 or about 12% of the total Yangtze River catchment, is situated between 28o43?29o32扤 and 112o54?113o8扙, and crosses Hubei and Hunan provinces in administrative division. The main tributaries include Xiangjiang, Zishui, Yuanjiang, Lishui rivers (4 Tributaries) and some local rivers, such as Miluo River, Xinqiang River and other little streams. In the nor…  相似文献   

14.
The sediments of the Dongting Lake come from four channels (one of them was closed in 1959), connected with the Yangtze River, four tributaries (Lishui, Yuanjiang, Zishui and Xiangjiang) and local area, and some of them are transported into the Yangtze River in Chenglingji, which is located at the exit of the Dongting Lake, some of them deposit into drainage system in the lake region and the rest deposit into the lake. The annual mean sediment is 166,555x104 t, of which 80% come from the four channels, 18% from the four tributaries and 2% from local area, whereas 26% of the total sediments are transported into the Yangtze River and 74% deposited into the lake and the lake drainage system. Based on topographic maps of 1974, 1988 and 1998, and the spatial analysis method with geographic information system (GIS), changes in sediment deposition and erosion are studied in this paper. By overlay analysis of 1974 and 1988, 1988 and 1998, erosion and sediments deposition areas are defined. The main conclusions are: (1) sediment rate in the lake is larger than erosion rate from 1974 to 1998. The mean deposition in the lake is 0.43 m; (2) annual sediment deposition is the same between 1974-1988 and 1988-1998, but the annual volume of deposition and erosion of 1988-1998 is bigger than that in 1974-1988; (3) before the completion of the Three Gorges Reservoir, there will be 7.82x108 m3 of sediments deposited in the lake, which would make the lake silted up by 0.33 m; (4) in the lake, the deposition area is found in the north of the east Dongting Lake, the south-west of the south Dongting Lake, and the east of the west Dongting Lake; while the eroded area is in the south of the east Dongting Lake, the middle of the south Dongting Lake, the west of the west Dongting Lake, as well as Xiangjiang and Lishui river flood channels.  相似文献   

15.
Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.  相似文献   

16.
陈骥  姜在兴  张万益  刘超  许文茂 《中国沙漠》2018,38(5):999-1008
湖泊滨岸带风成沉积一直是湖泊沉积研究的薄弱点。占据青海湖湖盆面积近五分之一的东岸沙丘为研究现代湖泊滨岸带提供了一个现代沉积实例。随着沉积学的不断发展,研究内容已经从"一元"的传统相模式逐渐过渡为"二元"的"源-汇"体系。以野外地质考察和遥感影像资料为基础,结合文献调研和相关测试分析资料,进行分析。结果表明：团保山前发育有相对广阔的滨岸沉积,指示了青海湖湖水曾经达到山前地带。大面积的滨岸沉积和古沙丘出露水面以及风对山体的吹蚀,为湖东风砂堆积提供了物源基础。干旱-半干旱气候为沙丘的形成提供了有利的气候条件。断陷湖盆的长条形地貌使得风沿着长轴方向吹扬,造成风成堆积物在长轴的一端堆积。高大的团保山/达坂山的山前复杂地貌特征为金字塔沙丘的形成提供了有利的地形条件。西北风产生的波浪和沿岸流作用于沙岛和海晏湾的滨岸带,导致滨浅湖的砂堆积形成沿岸沙坝。随着湖平面的下降,沿岸沙坝出露水面,并逐渐闭合形成障壁岛-泻湖沉积。  相似文献   

17.
High-resolution seismic reflection profile data show that the modern sediment cover (over the last 150 years) in Georgian Bay is thin and spatially discontinuous. Sediments rich in ragweed pollen, largely derived from siltation linked to land clearing and European settlement, form a thin, discontinuous veneer on the lakebed. Much of the lakebed consists of exposed sediments deposited during the late glacial or early postglacial. Accumulation rates of modern sediments range from < 0 mm/year (net erosion) to ∼3.2 mm/year, often within a few hundred metres spatially. These rates are much lower than those reported for the main basin of Lake Huron and the other Great Lakes, and are attributed to the low sediment supply. Only a few small rivers flow into Georgian Bay, and most of the basin is surrounded by bedrock of Precambrian gneiss and granite to the east, and Silurian dolostone, limestone and shale to the west. Thick deposits of Pleistocene drift, found on the Georgian Bay shoreline only between Meaford and Port Severn, are the main sediment source for the entire basin at present. Holocene to modern sediments are even absent from some deep basins of Georgian Bay. These findings have implications for the ultimate fate of anthropogenic contaminants in Georgian Bay. While microfossil assemblages in the ragweed-rich sediments record increased eutrophication over the last 150 years, most pollutants generated in the Georgian Bay catchment are not accumulating on the lakebed and are probably exported from the Bay.  相似文献   

18.
The recognition of ice-marginal deltas constructed during the formation of the Nakina II moraine and a previously unrecognized spillway, in the vicinity of Longlac, northern Ontario, indicates that existing concepts of ancestral lake level history and drainage systems in the Lake Superior–Lake Nipigon region is inadequate. Based on isostatically corrected digital elevation maps, ice-marginal deltas of the Nakina II moraine probably formed at the level of glacial Lake Minong, most likely Minong III, and not glacial Lake Nakina as has been commonly suggested. In addition, the presence of a spillway near Longlac indicates that lake water drained southward through the Mullet Outlet–Pic River system immediately following ice-marginal retreat from the Nakina II moraine and not eastward as previously proposed. Architectural-element analysis of exposures within the spillway indicates hyperconcentrated outbursts of meltwater produced thick channel-fill elements during flood conditions with peak-velocities exceeding 3 m/s. Subsequent retreat of ice from the Pic River valley to the east, may have allowed waters of Lake Agassiz, Lake Barlow–Ojibway, or both, to drain into post-Minong lake levels in the Lake Superior basin. These findings place major constraints on previously proposed concepts of northeastern or eastern outlets of Lake Agassiz.  相似文献   

19.
A common break was recognized in four Lake Superior strandplain sequences using geomorphic and sedimentologic characteristics. Strandplains were divided into lakeward and landward sets of beach ridges using aerial photographs and topographic surveys to identify similar surficial features and core data to identify similar subsurface features. Cross-strandplain, elevation-trend changes from a lowering towards the lake in the landward set of beach ridges to a rise or reduction of slope towards the lake in the lakeward set of beach ridges indicates that the break is associated with an outlet change for Lake Superior. Correlation of this break between study sites and age model results for the strandplain sequences suggest that the outlet change occurred sometime after about 2,400 calendar years ago (after the Algoma phase). Age model results from one site (Grand Traverse Bay) suggest an alternate age closer to about 1,200 calendar years ago but age models need to be investigated further. The landward part of the strandplain was deposited when water levels were common in all three upper Great Lakes basins (Superior, Huron, and Michigan) and drained through the Port Huron/Sarnia outlet. The lakeward part was deposited after the Sault outlet started to help regulate water levels in the Lake Superior basin. The landward beach ridges are commonly better defined and continuous across the embayments, more numerous, larger in relief, wider, have greater vegetation density, and intervening swales contain more standing water and peat than the lakeward set. Changes in drainage patterns, foreshore sediment thickness and grain size help in identifying the break between sets in the strandplain sequences. Investigation of these breaks may help identify possible gaps in the record or missing ridges in strandplain sequences that may not be apparent when viewing age distributions and may justify the need for multiple age and glacial isostatic adjustment models. This is the third in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.  相似文献   

20.
The Bunger Hills in East Antarctica occupy a land area of approximately 400 km2. They have been exposed by Holocene retreat of the Antarctic ice sheet and its outlet glaciers. The accompanying sea level rise flooded the marine inlets that now separate the northern islands and peninsulas from the major part of the hills. During deglaciation the continental ice sheet margin retreated south‐eastwards with several temporary halts, during which ice‐dammed lakes were formed in some valleys. These lakes were maintained long enough to permit formation of beaches of sand and gravel, and for the erosion of shore platforms and low cliffs in bedrock. Around the western end of Fish Tail Bay impressive shoreline features 20 m above sea level define a former ice‐dammed lake that was 5.5 km long. A similar 7 km long former ice‐dammed lake was formed at Lake Dolgoe. The more extensive and deeper glacial lake is revealed by well‐developed and preserved shoreline features cut at 29 m which is 16 m above present lake level. In addition, several small ice‐dammed lakes existed temporarily near Lake Shchel and in the valley to the west. Lake Fish Tail existed more than 6,900 14C years ago and Lake Shchel probably more than 6,680 14C years ago. It is inferred that the shore platforms and beaches were formed by lake ice and wave action over considerable periods when the lakes were impounded by steep cold ice margins. There appears to have been a balance between meltwater input and evaporative loss from the lakes in the cold dry continental climate. There is no evidence for rapid lake level fluctuations, and there was very little input of clastic sediment. This resulted in poor development of deltaic and rhythmically laminated lake floor deposits. It is suggested that such deposits are more characteristic of ice‐dammed lakes formed in association with wet‐based temperate ice than those associated with dry‐based polar ice.  相似文献   

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