Lithofacies characterization of fluvial sandstones from outcrop gamma-ray logs (Loranca Basin, Spain): the influence of provenance

Abstract

Natural gamma spectral (NGS) log motifs and cluster analysis were
used to characterize outcropping sandstone bodies formed in braided and highsinuosity
streams of the Tertiary Tórtola fluvial system of the Loranca Basin (Spain).
Five coarse-grained lithofacies comprise these deposits and determine distinct NGS
log motif. Cross-plots and cluster analysis of NGS log data of the lithofacies suggest
three distinct clusters. These clusters reflect distinct values for sandstones with small
sets of ripple lamination, cross-stratification, and conglomerates and pebbles.
Ripple-laminated sandstones show the most variability in NGS signature, whereas
conglomeratic sandstones show the most uniform signature. Such cluster analysis
may be used to assign NGS log data points of unknown origin to a specific fluvial
lithofacies under conditions of equal rock provenance and diagenetic history. A
sedimentaclastic (i.e. sedimentary parent rock) origin of sediments appears to be the
main control on detrital composition that, in turn, varies with grain size.