Taphonomy of shell beds and the open surface furrow origin in Cruziana (trace fossil): a test case in the Ordovician of NW Argentina.

ACEÑOLAZA, G.F., ARIS, J. Y MILANA, J.P.,

Córdoba

Congreso; 9no Congreso Argentino de Paleontología y Bioestratigrafía,; 2006

facultad de Cs. Exactas y Naturales, UNC

Shell beds occur randomly distributed along the Cambro-Ordovician succession of NW Argentina, and several papers have focused on them from different points of view (ej. Mastandrea and Leanza, 1975; Waisfeld, 1997; Aceñolaza et al., 2003 with references). In addition, within the shallow water facies of the Cordillera Oriental of northern Argentina, abundant trace fossils have been described from several localities (Aceñolaza and Aceñolaza, 2002 with references), but the association of both elements, shell beds and ichnofossils, and the  taphonomic significance of this association has never been analyzed. Cruziana is typically represented by shallow open furrows mostly produced by trilobites in Paleozoic strata.  The locality of Zenta is one of the most impressive outcrops with Ordovician Cruziana in western Gondwana due to the abundance and preservation of traces (Aceñolaza and Milana, 2005). Material comes from the upper sector of the Ordovician sequence of Zenta, and has been taxonomically assigned to the “rugosa group” of Seilacher (1970). The formation of Cruziana as open furrows or true burrows has been a mater of detailed work (ej. Baldwin, 1977; Goldring, 1984; Seilacher, 1985), and its interpretation is important for understanding the paleophysiology of the animal responsible for its production, its purpose and mode of locomotion. The stratinomical relationship of Cruziana from Zenta can be fully appreciated by its interpretation in terms of the sedimentological and biological processes involved in the formation, taking advantage of the unique presence of shell beds associated with the traces.  Shell beds are composed by fragmentary and fully preserved disarticulated brachiopod valves (two different taxa), in a similar manner than early described material from Mojotoro in Salta (Aceñolaza et al., 2003). Preferentially found within the trace cast, these are rarely found at the intertrace areas. The occurrence of shells within Cruziana indicates that the trace (actually a furrow) was acting as an hydrodynamic trap for the shells. Bioclasts represented by shells were larger than the sediment that host them, which is in general a fine grained sandstone, so shells were deposited preferentially in the irregularities of the ground when the flow started to loose its energy. Sand beds with Cruziana casts show wave micro crossbedding as well as wave reworking of their tops, suggesting  flows associated to orbital motion at least in its waning stage. Ichnofossils and associated shell beds are found in muddy-rich heterolithic intervals alternating with sheet-like and ribbon like sand bodies, which also shows shell beds as traditional hydrodynamic lags in a sandy bed. In spite of its resemblance to turbidites, the succession suggest a sedimentation within an outer-shoreface environment with frequent shoaling bars that become shallower towards the top. After the furrowing activity of trilobites, open traces were filled by shell beds, as a consequence of event–concentration accumulated in a very short time period and transported for little distances by tractive non-erosive flows. These findings thus support a furrow origin for most Cruziana at Zenta.   Aceñolaza, G. and Aceñolaza F., 2002. Ordovician trace fossils of Argentina. In: Aceñolaza, G. (Ed.) Aspects of the Ordovician System in Argentina. INSUGEO, Serie Correlación Geológica, 16: 177-194. Aceñolaza, G. and Milana, J.P., 2005. Remarkable Cruziana beds in the Lower Ordovician of the Cordillera Oriental, NW Argentina. Ameghiniana, 42 (3): 633-637. Buenos Aires. Aceñolaza, G., Emig, C. and Gutiérrez-Marcos, J.C., 2003. Lingulid shell beds from the Ordovician of Argentina, with notes on other peri-gondwanan occurrences. In: Albanesi, G., Beresi, M. and Peralta, S. (Eds.) Ordovician from the Andes. INSUGEO, Serie Correlación Geológica, 17: 237-244. Baldwin, C.T., 1977. Internal structures of trilobite trace fossils indicative of an open surface furrow origin. Palaeogeography, Palaeoclimatology, Palaeoecology, 21: 273-284. Goldring, R., 1984. The formation of the trace fossil Cruziana. Geological Magazine, 122: 65-72. Mastandrea, O. and Leanza, H.A. 1975. Los estratos con Lingula del norte argentino y su importancia como fuente de fosfatos. 2º Congreso Iberoamericano de Geología Económica,  2: 445-460. Seilacher, A. 1970. Cruziana stratigraphy of  “non-fossiliferous” Paleozoic sandstones. En: T.P. Crimes and J.C. Harper (eds.), Trace Fossils. Geological Journal Special Issue, 3: 447-476. Seilacher, A., 1985. Trilobite palaeobiology and substrate relationships. Transactions of the Royal Society of Edinburgh. 76: 231-237. Waisfeld, B.G., 1997. Concentraciones fosilíferas ordovícicas en las formaciones Acoite y Sepulturas, Cordillera Oriental jujeña. Análisis tafonómico y paleoecológico. Ameghiniana, 34 (3), 317-332.