The Upper Permian Boniches Conglomerates Formation: evolution from alluvial fan to fluvial system environments and accompanying tectonic and climatic controls in the southeast Iberian Ranges, central Spain

Abstract

The Boniches Conglomerates Fm. (Upper Permian) represents the first episode of sedimentation in the southeast border margin of the Iberian Basin, central Spain. The Iberian Basin commenced development as a rift basin running mainly NW-SE during the Early Permian and underwent its first general extensional period during the Late Permian. The Boniches Fm. was deposited as a small segment (less than 100 km long and 9-10 km wide) of the Iberian Basin that extended in a northeast direction during this period. Detailed mapping and palaeocurrent analysis have shown that the Boniches Fm. is of a curvilinear, wedge-shaped morphology. Sediments close to the southwest main faulted border are of a maximum thickness of 230 m. Clear lateral variation in thickness controlled by the activity of secondary transfer faults perpendicular to the main NW-SE axis may be observed. This network of faults permitted the formation of blocks, limited laterally by the transfer faults. These blocks experienced subsidence and rotation against the elevated southwest footwall block which resulted in the isolation of reduced sub-basins with different sedimentary characteristics. Based on facies and facies association, the Boniches Fm. is thought to have evolved vertically from two main alluvial fan systems in the lowest part of the formation, into fluvial braided systems flowing southeast. As expected, no flow orientations towards the footwall block are observed during the first episodes of the refill. This may be explained by erosion, or simply a lack of sedimentation due to the gentle slope. The change from alluvial fan to fluvial environments is interpreted as a relatively sudden tectonic reorganization of the whole segment favouring new longitudinal fluvial drainage from a transfer zone located to the northwest. Three different reorganization episodes, clearly marked in the Boniches Fm. as 'main boundary surfaces' (MBS) separate four members subdivided into 4-5 levels (34 m thick). Each level consists of fining-upward successions less than 1 m thick. Tectonics and climatic factors clearly controlled sedimentation. Tectonic activity was probably of greatest importance and was conditioned by the movement of the main NW-SE and NE-SW transfer faults. The role of climate in ancient conglomerate sediments has not been well established. Palaeogeographical, mineralogical, sedimentological and clast analyses 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 Boniches Fro. is not clear since, in many recent alluvial fans, braided fluvial systems are associated with fans.