Dynamic Formation of Rugosity on Mass Transport Complexes: Implications for Emplacement Dynamics

Rob Butler¹, Adriana Del Pino Sanchez¹, Bill McCaffrey¹, Joris Eggenhuisen¹, Peter Haughton², Simon Barker², Bill Hakes³, and Gillian Apps^4
1 University of Leeds, Leeds, United Kingdom
² University College Dublin, Dublin, Ireland
³ Britannia Operator Ltd, Aberdeen, United Kingdom
⁴ BHP Billiton, Houston, TX

Submarine mass transport complexes on the modern sea floor often display complex rugosity on their upper surfaces, defined by features interpreted to be rafted blocks, pressure ridges, detached faults and folds. This bathymetric rugosity clearly provides local accommodation space, and thus similar variations in thickness may be inferred for ancient MTCs in the subsurface, associated with varying thicknesses of overlying strata. However, a series of outcrop studies show how the depositional architecture of the overlying sediment may in addition amplify the inherited structure of underlying debrites. Our Macigno case study (Oligocene, Tuscany) is of a debrite (mean thickness 3m) that has necked in zones marked by stretched clasts and shear fabrics. These define down-palaeoslope extension. The sandstones deposited in the overlying hollows (kinematically equivalent to half graben) show growth, evidenced by fanning lamination and differential thickening. Thus, rugosity at the top of the debrite formed by gradual, post-emplacement thinning of the debrite during deposition of the overlying sand. Our Orcieres case study (Oligocene, French Alps) shows similar growth strata above a debrite (mean thickness c. 1m), albeit modified by injection of underlying sand. Both field examples (and others) indicate that substantial soft-sediment deformation can occur at a late stage in the emplacement of MTCs at slow strain rates (creep) such that other depositional processes can keep pace. Not only does this impact debrite recognition in modern seabed sections, it also makes novel predictions of sand body continuity in the subsurface. We compare our outcrop findings with subsurface examples.