Diversity and factors controlling widespread occurrence of syn-rift Ladinian microbialites in the western Tethys (Triassic Catalan Basin, NE Spain)

Abstract

The fault-block carbonate ramps of the Ladinian (Middle Triassic) Catalan Basin hosted a wide variety of microbial deposits which formed during a syn-rift stage. Stratigraphical and sedimentological analyses of these microbialites allow us to reconstruct two depositional models for such microbial deposits. Moreover new insights into the interplay between the intrinsic and extrinsic factors that controlled the widespread development of these microbialites are provided. Stromatolites, ooidal-muddy microbial laminites and thrombolites were analyzed on the basis of their geometry, lamination and textural attributes. These microbialites are distributed over two Transgressive–Regressive sequences and coexisted during the Fassanian (Early Ladinian) regressive stage. Later, stromatolites and ooidal-muddy microbial laminites developed during the Longobardian (Late Ladinian) transgressive stage. Three types of lamina couplets reflect distinct accretion processes linked to water chemistry, sediment supply and hydrodynamic conditions. Thrombolite textures were produced by accretionary, binding and encrusting processes (microbially mediated) and pervasive cementation.The widespread occurrence of microbial deposits is explained as a consequence of a biogeochemical cascade of events (e.g., anoxic/dysoxic seawater conditions, volcanic activity, upwelling of alkaline waters, nutrient concentrations, and microbial blooms), which promoted favorable conditions for microbial growth in the Tethys during the Ladinian. Furthermore, in the Catalan Basin, syn-rift fault induced subsidence and local water energy gradients exerted an essential role in the distribution of the three types of microbialites through space and time. Thrombolites were prone to grow in the hanging wall block of a half-graben (subtidal conditions), whereas stromatolites and ooidal-muddy microbial laminites flourished preferably in a foot wall block setting (intertidal–shallow subtidal conditions). However, the fact that thrombolites exhibit abundant botryoidal and isopachous fibrous marine cements, corrosion and significant microbial evidence allow us to hypothesize about the relationship between thrombolite occurrence and hydrothermal fault-controlled fluid circulation during the Triassic rifting.The two depositional models here proposed constitute a step forward the understanding of the platform-to-basin microbialite heterogeneity during the Ladinian. Furthermore this work sheds new light on the mechanisms that likely promoted microbialite development during a period of major ecological restructuration and complex oceanographic conditions.