Page 98 - Petrology of Sedimentary Rocks

revert to illite in the lab (also in sea water), whereas most volcanic montmorillonite
will not, and this serves to distinguish the two. To determine if weathering is
responsible for the particular clay mineral present in a sedimentary rock, the clay
mineralogy should be compared with other data, e.g., conditions of feldspars or kinds of
fossils as climatic indicators. It has been claimed that a prolonged time of weathering
under moderate conditions, may have the same mineralogical results as more extreme
heat and moisture for a shorter period. Thus, rarely, kaolin and alumina minerals may
form in very old “temperate” soils.

(3) Subaqueous weathering of acid volcanic ash usually produces montmorillonite
but sornetimes illite; the older bentonites tend to be more illitic, but these may have
been montmorillonite to begin with and changed over on diagenesis and deep burial.
Volcanism is a major source for montmorillonitic clays, but much montmorillonite
develops on weathering (= degraded illite, above). Basic volcanic material often
weathers to chlorite. To prove a volcanic origin, look for shards, volcanic fragments,
plagioclase, biotite, other mafic minerals, or idiomorphic zircon and apatite.

(4) Marine diagenesis starts altering certain clay minerals as soon as they are
dumped by fresh-water streams into the salty ocean, rich in Na, Mg, and K ions. Mixed-
layer montmorillonite-illite (i.e., partially to completely degraded illite, the latter also
termed “Weathering” mcntmorillonite) regains its K very quickly from sea water and is
partly or completely reconverted to illite. Fixation of K+ to form illite happens very
rapidly if the clay is ingested by certain animals and defecated (Jonas). Under certain
conditions, it may pick up Mg from sea water and go to chlorite instead. “Volcanic”
montmorillonite apparently undergoes no change on deposition in marine waters, or if
there is a change it is much slower. Detrital chlorite and illite probably also remain
stable in sea water, but minor chemical changes may occur (Ressman & Keller, JSP
1967). The role of kaolinite is uncertain. Because it usually decreases offshore, it was
once thought that it altered to other clays (e.g., illite) by reaction with sea water. This
view is now largely abandoned. Kaolinite is stable in continental, fresh water deposits
and in soils under-going weathering.

Clay minerals in recent sediments are often found to change systematically when
traced out to sea; kaolinite being present in continental and nearshore sediments, with
illite and montmorillonite farther out. To some, this is the result of marine diagenesis,
with the farthest-seaward clays having had more time to react with the saline waters.
To others (Riviere; Weaver) this represents a sorting effect, with clay minerals being
physically transported for different distances by currents as a result of their differ-
ences in particle size or degree of flocculation. The latter school also feels that clay
mineral variation both laterally and vertically depends chiefly on sorting processes like
that mentioned, or upon changes in petrographic character or climate of the source
area; clay minerals, like sand grains, are largely detrital hence chiefly controlled by
source area Iithology. The other school (Grim) feels that clay minerals are more
controlled by the chemical nature of their depositional environment. To determine if
depositional environment plays a role in clay mineralogy of a specific formation,
determine environment of successive beds by other criteria (fossils, glauconite,
sedimentary structures, size and shape criteria) and see if clay mineralogy changes with
these different environments --but realize that different clay composition in different
environments might be caused by sorting as well as diagenesis. If the clay minerals are
constant in beds of different environments, this indicates that the clays are probably
controlled by the source area.

(5) and (6) On deep burial and continued, intensified diagenesis, kaolin and
montmorillonite appear to be gradually destroyed; if the environment is rich in Fe or

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