Page 359 - Petroleum Geology

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Mudstone (shale) diapirs

Mudstone does not seem to develop into such clearly defined diapirs as
salt. The expressions of mudstone diapirism are typically two:
- True mudstone diapirs that are penetrative stocks. These may have a
surface expression in the form of mud volcanoes.
- Incipient diapirs, forming the cores of anticlines, similar to salt pillows.
True mudstone diapirs, and incipient diapirs, typically contain fluids at ab-
normally high pressures. The true diapirs are sheathed in compacted mudstone
or shale. Dips measured in boreholes that penetrate the diapir are usually
found to be steep but regular, the result of flow rather than folding. Geo-
physically, they are low-velocity, low-density features; and drilling has found
them to be undercompacted, with small mechanical strength and small equi-
valent viscosity. These indicate, of course, similar properties for the mother
layer. The preservation of these properties in the diapir suggest that the mud-
stone has not been intruded upwards from great depth, and the accumulation
of sediment over the diapir is strong evidence that the whole diapir was sub-
siding with the sedimentary basin.
Mud volcanoes occur commonly in younger sedimentary basins around
the world, and seem to be associated strongly with regressive sequences. They
are reported from Trinidad and northern South America, the Gulf Coast
province of North America, Asia Minor, Pakistan, Burma, Indonesia, Borneo,
New Guinea and New Zealand. The principle of uniformitarianism requires
us to postulate that they also occurred in older sedimentary basins, but sub-
sequent geological events have obscured them.
Salt or brackish water is the main fluid of mud volcanoes, but gas (mainly
methane and carbon dioxide) and oil also occur. The fluid is usually warm,
and the activity intermittent. The fluid is clearly the fluid of expulsion from
the mudstone, with some perhaps coming from other material incorporated
into the flow.
In the Mississippi delta, mounds and small islands appear from time to time,
due to mud volcanism with less than 100 m of overburden. These mud-lumps,
as they are called, have long been known, and their intermittent activity
(which can be a hazard to navigation) has been attributed to the variable de-
positional patterns of the delta (see Lyell, 1867, pp. 447-454 for an early
description and interpretation of them). Investigation with core holes revealed
folds and overthrusts that clearly have a purely gravitational origin (Morgan
et al., 1968).
Most mud volcanoes can be considered as mudstone diapirs that have
reached the surface, and their activity will continue until sufficient fluid has
been expelled from the mudstone to halt the diapiric tendency. The bedding
of the mudstone is commonly destroyed, and rock material may be included
that has demonstably been brought from another formation at depth. Some
mud volcanoes have apparently resulted from intrusion up a fault plane (Fig.