Data report: clay mineral assemblages in hemipelagic sediments entering the Sumatra subduction zone, IODP Sites U1480 and U1481, Expedition 362

This report documents the results of X-ray diffraction analyses of 132 mud and mudstone samples collected offshore Sumatra during International Ocean Discovery Program Expedition 362. The clay-size mineral assemblage consists of smectite, illite, chlorite, kaolinite, and quartz. The relative abundance of smectite at Site U1480 decreases downsection from a mean value of 33 wt% in Unit I to a mean of 19 wt% in Unit II; illite increases from a mean of 49 wt% to a mean of 59 wt%. Smectite in Unit III increases to a mean of 73 wt%, and illite decreases to a mean of 19 wt%. Mean values are subordinate (<16 wt%) for undifferentiated chlorite + kaolinite and <7 wt% for quartz in all units. A significant compositional discrepancy occurs between Subunit IIIA at Site U1480 (mean smectite = 64 wt%) and Unit III at Site U1481 (mean smectite = 36 wt%). At Site U1480, the expandability of illite/smectite mixed-layer clays increases downsection, which is opposite to the trend expected with burial diagenesis. The maximum value is 88% within smectite-rich samples from Unit III. Values of the illite crystallinity index are between 0. 42Δ°2θ and 0. 76Δ°2θ, with most data straddling the generic boundary between advanced diagenesis and anchimetamorphism. Illite (060) reflections yield bo values of 8.988 to 9.000, which are indicative of low phengite contents. Smectite (060) reflections display peak apex positions of 61.998°–61.798°2θ, which are consistent with the mineral structure of montmorillonite. The detrital illite fraction contains 46%–60% 2M1 polytype, and the remainder is 1M/1Md. Introduction International Ocean Discovery Program (IODP) Expedition 362 focused on recovering the sedimentary section that will be subducted to seismogenic depths in the northern Sumatran subduction zone, which is part of the 5000 km long Sunda subduction zone system (Figure F1). Regionally, the input materials of this subduction zone are composed largely of trench-wedge and Bengal-Nicobar Fan sediments with a total thickness of 4–5 km (McNeill et al., 2017b). Beneath the fan sediments is a basal pelagic interval that overlies the basaltic crust of the subducting oceanic plate (Figure F2). Two sites (U1480 and U1481) were drilled, cored, and logged about 250 km southwest of the subduction front (Figure F1). The primary drilling objective for Site U1480 was to recover a complete section of the incoming sedimentary section and uppermost basaltic basement (McNeill et al., 2017b). Six lithologic units were identified based on major changes in grain size, bed thickness, and composition (McNeill et al., 2017a; McNeill et al., 2017b). Unit I (0–26.42 meters below seafloor [mbsf ]) is early Pleistocene to Holocene in age and composed of calcareous clay with interbeds of fine-grained sand, silty clay, and minor volcanic ash. Unit II (26.42– 1250.35 mbsf) is late Pliocene to early Miocene in age and contains thin to medium beds of sandy silt and fine-grained sand (turbidites) that transition downsection into claystone, silty claystone, and muddy sandstone. Unit III (1250.35–1327.23 mbsf ) is late Paleocene to late Miocene in age and contains claystone and tuffaceous silty claystone. Unit IV (1327.23–1349.80 mbsf ) is Late Cretaceous to late Paleocene in age and contains basalt, tuffaceous and volcaniclastic sandstone, and volcaniclastic breccia. Unit V (1349.80– 1415.35 mbsf ) is Late Cretaceous in age and contains calcareous claystone, chalk, and intercalated basalt intrusions. Unit VI (1415.35–1431.63 mbsf) is also late Cretaceous in age but composed entirely of basalt, which was interpreted to be the top of igneous oceanic crust (McNeill et al., 2017a; McNeill et al., 2017b). K. Rosenberger et al. Data report: clay mineral assemblages in hemipelagic sediments At Site U1481, the primary drilling objective was to core and log the deeper part of the stratigraphic section because it contains intervals that may become the plate boundary décollement farther landward (McNeill et al., 2017b). Two lithologic units (McNeill et al., 2017a; McNeill et al., 2017b) were correlated with Units II and III at Site U1480 on the basis of age and lithology. Unit II (1149.70– 1360.12 mbsf ) is late Miocene in age and contains claystone, silty claystone, and muddy sandstone. Unit III (1360.12–1498.72 mbsf ) is early to late Miocene in age and consists of claystone (McNeill et al., 2017a; McNeill et al., 2017b). This report summarizes the results of X-ray diffraction (XRD) analyses of clay-size fractions from 132 core samples extracted from Sites U1480 and U1481. Samples were preferentially collected from clay-rich intervals of the cores rather than interbeds of turbidites. Most sample intervals in the split cores were positioned immediately adjacent to whole-round specimens that were extracted for studies of interstitial water geochemistry and frictional-hydrogeologic properties, and clay XRD specimens were co-located in “clusters” together with discrete specimens for bulk powder XRD, carbon-carbonate, grain-size analysis, and moisture and density; additional samples were taken to document significant local lithology changes (e.g., transition from coarser grained to finer grained intervals, decrease in carbonate content, or change in color) to ensure both “representative” and end-member clay mineral compositions. Our primary objective is to document changes in proportions of smectite, illite, chlorite + kaolinite, and quartz in mud and mudstone specimens relative to depth below seafloor and lithostratigraphy. Such information can be used to infer how paleoclimate, detrital provenance, and first-order patterns of sediment dispersal may have changed over time (e.g., Biscaye, 1965; Petschick et al., 1996; Thiry, 2000; Gingele et al., 2001; Phillips et al., 2014). Such information is relevant to the objectives of Expedition 362 because large variations in the proportions of clay minerals may have significant effects on frictional properties and dehydration reactions as Figure F1. Map of eastern Indian Ocean showing locations of Sunda-Sumatra subduction zone, Bengal and Nicobar Fans, and Sites U1480 and U1481. 20° 70°E 75° 80° 85° 90° 95° 100° 105° 110° 15° 10° 5° S 0° 5° 10° 15° 20° N India Bay of Bengal