Conditions for formation of massive turbiditic sandstones by primary depositional processes

It can be shown from bedform stability diagrams, that each turbidity current, regardless of whether its density is low or high, or whether its history is simple (e.g., surging flow) or complex (e.g., sustained flow), should pass through a final waning phase in which flow conditions are prone to bedform development. Yet, many thick deep-marine sandstones of supposed turbidity-current origin are totally devoid of tractional structures, such as plane-parallel lamination and ripple cross-lamination. TDURE, a mathematical model for the calculation of duration of sediment deposition from high-density turbidity currents (HDTCs), is used to show that the absence of Bouma T-b, and T-c. divisions cannot be explained by rapid temporal flow deceleration only. The model calculations imply that discernible tractional structures are formed, even at residence times within the upper-stage plane bed and ripple stability regimes of several seconds and several tens of seconds, respectively. Therefore, other explanations have to be sought for the absence of lamination in turbiditic massive sandstones. These include incompatible grain size, post-depositional removal or obstruction of lamination by bioturbation and liquefaction, and erosion of laminated divisions followed by rapid burial. (C) 2004 Elsevier B.V. All rights reserved.