An introduction to the analysis of ancient turbidite basins from outcrop perspective

- In peripheral foreland basins, turbidite systems are characterized by relatively simple depositional settings. The fundamental expression of these systems are huge accumulations of basinal, cyclically-stacked sandstone lobes with an impressive lateral extent. These accumulations, representing potential reservoirs for hydrocarbons, record the main depositional zone of large-volume turbidity currents originated from hyperpycnal flows emanating from flood-dominated deltaic systems along basin margins. Large-scale submarine erosional features, cut into shelfal and slope sediments, acted as conduits of turbidity currents during their basinward motion. Treating turbidite systems in terms of transfer and depositional zones of turbidity currents may avoid much confusion, both conceptually and in practice. Whatever the depositional model, transfer and depositional zones of turbidity currents will exist in each turbidite basin considered. Like in a fluvial system (in the sense of Schumm, 1977), turbidity currents originate (source zone), flow (transfer zone) and will eventually decelerate to the point that all their sediment load will be deposited (depositional zone). Regardless of their size, plan-view and cross-sectional geometry, and facies types, these deposits record the depositional zone of the currents considered. - Turbidite sandstone beds appear to be the deposit of bipartite turbidity currents consisting of a basal, high-density and overpressured granular flow overlain by a low-density, fully turbulent flow. These two flows are the basic components of a turbidity current whose sediment load includes coarse-grained sediment. This concept is supported by both theoretical and experimental work (e.g., Sanders 1965; Ravenne and Beghin, 1983; Norem et al., 1990). Conglomerate, pebbly-sandstone and relatively coarse-grained sandstone facies are the typical deposit of granular flows. The genuine Bouma sequence, as intended in these notes, forms farther basinward and is primarily the deposit of relatively low- density and fully turbulent flows which have outdistanced their parental inertia flows in basinal regions. - A basal, overpressured granular flow, mainly driven by inertial forces and excess pore pressures, is needed to form laterally extensive beds of relatively coarse-grained sediment, i.e. the turbidite reservoir facies. This kind of flow has been described and termed in many different ways (e.g., high-density turbidity current, modified grain flow, debris flow, hyperconcentrated flow, flow-slide), thus generating considerable confusion among sedimentologists and stratigraphers. - Coarse-grained granular flows, probably originated directly from flood-generated subaerial flowslides and their bulking through erosion and sediment failures in delta-front regions, are funneled in relatively steep submarine conduits along which they progressively accelerate until reaching a phase of catastrophic bed erosion. As a consequence, a large amount of fine-grained sediment is incorporated within the upper, turbulent part of each flow, thus increasing its density, thickness and velocity. At this point a fully-developed, bipartite turbidity current is formed. - All other things being equal, the processes controlling sediment transport and deposition within a turbidity current during its basinward motion are essentially related to the ability of the basal granular flow to maintain its excess pore pressure and to the amount of turbulent energy developed in the upper and more dilute part of the flow. The first factor controls the runout distance of the granular flow and therefore the lateral extent of coarse-grained facies. The second factor controls how much finegrained sediment (fine sand and mud) can be resuspended from the basal granular layer and transported farther basinward, and how much traction the turbulent flow can exert on the residual coarser-grained sediment of the granular layer. These two factors combine in determining the degree of efficiency of a turbidity current, i.e. its ability to carry its sediment load in a basinward direction and segregate the original grain populations of this load into distinct and relatively well-sorted facies types with distance. Flow efficiency is therefore of fundamental importance in controlling geometry and reservoir characteristics of turbidite sandstone beds. - Highly efficient turbidity currents dominate transport and deposition in large and elongate foreland basins. Facies tracts reconstructed from these basin fills through careful stratigraphic correlations permit the recognition of the many different processes (erosion, water escape, bypass, reworking, deposition) that characterize a turbidity current during its basinward motion. Facies and processes can thus be framed into models of fairly good predictive value. - Particularly during their earlier stages of development, foreland basins have apparently a relatively simple topography. Turbidity currents accelerate when moving along their steeply sloping conduits and decelerate at the exit of these conduits due to the lateral spreading of the flow. After moving across substantially flat basin-proximal regions (lobe region), where they progressively outdistance their parental granular flows, turbulent flows may reach sufficient thickness to experience reflection from bounding slopes and ponding in terminal basin-plain regions. Structurally complex settings require that facies distribution pattern be considered only on a case-by-case basis. - Turbidity currents may be forced to deposit much of their sediment load without developing a significant segregation of their load if: (1) local topographic obstacles decelerate the currents; (2) parental granular flows cannot reach the catastrophically erosive phase either because they are too thin (small volume) or move on insufficiently steep slopes; (3) parental granular flows cannot maintain their excess pore pressure long enough because of their low mud content. In all these cases, turbidity currents will be poorly efficient, generating facies tracts composed of a limited number of facies, each characterized by relatively poor sorting and, therefore, poor reservoir quality. - Sequence stratigraphy and sedimentary cyclicity of turbidite basin fills has to be treated with great caution. As amply discussed in these notes, turbidite systems of foreland basins are invariably associated with tectonically-induced angular unconformities along basin margins, suggesting that tectonic uplift must play a major role in producing large sediment availability and high-gradient fluvial systems. As suggested by Milliman and Syvitski (1992), Mulder and Syvitski (1995) and Mutti et al. (1996), sediment flux to the sea is dramatically increased in settings of this type through flooding. Sedimentary cyclicity of turbidite systems of foreland basins strongly suggests that climatically-induced high-frequency ciclicity (Milankowich cyclicity) punctuated low-frequency cycles governed by tectonic uplift, subsidence and denudation.