Examples of spatial relationships between organic matter and mineral groundmass in the microstructure of the organic-rich Dorset Formation rocks, Great Britain

Abstract

The Jurassic Dorset Formation consists of argillaceous to calcareous rocks that outcrop on the South Coast of Great Britain. This formation exhibits large variations in T.O.C. content (from 2 to about 50%). The organic matter is essentially algal, withh a small contribution of detrital organic debris (mostly ligneous). All the samples are thermally immature. The present study aims firstly to visualize the distribution of organic matter in the rock on a submicroscopic scale, and secondly to characterize its spatial relationships with the mineral matrix. Three samples with increasing T.O.C. contents, from about 9 to 48%, were studied using a combination of several microscopic technics: transmitted, reflected and fluorescence light observations on thin slides and polished sections; optical observations on isolated organic matter (palynologic residue) on strewn slides; scanning electron microscopy (SEM) observations (secondary electrons) on polished sections; pin-point analysis and multi-elemental mapping using EDS and image analysis coupled with SEM (secondary electrons) on polished sections; and Carbon mapping using EDS and image analysis coupled with SEM (back-scattered electrons) on polished sections. The interpretations are based on the comparison and the integration of the different photonic and electron microscopy images. The results are given in the form of composite maps describing the mutual spatial distribution of both the organic and mineral elements in the rocks. The organic matter is found distributed as either well individualized particles, such as ligneous continental fragments and planktonic algal bodies, or as diffuse unfigured organic matter. At the investigated scale, the diffuse organic matter constitutes a continous network through the organo-mineral groundmass. A part of it originates probably from the destruction of algal bodies through bacterial activity and compaction while another part should be attributed to the coccolith sedimentation. The sediment microstructure whatever the organic carbon content might be, exhibits small inorganic zones parallel to the stratification. These are interpreted as microburrows from which the organic matter has been removed. The abundance and distribution of sulphur (pyrite and organic sulphur) suggests that the rate of suphate-reducing activity is not simply related to the supply of metabolizable organic matter.