Microbial mats and their Role in the preservation of vertebrate footprints: modern tracks and their fossil counterparts

CARMONA, N.B.; BOURNOD C.N.; PONCE, J.J.; CUADRADO D. G.; WETZEL, A.

St. John?fs

Congreso; ICHNIA 2012; 2012

Studies of recent environments with microbial mats increased considerably during the last years, not only because their unique sedimentological and ecological characteristics, but also because they provide important implications for the understanding of fossil environments and palaeocommunities. The role of microbial mats in the preservation of bird tracks was studied in detail in lower-supratidal flats of the Bahia Blanca estuary, Argentina. For several months a number of selected footprints were photographed to record the morphological modifications they experienced with time. The footprints showed resistance to tide-and wind-erosion and also to heavy rains and storms. The strongly lowered erodability is clearly associated to the presence of the microbial mats, which biostabilize the sediment. In addition, microscopic analysis of the tidal-flat sediment revealed the presence of zeolites and calcite being suggestive of early cementation, which very likely enhanced the preservation potential of the footprints. Along with these observations, the study of fossil counterparts analyzed in the Rio Negro Formation (Lower Miocene-Pliocene), allowed the identification of various microbially induced sedimentary structures in tidal-flat facies. These Neogene deposits, in fact, represent classical localities for the study of vertebrate trace fossils in Argentina, while containing diverse, excellently preserved tracks of mammals and different birds. Thus, considering the data from the modern setting and the information from sedimentological studies in the rock record, it becomes evident that the presence of microbial mats played an important role in the preservation of the footprints mainly in three ways. First, the microbially stabilized sand behaved plastically, preserving the imprints better than pure cohesionless sand. Second, microbial mat decay induced early cementation processes, stimulating mineral precipitation, as it occurs in modern settings. And third, the near-surface cementation and the reducing conditions below the microbial mats also precluded the colonization by large endofauna, preventing therefore the obliteration of the vertebrate footprints.