REFINING CRUZIANA STRATIGRAPHY: THE CASE OF TRANSITIONAL ICHNOTAXA IN THE LOWER PALEOZOIC OF NORTHWESTERN ARGENTINA

MUÑOZ, DIEGO F.; MÁNGANO, M. GABRIELA; WAISFELD, BEATRIZ G.; BUATOIS, LUIS A.

Florianópolis

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

The Central Andean Basin comprising northwestern Argentina, northern Chile, western Bolivia, and southern Perú is well known for the presence of highly fossiliferous upper Cambrian-Lower Ordovician rocks. The occurrence of both body and trace fossils in the same outcrops allows to better constrain the age of animal-substrate interactions. The aim of this contribution is to evaluate the transition between the Cruziana semiplicata and Cruziana rugosa groups based on the presence of trace and body fossils. The biostratigraphy based on trilobites, conodonts, graptolites and chitinozoans in the basin is well established at high resolution [1-6]. Also, the presence of the Cruziana semiplicata and Cruziana rugosa groups has been extensively documented (see [7 ] and references therein, and [8]). The main trilobite authorship suspects are olenids and asaphids (Neoparabolina and Ogygynus for the Argentina specimens) for the C. semiplicata and C. rugosa groups respectively (see [9]). During the Tremadocian–Floian transition, a significant trilobite faunal turnover took place in northwest Argentina, as evidenced by the replacement of olenids by asaphids (see [10] and references therein). This is coincident by a turnover of trilobite trace fossils as illustrated by the replacement of the Cruziana semiplicata by the Cruziana rugosa groups in the same region [7, 11-12].The stratigraphic range of the Cruziana rugosa group (i.e., C. rugosa rugosa, C. rugosa furcifera, and C. rugosa goldfussi) have been traditionally considered Lower Ordovician indicators, but specimens from Bolivia triggered the debate about their younger range [8]. As a result, the C. rugosa group is now considered as Ordovician in age without further precision. On the other hand, the distinctive ichnospecies Cruziana semiplicata tends to be considered an upper Cambrian (Furongian)–Tremadocian indicator. In turn, Cruziana tortworthi and C. breadstoni were proposed as “transitional ichnotaxa” signaling Tremadocian strata in other Gondwana regions [11,13,14 ]. These last two ichnotaxa display morphologic features from both C. semiplicata (e.g. external fine scratch imprints and marginal ridge) and C. rugosa (e.g., deeper, regular, subparallel, multiple sets of scratch imprints). Particularly, the Rupasca Member (early late Tremadocian age; Bryograptus kjerulfi graptolite Biozone/Bienvillia tetragonalis trilobite Biozone) of the Santa Rosita Formation is host to C. semiplicata, directly associated with C. tortworthi and C. breadstoni. These deposits are restricted to the upper part of the B. tetragonalis Biozone according to the revised concept of Meroi Arcerito et al. [1]. A low diversity trilobite association dominated by the olenid Leptoplastides granulosus (Harrington) and with the asaphid Asaphellus stenorachis (Harrington) as a subordinate element occur in Cruziana-bearing strata. Finally, the presence of these “transitional ichnotaxa” in these well-defined age strata allow us to conclude that the trilobite trace fossils of the Rupasca Member are key to provide new precision on Cruziana stratigraphy for the Tremadocian.Acknowledgements: Project funded by: CONICET PIBAA 28720210100955, CONICET PIP 11220200102403CO.