Large-scale carbonate beds interbedded in basin-plain turbidites of the Eocene Hecho Group, northern Spain, are interpreted as giant turbidity-current deposits because of their internal organization and lateral extent. One such bed, the Roncal Unit, has been traced down the axis of the basin for 75 km and commonly reaches thicknesses in excess of 100 m. Internally, the Roncal Unit displays an overall upward decrease in grain size, from a basal megabreccia with large slabs of carbonate platform debris to a calcareous mudstone. The Roncal Unit was probably deposited from a highly mobile and competent sediment gravity flow generated by an earthquake shock, which led to the catastrophic collapse and subsequent transport of the upper part of a carbonate platform flanking the basin.

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