We spudded Hole 1136A at 1215 hr on 27 December 1998, using the RCB coring assembly and an RBI C-7 core bit. The depth of the seafloor adjusted to the rig floor datum was 1942.0 m. The 3.5 kHz PDR had indicated an adjusted seafloor depth of 1948.4 m, or 6.4 m deeper. Continuous wireline coring proceeded in homogenous white foraminifer-bearing nannofossil ooze interspersed with thin chert layers to a depth of 60.9 mbsf. Recovery for this interval was 74.7%, and the average rate of penetration (ROP) was 41.9 m/hr. The formation then graded into a pale brown foraminifer bearing calcareous ooze. The average recovery dropped in this 19.0-m interval to 41.7%, and the average ROP increased to 51.8 m/hr. The interval from 79.9 to 128.0 mbsf consisted of volcanic sand and silt. Recovery through this interval was an extremely poor 0.9%, and the average ROP decreased to 23.1 m/hr. At a depth of ~133.0 mbsf (Core 15R), the driller noted a dramatic slowing of the drilling rate, indicating that basement had been encountered. Coring continued in basalt to a depth of 161.4 mbsf or ~28 m into basement. Recovery in basement was 55.0%, but the ROP was very low (2.8 to 1.1 m/hr).
Because the ROP was so slow and the limited basement recovery allowed some of the scientific objectives to be achieved, a decision was made to abandon further coring into basement. The previously planned wireline logging program was also canceled because very little basement had been penetrated (~28 m) and the amount of hole that could be logged (~85 m) was limited compared with the time required. By making this decision, we were able to regain the time lost during the transit south, and we departed Site 1136 essentially on schedule for the five remaining "primary" drill sites of the leg.
While the drill string was being recovered, the positioning beacon was released and, incredibly, came up inside the lower guide horn of the ship. This required partially pulling the upper guide horn. The beacon was ultimately recovered at 1115 hr on 29 December 1998. The ship was secured for transit, and at 1130 hr on 29 December 1998, we were under way for Site 1137.
Only two icebergs were observed during operations at Site 1136. Neither posed any threat to the drilling operation. The closest point of approach for the two icebergs was 17.2 and 5.3 nmi.
Leg 183 Operations - Site 1137
Leg 183 Table of Contents