A reconnaissance of deltaic environment in the Middle Eocene of the south -central Pyrenees, Spain

Abstract

A regressive sequence of Eocene sediments is exposed in an area West of Tremp (South-Central Pyrenees, Spain). The sequence forms part of deposits formed in the Upper Cretaceous-Eocene marginal basin south of the Pyrenees. Insignificant amounts of continental and littoral deposits are preserved on the north coast of this marginal basin. In the studied areas, however, on the eastern margin of the basin, a variety of continental environments is found. Three formations axe distinguished within the Eocene deposits. At the base is the Roda Formation with a regressive marine series of limestone, marl, and sandstone. Partly overlying this formation and partly laterally transitional to it is the Montaña Formation, with continental and littoral deposits. The Santa Liestra Formation, formed after a major regressive phase, is the youngest. The distribution of these formations on the geological map and their main sedimentary facies are presented in figure 1. The Montañana Formation is regarded as a deltaic association. Two major sedimentary environments are distinguished: (1) a flood-plain environment with fluvial sandstones, conglomerates and finer sediments of the inter-distributary lagoons and swamps, and (2) a transitional environment with channel mouth, bay, tidal flat, and deltafront deposits, containing marine fauna. Two types of large-scale cross-bedding, with different origins are compared. One is interpreted as river subdeltaic formations in lagoons, while the other has been formed by lateral deposition in a migrating river channel. Several small sedimentary structures occur, of which "current crescent marks" and "longitudinal furrows-and-ridges" are discussed in some detail. Excellent exposure of the Montañana Formation provided the opportunity for an almost complete paleogeographic interpretation. The paleogeographic pattern of the floodplain is not that of the ideal delta, in which one major stream forms a fan-shaped deposit. Instead there is a concentration of supply from the north and east by several small rivers, caused by the configuration of the upland area.