Geoscience Reference
In-Depth Information
Figure 10.33.
Some mechanisms that can give rise to sedimentary basins.
(a) Heating of the lithosphere, which results in expansion and uplift. If erosion
takes place, a basin forms when the lithosphere cools and contracts. (b) Loading
of the lithosphere from above (thrust piles) or below (intrusions, phase changes).
(c) Extension, compression and faulting of the lithosphere.
10.3.3 Mechanisms for producing subsidence
Avariety of geological processes can lead to the formation of thick sedimentary
deposits on continental crust. They can, however, be divided into three main
classes: (1) those formed by thermal events, (2) those formed as the result of
flexure of the lithosphere by an imposed load and (3) those formed as the result
of extension, compression or faulting of the basement (Fig. 10.33). Basins of the
first class subside as a result of a heating event that causes uplift and erosion. Then,
as cooling and contraction begin, sedimentation occurs. Examples are the Illinois
and Michigan basins in the U.S.A. An example of the second class, a basin formed
as a result of flexure of the lithosphere under an imposed load, is the Canadian
Alberta basin. This foreland basin formed as a result of the loading provided
by the Rocky Mountain fold thrust belt. Examples of the third class include the
thick deposits laid down on the continental margins as the result of continental
rifting (e.g., the Atlantic continental margins), basins formed after stretching of
the continental lithosphere (e.g., the Tertiary North Sea basin), basins formed in
major fault grabens (e.g., the Basin and Range of the western U.S.A.) and the
fault-controlled basins that apparently formed as a result of compression of the
lithosphere (e.g., central Australian basins).
These classes of basins are by no means mutually exclusive or comprehensive.
Forexample, the subsidence of the North Sea basin is explained by extension and
thinning of the lithosphere by a factor of about 1.25, which gave rise to an initial
rapid subsidence. This was followed by cooling, which resulted in a subsequent,
slower, exponential thermal subsidence.
