THE COMPACTION GRADIENT THROUGH A CONTINUOUS MIOCENE TO RECENT FINE-GRAINED VOLCANICLASTIC SUCCESSION AT IODP SITE U1437, IZU-BONIN REAR ARC, JAPAN

Scientific drilling in the rear arc of the Izu-Bonin arc (IODP Expedition 350) recovered an exceptionally complete ~1,800 m-thick succession of volcaniclastic sediments recording deposition from the Miocene to the modern seafloor. Throughout this period, sedimentation in the rear arc Manji-Enpo volcano bounded basin (Site U1437), was remarkably monotonous and rapid, with sustained deposition of bioturbated silt-sized ash / tuff and bioturbated tuffaceous mud / mudstone (collectively “tuffaceous mudstone” lithofacies) at rates of 60 – 120 m/Myr. Tuffaceous mudstone, and its non-lithified equivalent tuffaceous mud, are intercalated with tephra fall deposits, and appear to have been deposited from density currents primarily. The monotonous nature of the tuffaceous mudstone lithofacies lends itself to understanding the role of compaction in lithification and diagenesis through a thick sedimentary succession. The tuffaceous mud and mudstone intervals are often very strongly bioturbated and worm burrows are common. Pumice dropstones (single, very well-rounded, anomalously large lapilli), ash pods (tuffaceous mud only), and fiamme (mudstone only) are uncommon but ubiquitous. All of these features can be used as 2-D strain markers to estimate the 1-D vertical (i.e. compaction) strain gradient. Preliminary analysis shows that these 1-D compaction profiles correlate with downhole measurements of porosity, density, and post-depositional flattening of paleomagnetic inclination. Furthermore, the compaction gradient is remarkably smooth and constant throughout the thickness analyzed, emphasizing the steady-state nature of deposition over a >8 Myr period, and the absence of major structural (compaction) breaks. Here we present an expanded analysis of compaction through the upper 1,300 m at U1437 with a greater density of strain markers, and a more thorough examination of the starting shapes of strain markers and the impact of vertical extension due to coring techniques in the upper ~400 m.