Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin, North-eastern Spain

Abstract

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North-eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three-dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium-grained to coarse-grained, moderately sorted lithic arenite. In cross-section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium-grained to coarse-grained sandstone with medium-scale trough cross-bedding and large-scale inclined stratasets. Each of Units A and C comprises a fining-up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine-grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low-discharge flows. The adjoining ‘wing’ consists of fine-grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large-scale inclined stratasets up to 3 m in thickness represent four bank-attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross-set in Unit A represents a mid-channel transverse bar. In Unit C, a very coarse-grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid-channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin-derived slump blocks cover the upper bar. The youngest deposit is fine-grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport-limited sediment discharges, possibly attaining transient hyperconcentrated conditions.