THE COAL-BEARING, CENOZOIC AS-PONTES BASIN (NORTHWESTERN SPAIN) - GEOLOGICAL INFLUENCE ON COAL CHARACTERISTICS

Lignite deposits in the Cenozoic As Pontes strike-slip basin (northwes tern Spain) were formed as a function of specific paleoclimatic condit ions and tectonic evolution of the basin. During the early evolutionar y stages, the presence of active normal faults and thrusts inside the basin resulted in two subbasins with distinct differences in sedimenta ry records, with respect to lignite seam occurrence, thickness, areal extent and lithotype development. In contrast, during the late evoluti onary stages the basin was not split and a more homogeneous sedimentar y record in terms of coal seam occurrence and lithotype characteristic s developed. A total of 26 lignite samples, distributed along the basi n infill, were analyzed by organic petrography and geochemistry. All a re lignite B (ASTM). The lignites deposited during lower basin infill sedimentation (units 1 and 2) are dark, matrix-rich, mainly huminitic brown coals, with minor bright, liptinitic-rich coal lithotypes. The d ark huminitic coals in these units show sedimentological and paleontol ogical evidence of lacustrine influence. Persistent, relatively modera te to low Tissue Preservation Index (TPI) values and high average Geli fication Index (GI) values are characteristic of these limno-telmatic coals. Lignites in the upper basin infill (Unit 4) are almost exclusiv ely matrix-rich, huminitic brown coals. Persistent, widespread, low TP I values and variable, but law, average GI values are also characteris tic of these telmatic coals. The overall coal petrological data trend (TPI, GI and widespread development or absence of liptinite-enriched l ignite layers) recorded from the lower to the upper basin infill units agrees with the stratigraphic and sedimentological data, which show a trend of increasingly drier conditions. Development of bright, liptin ite-enriched lignite layers was widespread during the early basin evol utionary stages and was influenced by punctuated water-table oscillati ons. Sedimentological, petrological and organic geochemical data point to the conclusion that, although the paleoenvironments where peat dep osition took place did not undergo dramatic changes, they were affecte d by distinguishable variations (i.e., water hydrochemistry and ground water-level stability), linked mostly to the evolution of basin morpho logy (depending mainly on tectonics) and basin water balance.