Some remarks on diagenesis
of Rugose coral skeletons
Kilka uwag
o diagenezie szkieletów koralowców Rugosa
Institute of Geology, Adam Mickiewicz University, Poznań
Abstract: Rugose coral microstructure exhibits striking similarity to that in the Scleractinia. The main difference lies in the mineral composition: calcitic in the former and aragonitic in the latter. Calcitic skeletons of the Rugosa are commonly better preserved than those in the Scleractinia, and therefore some of them have been interpreted as unaltered, a position rejected in this paper. The dual nature of septa, which commonly consist of a primary trabecular septum and secondary fibrous sheets, results in differently expressed diagenetic alterations in comparison to other structural elements.
It has been postulated that both early and advanced diagenesis may, in some circumstances, be distinguished in the Rugosa. In most instances the early diagenetic features were destroyed by the post-burial alterations. Replacement and recrystallization are the most important processes among the advanced chemical alterations. Both may either facilitate the recognition of original macro- and microstructures or obscure them. Surface replacement by silica promotes perfect preservation of shape and inner morphology, whereas pervasive replacement may destroy both. Selected replacement by hematite may help in exposing the trabecular microstructure of septa, whereas deep replacement may destroy the entire morphology. Physical alterations, such as crushing and flattening of skeletons are always destructive. They result from compaction, and their scale depends on skeletal morphology and on the relationship between the rate of infilling of intra-skeletal spaces and the accumulation of surrounding sediment.
Pre-burial events, such as overgrowth and penetration by borrowing organisms, their holdfasts or roots may aid in the recognition of early diagenesis, but may also lead to substantial pre- or post-burial skeletal alteration, resulting sometimes in total destruction. This depends mostly on the pH of pore fluids.
Key words: Rugosa, diagenetic alterations
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