Biostratinomy and Diagenetic Impact on Exceptional Preservation of Coccospheres from Lower Oligocene Coccolith Limestones

Agnieszka Ciurej,Marek Szczerba,Marta Bąk

doi:10.3390/min10070616

Abstract

Lower Oligocene coccolith limestones, known as Tylawa Limestones, in the Polish part of the Outer Carpathians have been analyzed using back-scattered electron (BSE) and charge contrast imaging (CCI) techniques and X-ray diffraction. The aim of the study was to reconstruct the fossilization history of the excellently preserved coccospheres, commonly observed in these sediments, deposited in the Paratethys basin. Multiple petrographic and geochemical analyses indicate that the exceptional preservation of coccospheres has been caused by the special coincidence of biostratinomic processes: pre- and post-depositional processes within the water–sediment interface and possible early cementation. We discuss the origin of the calcite and silica and provide some hypotheses regarding the environment and possible factors responsible for the spectacular preservation process. Based on the observed intermediate steps of calcite and silica alteration, we propose an extended model of biostratinomic processes and diagenesis. The possibility of finding exquisitely preserved coccospheres not only in soft clay-rich deposits but also in heavily lithified rocks extends the possibilities of research on the development and evolution of this group of organisms.

Highlights

Coccolithophores are a group of unicellular calcifying haptophyte algae living in the photic zone of the oceanic environment
Based on the observed intermediate steps of calcite and silica alteration, we propose an extended model of biostratinomic processes and diagenesis
Preserved coccospheres have been found in the Tylawa Limestones that occurred as an isochronic lithological horizon in the Lower Oligocene deposits of the Outer Carpathians

Summary

Introduction

Coccolithophores are a group of unicellular calcifying haptophyte algae living in the photic zone of the oceanic environment. Each coccolithophore cell secretes intracellularly minute calcitic particles, the coccoliths, which after completion are exocytosed and incorporated into the coccosphere. Individual coccospheres may consist of a few or a few dozen coccoliths. Individual coccolith plates are small (1−10 μm in diameter) biomineral structures that consist of interlaced single calcite crystals with a highly complex morphology [1]. Coccoliths are formed of low-Mg calcite, they have a high potential for fossilization process. Since the Mesozoic up to the present times, coccoliths are an important component of pelagic carbonates covering the basin floor above the calcite compensation depth [2] and a significant source of calcium carbonate in different types of marine deposits [3]. During the Late Cretaceous, a huge accumulation of coccoliths and coccolith debris played a significant role in formation of chalk and opokas lithofacies in most parts of the epicontinental European Basin [4,5,6,7,8]

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