Outcrop Characteristics of a Gradual Avulsion
We mapped three-dimensional outcrop exposures of the Miocene Huesca Fluvial Fan deposits, Ebro Basin, Spain. The Huesca Fan has a radius of ~60 km and drains from the high-relief Pyrenees into the low-gradient Ebro Basin. Fluvial fan deposits change gradually downstream from coarse high-energy braided river deposits, to more fine-grained meandering channels at the fluvial fan fringe.
River channel dimensions in the latter zone are estimated from sandstone body thickness and measured from clay plugs and range from 20-40 m width and 1-4 m depth. The fan fringe sediment bodies have a ribbon to sheet shape, depending on the amount of lateral accretion of the meandering channel prior to abandonment. The sediment bodies have a sharp erosional base and are incised in floodplain mudstone with sparse crevasse splay channel beds. The deposits show a continuous fining-upward grain-size succession from medium-to-coarse sandstone, to very-fine sandstone and siltstone at the top. Sigmoidal-shaped, siltstone-draped lateral accretion surfaces occur in the upper third part of the sediment bodies indicating a migrational pointbar sequence. In cross-sections in the lateral accre-tion direction the sediment bodies end in a clay plug of a thickness much smaller than that of the at-tached pointbar deposits. Towards the clay plug the erosional base of the sediment body gradually steps up. Sediment grain size at the channel base laterally decreases along this step-up, without any abrupt transitions. Channel sinuosity rapidly increases towards the last phase of fluvial channel activ-ity, to the point that the remnant channel erodes part of its previously-deposited point bar.
Observed 3D geometries and lithofacies characteristics are interpreted to reflect decreasing water dis-charge resulting in reduced transport capacity of the river, causing the shallowing of the channel (and consequently the step-up of the erosional lower surface) and the finer-grained sediment load. The sys-tem gradually evolved from an active accretional meandering system capable of carrying significant amounts of bedload, to a small underfit high-sinuosity channel with strongly reduced transport capac-ity. The gradual nature of the shift in channel activity lead us to interpret this as being caused by an upstream partial avulsion, which would point to a wet climatic regime. Our outcrop mapping provide guidelines for reservoir modelers to defining complex facies models in distal fluvial fan settings.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009