Triassic ichnofaunas in distal alluvial fans: Evidence from the Cerro Puntudo formation, Cuyo Basin, Argentina

摘要

Alluvial fan deposits of the Cerro Puntudo Formation (Triassic, Cuyo Basin, San Juan province) contain a low-diversity ichnofauna. The analyzed section indicates sedimentation in the most distal portion of an alluvial fan system. Palaeophycus tubularis Hall, Palaeophycus striatus Hall, and small sub-horizontal and inclined trace fossils occur in laterally extensive tuffaceous limestone, interpreted as playa-lake deposits originated by flash floods. They are attributed to the Scoyenia, ichnofacies. Massive and trough cross-stratified medium- and fine-grained sandstones, filling small channelized bodies, contains a monospecific association of sparse Skolithos isp. illustrating the Skolithos ichnofacies. Low diversity, moderate abundance, and the presence of simple dwelling structures suggest an opportunistic association. Based on cross-cutting relationships among biogenic structures and preservation of morphological details, an evolution from a soft to a relatively firm substrate is proposed. Two suites have been distinguished in connection to progressive dewatering of floodplain sediments: 1) Palaeophycus tubularis Hall and small sub-horizontal and inclined trace fossils, developed in a soft substrate, and 2) Palaeophycus striatus Hall associated with small sub-horizontal and inclined trace fossils, developed in a relatively firm substrate.