Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Abstract

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.