Organic matter in the 10 Cretaceous black shale samples from Sites 1257 and 1258 is thermally immature, as indicated by sterane and hopane biomarkers that remain in their most immature structural configurations. Distributions of n-alkanes, steranes, and hopanes extracted from the black shales indicate that they are derived from a mostly algal and microbial source with varying contributions of land-plant material. Good preservation of organic matter implies a strongly oxygen depleted depositional setting, especially considering the generally low sedimentation rates.