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doi:10.2204/iodp.proc.313.203.2019

Discussion and correlation across sites

Although XRF classification schemes for sedimentary rocks are less rigorously defined than for igneous formations (e.g., Taylor and McClennan, 1985; Rollinson, 2014), there are a number of long established principles, such as the response of Si/Al ratios to the clay (hydrodynamically light) versus quartz (hydrodynamically heavy) ratio and its use as a grain size indicator (e.g., Pettijohn et al., 1972; Potter, 1978). Zr/Rb ratios (e.g., Dypvik et al., 2001) and downhole spectral gamma radiation content (notably Th for clay; e.g., Rider, 1996) are widely accepted to be useful as grain size indicators in many circumstances. In general, there is a correlation in all three holes between Si/Al ratios, Zr, Rb, and Th with grain size. In both Holes M0027A (Zones H and F) and M0028A multiple series of offshore to shore-face cycles (Mountain et al., 2010) are reflected in fining-upward packages that are recognized in these ratios. In Hole M0029A, these ratios are generally lower and less variable. There are also a few instances where the “grain size” indicators do not vary equivalently. For example, in the clays of Zone E in Holes M0027A and M0028A, Th is more variable than either Si/Al or Zr/Rb ratios, which is reflected by poorer correlation between Si and Th in Hole M0028A where the majority of samples are from clay; Fig. F2B). In Zone B in Hole M0029A, Si/Al and Zr/Rb ratios differ, with a notably high Zr/Rb ratio in the mid-Zone B clay that is not matched by a corresponding increase in Si/Al ratios.

Alkali content is commonly a measure of feldspar content in a sediment (e.g., Rollinson, 2014). The New Jersey sediments throughout all three holes display an enrichment in alkali in comparison with shale standards (e.g., the Post-Archean Average Shale [PAAS]; Taylor and McClennan, 1985; Fig. F3). There is a very strong correlation (>0.9) between La and Th in all three holes (Fig. F2), which is suggestive of monazite. Alkali content falls below PAAS at only a few places that often correlate with very high Si in coarse beach sands (e.g., at the base of Zone I in Hole M0027A) and therefore may reflect a long residence time on the seafloor where feldspar has been lost. The observed enrichment in feldspar may help infer likely source regions of the New Jersey sediments.

Rare earth elements (REEs), of which Ce is illustrated in Figure F3, tend to be highly correlated with each other as well as with Th and V (Figs. F2; green shaded areas). These correlations can respond to the proportion of terrestrial to metalliferous sources (Murray et al., 1991). Here, because of their close correlation with alkali content, which in general covaries with Si/Al ratios, and inference that La and Th are coexisting in feldspar, the main contribution to their signal appears to be detrital.

P greatly increases in several places, notably in Hole M0027A (Zone F) but also to a lesser extent in Holes M0028A (Zones D and C) and M0029A (Zone D). In Hole M0027A (Zone F), thin section and scanning electron microscope (SEM) analysis shows a large number of bone fragments that would account for the increase (A. McGrath, pers. comm., 2016). In Hole M0028A, there are no equivalent spikes in P, with smaller increases observed at cemented nodules (e.g., Zone D; Fig. F4B). High Sr in Hole M0028A (Zone F) correlates with observed shell beds (Mountain et al., 2010). In both Holes M0027A and M0028A, Zone F lies beneath the distinctive clays found in Zone E, and it is possible that these beds were formed during a transgression, which is consistent with the interpretation in Proust et al. (2018) for the formation of the shell beds in Hole M0028A.

The high and variable magnetic susceptibility in Zone E of Holes M0027A and M0028A and its covariance with Fe is in contrast to the successions elsewhere where Fe and magnetic susceptibility generally correspond. Magnetic susceptibility is higher in the darker bands, whereas Fe/S ratios peak in the lighter bands (Fig. F3A). Nilsson et al. (2013) infer the formation of authigenic mineral greigite within this zone.

Elements and ratios classed as redox sensitive include Fe, Mn, Ni, V, Mo, and U, but some of these can be found in both detrital and authigenic minerals, and fully unraveling the geochemical evolution of these sediments is beyond the scope of this report. Mo has been used as an indicator of anoxia (e.g., Dahl et al., 2013). Here, Mo in the clays shows limited variability, although there is some correlation with U in the more proximal sites (Fig. F2A, F2B) and a moderate correlation with TOC measurements where available. Pyrite is an authigenic mineral observed within this zone (see the “Expedition 313 summary” chapter [Expedition 313 Scientists, 2010a]), which is consistent with As spikes (e.g., Zone D in Hole M0027A). The variability in elements and ratios, including Fe/S, Ni, V, Cr, and to a lesser extent Mo in the clay sequence in Holes M0027A and M0028A (notably Zone E) suggests that these sediments were affected by postdepositional redox reactions. In Holes M0027A and M0028A, geochemical measurements were obtained from successions characterized by freshwater (top of interval; 183 m CSF-B in Hole M0027A and 225 m CSF-B in Hole M0028A) (Fig. F3A, F3B, Zone B; Expedition 313 Scientists, 2010a). This is in contrast to Hole M0029A, where much of the succession is characterized by saline pore water (Fig. F3C). There are also intervals of diagenetic cement within these intervals (Pierre et al., 2017; Fig. F3). The movement of freshwater through clays is likely to contribute to postdepositional geochemical changes in the clays of the two more proximal sites.

The geochemical observations discussed here can be placed within the sequence stratigraphic context of the Expedition 313 sediments. Holes M0027A and M0028A are located in the clinoform topsets. The inferred location of the seismic reflector m4.1 surface is placed within Zone F in these more proximal boreholes and likely represents a concatenated or merged transgressive surface and sequence boundary (Miller et al., 2013). This sequence boundary is suggested to correlate with Miocene isotope event Mi4 δ¹⁸O increase (Browning et al., 2013). In Hole M0029A, the seismic reflector m4.1 surface is also likely a merged transgressive surface and sequence boundary (Miller et al., 2013). Here, the sediments were deposited further offshore in deeper water with generally less geochemical variation. In the sequence from seismic reflector m4.5 to seismic reflector m4.4, there is a general shallowing before water depths deepen by ~50 m across seismic reflector m4.1, more than accounted for by the glacioeustatic lowering (McCarthy et al., 2013). In general, geochemical data in conjunction with petrophysical and other observations suggest the clay sequence overlying seismic reflector m4.1 can be correlated across the two more proximal sites and were affected by similar geochemical processes. That these are not apparent in the more distal site could be explained by a potential change in source direction or by the significant sediment bypass above seismic reflector m4.1 noted by Karakaya (2012). By continued analysis of the geochemical story in relation to other observations, a more complete story of this interval is becoming established.

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