Sedimentary facies and sequence stratigraphy of coarse-grained Gilbert-type deltas within the Pliocene thrust-top Potenza Basin (Southern Apennines, Italy)

Abstract

Coarse-grained, fluvially-dominated Gilbert-type braid deltas represent a volumetrically significant component of the Potenza Basin sedimentary infill, a small thrust-top structural depression that developed during the Pliocene in the Southern Apennines (Italy). Excellent exposure shows vertical and lateral relationships among facies assemblages that identify a suite of deltaic and non-deltaic depositional environments occupying semi-confined marine embayments along the southern–western basin margin. The succession is around 50 m thick and consists mainly of shoreface and offshore sands and clays, moderately wave-worked and covered by shoal-water deltaic clinostratified gravels. The deltas are organized into two vertically-stacked sequences that display well-developed angular-to-tangential foresets and poorly-preserved topsets, and a suite of internal depositional architectures consisting of alternating progradational and aggradational geometries that were controlled by high-frequency, relative sea-level changes in a relatively slowly subsiding basin. The common element that characterizes all the depositional architectures detected within the two main studied sections is the constant influence of coastal uplift on the deltaic systems during sediment accumulation. The tectonic control of the basin margin from which the deltas were sourced forced a forward-stepping (basinward) arrangement of stacked Gilbert-type braid deltas, and these produced clinoforms that become progressively younger toward the basin depocentre. The deltaic sequences show different offsets in their along-dip arrangement suggesting that the coastal margin was inclined at varying angles depending on the rates of the tectonic uplift. Two end-members of Gilbert-type delta architectures are represented by concave- and flat-bottom deltas, prograding onto a mudstone and calcareous substrate, respectively. These two different Gilbert-type models reflect a different style of delta accretion during progradation, due to the response of the substrate to the erosion exerted from gravel avalanches.