Turbidity Currents as a Cause of Graded Bedding

Some types of graded bedding, especially minor or isolated occurrences and varved "clays," can be readily accounted for by normal processes of sedimentation. Volcanic eruptions, dust storms, annual and longer climatic rhythms, rejuvenation of relief at the source or filling in of the sedimentary basin, churning up of sediment by storm waves, are among the more obvious potential causes of graded deposits. But the majority of occurrences cannot be explained by any of these processes. The authors believe turbidity currents of high density may be invoked to have supplied the sediment and deposited it in graded beds. The absence or insignificance of current bedding and ripple marking in graded deposits, the deposition of coarse material on the unconsolidated fine-grained top of the preceding bed, the enormous extent of each individual member without apparent change in thickness or character, and the frequent inclusion of angular fragments (even composed of clay) and of redeposited fossils are among the chief characteristics opposing transport and deposition by normal agents. All these and several other features, such as the often observed combination of grading with slumping, are explained by turbidity currents of high density. The graded graywacke series of some geosynclines, coarse graded conglomerates and breccias deposited in deep water, and some deep-sea sands can be accounted for in this manner. Experimental investigations on the nature of turbidity currents and the production of graded bedding are described, and the formation of certain graded rocks from the Apennines are treated in detail. Finally, an attempt is made to give a general elucidation of the formation of graded sediments by the action of turbidity currents of high density.