Data report: reconnaissance of bulk sediment composition and clay mineral assemblages: inputs to the Hikurangi subduction system

Abstract

This report provides a reconnaissance-scale assessment of bulk mineralogy and clay mineral assemblages in sediments and sedimentary rocks that are entering the Hikurangi subduction zone, offshore North Island, New Zealand. Samples were obtained from three sites drilled during Leg 181 of the Ocean Drilling Program (Sites 1123, 1124, and 1125) and 38 piston/gravity cores that are distributed across the strike-length of the margin. Results from bulkpowder X-ray diffraction show large variations in normalized abundances of total clay minerals and calcite. The typical lithologies range from clay-rich hemipelagic mud (i.e., mixtures of terrigenous silt and clay with lesser amounts of biogenic carbonate) to calcareous mud, muddy calcareous ooze, and nearly pure nannofossil ooze. Basement highs (Chatham Rise and Hikurangi Plateau) are dominated by biocalcareous sediment, whereas most deposits in the trench (Hikurangi Trough and Hikurangi Channel) and on the insular trench slope are hemipelagic. Clay mineral assemblages (<2 μm) change markedly as a function of geographic position. Sediment entering the southwest side of the Hikurangi subduction system is enriched in detrital illite (>60 wt%) relative to chlorite, kaolinite, and smectite. Normalized proportions of detrital smectite increase significantly toward the northeast to reach values of 40–55 wt% offshore Hawkes Bay and across the transect area for Expeditions 372 and 375 of the International Ocean Discovery Program. Introduction Expeditions 372 and 375 of the International Ocean Discovery Program (IODP) drilled five sites on the overriding and subducting plates of the Hikurangi convergent margin, offshore North Island, New Zealand (Figure F1). The project’s overarching goal is to understand the behavior and spatial distribution of slow slip events (SSE) along the plate interface (Saffer et al., 2017). Drilling focused on recovery of sediments, rocks, and pore fluids, acquisition of logging-while-drilling data, and installation of long-term borehole observatories. Interpretations of new compositional results from the transect area are challenging, however, because comparable information is almost nonexistent from other sectors along the strikelength of the margin. The closest Ocean Drilling Program (ODP) sites (1123, 1124, and 1125) are located far seaward of the Hikurangi Trough (Figure F1). Shipboard X-ray diffraction (XRD) measurements were not completed during that expedition, and only one published report contains postcruise compositional data (Winkler and Dullo, 2002). Far to the southwest, XRD data from the Canterbury Basin and Canterbury slope (Land et al., 2010; Villaseñor et al., 2015) are of limited value to Hikurangi studies because those sites capture a different system of detrital sources and dispersal routes off the South Island of New Zealand (Figure F1). The motivation for regional-scale reconnaissance of sediment composition is to provide better context for forthcoming interpretations of detrital provenance, sediment dispersal, and temporal evolution of sedimentary systems. Quantitative compositional data are also important for several ancillary reasons. The geologic hosts for slow slip events near the Hikurangi IODP transect likely include lithified and variably altered volcaniclastic sediments of Late Cretaceous age, but incorporation of other rock types in the fault zone (e.g., siliciclastic mudstone, altered basalt, marl, and nannofossil chalk) is also possible (Davy et al., 2008; Barnes et al., 2010; see the Expedition 372B/375 summary chapter [Saffer et al., 2019]). If chalk and marl are volumetrically significant at the depths where slow slip occurs, then the subducting carbonates might modulate fault-slip behavior by crystal plasticity (Kennedy and White, 2001) or diffusive mass transfer (Rutter, 1976). The purity of the M.B. Underwood Data report: reconnaissance of bulk sediment composition and clay mineral assemblages marl/chalk is a critical variable, however, as is the extent of replacement of primary volcaniclastic constituents by expandable clay minerals (smectite group). How much clay is present in these lithologies, and which clay minerals are dominant? How variable are the lithologies along the strike-length of the margin? This report provides some preliminary compositional information to help answer those important questions. To build an archive of relevant compositional data, samples were acquired for XRD analyses from ODP Sites 1123, 1124, and 1125 (Figure F1) plus a representative distribution of piston/gravity cores along the strike-length of the Hikurangi margin. Prominent bathymetric features targeted by the sampling include (1) the Bounty Channel, which heads off the southeast coast of South Island and directs gravity flows toward the southeast (Lawver and Davey, 2005); (2) submarine canyons emanating from the Cook Strait sector between South Island and North Island (Mountjoy et al., 2009); (3) the Hikurangi Channel, which funnels gravity flows down the axis of Hikurangi Trough (toward the north-northeast) before bending sharply to the east (Lewis and Pantin, 2002); (4) the Ruatoria debris avalanche, which remobilized accreted trench sediments and slope deposits along the northernmost Hikurangi margin (Collot et al., 2001); and (5) two prominent basement highs on the subducting plate, Chatham Rise and the Hikurangi Plateau (Wood and Davy, 1994; Davy et al., 2008). In addition, the array of sampling sites encompasses the region’s most influential ocean current, the Pacific Deep Western Boundary Current (DWBC) (Shipboard Scientific Party, 1999a; McCave et al., 2004). Another reason for reconnaissance-scale sampling was to calibrate shipboard and shore-based XRD computations. The method used during IODP Expeditions 372 and 375 (see the Expedition 372B/375 methods chapter [Wallace et al., 2019]) depends on analyses of standard mineral mixtures (Fisher and Underwood, 1995; Underwood et al., 2003). Data from the standards were used to calculate a matrix of normalization factors with singular value decomposition (SVD), as well as a suite of regression equations that relate values of integrated peak area to mineral abundance (weight percent). Because of differences in XRD hardware, tube fatigue, and software, each individual instrument requires calibration and computation of its own set of normalization factors and/or regression equations. It is also important to blend the “correct” mineral mixtures using individual standards that match as close as possible to the natural mineral assemblages of a particular study area. The “wrong” blend of clay minerals, for example, or the “wrong” crystallinity of calcite will exacerbate errors in their calculated values of weight percent. Underwood et al. (in press) provided details regarding the standard mineral mixtures (both bulk powder and clay size), along with intralaboratory and interlaboratory tests of precision and comparisons of accuracy. The results reported herein were used to inform choices for the Hikurangi-specific standards. Methods Samples A total of 61 specimens from Sites 1123, 1124, and 1125 were acquired from the Gulf Coast Repository in College Station, TX (USA). The lithologies range from hemipelagic mud (defined here as silty clay to clayey silt, or lithified equivalent, with subordinate biogenic carbonate) to marl (defined here as muddy calcareous ooze to calcareous mud or lithified equivalents) and chalk (>75% carbonate). Sample spacing for those specimens was designed to cover a representative spread of burial depths and ages for each site. A total of 91 specimens from 38 piston and gravity cores were acquired Figure F1. Index map of the offshore eastern New Zealand region with major bodies of sediment accumulation and likely pathways of sediment dispersal. Ocean Drilling Program (ODP) sites were drilled during Leg 181. International Ocean Discovery Program (IODP) sites were drilled during Expeditions 372 and 375. CB = Canterbury Basin, CS = Cook Strait, RDA = Ruatoria debris avalanche. o o o o o o 4000m 00