Geology Reference
In-Depth Information
Fig. 18.15
Outcrop images of Sandstone Unit 5 of the Pallaresa
mbr of the Ametlla Fm. east of the old railroad station of the
town of Ametlla. (
a
) Overview of Sandstone Unit 5 behind the
railroad station. (
b
) Overview of a part of the stratigraphy with
Sandstone Units 4, 5 and 6 looking westward towards the rail-
road station. (
c
) Tabular cross-stratifi ed set with mud-draped
bottomsets in the outcrop of (
a
). (
d
) Tidal bundle succession at
the base of Sandstone Unit 5 fi lling up a scour in the channel
fl oor (2 km east of the railroad station). Note the reactivation
surfaces and the neap-spring cycles. (
e
,
f
) - Details of (
d
) show-
ing mud-draped foresets, reactivation surfaces and neap-spring
cyclicity
m to a km, both along depositional dip and depo-
sitional strike. They commonly have a gradational
base and are formed by stacked siliciclastic and bio-
clastic cross-stratifi ed, planar- and trough-bedded sets,
ripple-laminated sandstone and highly bioturbated
sandstone. No oscillatory wave produced sedimentary
structures have been reported. Cross-stratifi ed sets show
unidirectional or bi-directional paleocurrent directions
and have occasional mud drapes on the foresets. The
rippled sandstone is fi ner grained and contains thicker
mud drapes (Mutti et al.
1985b
; Olariu et al.
2008a,
b
) .
The bedforms shingled by migrating one over the other
and offl aping (Fig.
18.17d
).
Importantly, single dunes in the stacked sets
(compound dunes) are inclined in the same direction
(eastward) as the compound-dune master surfaces, that
is, the surfaces on which the larger compound dune
migrated by forward accretion (Olariu et al.
2008a,
b
;
Fig.
18.17
). This observation classifi es the sandstone
bodies as tidal compound dunes with their crest oriented
normal to the tidal currents and internal accretion
surfaces that dip in the same direction as the tidal
currents. This interpretation stands in contrast to the
interpretation as tidal bars proposed by Mutti et al.
(
1985b
) for sandstone bodies of the lower unit which
have their long axis parallel with the tidal currents
and internal accretion surfaces that migrate laterally
(at a high angle to the tidal currents). Note that Wonham
(
1993
) interpreted bedforms in the overlying upper
unit (up to 6-8 m) as compound tidal dunes on the
same grounds. In contrast, however, to the typical
orientation of compound dunes (sensu Olariu et al.
2008a,
b
) inferred to have been aligned parallel to
paleofl ow.
At basin scale, migration directions were primarily
controlled by seafl oor topography (dunes migrated to
fi ll adjacent deeper parts of the basin) and dominant
tidal current directions. Additionally, migration direc-
tions of the largest compound dunes were controlled
by relative sea-level changes because dunes respond to
