Lumpy limestones: a puzzling facies encased on a worldwide relevant stratigraphic succession

Extensive research has been dedicated to the Peniche section (Lusitanian Basin, west coast of Portugal), one of the western Tethys most complete and continuous sedimentary succession for the Pliensbachian/Toarcian interval,... [ view full abstract ]

Extensive research has been dedicated to the Peniche section (Lusitanian Basin, west coast of Portugal), one of the western Tethys most complete and continuous sedimentary succession for the Pliensbachian/Toarcian interval, recently distinguished by the Pliensbachian/Toarcian Global Stratotype Section and Point. Despite the worldwide relevance of this section, a portion of this succession showing a high facies heterogeneity, comprising an alternation of lumpy marls and limestones (member of Vale das Fontes Formation; lower Pliensbachian) still deserves further attention. A particular 10 meter interval was therefore chosen for a high-resolution geochemical evaluation (C and O-isotopes, major and trace elements), from which 20 hand-samples were collected, providing a total of 63 sub-samples. This collection of samples is representative of matrix micrite from limestone and marly horizons, intentionally covering a wide range of lump aspect (net, diffuse or altered rims) and distribution pattern (clustered or scattered on the matrix). Fairly preserved belemnite rostra and late cements also provided further arguments.
Regarding C and O stable isotopes, limestone matrix micrite and all lump samples fall within the same range of values. But an offset of about 2‰ in δ18O values in respect to fairly preserved belemnite rostra points towards a diagenetic stabilization under near-marine burial fluids. Marly matrix micrite samples show rather lowered C-isotope values when compared to the remaining samples (up to -3‰), a trend consistent with the influence of meteoric diagenesis for these specific samples. Furthermore, results obtained from intra-lump transects (up to 6 samples across 1 cm lumps) and from different lumps within the same hand-sample showed the lowest C and O-isotope scattering. Such trend suggests that the mechanisms generating lumpy limestone facies promote a higher geochemical preservation of lumps when compared to their encasing matrix. Preliminary major and trace element interpretations (Ca, Mg, Sr, Fe and Mn) support these findings. Accordingly, higher Mn content was obtained for lumps with diffuse boundaries or presenting alteration rims, coupled with lower Sr content. This elemental trend is consistent with the diagenetic alteration already suggested by C and O-isotope values. This high-resolution approach to intra-sample variability suggests that for lumpy limestone intervals, more reliable geochemical information can be obtained by targeting limestone intervals, preferably if these present lumps with net, unaltered boundaries.