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Sedimentary Geology, 17 (1977), 311-332 SEDIMENTOLOGY OF EARLY-ALPINE, FLUVIO-MARINE, CLASTIC DEPOSITS (VERRUCANO,
TRIASSIC) IN M. TONGIORGI1, A. RAU2 and I.P. MARTINI3 1Istituto di Geologia dell'Universita di Pisa (Italy)
The sedimentological investigation of the Middle and lower Upper Triassic
clastic rocks of the Mt. Pisani, Tuscany, Italy (type locality of the Verrucano)
indicates three main depositional sequences that constitute a fluvio-deltaic
shelf complex.The lower fluvial sequence is characterized by an upward transition from conglomeratic deposits of braided streams to sandy, shaly deposits of low- and high-sinuosity meandering are of the point-bar type, others are due to deposition predominantly in streams. In the top part of the fluvial sequence, within the cyclic deposits of these meandering rivers, several types of channel fills are to be found. Some transverse bars of crossover reaches; others are characterized by a highly variable texture and trough cross-bedding that are typical of the braided reaches of streams. The fine overbank interlayers of these meandering braided streams are usually capped by caliche profiles. The intermediate, fossiliferous, shelf sequence was formed in a wide, shallow shelf, locally and temporarily barred by sand ridges, and swept by seasonal storms. Several sedimentological sequences developed, such as wavy and lenticular beds, structureless storm layers, thin fining-upward 'tidalites' and shallow-water turbidites. The deposits of the bars show distinct traces of inclined accretionary surfaces and thin planar cross-beddings that indicate up slope migration of secondary longitudinal sand ridges. The sedimentary cycle is closed by a prograding deltaic sequence that grades from delta-front sand to silt and clay of deltaic plain-bays. Extensive and regular sandstone beds, coarsening-upward bar sequences and shallow, wide channel fills all characterize submerged parts of the delta. Thick, coarse-grained deposits of distributary channels, thin silt-shale interlaminations of flood plains and bays, well-developed thickening-upward sequences of crevasse-splay deposits, mud cracks and tracks of tetrapods are the most typical features of the deltaic plain. This Triassic sedimentary system developed during early rifting stages of the Alpine geosyncline. The active tectonic regime and related morphological rejuvenation of the lands are recorded in repeated interfingering of facies and in the reappearance of fresh Hercynian materials at different stratigraphic levels. Primary red beds, caliche profiles, salt ponds, the lack of vegetational traces and the ephemeral character of the streams substantiate semi-arid paleoclimatic conditions during Triassic time in southern Europe. ¡¡ |
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