Chapter Three - Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain)

Abstract During the early Silurian, widespread anoxic graptolitic black shales developed predominantly on the northern margin of Gondwana. These black shales are attracting a considerable amount of scientific interest, as it is nowadays hypothesized that their genesis might be closely related to major changes within the paleogeographical configuration of continents, the oceanic circulation patterns, and also drastic global climatic perturbations. Furthermore, these black shale successions can be of highest economic significance, as some form the source rocks of the world's major oil reserves. For the present study, high-resolution whole rock geochemical data obtained from an early Silurian black shale source rock analogue (Formigoso Formation, Cantabrian Mountains, NW, Spain) are used to develop a chemostratigraphic framework and to reconstruct and further constrain the paleoenvironmental conditions that prevailed at the time of deposition. These chemostratigraphic interpretations and paleoenvironmental reconstructions are used, together with the results of previously published studies, to develop a detailed generic depositional model for the black shale successions of the Cantabrian Basin. This analogue model is compared and contrasted to previously proposed models. The new high-resolution chemostratigraphic data are combined with detailed biostratigraphic information from the published literature. The present study demonstrates that chemostratigraphic methods provide not only complementary but even vital information, especially where biostratigraphic data are absent or scarce, for larger scale intra- and interbasinal correlations.