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151.
152.
Revised age of deglaciation of Lake Emma based on new radiocarbon and macrofossil analyses 总被引:1,自引:0,他引:1
Previous radiocarbon ages of detrital moss fragments in basal organic sediments of Lake Emma indicated that extensive deglaciation of the San Juan Mountains occurred prior to 14,900 yr B.P. (Carrara et al., 1984). Paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages. Subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early Holocene age, rather than a late Pleistocene age, for the basal sediments of Lake Emma. These new radiocarbon ages suggest that by at least 10,000 yr B.P. deglaciation of the San Juan Mountains was complete. The insect and plant macrofossils from the basal organic sediments indicate a higher-than-present treeline during the early Holocene. The insect assemblages consisted of about 30% bark beetles, which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the Lake Emma cirque, in which bark beetles comprise only about 3% of the assemblages. In addition, in the fossil assemblages there were a number of flightless insect species (not subject to upslope transport by wind) indicative of coniferous forest environments. These insects were likewise absent in the modern assemblage. 相似文献
153.
Late Pleistocene and Holocene Seasonal Temperatures Reconstructed from Fossil Beetle Assemblages in the Rocky Mountains 总被引:1,自引:0,他引:1
Scott A. Elias 《Quaternary Research》1996,46(3):311-318
Mutual Climatic Range (MCR) analysis was applied to 20 fossil beetle assemblages from 11 sites dating from 14,500 to 400 yr B.P. The fossil sites represent a transect of the Rocky Mountain region from northern Montana to central Colorado. The analyses yielded estimates of mean July and mean January temperatures. The oldest assemblage (14,500 yr B.P.) yielded mean July values of 10–11°C colder than present and mean January values 26–30°C colder than present. Postglacial summer warming was rapid, as indicated by an assemblage dating 13,200 yr B.P., with mean July values only 3–4°C cooler than modern. By 10,000 yr B.P., several assemblages indicate warmer-than-modern mean summer and winter values. By 9000 yr B.P., MCR reconstructions indicate that both summer and winter temperatures were already declining from an early Holocene peak. Mean July values remained above modern levels and mean January values remained below modern levels until 3000 yr B.P. A series of small-scale oscillations followed. 相似文献
154.
E. Wasmund Bubnoff Wilckens Jan Lewiński Leuchs Karl Krejci-Graf Archangelski Kalicki Orlow 《International Journal of Earth Sciences》1935,26(4):286-299
Ohne Zusammenfassung 相似文献
155.
Arnoud Slootman Poppe L. De Boer Matthieu J. B. Cartigny Elias Samankassou Andrea Moscariello 《Sedimentology》2019,66(4):1302-1340
Cool‐water carbonate sedimentation has dominated Mediterranean shelves since the Early Pliocene. Skeletal sand and gravel herein consist of remains of heterozoan organisms, which are susceptible to reworking due to weak early cementation in non‐tropical waters. This study documents the Lower Pleistocene carbonate wedge of Favignana Island (Italy), which prograded from a 5 km wide passage between two palaeo‐islands into a perpendicular, 10 to 15 km wide strait between the palaeo‐islands at one side and Sicily at the other during the Emilian highstand (1·6 Ma to 1·1 Ma). The clinoformed carbonate wedge, which is 50 m thick and 6 km long, formed by east/south‐east progradation of a platform on the submarine sill by currents that were funnelled between the two palaeo‐islands. Platform‐slope clinoforms evolved from initial aggradation (thin and low‐angle) into a progradation phase (thick and high‐angle). Both clinoform types are characterized by a bimodal facies stacking pattern defined by sedimentary structures created by: (i) subaqueous dunes associated with dilute subcritical currents; and (ii) upper‐flow‐regime bedforms associated with sediment‐laden supercritical turbidity currents. Focusing of episodic currents on the platform by funnelling between the islands controlled the downstream formation of a sediment body, here named carbonate delta. The carbonate delta interfingers with subaqueous dune deposits formed in the perpendicular strait. This study uses a reconstruction of bedform dynamics to unravel the evolution of this gateway‐related carbonate accumulation. 相似文献
156.
Elias SALAMEH Hani KHOURY W. Uwe REIMOLD Werner SCHNEIDER 《Meteoritics & planetary science》2008,43(10):1681-1690
Abstract— Triggered by re‐evaluation of a 1960s report on the regional geology of the northeastern border region of Jordan and following Landsat satellite image investigation, a 5.5 km diameter, complex, circular structure was discovered in the central eastern region of the Kingdom of Jordan. Initial ground truthing revealed complex geological structures involving Upper Cretaceous and Paleogene strata, and including a prominent outer rim rising up to 60 m above the surrounding plain, an intermediate ring of up to 20 m elevation within a ring syncline, and a central zone of stratigraphically uplifted sedimentary strata characterized by intense macroscopic (folding and faulting, widespread cataclasis) and locally mesoscopic (cataclasis) deformation. Ten sites with shatter cone development in fine‐grained sandstone or limestone have been mapped to date, mostly in the outer parts of the central uplifted area. This finding confirms that the Jebel Waqf as Suwwan structure was formed as the result of the impact of an extraterrestrial projectile. Search for impact‐diagnostic micro‐deformation has been rather unsuccessful: only 1 quartz grain with both planar deformation features and planar fractures has been detected in a sandstone sample to date. The overall majority of the approximately 70 samples investigated by micropetrographic analysis consist of extremely fine‐grained chert, siltstone, or marly limestone. Cataclasis is widespread in chert and limestone, also on the micro‐scale. Considering the severely limited amount of characteristic impact microdeformation, and the stratigraphic situation within the central uplift, it is likely that a relatively deep level of the central uplift is currently exposed. The extensive drainage demonstrated for this region supports the conclusion that this impact structure could be quite deeply eroded‐especially as its geology involves some relatively soft lithologies (marls, limestones). The age of this impact event is at present poorly constrained at post‐Middle to Lower Eocene. 相似文献
157.
158.
159.
J.L. Elliot J.H. Elias R.G. French Jay A. Frogel W. Liller K. Matthews K.J. Meech D.J. Mink P.D. Nicholson B. Sicardy 《Icarus》1983,56(2):202-208
Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15–16 August 1980 are compared. The α ring shows a “double-dip” structure; the η ring shows a broad and narrow component (similar to Saturn's F ring); and the ε ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles. 相似文献
160.
The shrinkage of the Lisan Lake (LL) to form the recent Dead Sea (DS) was mainly a result of the reduction of the catchment
area from around 157,000 km2 during Late Pleistocene to 43,000 km2 presently. The reduction in the catchment area resulted from the eruption and spread of the basalt flows of Jabal Arab-Druz
(JAD), which together with the resulting deposition of thick rock debris and gravels occupied the drainage system. The filling
of the pre-basalt drainage system, which used to feed the Dead Sea, with basalts and alluvial sediments blocked the inflows
from reaching the Dead Sea. Local base levels along the basalt flow boarders such as Azraq Oasis, Sirhan Basin and Damascus
Oasis, and numerous pools and mud flats were created. 相似文献