Downhole measurements in Hole 1213B were made after the completion of RCB coring to a total depth of 494.9 mbsf. Prior to logging, a wiper trip was made with the drill string, and the hole was flushed of debris and filled with sepiolite mud. During the wiper trip, the BHA reentered the igneous rocks in the lower 45 m of the hole only with some difficulty. The triple-combo tool string (porosity, density, resistivity, and natural gamma radiation) (see "Wireline Logging" in "Downhole Measurements," Fig. F8, and Table T7, all in the "Explanatory Notes" chapter) was successfully lowered to 431 mbsf, 64 m short of the total hole depth. During the uplog, between 312 and 324 mbsf, the hole apparently narrowed to <5 in wide; this obstruction had not been noted on the downward passage of the tool. High cable tension was encountered at 188 mbsf, and the logging run had to be stopped at this point, where it was also discovered that the caliper arm on the density tool would not close. After ~20 min of working the tool, it came free and was brought into the pipe and up to the ship. The most likely explanation for the difficulties in logging the hole is that pieces of chert fell out of the borehole wall and down the hole. There was no time for further logging runs because an approaching storm forced us to leave the site.
In the 240-m-long logged interval, the tools provided continuous, high-quality log data. The borehole diameter ranged from 11 in at the base of the hole to a maximum of 17 in at 305 mbsf. The hole contained ledges that formed at the hard chert layers, with the relatively soft ooze or chalk in between preferentially washed away. The wireline depth to seafloor was not determined.
Hole 1213B was divided into six units on the basis of the logs (Figs. F38, F39).
Unit 1 is characterized by high resistivity values, generally high densities, and a generally narrow hole diameter, all implying quite well lithified sediment.
Unit 2 is characterized by low resistivity values compared to the units above and below. The hole is wider in this unit, implying that it is more easily washed away and less lithified.
This unit has high resistivities and high gamma radiation values, including a double peak in uranium at 257.5 and 259.5 mbsf (Fig. F39). Potassium is also high in this interval, indicating that it is more clay rich than the other units. Units 1, 2, and 3 at this site bear similarity to Units 4, 5, and 6 at Site 1207 (Fig. F40).
This unit is characterized by generally low resistivities and low gamma radiation values.
This unit is characterized by generally high resistivities, high densities, and low gamma radiation values.
Unit 6 is characterized by higher resistivity values than the unit above, and uniformly high densities. It is probably a limestone.
The lower Aptian black shales recovered in Core 198-1213B-8R are represented in the logs by two large peaks in gamma radiation at 257.5 and 259.5 mbsf (Fig. F41). Uranium is the main contributor to the gamma radiation peaks; uranium peaks are often associated with black shales because the uranium is adsorbed by organic matter in the shale. Only the lower of the two uranium peaks is accompanied by low density values. Using the same equations presented in "Black Shales" in "Downhole Measurements" in the "Site 1207" chapter, the low density indicates that the peak has an organic matter content of ~9 wt%. This value falls within the range for TOC determined from core analysis (see "Carbonate and Organic Carbon" in "Organic Geochemistry"). The shale is ~0.9 m thick, according to the uranium, porosity, and density logs.
The interval in which the shales occur is relatively rich in potassium, which is in clay minerals like illite. If the potassium content is 1 wt% and all the potassium is in illite, the matrix would be ~20 wt% illite. The other major clay minerals have lower potassium contents. As at Site 1207, the black shale occurs within a high-resistivity unit (Fig. F40). The higher resistivity is probably caused by increased cementation of the chalk, or perhaps by increased concentration of chert layers, in this interval.
As in Hole 1207B, there are no other gamma radiation peaks of the same magnitude in the Cretaceous section at Hole 1213B. There are some minor peaks, such as the potassium peak at 361 mbsf, which may be related to the lower Valanginian shale found in Core 198-1213-19R. The shale recovered in Core 198-1213B-15R is not represented in the logs. However, the general conclusion is that, of the Cretaceous OAEs observed elsewhere, only OAE1a is significantly represented at Shatsky Rise.