Page 84 - Petrology of Sedimentary Rocks

B. Late Syngenetic (Shallow Burial). Cement develops while the sediment
is buried anywhere from a few feet to perhaps several hundred feet or
more beneath the sea floor. Precipitation occurs from connate
(trapped) sea water, which is essentially stationary or at most moved a
short distance.

2. Epigenetic Stage

A. Deep Burial. Cement develops while the sediment is buried a thousand
feet or much more beneath the sea floor, but prior to any structural
deformation of the beds. Precipitation occurs from subsurface water
which may have moved long distances through the formation or
adjoining formations, and which may be a mixture of fresh water with
connate water. Some silica may be supplied by waters pressed out of
shales on compaction. Source of silica can be the Si liberated when
montmorillonite changes to illite, or from solution of buried opaline
skeletons.

B. Post-Structural Deformation. Cement develops during or after uplift
or structural deformation of the depositional basin. “Deformation”
here includes anything from gentle warping to intense folding and
faulting; the essential point is that now sediments are no longer being
laid down in the same basin. Source of the silica may be far-travelled
subsurface waters; solution of detrital quartz grains under pressure
provides a small amount of silica (the so-called Riecke principle,
actually first described by H. C. Sorby in 1863; see Sibley and Blatt
I976 JSP).

C. Hydrothermal. Occurs generally during or after structural deforma-
tion, from waters heated by magmatic activity and carrying in solution
products sweated off from igneous bodies. These may penetrate a
formation and be carried along with the migrating subsurface waters.
D. Weathering. Occurs when the sediment is nearing the present ground
surface; silica may be carried by ground (meteoric) waters.

For any given formation, deciding when it became cemented with quartz is a very
difficult problem. Petrographic work must be supplemented with extensive field work;
in fact field work is probably the best method of attack. The microscope must be used
however to determine how the cement formed (by replacement or direct precipitation);
the nature of diagnostic inclusions which might indicate temperature or composition of
the precipitating solutions; whether the cement has spread the sand grains apart by
pressure of its growth; the relation of the cement to joints, stains, alteration or other
time markers. But field work combined with intelligent sampling is generally required
to obtain the areal and stratigraphic distribution of cement, which is generally the key
to the problem. For example, if a formation is cemented uniformly over tens of
thousands of square miles, the cement is probably syngenetic, formed from sea water at
the same time as or just after the sand grains were being deposited. Hydrothermal
sources are rather easy to establish or eliminate by noting if the formation is cemented
only in the vicinity of known hydrothermal activity; widespread cements cannot be
caused by this process. Whether or not the cement has formed on deep burial can be
determined by reconstructing the structural history of the area in which the formation
occurs; parts of the formation never deeply buried then should lack cement. Cements
quite frequently are related to the structural geology of a formation; many are

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