Contrasting styles of siliciclastic tidal deposits in a developing Thrust-Sheet-Top Basins - The lower Eocene of the central Pyrenees (Spain)

Abstract

Lower Eocene tidal deposits in the Tremp-Graus-Ager Basin in the southern Pyrenees (Spain) are well-developed and include typical examples of tidal bars, compound tidal dunes, tidal bundles and tide-dominated back-barrier lagoons as well as tidally-influenced fluvial systems. They occurred in a relatively narrow (up to 60 km) and long (up to 200 km in total) semi-enclosed sea which had an open connection to the Atlantic ocean in the west. Two groups of tidal deposits are recog-nised related to two stages of the obliquely migrating thrust-sheet-top basin which affected the position and relative dimensions of the foredeep and shelf sections. Compound tidal dune fields and large tidal bars developed mainly in the initially underfilled foredeep in relatively deep water (at least up to 40 m) during the Early Ypresian. Favourable conditions existed for basin scale tidal current circulation patterns, locally significantly amplified and modified by pronounced bathymetric variations (related to developing blind thrust related folds and blind ramps) and a variable, and probably distinct, structurally controlled, coastline morphology. Small shoal-water fan deltas and larger Gilbert-type delta(s) and associated tidal bars developed along the basin margins near, often long-lived, sediment entry points. During the Late Ypresian to Late Lutetian the basin shelf area filled-up by a rapidly developing axial east to west prograding alluvial to deltaic system. This reduced tidal amplification in the basin and shallow-water tidalites developed only in a narrow (approximately 10 km) zone, located above an oblique lateral ramp system, including the in-shore parts of the delta distributaries and the subaqueous part of the, partly barred, delta top.