Page 40 - Petroleum Geology

19

position of the sedimentary basin relative to the environments of the physiog-
raphic basin. Several classifications have been proposed (some are listed in
the references at the end of the chapter) but few are satisfactory because the
concepts of physiographic and sedimentary basins are confused. Classifica-
tions of continuous variables are always arbitrary and artificial to some
extent. It will be sufficient to make but a simple classification here.

Sedimentary-basin geometry

The geometry of sedimentary basins in section, in terms of isochronous
surfaces, may be classified as either symmetrical or asymmetrical (Fig. 1-7).
These may be further subdivided on the basis of the nature of their margins,
that is, faulted or unfaulted (but it must be realized that the nature of the
margin of a sedimentary basin is not always determinable because subsequent
geological events may have obscured it or it may be beyond the reach of in-
vestigation). The asymmetrical basin has received much attention in the liter-
ature, and was called a “half-graben” by Weeks (1952) when the asymmetry
is due to a fault or fault system.
The term “structural basin” is sometimes used for basins bounded on one
or both sides by faults, but no clear meaning has become attached to the
term. It is sometimes used for a basin of simple, but large, synclinal form.
It must be emphasized that the symmetry of sedimentary basins in this
context relates to the geometrical symmetry of isochronous surfaces, not to
the symmetry of facies that accumulated in the basin. It is dangerously fal-
lacious to suppose that deeper-water sediments belong to the deeper parts of
the sedimentary basin, and shallower-water sediments belong to the edges.

SYMMETRICAL ASYM ME T RlCAL

Q
(u

Fig. 1-7. Simple classification of sedimentary basins on basis of geometry of isochronous
surfaces.