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Pleistocene sediments in the Guaymas Basin, Gulf of California, have been intruded by sills and their organic matter thus subjected to thermal stress. Sediment samples from DSDP/IPOD Sites 477, 478, and 481, and samples of thermally unaltered materials from Sites 474 and 479 were analyzed to characterize the lipids and kerogens and to evaluate the effects of the intrusive thermal stresses. The lipids of the thermally unaltered samples are derived from microbial and terrestrial higher-plant detritus. The samples from the sill proximities contain the distillates, and those adjacent to the sills contain essentially no lipids. The pyrograms of the kerogens from the unaltered samples reflect their predominantly autochthonous microbial origin. When compared with the unaltered samples, the pyrograms of the altered kerogen samples reflect the thermal effects by a reduction in the complexity of the products. Kerogens adjacent to the sills produced little or no pyrolysis products. The effects of intrusions into unconsolidated, wet sediments resulted in in situ pyrolysis of the organic matter, as confirmed by these data. INTRODUCTION Drilling in the central Gulf of California concentrated on the Guaymas Basin and the continental slope east of that basin. The Guaymas Basin is an actively spreading oceanic basin, part of the system of spreading axes and transform faults extending from the East Pacific Rise to the San Andreas Fault System (Curray et al., 1979): It is therefore tectonically very active and consists of two rift valleys separated by a 20-km transform fault. The basin is characterized by a high sedimentation rate, which keeps its floor and rifts covered during the formation of complex ocean crust and intrusions (Curray et al., 1979). Site 477 is in the southern rift, and Site 481 is in the north (the locations of all Leg 64 sites are found on the area map in the introduction to shorebased geochemistry, this volume). The sediments are all late Quaternary diatomaceous ooze and turbidites intruded by various sills (Curray et al., 1979). Site 477 shows extensive hydrothermal alteration and high in situ heat flow. Site 478 is on the basin floor in the transform fault zone and consists of similar sediments with intrusives (Curray et al., 1979). Biogenic gas occurs at all sites; thermogenic hydrocarbons, ranging to C7 and derived from the endogenous organic matter, were also detected at Sites 477 and 481 (Curray et al., 1979). Site 479 is on the Guaymas Slope and consists primarily of diatomaceous ooze laminated below 250 meters sub-bottom (Curray et al., 1979). Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt. Printing Office). 2 Present address: School of Oceanography, Oregon State University, Corvallis, Oregon. It was therefore of interest to examine the organic matter to evaluate two major questions: (1) the sources and characteristics of the organic matter, and (2) the effects of the thermal stress on that organic matter. The sources and characteristics of the organic matter can be determined by analyzing the lipid fractions for homologous compound distributions, for structural types of molecular markers and the nature of the unresolvable mixtures (Simoneit, 1975, 1978b, in press), and by analyzing the kerogens for their bulk compositional properties; pyrolysis/gas chromatography (GC), and pyrolysis-GC/mass spectroscopy (MS) may also be used (e.g., van de Meent et al., 1980). The effects of thermal stress on the organic matter can also be evaluated by these same lipid and kerogen analyses. Such data can then be correlated with the results of the interstitial hydrocarbon gas (Q-C7) compositions. Herewith are the data from lipid analyses of the hydrocarbon and fatty acid (as methyl esters) with ketone fractions for selected samples (Table 1). The samples are listed sequentially by sites in Table 1, and their locations are indicated to the left of each lithological column by the numerals in Figure 1 (supplemental samples are indicated by A-F). For a variety of reasons, Curie-point pyrolysis (Cupy) of the kerogens was used to examine the effects of sills on the organic matter. In the past few years, pyrolysis has become a powerful analytical tool, especially in studying macromolecular materials. This rapid expansion in the use of pyrolysis is the result of fast microscale flash pyrolysis techniques that appear predominantly to yield primary degradation products (Jones and Cramers, 1977). Pyrograms, obtained either by Cupy/GC or Cupy/MS, are

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Organic Geochemistry of Lipids and Kerogen and the Effects of Basalt Intrusions on Unconsolidated Oceanic Sediments: Sites 477, 478, and 481, Guaymas Basin, Gulf of California