50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Abstract

Under the prevailing turbidite paradigm, the term turbidite (i.e., deposits of turbidity currents with Newtonian rheology and turbulent state) is used very loosely and is commonly applied to deposits of debris flows with plastic rheology and laminar state. For example, because high-density turbidity currents are defined on the basis of three different concepts (i.e., flow density, grain size, and driving force), there are no consistent criteria for recognition of their deposits. As a result, deep-water massive sands of debris-flow origin are routinely misinterpreted as high-density turbidites. The concept of waxing flow as a type of turbidity current is problematic because waxing flows are defined on the basis of velocity, not on fluid rheology and flow state. The waxing-flow concept allows inversely graded sands to be misinterpreted as turbidites. Perhaps, the most problematic issue is the use of alluvial channel traction bed forms observed in flume experiments as the analog for the five divisions of the Bouma Sequence (i.e., classic turbidites deposited from suspension). This is because flume experiments were conducted under equilibrium flow conditions, whereas natural turbidity currents deposit sediment under disequilibrium waning flow conditions. This and other problems of deep-water processes and facies models are addressed in this paper from the authors personal perspective. Classification of sediment-gravity flows into Newtonian flows (e.g., turbidity currents) and plastic flows (e.g., debris flows), based on fluid rheology and flow state, is a meaningful and practical approach. Although popular deep-water facies models are based on transport mechanisms, there are no standard criteria in the depositional record to reliably interpret transport mechanisms. According to existing turbidite-facies models, an ideal turbidite bed, which has normal grading, with gravel- to mud-size particles should contain a total of 16 divisions. However, no one has ever documented a complete turbidite bed with 16 divisions in modern or ancient deposits. Recognition of units deposited by deep-water bottom currents (also referred to as contour currents) is difficult. Traction structures are good indicators of bottom-current reworking, but distinguishing deposits of bottom currents from deposits of overbanking turbidity currents is difficult even though it has important implications for developing depositional models for hydrocarbon exploration and production. I consider sandy debris flows to be the dominant process responsible for transporting and depositing sands in the deep sea. Experiments on sandy debris flows suggest that low clay content (as little as 1%) is sufficient to provide the strength necessary for sandy debris flows. Deposits of experimental sandy debris flows are characterized by massive sand, sharp upper contacts, floating clasts, inverse grading, normal grading with clasts, and water-escape structures. As a counterpart to turbidite-dominated fan models suited for basinal settings, a slope model is proposed that is a debris-flow dominated setting with both non-channelized and channelized systems. Contrary to popular belief, deposits of sandy debris flows can be thick, areally extensive, clean (i.e., mud poor), and excellent reservoirs. High-frequency flows tend to develop amalgamated debris-flow deposits with lateral connectivity and sheet-like geometry. Submarine-fan models with turbidite channels and lobes have controlled our thinking for nearly 35 years, but I consider that these models are obsolete. The suprafan lobe concept was influential in both sedimentologic and sequence-stratigraphic circles because it provided a basis for constructing a general fan model and for linking mounded seismic facies with sheet-like turbidite sandstones. However, this concept recently was abandoned by its proponent, which has left the popular sequence-stratigraphic fan models with a shaky foundation. A paradigm shift is in order in the 21st century. This shift should involve the realization that thick deep-water massive sands are deposits of debris flows, not high-density turbidites. However, there are no standard vertical facies models that can be applied universally for either turbidites, contourites, or sandy debris flows. Science is a journey, whereas facies models terminate that journey and become the final destination.