Search

The 1:10,000 geological map of the Arera-Vedra Valley area covers an area of about 12 km2 in the Orobic Alps (Lombardy, N Italy), and is accompanied by five underground maps corresponding to different levels of historic mining works. The area is characterized by a Triassic stratigraphic succession that hosts one of the most important Zn-Pb mining districts in Italy (Gorno district). Original surface and underground geological mappings have been integrated with the analysis of drillhole stratigraphies and historic underground geological maps. Detailed geological cross sections correlating surface map data with those of the underground mining tunnels have been done. This work proposes a new map representation that is consistent with new interpretation of the geometry of the orebodies at depth, to serve as a tool for mining purposes (targets definition, mine design) and for a better understanding of the genesis of the Gorno ‘Alpine-type’ deposits.

This report deals with the Cretaceous sequence which overlies the "Maiolica" formation in che Bergamo area (Lombardian Basin, Northern ltaly). The sequence. mainly terrigenous, testifies a contemporaneous tectonic activity both at a locai as were regional scale. Between Aptian and Turonian at least, the facies discribucion suggcscs almost the same paleogeographic sctting as during the Jurassic. Some elevated areas bounded by normai rejuvenated faults are observed, which are M. Cavallo, M. Misma and the Zandobbio hills, oricntcd as "Giudicarie" trcnds. The first tcrrigenous turbiditcs are Aptian in age; but it is during the Cenomanian till to thc Turo­nian that the supply of clastic materiai develops and progrades from Centrai Bergamo arca toward the west. Strong mass movcments havc becn documcnted ali around the aforementioned elevated areas. The Aptian to Turonian turbidites are possibly linkcd to the paleoalpine compressive orogenic phases of the Eastern and Centrai Alps. The arenaceous turbidites Coniacian in age, are supposed to originate from rcscdimentation of "Gosau Molasse" sliding into the Lombardian Basin through the "Giudicarie" structural pattern. The clastic supply as well as subsidence were so strong that the originai paleogeographic setting was completely drowned. The calcarenitic turbidites as well as the silicoclastic ones which caractcrize the "Pietra di Credaro" and the "Bergamo Flysch" respectively, are thoughc to come from differenc areas, during thc time span elapsed between Santonian and Campanian.

The Lower Cretaceous Maiolica Formation represents the classic pelagic succession of the Apulian margin of the Tethys Ocean, in the Southern Alps. It consists of white calcilutites with minor marly layers and black shales. A multidisciplinary study including lithostratigraphy, nannofossil assemblage composition and organic geochemistry was performed on two composite sections in order to characterise the marl–shale layers that record production and storage of organic matter (OM) and terrigenous input in the Berriasian–Aptian interval. Such layers are associated with turbidite deposition and bottom current activity in the Late Berriasian–earliest Valanginian, Late Valanginian and Early Aptian. The uppermost Hauterivian–Barremian black shales alternate with pelagic calcilutites, forming an interval barren of redeposited layers. The quantity of organic carbon increases from the Berriasian to the Barremian; the background composition of the kerogen is continental-dominated, with varying proportions of marine-derived OM. Preservation of OM did not necessarily occur under anoxic conditions; on the contrary, storage of OM probably resulted from enhanced terrestrial input of nutrients, siliciclastics, woody and herbaceous fragments, inducing mesotrophic conditions and increased primary productivity. Higher fertility of surface waters during the Late Valanginian and Early Aptian is reflected by distinctive decreases in abundance of oligotrophic nannoconids and increase in abundance of mesotrophic Diazomatholithus in the Valanginian. During the Barremian, the rhythmic establishment of eutrophic conditions, documented by the highest amounts of marine OM, determined high-frequency fluctuations in nannoconid abundance. The Early Aptian nannoconid crisis is interpreted as the result of a major climate change and enhanced primary productivity. Variations of calcareous nannofloras, as well as changes of lithology and OM, which we describe in the Lower Cretaceous Maiolica Formation of the Lombardian Basin, can also be recognised in coeval units from other sites of the Apulian side of the Tethys. Similar episodes of siliciclastic and terrestrial OM accumulation and major changes in calcareous nannofossils are also recorded in the Upper Valanginian and Lower Aptian pelagic sediments of the Tethys, Atlantic and, partly, Pacific oceans. Such nannofossil variations are coeval with positive excursions of the δ13C curve and suggest perturbations of the bio-geochemical cycles at a global scale. Additional minor changes in siliciclastic and terrestrial OM accumulation as well as in nannofloras are probably the result of local events, strictly related to the physiography and evolution of the Tethyan margin. [Copyright &y& Elsevier]