K/T boundary geochemical anomalies have been used previously to support the impact event at the end of the Cretaceous. However, impact models and assessment of the extraterrestrial contribution to the boundary sediments should also consider the diagenetic alteration of the impact signatures. Mineralogical and geochemical studies centring on redox proxies reveal differences in trace‐element concentrations at Agost and Caravaca (SE Spain), two of the most complete K/T boundary sections. These differences probably derive from variations in the diagenetic evolution of the ejecta layer. Several redox proxies, such as extensive pyrite formation, high authigenic uranium concentration and positive Eu anomalies, indicate very strong reducing conditions at Agost. Positive Eu anomalies are extremely unusual in sediments, and in this case are interpreted to indicate a highly reducing environment. In such conditions, certain trace elements such as Ir, may have been remobilized, thereby masking the original signature of the impact.
A B S T R A C T:The spatial and vertical distributions of six clay mineral assemblages were identified in the pelagic marls and marly limestones and in the Ammonitico Rosso of the Lower Jurassic in the Betic Cordilleras (SE Spain). The six assemblages contain varying proportions of illite, kaolinite, chlorite, smectite and mixed-layer illite-smectite. The clay mineralogy and the geochemical data, particularly for the rare earth elements, indicate that the sediments were deposited in a pelagic environment influenced by a nearby continent. The source areas were many and lithologically varied. Clay mineralogy and geochemistry are shown to be most useful in the reconstruction of the palaeogeography of a basin in a passive continental margin.At the beginning of the Jurassic a considerable marine transgression took place in the Betic Cordilleras and a shallow carbonate platform developed with episodes of tidal-flat and, locally, open-sea facies. The environment is interpreted as being that of a very shallow carbonate platform, the varying textures of which reflect different energy regimes according to their position relative to the tidal level (Garcia Hern~indez et al., 1980). The deposition of sequences of the Ammonitico Rosso facies and thick marine series of marls and marly limestones show that pelagic conditions were established in the Subbetic Zone from the Domerian onwards.In this paper we study the mineralogy and geochemistry of these facies with a view to understanding the palaeogeographic conditions that controlled their sedimentation, and also to locating the source-areas of these sediments.
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