The Upper Permian Boniches Conglomerates Formation:udevolution from alluvial fan to fluvial system environments andudaccompanying tectonic and climatic controlsudin the southeast Iberian Ranges,udcentral Spain

摘要

The Boniches Conglomerates Fm. (Upper Permian) represents the first episode of sedimentation in the southeastudborder margin of the Iberian Basin, central Spain. The Iberian Basin commenced development as a rift basin runningudmainly NW-SE during the Early Permian and underwent its first general extensional period during the Late Permian. TheudBoniches Fm. was deposited as a small segment (less than 100 km long and 9-10 km wide) of the Iberian Basin thatudextended in a northeast direction during this period.udDetailed mapping and palaeocurrent analysis have shown that the Boniches Fm. is of a curvilinear, wedge-shapedudmorphology. Sediments close to the southwest main faulted border are of a maximum thickness of 230 m. Clear lateraludvariation in thickness controlled by the activity of secondary transfer faults perpendicular to the main NW-SE axis mayudbe observed. This network of faults permitted the formation of blocks, limited laterally by the transfer faults. Theseudblocks experienced subsidence and rotation against the elevated southwest footwall block which resulted in the isolation ofudreduced sub-basins with different sedimentary characteristics.udBased on facies and facies association, the Boniches Fm. is thought to have evolved vertically from two main alluvialudfan systems in the lowest part of the formation, into fluvial braided systems flowing southeast. As expected, no flowudorientations towards the footwall block are observed during the first episodes of the refill. This may be explained byuderosion, or simply a lack of sedimentation due to the gentle slope. The change from alluvial fan to fluvial environments isudinterpreted as a relatively sudden tectonic reorganization of the whole segment favouring new longitudinal fluvial drainageudfrom a transfer zone located to the northwest. Three different reorganization episodes, clearly marked in the Boniches Fm.udas 'main boundary surfaces' (MBS) separate four members subdivided into 4-5 levels (34 m thick). Each level consistsudof fining-upward successions less than 1 m thick.udTectonics and climatic factors clearly controlled sedimentation. Tectonic activity was probably of greatest importanceudand was conditioned by the movement of the main NW-SE and NE-SW transfer faults. The role of climate in ancientudconglomerate sediments has not been well established. Palaeogeographical, mineralogical, sedimentological and clastudanalyses suggest that the Boniches Fm. was deposited under humid conditions by running water. The differentiation between 'proper' alluvial fans and fluvial environments during the transition period of the BonichesudFro. is not clear since, in many recent alluvial fans, braided fluvial systems are associated with fans.