Primary Sedimentary Structures Formed by Turbidity Currents and Related Resedimentation Mechanisms
John E. Sanders, 1965. "Primary Sedimentary Structures Formed by Turbidity Currents and Related Resedimentation Mechanisms", Primary Sedimentary Structures and Their Hydrodynamic Interpretation, Gerard V. Middleton
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Primary sedimentary structures from so–called “turbidites” (deposits made by turbidity currents) may be divided into two groups: (1) those that were formed by deposition from turbulent suspensions, and (2) those that were not formed by deposition from turbulent suspensions. Structures in Group 1 include: (a) scour marks made on cohesive mud bottoms; (b) syndepositional deformation structures in fine–grained sediment; (c) structureless fine–grained beds; and (d) traction–plus–fallout structures (plane parallel laminae, “ripple drift with deposition from above,” and convoluted laminae). Structures in Group 2 include: (a) heterogeneous structureless beds; (b) cross–strata; (c) certain syndepositional structures in sands; (d) bedding surfaces showing linear tool marks without scour marks; (e) inversely graded beds; (f) coarse–grained graded beds; and (g) beds of coarse–grained sediment that are more delicately adjusted to bottom microrelief than interbedded layers of fine–grained sediment.
Because turbidity currents are defined as density currents caused by turbulently suspended sediment, it is here argued that only deposits of Group 1 should be classified as turbidites.
Previous usage has assigned structures from both groups to “turbidites,” but a revised nomenclature is presented that assigns structures of both groups to the process of resedimentation. Turbidity currents comprise the “suspended–load” part of resedimentation, whereas other processes, including slumps, Aowing–grain layers, and moving viscous suspensions, comprise the “bed–load” part of resedimentation. Structures of Group 2 are inferred to originate by “bed–load” resedimentation.
Three implications of the present interpretation are discussed: (1) The so–called fixed sequence of bedding types in “turbidites” should be modified to include more varieties, based on different proportions of “bed–load” parts and “suspended–load” parts of resedimented deposits as well as on differential distance of travel from the origin of resedimented deposits with equal proportions of these two parts. (2) The conditions of drag at the base of a turbidity current vary widely and range from low drag where the bottom is cohesive, to large drag where the bottom consists of cohesionless grains, and also include zero or “negative” drag where a more rapidly moving flowing–grain layer is in motion at the base of the turbidity current. (3) The distinctive features of resedimented deposits are the primary sedimentary structures of Group 2, which are here excluded from turbidites and assigned to the “bed–load” part of resedimentation.