Geology Reference
In-Depth Information
Fig. 13.28
Inferred influence of sea-level change on the
architecture of tidal deposits infilling a Jurassic seaway that
occupied a North Sea rift basin. The succession consists of an
alternation of thick crossbedded deposits formed by large
dunes migrating along the axis of the seaway that form a tidal
sand ridge during transgressive periods, and finer-grained
and more thinly bedded sandstones that accumulated during
highstands when the water depth was greater and the currents
speeds were less. Each sea-level fall and the start of the subse-
quent transgression is marked by a discrete pebble lag and bio-
turbated, glauconitic horizon that underlies the giant crossbeds.
Labels in the margin refer to deposit attributes (
SB
sand bank,
SS
sand sheet,
arrows
palaeocurrents) (From Surlyk and Noe-
Nygaard
1991
)
13.8.2 Long-Term Changes in Basin
Morphology
It is expected that, in the course of an overall first-
or second-order transgression, the shelf will gradually
grow wider, with the progressive development of a
more complex coastline, including tidal embayments
that can extend many hundreds of kilometers inland
(Houbolt
1982
; Houthuys and Gullentops
1988a, b
;
André et al.
2003
). With continued sea-level rise,
these embayments can eventually evolve into tidal
seaways and straits with a marine connection at both
ends (e.g. Anastas et al.
1997
; Besson et al.
2005
;
Longhitano and Nemec
2005
). Whereas tidal currents
must decrease at the head of an embayment, they can
be accelerated through a seaway, making possible the
propagation of a progressive tidal wave and the main-
tenance of strong tidal currents a long distance into
the continental interior. This seems to have been the
case for the Peri-Alpine, Miocene seaway of southern
Europe, which formed a short-lived connection
between the Atlantic and the Paratethys during the
Burdigalian (Allen et al.
1985
; Martel et al.
1994
;
Bieg
2005
; Fig.
13.30
). If, however, tidal resonance
occurs at the embayment stage, the connection of the
head of the embayment to another tidal basin as a
On the much longer time scale of first- to second-
order sea-level changes (up to a few hundreds of
meters of relative sea-level change, stretching over
tens to hundreds of millions of years), two generic
end-member situations arise (Fig.
13.29
). During
overall low sea-level periods (e.g. during the late
Cenozoic and present), there is limited flooding of
continental interiors. Most shallow-marine sedimen-
tation occurs on narrow shelves at the margins of the
continents. Large-scale embayments are restricted
primarily to tectonically structured seaways along
collisional or transform margin (Kamp et al.
1988
;
Hoppie
1996
). During overall high sea level, such as
in the Upper Cretaceous, by contrast, a much larger
part of the continents is flooded, creating extensive
semi-enclosed seas with a complex topography.
Because of their complex paleogeography, these seas
experience very complex interactions between fric-
tion forces and tide-enhancing processes that cannot
be solved without the help of paleotidal modeling.
