Geolipids of black shales and claystones in Cretaceous and Jurassic sediment sequences from the North American Basin

Abstract

Summary Aliphatic hydrocarbon distributions in DSDP black shales and green claystones from the North American Basin have been examined by gas chromatography—mass spectrometry (GC-MS). The sediment sequences studied were: (i) black shale and claystone horizons (Callovian-Cenomanian) from the Blake-Bahama Basin (Leg 76, Site 534), and (ii) black shales and associated green claystones (Cenomanian-Turonian) from the US East Coast Continental Rise (Leg 93, Site 603). In both series of black shales, specific biological marker compounds provide evidence for contributions of organic matter from terrestrial, marine and bacterial sources. In addition, several of these geolipid distributions (e.g. hopanoids) are markedly similar for both sequences, indicating some common inputs of organic matter, and comparable diagenetic histories. For the Leg 93 samples, the relatively organic-lean green claystones are found to contain mainly terrigenous marker compounds, in constrast to their interbedded black shales which show a larger contribution from marine sources. All the sediments are immature, with a trend of increasing diagenesis evident from changes in the geolipid distributions down the Leg 76 sequence. One sample, however, is anomalous; its enhanced maturity may be due to an input of reworked, older organic material at the time of deposition.