Ordovician Jeleniów Claystone Formation of the Holy Cross Mountains, Poland - redox condition reconstruction using pyrite framboid study

Abstract

Jeleniów Claystone Formation (JCF) is one of the 17 lithographic formations distinguished in Ordovician of the Holy Cross Mountains. The formation, with a thickness of up to 120 m, consists of dark-gray to black claystones and locally occurring gray claystones with a green shade. The stratotype of JCF (cores from the boreholes: Jeleniów-2, Wilków IG–1 and Daromin IG–1) Caradoc in age is stored in the archives of the Polish Geological Institute - National Research Institute, Holy Cross Mountains Branch in Kielce. The aim of this research is to reconstruct palaeoredox conditions during sedimentation of the Jeleniów Claystone Formation deposits, using framboid pyrite diameter measurements. Analysis of pyrite framboids diameter distribution is an effective method in the palaeoenvironmental interpretation which allow for a more detailed insight into the redox conditions, and thus the distinction between: oxic, dysoxic, anoxic and euxinic conditions in the water column. Ten samples were collected from the Zbrza PIG 1 borehole, comprising various claystones and black shales. Pyrite framboids are common in all the Sandbian claystones. In every sample 100 measurements have been carried out. Most of the samples is characterized by framboid indicators typical for anoxic/euxinic conditions in the water column, with average (mean) values ranging from 5.29 to 6.02 μm and quite low standard deviation (SD) values ranging from 1.49 to 3.0. The remaining samples have shown slightly higher values of framboid diameter typical for upper dysoxic conditions, with average values (6.37 to 7.20 μm) and low standard deviation (SD) values (1.88 to 2.88). From the depth of 75.5 m till the shallowest part of the Jeleniów Claystone Formation, two samples have been examined and no framboids has been detected. Because secondary weathering should be excluded due to core samples, the lack of framboids possibly indicates oxic conditions in the water column. Oxic conditions continue within the Wólka Formation based on the lack of framboids in the examined samples. Our data is consistent with that obtained on the basis of trace element study (Zhang et al., 2011).

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015