Comparison of Pliocene organic-rich lacustrine sediments in twin craters

The organic matter of Pliocene oil shales from maar-type twin craters (Egyhazaskeszo and Varkeszo) in Hungary was studied by different analytical techniques (Rock-Eval pyrolysis, bitumen analysis, FTIR, elemental analysis and pyrolysis of the insoluble material). The organic-rich, alginitic layers were deposited at the same time, under the same palaeoclimatic conditions and have basically similar lithologies. Despite this, the oil shale deposits from each crater show distinct differences. Furthermore, within each crater, the older oil shale deposits are different from the younger. This phenomenon can be explained both by variations in organic matter input and changes in the depositional environment. The principal source of the organic matter is the microalgae Botryococcus braunii, but the terrestrial contribution is also significant. The prevalence of the algal material is supported by the elemental composition and kerogen pyrolysis data. The pyrograms show that there is considerably more algal material in the Varkeszo samples. Kerogens in the Egyhazaskeszo crater contain much more organic sulphur and pyrite is more abundant. The nominally Type II kerogens in the twin craters are the products of diverse processes. Varkeszo kerogens are in fact mixtures of Type I and Type III organic matter and are preserved relatively well. Egyhazaskeszo kerogens must have suffered biological degradation and chemical alteration during pyrite formation, resulting in medium sulphur-rich Type II kerogen formation. (C) 2000 Elsevier Science Ltd. All rights reserved.