Rare earth elements as indicators of different marine depositional environments in chert and shale

Differences in rare earth element (REE) relative fractionations and total abundances in marine cherts and shales reflect the depositional location of the sediments. Based on cerium anomaly (Ce/Ce*) and total REE abundance (ΣREE) variations preserved in interbedded chert and shale sequences of the Franciscan assemblage, the Claremont Formation, and the Monterey Formation of coastal California, we can resolve three depositional regimes: spreading ridge proximal, ocean-basin floor, and continental margin. This resolution of depositional environment, previously unattainable in otherwise physically indistinct marine sedimentary rocks, provides a potentially powerful new tool for tectonic and stratigraphic reconstructions. Chert and shale deposited near the spreading ridge (within ∼400 km) under the influence of metalliferous activity are characterized by extremely low Ce anomalies (Ce/Ce* ∼0.29), those from an ocean-basin floor setting by well-developed yet less extreme Ce anomalies (Ce/Ce* ∼0.55), and those from continental margin regimes by no or slight anomalies (Ce/Ce* ∼0.90 to 1.30). ΣREE in chert and shale are extremely low in regimes greatly influenced by continental input, owing to the high sedimentation rate, which minimizes exposure time to seawater and, thus, the adsorption of REE. Preliminary results indicate that the effect of diagenesis does not mask the primary REE depositional signature.