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:
2 ERM (Environmental Resources Management), Pau Claris 96, 3o, 1a 08010
Barcelona, Spain


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.