The Depositional Record (Feb 2022)
A multi‐scale approach to laminated microbial deposits in non‐marine carbonate environments through examples of the Cenozoic, north‐east Iberian Peninsula, Spain
Abstract
Abstract This contribution focusses on stromatolites and oncolites as tools to seek diverse environmental and climate information at different temporal scales. The scales are: (a) Low frequency, dealing with macroscopic and megascopic scales, and (b) high frequency, involving calendar and solar frequency bands. Two depositional environments are used for this purpose: (a) Fluvial and fluvial–lacustrine, which can develop under high to moderate gradients, and in low‐gradient conditions, and (b) lacustrine, subject to low‐gradient, hydrologically closed lake conditions. Several current and ancient examples in the Iberian Peninsula allow high‐frequency and low‐frequency analyses. Within the wedge‐shaped depositional units that fill the high‐ to moderate‐gradient, stepped fluvial systems, stromatolites form half domes and lenticular bodies, commonly at the wedge front. Oncolites are uncommon. These stromatolites developed in moderate to fast‐flowing water in stepped cascades and rapids. Their geometry and extent reflect the topography of the bedrock and later ongoing growth. In low‐gradient fluvial and fluvial‐(open) lacustrine systems the depositional units are tabular, low‐angle wedge‐shaped and lenticular and have great spatial facies variability. The dominant oncoid and coated‐stem limestones form gently lenticular stacked bodies, developed in wide, low to high‐sinuosity channels within wide tufaceous palustrine areas and small lakes. In the Ebro Basin saline carbonate lacustrine systems, stromatolites form thin planar to domed and stratiform bodies and are associated with muddy‐grainy laminated carbonates and very rare oncolites, together forming ramp‐shaped units that represent the inner fringes of high lake‐level deposits. This geometry reflects low‐gradient lake surface and shallow water conditions. Textural and structural features allow different ranks of laminae and types of lamination to be distinguished. Texture, together with the δ13C and δ18O values of consecutive laminae, are useful in distinguishing environmental and climate changes operating over different time spans. Periodicity analysis of lamination can help to discern any temporal significance in the lamination.
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