Geochemical compositional mapping of Lower Jurassic trace fossils: palaeoenvironmental significance and methodological implications

Abstract

Analyses of X-ray microfluorescence (XRF) elemental maps of the ichnofossil assemblages from the upper Pliensbachian-lower Toarcian marl-limestone rhythmite of the South Iberian Palaeomargin (Betic Cordillera, SE Spain) show that the compositions of burrow infills are geochemically and mineralogically different with respect to the surrounding sediment. XRF elemental maps and back-scattered electron images show pyrite framboids within the burrows and enrichment in Fe and S relative to the host sediment. This is congruent with the position of burrows beneath the redox boundary at the time of their active or passive sedimentary infilling. Organic matter that accumulated at the seafloor was not preserved due to oxidation and consumption by benthic organisms. However, burrows located beneath the redox boundary served as traps for organic matter preservation. The composition of burrow fills reflects the original signal of the redox conditions of pore-waters below the sediment-water interface, which was not preserved in the surrounding sediment. The XRF elemental maps also show that some trace fossils from limestone and marly-limestone intervals contain high concentrations of Si, Al, K, Zn, Cr, and Ti with respect to the surrounding sediment. This may reflect enrichment of clay minerals in the burrow fills derived from overlying marly layers. This work proposes the use of elemental imaging of trace fossils prior bulk-rock geochemical analyses in order to evaluate the potential for biogenic heterogeneity and elemental partitioning before sampling. Geochemical analysis of trace fossils represents an additional tool that may be of use in hydrocarbon exploration.