A new model for the formation of microbial polygons in a coastal sabkha setting

Abstract

The stratigraphic record of microbially induced sedimentary structures spans most of the depositional record. Today, microbes continue to generate, bind and modify sediments in a vast range of depositional environments. One of the most cited of these settings is the coastal microbial mat system of the Persian/ Arabian Gulf. In this setting, an extensive zone of microbial mat polygons has previously been interpreted as resulting from desiccation-related contraction during episodic drying. This study employs 15 years of field-based monitoring of the interaction between environmental factors and the development and evolution of polygon morphologies to test the desiccation model in this setting. On the basis of these observations, a new model is proposed that accounts for the genesis and development of microbial polygons without the need for desiccation-induced shrinkage. Conversely, the formation, development and erosion of microbial polygons is a direct result of the production of large amounts of organic matter in a healthy, yet spatially limited, microbial community. The recognition of microbial polygons has previously been applied as a diagnostic tool for the reconstruction of ancient depositional environments. The present study calls these interpretations into doubt. It is inferred that preservation of the microbial polygons as a recognizable form would be rare. Biological degradation and compaction will reduce polygons to produce the 'wispy' laminae that are a common feature of ancient sabkha lithofacies.

A microfacies analysis of arid continental carbonates from the Cedar Mesa Sandstone Formation, Utah, USA

“…MF3), were likely transported from these shallower areas, possibly similar to the transport of sediment in sabkha environments through higher-energy events as described by Lokier et al (2017). It is suggested that these clasts may be reworked from partially lithified sediments from the proximal sedimentary environment.…”

“…These most likely represent the deposits of laminated microbial mats (Van Gemerden, 1993;Riding, 2000;Lokier et al, 2017). Microbial mats are present in many environments, however, the spatial relationships seen here indicate a likely evaporitic depositional setting, either a marginal marine, or continental sabkha.…”

“…The carbonate grains formed of micro-spar may originate from reworked carbonate material, potentially transported into the system with the clastic component (cf. Lokier et al, 2017). The general lack of evaporites or of any desiccation structures in association with this microfacies suggests sufficient water to maintain long-standing pools/puddles of water suitable for carbonate development.…”

“…The presence of coeval lacustrine clastic sediments, and the framework of a continental playa-lake discussed by other authors (Langford and Massad, 2014) also support this argument. The distorted and fensetral form of the microfacies suggests lateral growth Lokier et al, 2017), loading and degassing. This could be driven by the encroachment of aeolian dunes, which would enhance preservation potential (as seen within MF1) during an arid climate (cf.…”

“…Lokier et al, 2017). The carbonate mud matrix along with the occasional carbonate grains may represent many depositional environments, however, the close lateral and stratigraphic proximity to sub-aerial deposits (Stxb, Sm) probably indicates extremely shallow (few centimetres) quiet waters.…”