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Keywords:

  • Autocyclicity;
  • compensation;
  • scouring;
  • turbidity currents

Abstract

Erosion by turbidity currents changes the morphology of the sea floor. The relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition; this could, in turn, affect flow and deposition patterns in subsequent events. This study investigates shallow, centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the Oligocene Macigno Formation of North-west Italy, where erosion and deposition are variably coupled at the bed scale in a net-aggradational setting. The research focus was on: (i) the recognition of scour edges and erosive surfaces; (ii) quantification of spatial differences in the amount of erosion; and (iii) an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces. Where they can be observed, scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded. A consequence of this de-laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges, giving the appearance of normal conformable bed bases. Despite their cryptic nature, such bedding-parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs (here, 16 bed-parallel scour surfaces were identified in a succession comprising 95 beds). Different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief. It is found that differential erosion is on average under-compensated by differential sedimentation. In some cases, the overlying deposits anti-compensate, being thinner at the location where more erosion has occurred. Unequal spatial distribution of differential erosion in the study area combines with sedimentary under-compensation to result in a trend of accumulating section thickness differences over multiple beds. In one ca 25 m thick package, the maximum cumulative change in lateral gradient during some 20 events reached 0·17°, before being reset by a single event. This process can be interpreted either as a lobe compensation effect, or as a scour enhancement effect, depending on the orientation of the palaeohorizontal datum. If allowed to proceed, the latter process could force the system past a channellization threshold, prompting a change from sheet to channelled architecture. This type of shallow substrate scouring and differential deposition is likely to be an important process in the build-up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea-floor gradients.