| Abstrakt: |
Abstract: Calcite concretions from both Přední Kopanina (Prague suburb) and Hředle quarries were the subject of the reported study, or more exactly, insoluble residues from conc retions occurring in totally decalcified lower Turonian "marly" silicites (opuka in Czech and Plänerkalk in German languages). Both localities be long stratigraphically to Bílá hora formation - member of the Bohemian Cretaceous Basin. Mutual comparison of chemical analyses recalculated to "normative" minerals reveals that in the course of diagenesis initial compositions of sediments have beench anged remarkably. The original content of normative feldspars preserved in concretions and nodular beds ranges between 9.4 and 18.5 %, remarkably decreases to 4.3 - 10.6 % in host rocks affected by diagenesis. At the same time the content of clay and mica minera ls (kaolinite, chlorite, illite or glauconite) in host sediment s increases two or three times (from 7.3 - 11.2 % to 18.1 - 22.7 %). Comparison of concretions insoluble residues (I.R.) with decalcified host sediments from two quarries near village Hředle showed a different picture. The I.R. of concretions contain 10.2 - 25.3 % of feld spars comparing with 6.2 - 7.9 % in host sediments. However, the content of clay and mica minerals, which in I.R. of concretions forms about 28.8 - 33.9 %, decreases three up to five times (to 6.8 - 7.3 % content) in host rocks. Reasons for this irregularity are supposed to be attributed to the late diagenesis and epigenesis, which not only completely remove calcite, but also dissolute feldspars and clay and mica minerals in host sediments. Microsparitic calcite concretions, which are much denser and less porous than the host rocks, preserve the original composition of the sediment, because the removal of calcite from them passed very slowly. |
