Protection of organic matter by mineral matrix in a Cenomanian black shale

Three types of pathways (degradation-recondensation, natural sulphurization and selective preservation) are commonly considered for the formation of k erogen dispersed in sedimentary rocks. A fourth pathway has been recently p ut forward, however, from studies on Recent marine sediments, the so-called sorptive protection mechanism. This pathway is based on the adsorption of otherwise labile organic compounds onto minerals, thus preventing their dia genetic degradation and promoting their subsequent condensation into keroge n. The main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a Cenomanian kerogen. They pro vide (i) evidence, on an ancient material, for a crucial role of the minera l matrix both in organic matter (OM) preservation during kerogen formation and in kerogen stability once formed, (ii) indications that the dominant pr otective process likely involves physical protection by minerals, resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness, rather than OM adsorption as molecular monolayers and (iii) obs ervations of the relatively poor stability or an isolated kerogen, contrary to the inertness commonly assumed for fossil macromolecular organic matter . (C) 2000 Elsevier Science Ltd. All rights reserved.