Channel Avulsion and Sediment Aggradation Rate Controls on Fluvial Sandstone Body Stacking Patterns, Miocene, North Spain

Nichols, Gary

Determination of the rates of processes responsible for the creation of the architecture of fluvial channel sandstone bodies is generally difficult. However, in the Miocene of the Ebro Basin of North Spain a detailed magnetostratigraphy of a lacustrine succession in a largely undeformed basin-fill provides a means of determining the rate of aggradation of coeval fluvial deposits. The basin was endorheic, so in order to maintain an approximately constant slope of fluvial profile, lacustrine and fluvial aggradation need to have been in balance: for part of the fluvial succession in the early Miocene an aggradation rate of approximately 90 mm/ky can be determined. The succession was formed by a terminal distributive fluvial system (DFS) which can be completely mapped over an area of 1,800 km2 at the same stratigraphic level. Rates of sediment supply to the DFS can therefore be estimated to be approximately 162,000 m3/y. This is equivalent to a sediment load of 13.6 kg/s for the Miocene river supplying the DFS.

A timescale for the development of the architecture of channel sandstone bodies and alluvial plain deposits can also be derived. The vertical interval between channel sandstone bodies at a point on the DFS is equivalent to the time interval between events when the river channel occupied the same position on the DFS following episodes of avulsion. These avulsion reoccupation intervals vary from approximately 130 ky in medial areas to 270 ky in distal locations, reflecting the increased spacing of channels distally across the radial planfom of the DFS.

In a system like this a decrease in the proportion of suspended load in the system would result in a decrease in aggradation at the toe of the DFS and hence a relative increase in the proportion of channel deposits (and hence connectedness) at any point across the system. The proportion of channel deposits decreases distally and therefore the relative position on the DFS is also important. The architecture - and sandstone body connectedness - in an endorheic basin is hence determined by sediment supply and in particular the proportion of bedload and suspended load supplied to the system. Differential subsidence across the basin may modify the profile, but accumulation in an endorheic basin is largely independent of regional subsidence.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013