Tertiary paleoweatherings and silcretes in the southern Paris Basin

Abstract

The southern border of the Paris Basin has undergone successive weathering events during the Tertiary. Fluviatile and lacustrine deposits are scattered, being mainly confined to several North-South trending grabens. These are generally devoid of fossils which would allow them to he dated. A discontinuous blanket of weathering products is the main record of this long period of continental evolution. These weathered materials result mainly from alteration of the Cretaceous flint-bearing chalk and are composed of corroded flints enclosed in brown and red mottled kaolinitic clays. In places, the succession of Cretaceous layers has been preserved in the paleoprofiles, testifying of in situ paleoprofiles, but most profiles are the result of reworking as shown by the mixture of materials coming from different stratigraphic horizons. These paleoweathered materials led to various residual and sedimentary deposits, from flint breccia along the graben scarps to flint gravels, clay deposits and lacustrine limestones in the grabens. These weathering products correspond to warm and humid climatic periods. Their reworking results from a renewal of tectonic activities combined with a period of drier climate. A later silicification process has affected the weathering cover and the Tertiary deposits of the graben as well. Extensive pedogenic silcrete crusts, with typical columnar structures, developed on the plateaux. This period of silicification corresponds to a climate with a marked dry season which led to the development of glacis. Massive silicification, preserving the primary sedimentary structures, developed in the grabens. This was sub-contemporaneous with the pedogenic silcretes which lie over them and which were developed at their expense. The significance of the massive silicification is not yet clear, nor the geochemical mechanism of their formation. In the process, primary clay minerals are destroyed and silica must have been fixed while aluminium was leached out of the profile. The silcretes are covered by lacustrine limestones and calcretes which accumulated in the lowest parts of the paleolandscape and toward the centre of the basin. They seal the silcrete development and are dated from Upper Eocene, a period of very dry climates as shown by thick gypsum deposits in the center of the basin. No remnants from Oligocene-Miocene times are preserved in the region studied. During Late Miocene, the disruption of these landscapes took place because a very low sea level led to the renewal of erosion. As a result residual landforms with siliceous crusts now cap the higher elevations.