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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 50 (1966)

Issue: 3. (March)

First Page: 635

Last Page: 635

Title: Mud into Shale: When and How?: ABSTRACT

Author(s): Raymond Siever

Article Type: Meeting abstract

Abstract:

Coring of surficial sediments on the sea floor to date has not revealed the presence of indurated, clayey sediments that are worthy of the name mudstone or shale, even though some clay as old as 50 million years has been recovered. In contrast, very young, argillaceous rocks on continental platforms are in many places indurated and cemented. Shale, in the sense of a fissile argillaceous rock, is uncommon in the younger part of the geological record though it is common in older rocks. Therefore, one can conclude that the induration to mudstone and the development of fissility of shale have two different time scales and thus result from two different processes.

The initial process of induration is clearly linked to simple mechanical compaction under load where water can be expelled, as studies of marine sediments have shown. During this stage there is little gross mineralogical or chemical change in the sediment. Later stages include chemical additions that fill pore space and increase bulk density. Chemical additions are restricted to sediments that do not remain buried permanently under deep water; the additions result from ground-water movement through the mud, a movement that is initiated by the bed being raised above sea-level and thus making possible a hydraulic head in the outcrop area. Much of the chemical precipitation in this stage is simply redistribution of carbonate, but small amounts of silica generally are added too.

Fissility, as was noted long ago, is correlated directly with the growth of phyllosilicate minerals in the ab plane, parallel with the bedding. It appears that the 10 A.U. micas and the chlorites are the principal causes of fissility. Accompanying this growth is a general increase in cementation that reduces porosity to low levels. However fissility does not always accompany cementation.

Fissility, then, is the result of preferential growth of the 10 A.U. micas and the chlorites under load that is induced by the geochemical environment. That environment may be characterized as one whose K+/Na+, Mg++/Na+, and alkali/H+ ratios are relatively high compared with sea water, and whose dissolved silica concentration is close to that in equilibrium with quartz. The environment that has these characteristics is that of concentrated subsurface brines typical of most ancient sedimentary basins. The correspondence of the time scales of the production of fissility in shales and the formation of brines is important support for this hypothesis.

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Copyright 1997 American Association of Petroleum Geologists