Permian nonmarine invertebrate trace fossils from southern Patagonia, Argentina: Ichnologic signatures of substrate consolidation and colonization sequences

Abstract

Floodplain deposits of the Permian Laguna Polino Member (La Golondrina Formation) of southern Patagonia, Argentina, contain a moderate diversity invertebrate ichnofauna. This member consists of up to 500 m of ripple cross-laminated and trough cross-bedded, medium-grained sandstone and mudstone, with some conglomerate. It records sedimentation in braided fluvial environments with moderate channel sinuosity. The system was characterized by the development of multistory channels, transverse or linguoid bars, and levees. At the Laguna Grande locality, trace fossils were recovered from current-rippled, very fine-grained muddy sandstone beds formed in a pond adjacent to the levee. The ichnofauna consists of a mixed pascichnia, repichnia, and domichnia assemblage composed of Cochlichnus anguineus, Ctenopholeus kutscheri, Helminthoidichnites tenuis, Helminthopsis abeli, and Palaeophycus striatus. These low-energy ponds were more suitable for development of benthic fauna and preservation of ichnofossils than were fluvial channels and point bars. Accordingly, preservation of distinct trace fossil-bearing pond horizons within an otherwise unfossiliferous fluvial succession may be regarded as taphonomic and colonization windows. The colonization history of these beds can be traced through the study of cross-cutting and substrate relationships. Detailed ichnologic analysis suggests the following succession: 1) large Cochlichnus tracemakers colonizing transitional soupgrounds to softgrounds; 2) large and small Cochlichnus, Helminthopsis and Helminthoidichnites formed in softgrounds; and 3) Palaeophycus and Ctenopholeus emplaced in transitional soft to firm substrates. The colonization sequence suggests an ecologic replacement of a surface mobile fauna dominated by nematodes, nematomorphs, and larval insects by a more stationary infauna consisting of adult insects as a result of increasing consolidation. Presence of Ctenopholeus is significant because it represents the second report of this ichnogenus and the first from nonmarine settings.