Ichnofauna of a Cambrian–Ordovician delta complex: The Los Cabos Group of Asturias, Spain.

PAZ, MAXIMILIANO; MÁNGANO, M. GABRIELA; BUATOIS, LUIS A.; CAMPETELLA DEBORA MICAL; GOUGEON ROMAIN; PÉREZ-PUEYO, MANUEL; SHILLITO, ANTHONY; PIÑUELA, LAURA; GARCÍA-RAMOS, JOSÉ CARLOS

Florianopolis

Congreso; ICHNIA 2024 - The 5th International Congress on Ichnology; 2024

Unisinos - International Ichnologist Association

Los Cabos Group comprises 3000-4500 m-thick middle Cambrian to LowerOrdovician shallow-marine deposits in Asturias, Spain [1], constituting distal deposits of the wellknown"Armorican Quartzite" of Iberia and France. The formation holds enormous potential toanalyze paleoecological controls and the evolution of trace fossils in mud-rich shallow-marineareas due to its great thickness and prolonged time span. Moreover, it contains an impressiverecord of animal-substrate interactions that have been intensively studied in the seventies [2].However, deltaic and sediment-gravity flow facies were overlooked in previous analyses [3], andtherefore, there is a need to consider this riverine influence on the development of the Los Cabosichnofauna.For this purpose, we undertook a sedimentologic and ichnologic analysis of the mostcomplete and less tectonically affected succession of the Los Cabos Group (>5000 m of section,between the towns of Barcia and Cadavedo). Trace fossils were analysed in detail in terms of theirmorphology (and related ichnotaxobases), size, and ethology. Bioturbation intensity was assessedas well.A preliminary sedimentologic analysis indicates that the succession contains storm- andflood-influenced deltaic and tide-dominated shallow-marine deposits. The storm- and floodinfluenceddeltaic deposits are characterized by thin- to thick-bedded, massive and normalgraded,mudstone and very fine-grained quartz and lithic greywacke; and combined-flow, current-, and wave-ripple cross-laminated, hummocky and swaley cross-stratified, planar, trough, andlow-angle cross-bedded, and horizontal-laminated, very fine-grained quartz and lithic greywacke,siltstone and very fine- to fine-grained quartz arenite. The mudstone and greywacke showmudstone and arenite intraclasts, slump folds, arenite and mudstone pseudonodules, convolutebedding, ball and pillow structures, and load casts. Bioturbation index is 0 to 3, and Bifungites,Cruziana, Monomorphichnus, Planolites, Rusophycus, Skolithos, and Teichichnus occur.The tide-dominated shallow-marine deposits consist of very thin- to medium-bedded,massive and horizontal-laminated mudstone; and current-ripple and low-angle cross-laminated,planar and trough cross-bedded, and horizontal-laminated very fine- to fine-grained quartz arenitewith abundant very thin mud drapes. Mudstone and arenite are typically interbedded as lenticular,wavy and flaser heterolithic successions or as sand-rich intervals. Bioturbation index is 0 to 5,and Arenicolites, Diplocraterion, Lingulichnus, Planolites, and Rosselia are observed.Preliminary results indicate the presence of two trace-fossil assemblages. The storm- andflood-influenced assemblage contains a larger proportion of horizontal trace fossils over verticalstructures, whereas the opposite is true in the tide-dominated intervals. This trend could be relatedto the detrimental effect of high-water turbidity in deltaic systems upon filter feeding organismsthat create vertical trace fossils. Further ichnologic work will improve our understanding of stress factors controlling the ichnofauna, generate a comprehensive depositional model of the CabosGroup, and help to delineate patterns of evolutionary significance in Cambrian-Ordovician strata.