The coal-bearing, Cenozoic As Pontes basin (northwestern Spain) : geological influence on coal characteristics

Abstract

Lignite deposits in the Cenozoic As Pontes strike-slip basin (northwestern Spain) were formed as a function of specific paleoclimatic conditions and tectonic evolution of the basin. During the early evolutionary stages, the presence of active normal faults and thrusts inside the basin resulted in two subbasins with distinct differences in sedimentary records, with respect to lignite seam occurrence, thickness, areal extent and lithotype development. In contrast, during the late evolutionary stages the basin was not split and a more homogeneous sedimentary record in terms of coal seam occurrence and lithotype characteristics developed. A total of 26 lignite samples, distributed along the basin infill, were analyzed by organic petrography and geochemistry. All are lignite B (ASTM). The lignites deposited during lower basin infill sedimentation (units l and 2) are dark, matrix-rich, mainly huminitic brown coals, with minor bright, liptinitic-rich coal lithotypes. The dark huminitic coals in these units show sedimentological and paleontological evidence of lacustrine influence. Persistent, relatively moderate to low Tissue Preservation Index (TPI) values and high average Gelification Index (GI) values are characteristic of these limno-telmatic coals. Lignites in the upper basin infill (Unit 4) are almost exclusively matrix-rich, huminitic brown coals. Persistent, widespread, low TPI values and variable, but low, average GI values are also characteristic of these telmatic coals. The overall coal petrological data trend (TPI, GI and widespread development or absence of liptinite-enriched lignite 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, liptinite-enriched lignite layers was widespread during the early basin evolutionary stages and was influenced by punctuated water-table oscillations. Sedimentological, petrological and organic geochemical data point to the conclusion that, although the paleoenvironments where peat deposition took place did not undergo dramatic changes, they were affected by distinguishable variations (i.e., water hydrochemistry and groundwater-level stability), linked mostly to the evolution of basin morphology (depending mainly on tectonics) and basin water balance.