Page 178 - Petrology of Sedimentary Rocks

DIAGENESIS

Diagenesis, the conversion of carbonate sediment into rock, takes place by a
multitude of processes, described by a recent avalanche of papers (see Folk 1974 JSP).
Three processes go on: (I) solution of the more unstable minerals, particularly
aragonite; (2) filling of pores by precipitated minerals of all kinds, but most abundantly
calcite; (3) alteration of original minerals to newer ones, stable under changed
conditions, by a complex of processes such as expulsion of Mg from mg-calcite,
inversion of aragonite to calcite, recrystallization of fine calcite to coarser calcite, or
replacement by dolomite (or silica, pyrite, etc.).

Most diagenesis takes place near the contact zones between two or three of the
following phases: air, fresh water, sea water, and sediment. Diagenesis and
precipitation of cements is particularly active where two solutions of different
composition, temperature, CO content, etc. mix. If Karl Marx will forgive us, we may
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state the Geochemist Manifesto: “Waters of the World, Unite! You have nothing to
lose but your ions.” The accompanying diagram shows common environments of
carbonate diagenesis.

The minerals that form upon diagenesis are obviously controlled by chemistry of
the environment. The main features can be explained by making a plot of the salinity
vs. Mg/Ca ratio of the diagenetic solutions. Two ideas are fundamental:

( I) Calcium Carbonates. The presence of MgICa ratios over about 2:1 forces
CaCo3 to be precipitated as either aragonite or magnesian-calcite. Mg++ poisons
sideward growth of these crystals, so they tend to be fibers or steep-faced rhombs.
Only when Mg/Ca ratios drop below about 2:1 can equant, sparry calcite form. This
means that in sea water, hypersaline brines, beach-rock, etc. with Mg/Ca ratios varying
f rorn 3: I to as much as 100: I in some sabkhas, that aragonite and magnesian calcite
fibers or micrite form. In fresh surface waters, and in most subsurface brines, where
Mg/Ca ratios generally are I:10 to l:2, equant to bladed sparry calcite forms.

(2) Dolomite is difficult to crystallize because of the precise ordering required to
form the alternating sheets of Ca and Mg. Thus it forms best when crystallization is
slow, precipitating from dilute solutions. Dolornite formation is also favored by a high
Mg/Ca ratio; in hypersaline solutions, dolomite forms at MgICa over 5 or lO:l, but in
“fresh” waters it can form at Mg/Ca values as low as I :I. Thus the dolomite boundary
forms a sloping line on this graph: Its formation is favored by reducing salinity, or by
increasing the Mg/Ca ratio. Fresh water dolomite, because of its slow crystallization,
is characteristically euhedral and “limpid” (water-clear with sharp, mirror-like facets).
Limpid dolomite is much more slowly soluble than ordinary dolomite. Hypersaline
dolomite is in very tiny crystals, poorly developed.

The following are the main diagenetic realms:

A. Saline waters
I. Beachrock. Cementation mostly by evaporating sea water in inter-
tidal zone, at the sea/sediment/air contact zone. Salinity normal marine to
somewhat hypersaline, MgICa ratio about 3:1 or a little higher. Aragonite
and Mg-calcite cements, fibrous or micritic.

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