Sand Composition and Sedimentary Evolution of a Late Quaternary Depositional Sequence, Northwestern Adriatic Coast, Italy

Abstract

ABSTRACT In order to detect temporal and spatial variations in detrital supply in the upper part of the last depositional sequence of late Pleistocene-Holocene age, we conducted a combined petrographic and stratigraphic study in the Romagna coastal plain, south of the Po delta, Northern Italy. We analyzed heavy minerals and the bulk composition of sands from cores and surface samples. By using petrographic and stratigraphic data on composition of modern beach and major river deposits, new aspects of detrital dispersal mechanisms and the depositional history of the study area are documented. The heavy-mineral distribution, coupled with the dolostone and volcanic rock fragment contents of bulk sands, allow identification of three different petrofacies: Petrofacies A, Petrofacies B, and Petrofacies C, which have been interpreted as of apenninic, mixed Eastern Alps/Po River basin, and Po River catchment basin provenance, respectively. Distribution of the three petrofacies changed through time in response to landward and seaward shifting of the coastline. During the late Pleistocene lowstand, a pure Apenninic provenance characterized the Romagna coastal plain (Petrofacies A). During the Holocene transgression, when the shoreline migrated tens of kilometers west of its present position, eastern Alpine sediment sources fed littoral facies (Petrofacies B), probably as a result of southward transport by littoral drift. This sediment supply continued during the early regressive phases and was cut off by a change in coastal morphology related to the development of the early Po delta. This led to the establishment of a sediment supply entirely related to the Po River catchment basin (Petrofacies C). Changes in the compositional signature of sand in the youngest beach ridges mark the abandonment of the early Po delta due to an avulsion event and testify to the establishment of a coastal system fed by rivers draining the Apennines (Petrofacies A). We emphasize sand petrography as an important tool in studying the internal architecture of sandy clastic depositional units on detailed spatial scales, and its use in deciphering the depositional history of complex sedimentary successions. FIG. 1. Location of the study area. End_Page 829------------------------