Geneza struktury gruzłowej w górno dewońskich wapieniach okolic Olkusza i Zawiercia

Abstract

Analiza petrograficzna i sedymentologiczna wapieni gruzłowych z górnego dewonu okolic Olkusza i Zawiercia ujawniła obecność zróżnicowanej cementacji w przydennej warstwie pierwotnego mułu wapiennego. Selektywna cementacja wokół uprzywilejowanych geochemicznie ośrodków prowadziła do powstawania gruzłów mikrytowych, otoczonych marglistym tłem skalnym. GENESIS OF NODULAR STRUCTURE IN UPPER DEVONIAN LIMESTONES OLKUSZ – ZAWIERCIE AREANodularity is a common feature of two Upper Devonian lithostratigraphic units described earlier by the author (M. Narkiewicz, 1978) in the subsurface of the Olkusz – Zawiercie area, southern Poland. The units in question are as follows: the Frasnian nodular limestones unit and the Famennian detrital nodular limestones unit.The nodular limestones do not show any strong evidence of high depositional energy. Furthermore, there is no evidence of slumping processes, load-casts or extensive bioturbation (with some minor exceptions, however – see PI. 1. Fig. 3) along with tectonic and/or compactional breakage.Thus, the, nodularity is assumed to be due not to destructive but rather to constructive processes, e.g. accretion or cementation. There are the following evidences that the micritic nodules were considerably lithified within the bottom sediment layer (i.e. within eogenetic environment) prior to the cementation of marly matrix:1. The nodules contain well-preserved burrows and delicate organic skeletons, while the marly matrix was strongly affected by the compaction phenomena.2. The nodules build up evident elevations at numerous intraformational erosion surfaces within the detrital-nodular limestones unit. At the same time, the marly martix was apparently more susceptible to erosion and it formed depressions at the erosional surfaces (PI. II, Fig. 7; PI. Ill, Fig. 10).3. The redeposited nodules frequently observed within the intraformational conglomerates, generally retained their original shape while the primary marly matrix is usually disintegrated and disseminated within the conglomerates matrix.The development of erosional surfaces together with the general composition of intraclasts suggest that the intraformational erosidn did not reach very deep into the sediment and that it rarely involved older detrital beds (cf. Fig. 2).Thus, one may assume that the nodules that were exhumed by occasional erosion, had mostly been cemented during the time deposition of the uppermost eroded nodular bed. One can make a rough estimate of that period having in mind that there are about 160 intraformational horizons within the investigated detrital-nodular limestones unit, and assuming that the total time of deposition of this unit did not exceed 5 m.y. (i.e. duration of the Famennian stage according to Van Eysinga, 1975). The estimated maximum time necessary for nodules lithification is of order of 30000 years. This number may be, however, strongly overestimated.The selective cementation hypothesis (J. P. A. Noble, K. D. M. Howells, 1974) is considered the most adequate genetic explanation of described nodular structure. The above hypothesis implies more straightforward geochemical processes than the rival diagenetic segregation model (e.g. J. Grlindel, H. J. Rosler, 1963; H. C. Jenkyns, 1974). Moreover it is confirmed to some extent by the Recent evidence of the lithification of the deep-sea carbonates (e.g. A. G. Fischer, R. E. Garrison, 1967; J. Mòller, F. Fabricius, 1974).The described nodular structure was subsequently modified by some late diagenetic processes, mostly by pressure-solution (PI. III, Fig. 11) and selective dolomitization of marly matrix (PI. I, Fig. 6).