Evolution of Sedimentary Basins as Recorded in Silica Concretions: An Example from the Ionian Zone, Western Greece

Georgia Pe-Piper,Avraam Zelilidis,David J W Piper,Nicolina Bourli

doi:10.3390/min11070763

Abstract

Chert concretions in thick limestone successions preserve a more complete paragenetic sequence of diagenetic minerals than their host limestone and interbedded shale. The goal of this study was to test the possible presence of a high-temperature mineralising system in the Ionian basin of western Greece. Upper Cretaceous chert nodules were sampled at Araxos, where rocks are highly faulted and uplifted by salt diapirism, and on Kastos Island, on the flanks of a regional anticline. Chert concretions have microporosity produced by recrystallisation of opal to quartz and fractures produced in the brittle chert during basin inversion. Diagenetic mineral textures were interpreted from backscattered electron images, and minerals were identified from their chemistry. Diagenetic minerals in pores and veins include sedimentary apatite (francolite), dolomite, Fe-chlorite, Fe oxide-hydroxide mixtures, sphalerite, barite and calcite. Sphalerite is restricted to Araxos, suggesting that inferred basinal fluids were hotter and more saline than at Kastos. At Araxos, the Fe oxide-hydroxide also includes minor Cu, Zn, and Ni. Whether the transported metals were derived from sub-salt clastic rocks and basement, or from enriched Mesozoic black shales, is unclear. The effectiveness of this novel approach to understanding fluid flow history in thick limestone successions is validated.

Highlights

Siliceous concretions in limestones provide an important record of diagenetic, tectonic, and fluid-flow events in limestone-dominated sedimentary basins
Mineral phases precipitated in veins and in the microporosity of chert concretions have the potential to preserve a record of basinal fluid flow first during burial and later during tectonic inversion
We have investigated the record of fluid flow in Cretaceous deep-water limestones from the Ionian zone of western Greece using the record preserved in chert concretions

Summary

Introduction

Siliceous (chert) concretions in limestones provide an important record of diagenetic, tectonic, and fluid-flow events in limestone-dominated sedimentary basins In these limestones, opal precursors of many chert concretions and beds form early in the diagenetic paragenesis at redox controlled boundaries [1], but recrystallisation of opaline silica to quartz is unusual in rocks younger than Miocene in the modern ocean [2] and takes place at elevated temperatures following burial [3]. Opal precursors of many chert concretions and beds form early in the diagenetic paragenesis at redox controlled boundaries [1], but recrystallisation of opaline silica to quartz is unusual in rocks younger than Miocene in the modern ocean [2] and takes place at elevated temperatures following burial [3] This replacement of opal by quartz creates pervasive microporosity in many cherts [4,5,6]. Mineral phases precipitated in veins and in the microporosity of chert concretions have the potential to preserve a record of basinal fluid flow first during burial and later during tectonic inversion

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