Internal structure and environmental reconstruction of eocene transitional fan-delta deposits, monllobat-castigaleu formations, Southern Pyrenees, Spain

Abstract

A detailed study has been made of a part (500 × 200 × 15 m) of the Eocene Monllobat Formation. The conglomeratic bottomset—foreset—topset build up of a small fan-delta passes into sand- and mudstone layers. Ultimately the sandstone layers wedge into the mudstone due to concave-upwards basal surfaces and convex top surfaces or due to thickening of intercalated mud layers. Sequences vary accordingly from coarsening upwards in the proximal parts to fining upwards distally. Both sequences are topped by a thin layer fining upwards. The proximal fan-deltaic part consists of interconnected lobes. Crescentic, transverse bars deposited gravel on a slightly inclined top. Conglomerates of the moderately inclined foreset show some imbrication. The bottomset consists of tabular cross-bedded and cross-laminated sandstone. Parallel-laminated and tabular cross-bedded sandstone layers drape rises of fine sediment in front of the fan-delta face. Continued progradation brought about truncation of these backset sediments. Sandstone lobes of the distal part are not connected. Tabular croos-bedding and parallel lamination are the main sedimentary structures. Climbing, large-scale bedforms are prominent on top surfaces. Most of the lobes are enveloped by blue-grey mud. Layers covered by mottled mud terminate in foreset faces. Highly energetic inflow took place from a shallow, braided stream with longitudinal bars. The low sedimentary relief caused jetflow. The high flow energy, the large grain-size range and the large quantities of sediment caused great differences from inflow models for the deltaic environment. Sedimentation was effected by flow deceleration. Expansion, inertial forces, friction or a combination of these phenomena were responsible for the deceleration. Expansion and inertial forces caused development of moderately inclined foreset faces. Due to expansion, steeply inclined foresets were formed. Sand was deposited in shallow marine water and in scour pits in continental deposits during flow deceleration caused by frictional and inertial forces. Parallel-laminated backsets connected with flat, tabular sets developed due to flow deceleration by expansion and frictional forces. The deposits were formed in the transition zone of a fan-delta under a tropical climate with marked dry and wet periods. Wide, braided streams entered a restricted, shallow marine basin at right angles to the basin axis. Aggradation exceeded water depth. Progradation processes were influenced by large- and small-scale tectonic activity.