Catchment inversion during the Silurian of SW Wales: Pediment plains, embryonic drainage networks and incised valley fills in a dryland range front alluvial system

Abstract This paper presents a comparison between the system tract architecture and the reservoir geometries of the Gironde and Leyre (Arcachon) incised-valley fills, both located within the Bay of Biscay 100 km apart. This study, based on high resolution seismic lines acquired by Ifremer on the continental shelf and onshore core and well data, illustrates that some features of the Gironde and Leyre valleys fills are similar while some others are not. The architecture of both valley fills is characterized by fifth order depositional sequences (corresponding to an interval from 120000 yr B.P. to present day). Both valleys are filled predominantly with transgressive systems tract, with associated poorly developed lowstand and highstand systems tracts. Key stratigraphic surfaces punctuate the valley-fill architecture and comprise deeply eroding tidal ravinement surfaces merged with and enhancing, earlier formed, fluvial-related erosive sequence boundaries. These tidal ravinement surfaces can be undulatory in form and in most places mark the basal boundary of the incised valleys. In contrast, nearly horizontal wave ravinement surfaces cap the incised-valley fills, extending over the adjacent interfluves. The Gironde and Leyre (Arcachon) valley fills exhibit two main stratigraphic differences: 1) transgressive systems tract sand bodies are ribbon shaped within the Gironde and tabular shaped within the Leyre; 2) lowstand systems tract deposits, represented by fluvial sediments, are preserved within the Gironde but absent within the Leyre. In a wave- and tide-dominated environment, the geometry of the sandbodies within the transgressive systems tract is a function of the tidal ravinement processes, which characterizes the estuary inlet. Two categories of tidal ravinement processes can be distinguished here: “anchored tidal ravinement” and “sweeping tidal ravinement”. The Gironde estuary is characterized by an “anchored tidal ravinement”. The tidal inlet has remained largely in a fixed location; littoral drift has not shifted the tidal inlet to the south because it is constrained by resistive Eocene carbonates that define the margins of the Gironde incised valley. In contrast, the Leyre estuary is characterized by a “sweeping tidal ravinement”. The inlet has been shifted approximately 30 km to the south by the formation of a littoral drift associated spit. This extensive lateral shifting was made possible by the fact that the incised valley was cut into unconsolidated, easily eroded Pleistocene sands. Within a wave- and tide-dominated environment, the preservation potential of the lowstand systems tract is a function of the size of the fluvial drainage basin. During lowstand time, the erosive power of the fluvial discharge was much greater within the much larger Gironde valley, consequently the fluvial sequence boundary was cut much deeper in the Gironde valley than within the Leyre valley and, correspondingly, the thickness of the associated fluvial deposits was commensurately greater. In response, the lowstand systems tract was not preserved within the Leyre valley fill because the depth of tidal ravinement erosion formed during the sea-level rise and associated transgression was greater than that associated with fluvial incision generated during the sea-level fall.

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