COMPOSITIONAL CHANGES IN A MID-STEPHANIAN (KASIMOVIAN) FLORA IN RELATION TO ALLUVIAL PLAIN DEPOSITS DERIVED FROM WESTWARD-RECEDING MOUNTAINS AND BORDERED BY THE PALEOTETHYS: LA MAGDALENA COALFIELD, NORTHWESTERN SPAIN

Abstract

Upper Barruelian, “Saberian,” and Stephanian B (Kasimovian–lower Gzhelian) strata in northwestern Spain represent a strongly subsiding alluvial plain connected to the Paleotethys and adjacent to a mountainous hinterland. The rocks have been deformed into steeply dipping, often isoclinal synclines delimited by foreland-directed thrust faults; a steep basin margin with paleovalleys occurs on the hinterland side. One of these tectonic outliers is the La Magdalena Coalfield, of “Saberian” age, where the quantification of a substantial floral record (140 taxa) from 85 localities in 1200 m of strata allows us to observe changes in floral composition through time matched with broad changes in sedimentary facies corresponding to increased remoteness from a receding basin margin. Within the context of a humid environment, a predominantly fluvial facies changed upward and in time into more generalized semi-lacustrine and peat-forming facies. Although pteridosperms and marattialean tree ferns (pecopterids) are dominant throughout, a proportional increase in tree ferns is observed for the higher part of the succession where calamitaceans become less common and lycopsids more noticeable. These floral changes reflect wetter sedimentary conditions, conducive to the production of coals (high ash) of limited lateral extent.