An analogue of dominance of tectonic over climatic forcing in intermontane coal-bearing basins: Padul (SE Spain)

Abstract

A multiproxy study based on sedimentology, mineralogy, magnetic susceptibility, organic geochemistry and fossil content allowed to establish the sedimentary evolution of palustrine basins and to study the interplay between tectonic and climatic forcings in the sedimentation and facies arrangement that can be extrapolated to other coal-bearing basins. The reinterpretation of the data from 18 cores allowed to determine the facies distribution and to model the sedimentary evolution of the Padul Basin, with the longest continuous continental Quaternary record in the Southwestern Mediterranean region. The Padul Basin sediment record and facies successions provide an outstanding example of tectonically and environmentally controlled sedimentation. The sedimentary characteristics of the cores and thickness distribution revealed that recent tectonics was a more important forcing than other processes (e.g. climate). The influence of recent tectonics was determinant in facies arrangement, that is to say that the activity of a rotated fault produced a differential subsidence, causing more than 100 m of palustrine deposits to stack. On the basis of the sedimentological record combined with data on the mineralogy, fossil content, magnetic susceptibility and total organic carbon, three main sedimentary units were identified, linked to diverse subsidence rates and paleoenvironmental oscillations. The magnetic susceptibility was likely to have been controlled by the organic matter content (TOC values), linked to oxic/anoxic conditions. In the lower part of the record, Unit A (107.0–68.7 m), shallow lacustrine conditions were dominant, with an important influx associated with considerable runoff from active alluvial fans, linked to a higher subsidence rate. Unit B (68.7–37.6 m) had a transitional character, with a considerable decrease in the influence of the alluvial fan system. The lake recharge through overland flow markedly diminished and pre-existing groundwater recharge was dominant. A sudden change to peaty materials indicated that surface runoff was directly diverted to the recently excavated Dúrcal River gorge, thereby precluding the basin from becoming a swampy environment with stagnant waters. Unit C (upper 37.6 m) was almost exclusively fed by bicarbonate groundwater and subsurface runoff through the highly permeable coarse alluvial fan deposits. Within these main sedimentary units, minor mud-peat shallowing-upward sequences were identified.