Geology Reference
In-Depth Information
changes in water depth. Olariu et al. (
2008a
) suggested
water depths between 25 and 36 m at a minimum
average for the lower unit. The sandstone bodies are
intercalated with strongly bioturbated muddy sand-
stones up to tens of meters thick that represent low-
energy fringes of amalgamated dune fi elds or periods
of drowning.
Wonham (
1993
) and Olariu et al. (
2008a
) compared
the depositional setting of the Baronia Fm with the outer
part of the San Francisco Bay where the sea fl oor is cov-
ered by a tidal dune fi eld (Rubin and Hunter
1982
;
Barnard et al.
2006
) . Berné et al. (
1988
) used the Baronia
bedforms as an ancient analogue for the modern com-
pound dunes of Surtainville in the English Channel that
are formed by strong tidal currents. It is at present, how-
ever, unclear what the geography and morphology of the
south-eastern section of the T-G-A Basin was at the time
of deposition of the Baronia Fm. If the basin was closed
towards the southeast this closure must have been
located at least 10 or more kilometres away from the
location of the (preserved) Baronia bedforms.
tions from the expected equality in the bundle sequence
were interpreted to refl ect incidental storm infl uence.
A universally applicable dependency between
tidal current velocity and the tidal range was used to
estimate the tidal range. However, only a linear rela-
tionship between the volume of water fl owing through
a tidal channel during the dominant tidal period and
the wet cross-sectional surface of a tidal channel below
mean water level has been proven (O'Brien
1931
;
van de Kreeke and Haring
1979
; Van den Berg
1986
) .
Although maximum current velocities vary between
spring tide and neap tide, and deeper channels are
associated with somewhat higher local current veloci-
ties than shallow channels, theoretically equal tidal
current velocities for all channel depths can be expected
if tidal current velocity is replaced with shear velocity
(J.H. van den Berg, personal communication 2010).
Thus, no relationship exists between tidal shear veloc-
ity and tidal range and, hence, the estimated tidal range
derived from the Roda bundle succession is
questionable.
López-Blanco et al. (2003, his Fig. 6) suggested that
the tidal dunes migrated over the lower part of the sandy
delta front as part of attached tidal bars and that they
were driven generally towards the NW by tidal currents.
Low-amplitude, gentle NW-SE oriented folds resulted
in a seafl oor topography that caused funnelling of tidal
currents in a NW-SE direction; low water depth would
have contributed to the effectiveness of this process
(López-Blanco et al. 2003). However, from observa-
tions in the Rhine-Meuse delta, including the
Oosterschelde (Siegenthaler
1982
, his Fig. 1), it appears
that offshore tidal currents close to the coastline follow
a rotary path in contrast to inshore estuarine tidal cur-
rents that show distinct reversals of current direction
approximately along a linear fl ow path. The latter situ-
ation requires a funnel-shaped land constriction and,
consequently, it is concluded that the Roda tidal bun-
dles, exposed along the Isábena River close to Roda de
Isábena, formed in a NW-SE oriented, restricted,
inshore, tide-dominated environment such as an embay-
ment (following Nio and Siegenthaler
1978
; Yang and
Nio 1985). Additionally, it is doubtful whether gentle
seafl oor topography could cause suffi cient funnelling
and reversals of tidal fl ow. The indented coastline
morphology resulted from movements along the same
pre-existing NW-SE oriented blind thrust and associ-
ated faults mapped by López-Blanco et al. (
2003
) and
which also controlled the location of the 'Serraduy
18.4.6 Tidal Bundles
The distally located toesets of almost all progradational
Gilbert-type delta lobes of the Roda Sandstone Member
in the Isábena valley were modifi ed by tidal currents dur-
ing periods between fl uvially-derived sediment infl uxes
and are represented by tidal dunes. These are small (up to
20 cm) in the lowermost three lobes and increase in size
in the upper three lobes (preserved thickness up to 1 m).
All have mud-draped foresets (Figs.
18.18
and
18.19
).
Bottomsets of the Gilbert-type delta lobe that reached
farthest into the basin overlie an approximately 100 m
wide belt formed by an at least 5 m thick preserved suc-
cession of tidal dunes with distinct neap-spring-neap
cyclicity (Yang and Nio
1985,
1989
; Nio and Yang
1991
). This is well displayed in an outcrop along the
Isábena River close to Roda de Isábena.
Based on a comparison with the thickness and
characteristics of bundle successions in tidal dunes
formed in channels of the modern Oosterschelde estuary,
Yang and Nio (
1985
) estimated that the tidal bundles
were formed in an estuary with water depths of about
15 m. Tidal periodicity analysis indicates that the tidal
bundles were formed in a meso- to macrotidal semi-
diurnal regime (M2 dominant) with an estimated mean
tidal range of 3.6 m (Fig.
18.19
). Large irregular devia-
