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Abstract: Analysis of a deep-sea mass-transport deposit exposed as a nearly 1.6 km continuous outcrop reveals heterogeneous internal structures and existence of a compressional stress field during transportation and deposition. Deposit of gravelly mudstone, containing large deformed sedimentary blocks (long axis from tens of centimeters up to 100 m), occurs in the Upper Cretaceous (Maastrichtian) to Paleocene Akkeshi Formation, Hokkaido Island, northern Japan. The outcrop was photographed and sketched, and clast sizes were measured to study quantitatively the internal structure of this mass-transport deposit. The size distribution of sedimentary blocks exhibits a power-law distribution, but mean size and concentration of blocks exhibit highly variable, local fluctuations.

This mass-transport exposure exhibits three facies, based on size and spatial arrangement of accumulated blocks. Facies A consists of relatively small blocks (long axes approximately 1 to 10 m), supported by a gravelly mudstone matrix. Facies B consists of clast-supported moderate blocks (long axes & 30 m). Generally, blocks in Facies B are deformed significantly. Facies C comprises mainly large blocks with long axes up to 100 m. Considering the evidence of turbidites in blocks of Facies C, these blocks not only slid but also rotated both horizontally and vertically. In some cases, original stratigraphy found within these blocks is inverted. Facies A and B alternate downcurrent, while Facies C occurs only at the more distal end of the exposure.

Usually, long axes of blocks are oriented parallel to the bedding surface, suggesting a laminar state of flow. In addition, application of the multiple inverse method to mesoscale faults observed in the blocks reveals possible internal paleostress fields that existed before deposition. This analysis suggested two different stress fields: (1) a uniaxial compressional stress field, where the a1 axis is oriented normal to bedding surface, and (2) a triaxial compressional stress field, where the a1 axis is oriented parallel to the paleocurrent direction. This mass-transport deposit apparently experienced the first stress field when it moved downslope, thereby expanding its surface area. It then experienced the second stress field as it decelerated, because of compression parallel to paleocurrent direction. A heterogeneous nature of internal structures and compressional stress fields appear to be common features of mass-transport facies of deposits.

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