TRAZAS FOSILES E ICNOFABRICAS EN DEPOSITOS CARBONATICOS CRETACICOS, LAS CUEVAS, ALTA CORDILLERA DE MENDOZA.

Abstract

An upper Neocomian carbonate succession, assigned to the Agrio Formation (Mendoza Group), crops out at Quebrada de La Avalancha, Las Cuevas (Mendoza, Argentina). The whole section shows a general regressive trend. It is mainly composed of stacked shallowing-upward cycles. Each cycle consists, from base to top, of three facies: (A) Fossiliferous Limestones deposited in an offshore setting; (B) Nodular limestones formed in an offshore-shoreface transition and (C) Massive limestones interpreted as lower shoreface deposits. Facies B shows pervasive bioturbation evidenced by a Thalassinoides paradoxicus ichnofabric of bioturbation index 4. This form is considered an elite trace fossil created by both biologic and diagenetic processes. Mobile deposit-feeders, inhabiting medium tiers, obliterated shallower tier structures. Skolithos linearis, Schaubcylindrichnus isp. and Phycodes aff. P. palmatus are present in facies C. This facies displays lower ichnofabric indices (BI 2-3). Burrows are commonly isolated, but locally overlap. The ichnogenus Schaubcylindrichnus is only known from the Cretaceous of United States and the Neogene of Taiwan. This ichnogenus has only been reported from shallow water deposits. The occurrence of the present examples in lower shoreface deposits is consistent with other Cretaceous reports.