ABSTRACT

Upper Oligocene to lower Miocene deposits of the Tortola fluvial system, Loranca Basin, central Spain, display a labyrinthine stratigraphic architecture comprising isolated sandstone bodies embedded in mudstone and siltstone. These sandstone bodies are subdivided into seven types, which are interpreted to have formed in rivers with different discharges, sediment yields, and channel planform patterns. On the basis of their characteristics and spatial distributions, four facies zones are defined. These formed a continuum along the Tortola fluvial system profile, each being characterized by specific depositional conditions and (consequently) channel-pattern styles. Gravel-rich braided streams dominated Zone 1. Down gradient, these streams graded into wide, sand-dominated, braided streams of Zone 2, which contain large bedforms and were typified by significant discharge variations. Zone 3 comprises belts of mixed-load-dominated meandering and low-sinuosity channels. Zone 4 represents distal floodplain areas at the toe of the fluvial system. Depositional conditions in a facies zone varied between an upper and lower bound, or geomorphic threshold. A crossing of the upper or lower threshold is indicated by relatively sudden but orderly lateral and vertical changes in channel pattern that in turn reflect superposition of facies zones. A facies cycle is formed by superposition of two successive downstream facies zones. Superposition of facies zones is inferred to have been caused by changes in sediment supply and hydraulic regime and consequently the regional gradient of the system. It is argued that the Tortola fluvial system developed under a weak tectonic regime during the later stages of basin filling and was possibly affected by climatic changes. Changes in size of the facies zones typified the long-term development of the Tortola fluvial system and determined the three-dimensional architecture of the succession.