The Upper Cretaceous chalk of northwest Europe was deposited in
an open epicontinental
sea during a period of high global eustatic sea-level – a unique
depositional setting requiring modification
of the sequence-stratigraphical approach. In this paper, a framework for
the sequence-stratigraphical
analysis of the Upper Cretaceous chalk is discussed, and an ideal sequence
model presented.Analysis of the Coniacian succession has identified eight third-order
(short-term) cycles of relative
sea-level change (∼400 Ka duration), superimposed upon a second-order
(longer term) cycle (∼3.2
Ma duration). The third-order cycles represent frequent, basin-wide oceanographic
changes for which
there are no known tectonic mechanisms. These cycles show a visual correlation
with both the δ13C
and the δ13C stable isotope curves, indicating a climatic
control on sedimentation, probably linked to a
Milankovitch eccentricity rhythm.The long-term trend in independently derived δ13C stable
isotope values parallels the long-term relative
sea-level curve, reflecting increased production as the shelf area expands.
By contrast, the δ18C stable
isotope values show little change over the long term, suggesting that tectonics,
rather than climate,
was the controlling factor on second-order relative sea-level change. This
is corroborated by the fact
that the long-term cycle is coincident with a phase of increased activity
at the mid-ocean ridges, which
is thought to allow for a 60 m rise in global sea level.
Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr
