Origin of a zone of anomalously high porosity in the subduction inputs to Nankai Trough

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doi: 10.1016/j.margeo.2015.01.004
Author(s): Hüpers, A.; Ikari, M. J.; Dugan, B.; Underwood, M. B.; Kopf, A. J.
Author Affiliation(s): Primary:
University of Bremen, Center for Marine Environmental Sciences, Bremen, Germany
Rice University, United States
New Mexico Institute of Mining and Technology, United States
Volume Title: Marine Geology
Source: Marine Geology, Vol.361, p.147-162. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0025-3227 CODEN: MAGEA6
Note: In English. 61 refs.; illus., incl. 1 table, sketch map
Summary: One poorly understood feature of the subduction inputs to the Nankai Trough subduction zone (SW Japan) is a stratigraphic interval with an anomalously high porosity zone (HPZ), which is up to 240 m thick and located within the clay- and volcanic ash-rich Shikoku Basin facies. To investigate the origin of their peculiar physical properties, we integrated logging-while-drilling (LWD) data, shipboard density measurements, and visual descriptions of core samples recovered from four drill sites of the Ocean Drilling Program and Integrated Ocean Drilling Program. We combined those observations with scanning electron microscopy (SEM) and laboratory consolidation tests on both HPZ samples and artificial mixtures of ash (glass shards) + smectite and vesicular pumice + smectite. LWD data indicate that the HPZ mudstones have a large proportion of dispersed volcanic ash (∼ 20-30%). The consolidation tests show that the rate of porosity loss with increasing effective stress (consolidation behavior) is consistent among HPZ specimens and matches artificial mixtures containing up to 60% volcanic material. However, absolute values of porosity remain higher for HPZ samples compared to artificial mixtures, so processes in addition to the mechanical effects of volcanic clasts must be contributing. We suggest that hydration and partial dissolution turns clusters of volcanic glass into aggregates with distinct microfabric. SEM images confirm the presence of strengthened grain-to-grain contacts, which probably inhibit the collapse of the intra-aggregate pore space. The aggregates behave like grains, so that cohesive strength of the bulk sediment and rate of porosity loss remain nearly unchanged during burial. The two-step diagenetic process of dissolution and precipitation depends critically on a threshold abundance of fine-grained dispersed volcanic ash/pumice. Older units in the Shikoku Basin with only traces of dispersed ash show no such effects. HPZs should be expected in other subduction zones with similar compositional and diagenetic prerequisites. Abstract Copyright (2015) Elsevier, B.V.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Asia; Cenozoic; Clastic rocks; Consolidation; Continental margin; Cores; Electron microscopy data; Expedition 319; Expedition 322; Expedition 333; Expedition 338; Experimental studies; Far East; Gamma-ray spectra; IODP Site C0011; IODP Site C0012; Integrated Ocean Drilling Program; Japan; Laboratory studies; Leg 190; Leg 196; Lithostratigraphy; Marine sediments; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; ODP Site 1173; ODP Site 1177; Ocean Drilling Program; Pacific Ocean; Physical properties; Porosity; Quaternary; SEM data; Sedimentary rocks; Sediments; Shikoku Basin; Spectra; Strength; Subduction zones; Tertiary; Volcaniclastics; Well logs; West Pacific
Coordinates: N321500 N321500 E1350200 E1350200
N313900 N314000 E1340100 E1340000
N324944 N324945 E1365256 E1365254
N324453 N324453 E1365501 E1365501
Record ID: 2015083982
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands