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- volume 17 (2014)
- number 3-4
- Facies characterization of organic-rich mudstones from the Chokier Formation (lower Namurian), south Belgium
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Facies characterization of organic-rich mudstones from the Chokier Formation (lower Namurian), south Belgium
Abstract
In a case study of two wells from the Namur Synclinorium, theblack shale-dominated Chokier Formation was analyzed for petrography, mineralogy and organic geochemistry. Thin section petrography revealed a different facies assemblage for each well and a total of six microfacies types. Whole rock mineralogical data largely confirms a facies-dependent mineral composition. All different microfacies types of locality 1 (MFT-1, laminated silty mudstone; MFT-2, laminated mud-clast-rich mudstone; MFT-3, calcareous bioclast-rich mudstone) and locality 2 (MFT-4, lenticular mudstone; MFT-5, burrow-mottled mudstone; MFT-6, burrowed silty laminated mudstone) show ample evidence of a distal shelf environment that is sourced by currents, which may be linked to seasonal (monsoonal) transport of sediment from land to sea. Erosive bedload transport is an important mechanism of sedimentation whereas accumulation by settling from the water column can be excluded for the majority of investigated strata. Frequently, endobenthic activity and other processes after deposition, e.g. winnowing and reworking, caused strong fabric modifications. Various, but simple ichnofabrics prove at least temporary dysoxic conditions as confirmed by relatively low TS/TOC ratios. With regard to sedimentary features, organic geochemistry data hints to organic matter preservation due to rapid burial rather than intense anoxity. Silicification is a widespread diagenetic feature independent from facies and locality. It is most likely linked to a high supply rate of terrestrially dissolved silica as indicated by SEM observations, paleogeographic constraints and sedimentary features.