Forming conditions of the Graikas lignite, N. Peloponnese

Résumé

In Northern Péloponnèse (southern Greece) several lignite seams occur, many of which were exploited in the past decades. In the small Graikas basin, in the upland Aigialia, a 2.5-m-thick lignite seam outcrops within the Pliocene/ Pleistocene sediments. The pre-Neogene margins and the basement of the basin consist of radiolarites, pelagic limestones, and flysch of the Pindos isopic zone. The sediments filling the basin include marly, sandy and clay layers, which deposited under marine, brackish and lacustrine conditions, during the rifting of the Corinth graben (Late Pliocene-Early Pleistocene). The seam consists of lignite and clay-rich lignite layers, of matrix lithotype, with total thickness of 1.4 m, intercalating with thin marly, silty, clay and humic clay layers. Fourteen lignite samples were obtained for proximate and ultimate analyses and coal-petrography studies. The ash contents of the Graikas lignite range up to 46.2% on dry base revealing intense inorganic input. The total sulphur contents are also high up to 4%. The fixed carbon and volatile matter contents (33.2-46% and 53.9-66.8% on daf, respectively), as well as the H/C and O/C atomic ratios and the reflectance of Eu-ulminite Β (Rm 0.26-0.30%), indicate a low rank (soft brown coal, Weichbraunkohle). Micropetrographic studies revealed a Huminite content >73%, Liptinite <23%, and Inertinite <12%. Attrinite and Densinite are the most prevailing macérais, while Texto-ulminite A and Eu-ulminite A show moderate values. Inertodetrinite and Semi-fusinite dominate within the Inertinite macerai group. Cutinite and Liptodetrinite are the most abundant macérais of the Liptinite group. Mainly clay and carbonate minerals constitute the inorganic matter (7-30%). However, pyrite content is also significantly high, up to 10%, revealing anoxic marine influence. Factor analysis suggests that peat accumulation started under anoxic conditions and intense bacterial activity. The peat-forming vegetation was mostly herbaceous with minor contribution of arboreal vegetation. Moreover, negative correlation is revealed between the gelification degree and the inorganic input. Coal-facies diagrammes suggest low preservation of the organic tissues and highly scattered gelificationindeces. The peat in the Graikas basin started accumulating in a lagoonal environment under brackish conditions behind a barrier. Progressively, sea regraded and the conditions became limnic to limnotelmatic