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1.
《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
2.
《Quaternary Science Reviews》2007,26(17-18):2067-2089
The continental vertebrate fauna of Cúllar Baza-1 (Granada, Spain) occur immediately above the Matuyama/Brunhes polarity boundary, and therefore represent an initial record for the Middle Pleistocene. All polarity zones for the Late Pliocene through Middle Pleistocene are found in an 80 m section, which includes this fossil quarry. The existing collection of abundant remains of micro-mammals, macro-mammals and lithic artifacts (indicating human activity) can now be assigned an earliest Middle Pleistocene age of 0.75 Ma. Another section, 25 km across the Neogene Baza Basin, also has the Matuyama/Brunhes polarity boundary. However, in this case the Huéscar-1 fossil quarry and the Puerto Lobo sites are both below the polarity boundary, thus representing a record of the late Early Pleistocene at ∼0.9 Ma. Differences in micro-mammal species across these Matuyama/Brunhes boundaries are significant and justify an approximately coincident biostratigraphic boundary between the Biharian and Toringian stages. 相似文献
3.
The Cambrian Maotianshan Shale in Yunnan Province, China contains the well-preserved soft-body fossils of the Chengjiang Biota. The high quality preservation of the non-mineralizing biota (soft tissues and whole carcasses) shows regional and temporal differences, suggesting that paleogeography and local environmental conditions might have contributed to the taphonomy of these fossils. In this paper we present new results from petrographic, geochemical and detrital zircon analyses, and provide a new interpretation about the provenance of the Maotianshan Shale, as well as add to the understanding of the paleogeography of the South China Block during the Cambrian Stage 3. Results from petrographic analysis indicate that the provenance of the Maotianshan Shale is a recycled orogen overall, bordering the western and southwestern margin on the Yangtze Block. The most likely source of the terrigenous material is an exhumed area extending from the Kangdian paleoland to the southeast, paralleling the Song Ma fault zone. Minor regional differences in geochemical and petrographic proxies between the northwestern Jianshan/Ercai area and the southeastern Maotianshan/Xiaolantian area suggest influence of local sources. Sediments of the southeastern province are less mature and samples include minor elements commonly associated with mafic sources. Sediments from the northwestern province are more mature, largely lack mafic components and are enriched in Zr and Hf. The major population of the Maotianshan Shale detrital zircons group at ~ 800 Ma. This crystallization age matches well with the age of a widely spread felsic volcanic and intrusive event associated with the Neoproterozoic Kangdian rift, suggesting that these igneous rocks are most likely a major provenance for the Maotianshan sediments. The youngest zircon population yields consistent Concordia ages of ~ 520 Ma, representing a maximum age constraint on the timing of deposition of the Maotianshan Shale. The zircon crystals of the ~ 520 Ma populations are euhedral with magmatic zoning, indicative of short-distance transport. Volcanic activity along the Song Ma suture zone is a potential source for the ~ 520 Ma detrital zircon suite. 相似文献
4.
《Journal of Asian Earth Sciences》2002,20(3):255-266
Palynological and sedimentological studies of a series of slimes collected from a 284 m-long drill-well from the Kathmandu Basin reveal paleoclimatic records and environmental changes within the Kathmandu Valley during the last 2.5 myr. The slimes are composed of fluvio-deltaic and lacustrine sediments comprising sand beds of 66.3 m and mud beds of 218 m in length. Pollen analyses show Quercus and Cyclobalanopsis are predominant, with frequencies exceeding 70%. Pinus, Alnus and Gramineae are the next dominant taxa. Three fossil pollen zones were discriminated; each zone reflects major climatic change: Zone I, the oldest stage, indicates a cool and rather wet climate during 400 kyr from ca. 2.5 to 2.1 Ma; Zone II, the middle stage, reflects a warm and relatively dry climate without remarkable fluctuation; Zone III is characterized by seven cycles of warm-and-wet and cold-and-dry climate, which reflect the alternation of glacial and interglacial periods. The last cold maximum, 11 m deep, corresponds to the last glacial age around 20 kyr bp, judging from the 14C dating of the uppermost part of the lacustrine sediments.The Paleo-Kathmandu Lake is likely to have been initiated at around 2.1 Ma and to have been filled with black organic mud, the Kalimati Clay. The top of the Kalimati Clay is eroded and was overlain by fluvial sand after the last glacial age. The abrupt appearance of a 4 m-thick fossiliferous sand bed at the top of the middle member suggests a lowering of water level at around 1 Ma. 相似文献
5.
Irene Zembo 《Sedimentary Geology》2010,223(3-4):206-234
The sedimentary record of the Val d'Agri basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in the Southern Apennines. Changes in tectonic controls, sediment supply and climatic input have been identified. The interval from ～ 56 to ～ 43 ka was associated with asymmetric subsidence restricted to the north-eastern actively faulted margin of the basin and development of axial braided river and transverse alluvial fan systems. Short-lasting Mediterranean-type pedogenesis between ～ 43 and ～ 32 ka (MIS Stage 3) coexisted with progradation–aggradation of the southern alluvial fan deposits and southwards tilting of the basin floor. Aggradation ended with consumption of accommodation space after 32 ka. During a subsequent stage of decline of vegetation cover, possibly as a consequence of climatic cooling (probably MIS Stage 2), active progradation of alluvial fans occurred. Breakthrough of the basin threshold and entrenchment of the drainage network must therefore be attributed to a latest Pleistocene to Holocene age. The first stages of basin opening and fill, predating ～ 56 ka have only been inferred by stratigraphic considerations: the earliest lacustrine sedimentation should be middle Pleistocene or older in age. The following south-eastward basin widening allowed progradation of alluvial fan systems, which completely filled the lacustrine area (tentatively late middle Pleistocene). Pedogenesis in “Mediterranean-like” climate conditions caused the final development of a highly mature fersiallitic paleosol at the top of the fan surfaces, in areas of morpho-tectonic stability, plausibly during MIS Stage 5. The study results demonstrate the potential of applying a multidisciplinary approach in an intermontane continental settings marked by a relative rapid and constant tectonic subsidence and a high rate of sediment supply during the Pleistocene glacial–interglacial cycles. 相似文献
6.
《Gondwana Research》2014,25(2):764-774
The Marsili Seamount (MS) is an about 3200 m high volcanic complex measuring 70 × 30 km with the top at ~ 500 m b.s.l. MS is interpreted as the ridge of the 2 Ma old Marsili back-arc basin belonging to the Calabrian Arc–Ionian Sea subduction system (Southern Tyrrhenian Sea, Italy). Previous studies indicate that the MS activity developed between 1 and 0.1 Ma through effusions of lava flows. Here, new stratigraphic, textural, geochemical, and 14C geochronological data from a 95 cm long gravity core (COR02) recovered at 839 m bsl in the MS central sector are presented. COR02 contains mud and two tephras consisting of 98 to 100 area% of volcanic ash. The thickness of the upper tephra (TEPH01) is 15 cm, and that of the lower tephra (TEPH02) is 60 cm. The tephras have poor to moderate sorting, loose to partly welded levels, and erosive contacts, which imply a short distance source of the pyroclastics. 14C dating on fossils above and below TEPH01 gives an age of 3 ka BP. Calculations of the sedimentation rates from the mud sediments above and between the tephras suggest that a formation of TEPH02 at 5 ka BP MS ashes has a high-K calcalkaline affinity with 53 wt.% < SiO2 < 68 wt.%, and their composition overlaps that of the MS lava flows. The trace element pattern is consistent with fractional crystallization from a common, OIB-like basalt. The source area of ashes is the central sector of MS and not a subaerial volcano of the Campanian and/or Aeolian Quaternary volcanic districts. Submarine, explosive eruptions occurred at MS in historical times: this is the first evidence of explosive volcanic activity at a significant (500–800 m bsl) water depth in the Mediterranean Sea. MS is still active, the monitoring and an evaluation of the different types of hazards are highly recommended. 相似文献
7.
Establishing firm radiocarbon chronologies for Quaternary permafrost sequences remains a challenge because of the persistence of old carbon in younger deposits. To investigate carbon dynamics and establish ice wedge formation ages in Interior Alaska, we dated a late Pleistocene ice wedge, formerly assigned to Marine Isotope Stage (MIS) 3, and host sediments near Fairbanks, Alaska, with 24 radiocarbon analyses on wood, particulate organic carbon (POC), air-bubble CO2, and dissolved organic carbon (DOC). Our new CO2 and DOC ages are up to 11,170 yr younger than ice wedge POC ages, indicating that POC is detrital in origin. We conclude an ice wedge formation age between 28 and 22 cal ka BP during cold stadial conditions of MIS 2 and solar insolation minimum, possibly associated with Heinrich event 2 or the last glacial maximum. A DOC age for an ice lens in a thaw unconformity above the ice wedge returned a maximum age of 21,470 ± 200 cal yr BP. Our variable 14C data indicate recycling of older carbon in ancient permafrost terrain, resulting in radiocarbon ages significantly older than the period of ice-wedge activity. Release of ancient carbon with climatic warming will therefore affect the global 14C budget. 相似文献
8.
《Gondwana Research》2014,25(2):820-841
The Huoshishan–Niujuanzi ophiolitic mélange (HNO) is located near the central part of the Beishan Orogenic Belt in the southernmost Altaids. The HNO consists of ultramafic rocks, cumulate gabbros, gabbros, plagiogranites, diorites, diabases, basalts, andesites, rhyolitic volcaniclastic rocks and siliceous sedimentary rocks, many of which are in a schist matrix (Gongpoquan Group). Geochemical data of the mafic rocks indicate a calc-alkaline or a mixture of calc-alkaline and tholeiitic rocks with negative Nb, Ta and positive Pb, Ba and La anomalies, suggesting formation in an island arc or supra-subduction zone setting. A gabbro from a block in the mélange in the Niujuanzi area has a zircon age of 435.0 ± 1.9 Ma and a plagiogranite with an age of 444.3 ± 1.9 Ma, and another gabbro from the Huoshishan area has an age of 410.5 ± 3.7 Ma. The schist matrix has a zircon age of 512 ± 5.3 Ma and contains Silurian, Devonian and Carboniferous fossils, thus the mélange formed in the late Carboniferous or later. Our structural analysis of fault planes in the HNO, the crenulation cleavages (S2) of the schist, and fold axial planes of early Permian sandy limestone/quartz veins and late Permian sandstones indicates that the mélange underwent a north-to-south compression, and the orientation of stretching lineations, slickensides and fold hinge lines implies that the HNO experienced top-to-the north (or -northwest) movement. The entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late Paleozoic. The emplacement age of the HNO was probably near the end-Permian based on the age of the youngest rocks in the ophiolitic mélange, and by the presence of a late Permian unconformity. From our work, integrated with published regional data, we outline a comprehensive geodynamic model for the central BOC. 相似文献
9.
《Journal of South American Earth Sciences》2007,23(1):61-80
Fossil content (vertebrate paleofauna and palynology) indicates that the sediments of the Solimões Formation in Acre (SW Brazilian Amazonia) are continental, having been deposited by avulsive fluvial belts in a floodbasin–floodplain environment. The main source area was the Andes chain. Widespread lacustrine swampy deposits, stacked channel deposits, and paleosoils are typical elements that characterize the Solimões Formation sediments that outcrop in southwestern Brazilian Amazonia. New data on fossil vertebrate assemblages and palynology corroborate the Late Miocene age suggested previously and assign the fossils to the Huayquerian mammalian biozone, spanning 9–6.5 Ma. These geological and paleontological data show that the existence of an intracontinental seaway through SW Amazonia during the Late Miocene (11–10 Ma), connecting the Caribbean Sea with the Parana Basin as previously proposed is unsustainable, because the sediments used by previous authors to propose the seaway were deposited in a continental environment and are younger than 11–10 Ma. 相似文献
10.
The Western foreland basin in Taiwan originated through the oblique collision between the Luzon volcanic arc and the Asian passive margin. Crustal flexure adjacent to the growing orogenic load created a subsiding foreland basin. The sedimentary record reveals progressively changing sedimentary environments influenced by the orogen approaching from the East. Based on sedimentary facies distribution at five key stratigraphic horizons, paleogeographic maps were constructed. The maps highlight the complicated basin-wide dynamics of sediment dispersal within an evolving foreland basin.The basin physiography changed very little from the middle Miocene (∼12.5 Ma) to the late Pliocene (∼3 Ma). The transition from a passive margin to foreland basin setting in the late Pliocene (∼3 Ma), during deposition of the mud-dominated Chinshui Shale, is dominantly marked by a deepening and widening of the main depositional basin. These finer grained Taiwan derived sediments clearly indicate increased subsidence, though water depths remain relatively shallow, and sedimentation associated with the approach of the growing orogen to the East.In the late Pleistocene as the shallow marine wedge ahead of the growing orogen propagated southward, the proximal parts of the basin evolved into a wedge-top setting introducing deformation and sedimentation in the distal basin. Despite high Pleistocene to modern erosion/sedimentation rates, shallow marine facies persist, as the basin remains open to the South and longitudinal transport is sufficient to prevent it from becoming overfilled or even fully terrestrial.Our paleoenvironmental and paleogeographical reconstructions constrain southward propagation rates in the range of 5–20 km/Myr from 2 Ma to 0.5 Ma, and 106–120 km/Myr between late Pleistocene and present (0.5–0 Ma). The initial rates are not synchronous with the migration of the sediment depocenters highlighting the complexity of sediment distribution and accumulation in evolving foreland basins. 相似文献
11.
Recently, a new organic geochemical paleothermometer based on the relative abundance of long chain alkyl 1,13- and 1,15-diols, the so-called long chain diol index (LDI), was proposed. Because of its novelty, the proxy has not been reported for sediments older than 43 ka. We therefore determined the LDI for 14 sediment samples from the early Pleistocene between 2.49 and 2.41 Ma, comprising Marine Isotope Stage (MIS) 98 to 95, and converted the values to sea surface temperature (SST) estimates to test whether the LDI could be applied or not to the early Quaternary. We show that the long chain diols can be preserved in marine sediments from the early Pleistocene, although at our study site this is limited to periods of increased biomarker accumulation (glacials). Although the results are based on a limited time interval and number of samples, the similarity between LDI-based SST and alkenone-based SST from the same samples suggests that the LDI proxy may have potential for studies covering the entire Quaternary. 相似文献
12.
M.C. Meyer R.A. Cliff C. Spötl M. Knipping A. Mangini 《Quaternary Science Reviews》2009,28(15-16):1374-1391
Ancient cave systems in the Northern Calcareous Alps, today located well above the timberline at altitudes of 2400–2500 m, host U-rich speleothems that preserved growth layers on the microscopic scale of presumably annual origin. Two flowstone samples were dated to 2.019 + 0.037/?0.069 Ma and 1.730 + 0.032/?0.068 Ma, respectively, using U–Pb isochron techniques. These ages are corroborated by the Late Pliocene to Early Pleistocene pollen spectrum extracted from one of the samples. We use a multiproxy approach and exploit laminated speleothem sequences to tie high-resolution stable isotope data to a floating lamina-counted chronology. O isotope values of growth intervals when calcite deposition was close to isotopic equilibrium are low compared to modern and Holocene speleothems from other alpine caves and are inconsistent with the current altitudinal setting of the caves. A vegetated but geomorphologically stable alpine catchment (i.e. ～2000 m asl., no (peri)glacial processes) combined with a deep-seated cave (the thickness of the vadose zone might have exceeded 1000 m) is required in order to reconcile the isotopic data with the pollen record and the petrographic evidence. Furthermore, the data can be used to constrain the rate for Quaternary rock-uplift to ≤0.8 mm/annum for this frontal part of the European Alps. Collectively, the data suggest that these speleothems formed both during interglacials (MIS 59 or 61) and interglacial–glacial transitions (MIS 75/74 or 77/76), but the seasonal precipitation pattern was arguably markedly different from today's. Provided that the highly regular microscopic laminae are indeed annual, lamina counts suggest a minimum length of ca 6 ka for interglacials during the earliest Pleistocene. 相似文献
13.
《Precambrian Research》2006,144(3-4):199-212
LA-ICP-MS U–Pb zircon dating and cathodoluminescene (CL) image analysis were carried out to determine the protolith and metamorphic ages of high-grade Al-rich gneisses, named as “khondalites”, from the Jining Complex of the North China Craton (NCC). The analytical results of more than 200 detrital zircon grains from the khondalites show three main age populations: 2060 Ma, 1940 Ma and 1890 Ma. These data indicate that the provenance of the Jining khondalites is Paleoproterozoic in age, but not Archean as previously suggested, and the sediments were derived from a provenance different from the Eastern Block and the Yinshan Terrane of the NCC. The nearly concordant youngest age of 1842 ± 16 Ma (207Pb/206Pb age) for the detrital zircons is interpreted as the maximum depositional age of the khondalites. Overgrowth rims of detrital zircons yield an age of 1811 ± 23 Ma, which we interpret as the metamorphic age. The new age data are consistent with the recent three-fold tectonic subdivision of the NCC and support that the Eastern and Western Blocks collided at ∼1.8 Ga to form the coherent NCC. 相似文献
14.
Denis Lacelle Melanie St-Jean Bernard Lauriol Ian D. Clark Antoni Lewkowicz Duane G. Froese Stephen C. Kuehn Grant Zazula 《Quaternary Science Reviews》2009,28(27-28):3401-3413
This study describes the origin and age of a body of massive ground ice exposed in the headwall of a thaw slump in the Red Creek valley, central Yukon, Canada. The site is located beyond the limits of Pleistocene glaciation in central Yukon and within the southern limit of the modern continuous permafrost zone. The origin of the massive ground ice, which is preserved under a fine-grained diamicton containing thin layers of tephra, was determined through ice petrography, stable O-H isotope composition of the ice, and gas composition of occluded air entrapped in the ice. The age of the massive ground ice was established by identifying the overlying tephra and radiocarbon dating of a “muck” deposit preserved within the ice. Collectively, the results indicate that the massive ground ice formed by snow densification with limited melting-refreezing and is interpreted as being a buried perennial snowbank. The muck deposit within the ice, which yielded an age of 30,720 ± 340 14C a BP, and the Dawson tephra (25,300 14C a BP) overlying the perennial snowbank, indicates that the snowbank accumulated at roughly the transition between marine isotope stages 3 and 2. Dry climatic conditions at this time and possibly high winds enabled the snowbank to accumulate in the absence of extensive local valley glaciation as occurred in the mountains to the south. In addition to documenting the persistence of relict permafrost and ground ice to warming climate in regions where they are predicted to disappear by numerical models, this study presents evidence of an isotopic biosignature preserved in a body of massive ground ice. 相似文献
15.
《Journal of Asian Earth Sciences》2002,20(2):157-175
The biostratigraphically constrained Permo-Triassic Emeishan Volcanic Province (EVP), extending over wide areas in southwest China, has been recently considered as a Large Igneous Province contemporaneous with the Siberian Traps and the siliceous tuffs at the P–T boundary in South China. We report the first 40Ar/39Ar ages on this igneous province. Minimum ages have been obtained on phenocrystic plagioclase of the Emeishan basalt, which has undergone a pervasive metamorphism, most likely during subsequent tectonization as a consequence of terrane amalgamation. Comparison between the Ar–release spectra obtained on clear vs. cloudy plagioclase indicates a 40–30 Ma sericite resetting time. A minimum apparent age of 246 ± 4 Ma for plagioclase from a plagiogranite, a late-differentiate of the Panzhihua Layered Complex, and an age of 254 ± 5 Ma for phlogopite from a pyroxenite near Lake Erhai, provide the first absolute age constraint on this igneous province. Additional Ar–Ar age measurements on post-Emeishan alkaline and mafic magmatism yielded 104 ± 2 and 100 ± 2 Ma for an alkaline complex near Panzhihua, and 42 ± 1 Ma for a gabbro sill emplaced near the Ertan Dam. Further study is still needed to determine the age of the Emeishan volcanic emission accurately, and to test the validity of the assumed short duration of the eruption. 相似文献
16.
S.J. Preece N.J.G. Pearce J.A. Westgate D.G. Froese B.J.L. Jensen W.T. Perkins 《Quaternary Science Reviews》2011,30(17-18):2069-2090
Old Crow tephra is the largest and most widespread Quaternary eruption presently known in eastern Beringia. Its major- and trace-element geochemistry, Fe-Ti oxides, and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption. The proximal region may well have experienced tephra fallout from small eruptions just prior to or after the Old Crow event, but there is no evidence to indicate that the distal area was affected. We recalculate the glass fission-track age at 124 ± 10 ka, which, coupled with stratigraphic and paleoenvironmental reconstructions, indicates that deposition occurred prior to development of the last interglacial boreal forest, which suggests a latest Marine Isotope Stage (MIS) 6 age. The bulk tephra volume erupted is estimated by three different approaches, that are in broad agreement at ～200 km3, but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area. The source caldera, although presently unrecognized, is located in the eastern Aleutian arc, possibly at or near the Emmons Lake volcanic center. 相似文献
17.
Ronald L. Bruhn Francis H. Brown Patrick N. Gathogo Bereket Haileab 《Journal of African Earth Sciences》2011,59(2-3):295-312
The distribution of hominin fossil sites in the Turkana Basin, Kenya is intimately linked to the history of the Omo River, which affected the paleogeography and ecology of the basin since the dawn of the Pliocene. We report new geological data concerning the outlet channel of the Omo River between earliest Pliocene and final closure of the Turkana Basin drainage system in the latest Pliocene to earliest Quaternary. Throughout most of the Pliocene the Omo River entered the Turkana Basin from its source in the highlands of Ethiopia and exited the eastern margin of the basin to discharge into the Lamu embayment along the coast of the Indian Ocean. During the earliest Pliocene the river’s outlet was located in the northern part of the basin, where a remnant outlet channel is preserved in basalts that pre-date eruption of the Gombe flood basalt between 4.05 and 3.95 Ma. The outlet channel was faulted down to the west prior to 4.05 Ma, forming a natural dam behind which Lake Lonyumun developed. Lake Lonyumun was drained between 3.95 and 3.9 Ma when a new outlet channel formed north of Loiyangalani in the southeastern margin of the Turkana Basin. That outlet was blocked by Lenderit Basalt lava flows between 2.2 and 2.0 Ma. Faulting that initiated either during or shortly after eruption of the Lenderit Basalt closed the depression that is occupied by modern Lake Turkana to sediment and water.Several large shield volcanoes formed east of the Turkana Basin beginning by 2.5–3.0 Ma, volcanism overlapping in time, but probably migrating eastward from Mount Kulal on the eastern edge of the basin to Mount Marsabit located at the eastern edge of the Chalbi Desert. The mass of the volcanic rocks loaded and depressed the lithosphere, enhancing subsidence in a shallow southeast trending depression that overlay the Cretaceous and Paleogene (?) Anza Rift. Subsidence in this flexural depression guided the course of the Omo River towards the Indian Ocean, and also localized accumulations of lava along the margins of the shield volcanoes. Lava flows at Mount Marsabit extended across the Omo River Valley after 1.8–2.0 Ma based on estimated ages of fossils in lacustrine and terrestrial deposits, and possibly by as early as 2.5 ± 0.3 Ma based on dating of a lava flow. During the enhanced precipitation in latest Pleistocene and earliest Holocene (11–9.5 ka) this flexural depression became the site of Lake Chalbi, which was separated from Lake Turkana by a tectonically controlled drainage divide. 相似文献
18.
《Precambrian Research》2006,144(1-2):69-91
By using unusual combinations of demagnetization techniques, Proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post-tectonic Proterozoic units near Armstrong, northern Ontario, and also for well-dated Gunflint Formation, which by previous techniques yielded problematical paleomagnetic data. The first paleomagnetic data are provided also for the Seagull Pluton and Inspiration Sills. Characteristic remanent magnetizations (ChRM) for the Pillar Lake Lavas indicate a Keweenawan age, more specifically ∼1000–1040 Ma by comparison with the well-established APWP for the Late Proterozoic Superior craton. Four combinations of demagnetization techniques yield declinations in the range 108–133° and inclinations in the range −65 to −70° (n = 100), which define paleopoles near 200 W/48 N corresponding to a location on the Keweenawan APWP near ∼1040 Ma. In the underlying basement a recently discovered Proterozoic igneous complex, the Waweig Troctolitic Complex, yields new paleomagnetic data with declination and inclination 42/−54 (n = 14) defining a paleopole at 238 W/09 N. Its ages may be 1400–1600 or ∼2000 Ma by comparison with the presently available, ambiguous and sparsely populated APWP. The first paleomagnetic results for the Seagull Pluton (U–Pb age 1113 Ma) yield a mean declination of 87.4/−75.7 (n = 32) corresponding to a Keweenawan paleopole near 233/42 N, consistent with other paleopoles near ∼1200 Ma. Tuffs of the oft studied but problematical Gunflint Formation (U–Pb age1878 Ma) yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens. Their mean primary declination and inclination ∼303/+48.8 (n = 17) yields a paleopole now located near 178 W/42 N, comparable with the published locations of paleopoles of ∼2000 Ma. Of broader interest, we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors. Combining the conventional techniques (thermal and AF demagnetization) also improved the resolution of characteristic vectors not achieved by other means. Low grade metamorphism affected the non-tectonized Proterozoic cover to the Canadian shield, due to burial or hydrothermal effects, obfuscating or erasing primary vectors in some lithologies and especially at certain sites. 相似文献
19.
A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8 ± 0.46 Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D = 348.0°, I = 47.3°, k = 51.0, α95 = 5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A95 = 5.7° and yielding a paleolatitude of 29.3° ± 5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~ 100 Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D = 312.6°, I = 47.7°, k = 109.7, α95 = 3.0°, N = 22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A95 = 3.2° (yielding a paleolatitude of 28.7° ± 3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~ 100 Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~ 550 km N–S convergence between the Qiangtang and Lhasa terranes happened after ~ 100 Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~ 100 Ma, which is most likely caused by the India–Asia collision. 相似文献
20.
《Gondwana Research》2013,24(4):1599-1606
Direct radiometric dating of the Lower/Middle Permian epochs has not been well accomplished. Shales and bedded cherts of the geologically well-documented Middle Permian Gufeng Formation are exposed in the Chaohu area, Anhui province, South China. Through detailed field examination and mapping of the Gufeng stratigraphic section, we found at least four volcanic ash beds within the basal shale strata. This new discovery indicates the existence of prominent volcanic activity during Gufeng sedimentation and provides the opportunity to precisely date the age of the Middle Permian. Zircon grains separated from two near-basal horizon yield LA‐ICP‐MS U–Pb ages of 272.0 ± 5.5 Ma (MSWD = 2.6) and 271.5 ± 3.3 Ma (MSWD = 1.7). As the first precise isotopic age (272 Ma) of the Middle Permian Gufeng Formation in South China, our data offer precise geochronological constraints for the division and correlation of Middle Permian not only in South China but also worldwide. 相似文献