Conceptual consideration and outcrop interpretation on early stage deformation of sand and mud in accretionary prisms for chaotic deposit formation

Abstract

This paper dealt with the summary of the outcrop observation of chaotic deposits (mélanges) in the present and past accretionary prisms and complexes with comparative consideration from the conceptual results of deformation under un- and semi-lithified (weakly lithified) conditions. I stressed the significance of pore-fluid pressure, mud pressure (both are treated as head), and frequency of stress period in order to interpret chaotic deposits and related structures on outcrops. Most important results were to point out that the vice versa relationships in sandstone and mudstone, as in one case sandstone is a clast within mudstone matrix, but in another case of early stages of deformation, mudstone is a clast within sandstone matrix. This is interpreted due to sudden shake of high frequency of period like earthquake shake. In addition, new ideas of explanation were adoptable using different stress – effective stress diagrams as well as the conventional stress – strain diagrams as applied by the experimental results of solid mechanics, e.g. critical state deformation in uniform flow for slump and convolution, and scaly clays that are commonly developed in accretionary prisms. The different modes of deformation in sandstone and mudstone in case of hydraulic fracturing, liquefaction and fluidization, and also unique structures like vein structure and scaly clay formation are implied. These concepts are available to interpret various chaotic deposits occurrences, and to discuss their origin from on land and under the sea.