Jurassic limestone-marl sequences in northern spain: detecting diagenetic signals using rare earth and trace elements
Mesozoic rhythmically bedded limestone and marl sequences have been traditionally interpreted to be the result of eustatic sea-level changes related to Milankovitch cycles. An alternative interpretation supports the idea that these calcareous alternations are a product of diagenetic bedding. Stratigraphic analysis is typically used to determine the extent of diagenesis and detect any primary depositional signal in these limestone-marl rhythmites. Rare earth and trace element geochemistry is an overlooked method to distinguish the presence of a primary signal from a diagenetic one. The main objective of this study was to test the validity of using rare earth and trace elements as a method for determining diagenesis in calcareous rhythmites that are part of the Rodiles Formation (Pliensbachian, Lower Jurassic) from coastal outcrops in NW Spain. Data from the application of this geochemical approach, in conjunction with stratigraphic analysis, provides insight into the distribution of elements that are facies-controlled, chemically-controlled, and elements that are independent of both. Results presented here for the use of rare earth elements as a test for a primary signal are inconclusive. Problems with using rare earth elements in this study include the possibility of unrealized REE mobility and signal interference from strontium, which many rare earth elements commonly substitute for. Thorium and strontium presented primary signals that are independent of facies and diagenesis. Of the two, thorium is the best element for use in the detection of a primary, externally controlled signal. Using thorium as a geochemical proxy, it may be concluded that despite diagenesis, diagenetic bedding is not the mechanism behind the formation of these carbonate rhythmites. Therefore, it can be concluded that the Rodiles Formation is the result of eustatic sea-level changes. Furthermore, thorium could be an ideal element for chemostratigraphy of Jurassic and Cretaceous deposits within the same paleogeographic setting as the Rodiles Formation.