Page 85 - Petrology of Sedimentary Rocks

cemented tightly in the vicinity of faults (where shattering has aided migration of
silica-precipitating fluids), and some are cemented in tightly folded areas but loose in
non-folded ones. In large scale folds a thermal gradient may be established (because of
the general increase of temperature with depth), and this affects the solubility of
minerals and may cause selective cementation at anticlinal crests or synclinal troughs.
Badly jointed areas (as at the crests of folds) may also become tightly cemented
because of easier fluid migration. Weathering may be easily determined as the cause by
noting correlation of cementation with the actual outcrop of the formation versus its
condition in deep cores or spots inaccessible to weathering. If one sets out to solve a
problem of this type, sampling is the chief concern. He should take weathered outcrop
samples and deep cores; sample folded areas, jointed areas, faulted areas, and flat-lying
areas, obtain samples near and far from hydrothermal activity; study the correlation of
grain size of the sand with cementation; the relation of cemented zones to overlying
and underlying formations, etc.

In general, older formations are more likely to be cemented with quartz than
younger ones; yet there are many uncemented Cambrian or Ordovician sands, and some
Cretaceous or Tertiary ones are tightly cemented with quartz. In general, deeply
buried or strongly folded sands are more apt to be cemented, but many horizontal,
unfaulted beds that have never been buried over a few hundred feet are also cemented
tightly. The problem is an interesting and difficult one.

Chert and Opal

Sedimentary quartz assumes three forms: (I) megaquartz, a general term for
quartz overgrowths, crystals, geode and vein fillings, composed of equant to elongated
grains larger than 20 microns; and (2) microquartz, divided into (2A) microcrystalline
quartz, forming a pinpoint-birefringent aggregate of equidimensional grains usually
ranging from l-5 microns in diameter (but ranging from a fraction of a micron--in the
apparently isotropic cherts--to 20 microns, the arbritrary upper limit); and (2B)
chalcedonic quartz, forming sheaf-like bundles of radiating extremely thin fibers, which
average about 0.1 mm long but may range from 20 microns to a millimeter long. All
three types are transitional in some degree. Both forms of microquartz consist of
nothing but finely crystalline quartz containing variable amounts of very minute Iiquid-
filled spherical bubbles, averaging 0.1 micron in diameter, which look brownish in
transmitted light and silvery in reflected light (Folk and Weaver). These bubbles are
responsible for lowering the index of chalcedony so that it varies from 1.535 in most
bubbly specimens up to 1.544 in those with few bubbles. They also decrease the density
of microquartz and provide a sponge-like character so that solution is quicker, and this
type of quartz consequently weathers faster than megaquartz.

Two length-slow varieties of fibrous silica, quartzine and lutecite, apparently
occur as a replacement of evaporites or in sulfate-rich, alkaline environments such as
semi-arid paleosoils (Folk & Pittman, 197 I, J.S.P.; Milliken, 1979, J.S.P.). Always
check optical elongation of fibrous silica to determine this. Zebraic chalcedony
(Milliken) is also a common evaporite associate.

Chert is defined as a chemically-precipitated sedimentary rock, essentially mono-
mineralic and composed chiefly of microcrystalline and/or chalcedonic quartz, with
subordinate megaquartz and minor amounts of impurities. Common impurities are clay
minerals, silt, carbonate, pyrite, and organic matter.

Some cherts contain opal. Chert nodules consist very largely of micro-crystalline
quartz, while chalcedonic and megaquartz usually form as cavity-fillings within the

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