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In northwestern Argentina, mid-Pleistocene out-of-sequence thrusting further disrupted the Andean foreland, giving rise to the Lerma piggyback basin within the Eastern Cordillera. Emergent topography along the eastern edge of the basin, as well as in its interior, interfered with preexisting eastwardfl owing river courses. In the northern part of the Lerma basin, rivers temporarily incised across the new topography, yielding wind and water gaps, and in places preserving straths with treads tilted upstream, revealing their tectonic origin. Application of Hack's law shows correspondence between the active and the abandoned channel profi les. Bed profi les of wind gaps are distinct from those of nearby consequent streams. Defeated and defl ected northern streams coalesced into a trunk stream, which maintained an open channel across the eastern bounding ranges. River defeat may have been related to rain shadowing by the growing topographic barrier and retention of easterly derived moisture outside the Lerma basin. In the southern Lerma basin, river courses were not generally capable of sustaining active water gaps. Instead, they were defl ected southward, and their discharges fed ancient Lake Lerma. Lake expansion ensued until water level reached a structural low, through which lake waters outfl owed from the basin and subsequently incised across the eastern bounding ranges. The relative success of northern rivers in incising across the rising topographic barrier is mainly attributed to their greater channel gradients at the point of incision, combined with high rainfall levels. Longitu-dinal channel profi les show that rivers in the northern Lerma basin had approached their base level of erosion before uplift of the eastern bounding ranges. After this disturbance, the system continues to approach a new base level of erosion, modifi ed by sediment aggradation within the basin. Speculatively, mid-Pleistocene out-of-sequence thrusting is attributed to basement uplift in the distal foreland.

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.

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