Statistical evaluation of elemental concentrations in shallow-marine deposits (Cretaceous, Lusitanian Basin)

Abstract

The elemental geochemistry of ancient shallow-marine carbonates represents an often underexplored archive of paleoenvironmental change and diagenetic pathways. Complex multi-parameter datasets from a lithologically heterogeneous sedimentary succession in the southern part of the Lusitanian Basin (Ericeira, Portugal) are presented and interpreted. The section comprises marly, carbonate, and sandstone-rich coastal-marine deposits of early Albian to early Cenomanian age. Principal component analysis of bulk elemental abundance (Ca, Mg, Sr, Fe, and Mn) lead to define four geochemical clusters based on common elemental attributes. Siliciclastics and dolomitized limestones facies yield evidence for significant diagenetic alteration, but still preserve information on paleoshoreline position. Mixed carbonate-siliciclastics and limestone facies bear evidence for relative sea-level position and relevant paleoclimatic information in their elemental record. Accordingly, mid-Albian mixed carbonate-siliciclastic deposits present variable degrees of continental influx. Specifically, a sharp increase of Fe and Mn concentrations in relation to continental input is in agreement with periods of enhanced hydrological cycling and increased weathering on adjacent emerged lands. The progressive transition towards more marine conditions is accompanied by higher Ca and Sr content and represented by limestone facies corresponding to a higher sea-level position during late Albian to early Cenomanian times. Moreover, this facies records a trend towards warmer and more arid conditions. Early diagenetic stabilization and /or dissolution of aragonite into low-Mg calcite can account for major elemental variations, including higher Sr content strongly coupled with Ca content, along with lower Mg, Fe and Mn concentrations. Data shown here provide solid evidence that the statistical analysis of the elemental record of ancient marine deposits, when combined with conventional sedimentology and thin section petrography, allow for elaborate conclusions on their depositional environment and diagenetic pathways. Cored wells and facies-specific reservoir properties can be addressed by these methodologies, able to detect and quantify changes in patterns. Both industry and academy can therefore benefit from approaching such a complex interplay.