A synthetic view of the results for Hole 1118A is given in Figure F3, where core recovery, static FMS image, FMS structural interpretation tadpoles, shipboard lithologic units (Table T2), logging units, and structural domains are shown together. Further details on shipboards units can be found in figures F1 (lithologic units), F42 (structural domains), F71 (logs and logging units), and F76 (synthesis) of Shipboard Scientific Party (1999e).
The structural measurement orientation is shown on stereographic projection and on strike and dip histograms for bedding (Fig. F4), fracture (Fig. F5), and unknown structures (Fig. F6).
Only 24 fractures are identified, and all of them belong to the fracture 2 set (Table T3), which means that the determination always had some ambiguity. The fracture distribution is scattered but shows steep 35°-80° dips (Fig. F5).
The FMS bedding dip distribution, based on 589 measurements (Table T3), shows that most dips are <10° (Fig. F4) and are compatible with those obtained from 301 measurements in cores (fig. F43 in Shipboard Scientific Party, 1999e). The dip direction is scattered, with a slight trend toward the south (strike peak = ~90°). The less well defined bed 2 distribution is more dispersed than the bed 1 distribution, as expected, but displays similar mean orientations. The depth dependence of these structures is illustrated in the tadpole plots of Figure F3. Five intervals contain most of the beddings that deviate from the horizontal: 250-340, 390-410, 565-596, 630-651, and 840-890 meters below seafloor (mbsf) (Fig. F3; column 7).
Four of these intervals (250-340, 390-410, 565-596, and 840-890 mbsf) correspond to locations where soft sediment deformation or faulting was observed in core (shown in pink in the structure column of Fig. F3), and three among these (250-340, 565-596, and 840-890 mbsf) strikingly correspond to locations where faults are identified on FMS images. This association of folding with faulting was also noted in the shipboard structural analysis of cores.
Further analysis requires focus on specific depth intervals. Intervals where either larger-scale images or where detailed stereograms and histograms of structure will be introduced in this discussion are marked in columns 5 and 7 of Figure F3, respectively. The discussion will proceed from uppermost to lowermost. The ~1/200 vertical scale full coverage of the hole with a dynamic image given in the "Appendix" (Fig. AF1) can be used to bridge the gap between Figure F3 and the larger-scale figures presented below.
Depth shifts of FMS with respect to conventional log data can reach 9 m near the top but become negligible at the bottom of the hole (see "Appendix"). As a consequence, the discussion below will first describe FMS images and their depth and then correlate them to core and log units with their own depth. In the lower part of the hole, this distinction will vanish.
Bedding is poorly defined (Fig. F7), and few structural measurements are therefore available (Fig. F3) in this interval, which was not cored and is therefore known only from logs. Logging Unit L1 (100-204 mbsf) is characterized by large divergence between neutron and lithodensity porosity, which indicates a high clay content.
The borehole diameter is often reduced below the bit size of 25 cm (9.875 in) in this interval (Fig. F3). The FMS static image is more conductive than in the previous interval and is even more so where the borehole diameter is reduced (Fig. F3). This suggests that clay swelling may have caused these restrictions. Bedding remains poorly defined on the dynamic FMS image (Fig. F8).
This interval corresponds to logging Unit L2 (204-255 mbsf), defined by a higher bulk density than Units L1 above and L3-L7 below and the major seismic reflector (Light Green 1), discussed in Goodliffe et al. (this volume). It also corresponds to the upper part of lithologic Unit I, which begins at 205 mbsf with the first recovered core.
This interval, also within lithologic Unit I (205-378 mbsf), corresponds to logging Units L3 (255-292 mbsf), L4 (292-347 mbsf), and the upper part of L5 (347-402 mbsf).
Within the 247- to 286-mbsf interval, the static FMS image is more resistive than above (Fig. F3) and the dynamic image shows clearly defined thin resistive beds (Fig. F8). These beds are often deformed; dipping and sometimes nonplanar structures are observed, such as those around 281 mbsf (Fig. F9). The corresponding logging Unit L3 was defined by a separation of the two porosity logs, with no detectable clean beds and a slightly elevated PEFL, indicating high clay and carbonate content. The thin resistive beds seen on the FMS may then be due to higher carbonate content. Alternatively, if they are related to lower clay content, their deformation may partly explain why they are not identified on the lower-resolution density and neutron logs.
The static image shows a downward-decreasing resistivity in the 286- to 341-mbsf interval (Fig. F3). Evidence of deformation can be observed but not correlated from pad to pad because of poorly defined bedding (Fig. F10). This corresponds to logging Unit L4, where the two porosity logs are separated but by less than in Unit L3, converging often, indicating slightly lower clay content and thin clean beds. PEFL is lower than in Unit L3, indicating lower carbonate content. The likely cause of lower resistivity is then lower carbonate content because indications point to a clay content decrease.
Soft sediment deformation was observed in core from this interval, with centimeter-sized folds in structural domain Ic (272-320 and 335-396 mbsf) and decimeter-sized folds in domain Ib (320-335 mbsf) (fig. F4 in Shipboard Scientific Party, 1999e).
The numerous dipping beds and the few fractures on the FMS tadpole plot within the 250- to 340-mbsf interval underscore this zone of deformation (Fig. F3). The small scale of the structures observed in core suggests reduction of the analysis interval. Nearly flat beds at 271-277 mbsf allow the definition of two subintervals: an upper interval at 250-280 mbsf, mainly associated with folding, and a lower 275- to 340-mbsf interval associated with both folding and faulting.
A stereographic projection of the poles to bedding in the 250- to 280-mbsf interval suggests a poorly defined horizontal folding axis trending N67°E (Fig. F11).
The same projection of the poles to bedding within the 275- to 340-mbsf interval shows a large scatter with a slight trend along a great circle with a horizontal axis trending N52°E (Fig. F12). That circle almost contains a south-dipping fracture (#3 on Fig. F12) that is located at 309 mbsf (Fig. F3). This suggests a genetic link between folding and fracturing. The fact that most beds dip to the north while the fracture dips to the south suggests reverse drag (i.e., roll over) along the probably normal fault. If we follow this hypothesis, then the northeast fold axis trend with a south-dipping fault would suggest a significant strike component of slip. However, the scatter of the data, the fact that the bed 2 category dominates the bedding measurements, and the fact all the fractures belong to the fracture 2 category, make this highly hypothetical.
The more conductive static FMS image observed in the 341- to 372-mbsf interval (Fig. F3) is consistent with the increased clay content inferred from the higher neutron and lithodensity porosity separation, which defines logging Unit L5 at the bottom of lithologic Unit I.
This interval corresponds to lithologic Unit II (378-492 mbsf) once the depth offset is taken into account. Unit II is described as more sandy in cores. Further subdivision, defined by logs and core structural observations, can be recognized on the FMS images.
The upper interval (372-395 mbsf) corresponds to the bottom part of logging Unit L5. The FMS static image shows a sharp increase in resistivity in the 383- to 392-mbsf interval (Fig. F3), which correlates with the 390- to 399-mbsf interval of Unit L5, where the neutron and density porosity curves get closer and the photoelectric effect increases, suggesting the presence of calcareous sands.
In the next 395- to 401-mbsf interval, the image becomes conductive again (Figs. F3, F13). This corresponds to the uppermost 402- to 408-mbsf interval of logging Unit L6 (402-438 mbsf), which is characterized by high gamma ray, low PEFL, and neutron and density porosity convergence, suggesting a radioactive, relatively clean, and therefore sandy or silty, formation.
The resistive 401- to 431-mbsf interval on the static FMS image (Figs. F3, F13) corresponds to the bottom 30 m of logging Unit L6 (408-438 mbsf), which combines high PEFL and neutron density porosity divergence, indicating significant carbonate and clay content. A change is also seen in the dynamic FMS image, which shows flat-lying thinly laminated sediments below 404 mbsf (Fig. F13).
In this instance, the silty upper interval (395-401 mbsf) is conductive because of porosity, whereas the lower interval (401-431 mbsf) is resistive because of its carbonate content and despite its clay content.
A spectacular decimetric slump structure can be observed on the FMS image at ~401 mbsf (Figs. F13, F14). This interval corresponds to high recovery, and many similar structures are observed on the corresponding core Section 180-1118A-21R-4. However, only that seen at 125-148 cm and shown on figure F9 of Shipboard Scientific Party (1999e) matches the size of that observed on the FMS image. It is therefore very likely that figure F9 of Shipboard Scientific Party (1999e) and Figure F14 represent the same object. This would locate it at the bottom of the radioactive silt discussed above.
Dipping beds in the 390- to 410-mbsf interval (Fig. F3) outline a deformed zone that was also observed in core with centimeter-sized folds (domain Ic; 335-397 and 406-474 mbsf) and decimeter-sized folds (domain Ib; 397-406 mbsf) (fig. F9 in Shipboard Scientific Party, 1999e). No fault is identified on FMS images within this interval, but one fault is identified below it at ~415 mbsf (Fig. F3). The stereographic projection of the poles to bedding shows a cluster along a north-south vertical plane that suggests an east-west horizontal fold axis (Fig. F15) that remains poorly constrained because of the small number of data points. The steepest dip (#5 on Fig. F15) corresponds to a limb of the slump at 401 mbsf, as can be seen on the tadpole plot of Figure F14.
The bottom interval (431-485 mbsf), clearly marked by a more conductive static image, corresponds to the upper part of logging Unit L7 (438-682 mbsf), which is characterized by lower PEFL, and thus carbonate content.
This interval is clearly defined by a resistive static image (Fig. F3) and corresponds closely to lithologic Unit III (492-679 mbsf). This higher resistivity is correlated with higher sand content and a downward increase in carbonate content documented in the core and in logging Unit L7. Larger-scale images show the intercalation of conductive layers in a generally resistive formation (Fig. F16).
Curved boundaries observed at 527-528 mbsf (Fig. F17) suggest small-scale slumping.
Dipping structures at 570-610 mbsf (Figs. F3, F18, F19) correspond closely to a zone of sediment folding (domain Ic; 560-584 mbsf) and faulting (domain IIa, fault zone 1; 584-608 mbsf) observed in core (figs. F16 and F46 in Shipboard Scientific Party, 1999e). The interval is reduced to 570-596 mbsf after eliminating unknown measurements. It presents one of the clearest association of dipping beds and fractures on the FMS images (Figs. F3, F18, F19). The six fractures identified on FMS in this interval represent a quarter of all fractures identified in Hole 1118A (Table T3).
The orientation plots (Fig. F20) show a cluster of bedding poles along a great circle with a horizontal axis trending N97°E. This circle contains the pole to the fracture at 578.1 mbsf and is close to that at 586.8 mbsf. This again suggests the folding may be related to drag along the faults. The north-northeast bedding dip with a north-northeast dipping fault suggests a normal drag fold, and the east-west horizontal fold axis suggests a mainly dip-slip movement.
The bottom of lithologic Unit III shows very conductive thin beds (Fig. F21) with a different facies than that observed higher uphole (Fig. F16).
A few dipping beds are measured in the 630- to 651-mbsf interval where no deformation is observed in core (domain Ia; 608-714 mbsf). The orientation plot shows a slight (at most 15°) dip toward the southeast (Fig. F22).
The transition at ~679 mbsf is marked by lower resistivity on the static FMS image (Fig. F3).
This interval corresponds to lithologic Unit IV (679-811 mbsf) and logging Unit L8 (682-857 mbsf), with lower carbonate content.
The poorly recovered low-resistivity 692- to 732-mbsf interval (Fig. F3) corresponds to the 690-to 730-mbsf interval on logs where neutron and density logs converge, indicating a sandy formation (fig. F71 in Shipboard Scientific Party, 1999e). The typical FMS facies of this sandy formation is displayed in Figure F23, where bedding contrast disappears below 727 mbsf. A fracture at 717 mbsf falls into the core-defined fracture zone 2 (domain IIb; 714-716 mbsf).
Around 775 mbsf, dipping structures are observed (Fig. F3), but the poorly defined bedding makes it difficult to assess their nature (Fig. F24). This interval had low recovery and is located just above fault zone 3 (domain IIb; 784-813 mbsf) described in core.
A progressive downward resistivity increase (Fig. F3) in the 811- to 856-mbsf interval corresponds to lithologic Unit V (811-857 mbsf) and to increasing PEFL in logging Unit L8.
The resistive interval at 856-858 mbsf (Fig. F25) correlates with the limestone of lithologic Unit VI (857-859 mbsf) and with a high PEFL and density at the top of logging Unit L9 (857-873 mbsf).
The 859- to 861-mbsf more conductive image (Fig. F25) can be correlated with a lower PEFL and density and high gamma ray levels within logging Unit L9, suggesting a radioactive sand. The dynamic image facies (Fig. F25) is indeed similar to that of a sand, such as that at 780 mbsf (Fig. F24).
The 861- to 873-mbsf resistive interval corresponds to the paraconglomerate of lithologic Unit VII (859-873 mbsf) and to the lower part of logging Unit L9, where a high PEFL indicates high carbonate content. The dynamic image shows various facies, from laminated at the top (Fig. F25) to sandy at the base, where bedding disappears (Fig. F26).
Below 873 mbsf, the dolerite conglomerate of lithologic Unit VIII (873-930 mbsf) and logging Unit L10 (873-890 mbsf) is clearly identified on the FMS dynamic images where individual cobbles can be seen (Fig. F26).
Dipping structures are identified on the FMS images in the 840- to 890-mbsf interval (Fig. F3). Bedding can be identified in the sediments down to 860 mbsf (Fig. F25). Below that depth, pseudoplanar structures were identified in the paraconglomerate and conglomerate (Fig. F26) in an attempt to define the layering, but their significance as bedding is unsure and they are labeled as unknown. Core analysis showed undeformed sediments (domain Ia) above alternating brecciated (domain IIIB) and unbrecciated (domain IIIa) conglomerate. However, the conglomerate was poorly recovered.
The poles to bedding and to a few of the unknown structures seem to cluster around a north-northwest-striking plane steeply dipping to the west (Fig. F27). This plane contains the pole of the fracture located at 853 mbsf. This is poorly constrained but is consistent with folding around a horizontal axis trending N77°E, which suggests a mainly dip-slip movement along the south-southeast-dipping fault. The dominant north-northwest bedding dip suggests reverse drag (i.e., roll-over folding).