A comparative organic geochemical and stable isotope study of the Cenomanian-Turonian organic-rich sediments from Tunisia, Germany and the UK

Abstract

A comparative multidisciplinary study has been undertaken of organic-rich Cenomanian-Turonian (C-T) sediments from Tunisia, Germany and the UK. Three hundred samples were taken from a total of 7 field localities and have been analysed using bulk and stable carbon isotope geochemistry (all samples), palynofacies (most samples) and molecular biomarker and pyrolysis geochemistry (selected samples). Most of the sections are thermally immature. All but one of the sections could be correlated using the organic carbon S13C curve, which shows a strong (<3%o) excursion at the C-T boundary, followed by a gradual return to background values (ca. -25.5%o). Relative to this excursion, there is a slight apparent difference in the stratigraphic development of the organic-rich sediments at different locations. Of the sediments studied, the Balhoul Formation of Tunisia shows the highest concentration and best preservation of organic matter (0.2-8.0% TOC, 50-850 hydrogen index); together with darker colours and more frequent lamination, this suggests a more poorly oxygenated regime than in NW Europe (0.2-4.0% TOC, 6-140 HI), probably related to an upwelling intensified oxygen minimum zone (OMZ) on the south Tethyan margin. Data from the two northernmost (offshore) Tunisian sections indicate poorer preservation, which by modem analogy is attributed to deeperwater deposition below the suboxic core of the OMZ. Methylhopanes and bisnorhopane are abundant in the richest (OMZ core) facies. Principal Component Analysis of kerogen pyrolysates from Py-GC and Py-MS analyses are dominated by the relative variations of aliphatic and aromatic compounds between the sections which largely reflect differences in the preservation of the predominantly marine organic matter (30-98% AOM).