DEEP-SEA UPPER ORDOVICIAN LINGULICHNUS VERTICALIS AND THEIR ASSOCIATED BODY FOSSILS SUGGEST AN EARLY COLONIZATION OF PROXIMAL TURBIDITE SYSTEMS

PAZ, MAXIMILIANO; MÁNGANO, M. GABRIELA; BUATOIS, LUIS A.; CAMPETELLA DEBORA MICAL; SPROAT, COLIN; PÉREZ-PUEYO, MANUEL; PIÑUELA, LAURA; GARCIA-RAMOS, JOSÉ CARLOS

Florianopolis

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

Unisinos - International Ichnologist Association

High-energy zones of deep-marine systems such as channel-levee complexes andproximal areas of turbidite splays may show the development of the Ophiomorpha rudisichnosubfacies of the Nereites Ichnofacies, dominated by callianassid shrimp burrows [1].However, this ichnosubfacies was not established in the deep sea until the Late Jurassic. Duringthe early and mid Paleozoic, high-energy deep-marine areas were typically barren or display lowlevels of bioturbation [2]. These areas probably remained sparsely bioturbated until the LateJurassic, although a comprehensive evolutionary review of this subenvironment is currentlyneeded.An unusual, dense occurrence of Lingulichnus verticalis and their associated brachiopodbody fossils (lingulides) is recorded in an Upper Ordovician channel-overbank complex in Spain,constituting the first report of Lingulichnus from deep-marine environments [3]. The study area isin the Frejulfe beach, Asturias, Spain, and belongs to the West Asturian-Leonese Zone, a Variscantectonostratigraphic domain of the Paleozoic Iberian Massif. The trace fossils were found in theAgüeira Formation, a 300–3000 m-thick turbiditic sandstone, siltstone, and mudstone successioninterpreted as a submarine fan [4,5].The study interval (35 m thick) preserves fining- and thinning-upward channel andoverbank deposits. The channel deposits comprise 10–100 cm-thick, massive, and parallel- tosinusoidal-laminated, very fine-grained sandstone. Bioturbation index is typically 0 and rarely 1-2, with Lingulichnus verticalis and Phycosiphon incertum. The overbank deposits consist of 1-20cm-thick, climbing-ripple to current-ripple cross-laminated, and parallel-laminated, very finegrainedsandstone to siltstone, interbedded with mudstone. It contains Lingulichnus verticalis,Nereites isp., Phycosiphon incertum, and Planolites isp., generating a bioturbation index of 1-3and rarely 4 or 5. Calcium phosphate and poorly preserved pyrite-replaced lingulides (10-55 mmin length and 3-25 mm in width) occur in both deposits.Lingulichnus verticalis [6] consists of oblique (35-45º of inclination) to rarely vertical,passively filled burrows showing a lower rounded to ellipsoidal structure (1-20 mm diameter),and an upper, straight, tubular structure (1-5 mm wide and 10-65 mm long) surrounded byconcave-up, cone-in-cone spreite (1-13 mm diameter). Rarely, the upper structure has spademorphology. Plan views are ovoid to oval with rounded and pointed edges. The lower structureis typically preserved in mudstone (hemipelagite), below sandstone or siltstone (turbidite),whereas the upper structure is preserved in sandstone or siltstone (turbidite). Lingulides occurbelow the spreite of, or laterally associated to Lingulichnus verticalis with their anterior endfacing up.Ellipsoidal burrow cross-section, cone-in-cone spreite, and the existence of spademorphologies suggest the trace fossil can be attributed to Lingulichnus Hakes, 1976, a dwelling, equilibrium, and escape structure of lingulide brachiopods. The burrows are inclined in thedirection of cleavage, indicating that tilting is produced by tectonic deformation and supportingan assignment to Lingulichnus verticalis. Escape behaviour is inferred from the specimenscrosscutting thin-bedded turbidites. Dwelling behaviour can be interpreted from the exampleswhere the lingulide is preserved below the burrow, as it represents an organism that retracted intoits burrow during a high hydrodynamic energy event and was unable to reposition to the surfaceafter sedimentation and burial associated with the high energy event. Equilibrium behaviour isinterpreted in spreite observed within mudstone, indicating gradual vertical migration on a slowlyaccreting seafloor.The present report expands the variety of behaviours and feeding styles in early Paleozoicdeep-marine environments and the bathymetric range of Lingulichnus. The abundance oflingulide trace fossils may reflect an environment rich in suspended food due to the occurrenceof continuous bottom currents. As lingulides are opportunistic organisms adapted to stressed,relatively high-energy environments such as intertidal settings [7], both their suspension-feedingstrategy and the ability to burrow responding to erosion and rapid sedimentation probably enabledlingulide populations to thrive in these unusual proximal settings of the deep sea.