Procesy neomorficzne w utworach onkolitowych wapienia cechsztyńskiego syneklizy pery bałtyckiej

Abstract

Omówiono procesy neomorficzne (jak również towarzyszące im inne procesy diagenetyczne) występujące w onkolitach wapienia cechsztyńskiego syneklizy perybałtyckiej. Stwierdzono, że neomorfizm agradacyjny najintensywniej zachodził w wewnętrznych częściach onkoidów. Przedstawiono sekwencję procesów, w wyniku których skała onkolitowa zmienia się stopniowo w coraz bardziej bezstrukturalną skałę o stosunkowo małym zróżnicowaniu wielkości kryształów. Sądzić można, te co najmniej część sparytów sąsiadujących z onkolitami ma genezę poonkolitową. Przedstawiono kryteria pozwalające w pewnych przypadkach odróżnić sparyty pomikrytowe od sparyrtów poonkolitowych oraz omówiono genezę struktury grudkowej.

NEOMORPHIC PROCESSES IN ONCOLITIC DEPOSITS OF THE ZECHSTEIN LIMESTONE OF THE PERI-BALTIC SYNECLISE (NE POLAND)

The Zechstein carbonate rocks and especially oncolites were subjected to very complex diagenesis, and one of the most important diagenetical processes were neomorphic changes.

Other diagenetical processes associated with neomorphic changes (first of all 1. solution and filling and 2. replacement) modified the action of neomorphic processes in a different manner. The neomorphic changes of oncoid forms (Tables I-III) and of matrix (Tables V-VII; Table VIII, Fig. 19) are detaily discussed in connection with other diagenetical processes. It was found that in the case of oncoids aggrading neomorphism (as well as micritization) is the most intensive in internal parts of oncoids . whereas the most external envelopes are often well preserved and relatively little changed (Table III, Fig. 7); in some cases, especially when rather big oncoids occur, it is related to relatively high content of clayey material in the external envelopes of oncoid forms compared with the internal envelopes.

If the sequences of neomorphic processes of matrix and oncoid forms are similar and if oncoids underwent a very weak micritization and/or aggrading neomorphism during the earlier phases of diagenesis, the rock characterized by the primary oncolitic structure changes gradually into the more and more structureless rock of relatively small differentiation of crystal sizes in the result of continuous aggrading neomorphism (Table IV, Fig. 10, 11). Therefore it seems probable that at least a part of the seemingly massive carbonate rocks abuting the oncolites was primarily the oncolites. The presence of oncoid relicts (Table IV, Fig. 10; Table V. Fig. 12) is the criterion of discrimination between sparites after micrites and sparites after oncolites; The genesis of clotted texture (Table VIII, Fig. 18) is also discussed; taking into account the close relationship between that texture and associated different types of oncolitic deposits, the genetical interpretation of R. Bathurst (1970) was accepted for studied cases.