Previous   |    Next

doi:10.2204/iodp.proc.313.203.2019

Results

To facilitate the description and discussion in this report, a selection of key elements, elemental ratios, and petrophysical logs are plotted downhole for each site. The analyzed interval is divided into zones (Fig. F3). Downhole plots summarizing the complete set of XRF measurements are shown in “Appendix B” (Figs. BF1, BF2, BF3 for XRF-sample measurements).

Hole M0027A

The 63 m interval analyzed in Hole M0027A (238–174 m CSF-B; Fig. F3A) ranges upward from clay at the base (Zone K) through a series of alternating sands, silts, and clays (from Zone J to Zone H) before a final sand-silt-silty clay sequence (Zone G) followed by a longer interval of clay between 208 and 192 m CSF-B (from Zone F to Zone C). Above this clay interval is an interval of poor recovery (10%) with thin clays (Zone B) succeeded by sands and clays shallower than 180 m CSF-B (Zone A). The middle section (Zones C–G) have been densely sampled for XRF-sample measurements (66 of 118 samples), enabling high-resolution pattern recognition.

From the base of the analyzed interval, the change from Zone K (clay) to Zone J (sand) corresponds with a increase in Si/Al and Zr/Rb ratios and a decrease in Th across the boundary. The sands of Zone J display the highest Si/Al ratios. There are several magnetic susceptibility peaks in Zone J, but the resolution of the geochemical measurements makes it difficult to establish the nature of any correlation with Fe. At the top of Zone J, a sample with very high SiO₂ correlates with a clean, coarse sand. Zone I has less data because of low recovery, but the spectral gamma ray downhole logs show a decrease consistent with a sandier lithology. Zone H has a series of fining- upward cycles from sands to clays that are reflected in Si/Al and Rb/Zr ratios and Th values. Zone G is marked by another fining-upward cycle as well as increased Mo, U, V, and Sr. At the upper limit of this zone is a large spike in P, which is evident in both the XRF-core and XRF-sample measurements. The narrow Zone F is defined by a significant increase in magnetic susceptibility and Fe/S ratios before a further increase into Zone E.

Zone E is characterized by considerable variability in a number of elements, including Fe, Mn, Mg, As, Co, and Ni. Although this may be in part because this zone is the most densely sampled for XRF-sample measurements, the equivalent variability in the continuous core measurements (e.g., Fe/S ratios) and the downhole magnetic susceptibility log along with observed banding in the clays suggests this variability is not purely a function of the higher sampling density. A few peaks in As and Co coincide with magnetic susceptibility highs.

Above, in Zone D, magnetic susceptibility, Fe, and Fe/S ratios remain high but are noticeably less variable and are followed by significant decreases in Zone C. Sr increases across this zone. The limited core recovery in Zones B and A make observations more difficult, but Si/Al and Zr/Rb ratios are low in the recovered clays of Zone B and high in the sands of Zone A. Fe/S ratios are high in these clays and increase more in comparison to underlying zones than magnetic susceptibility, but the lower sampling resolution makes it harder to precisely identify trends.

Hole M0028A

In Hole M0028A, a shorter interval (26 m; 254–223 m CSF-B; Fig. F3B) was analyzed than in Hole M0027A, beginning with a fining-upward sequence from sand to silt and clay (Zone G) overlain by a coarse sand in a lower recovery sandier interval (Zone F). The remainder of the analyzed interval is composed of a clay succession between 244 and 223 m CSF-B (from Zone E to Zone A). XRF-sample measurement sampling density is fairly consistent throughout the clays, and XRF-core measurements were taken on all suitable cores.

In Zones H, G, and F, the low resolution of XRF-sample measurements combined with no XRF-core measurements from the sand and silt cores makes trends harder to identify, although there is some variability apparent in Mo, As, Ce, and Si/Al ratios. In Zone F, Si/Al ratios are higher and Sr is high. Magnetic susceptibility is low throughout Zones H–F.

In Zone E, Si/Al ratios are low and Th increases in the clay. Magnetic susceptibility suddenly increases significantly and is variable throughout the zone, with low apparent correlation with Fe when studied at higher resolution. Fe/S ratios display similar variability, and there are some depths where higher As is observed. Mo is higher in this zone.

In Zone D, magnetic susceptibility decreases, although some variability remains. One significant spike matches a spike in Fe/S ratio. Magnetic susceptibility correlates very well with Fe in Zone D and the overlying zones. There are minor increases in P in this zone. The upper boundary with Zone C is marked by a change in Fe/S ratios to an interval of significantly lower variability. A larger P spike occurs in this zone. Zr/Rb ratios and, to a lesser extent, Th and Si/Al ratios increase slightly and are more variable. High As content is observed in one XRF-sample measurement. Zone B is marked by a return to more variable Fe/S ratios. Ce decreases midway through this zone. Zr/Rb ratios are slightly higher and more variable. The base of Zone A is marked by a spike in Si/Al and Zr/Rb ratios and a decrease in Fe, although it is otherwise fairly indistinguishable geochemically from the underlying zone.

Hole M0029A

In Hole M0029A, a greater depth interval (153 m; 479–326 m CSF-B; Fig. F3C) was analyzed than in the more proximal sites. The interval is characterized by sands alternating with thin silts at the base (Zone J) succeeded by a silty sequence with occasional thin sands (Zone I), clays, silts, and thin sands (Zones H–C) before reencountering sandier lithologies near the top of the interval (Zones B and A). In general, silts and silty clays predominate. Continuous intervals of clay are found between 417 and 404 m CSF-B (Zones H and G) and between 399 and 387 m CSF-B (Zone E), as well as shorter sections in the upper 15 m of the interval (in Zones B and A). The density of XRF-sample measurements is reduced in the lower part of the analyzed interval (Zones J and I) and in the sandier lithologies shallower than 360 m (Zones B and A). XRF-core measurements were restricted to a single clay core at 344 m CSF-B (across the Zone B/A boundary), which is inferred to either include the seismic reflector m4.1 surface (see the “Site M0029” chapter [Expedition 313 Scientists, 2010e]) or be located immediately above (Miller et al., 2013).

The base of the analyzed interval in Hole M0029A is marked by a sample from the basal silt with high alkali content and correspondingly lower Si/Al and Zr/Rb ratios than the overlying sands, although the low resolution of XRF-samples in Zones K–I make it difficult to interpret detailed trends. A peak in Fe that correlates with P and magnetic susceptibility peaks identified in both downhole logging and core data) occurs within the silts of Zone I (448 m CSF-B). This peak also corresponds to a cemented layer in the sediments (see the “Site M0029” chapter [Expedition 313 Scientists, 2010e]). In Hole M0029A, parts of the metal core catchers were noted in almost one out of two cores in the analyzed section, so the core magnetic susceptibility must be interpreted with caution (black bars across the magnetic susceptibility track in Fig. F3C). However, where variation can be corroborated by the downhole magnetic susceptibility log (acquired deeper than 404 m CSF-B), the trends can be established with more confidence, but they are notable for generally low values and lack of variability.

The deepest occurrence of clays in the studied interval is in Zone H, where Si/Al ratios increase, alkali elements decrease, and a small peak in Mo and high TOC occurs. Above Zone H, Zones G–D are relatively geochemically homogeneous, although the average sample resolution of one XRF-sample measurement per 1.5 m, with no corroboration from XRF-core measurements, needs to be considered. Significant observations from these zones include an interval with high P and Mo (Zone G), an extremely geochemically consistent interval (Zone F), a slight decrease in Zr/Rb ratios and corresponding increase in the Th log (Zone E), another P peak, and gradually increasing U uphole (Zone D). The changes between clays and silts are rarely coupled with major geochemical changes.

Zone C is marked by an increase in alkali elements from the underlying zone and a small increase in Si/Al and Zr/Rb ratios. This zone is characterized by more variability in P, Fe, and Mo than in the underlying zones. Zone B has lower alkali content (although note the reduced XRF-sample resolution), particularly in one sample near the top, and an increase in Si/Al and Zr/Rb ratios, albeit with differential increases between these ratios in different samples. Mo content is higher at the base of this zone. The change from Zone B to Zone A is marked by a decrease in Si/Al and Zr/Rb ratios, an increase in alkali elements, and a small rise in P. Changes across the boundary are supported by XRF-core measurement data. The remainder of Zone A has low Si/Al and Rb/Zr ratios, high alkali content, some magnetic susceptibility and Fe variation, and gradually increasing As with higher Ce than in the underlying succession.

General observations and correlations

The correlation matrixes of XRF-sample measurements of major element oxides and trace elements show that there are some similar correlations across Holes M0027A, M0028A, and M0029A (left panels in Fig. F2). In particular, Si has a high covarying correlation (>0.75) with Al, K, LOI, and Fe (red squares over element names in Figure F2). Rare earth elements Ce, La, Nd, Sc, and Y (green squares over element names in Fig. F2) are highly correlated in Holes M0027A and M0028A (>0.8) and moderately (>0.6) to highly correlated (>0.9) in Hole M0029A. In all three holes, these elements are moderately to highly correlated with Th, V, Ga, and Ba (pale green squares over element names in Fig. F2). Elements and oxides that have lower correlations (lighter shading on matrix plots, typically <0.4) are Mo, P, U, Ca, Na and Sr (blue squares over element names, Fig. F2).

In Hole M0028A, the sampled sequence was predominantly clay (all but 6 of 21 samples), which results in some different characteristics to the correlations between elements. The correlation matrix shows a general delineation between high to very high correlations among many of the trace elements (Fig. F2B) but with generally low correlations between these and the major element oxides. Correlations among the major elements oxides are similar to the other holes. In Hole M0029A,correlations are typically lower between elements, reflecting the greater range of sampled lithologies.

PCA for Holes M0027A, M0028A, and M0029A (left panels in Fig. F2) indicates that much of the variance in the data is carried by the first principal component (PC1), which is comprised of a large number of elements. However, because PC1 is composed of a fairly even distribution of a number of the elements studied, this does not lead to a simple interpretation, although it likely reflects a detrital component to the signature of most elements. The second principal component (PC2) in both Holes M0027A and M0029A has a grouping of major element oxides and trace elements that include CaO, P₂O₅, Sr, and Na₂O, with a slight difference in Hole M0028A where, in addition to Na₂O and P₂O₅, this grouping also includes As and U (purple shaded areas in Fig. F2). PCA clearly shows the clustering of rare earth elements, which are all toward the top of PC1 and near the middle of PC2 (green shaded areas in Fig. F2). In Hole M0028A, the fact that the analyzed sediment is predominantly clay produces an additional grouping of the major element oxides, including Fe₂O₃ and Al₂O₃ (brown shaded area in Fig. F2B).

Clay sequences

The clay sequences contain much of the petrophysical and geochemical variations observed in the analyzed successions but also include intervals that are extremely homogeneous.

In the clays observed in the two proximal holes (Zone E in Holes M0027A and M0028A), which is distinctive for its high, variable magnetic susceptibility, geochemical data, in particular Fe/S ratios, indicate that although the degree of variation is similar, the magnetization and geochemical signal are responding equivalently to the characteristics of the sediment. In Hole M0027A (Fig. F4A) light and dark bands are very clear in these clays, and it is apparent that the darker bands display higher magnetic susceptibility and the lighter bands show peaks in Fe/S ratios in combination with high Fe and Mn, although Fe/Mn ratios (not shown) are near constant. Si/Al ratios show limited variation even at very fine scales.

There are also intervals of clays that display high but more constant magnetic susceptibility where, in general, magnetic susceptibility and Fe have similar trends. In Hole M0028A (Fig. F4B), clays overlying the interval of highest magnetic susceptibility (Zone D) contains two nodules that display an increase in magnetic susceptibility, Fe, P, and, to a lesser extent, Si/Al ratios. This interval also correlates with an impedance contrast and is noted to be more indurated (Fig. F3B).

Other clays, in particular in Hole M0029A, are more geochemically homogeneous. For example, in Hole M0029A (Fig. F4C) an interval of silt changing to a very homogeneous clay occurs near the top of the analyzed interval (Zone B/A boundary) in the vicinity of the seismic reflector m4.1 boundary. Other than the clear change observed in most geochemical data as the lithology changes from silty at the base to clay just just shallower than 343 m CSF-B, this clay is sedimentologically and geochemically homogeneous.

Top of page   |    Previous   |    Next