Unconfined flow deposits in distal sectors of fluvial distributary systems: Examples from the Miocene Luna and Huesca Systems, northern Spain

Thin sheet sandstone beds in a continental succession may be the products of unconfined overbank flow or deposition in lacustrine environments. In the distal parts of fluvial distributary systems these two settings may be intercalated, recording fluctuations in lake level in response to climatic changes. Field studies of the Luna and Huesca fluvial distributary systems in the Miocene of the Ebro Basin, Spain, are here used to characterise sheet sandstone deposited in distal fluvial and lake margin environments. In this study we document the facies and relationships between 322 individual sandbodies deposited in both alluvial settings as sheetfloods and in lacustrine settings as deltaic lobes. The alluvial sandstone sheets were deposited from lateral and frontal sheetflood events when unconfined flow replaced channelised flow as the main transport mechanism. The term sheetflood is used here to refer to sub-aerial, unconfined, turbulent flow events that undergo expansion, thinning and deceleration with increasing radial distance from source. The depositional process is similar to modern “floodouts” and “terminal splays” of the Channel Country and Lake Eyre, central Australia. Such a process was thought to be the dominant depositional mechanism during lake lowstands. The sheets formed in the lacustrine setting are interpreted as deltaic sediments deposited in a shallow-gradient lake similar to the modern day Volga Delta, Caspian Sea. Shallow-water, friction-dominated, deltaic processes are thought to have been the primary depositional mechanism in the distal sectors of the fluvial systems during lake highstands.