Increasing size and abundance of microbialites (oncoids) in connection with the K/T boundary in non-marine environments in the South Central Pyrenees

Abstract

A layer rich in giant microbialites, developed within continental deposits of the Tremp Formation (South-Central
Pyrenees) in the aftermath of the end-Cretaceous mass extinctions, is here reported for the first time. Its stratigraphic
location just above the Latest Maastrichtian Reptile Sandstone unit, and a characteristic δ¹³C anomaly,
are clear proofs linking this so-called “z” layer to the K/T boundary. The great abundance of microbialites, their
outsized dimensions (oncoids with coating thickness up to 1.2m; average >50mm), and the great lateral continuity
of the “z” layer in the study zone, support the hypothesis that these giant microbialites may represent disaster
forms linked to the latest Mesozoic crisis.
To our knowledge, this is the first time a record of this type is reported in continental realms. Other large continental
oncoids and stromatolite constructions (e.g., in the Late Palaeogene deposits of the Pyrenees, Ebro basin,
and Mallorca island) differ from those of the “z” layer in occurring in carbonate rocks and in having less lateral
continuity, but they could also have been favoured by palaeoenvironmental crises, such as the global Eocene
hyperthermals, Early Eocene Climatic Optimum, Middle Eocene Climatic Optimum, and the climatic cooling of
the Eocene-Oligocene transition. It is thus possible that some layers of microbialites may record biotic crises in
continental series, as they do in the marine record.