The deep-water Lasarte megabreccia (mid-Cretaceous, Basque-Cantabrian Basin)sedimentological evidences of a growth structure denudation

Abstract

The deep-water Lasarte megabreccia (NE margin of the Basque-Cantabrian Basin) is a 427 m thick unit composed of chaotic, massive breccias with olistoliths up to 75 m long. Planktic foraminifers and ammonites indicate an Upper Albian S. dispar Zone-Lower Cenomanian age for these deposits. Their sedimentological analysis suggests slope failure and rockfall/avalanching and slumping processes, implying erosion of vast amounts of sediment of the upper slope and their accumulation on a deeper slope apron environment. Resedimented deposits show an inverted clast stratigraphy with fragments derived from underlying rock units (up to the Jurassic), suggesting a minimum denudated thickness of 305 m. Structural analysis of a slump indicates a mean palaeoslope dip direction of N147E. In addition, the megabreccia deposits show a southward overall fining and interbedding with contemporaneous mudstones and sandstones, indicating a southward increasing distality. These results together with preliminary mapping of the area suggest the deposition of the Lasarte megabreccia took place in a southfacing slope adjacent to and controlled by the east-trending Errekalde growth structure.