(Table 1) Kerogen geochemistry at DSDP Leg 64 Holes

Twenty-seven kerogen samples from the Guaymas Basin were examined to determine the effects of dolerite sill intrusions on recently deposited organic matter. ESR spin density and line width were observed to pass through maxima during the course of alteration. ESR g value, however, showed no correlation with maturity. Rather surprisingly, d13C decreased by 1 to 1.5 per mil in the vicinity of the sills at Sites 477 and 481. In addition, atomic N/C decreased only slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Sites 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to the chemical and isotopic heterogeneity of Guaymas Basin protokerogen.