Role of mass transport complexes (MTCs) in evolution and growth patterns of confined basin-floor submarine fans (seismosequences), Mid Eocene, south Spanish Pyrenees Kevin T. Pickering 1 & Jordi Corregidor 2 1 Department of Earth Sciences, UCL (University College London), Gower Street, London WC1E 6BT, U.K., E.mail: email@example.com 2 ERM (Environmental Resources Management), Pau Claris 96, 3o, 1a 08010 Barcelona, Spain ABSTRACT Mass transport complexes (sediment slide - debris flow complexes, or MTCs) in the Mid Eocene deep-marine Ainsa fan systems, Spanish Pyrenees, can be divided into 3 generalised types: (1) Type I MTCs are spatially associated with the most strongly erosive types in terms of total cut-down (up to 10s m for a single complex which may represent multiple stacked events), and appears stratigraphically restricted to the succession around the Ainsa I Fan. The incorporated sediments are entirely intraformational marly and heterolithic sediments. (2) Type II MTCs are characterised by a more sheet-like geometry compared with Type I MTCs. Cumulative erosion at the base is typically only metres, i.e. they are less-confined than the type I. Thin packets (typically dm to a few m) of heterolithics occur along the upper, irregular, surface of these MTCs, infilling the residual topography. (3) Type III MTCs occur as the lateral equivalent of the sandy packets of the Ainsa II Fan, particularly in the older part. This type may contain extra-formational material, e.g. very well-rounded pebbles. Also, this type of MTC contains incipient slide units as disrupted, essentially in situ, fine-grained faces-associations. In the area just north of Barranco del Esta˝uelo, the orientation of the slide folds suggests a down-slope translation from east to west. Since the base of the immediately overlying sandstone packets appears essentially non-erosive, these MTCs probably were the most cohesive type, creating seafloor topography of at least several metres. Thus, the sandy packets do not represent channels, sensu stricto, but highly-confined to confined sandy facies-associations constricted between Type III MTCs. We show that MTCs are the most erosive sediment transport processes within the basin (rather than sandy turbidity currents), and that they exerted a major control on sandy turbidity-current flow routes and depositional architecture within the Ainsa basin.