Silurian dolostones of eastern Lithuania

A Brazauskas,D Kaminskas,G Bičkauskas

doi:10.3176/earth.2010.2.07

A Brazauskas, D Kaminskas + Show 1 more

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https://doi.org/10.3176/earth.2010.2.07

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Abstract

Silurian dolostones representing the Jaani (Verknė and Jocionys formations (Fms)) and Minija (Pabradė Formation (Fm.)) regional stages were studied in the Jocionys 299 borehole located in eastern Lithuania. In addition to petrological studies, dolostones were subjected to XRD, XRF and ICP-MS analyses. X-ray diffraction analysis revealed that dolomite crystals in dolostones were very close to stoichiometric and well ordered and could be interpreted as early diagenetic. Strontium in dolostone of the Pabradė Fm. is comparable to that of other ancient dolomites, and is much lower than Sr concentrations in typical modern marine dolomites. Slight enrichment in Sr and S in the Verknė and Jocionys Fms is due to the presence of celestine (SrSO4) and gypsum. Evaporative (sabkha), seepage-reflux, mixing-zone, burial and seawater dolomitization models of modern and ancient examples from literature were considered. For the Jocionys Fm. we suggest seepage-reflux and burial (?) models. Evaporative (sabkha) and mixing-zone dolomitization models may be applied to the Verknė Fm. and the Pabradė Fm., respectively.

Highlights

Dolomitization of Silurian rocks of the southern part of the Baltic Silurian Basin is poorly understood. Lapinskas (2000) and Paškevičius (1997) report epigenetic dolostones in the lower and upper Silurian, which originated from various types of limestone and dolomitic marl
In this paper we study the dolomitization of the Silurian carbonate sequence in eastern Lithuania by means of XRD, geochemical major and trace element analysis and stable isotope (δ18O and δ13C) composition of dolomitic rocks in order to reveal the mechanisms of dolomitization, including distinction between meteoric, marine or evaporitic environment and possible dolostone precursor sediment
Three types of 20 sampled dolostones were differentiated on the basis of distinctive stratigraphic and petrological characteristics (Figs 6–8)

Summary

Introduction

Dolomitization of Silurian rocks of the southern part of the Baltic Silurian Basin is poorly understood. Lapinskas (2000) and Paškevičius (1997) report epigenetic dolostones in the lower and upper Silurian, which originated from various types of limestone and dolomitic marl. Much research has been done on dolomitization and a number of models have been proposed to explain the processes involved Tucker & Wright 1999; Machel 2004). Different dolomitization models rely on three basic factors: the source of Mg (generally seawater), water/fluid movement through the sediment package and concentration of kinetic inhibitions to dolomite precipitation. Dolomite should be a stable and widespread precipitate from seawater. Several kinetic factors (hydration of Mg2+ ions in seawater, high ionic strength of seawater, relative efficiency of aragonite and high-Mg calcite precipitation, inhibition effects of SO42– ions) mitigate its formation (Scholle & Ulmer-Scholle 2003). The role of sulphate as an inhibitor according to Machel (2004) is rather overrated

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