Ichnofabric análisis of a shallow marine deposit, Chenque Formation, Lower Miocene, Punta Delgada, Patagonia, Argentina.

CARMONA, N.B.; BROMLEY, R.G.; BUATOIS, L.A.; MÁNGANO, M.G.; BELLOSI, E.

Basilea, Suiza

Workshop; VII International Ichnofabric Workshop; 2003

Analysis of a well-preserved ichnofauna exposed at Punta Delgada locality (Chenque Formation, lower Miocene of Patagonia, Argentina) allowed a detailed description of the ichnofabric structure of these shallow marine deposits. The analysed interval corresponds to the lower portion of a coarsening upward parasequence, recording deposition in a lower shoreface environment. The totally bioturbated aspect of the sediment suggests a slow rate of sedimentation. The ichnofauna is dominated by Ophiomorpha and Asterosoma. Six ecological units or ichnoguilds have been identified: 1- Planolites montanus ichnoguild: this represents the shallowest-tier (recorded as the background mottling) and consists of vagile, deposit-feeder structures; 2- Asterosoma - Rosselia ichnoguild: this includes semi-vagile, shallow-tier, detritus and deposit-feeder structures; 3- Ophiomorpha ichnoguild: this ichnoguild contains stationary, middle-tier, suspension-feeder structures; 4- Thalassinoides - Schaubcylindrichnus ichnoguild: this represents semi-vagile to vagile, middle-tier, deposit-feeder structures; 5- Phycosiphon ichnoguild: this ichnoguild includes vagile, deep-tier, deposit-feeder structures; 6- Chondrites  ichnoguild: this represents non-vagile, deep-tier, deposit-feeder or chemosymbiont structures. The tiering structure is complex with at least seven preserved tiers. The dominance of deposit-feeder organisms, the poor development of the suspension-feeder ichnoguilds, and the extremely good vertical partitioning of the infaunal ecospace, reinforce the interpretation of this assemblage as a climax paleocommunity, with high ichnodiversity and abundance. Comparing this ichnofauna to that from Playa Las Cuevas locality (emplaced in a distal lower-shoreface environment), an accentuation of the ichnofabric trends (poorer representation of the suspension-feeders ichnoguilds and greater development of the deposit-feeder structures) displayed by the Punta Delgada ichnofauna is detected. The coexistence of these deposit-feeder structures could be interpreted by the “niche heterogeneity model”, which holds that each (ichno)species does best in one part of the niche space and, therefore, has a particular “place” within the community in a spatial and temporal context, allowing the development of this complex tiering structure.