RECOGNIZING MICROBIAL MATS IN FLUVIAL-TIDAL SILICICLASTIC DEPOSITS AFFECTED BY VERY LOW-GRADE METAMORPHISM: COMPARISON OF THE DINOSAUR TRACK-.BEARING ONCALA GR. (LOWER CRETACEOUS, N SPAIN) WITH PRESENT -DAY EXAMPLES FROM ARGENTINA

EMMA QUIJADA; DIANA CUADRADO; MARIBEL BENITO; LUCIA MAISANO

LONDRES

Congreso; LYELL MEETING2017. STICKING TOGETHER: MICROBES AN DTHEIR ROLE IN FORMING SEDIMENTS; 2017

THE GEOLOGICAL SOCIETY

Present-day tidal flats are commonly colonized by microbial mats, which alter sediment properties as a consequence of the interaction between microorganisms and sediments. Thus, their recognition in ancient siliciclastic tidal deposits is important to understand their impact on sediment dynamics and their role on the formation and preservation of sedimentary structures, such as vertebrate footprints.This is the case of the Oncala Gr, which was deposited during the Berriasian in the Cameros Basin, located in northern Spain. The Oncala Gr contains siliciclastic deposits made up of laterally continuous layers of interlaminated mudstone, siltstone and very fine to medium-grained sandstone, and meander loop bodies showing inclined heterolithic stratification, which are interpreted as formed in broad fluvial-tidal flats traversed by meandering channels. These deposits are characterized by ubiquitous tracks of dinosaurs and other vertebrates, and desiccation cracks. The Oncala Gr was buried under more than 5000 m of Cretaceous sediments, and was affected by very low-grade metamorphism. Despite the important compaction and metamorphism of the Oncala Gr, the comparison of the laterally continuous, laminated siliciclastic deposits of this unit with present-day microbial mats from the upper intertidal-supratidal flats of the Bahía Blanca Estuary and from a supratidal plain in Paso Seco (Argentina) allowed the identification of the following characteristics and microbially-induced sedimentary structures (MISS): 1) Extremely thin (100 µm-1 mm) lamination made up of alternating siliciclastic mudstone and siltstone/fine-grained sandstone, occasionally showing pinch and swell structures, which resembles present-day biolaminites (Figs. 1A-D). 2) Mudstone laminae show 0,3-1 mm tall pinnacle microstructures (Fig. 1C), strikingly similar to the erected bundles of cyanobacteria filaments developed in areas of the supratidal plain of Paso Seco that remain flooded for long periods of time. 3) Sand grains floating within the mudstone laminae, resembling the grains included within the biomass layers of the biolaminites of the upper intertidal-supratidal flats of the Bahía Blanca Estuary (Figs. 1E-F). 4) Rippled sandstone layers showing thin mud laminae draping the foreset laminae, like the ones in the upper intertidal-supratidal flats of the Bahía Blanca Estuary resulting from repetitive rapid colonization and production of biomass on the ripple lee faces, followed by a new sand input covering the leeward slopes. 5) Rectangular mud chips, similar to fragments of present-day desiccated microbial mats reworked by storms in the upper-intertidal flats of the Bahía Blanca Estuary or wind currents in the supratidal plain of Paso Seco. 6) Lamina top surfaces specked with rounded particles, which could be similar to the dispersed grains stuck on the surface of the present-day mats. In addition to these, typical MISS from ancient sediments, such as wrinkle structures (Fig. 1G) and irregular polygonal textures (Fig. 1H) on lamina top surfaces, are also present.All these evidence indicate that the siliciclastic flats of the Oncala Gr were likely colonized by microbial mats. Thereby, the biostabilization of the sediments allowed the preservation of delicate sedimentary structures (such as thin lamination, desiccation cracks at the top of every submillimetre-thick mudstone lamina, consecutive ripple trains, and tracks) in an environment affected by tide and wave currents that would have otherwise eroded them. Moreover, microbial mats probably prevented the formation of deep dinosaur tracks in the predominantly fine-grained sediments of the Oncala Gr, as occurs nowadays when stepping on modern, cohesive microbial mats (Fig. 1I-J).