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Carbonate productivity and glaciomarine deposits of the Ordovician–Silurian transition display different sedimentary architectures in the Iberian and Hesperian Chains of NE Spain, as a result of quiescent and active extensional tectonics on platforms fringing North Gondwana. The late Katian carbonate productivity of the Iberian platform reflects the onset of bryozoan–pelmatozoan meadows and mud-mound complexes throughout an intra-shelf ramp, whereas carbonate nucleation of prominent carbonate factories took place on the top of isolated palaeo-highs in the Hesperian platform. In both cases, the end of carbonate productivity is associated with glacioeustatic regression, subaerial exposure and karstification, pre-dating widespread precipitation of iron ore deposits in the vicinity of palaeo-highs. The Hirnantian glacioeustatic transgression is represented lithostratigraphically by the Orea Formation. In the Iberian platform, the formation consists of two distinct depositional sequences bounded by the progradation of conglomeratic channels, and is dominated by the record of massive and crudely stratified diamictites, with tabular geometries and deposited subaqueously as ‘rain-out’ facies. In contrast, the Hesperian platform is rich in disrupted diamictites, which form strongly deformed units interpreted as submarine slumps associated with active synsedimentary faults. In both cases, the anomalous occurrence of massive diamictites, rich in boulder- to sand-sized carbonate dropstones, and displaying rapid variations in density and size, suggests that distinct iceberg ‘drift lanes’ existed, indicating current activity in the open sea.

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