Facies Sequences and Downcurrent Changes in Large-Volume Debris and Turbidity Flow Deposits

Pierre Souquet

The thickest sediment gravity flow deposits from the Pyrenean Cretaceous Series are (1) the Breche de Campo in a basin-margin setting, deep valley fill and base-of-slope apron onlapping a base discordance of a third-order depositional sequence; and (2) the Megaturbidites basco-bearnaises in a basin-plain setting (60 m thick and 50 km long), which are carbonate turbidites similar to a Bouma sequence model. Their facies analysis and the use of a few other local summaries allows development of a depositional model.

Various assemblages of constituents and internal structures characterize different kinds of sequences that can be connected with a single transport event and interpreted as debrites, welded debrite-turbidite couplets, and turbidites. Their comparison and their relationship with enclosing strata reveal changes in a set of descriptors that are a function of transport distance by liquefaction and phase separation in large subaqueous flows: matrix, soft-sediment deformation, graded bedding, clast separation, type of turbidites, basal truncation of Bouma turbidites.

From previous field and composition data, a proximal-distal scheme is inferred. It points out a spectrum of sequence types between the two end-members of a coarse-textured debrite, occurring in both lobe and channel forms, and an unchanneled base-missing thick turbidite.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.