High resolution stratigraphy and evolution of the Rhône delta plain during Postglacial time, from subsurface drilling data bank
Résumé
This study is based upon a large set of core drilling data (lithology and well logs) of the Rhône delta plain extracted from the French geological survey (BRGM) databank. The main goals are to set up a sequence stratigraphy model of the postglacial sedimentary bodies of the Rhône delta plain and to evaluate the relative importance and influence of the main controlling factors and processes (subsidence, eustasy, sediment yield, climate, anthropic influence) that led to their formation.
First, a detailed study (60 drill holes with tenth meters spacing) was performed on a restricted area (Saint Ferréol lobe, 1.5 km2). Correlation between well logs data and lithological data allows to identify the main sediment lithofacies and key surfaces, and to set up a detailed stratigraphic framework. Postglacial deposits are subdivided into a lower member of coastal plain and backbarrier deposits and an upper member of marine deposits (coastal barrier environments).
Following, the analysis of 160 wells distributed on the whole delta plain depicts the large scale architecture of postglacial deposits. In the upper delta plain, fluviatile coastal plain and palustral deposits prevail. In the lower delta plain, the deposits are made of a lower part of coastal plain deposits organized into four units (T1–T4) stacked in a retrograding pattern. This lower member is truncated by an erosional surface progressively deepening seaward. Above, an upper member of marine deposits thickening seaward and coarsening upward is organized into five units (P1–P5) arranged in a prograding pattern.
The core drilling data have been correlated with high resolution seismic data on the adjacent shelf and are interpreted in sequence stratigraphy terms. The retrogradational units (T1–T4), with coastal plain deposits, are correlative of shelf units and constitute together trangressive parasequences deposited during phases of slowing down of postglacial sea level rise. The upper marine units (P1–P5), arranged in a prograding pattern, represent parasequences of the Highstand Systems Tract which have prograded since the period of reduced rate of sea level rise at the end of the Holocene. During this period, the evolution of the sedimentary Rhône system was controlled by major changes of the hydrologic regime. Periods of increased sediment discharge, aggradation of the floodplain and progradation of the coastline are correlated with the development of the deltaic lobes of the Rhône d'Ulmet (unit P3) and of the Rhône du Bras de Fer (unit P4). Unit P5 correlates with adjacent prograding shelf unit that developed seaward of the present Rhône river mouth.
Comparison of sea level history between the Eastern part of the Rhône delta plain and the rocky coast to the east of Marseille suggests that tecto-subsident movements account for part of the changes in relative sea-level. Moreover, the truncation of transgressive and progradation surfaces, the unevenness of the MFS, the dilatation of sedimentary record and deformations of key surfaces are also in favor of differential subsidence.