About the Microstructure of the Graptolite Periderm - Examples from the Holy Cross Mountains (Poland)

Abstract

Graptolite reflectance is one of the main parameters for assessing thermal maturity of pre-Devonian rocks in which vitrinite does not occur. However, microstructure of the graptolite periderm has been known only recently, and still little is known about low-reflectance graptolites. The purpose of the work was to determine microstructural features of the graptolite periderm in the oil window shales and compare them to those known from the previous studies, in which more matured organic matter was investigated. Four samples were collected in the Holy Cross Mountains (Central Poland), in two localities (Bardo Stawy and Prągowiec ravine). Reflectance (Rr) measurements as well as micro-Raman analysis were carried out on the sections parallel to the bedding. Micro-Raman analysis was performed on unpolished surfaces to exclude influence of polishing on microstructure. The obtained spectra were curve-fitted using four Lorentzian (G, D1, D2 and D4 band) and one (D3 band) Gaussian lines. Graptolites (Rr=0.70-0.77%; Vitrinite Reflectance Equivalent VRE=0.67-0.72%) occur in the Silurian dark grey clay shales of the Llandovery (Bardo Stawy) and Wenlock - Lower Ludlow (the Prągowiec ravine) age. Graptolite periderm represents poorly organized carbonaceous material, which is inferred from the occurrence of the D3 and D4 bands, and overlapping D2 and G bands. Microstructure does not vary significantly between the samples from the two considered localities (the exception being the D1 band FWHM), which corresponds to the narrow range of graptolite reflectance. Compared to the Raman spectra obtained from graptolites with higher reflectance, the G peak is significantly moved to the lower and the D1 peak to the higher wavenumbers. The FWHM of the G band is higher, while the ID1/IG ratio usually smaller. Taking into account the results from the previous study, it can be concluded that in the reflectance range of 0.7-1.8% the G band position first moves to the higher, and at Rr>1.2% to the lower wavenumbers, while the D1 peak gradually shifts to the lower wavenumbers. The G band FWHM decreases with increasing graptolite reflectance, which reflects improvement in structural ordering, while the ID1/IG ratio starts to increase at Rr≈1.5%, indicating the growth of the coherent domains within the graptolite periderm.