Samples can be divided into two groups (Table T1). Group I contains distinct sedimentary structures derived from such episodic currents as turbidity currents or benthic storms, and Group II contains sedimentary structures associated with a change in color or magnetic susceptibility but not associated with distinct structures in visible inspections compared to Group I. Group II is considered to represent more common sedimentary processes in this region because the frequency of occurrence of these structures is much higher than that of Group I.
In general, features observed on the radiographs are (1) the images getting lighter as the nannofossil (CaCO3) content increases and (2) the images getting lighter as the sediments become coarser. In the following sections, characteristic features observed in X-radiographic prints are described in detail. In this paper, the sediments are described starting at the base of a section and proceeding uphole.
This sample was taken where interbedded silt layers and dark greenish gray (8.5Y 4.8/1.5) clayey silt scour the underlying light greenish gray (2.7GY 5.1/1.3) silty clay (Fig. F1). Magnetic susceptibility values show a relatively large fluctuation in this interval (Fig. F2). Color reflectance values, especially L* and b*, also show a large change in this interval (Fig. F3).
Description of X-Radiographic Print. The characteristic features of this sample are sharp surfaces, which can be interpreted as erosional surfaces, and distinct sharp lines associated with upward gradation of shading in the radiograph, which seems to show fining-upward structures (Fig. F4).
According to the visual core description, the lowermost 0.9-cm-thick layer (Fig. F4, interval a) is massive silty clay. This silty clay is eroded by upper layers. The 2.2-cm interval above this erosional surface (Fig. F4, interval b) contains distinct, sharp light-colored lines. These distinct lines show that the silty sediments were deposited as plane beds. The lower 1.2 cm of this interval (Fig. F4, interval b1), in particular, consists of four layers with lower boundaries that are sharp and bright but gradually darken a few millimeters uphole. This feature in the radiograph seems to show fining-upward structures, although these fining-upward structures were not recognized in visual inspection of the core (Fig. F1). The 1.6-cm interval above this zone (Fig. F4, interval c) is structureless. Further uphole, the 2.2-cm interval consists again of distinct lines showing fining-upward structures (Fig. F4, interval d). The top of this zone is an erosional surface (the boundary between intervals d and e in Fig. F4).
The interval from 90.2 to 92.0 cm (Fig. F4, interval e) consists of coarser sediments. Sharp lines observed in the radiograph seem to show that the coarser material was deposited as discrete beds. The upper contact of this zone is also an erosional surface, but it is fairly undulated. In a 3.0-cm interval above this erosional surface are fining-upward layers ~3 mm in thickness (Fig. F4, interval f). The upper contact of this zone is also a sharp erosional surface that is overlain by coarser material ~1.5 cm thick (Fig. F4, interval g). These coarser sediments do not show distinct sedimentary structures relative to interval e. Further uphole, the uppermost 1.5 cm of this sample shows distinct lines that are destroyed at the center of the core by bioturbation (Fig. F4, interval h). Except for this interval, no significant bioturbation is recognized elsewhere in this sample, which suggests that this 15-cm interval was deposited in a short time.
Distinct sedimentary structures consisting of coarser sediments are not often found in the upper part of Unit I at Site 1056 but are present below 48 mbsf. Sample 172-1056B-6H-5, 51-79 cm (Fig. F5) is a most distinct one (Keigwin, Rio, Acton, et al., 1998, chap. 4, fig. 2). In the lowermost part of the sample (i.e., at 78.5 cm), greenish gray (5GY 5/1) nannofossil clay is scoured sharply by coarser beds of grayish olive (10Y 4/2) sand-sized siliciclastic and biogenic components interbedded with silt and clay. Magnetic susceptibility values show sharp peaks (Fig. F6).
Description of X-Radiographic Print. The lowermost erosional surface is a sharp boundary (Fig. F7B, between intervals a and b), although there is a crack at that contact. Directly above this erosional surface (Fig. F7B, interval b), there are three fining-upward layers between 77.4 and 78.5 mbsf (Fig. F7B, interval b1) in the X-radiographic print, although only two layers can be recognized by visual inspection of the split core (Keigwin, Rio, Acton, et al., 1998, chap. 4, fig. 2). The lowermost layer contains a series of very thin lines, suggesting the existence of multiple plane beds. Around 76 cm in this section (Fig. F7B, interval c), visual core inspection reveals a pair of distinct fining-upward layers; however the X-radiographic print shows that the lower layer (Fig. F7B, interval c1) consists of three fining-upward layers, and furthermore, the lowest layer in interval c1 contains a dipping structure like ripple cross-laminae. Above these layers, there is a 1.3-cm-thick clay layer (Fig. F7B, interval d) containing sharp "hairlines", which are very thin but clear light-colored lines. Further upcore, three fining-upward layers are recognized (Fig. F7B, interval e) and the X-radiographic print shows that the middle layer contains ripple cross-laminae that are 2 mm high (at 73.8 cm). The overlying clay layer (Fig. F7B, interval f) consists of two fining-upward layers and is truncated by overlying coarser layers (Fig. F7B, interval g) that contain dipping cross-laminae. These coarser layers are overlain by a 2-mm-thick mud layer (Fig. F7B, interval h). This clay layer is again overlain by coarser sediments (Fig. F7B, interval i), which thin laterally. The uppermost clayey part (Fig. F7B, interval j) of the interval contains sharp, distinct lines every 2-3 mm that are not clear in visual inspection of the core. Little bioturbation occurs in the interval from 69-79 cm, suggesting that multiple deposition events occurred over a relatively short time.
There is a gap of 3 cm between the intervals shown in Figures F7A and F7B that contains a clay layer continued from the fining-upward layer at the upper end of the lower interval (Fig. F7B, above interval j; shown only in the right part). Thin silt layers appear in the upper part of this clay layer, which are observed in the lowermost part of the interval shown in Figure F7A, interval l, as two light lines. Some burrows filled with silt- and sand-sized materials are observed directly above these silt layers in the close-up photograph (Keigwin, Rio, Acton, et al., 1998, chap. 4, fig. 2). This clay layer in the lowermost part of Figure F7A, interval l, is overlain by 7.6 cm of silt- and sand-sized sediments (Fig. F7A, interval m; 56.9-64.5 cm) including mud clasts. In the X-radiographic print, the lowermost 0.7 cm of this interval (Fig. F7A, interval m1) is slightly lighter-colored and seems to be a little bit coarser material. The overlying 1.9-cm interval (Fig. F7A, interval m2) seems to have plane beds, although they are not clearly defined. There is a burrow in the lower part of this interval. The upper 5-cm interval (Fig. F7A, intervals m3, m4) contains clear lines, suggesting plane bed formation during deposition, but the uppermost 2 cm (Fig. F7A, interval m4) seems to be slightly bioturbated. This coarser interval (Fig. F7A, intervals m1-m4) is overlain by a clayey layer (nannofossil clay mixed sediments) interbedded with silt layers (Fig. F7A, interval n). Linear structures appear in the X-radiographic print in this interval. The uppermost 2.5 cm (Fig. F7A, interval o) of Figure F7A consists of a bioturbated clay-sized sediment (nannofossil clay mixed sediments) layer that becomes gradually lighter in the radiograph. The lower, coarser part of the sample (Fig. F7A, interval m) seems to have been deposited quickly, and this fast deposition of coarse material seems to end gradually toward the uppermost part of this sample.
This interval includes the silt layer that is situated in early marine isotope Stage 8 and is correlatable from Sites 1055 to 1062. Silt layers, <1 mm thick, are observable by visual inspection (Keigwin, Rio, Acton, et al., 1998, chap. 4, fig. 13). The silt layers are interbedded in the interval of light greenish gray (2.5GY 5/1) nannofossil clay and are overlain by greenish gray (7.2Y 4/1) clay with silt and nannofossils. Magnetic susceptibility shows a peak in this interval (Fig. F8), and L* color reflectance values show a negative peak (Fig. F9).
Description of X-Radiographic Print. The silt layers are observed as bright lines ~2-3 cm above the bottom of the sample (Fig. F10, interval b). These lines show lateral changes in lightness, possibly because of the coarseness of the silt grains. These silt layers are overlain by a slightly bioturbated 1.3-cm-thick clay layer (Fig. F10, interval c). The silt layers, which overlie the clay layer, consist of three layers (Fig. F10, interval d). The top layer appears clearest compared to the lower two layers. These silt layers are overlain by a bioturbated 1-cm-thick clay layer (Fig. F10, interval e). This clay layer is again scoured sharply by the silt layer at 49.7 cm in depth. This silt layer appears most clearly in the radiograph. Above this silt layer, at least five sharp hairlines are observed in the X-radiographic print, although they are bioturbated a little (Fig. F10, interval f). The remaining upper part of this sample (Fig. F10, interval g) is heavily bioturbated so that no clear structures are observed. This set of silt layers (49-55 cm) may have been deposited by multiple events that occurred over a relatively short time.
Unit II in Hole 1062A (75-150 mbsf) often includes carbonate turbi-dites, which consist of well-preserved foraminifers (e.g., Keigwin, Rio, Acton, et al., 1998, chap. 5, fig. 10). The overlying Unit I does not include foraminiferal sand. The foraminiferal sand layers (i.e., carbonate turbidites) appear again at 177 mbsf and in the deeper part of Unit III in Hole 1062B as well. These carbonate turbidites appear at both the eastern and western flanks, and some of them can be correlated to each other (Keigwin, Rio, Acton, et al., 1998, chap. 5, table 3). These carbonate turbidites become thinner downhole, and this sample is taken from one such thin layer (Fig. F11). By visual inspection, at least five foraminiferal sand layers are recognized (Fig. F11, intervals i-v). Based on the biostratigraphic tie points, the age of this interval is ~1.4 Ma (early Pleistocene) (Keigwin, Rio, Acton, et al., 1998, chap. 5, fig. 66).
Description of X-Radiographic Print. All five visible foraminiferal sand layers have sharp bases (at 117.5 cm [Fig. F12, interval v], 120.3 cm [Fig. F12, interval iv], 121.0 cm [Fig. F12, interval iii], 121.5 cm [Fig. F12, interval ii], and 122.7 cm [Fig. F12, interval i]), suggesting that the flows had some degree of intensity. There are many hairline structures between these layers, especially between the upper two layers (117.5-120.3 cm). The lower parts of these foraminiferal sand layers, especially the lower three layers, are relatively unclear because of bioturbation. This means that these five foraminiferal sand layers were not deposited all at once but at intervals between layers. The lowermost part (Fig. F12, interval a) of this sample is massive, bioturbated clay. This clay is truncated very sharply, and there are some hairline structures in a 2-mm interval between the erosional surface of the top of the clay and below the first foraminiferal sand layer (Fig. F12, interval i). This seems to indicate that a fairly intense flow preceded the first sedimentation of carbonate turbidites. The middle 6 cm (Fig. F12, interval b) of this sample consists of the carbonate turbidites described above. The overlying, uppermost part (Fig. F12, interval c) of this sample has obscure line structures that are destroyed by intense bioturbation. The middle foraminiferal sand layers seem to have been deposited quickly compared to the upper and lower parts of this sample.
The sediments of Site 1064 are characterized by sharp color changes (e.g., Keigwin, Rio, Acton, et al., 1998, chap. 6, fig. 7) compared to other deep sites (i.e., Sites 1062 and 1063). The core that contains these samples consists of brown to reddish brown clay. Some of the sharp color changes are overlain by silt laminae or silty intervals (Fig. F13A). Sample 172-1064A-1H-6, 88-103 cm, is taken from the sharp color contact with overlying silt layers, and Sample 172-1064A-1H-4, 120-130 cm, is taken from the sharp color contact without overlying silt layers in the visual inspection (Fig. F13B).
Magnetic susceptibility values show large peaks at both of these sharp contacts (Fig. F14). Color reflectance shows that both a* and b* values increase rapidly. On the contrary, L* values decrease rapidly uphole (Fig. F15; solid arrows). These changes in color reflectance are common for both samples, although the change in Section 172-1064A-1H-4 is smaller in scale. Moreover, color reflectance indicates other sharp boundaries can be predicted in Section 172-1064A-1H-4 (Fig. F15; dashed arrows). Therefore, this kind of change in color reflectance may indicate the abrupt inflow of sediments associated with the turbidity currents at this site.
This sample contains a sharp contact between pale brown (2.0Y 4.4/0.8) clay and overlying reddish brown (0.7Y 4.0/0.9) clay. The contact is overlain by visible silt laminae, ~1 mm thick (Fig. F13A).
Description of X-Radiographic Print. The lowermost 2.4-cm interval of this sample is massive (bioturbated?) clay (Fig. F16A, interval a). The upper contact of this clay is sharply truncated. This part is correlatable to the color change (Fig. F13A). A distinct but discontinuous hairline directly overlies the erosional surface. Above this line, visible silt layers appear as two light lines in the radiograph (Fig. F13A, bottom of interval b). The upper line shows a fining-upward structure. Above these lines, at least 12 units of light and dark layers are found in the 8-cm-thick interval (92-100 cm). These structures can be recognized only on the X-radiographic print, not by visual inspection. Distinct fining-upward layers are recognized at 92-93 cm and 97-98 cm. Some of the units have scoured lower boundaries (s marks on Figure F16A at around 92.5 and 97.5 cm). These scoured surfaces may indicate bigger pauses in the turbidity currents. The uppermost 2.5-cm interval (Fig. F16A, interval c) of this sample appears to be highly bioturbated.
The interval of this sample seems to be a single event in magnetic susceptibility (Fig. F14), but the X-radiographic print reveals that it consists of several events that occurred in a relatively short time.
This sample contains a sharp contact (at 125 cm) between reddish brown (0.5Y 4.1/1.1) clay and overlying greenish brown (5.2Y 4.3/0.7) clay. No visible structures are associated with this contact (Fig. F13B).
Description of X-Radiographic Print. The lower 4.9 cm (Fig. F16B, interval a) of this sample is massive (bioturbated?) clay. This clay is truncated sharply by the overlying 2-mm-thick bright line. The base of this line is correlatable to the visible color change. This layer gradually darkens uphole, suggesting a fining-upward structure. In addition to this layer, a 1.9-cm interval above the erosional surface contains at least eight sharp, bright hairlines (Fig. F16B, interval b). The upper contact of this zone is also a sharp boundary, overlain by a 2-mm-thick dark layer (Fig. F16B, interval c), which has a sharp upper contact. Above this contact, the 2.9-cm uppermost part (Fig. F16B, interval d) of this sample is massive (bioturbated?) clay again. The middle 2.1-cm part (Fig. F16B, intervals b, c) of this sample has no bioturbation, suggesting a high sedimentation rate because of the turbidity currents.
This X-radiographic print tells us that sharp color contacts in this site may be accompanied by sharp plane structures derived from turbidity currents, even if these sharp plane structures are not observed by visual inspection.
Samples in Group II are taken from the sediments deposited during marine isotope Stages 8-10 for the purpose of observing sedimentary structures associated with changes in color or magnetic susceptibility but not associated with visible, distinct structures, as compared to Group I. The samples in this group are considered to represent the more common sedimentary processes in this region.
The following X-radiographic print descriptions start at the bases of samples and proceed uphole.
Sample 172-1055D-5H-5, 19-34 cm (Sample 1), is an interval from medium greenish gray (5.4GY 4.5/0.6) clayey silt to medium dark olive-gray (0.8GY 4.2/1.0) silty clay. Sample 172-1055C-5H-5, 3-18 cm (Sample 2), shows no perceptible color change. This sample consists of medium olive-gray silty clay (from 3.2GY 4.1/1.1 to 4.0GY 4.2/1.0 in the uphole direction). Sample 172-1055D-6H-6, 9-24 cm (Sample 3), consists of light greenish gray silty clay. The visible color changes slightly (from 1.5GY 4.9/1.2 to 1.5GY 4.6/1.1 in the uphole direction). The first two samples are located in marine isotope Stage 8, and the third sample is in marine isotope Stage 9. Magnetic susceptibility values increase in the second and third samples and show a small negative peak in the first sample (Fig. F2).
Description of X-Radiographic Print. Sample 172-1055D-6H-6, 9-24 cm, was bioturbated but not as much as were the other two samples (Fig. F17). There are sedimentary structures, but they are obscure in this sample. The lowermost 2.5-cm interval is relatively light in color, which may reflect relatively higher CaCO3 content, but the upper contact is obscure because of bioturbation. The uppermost 2.5-cm interval of this sample contains obscure dark and light stripes, which suggests that there were primary sedimentary structures, although they have been destroyed by bioturbation.
Sample 172-1055C-5H-5, 3-18 cm, looks homogeneous throughout. No primary sedimentary structures can be observed. Obscure color changes suggesting bioturbation are observed in the lower part of this sample. The upper part is considered to be homogenized by intense bioturbation.
In the lower part of Sample 172-1055D-5H-5, 19-34 cm, a fan-shaped color change is observed. It does not appear to be a primary sedimentary structure but a secondary deformed structure. There is clear trace fossils in this part. The rest of upper part looks homogeneous. There are no distinct primary sedimentary structures, but obscure traces of bioturbation are observed. This may be the result of the intense activity of organisms.
It is notable that the magnetic susceptibility (Fig. F2) and color reflectance (Fig. F3) values are not homogeneous in these 15-cm-long intervals, even though all samples are intensely bioturbated as described above.
Samples 172-1060C-9H-4, 115-130 cm (Sample 1), and 172-1060B-10H-6, 9-24 cm (Sample 4), show no visible color changes, but the change in magnetic susceptibility values is quite large (Fig. F18). In contrast, Samples 172-1060A-9H-4, 10-24 cm (Sample 2), and 172-1060B-10H-5, 25-40 cm (Sample 3), show visible color changes. Sample 4 consists of dark greenish gray clay. No visible color change is observed in this interval (3.8GY 4.5/0.6 to 8.0GY 4.1/0.6). Sample 3 contains color change intervals from reddish brown (2.4GY 4.8/0.6) clay to greenish gray (4.3GY 5.6/0.7) clay with nannofossils. The contact of these lithologies is bioturbated and gradual. Sample 2 also contains a color change interval from relatively light greenish gray (5.7GY 4.9/0.6) nannofossil clay to greenish gray (5.4GY 4.2/0.7) clay with nannofossils. The color change is gradual. Sample 1 is greenish gray clay. There is no visible color change in this interval (5.3GY 4.1/0.3 to 5.1GY 4.0/0.4). L* values change in this interval but not as much as seen in Samples 2 and 3 (Fig. F19). Sample 4 is situated in the end of marine isotope Stage 10. Magnetic susceptibility values reach a slight peak and then decrease abruptly uphole (Fig. F18). Sample 3 is situated in the very beginning of marine isotope Stage 9, where magnetic susceptibility decreases abruptly. Sample 2 is situated ~30 cm above the silt layer, which is in marine isotope Stage 8 and can be correlated from Sites 1055 to 1062 (regarding the correlatable silt layer, see previous description of Sample 172-1059C-7H-6, 41-56 cm, in "Site 1059"). In Sample 2, we begin to observe the increase in magnetic susceptibility. Sample 1 is situated in marine isotope Stage 8.
Description of X-Radiographic Print. Sample 4 contains two light lines in the middle (Fig. F20). These lines coincide with the peak in magnetic susceptibility, suggesting the inflow of sediments that have high magnetic susceptibility. However, these lines are not as sharp as the lines seen in Group I, suggesting that the flow is not fast enough to make plane beds or erosional surfaces. Dark and light stripes are also seen in the lowermost 4-cm interval of Sample 4, although it is bioturbated. The lowermost 4 cm of Sample 3, although bioturbated, shows dark and light stripes. The entire sample is highly bioturbated. The uppermost 2.5 cm of the sample shows a slightly lighter color in the radiograph. In Sample 2, the lowermost 5-cm zone of nannofossil clay is relatively light in the X-radiographic print and becomes darker upcore. It is highly bioturbated and homogeneous throughout. White shadows at both ends of the sample may be pyrite pieces. Sample 1 is highly bioturbated and homogeneous throughout. This interval shows a slight peak in magnetic susceptibility but no remaining sedimentary structures. It is notable, here as well as in Site 1055, that the magnetic susceptibility and color reflectance values are not homogeneous in these 15-cm-long intervals, even though the samples are intensely bioturbated as described above.
Samples 172-1062B-6H-2, 123-138 cm (Sample 1), and 172-1062B-7H-3, 117-132 cm (Sample 4), show no visible color changes, but the change in magnetic susceptibility values is quite large, especially in Sample 4 (Fig. F21). In contrast, Samples 172-1062B-6H-6, 8-23 cm (Sample 2), and 172-1062B-6H-6, 46-61 cm (Sample 3), show visible color changes (Fig. F22) related to the nannofossil abundance, which is typically seen in Unit I of Site 1062. Sample 4 consists of olive-gray clay with nannofossils. No visible color change is observed in this interval (3.6Y 3.6/0.4 to 3.3Y 3.4/0.6). Sample 3 contains a color change interval from light olive-gray (8.6Y 5.1/0.8) nannofossil clay to olive-gray (0.9GY 4.7/0.5) clay with nannofossils. The contact of these lithologies is highly bioturbated. Sample 2 also contains a color change interval from olive-gray (1.1GY 4.5/0.6) clay with nannofossils to light olive-gray (3.2GY 5.4/0.8) nannofossil clay mixed sediments. The contact of these lithologies is also highly bioturbated. Sample 1 is olive-gray clay. There is no visible color change in this interval (0.7GY 4.8/0.2 to 2.0GY 4.6/0.4). Sample 4 is situated in the beginning of marine isotope Stage 9. Magnetic susceptibility values reach a large peak at this point and then decrease abruptly uphole. Samples 2 and 3 are also in marine isotope Stage 9. Samples 2 and 3 are on either side of a peak in magnetic susceptibility (Fig. F21). This peak is situated about 3 m below the silt layer that is situated in marine isotope Stage 8 and can be correlated from Sites 1055 to 1062 (regarding the correlatable silt layer, see previous description of Sample 172-1059C-7H-6, 41-56 cm, in "Site 1059"). Sample 1 is situated in marine isotope Stage 8.
Description of X-Radiographic Print. Sample 4 is highly bioturbated throughout (Fig. F23). There is only one dark line structure, at 4 cm below the top of the sample. This dark line almost coincides with the large peak in magnetic susceptibility values in this sample. The abrupt increase in magnetic susceptibility could have resulted in more structures, but all other structures presumably have been destroyed by bioturbation. In other words, this kind of peak in magnetic susceptibility is not eliminated by bioturbation. The lower 3.5 cm of Sample 3 is lighter than the upper part in the X-radiographic print. The contact with the upper, darker part is highly bioturbated. At 1.3 cm below the top of the sample is a dark line structure, the only one in the upper dark part of Sample 3. The rest is bioturbated. In Sample 2, the lower 6 cm is darker and the remaining, upper part is light. The lower, darker part is intensely bioturbated, and the contact with the upper part is gradual. At 4-5 cm below the top of the sample, there are many sharp, light-colored hairline structures, suggesting fairly intense flows and high sedimentation rates. The overlying part above these hairline structures also shows traces of line structures, although these are destroyed by bioturbation. The lower half of Sample 1 is bioturbated and homogeneous. There is a pair of sharp, dark lines 8.3 cm from the bottom of the sample. Moreover, dark lines are observed upward every 1 cm, although these lines are not as sharp as the lowermost lines and are sometimes bioturbated. These lines are presumed to coincide with the inflow of the same sediments having high magnetic susceptibility as in Sample 4. Samples 1 and 4 indicate that large peaks of magnetic susceptibility may accompany a change in flow conditions, which in turn can leave sedimentary structures in these fine-grained sediments even if the visible sedimentary structures do not exist. Further investigation is needed to determine the intensity of the flows that make these kinds of structures. Comparing Samples 2 and 3 shows a different pattern related to color change. In Sample 3, the color change is very gradual and bioturbated and is not accompanied by any sharp lines. In contrast, in Sample 2 the color change is associated with sharp hairlines. This difference seems to coincide with the variation in both magnetic susceptibility and color reflectance. The rates of change for both variables are greater in Sample 2. The L* values decrease in steps in Sample 3 but increase abruptly in Sample 2. This pattern seems common in marine isotope Stage 9 of Site 1062 and may mean that the amount of nannofossils decreases in steps but increases abruptly. In other words, relatively intense inflows of sediments with large amount of nannofossils might result in the hairline structures observed in Sample 2.