Comparison of Pliocene organic-rich lacustrine sediments in twin craters

The organic matter of Pliocene oil shales from maar-type twin craters (Egyh azaskeszo and Varkeszo) in Hungary was studied by different analytical tech niques (Rock-Eval pyrolysis, bitumen analysis, FTIR, elemental analysis and pyrolysis of the insoluble material). The organic-rich, alginitic layers w ere deposited at the same time, under the same palaeoclimatic conditions an d 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 phenome non can be explained both by variations in organic matter input and changes in the depositional environment. The principal source of the organic matte r is the microalgae Botryococcus braunii, but the terrestrial contribution is also significant. The prevalence of the algal material is supported by t he elemental composition and kerogen pyrolysis data. The pyrograms show tha t there is considerably more algal material in the Varkeszo samples. Keroge ns in the Egyhazaskeszo crater contain much more organic sulphur and pyrite is more abundant. The nominally Type II kerogens in the twin craters are t he products of diverse processes. Varkeszo kerogens are in fact mixtures of Type I and Type III organic matter and are preserved relatively well. Egyh azaskeszo kerogens must have suffered biological degradation and chemical a lteration during pyrite formation, resulting in medium sulphur-rich Type II kerogen formation. (C) 2000 Elsevier Science Ltd. All rights reserved.