Heterogeneity of Internal Structures in a Mass-Transport Deposit, Upper Cretaceous to Paleocene Akkeshi Formation, Hokkaido Island, Northern Japan
Published:January 01, 2011
Hajime Naruse, Makoto Otsubo, 2011. "Heterogeneity of Internal Structures in a Mass-Transport Deposit, Upper Cretaceous to Paleocene Akkeshi Formation, Hokkaido Island, Northern Japan", Mass-Transport Deposits in Deepwater Settings, R. Craig Shipp, Paul Weimer, Henry W. Posamentier
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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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Mass-Transport Deposits in Deepwater Settings
Historically, submarine-mass failures or mass-transport deposits have been a focus of increasingly intense investigation by academic institutions particularly during the last decade, though they received much less attention by geoscientists in the energy industry. With recent interest in expanding petroleum exploration and production into deeper water depths globally and more widespread availability of high-quality data sets, mass-transport deposits are now recognized as a major component of most deep-water settings. This recognition has lead to the realization that many aspects of these deposits are still unknown or poorly understood. This volume contains twenty-three papers that address a number of topics critical to further understanding mass-transport deposits. These topics include general overviews of these deposits, depositional settings on the seafloor and in the near-subsurface interval, geohazard concerns, descriptive outcrops, integrated outcrop and seismic data/seismic forward modeling, petroleum reservoirs, and case studies on several associated topics. This volume will appeal to a broad cross section of geoscientists and geotechnical engineers, who are interested in this rapidly expanding field. The selection of papers in this volume reflects a growing trend towards a more diverse blend of disciplines and topics, covered in the study of mass-transport deposits.