Hydrocarbon-seep carbonates associated with Mesozoic environmental perturbations: Example from the Lower Bajocian of Morocco

Abstract

In the Lower Bajocian of the Jebel Bou Kendill section (Central High Atlas Mountain), three peculiar levels of authigenic carbonates are observed within the carbonate-poor Agoudim III Member. Morphological, sedimentological and geochemical evidences indicate that the two upper levels have been deposited at a hydrocarbon seep site, whereas the lowermost level likely underlines the zone where hydrocarbon fluids were generated. The first seep carbonate level is made of mineralized burrows that have acted as fluid conduits leading to a conspicuous cap carbonate bed on top of which several chimneys and chemoherm structures are embedded. Hence, this carbonate bed is a fossilized sea-floor carbonate crust. The second seep carbonate level is made of chemoherm structures buttressed against the first carbonate beds of the Agoudim IV Member. They were probably formed within the sedimentary column, close to the sea floor.Similar seep carbonates are observed regionally within the Agoudim III Member in the highly subsiding Central High Atlas troughs. A relationship between these seep carbonates occurrence and the late Early Bajocian global environmental perturbation can be deduced, as this latter has led to the deposition of a relatively organic-matter rich sedimentary sequence in these troughs via seawater eutrophication. The decomposition by bacterial activity of this organic matter, aided by rapid burial rates, has led to the generation of alkaline-rich fluids promoting the precipitation of the seep carbonates. A similar relationship between transient environmental perturbations and seep carbonates occurrence is observed in numerous Mesozoic examples and can be explained by the fact that hydrocarbon-seeps are supply-limited systems. Transient deviation from the Mesozoic greenhouse climate norm, either toward hothouse or coldhouse climates, favours the deposition of organic-rich sediments or transient methane storage, respectively, hence increasing the likelihood for the formation of hydrocarbon seepage.