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The Montehermosan Land Mammal Age has been etablished by fission-track dating of the mammal deposit of Farola Monte Hermoso. The fossil bones were found in the lowest part of the outcrop, consisting of fluvially derived clay and silt. The erosional surface between the lowest portion and the overlying aeolian sands is charcterised by a compact unit of rounded volcanic glass shards; a second fresh volcanic ash level is about 1 m above. EDS analyses and fission-track age analyses were performed on the two levels. The age determined for the lower volcanic ash level, 1.7 ± 0.4 Ma, is a minimum age for the Chapadmalalan mammal assemblage and a maximum age for the Uquian mammal assemblage in the Bonaerense area. 相似文献
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FRANCESCO MASSARI 《Sedimentology》1997,44(5):939-958
Middle Pleistocene sedimentation in the Bradanic area (foreland basin of the southern Apennines) took place in a setting in which tectonic tilting created anomalous shelf gradients and where slope failures repeatedly occurred at the front of quickly prograding sedimentary wedges. A coarse-grained wave-dominated coastal/deltaic complex consisting of two downstepping, forced-regressive wedges prograded during the falling stage of a middle Pleistocene glacio-eustatic fluctuation (P. lacunosa Zone). Conglomerate clinoforms show a dramatic increase in relief and steepness as a result of the infill of collapse scars. Compaction of prodelta mudstones, due to the load of the scar-filling conglomerates, resulted in a slight deformation of the fills into broad synformal structures, with gentle anticlinal closures at their distal ends. Accommodation space was created, resulting in splitting of the prograding bodies into high-frequency cycles which are thought to reflect the interaction of localized compactional subsidence with autocyclic shifting of river mouths and/or climatically induced variations in sediment supply. Available chronologic constraints suggest average durations of small-scale cycles on time-scales shorter than those of Milankovitch orbital frequencies. This case history highlights the importance of some factors which are often neglected in sequence stratigraphic models, e.g. physiography of the sea floor and localized sediment loading. On one hand, the bulk of falling-stage wedges can be preserved as they prograde into intraplatform depressions represented by collapse scars; on the other, significant accommodation space is created by compactional subsidence, leading to the splitting of a simple progradational unit into multiple, small-scale cycles. 相似文献
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RICHARD G. WOODFINE HUGH C. JENKYNS MASSIMO SARTI† FRANCESCO BARONCINI† CRESCENZO VIOLANTE‡ 《Sedimentology》2008,55(4):1011-1028
Chemostratigraphic analyses (87Sr/86Sr, δ13Ccarb) of limestones from two Jurassic platform‐carbonate sequences in Italy (Trento and Campania–Lucania Platforms) illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well‐calibrated sections elsewhere in Europe. Chemostratigraphic correlations between the platform‐carbonate successions and appropriate intervals from well‐dated reference sections allow the application of high‐resolution stratigraphy to these shallow‐water peritidal carbonates and, furthermore, elucidate the facies response to the Early Toarcian Oceanic Anoxic Event (OAE). Lower Jurassic (Toarcian) levels of the western Trento Platform (Southern Alps, Northern Italy) contain spiculitic cherts that appear where rising carbon‐isotope values characterize the onset of the OAE: a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near‐surface waters. There is a facies change to more clay‐rich facies at the level of the abrupt negative carbon‐isotope excursion, also characteristic of the OAE, higher in the section. The Campania–Lucania Platform (Southern Apennines, Southern Italy) records a change to more clay‐rich facies where carbon‐isotope values begin to rise at the beginning of the OAE but the negative excursion, higher in the section, occurs within oolitic facies. Although, in both examples, the Early Toarcian OAE can be recognized by a change to more clay‐rich lithologies, this facies development is diachronous and in neither case did the platform drown. Although the Trento Platform, in the south‐west sector studied here, was adversely affected by the OAE, it did not drown definitively until Late Aalenian time; the Campania–Lucania Platform persisted throughout the Jurassic and Cretaceous. Differential subsidence rates, which can be calculated using comparative chemostratigraphy, are identified as a crucial factor in the divergent behaviour of these two carbonate platforms: relatively fast in the case of the Trento Platform; relatively slow in the case of the Campania–Lucania Platform. It is proposed that where water depths remained as shallow as a few metres during the OAE (Campania–Lucania Platform), dissolved oxygen levels remained high, nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable, whereas more poorly ventilated and/or more nutrient‐rich waters (Trento Platform) adversely influenced platform growth where depths were in the tens of metres range. The stage was thus set for drowning on the more rapidly subsiding western margin of the Trento Plateau and a pulse of oolite deposition post‐dating the OAE was insufficient to revitalize the carbonate factory. 相似文献
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PAOLO ROBERTO FEDERICI MARTA PAPPALARDO 《Geografiska Annaler: Series A, Physical Geography》2010,92(3):361-373
In the southernmost tract of the Alps (Italian‐French Maritime Alps), extensively covered by glaciers during the Last Glacial Maximum, about 30 small glaciers were present by the end of the Little Ice Age. The aim of this paper is to document the progressive decrease towards exhaustion of these glaciers, located at the latitude of 44° N, highlighting the factors affecting their retreat. All available data sources were investigated for this work including: the annual glaciers fluctuations record, comparative analyses of historical maps and multi‐temporal oblique photographs and direct surveys in the field. The history of the Maritime Alps glaciers fluctuations was thoroughly reconstructed. Stationary conditions were observed from 1896 up to the beginning of the 1930s; since then they underwent phases of withdrawal with variable intensity. In the early 1990s, only six glaciers were still present, the extent of which were all was dramatically reduced. In the past two decades, the Maritime Alps glacier fronts experienced a global retreat of about 100 m, with a sharp acceleration after 2002. Currently ice patches along cirque walls and/or semi‐buried lenses of ice are still present; morphological evidence of permafrost creeping in the glacier forefield accounts for the incipient transition to periglacial landforms (i.e. rock glaciers). The main factors controlling glaciers retreat seem to have been their original extent at the beginning of the current regressive phase and their distance from the main chain divide. From a climatic point of view unfavourable factors for glaciers persistence have been in the last decades a remarkable and sharp temperature increase, a decrease in winter snowfall and a shift of the rainfall peak from autumn to spring. 相似文献