Dominance of Phycosiphon incertum vs Zoophycos in shelf environment: example from the transgressive Palermo Aike black shale deposits of the Austral-Magallanes Basin, Argentina

Abstract

The Zoophycos ichnofacies and its main components, Zoophycos isp., Phycosiphon incertum and Chondrites isp. have been related to marine environments of low energy and oxygen. This situation is very common in shelf environments of the past, the most important oil source rocks and/or unconventional reservoir. The Early Cretaceous Palermo Aike Formation of the Austral-Magallanes Basin (Southern Argentina) records marine sedimentation, composed mainly by black shales, with subordinate fine-grained sandstones, and redeposited tuff, deposited in a shelf environment. This succession is the main source rock and constitute a latent unconventional reservoir of the basin. In this paper we present the sedimentological and ichnological study of two well cores from the Tierra del Fuego area. These cores record the initial stage of the marine transgression, and the development of a distal shelf environment, with an alternation of black shales and, fine-grained tuff sandstones hyperpycnal deposits. The ichnological content that records this transgression shows three different ichnofacies: i) autochthonous Cruziana ichnofacies represented by Thalassinoides isp., Teichichnus isp., Planolites isp., Chondrites isp. and Rizocorallium isp. This trace fossil association is rapidly replaced by ii) Zoophycos ichnofacies represented by Phycosiphon incertum, Chondrites isp., and small Zoophycos isp. Finally, another allochthonous Cruziana ichofacies is observed related to the low-density turbiditic deposits, composed by Thalassinoides isp., Gyrolithes, Teichichnus isp., Scolicia isp. and Haenzchellinia isp. In the Santa Cruz area, a large Zoophycos isp. dominance is observed in the fine-grained deposits, occasionally interbedded with coarser-grained low-density turbiditic deposits with doomed pioneers. In the study area (Tierra del Fuego), on the contrary, a dominance of Phycosiphon incertum is observed instead of Zoophycos isp. which when is present have very poor development in size. This could be due to the fact that low-density turbiditic flows are very frequent in the study area, which generates a very unstable environment that does not allow the development of large Zoophycos isp. This environmental differences between the outcrops of Santa Cruz in the north of the basin and the subsurface study area in Tierra del Fuego may be triggered due to climatic, volcanic, or tectonic allogenic control factors.