Ichnology of an Intertidal Palaeosurface: The Record of Tidal Water Fluctuations, Palaeotopography, Microbial Mat Variability and Trace Fossil Distribution in the Lower Cretaceous of Patagonia, Argentina

PAZOS, PABLO J.; FERNÁNDEZ, DIANA E.; COMERIO, MARCOS; OTTONE, E. GUILLERMO

Saint John's

Congreso; Ichnia, Third International Congress on Ichnology; 2012

Memorial University

Tidal flats are complex environments influenced by tidal fluctuations, fluvial discharge andclimate; the interplay of such factors affects the time of subaerial exposure, microbial matdevelopment and preservation of trace fossils in recent and ancient records. Microtopography related to the lateral migration of a tidal channel enabled the analysis of trace fossils with physical and biologically induced sedimentary structures. In the depressed areas, tidal ebb ripples are well preserved. These contain delicate xiphosuran trackways developed on the ripple crests before the water level fell. In the ripple troughs, exquisite meandering and looping trace fossils are documented. The preservation is enhanced by microbial mats that favoured undermat miners, but precluded vertical burrowers. Water table fluctuations are evidenced in flattened ripple crests, longitudinal marks of temporary water level positions that vary laterally, indicating inclined palaeotopography. The shallow palaeotopographic areas show almost complete erosion of ripples and development of epistratal mats where slippery conditions are documented by vertebrate tracks, tentatively assigned to dinosaurs. The examination and comparison of the microbial mats under SEM, permits to conclude that these were unaltered where the invertebrate trace fossils are  recorded, but they were disturbed and remobilized in the rim of the slippery tracks, confirming an epistratal nature of the biofilm but also the disruption of the mat without conclusive evidence of mat exposure. Moving laterally in the direction of the shallowest area, only deformation on the sedimentary induced structures is recognized, but organized in a perpendicular pattern of deformation, documenting complex water circulation during tidal ebb. An absence of ebb ripples and dinoflagellates in the overlying interval suggests that the trace fossils were produced most probably during the tidal flood, while reduction of the water level related to the palaeotopography took place, without any evidence of fluvial interaction on the tidal flat during the ebb part of a tidal cycle.