IODP Logging Summaries

Site U1316 (965 m water depth, 51° 22.56' N, 11° 43.81'W) is located in the downslope sediment deposits approximately 700 meters to the southwest of Challenger Mound. Sediments recovered from Site U1316 located basinward of the Challenger Mound contain a sedimentary suite of post-, syn- and pre-mound growth phases that correspond to three lithological Units. The uppermost Unit 1 is 52-58 m thick, and mainly composed of grayish-brown silty clay. Unit 2 is a coral bearing facies of 10-13 m in thickness and underlies Unit 1 with an erosional surface. The age of this unit is mostly early-middle Pleistocene, which corresponds to the age of the thick coral mound at the Site 1317. This unit rests on the Unit 3 with a distinct unconformity surface. The Unit 3 consists of the 92-m thick (Hole U1316C) heterogeneous, dark green colored, glauconitic siltstone, and is calcareous in the lower part. Dolomite precipitation formed lithified layers around 72 mbsf.

After an unsuccessful attempt to log Hole U1316A, Triple Combo and FMS sonic downhole logs were acquired between 60 and 140mbsf in Hole U1316C. The density, resistivity, and acoustic velocity logs show a steady downhole increase due to compaction, interrupted by 1-5-m-thick intervals of higher values, indicating the presence of more lithified layers (Figs 5 and 6). The PEF values for these layers indicate they are carbonate-rich. These lithified layers are the cause of several strong reflections in the sigmoidal package in the seismic section at this site.

Figure 7. Log stratigraphy at Hole U1317D. (a) Hole shape, total gamma ray (HSGR) and Potassium content (HFK), (b) thorium (HTHO) and uranium (HURA) contributions to natural radioactivity, (c) deep (IDPH), intermediate (IMPH) and shallow (SFLU) resistivities, (d) porosity (APLC) and formation density (RHOM), (e) capture cross-section (SIGF) and photoelectric factor (PEFL), (f) downhole compressional velocities (VP) and interval velocity determined by the check-shot survey, and (g) log sub-units.

Figure 8. Core-log integration at Hole U1317 gamma-ray, porosity, density, velocity. Post-cruise correlation between features recorded in these logs (especially acoustic, density and gamma-ray) offer potential to provide in-situ ground truth for core data. Initial examination shows that the core depths are offset 4-6 m downwards from the log depths.

Figure 9. Thermal measurements at Site U1317. (a) Drilling mud temperature measurements using the EMS (blue, down logging) and TAP tool (red, down and up logging) compared to ADARA measurement, (b) thermal conductivity measurements on core (dot) and in-situ (ADARA measurement, star).

Site U1317 is located on the northwest shoulder of the Challenger Mound (51º 22.8' N, 11º 43.1' W, in 781 to 815 m water depth). Sediments from the on-mound Site U1317 can be divided into two units; the Pleistocene coral-bearing unit (Unit 1) and the Neogene siltstone (Unit 2). Unit 1 consists mainly of coral (mostly identified as Lophelia pertusa), floatstone, rudstone, wackestone, and packstone, and repeats cyclic color change between light grey and dark green. This coral mound unit rests on Unit 2 with a sharp erosional boundary that appears identical to the boundary between Units 2 and 3 of Site U1316. Unit 2, consists of glauconitic and partly sandy siltstone. It is lithologically correlated with Unit 3 at Site U1316.

Triple Combo, FMD-Sonic downhole logs, and a zero-offset VSP were between 80 and 245 mbsf in Hole U1317D. The density, resistivity, and acoustic velocity logs show a steady downhole increase due to compaction, interrupted by 1-5-m-thick intervals of higher values, indicating the presence of more lithified layers similarly to Hole U1316C. The PEFL values for these layers indicate they are carbonate-rich. These lithified layers are the cause of the high amplitude sigmoidal reflectors observed in the seismic profiles (Figs 7 and 8). Interval velocities were calculated from the checkshot survey (Table 2): they confirm the values of the acoustic velocity logs, but show that the physical property measurements made on the cores significantly underestimate the in-situ velocity.

Site U1318 (423 m water depth, 51° 26.16' N, 11° 33.0'W) is located in on the eastern slope of the Porcupine Seabight on the southwest continental margin of Ireland and is upslope from the Belgica Mound Province, including Challenger Mound. Sediments from the up-slope Site U1318 were divided into three units based on sediment colors, erosional surfaces, and biostratigraphy. The uppermost Unit 1 is 79.9 - 82.0 m thick, and consists of brown-colored silty clay with black motted structure, which is partly laminated and bioturbated. Dropstones are common in this unit. Across a distinct erosional surface, Unit 2 of 4-6 m thick underlies. This unit mainly consists of olive-gray, medium-fine sand interbedded with dark yellowish-brown silty clay. The sand beds are normal graded with sharp lower and upper boundaries. Dropstones, up to 3 cm in diameter, are found in both sand and clay horizons. The base of this unit is a conglomerate resting on a distinct erosional surface. It is 5-10 cm thick, and associated with black-colored apatite nodules. The Unit 3 of 155 m thick (Hole U1318B) consists of dark green siltstone, which frequently intercalates with sandstone layers in the upper and lower horizons. The siltstone tends to become calcareous to downward.

Triple Combo and FMS sonic downhole logs were acquired between 70 and 240 mbsf in Hole U1318B. The downhole logs are characterized by low amplitude variations in lithological subunits 3A and 3B (92-192 mbsf), and by increased velocity and thin lithified layers in subunit 3C (below 192 mbsf) (Figs 10 and 11). The hiatus represented by the oyster bed at the base of unit 2 is rich in uranium (as seen in the natural gamma radiation logs), which tends to accumulate at hiatuses and condensed intervals. Major changes in physical properties were observed at lithological unit boundaries that can be directly related to reflectors in the seismic section. The sand layers, silty clays, dropstones and oyster bed of lithological Unit 2 create a high amplitude reflector in the seismic profiles, and this erosive reflector has been tentatively identified as the upslope continuation of the moundbase reflector. The enigmatic low amplitude seismic package, whose identification was one of the main aims of drilling this site, corresponds to homogeneous calcareous silty clays. Lithostratigraphic Subunit 3C, below 192 mbsf, is characterized by a slight general increase in density in combination with some high density thin beds, and corresponds with high amplitude, high frequency parallel reflectors which can be traced along the seismic profile to the sigmoid unit at Site U1316.