Page 86 - Petrology of Sedimentary Rocks

chert nodule. In hand specimens, the following varieties of chert are recognized:
(I) flint, black d ue t o organic matter inclusions; (2) green jasper, green because of chlorite
inclusions; (3) red jasper, red because of hematite inclusions; (4) novaculite, a pure,
massive white chert occurring in thick beds, the color due to abundant water inclusions;
(5) agate, simply banded chert with different colors, usually a cavity-filling; (6)
chalcedony, colorless to pale gray or white, translucent, usually a cavity-filling and
usually composed of chalcedonic quartz; (7) moss agate, which is usually chalcedony
permeated by manganese oxides in dendritic form.

Chert may form beds hundreds of feet thick; nodules; sporadic replacements of
fossils within a carbonate matrix; geode and vein fillings; as a cement in sandstones;
and as tiny disseminated specks and dolomolds in carbonate rocks. The latter
occurrence is volumetrically far more important than all the others put together.

The origin of bedded cherts is diverse, some are probably carbonate replacements
and some are siliceous oozes made of radiolaria or spicules (McBride & Folk, ‘77-79
JSP). Nearly all nodular cherts result from replacement of carbonate rock while the
carbonate ooze was still soft. Evidence for this is the preservation of allochem
limestone structures such as fossils, oolites or fecal pellets and bedding laminae in the
chert nodule, and disruption of chert by mudcracking and soft-sediment slumping.

Replacement usually results in the formation of microcrystalline quartz, while chalce-
donic and megaquartz are usually cavity fillings; however the latter two occasionally
form by replacement and the former even more rarely as a cavity filling. A little chert
forms by hydrothermal activity and some by weathering.

Opal is isotropic, hydrous silica gel with index varying from 1.40 to 1.47. It has a
very strong relief, often thus appears brownish, and sometimes shows weak anisotro-
pism. X-Ray study shows that it contains a little high-cristobalite. It is rare in
sediments, and allegedly none has been found in rocks older than the Permian. It may
form biogenically as diatom ooze, radiolarians, or spicules; and may form inorganically
as cements in sandstones, nodules, weathering crusts, and geode and vein fillings. It is
supposed to recrystallize to chert with time. It is often associated with continental
volcanic sediments, derived from the decomposition of volcanic ash.

Reworked Detr i tal Chert

Chert may be eroded out of older limestones and chert beds, and the elastic
particles contributed to form a terrigenous constituent of sandstones. Chert may be in
turn reworked from these older chert-bearing sandstones. Such reworked chert forms
l-4% of the terrigenous fraction of sediments. It is always diagnostic of an older
sedimentary source, hence should be searched for diligently in determining the source
area of a given sandstone. It is slightly less durable than quartz (slightly softer, a little
more brittle), but despite this, usually occurs in pebbles because the nodules simply
started out so big. E. Sneed (U.T.M.A. 1955) has shown that chert pebbles are more
difficult to round than quartz because they tend to chip and split; but sand-sized chert
rounds faster than quartz because it is softer. It is also somewhat less stable than
quartz, because the fine grain size and bubble content presents a large surface area to
chemical attack. Weathered cherts develop a whitish outer band or patina due to
solution of carbonate or organic inclusions and sometimes spongy solution of the chert
itself.

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