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1.
The Dawson Cut Forest Bed lies in the lower part of thick, late Cenozoic loess deposits in the Fairbanks area. It is associated with several distal tephra beds that provide age control and offer the opportunity of its recognition elsewhere in central Alaska. EC tephra (named herein) occurs in the uppermost part of the Dawson Cut Forest Bed and its petrographic and chemical properties point to a co-magmatic relationship with PA tephra, which has not been found in direct association with the forest bed. Both tephra beds are pink and have unusually high Cl in their glass shards, which readily separates them from all other tephra beds in the Fairbanks area. They were produced by discrete eruptions, closely spaced in time. PA tephra has a glass-fission-track age of 2.02 ± 0.14 myr, indicating that the Dawson Cut Forest Bed must be about 2 million years old. The Palisades tephra (named herein) has very similar properties to these two tephra beds, suggesting that the buried forest bed just above it at the Palisades site on the Yukon River, about 250 km west of Fairbanks, correlates with the Dawson Cut Forest Bed.  相似文献   

2.
Radiocarbon‐dated sediment cores from six lakes in the Ahklun Mountains, south‐western Alaska, were used to interpolate the ages of late Quaternary tephra beds ranging in age from 25.4 to 0.4 ka. The lakes are located downwind of the Aleutian Arc and Alaska Peninsula volcanoes in the northern Bristol Bay area between 159° and 161°W at around 60°N. Sedimentation‐rate age models for each lake were based on a published spline‐fit procedure that uses Monte Carlo simulation to determine age model uncertainty. In all, 62 14C ages were used to construct the six age models, including 23 ages presented here for the first time. The age model from Lone Spruce Pond is based on 18 ages, and is currently the best‐resolved Holocene age model available from the region, with an average 2σ age uncertainty of about ± 109 years over the past 14.5 ka. The sedimentary sequence from Lone Spruce Pond contains seven tephra beds, more than previously found in any other lake in the area. Of the 26 radiocarbon‐dated tephra beds at the six lakes and from a soil pit, seven are correlated between two or more sites based on their ages. The major‐element geochemistry of glass shards from most of these tephra beds supports the age‐based correlations. The remaining tephra beds appear to be present at only one site based on their unique geochemistry or age. The 5.8 ka tephra is similar to the widespread Aniakchak tephra [3.7 ± 0.2 (1σ) ka], but can be distinguished conclusively based on its trace‐element geochemistry. The 3.1 and 0.4 ka tephras have glass major‐ and trace‐element geochemical compositions indistinguishable from prominent Aniakchak tephra, and might represent redeposited beds. Only two tephra beds are found in all lakes: the Aniakchak tephra (3.7 ± 0.2 ka) and Tephra B (6.1 ± 0.3 ka). The tephra beds can be used as chronostratigraphic markers for other sedimentary sequences in the region, including cores from Cascade and Sunday lakes, which were previously undated and were analyzed in this study to correlate with the new regional tephrostratigraphy. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

3.
The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments.  相似文献   

4.
The fossil vole Microtus deceitensis occurs in Early Pleistocene deposits at Fort Selkirk, Yukon Territory, and Late Pliocene beds at the type locality, Cape Deceit, Alaska. Analyses of simple vs complex morphotypes in the cheek teeth, and of differentiation of tooth enamel, show that the Cape Deceit sample of M. deceitensis is less derived, and thus appears to be older, than the Fort Selkirk sample. The fossiliferous deposits at Fort Selkirk are well constrained by fission-track and radiometric dates and are 1.5 to 1.7 myr. Sediments at Cape Deceit bear a normal magnetic polarity, are correlated with the Olduvai subchron, and probably are latest Pliocene.  相似文献   

5.
Near Summer Lake in southern Oregon, 54 tephra beds of late Quaternary age are exposed in pluvial lake sediments of Lake Chewaucan. Seven of the tephra beds near the top can be correlated with tephra deposits younger than 117,000 yr at Mount St. Helens, Washington, at Crater Lake, Oregon, and in northwestern Nevada in the deposits of pluvial Lake Lahontan. However, most of the section at Summer Lake lies below the correlated units, and contains 39 tephra beds older than 117,000 yr.Major-element chemistry of tephra glasses was determined by electron microprobe analysis; petrography supports the correlations made from chemical evidence. Compositions correlated range from 70 to 76% SiO2; the least silicic Summer Lake glass contained 57%.Extrapolation of depositional rate suggests that most of the sediments at Summer Lake are younger than about 335,000 yr, but older lake beds containing tephra layers occur at one place. The long lacustrine record suggests that Lake Chewaucan persisted through the last interpluvial stage, and that the lake may have dried up at the end of the Pleistocene due to diversion of the Chewaucan River by relict shore features.  相似文献   

6.
The age of the Sheep Creek tephra (SCt), a widespread marker ash bed in eastern Alaska and western Yukon Territory, has been ambiguous and controversial. We have obtained three reliable thermoluminescence age estimates from bracketing loess near Fairbanks that imply a deposition age of about 190,000 ± 20,000 yr for SCt. Three of six loess samples near and closely bracketing the SCt beds near Fairbanks yielded younger age estimates (∼117,000 and ∼135,000 yr), most likely (based on field aspects) because of reworking and contamination by translocated grains. The new, reliable age assignment of 190,000 yr confirms independent stratigraphic evidence of a pre-last interglaciation age, and stratigraphic evidence from one site (Upper Eva Creek) that SCt is older than the more-widespread 140,000-yr-old Old Crow tephra. The SCt age also has implications for regional correlations of glacial and nonglacial deposits. In particular, it supports the stratigraphic and geomorphic interpretation that the Delta Glaciation in the east-central Alaska Range and the Reid Glaciation in western Yukon Territory are older than the last interglaciation (isotope substage 5e).  相似文献   

7.
《Earth》2006,74(1-4):245-270
New tephrochronologic, soil-stratigraphic and radiometric-dating studies over the last 10 years have generated a robust numerical stratigraphy for Upper Neogene sedimentary deposits throughout Death Valley. Critical to this improved stratigraphy are correlated or radiometrically-dated tephra beds and tuffs that range in age from > 3.58 Ma to < 1.1 ka. These tephra beds and tuffs establish relations among the Upper Pliocene to Middle Pleistocene sedimentary deposits at Furnace Creek basin, Nova basin, Ubehebe–Lake Rogers basin, Copper Canyon, Artists Drive, Kit Fox Hills, and Confidence Hills. New geologic formations have been described in the Confidence Hills and at Mormon Point. This new geochronology also establishes maximum and minimum ages for Quaternary alluvial fans and Lake Manly deposits. Facies associated with the tephra beds show that ∼3.3 Ma the Furnace Creek basin was a northwest–southeast-trending lake flanked by alluvial fans. This paleolake extended from the Furnace Creek to Ubehebe. Based on the new stratigraphy, the Death Valley fault system can be divided into four main fault zones: the dextral, Quaternary-age Northern Death Valley fault zone; the dextral, pre-Quaternary Furnace Creek fault zone; the oblique–normal Black Mountains fault zone; and the dextral Southern Death Valley fault zone. Post − 3.3 Ma geometric, structural, and kinematic changes in the Black Mountains and Towne Pass fault zones led to the break up of Furnace Creek basin and uplift of the Copper Canyon and Nova basins. Internal kinematics of northern Death Valley are interpreted as either rotation of blocks or normal slip along the northeast–southwest-trending Towne Pass and Tin Mountain fault zones within the Eastern California shear zone.  相似文献   

8.
The “upper pit” at the Lost Chicken placer gold mine in east central Alaska contains fossils that provide information on the flora and insect fauna of interior Alaska just before the onset of global cooling at 2.5 myr. Fossils come from sediments interbedded with the Lost Chicken tephra (dated at 2.9 ± 0.4 myr—early Late Pliocene) and portray the floodplain and valley of a small creek within a region dominated by a coniferous forest richer in genera and species than the present one. Climate was wetter and less continental, and there was probably little or no permafrost. At least one other Pliocene tephra (the Fortymile tephra) occurs at the site and is also associated with plant and insect fossils. Among these fossils are extinct plants and insects like those found at other Tertiary sites in northern Canada and Alaska. The Lost Chicken sequence is the same age as the Beaufort Formation on Meighen Island, more than 1000 km to the north. Like Lost Chicken, Meighen Island sediments contain fossils representing a diverse boreal environment. This shows that the latitudinal climate gradient during early Late Pliocene time was shallower than at present and the boreal forest had a far greater latitudinal span than now.  相似文献   

9.
Two widespread tephra deposits constrain the age of the Delta Glaciation in central Alaska. The Old Crow tephra (ca. 140,000 ± 10,000 yr), identified by electron microprobe and ion microprobe analyses of individual glass shards, overlies an outwash terrace coeval with the Delta glaciation. The Sheep Creek tephra (ca. 190,000 yr) is reworked in alluvium of Delta age. The upper and lower limiting tephra dates indicate that the Delta glaciation occurred during marine oxygen isotope stage 6. We hypothesize that glaciers in the Delta River Valley reached their maximum Pleistocene extent during this cold interval because of significant mid-Pleistocene tectonic uplift of the east-central Alaska Range.  相似文献   

10.
Alluvial and lacustrine sediments exposed beneath late Pleistocene glaciolacustrine silt and clay at two sites along the Old Crow River, northern Yukon Territory, are rich in fossils and contain tephra beds. Surprise Creek tephra (SZt) occurs in the lower part of the alluvial sequence at CRH47 and Little Timber tephra (LTt) is present near the base of the exposure at CRH94. Surprise Creek tephra has a glass fission-track age of 0.17 ± 0.07 Ma and Little Timber tephra is 1.37 ± 0.12 Ma. All sediments at CRH47 have a normal remanent magnetic polarity and those near LTt at CRH94 have a reversed polarity — in agreement with the geomagnetic time scale. Small mammal remains from sediments near LTt support an Early Pleistocene age but the chronology is not so clear at CRH47 because of the large error associated with the SZt age determination. Tephrochronological and paleomagnetic considerations point to an MIS 7 age for the interglacial beds just below SZt at CRH47 and at Chester Bluffs in east-central Alaska, but mammalian fossils recovered from sediments close to SZt suggest a late Irvingtonian age, therefore older than MIS 7. Further studies are needed to resolve this problem.  相似文献   

11.
Cyclostratigraphic analysis of the Pliocene Zújar section (Guadix-Baza Basin, southeastern Spain) has enabled the recognition of a number of climatically forced cycles reflecting alternating dry and wet periods. Peaks of aridity are recorded at ca. 3.95, 3.55, 3.2, 2.8, and 1.8 myr B.P. The first dry period at about 4.0 myr B.P. corresponds to the early Ruscinian Mammal age, while the second arid interval at about 3.6 myr B.P. corresponds to the establishment of the Mediterranean double seasonality. The significant mammal turnover between the late Ruscinian and early Villanyian stages is placed between chron 2An.2n and the very base of chron 2An.1n, coincident with the dry phase recognized at about 3.2 myr B.P. The fourth aridity maximum at 2.8 myr B.P. roughly coincides with the Equus event in western Europe and is probably related to the beginning of the glacial–interglacial dynamics in the Northern Hemisphere. Finally, the last dry peak at about 1.8 myr B.P. is probably related to the set of mammalian events characterizing the transition from the late Pliocene faunas to those of the early Pleistocene.  相似文献   

12.
Catastrophic floods from glacial Lake Missoula entered the Pasco Basin in south-central Washington and backflooded its marginal valleys. Badger Coulee, one such valley, contains beds of fine-grained slackwater sediment deposited by these floods. The slackwater sediment contains two ash layers of the Mount St. Helens set S tephra, about 13,000 yr old. The ash was deposited on a ground surface developed atop slackwater sediment deposited during preash flooding. Evidence of the former ground surface includes the reworked ash, inferred trace fossils, stream and debris-flow deposits, slopewash and/or eolian sediment, and colluvium at the ash horizon. These features and the ash were buried by slackwater sediment deposited during postash flooding. Nonflood, subaerial deposits are not present atop other beds. Instead, beds commonly are reversely graded across “contacts,” suggesting that multiple beds were continuously deposited. The exposed beds thus record at least two late-Wisconsin floods, one preash, the other postash. The pre- and postash floods may be correlative with earlier-reported floods thought to have occurred 17,500-14,000 and 14,000–13,000 yr B.P., respectively.  相似文献   

13.
Pumice layers of set S from Mount St. Helens can be correlated with certain ash beds associated with young flood deposits of the channeled scabland. The correlation points to an age of about 13,000 14C yr B.P. for the last major flood to have crossed the scabland. Until recently, the last major episode of flooding was thought to be closer to 20,000 yr B.P., an age inferred chiefly from the relation of the flood to glacial events of the northern Rocky Mountains. Several investigations within the last few years have suggested that the last major flood occurred well after 20,000 yr B.P. Tentative correlations of ash beds of the scabland with set S pumice layers, the relations of flood and glacial events along the northwestern margin of the Columbia Plateau, and a radiocarbon date from the Snake River drainage southeast of the plateau all indicate an age much younger than 20,000 yr. The postulated age of about 13,000 yr B.P. is further supported by a radiocarbon date in the Columbia River valley downstream from the scabland tract. Basal peat from a bog on the Portland delta of Bretz, which is a downvalley deposit of the last major scabland flood, has been dated as 13,080 ± 300 yr B.P. (W-3404).  相似文献   

14.
A Glacier Peak tephra has been found in the mid-Holocene sediment records of two subalpine lakes, Frozen Lake in the southern Coast Mountains and Mount Barr Cirque Lake in the North Cascade Mountains of British Columbia, Canada. The age–depth relationship for each lake suggests an age of 5000–5080 14C yr B.P. (5500–5900 cal yr B.P.) for the eruption which closely approximates the estimated age (5100–5500 14C yr B.P.) of the Dusty Creek tephra assemblage found near Glacier Peak. The tephra layer, which has not been reported previously from distal sites and was not readily visible in the sediments, was located using contiguous sampling, magnetic susceptibility measurements, wet sieving, and light microscopy. The composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid-Holocene tephra layer. Using the same methods, the A.D. 1481–1482 Mount St. Helens We tephra layer was identified in sediments from Dog Lake in southeastern British Columbia, suggesting the plume drifted further north than previously thought. This high-resolution method for identifying tephra layers in lake sediments, which has worldwide application in tephrachronologic/paleoenvironmental studies, has furthered our knowledge of the timing and airfall distribution of Holocene tephras from two important Cascade volcanoes.  相似文献   

15.
Stratigraphically important Quaternary rhyolitic tephra deposits that erupted from the Okataina and Taupo volcanic centers in New Zealand can be geochemically identified using the FeO and MgO contents of their biotite phenocrysts. The FeO/MgO ratio in biotite does not correlate with FeO/MgO in the coexisting glass phase so that tephra beds with similar glass compositions can be discriminated by their different biotite compositions. Some individual tephra deposits display sequential changes in biotite composition that allow separate phases of the eruption to be identified, greatly increasing the potential precision for correlation. In addition, devitrified lavas that are unsuitable for glass analysis can be correlated to coeval tephra deposits by their biotite compositions. Biotite is common in high-K2O (>4 wt%) tephra beds and is widely dispersed in ash plumes because of its platy form, thus making it important in correlation studies.  相似文献   

16.
A tephra record is presented for a sediment core from Llyn Llech Owain, south Wales, spanning the early- to mid-Holocene. Seven cryptotephra deposits are discovered with three thought to correlate with known eruptions and the remaining four considered to represent previously undocumented events. One deposit is suggested to correlate with the ~6.9 cal ka bp Lairg A tephra from Iceland, whereas more distant sources are proposed as the origin for two of the tephra deposits. A peak of colourless shards in early-Holocene sediments is thought to tentatively correlate with the ~9.6 cal ka bp Fondi di Baia tephra (Campi Flegrei) and a second cryptotephra is tentatively correlated with the ~3.6 cal ka bp Aniakchak (CFE) II tephra (Alaska). The Fondi di Baia tephra has never been recorded beyond proximal sites and its discovery in south Wales significantly extends the geographical distribution of ash from this eruption. The remaining four cryptotephra deposits are yet to be correlated with known eruptions, demonstrating that our current understanding of widespread tephra deposits is incomplete. This new tephra record highlights the potential for sites at more southerly and westerly locations in northwest Europe to act as repositories for ash from several volcanic regions.  相似文献   

17.
Throughout most of its geological evolution Etna has been characterized by the eruption of lava flows of a predominantly hawaiitic composition, but within the stratigraphical record there are four major sequences of pyroclastic materials: the Acireale tephra and lahars (˜100000 B.P.); the ‘lower tephra’ and Milo lahars (both ˜26000 B.P.); the Biancavilla ignimbrites (15–15500 B.P.) and the ‘upper tephra’ (˜5000–6000 B.P.). This paper reports investigations carried out on these deposits in order to determine their stratigraphy, petrology, sedimentology, and likely origins. Whereas the Biancavilla ignimbrites were generated when a more evolved, gas-charged magma (benmoreite) was being produced by the volcano, the other suites of pyroclastic deposits were erupted from hawaiitic magmas—similar to those that have characterized the volcano during historical times. These deposits resulted from two processes: violent strombolian activity producing lapilli-rich. coarse, but well-sorted sediments, and hydrovolcanism when the mixing of water and magma in the conduit, brought about more violently explosive activity, giving rise to highly fragmented, poorly sorted, airfall tephra and lahars. Conditions favouring hydrovolcanism occurred at times in the volcano's history when palaeoenvironment and palaeogeography were conducive to the retention of large amounts of surface and subsurface water. Although climates favouring the retention of water at high levels on the volcano have occurred on many occasions in the history of the volcano, at ˜26.000 and ˜5000-6000 B.P. these occurred in conjunction with a construct of sufficient height and suitable configuration to allow storage of water and give rise to hydrovolcanic activity. The nature of the mechanisms responsible for the emplacement of these hydrovolcanic deposits is considered and it is concluded that airfall is the most probable process. Finally, the implications of this research for the assessment of hazard are reviewed.  相似文献   

18.
The Mt. Edgecumbe Volcanic Field (MEVF), located on Kruzof Island near Sitka Sound in southeast Alaska, experienced a large multiple-stage eruption during the last glacial maximum (LGM)-Holocene transition that generated a regionally extensive series of compositionally similar rhyolite tephra horizons and a single well-dated dacite (MEd) tephra. Marine sediment cores collected from adjacent basins to the MEVF contain both tephra-fall and pyroclastic flow deposits that consist primarily of rhyolitic tephra and a minor dacitic tephra unit. The recovered dacite tephra correlates with the MEd tephra, whereas many of the rhyolitic tephras correlate with published MEVF rhyolites. Correlations were based on age constraints and major oxide compositions of glass shards. In addition to LGM-Holocene macroscopic tephra units, four marine cryptotephras were also identified. Three of these units appear to be derived from mid-Holocene MEVF activity, while the youngest cryptotephra corresponds well with the White River Ash eruption at ∼ 1147 cal yr BP. Furthermore, the sedimentology of the Sitka Sound marine core EW0408-40JC and high-resolution SWATH bathymetry both suggest that extensive pyroclastic flow deposits associated with the activity that generated the MEd tephra underlie Sitka Sound, and that any future MEVF activity may pose significant risk to local population centers.  相似文献   

19.
A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km3, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km3 (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km3). Total tephra volume in the past 90,000 years is estimated at about 18.1 km3 (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km3, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km3/ky, which is similar to the rates of other Quaternary volcanoes in Japan.  相似文献   

20.
Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct 14C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic history of Antarctica.  相似文献   

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