Page 8 - Petrology of Sedimentary Rocks
P. 8
TERRIGENOUS
10%
ALLOCHEMICAL ORTHOCHEMICAL
c J
CHEMICAL
T. Terrigenous Rocks. Example: most mudrocks, sandstones, and con-
glomerates. Comprise 65% to 75% of the stratigraphic section. Most
terrigenous rocks occur in the shaded area.
IA. Impure Allochemical Rocks. Example: very fossiliferous shales; sandy
fossiliferous or oolitic limestones. Comprise I O-15% of the strati-
graphic section.
IO. Impure Orthochemical Rocks. Example: clayey microcrystalline lime-
stones. Comprise 2-5% of the stratigraphic section.
A. Allochemical Rocks. Example: fossiliferous, oolitic, pellet or intra-
elastic limestones or dolomites. Comprise 8-15% of the stratigraphic
section.
0. Orthochemical Rocks. Example: microcrystalline limestone or dolo-
mite; anhydr i te; chert. Comprise 2-8% of the strat igraphic section.
Collectively, “IA” and “10” are classed as Impure Chemical Rocks, and “A”
and “0” as Pure Chemical Rocks.
