Geoscience Reference
In-Depth Information
The Late Jurassic materials were formed in an arm of the sea that became increas-
ingly shallow and coastal as time passed. By the Early Cretaceous, the local environ-
ments were changing and the Wealden deposits were formed in lakes that were clearly
separated from the sea and were periodically invaded from the north by sand-carrying
rivers. Eventually the sea invaded again from the south, and in younger Early Creta-
ceous times sands (later forming the Greensands) and muds accumulated in shallow
coastal seas. Eventually these coastal environments became the remarkably uniform
seas of the Late Cretaceous in which algal calcareous muds formed, ultimately turning
into the Chalk after burial and compression by further sediments.
FIG
94.
Sketch section showing the Western Unconformity visible in the layers across the
Southeast England Basin. The zigzag lines represent the unconformities.
Above the Chalk, the succession contains a distinct time gap of about 20 million
years in the record of sedimentation, during which the already deposited Jurassic and
Cretaceous bedrock was subjected to crustal movements, and the pattern of sediment
accumulation was interrupted. When the record starts again in Early Tertiary times, the
material deposited was being formed in seas that extended only over an area now rep-
resented by the Hampshire and London basins. The succession of Tertiary layers pre-
served in the Hampshire Basin (located on Fig. 92) is beautifully visible in some loc-
alities, for example Alum Bay at the western end of the Isle of Wight. The succession
in the Hampshire Basin spans the time from about 58 million years ago to 28 million
years ago (mid Paleocene to mid Oligocene). The layers vary from about 200 to 700
m in total thickness from west to east, and the proportion of sediment that was depos-
ited in the sea rather than on land also increases eastwards, confirming the idea that the
eastern part of the Southeast England Basin was the most strongly subsiding.
