Geochemistry and organic petrography of Cretaceous sediments of the Calabar Flank, southeastern, Nigeria

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• 作者：B.O. Ekpo^a ; ^{baekpo@yahoo.com} ; U.J. Ibok^a ; N. Essien^b ; H. Wehner^c
• 关键词：Geochemistry ; Organic petrography ; Sulphate reduction ; Paleoenvironment ; Calabar Flank ; Sediments ; Niger Delta ; Nigeria
• 刊名：Marine and Petroleum Geology
• 出版年：2012
• 期刊代码：37_02648172
• 类别：pw
• 出版时间：August, 2012
• 卷：35
• 期：1
• 页码：252-268
• 文件大小：2275 K

摘要

Detailed bulk geochemistry and organo-petrography of outcrop Cretaceous sediments (with no significant effects of weathering) from the Calabar Flank, southeast Nigeria were performed to understand the organic carbon source, accumulation and degradation, and paleo-climatic, paleoceanographic and paleoenvironmental conditions in West Africa during Early Cretaceous (Aptian) to Maastrichtian times. This study was based on microscopic, elemental analyses (organic carbon, nitrogen, iron and sulphur), Rock-eval pyrolysis and carbon-isotope analyses. In general, the Calabar Flank shales are characterised by highly variable total organic carbon (TOC) contents, which range between 0.1%in Aptian-Albian Mfamosing Limestone and 9.9%in the Awi Formation sediments. The organic matter (OM) is a mixture of immature to early-mature marine and terrigenous OM of types III and IV. This is indicated by low hydrogen indices (HI value (10-190聽mg HC/g TOC), T_max (417-460聽掳C), vitrinite reflectance%R_o (0.39-0.62%R_o), low to high C/N ratios (3.4-1158.0) and high amounts of terrigenous macerals (vitrinite聽+聽inertinite). Based on carbon isotope, C/N ratios and sulphate reduction index (SRI), OM degradation (up to 70%, SRI聽>聽2.5) is most pronounced for shales deposited in a marine environment. The geochemical and petrographic data indicate that local factors such as low bioproductivity, down slope transport and redeposition of sediments from a fluvial-deltaic basin to nearshore facies, shallower, oxic and mildly oxygen-deficient environments, humid-arid paleogeographic conditions, specifically controlled the amount and quality of the OM during Aptian-Mastrichtian stages where marine sediments have been assumed to be deposited during the global anoxic events. Therefore, the order of the main factors controlling OM content in sediments are: input of terrigenous material transported from the land聽>聽low OM productivity by marine photoautotrophs聽>聽low preservation.