STRATIGRAFIA E PALEOGEOGRAFIA DEL NORICO NELLE PREALPI BERGAMASCHE OCCIDENTALI

Abstract

The stratigraphy of the thick and well - bedded Norian carbonate sequence cropping out in the Southern Calcareous Alps NW of Bergamo is reviewed. The formal name Aralalta Group is introduced to include the Zorzino Limestone, the Dolomie Zonate formations and the "Brecce sommitali della Dolomia Principale" member. Five regional (A - E) and four local (Al, El, E2, E3) facies have been distinguished in the Aralalta Group: chaotic massive breccias (A), locally bounded by encrusting organism (Al); debris flow breccias and proximal torbidites (B); torbiditic dolomitic arenites (C); thin bedded laminites - microtorbidites (D); micritic limestones, distal torbidites (E), locally thin bedded with black clay (El), bioclastic limestones with slumpings (E2) and patch reefs (E3). The Aralalta Group is time - equivalent with the upper part of the Dolomia Principale, a classic example of inner platform. The detailed study of sedimentary structures and microfacies demonstrates that the evolution of the Dolomia Principale platform margin was in Artavaggio - Val Taleggio area structurally controlled. This interpretation is supported by the following considerations: 1) The Dolomie Zonate and Calcare di Zorzino sediments, formerly thought to represent a lagunal environment, are here reinterpreted as internal basin - slope deposits because of the frequent graded cal-carenites and rudites intercalated in the basinal sequence. Fine Ta - Tj, Bouma sequences can be common. Near the platform margin breccias and megabreccias represent the most common deposit (Fig. 4, sez. 2, 3, 6; Fig. 6). 2) The upper slope deposit consists of megabreccias whose components are both platform derived sediments with huge blocks, calcarenites and slope derived sediments. They have the maximum thickness near the margin. The source area of these deposits was a SW - NE orientated fault escarpement which yielded a great amount of material transported mainly as debris flow into the basin. 3) The margin of the Dolomia Principale as well as the overlying breccias, are crossed by sedimentary dykes and tensional fractures. 4) The margins of the carbonate platform are represented by lagoonal -peritidal facies with a few patch reefs at the top of Calcare di Zorzino - bottom of the Argilliti di Riva di Solfo. This further demonstrates that the main source of the slope deposits were an active inner platform, as well as platform facies exposed by tectonic faults. The Dolomia Principale platform margin in the NW Bergamasc Alps belongs to an Upper Triassic geodynamic model where the platform underwent a tensional period with conseguent development of highly subsiding intraplatform basins (Fig. 1), delimitated by tectonic escarpments or flexures. This peculiar structural setting was due to a Norian rifting, soon interrupted during the Rhaetian. Nevertheless this event of crustal thinning can be considered in Lombardy a praecursor of the Tcthyan Liassic rifting.