Geology Reference
In-Depth Information
1 8
Contrasting Styles of Siliciclastic
Tidal Deposits in a Developing
Thrust-Sheet-Top Basins - The Lower
Eocene of the Central Pyrenees
(Spain)
A. W. Martinius
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-infl uenced fl uvial 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 fi elds and large tidal bars developed mainly in the initially
underfi lled 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 signifi cantly amplifi ed and modifi ed 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 fi lled-up by a
rapidly developing axial east to west prograding alluvial to deltaic system. This
reduced tidal amplifi cation 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.
18.1
Introduction
Dutch (Mey et al.
1968
; van Eden
1970
) , soon
followed by the Spanish, Italian and French workers,
began to study the well-exposed outcrops in detail.
A major step forward was the understanding of
tidal bundle successions based on studies of excava-
tions in the modern and historically well-documented
Oosterschelde estuary (The Netherlands) during the late
1970s and early 1980s (Nio et al.
1980
; Visser
1980
;
van den Berg
1981,
1982
) as well as sedimentological
studies in the Wadden Sea (Sha
1990
; Oost
1995
) that
The central Pyrenean Eocene thrust-sheet-top basins
in Spain have been well known for their siliciclastic
tidal deposits since the mid 1960s when fi rst the
