Pleistocene sediments in Guaymas basin, Gulf of California, have been intruded by sills. Their organic matter was thus subjected to thermal stress. Samples from DSDP/IPOD Sites 477, 478, and 481 and from unaltered material from Sites 474 and 479 were analyzed to characterize the lipids and kerogens.

The lipids of the thermally unaltered samples are derived primarily from algal and bacterial detritus. The samples near the sill contain the distillates; those closest to the sills contain essentially no lipids. The pyrograms of the kerogens from the unaltered samples reflect their predominantly autochthonous microbial origin. The pyrograms of the kerogens of the altered samples reflect the thermal effects by demonstrating a reduction in the complexity of the products, when compared with the unaltered samples. Kerogens adjacent to the sills produced little or no pyrolysis products.

The effects of intrusions into unconsolidated sediments resulted in in-situ pyrolysis of the organic matter as confirmed by these data.

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