Ichnolithological associations from the Huamampampa Formation: a methodological approach to understand open shelf and estuarine sedimentation during the Devonian of the Sub Abdean Foothill, Bolivia

Abstract

The Huamampampa Formation has been classically interpreted as the product of shallow open-marine sedimentation in a Devonian shelf. However, recent ichnological and sedimentological studies conducted on six cores and some outcrops have allwed the recognition of the occurrence of brackish water sedimentation during the deposition of the Huamampampa Formation. Trece fossils identified in cores and outcrops include Arenicolites, Asterosoma, Bergaueria, Chondrites, Cylindrichnus, Diplocraterion, Helminthopsis, Macaronichnus, Neonereites, Palaeophycus, Phycodes, Rhizocorallium, Rosselia, Skolithos, Teichichnus, and Zoophycos. These ichnogenera are grouped in the Cruziana, Skolithos and an impoverished ichnofacies from sections of the Huamampampa Formation analysed along the Sub-Andean foothills in east-southern Bolivia.The aim of this contribution is to show the methodology applied to carry out this study, which includes the measurement every 5 cm of Numerical Facies (NF), Bioturbation Index (BI), Ichnodiversity Index (IdI), Ichnogenus Size Ordening (ISO), and the geostatistic analysis of their obtained curves. Then, ichnofacies and ichnolithological associations have been used to recognise different subenvironments in a tide-dominated delta (or estuarine)- shallow marine environment. Nine ichnolithological associations were identified. The ichnolithological associations identified are i) Sandy facies with Cruziana ichnofacies (S-Cr), high bioturbation and ichnodiversity (assigned to shallow marine, upper shoreface); ii) Sandy facies with Impoverished ichnofacies (S-Imp), low bioturbation and ichnodiversity, general size reductions of ichnogenera (tide-dominated delta or estuarine, longitudinal bars, brackish water); iii) Sandy facies with Impoverished-Skolithos ichnofacies (S-Imp-Sk), low bioturbation and ichnodiversity, isolated Skolithos (top of longitudinal bars, high-energy pulses); iv) Sandy facies with belts of Arenicolites (S-bAr), horizontal levels of just Arenicolites up to 5 cm thick (high energy pulses crossing different facies, in the delta front or in the shoreface); v) Sand-rich heterolithic facies with Cruziana ichnofacies (Hts-Cr), moderate bioturbation and ichnodiversity (shallow marine, upper to middle shoreface); vi) Sand-rich heterolithic facies with Cylindrichnus and Rosselia (Hts-Cyl-Ros), moderate bioturbation and low ichnodiversity (shallow marine, upper shoreface); vii) Heterolithic facies with Cruziana ichnofacies (Ht-Cr), high bioturbation and ichnodiversity (shallow marine, transition zone between lower shoreface and offshore, open shelf); viii) Heterolithic facies with Impoverished ichnofacies (Ht-Imp), low bioturbation and ichnodiversity, reduced size of trace fossils (tide-dominated delta or estuarine, delta front, brackish water); and ix) Sandy facies with no bioturbation (S-Not Biot), interpreted as fluvial channels.