Some Notes on the Insoluble Residues in Limestones

Abstract

Most carbonate rocks contain insoluble residue to various extent, whereby the clay-content forms the basis for a classification of the limestones (Correns, 1939, p. 200). Rocks with a clay mineral content of up to 25% are hereby designated as limestones. This classification is rarely come across in geologic literature, however; most of the time they are referred to as clayey limestones. Along with organic remains (conodonts, fossil teeth, spicules, Radiolaria, chitinous periderma of graptolithes, pyritized or silicified fossils), insoluble residues may consist of authigenic components (quartz, feldspar, pyrite, dolomite, zeolite, glauconite, etc.) on the one hand, and of detrital components supplied by the action of wind or water (illite, chlorite, kaolinite, quartz, feldspar, heavy minerals, etc.) on the other. The delineation between both of the latter components is particularly difficult within the realm of the clay minerals, since it may also be a matter here of synsedimentary, epigenetic or diagenetic modifications and authigenic products. The percentage of such authigenic products is, however, relatively small, since the clay content in present seas is for the most part detrital (Biscaye, 1965). Nevertheless, this uncertainty makes a genetic interpretation of the mineral composition of the clayey residue more difficult, since certain lateral or vertical changes in the composition of clay residue do not necessarily have to be connected with a change in the source area, but might just as well be of a secondary nature. On the other hand, however, it are precisely such often short-lasting, qualitative and/or quantitative changes in the residue, which contribute to the clarification of definite geologic questions. The clarification of stratigraphie hiata represents such a case. These hiata are associated in part with a temporary local or regional regression and exposure to the air of an up till there marine realm of sedimentation. This may lead to soil formation as controlled by climate, in which case the soils which develop largely fall victim to the next-following transgression and are reworked, but in the process, however, can also change the insoluble residue picture within the transgression strata quantitatively and qualitatively over short distances.