Enterolithic folds in evaporites as microbially induced sedimentary structures: New model of formation and interpretation in the geological record

Abstract

Enterolithic structures are stratigraphically localized folds in gypsum beds found in certain saline evaporitic sedimentary units in a wide variety of basins. Different models of formation have been proposed, all related to inorganic processes. These models include: diagenetic transformation of gypsum beds producing either displacive growth of crystals or volume changes; mechanical folding caused by compressional stress; and folding produced by slumping. The analysis of three Cenozoic evaporite sequences in Spain reveals that none of the previous models explains their origin and existence. In these outcrops, gypsum enterolithic structures occur in horizontal beds with parallel troughs and crests of the folds. They appear in shoreline facies of lacustrine environments and did not undergo major diagenetic transformations after the primary lithification of the original sediment. Based on these observations, together with the study of a modern analogue in Minorca, Spain, a new model is proposed for the genesis of enterolithic structures. This new model is based on the existence of a microbial mat exposed to brine concentration–dilution cycles and strong wind events. The high wind flow events enhanced folding of the microbial mat that became subaerially exposed and lithified due to subsequent evaporation. Therefore, the presence of enterolithic structures could be used as an indicator of shallow water environmental conditions subject to variations in brine concentration in areas with strong wind flow events. Previous studies of some evaporitic successions should be revisited, taking into account the proposed model, which would imply new depositional environment interpretations. At the same time, the proposed model could explain the existence of Kinneyia‐type structures, also known as wrinkle structures, formed beneath microbial mats in peritidal zones. Moreover, considering enterolithic structures as microbially induced sedimentary structures could be useful as evidence of microbial life in the ancient geological record and on other planets such as Mars.

A model for the early diagenesis of humid climate, fluvial strata influenced by adjacent salt deposits: Grande Anse Formation, Cumberland Basin, Canada

“…3) documented radial quartz crystals growing out from a central void, which she interpreted to have been a replacement of anhydrite; prismatic quartz was also encountered in her Carboniferous ‘silicified evaporites’. Escavy & Herrero (2019) noted that enterolithic structures are usually composed of gypsum and produced in saline shallow‐water environments, but similar SIII structures in the lower member of the Grande Anse Formation are of fibrous, chalcedonic quartz (Fig. 9A and B).…”

The onset of the Messinian Salinity Crisis in the central Mediterranean recorded by pre‐salt carbonate/evaporite deposition

“…However, in the observed outcrops the enteroliths do not show evidence of significant diagenetic alteration and they are limited to restricted and precise layers underlain and overlain by generally flat microbialite beds (FA-1) and gypsum deposits. This evidence has led the authors to consider the morphology as the result of a syn-sedimentary alteration process, such as due to storm events as proposed for some modern and ancient examples in Spain by Escavy & Herrero (2019). In particular, strong winds that may occur seasonally, can detach and fold a microbial mat that formed subaqueously and can bring it into subaerial conditions; gypsum cement may then be precipitated, lithifying the contorted layers.…”