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lead to structures that range from minor displacements of plastic material to
major intrusions of large volumes of material through considerable thicknesses
of overlying rocks. Not all the structures that are called diapiric are intrusive,
but the development of a true diapir probably passes through stages of inci-
pient diapirism in which the mobile material forms ridges and low domes
(Fig. 15-1). In plan, diapirs tend to acquire a more or less circular outline: in
section, the amplitude may achieve dimensions of several kilometres. The
scale of diapirs ranges from kilometres down to centimetres (e.g., load casts).
They commonly occur in groups, in lines, or in lines of groups. They may be
intimately associated with folding and faulting, and they demonstrate that
certain rocks under stress will flow as a quasi-fluid or viscous solid. The ma-
terials of diapirs include ice, peat, evaporites (especially salt), mudstones and
marls, occasionally sands, and some igneous rocks.

A 0
Fig. 15-1. Diapir (A) and incipient diapir (B).

Confining our attention to diapirs of sedimentary rocks, we find them
only in sedimentary basins, of course, in rocks of most geological ages from
Proterozoic to Holocene, and in all continents except Antarctica (so far).
They are common in the petroleum provinces of the Gulf Coast of the United
States of America and offshore in the Gulf of Mexico, in the Middle East,
the Caucasus and adjoining regions to the north of the Caspian Sea, and in
north-west Europe (Braunstein and O’Brien, 1968). They may be equally
common in non-petroleum provinces that have not received the same intensity
of geological and geophysical investigation. Geophysical investigation has re-
vealed salt diapirs in sedimentary sequences off the continental shelf of West
Africa in water depths to 4 km (13,000 ft) (Beck, 1972; Beck and Lehner,
1974), and in the submarine parts of most of the major deltas.
Diapirs commonly, but not invariably, occupy areas of gravity minima. A
local gravity minimum over a diapir indicates a deficiency of mass despite
the intrusion of deeper material to shallower depths.