INTRODUCTION TO SEDIMENTARY ROCKS

1. INTRODUCTION TOSEDIMENTARY ROCKS Prepared by Dr. F. Clark Department of Earth and Atmospheric Sciences, University of Alberta August 06

2. 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.

3. 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.

4. 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.

5. 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.