A synthetic view of FMS interpretation and core- and log-defined units for Hole 1115C is given in Figure F54. Hole 1115C was not cored above 283mbsf, and therefore the upper lithologic and structural units are defined in Hole 1115B. Further details on shipboards units can be found in figures F1 (lithostratigraphy), F30 (structures), F59 (logging), and F62 (synthesis) in Shipboard Scientific Party (1999d).
The structural measurement orientation is shown on stereographic projection and on strike and dip histograms for bedding (Fig. F55), fracture (Fig. F56), and unknown structures (Fig. F57).
The global distribution of bedding orientation is more dispersed for bed 2 than for bed 1 but otherwise has similar trends: the orientation is subhorizontal with most dips within [0°,15°] and with a west-northwest preferential dip direction (Fig. F55). This distribution, based on 416 data points (Table T3), is compatible with the dip distribution based on 220 core measurements (fig. F31A in Shipboard Scientific Party, 1999d).
Core analysis yielded 127 fracture measurements concentrated in three fracture zones, fracture zone 1 (domain IIa; 552-566 mbsf), fracture zone 2 (domain IIb; 604-696 mbsf), and fracture zone 3 (domain IIc; 725-802 mbsf), with a dip distribution with a 45°-50° and a 70°-75° peak (figs. 30B and F31B in Shipboard Scientific Party, 1999d). The FMS analysis clearly underestimates the fracture population because only ten fractures are identified (Fig. F56). Nine of them are located within the three fracture zones defined in cores (Fig. F54). These fractures are steeply dipping, eight of them dipping between 65° and 80°, with a dip direction to the south. The underestimation does not seem solely due to the incomplete borehole wall coverage, which lessens the chance of detecting steeply dipping fractures, because both the moderately and the steeply dipping fractures are underrepresented.
From the top of the logged section down to 475 mbsf, most beds dip <10° with only rare exceptions (Fig. F54). Most dipping beds and unknown structures and all fractures are found below 475 mbsf, either in the 500- to 570-mbsf interval of early synrift sediments, where the borehole is enlarged, or in the prerift units. Eight intervals with dipping beds are identified and further discussed in the next section: 245-265, 480-505, 506-527, 527-532, 552-572, 615-620, 622-634, and 690-785 mbsf.
Fractures identified on the FMS images strikingly correspond to intervals where bedding or unknown structures are dipping >10° (Fig. F54). However, bedding dips remain moderate (<35°), which is consistent with core observation (figs. F32A and F33 in Shipboard Scientific Party, 1999d), whereas unknown structure dips are in the 10°-60° range (Fig. F57). The low recovery within fracture zones 1 and 2 makes it possible for some of these structures to correspond to unrecovered steep beddings.
Further analysis from top to bottom of the hole is presented below. The maximum depth differences between the conventional logs and the processed FMS data is ~1.2 m and occurs around middepth in the hole (see "Appendix").
The lower section of this hole (573.5-802 mbsf) is within forearc prerift sediments that were reached in this hole only during Leg 180. Prerift and synrift bedding orientations differ only slightly; the prerift beddings dip mostly <15° toward the northwest (Fig. F58), whereas the synrift beddings dip <10° toward the west (Fig. F59). The unconformity between prerift and synrift sediments is therefore not associated with a significant angular change in bedding orientation. This confirms the conclusion reached on the basis of core bedding dip measurements (fig. F34 in Shipboard Scientific Party, 1999d). The number of prerift and synrift measurements are 128 and 288 for the FMS and 151 and 69 for the cores.
The conductive formation seen on the FMS static image in the 119- to 150-mbsf interval corresponds to lithologic Unit II (36-150 mbsf) and logging unit L1 (79-153 mbsf), which are dominated by clay, as indicated by neutron and density porosity log separation. The dynamic image shows low bedding contrast (Fig. F60), and therefore very few structural measurements are available in this interval.
This interval corresponds to logging Unit L2 (153-413 mbsf). This unit is differentiated from Unit L1 by a neutron and density porosity log with less general separation and frequent punctual convergence, which suggests lower clay content and thin clean beds. A slightly increasing resistivity of the static image, especially at 192 mbsf (Fig. F54), is well correlated with a downward-increasing photoelectric effect, which indicates an increasing carbonate content (fig. F59 in Shipboard Scientific Party, 1999d).
Most of this interval is within lithologic Unit III (150-389 mbsf), which is distinguished from the unit above by the apparition of silty calcareous beds in cores and of thin resistive beds on the FMS dynamic image below 149 mbsf (Fig. F60). The general pattern of thin resistive beds against a conductive background is sometimes reversed with thin very conductive beds (Fig. F61). The very resistive beds at 247.3, 248.9, and 256.7 mbsf (Fig. F61) were identified as carbonates on conventional logs. A third very resistive bed at 292.2 mbsf (Figs. F3, AF3) corresponds to dolomite and is located at the depth where coring had to switch from extended core barrel to rotary core barrel.
The 245- to 265-mbsf interval provides the only nonisolated beds dipping >10° in the upper section of the hole (Fig. F54). It contains the resistive carbonate beds just mentioned and the depths where cores were noted to become indurated (256 mbsf) and lithified (264 mbsf). No significant deformation was noted in core. The dipping beds are overlaying and being overlain by more horizontal beds, suggesting synsedimentary structures (Fig. F61). The orientations are not sufficiently diverse to provide good constraint on a possible cylindrical axis trending N138°E (Fig. F62).
In the bottom (389-413 mbsf) of logging Unit L2, increasing lithification and grain size change was observed in core-defined lithologic Unit IV (389-417 mbsf). The FMS images, like the conventional logs, do not register a significant change (Figs. F54, F63).
Resistivity increases on the static image (Fig. F54), and bedding becomes more diffuse on the dynamic image (Fig. F63) in the 413- to 474-mbsf interval corresponding to logging Units L3 (413-474 mbsf) and lithologic Unit V (417-475 mbsf). Elevated PEFL and neutron and density porosity log moderate separation with intervals of convergence within Unit L3 indicate an increase in carbonate content but a still significant clay content with thin clean beds. In core, lithologic Unit V is marked by a grain size increase. The lower resistivity below 440 mbsf, which correlates with higher gamma ray and slightly lower PEFL logs, may correlate with an increased sand content.
Higher resistivity and a more homogeneous static image (Fig. F54) characterizes the 474- to 506-mbsf interval corresponding to logging Unit L4 (474-506 mbsf) and most of lithologic Unit VI (475-513 mbsf). Higher sand content in a coastal depositional environment were deduced from core observation, whereas high PEFL and gamma ray as well as neutron and density porosity convergence indicate low clay content and mixed sands and carbonates. The dynamic image oscillates indeed between the laminated carbonate and grainy sandstone facies (Fig. F64).
Within the 480- to 505-mbsf interval, bedding dips reach up to 30° toward the southwest but do not define a clear structure (Fig. F65). However, bedding is not sharply defined, so that the determinations are in the bed 2 category, and no significant deformation was noted in cores from this interval.
The 506- to 535-mbsf interval, which corresponds to logging Unit L5 (506-535) and lithologic Unit VII (513-552 mbsf), is marked by borehole enlargements that degrade the log data (Fig. F54). The FMS tracks along the shorter diameter of the enlargements, where the pad is in contact with the formation, show conductive layers that are not visible on the orthogonal tracks that are along the larger diameter (Fig. F66). The dynamic image grainy facies at 517-519mbsf (Fig. F66) is consistent with the presence of sandy intervals suggested by conventional logs and the siltstone recovered in core.
In the short intervals where the hole is not enlarged, a few dipping beds are identified. In the enlarged intervals, where only two FMS pads are clearly in contact with the formation, the nature and geometry of structures is poorly constrained and those that are tentatively interpreted are attributed to the unknown category. The dipping structures are separated into two intervals.
In the 506- to 527-mbsf interval, the orientation plot shows scattered data with no clear organization (Fig. F67). A few of the unknown planes have an orientation close to that of bedding, but the nature of those more remote from bedding remains uncertain. The fracture is the only fracture identified on FMS in this hole that does not belong to a fracture zone defined from core. However, it belongs to the lower confidence frac 2 category, and core structural observations were hampered by the poor recovery.
In the 527- to 532-mbsf interval the orientations appear organized within a cylindrical structure with a north-south horizontal axis (Fig. F68). However, too few data points are available and the nature of too many of them is undetermined to further interpret this geometry.
In the 535- to 565-mbsf interval, which corresponds to logging Unit L6 (535-565 mbsf), the borehole enlargements are more developed than in the above interval and severely affect FMS images (Fig. F54) and the scalar logs that suggest clay and sand layers. This interval also corresponds to the lower part of lithologic Unit VII and the organic-rich claystone and bioclastic limestone of lithologic Unit VIII (552-566 mbsf).
The 565- to 573.5-mbsf interval, which corresponds to logging Unit L7 (565-573.5 mbsf) and lithologic Unit IX (566-572 mbsf), is radically different (Fig. F69).
In the upper 2 m (565.5-567.5 mbsf), the hole diameter is restricted below bit size (9.875 in or 25 cm). High PEFL logs indicate significant carbonate content, whereas high porosity logs as well as the hole restriction suggests significant clay content. The FMS dynamic image facies is compatible with the carbonate, sand, and clay mixture seen in other intervals. This unit appears separated from that just below by a steep south-dipping fault.
In the next 567.5- to 573.5-mbsf interval, the FMS dynamic image shows a conglomerate (Fig. F69) with one of the most resistive static images of the borehole (Fig. F54). The northwest-dipping structure at the bottom of this conglomerate at 573.5 mbsf is interpreted as the rift onset unconformity separating the synrift (Units I-IX) from the prerift forearc (Units X-XII) sediments.
The poles distribution of the structural measurements within the 552- to 572.5-mbsf interval does not show an interpretable organization (Fig. F70). Core analysis identified fracture zone 1 (domain IIa; 552-566 mbsf) and synsedimentary deformation in this interval, despite poor recovery.
This interval corresponds to logging Unit L8 (573.5-628 mbsf), where PEFL, neutron density porosity, and gamma ray indicate succession of two similar sequences of sands overlaying carbonates.
The upper sequence can be recognized on the FMS images (Fig. F71); the upper sand corresponding to lithologic Unit X (572-604 mbsf) is more conductive than the very resistive basal carbonate at 604-618 mbsf.
In the lower sequence, the upper sand interval at 618-625 mbsf corresponds to a borehole elongation where only two FMS pads are in contact with the formation and the basal carbonates at 625-629 mbsf are not as resistive as those of the above sequence (Fig. F72). Lithologic Unit XI (604-658 mbsf) includes the lower sequence and the carbonate base of the upper sequence.
Many dipping structures are observed on the FMS image within the core-defined fracture zone 2 (domain IIb; 604-697 mbsf) (Fig. F54). Those that are identified as bed or fractures can be grouped into two subintervals separated by the borehole elongation (615-620 and 622-634 mbsf). The orientation plots show north-dipping beds (Figs. F73, F74). In the lower interval, four south-dipping fractures are identified and bedding seems organized in a cylindrical structure with a horizontal axis trending N88°E that almost contains the fractures (Fig. F74). This suggests a fault-related folding with a mainly dip-slip movement that is consistent with core observations. The north-dipping beds with the south-dipping fractures suggests a reverse drag (i.e., roll over) fold.
The last interval corresponds to logging Unit L9 (628-784 mbsf), which was defined by a higher clay content than the above unit but with still significant carbonate content. It also corresponds to the lower part of lithologic Units XI and XII (658-802 mbsf). The resistivity on the static image is lower than above but remains high (Fig. F54). The dynamic image (Fig. F75) shows rare poorly defined bedding in a facies that is different from that of the above carbonates of Figure F71. Many dipping unknown structures are present within the bottom part of the core-defined fracture zone 2 (domain IIb; 604-697 mbsf), within which slumping was also observed at ~690 mbsf.
A change is noted below 695 mbsf; resistive, probably calcareous, thin beds are clearly identified on the FMS image (Fig. F76) and are consistently tilted 10°-15° toward the northwest (Fig. F54), as confirmed by the orientation plot (Fig. F77). The bedding of the overlaying forearc sequence dips toward a more northerly direction (Fig. F78). This suggests that a slight angular unconformity may separate these two forearc sequences.
Three faults are identified on FMS within fracture zone 3 (domain IIc; 725-802 mbsf) and dip toward the south (Figs. F54, F77). The orientation of the unknown planar data spans a large space that includes that of bedding but also gets close to that of fractures, suggesting that this set contains both types of structures.