Online Journal for E&P Geoscientists

Facies analysis performed during more than ten years in a number of lacustrine and marine basins allowed the distilling of a genetic and predictive facies tract of general application to the analysis of long-lived and coarse-grained hyperpycnal deposits. The understanding of facies types related to hyperpycnal flows represents a deep challenge for sedimentology since river discharges and their associated facies types could be very different with respect to those related to conventional (surge-like) turbidity flows (Mulder and Alexander, 2001; Zavala et al., 2006a). A hyperpycnal flow has a distinctive behavior which will result in the accumulation of non-conventional beds from a classical point of view. Its origin related to a direct fluvial discharge results in a subaqueous flow having characteristics often considered as typical of alluvial sedimentation. A hyperpycnal flow is a land-derived, relatively slow moving, and fully turbulent sediment gravity flow (Mulder et al., 2003), having the ability of carrying basinward interstitial freshwater In contrast to surge like (“classical”) turbidites, hyperpycnal flows have a slow moving and more diluted leading head which will be very sensitive to the pre-existing subaqueous topography. The moving of a hyperpycnal flow will not necessarily require steep slopes since the flow could be maintained as long as the high-density fluvial discharge continues. Therefore, the distance reached by a hyperpycnal flow traveling along a gentle-dipping or near-flat sea bottom will be more dependent on the duration of the related flood event. In contrast with surge-like flows where deposition is dominated by the head, in hyperpycnal flows deposition is dominated by the body (De Rooij and Dalziel, 2001; Peakall et al., 2001). These characteristics allow the preservation in the hyperpycnal deposit of evidences of flow fluctuations that occurred during the passing-by discharge, resulting in the accumulation of composite beds (Zavala et al., 2007). In contrary with classical models of turbidity sedimentation, coarse-grained materials are not transported at the flow head but are dragged at the flow base as bedload related to shear forces provided by the overpassing long-lived turbulent flow (Plink-Björklund and Steel, 2004; Zavala et al. 2006b).  The facies tract here discussed is composed of three main genetically-related facies groups termed B, S and L, corresponding to bedload, suspended load and lofting transport processes, respectively (Figure 1).