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INTRODUCTION TO
SEDIMENTARY ROCKS
Prepared by Dr. F. Clark
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
THE OLD WAY – TWO CHOICES
Until the last few decades, sedimentary rocks were
conventionally subdivided into two large groups, namely
clastic and chemical sedimentary rocks. In simplest
terms, clastic rocks were those of a fragmental nature
(“clast” is derived from the Greek word klastos, meaning
broken in pieces), in which individual fragments of the
parent or source rock for the sediment were evident, and
had been moved as discrete particles. The chemical
rocks had formed by precipitation or extraction of
dissolved materials, mostly from marine and saline lake
waters, to produce the mineral grains or fossil shell
material.
THE NEW WAY – THREE CHOICES
We presently deal with sedimentary rocks as being
siliciclastic, biochemical, or chemical. Recognizing that
rocks usually excluded from the clastic group may also
have clastic textures, we now use the term siliciclastic to
denote those clastic sedimentary rocks composed of
generally stable silicate mineral grains. Many texts will
still use the term clastic rather than siliciclastic. We also
recognize that many rocks that used to be referred to as
chemical are in fact the result of direct organic extraction
from water, usually to form shells or hard parts, or
precipitate as a result of biological influence on water
chemistry, and these are referred to now as biochemical.
Those sedimentary rocks formed by precipitation without
organic influence are now the chemical ones.
Clastic Textures Compared
Both rocks exhibit clastic textures, but belong to different groups. The
left specimen has clasts of quartz and chert, stable silicate grains,
and is thus siliciclastic. The specimen on the right has clasts
consisting of intact and fragmented mollusc shells. The calcium
carbonate shells are formed by organic extraction of calcium and
bicarbonate ions from sea water, and so the rock is biochemical.
Fossils Don’t Make It Biochemical
We’ve suggested that fossil shells represent biochemical sedimentation,
but a rock must consist in the majority of such material before it is
considered biochemical. Thus, the left sample, consisting almost
wholly of shells of the bivalve mollusc Coquina, is biochemical,
whereas the right sample, with the few fossil shells noted, is
dominated by durable silicate grains and is therefore siliciclastic.