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Geochemical well logs were obtained in the sedimentary sequences drilled at Sites 798 and 799 in the Sea of Japan. The recorded data have been corrected for variations in borehole size, borehole fluid composition, downhole temperature, and logging speed and were processed to derive the concentration of the elements Th, U, and Gd and the dry weight percentages of oxides of the elements Si, Ca, Al, Fe, Ti, and K at 0.1524-m intervals (6 in.). The log-derived geochemistry shows fair agreement with XRF core measurements for Hole 798B. INTRODUCTION Leg 128 was the second of two Ocean Drilling Program (ODP) cruises devoted to probing the history of the Japan Sea. The unifying objectives of this cruise and that of Leg 127 were to assess the style and dynamics of rifting and marginal sea formation in a continental arc setting and to decipher the parallel paleoceanographic evolution of the sea. During Leg 128, two structural highs (Sites 798 and 799) and one basin site (Site 794) were drilled, with the basin site first occupied during Leg 127. Overall, logging operations were highly successful; the geochemical logging tool string and three other logging strings were run in Holes 798B, 799A, and 799B. Only at Hole 794D was the logging program not completed because of poor hole conditions. Logging operations have been detailed in Ingle, Suyehiro, von Breymann, et al. (1990). Geochemical logs provide continuous in-situ measurements of the major rock-forming elements at 0.1524-m intervals (6 in.) down the borehole. These provide a large data base of geochemical information unaffected by several problems that inhibit core-based studies, such as incomplete or preferential core recovery and core disturbance. This report describes the basic principles of the geochemical tool configuration and the post-cruise processing techniques employed on the data. This is followed by a brief qualitative comparison of the geochemical logs with the lithologic description and available core data. GEOCHEMICAL LOGGING TOOL Nuclear geochemical logging involves three separate modes of gamma-ray spectroscopy to obtain a comprehensive elemental analysis of the formation. The geochemical logging tool (GLT) deployed in ODP operations consists of four tool components, as shown in Figure 1. At the top of the string is the natural gamma-ray spectrometry tool (NGT, a trademark of Schlumberger Inc.), which measures the spectrum of natural gamma radiation of the formation using a sodium iodide (Nal) scintillation detector, from which the concentrations of potassium (K), thorium (Th), and uranium (U) are derived (Lock and Hoyer, 1971). This tool is positioned at the top of the tool string so that the measurement is taken before irradiation of the 1 Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al., 1992. Proc. ODP, Sci. Results, 127/128, Pt. 2: College Station, TX (Ocean Drilling Program). 2 Borehole Research, Department of Geology, University of Leicester, LEI 7RH, England. Borehole Research Group, Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A. formation by the sources below. The second measurement of the tool string is made by the aluminium activation clay tool (AACT, a trademark of Schlumberger Inc.). This measures the gamma rays produced by delayed neutron activation induced by a Cf (Californium) source, carried by a dual porosity compensated neutron tool (CNT-G) positioned above the AACT, from which the concentration of aluminium (Al) in the formation is derived (Scott and Smith, 1973). The gamma-ray spectrometry tool (GST), carried at the bottom of the tool string, consists of a "minitron" tritium source of pulsed 14 MeV neutrons and a gamma-ray scintillation detector. Incident neutrons lose energy through scattering interactions and upon reaching thermal energies, are captured by elemental nuclei, causing the emission of gamma rays of capture. The spectrum of gamma rays detected can be used, in conjunction with elements derived from the NGT and AACT, to derive the weight percentages of the elements silicon (Si), iron (Fe), calcium (Ca), sulfur (S), titanium (Ti), and gadolinium (Gd). The GST also measures the contribution of chlorine (Cl) and hydrogen (H) to the detected spectrum, from both the borehole and the formation. The only major elements in the formation not measured by the geochemical tool string are magnesium (Mg) and sodium (Na). Neither of these elements has a sufficiently large capture cross section (relative to the typical cross section) to be detected by the GST (Hertzog et al., 1989). An estimate of their concentrations can be derived from the photoelectric factor (PEF) from the lithodensity tool. The recorded PEF log is compared to a computed PEF derived from a summation of the photoelectric contributions from the GLT-measured elements. Any difference is attributed to unmeasured elements in the formation (i.e., Mg and Na). This calculation was performed on data from Sites 798 and 799, but it only induced noise into the elemental calculation and was therefore omitted. DATA REDUCTION During logging operations aboard the JOIDES Resolution, the data are transmitted electronically up the conductive logging cable and recorded digitally in the Schlumberger cyber service unit (CSU). Some preliminary processing of the data occurs at this time to produce "field logs" that are immediately available to the shipboard scientific party for initial interpretation. Subsequent post-cruise processing on shore allows for more comprehensive environmental corrections for borehole size, logging speeds, borehole fluid salinity, temperature, and pipe effects. Processing of the GST spectrometry data is required to transform them into elemental oxide weight fractions. The processing is performed using a series of programs written by Schlumberger Inc.; these are described in the following discussion. The processing flow is summarized in Figure 2.

Data Report: Geochemical Logging Results from the Sea of Japan: Sites 798 and 799