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The Jurassic succession of Rocca Busambra consists of two lithostratigraphic units: a pile of peritidal limestones several hundreds of metres thick (Inici Formation: Hettangian to Sinemurian) and a 2 to 15 m thick sequence of Rosso Ammonitico‐type pelagic limestones (Toarcian? to lowermost Berriasian). An extensive interval of non‐deposition is evidenced by a thick Fe–Mn oxide crust on the bounding disconformity and is recorded partially in the material contained within a complex network of neptunian dykes and sills. Seven lithofacies are distinguished in the Rosso Ammonitico. These lithofacies show that the Rosso Ammonitico limestones differ from most analogues both in Sicily and elsewhere: sediments are mostly grain‐supported and non‐nodular; obviously bottom currents were important during deposition of these sediments. These currents were pulsating at different frequencies and induced winnowing, intraclast production and early cement precipitation. Other Rosso Ammonitico lithosomes of Late Jurassic to earliest Cretaceous age, usually decimetre thick and discontinuous, overlie the Inici Formation without any Fe–Mn crust; their anomalous stratigraphical and geometrical relationships show that they were deposited on an inclined, stepped, erosional surface incised in the sub‐horizontal Inici Formation. This ancient escarpment is interpreted as the result of a mainly gravitational collapse of the margin of a pelagic plateau. Such mass wasting was probably due to the backstepping of the tectonic plateau–basin margin that is not observable directly, but may be inferred from circumstantial evidence. This observation clearly shows that tectonic activity affected the Rocca Busambra sector of the West Tethys continental margins a few tens of millions of years after the end of the rifting stage. The anomalous Rosso Ammonitico sediments are the only indication of the escarpment and their occurrence in the stratigraphic record is probably more widespread than reported in the literature. More accurate palaeoenvironmental and palaeogeographic reconstructions may depend on the identification of these sediments. 相似文献
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LOUISE HANSEN ACHIM BEYLICH VALENTIN BURKI† RAYMOND S. EILERTSEN‡ OLA FREDIN EILIV LARSEN ASTRID LYSÅ ATLE NESJE† KNUT STALSBERG JAN F. TØNNESEN Associate Editor – Nick Eyles 《Sedimentology》2009,56(6):1751-1773
The sediment fill of a silled bedrock valley in Western Norway has been investigated with respect to stratigraphy and infill history using a combination of mapping, georadar, seismic profiling and drilling. A small outlet glacier occupies the head of the valley that displays a stepwise down-valley profile and terminates in a lake at 29 m above sea-level. The valley is surrounded by high, steep bedrock slopes and is characterized by a series of filled basins each limited by sills of bedrock or moraine accumulations. Till, glacial outwash and/or rockslide deposits fill in the lower half of the two larger basins. (Fan) delta deposits fringed by the deposits of alluvial fans and colluvial cones dominate the upper fill of most basins. (Fan) delta deposits interfinger downstream with lake sediments in the larger basins and fluvial deposits comprise the top fill. The overall infill pattern was controlled by deglaciation as well as basin size and shape. An overall decreasing sediment supply following deglaciation is shown in the fill of a larger basin down-valley, whereas a recently increasing sediment supply during glacier growth is reflected primarily in an upstream basin. Only the lowermost basin was exposed to a sea-level drop from 75 m above sea-level to the present lake level associated with incision and river migration. This observation is in contrast to the basins above marine influence where incision has been limited due to fixed downstream sills resulting in insignificant erosion except for some fan-head entrenchment. It follows that the fills of these small valley basins display progradational and aggradational trends of deposition and paraglacial reworking has been limited. Additionally, the study demonstrates that georadar profiling, combined with other methods, is very useful for comprehensive investigation of valley basins. 相似文献
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Tim Sumner of Imperial College summarizes the community report on fundamental physics in Britain prepared for PPARC.
Click HERE to view the article. 相似文献
Click HERE to view the article. 相似文献
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