Page 83 - Petrology of Sedimentary Rocks

IV. Authigenic Quartz

These varieties of quartz generally appear to form from solutions at !ow
temperatures and pressures: either in sea water at the time of deposition, from
connate water, subsurface water, or ground water. They generally have no connection
with magmatic or hydrothermal waters (although such waters can form vein quartz).

Overgrowths form on the quartz grains of sandstones or limestones, especially
those of Paleozoic age. They are in optical continuity with the sand grain nucleus, and
are separated from it usually by vacuoles, sometimes by clay or hematite; sometimes
there is no visible demarcation line and the presence of quartz overgrowths must be
inferred by the apparent interlocking of adjoining quartz grains. If pore spaces are
large and there is little quartz precipitated, the overgrowths are idiomorphic with nice
crystal faces; if too much silica is precipitated so that the pore spaces are completely
filled, the overgrowths have irregular boundaries. Overgrowths must be searched for
under high power and are often missed because they are difficult to see. Luminescence
Petrography reveals them in some examples. Overgrowths formed by direct precipita-
tion usually lack inclusions; those that form by replacement of carbonate always have
carbonate inclusions. Zoned quartz overgrowths are common in silcretes and may
indicate vadose conditions with alternate wetting and drying.

Idiomorphic crystals averaging .05-.I5 mm are common in limestones, where they
grow by replacement of the calcite, hence are often loaded with microcrystalline
calcite inclusions and have pitted surfaces. They may or may not have sand grain
nuclei, and frequently replace fossil or oolite grains.

Nodular Masses of quartz are found occasionally in dolomites. They may form
cauliflower-shaped bodies from 0.5 mm to a foot or so in diameter, or even thin beds.
They are made up of flamboyant, crudely radiating quartz, and form chiefly by
replacement of evapori te nodules. Many have pseudo-cubic zoned quartz (McBride and
Folk, 1977, JSP), and associates are length-slow chalcedony. See Milliken (I 979 JSP).

Vein and Geode quartz occur as cavity fillings. Crystals are often idiomorphic
(mosaic if many are crowded together), and have straight extinction or sometimes
develop undulose extinction even without the application of stress,

The time of origin of these sedimentary quartz types has not yet been solved and
is a very difficult and economically important problem. Obviously the movement of
fluids through a formation will be affected by how tightly that formation is cemented;
thus it is essential to determine when cementation occurred as that will impede
migration of fluids. Furthermore if one could determine why certain parts of a
formation become cemented while others remain porous, one would have a valuable
prospecting tool.

These are several possible stages in the history of a sediment wherein cementa-
tion might occur. It is not known yet which stages are most important; all stages
probably occur in one formation or another. The following classification is based on
that of Krynine.

I. Syngenetic Stage

A. Early Syngenetic (Contemporaneous). Cement develops by precipi ta-
tion from freely migrating, essentially normal sea water while the
sediment is at the sea bottom or at most buried a few feet.

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