Eocene sheetflood systems and transitional fan‐deltas, Southern Pyrenees, Spain

The Eocene Monllobat and Castigaleu Formations in the Southern Pyrenees consist of continental and shallow marine deposits respectively. The Monllobat Formation contains conglomerate sheets with sandstone tails, symmetrical channel fills with wings and various types of lobes, accreted laterally in channels as well as by progradation. These deposits are intercalated in large amounts of flat‐bedded silt and mudstone with mottling and caliche. The high energy alluvial flow, which deposited these sediments, was initially at right angles to the axis of the elongated basin, becoming near parallel to the axis distally. The deposits are less gravelly distally. Upstream wide, gravelly braidplains were developed. Gravel lobes and muddy sheetfloods prograded over the distal floodplain. Meandering rivers were restricted to a marginal position in the sheetflood environment. The Castigaleu Formation consists of mudstone with intercalations of sandstone. Lobes of conglomerate and sandstone dominate the contact zone with the Monllobat Formation. Flat sandstone layers with tabular cross beds and parallel lamination and massive sandstone beds are distributed throughout the other parts. The lobes were formed at the coastline during high energy inflow in the marine basin. Flow deceleration caused the deposition of the coarsest load at this site, building a wedge with an inclined front during major aggradation and progradation. The other deposition types originated during hyperpycnal inflow in a marine environment. Massive sandstones originated partly because of bioturbation. Reworking by physical marine processes did not occur. Synsedimentary tectonics determined the type of sedimentation at times.