Abstract
A wide variety of sub-ripple-scale sedimentary surface textures are known from bedding planes in the sedimentary rock record. Many of these textures were traditionally ascribed an abiotic origin (e.g., due to rain drop impact, adhesion, etc.), but in recent decades the role of microbial mats and biofilms in sculpting and mediating some forms has become increasingly recognized. Microbial sedimentary textures are now well-described and understood from modern tidal environments and biological soil crusts, but descriptions from fluvial settings are less common, despite their known occurrence in ancient alluvium. This paper reports a suite of primary sedimentary surface textures which were observed forming in discrete bodies of standing water in the lower reaches of the ephemeral Murchison River, Western Australia. Microbial sedimentary signatures included bubble impressions (burst and intact) and roll-ups, in addition to reduced horizons. Many of these features exhibited rapid temporal evolution of their morphology in the dry days following an interval of heavy rain. Significantly, these microbial features were witnessed in close spatial proximity to other abiotic and biotic sedimentary surface textures including raindrop impressions, adhesion marks, desiccation cracks, and vertebrate and invertebrate traces. Such proximity of abiotic and microbial sedimentary surface textures is rarely reported from bedding planes in the rock record, but these modern observations emphasize the fact that, particularly in non-marine environments, such structures should not be expected to be mutually exclusive. An appreciation of the fact that primary sedimentary surface textures such as these develop during intervals of stasis in a sedimentation system is crucial to our understanding of their significance and diversity in the rock record.
Highlights
The contribution of microbial mats and biofilms towards shaping textures on siliciclastic bedding planes in the geological record has been the focus of much research in recent decades (e.g., Krumbein 1994; Hagadorn and Bottjer 1997; Pfluger 1999; Gerdes et al 2000; Schieber et al 2007; Noffke 2010; Seckbach and Oren 2010; Noffke and Chafetz 2012) and many such textures can be considered ‘microbially induced sedimentary structures’, or ‘MISS’ (Noffke et al 2001)
Fossilized examples of such features have commonly been explained through analogy with modern textures that can be seen forming under a microbial influence (e.g., Grazhdankin and Gerdes 2007), and modern MISS are well-documented from mats forming in tidal (e.g., Gerdes and Klenke 2007; Cuadrado et al 2014), ephemeral lacustrine (e.g., Beraldi-Campesi and Garcia-Pichel 2011), and desert settings (e.g., Beraldi-Campesi et al 2014)
Where records of ancient MISS have been reported, it is only rarely that they have been recorded as co-occurring with abiotic textures such as adhesion marks or raindrop imprints, despite this association being common in modern settings (Davies et al 2016)
Summary
The contribution of microbial mats and biofilms towards shaping textures on siliciclastic bedding planes in the geological record has been the focus of much research in recent decades (e.g., Krumbein 1994; Hagadorn and Bottjer 1997; Pfluger 1999; Gerdes et al 2000; Schieber et al 2007; Noffke 2010; Seckbach and Oren 2010; Noffke and Chafetz 2012) and many such textures can be considered ‘microbially induced sedimentary structures’, or ‘MISS’ (Noffke et al 2001). The purpose of this short paper is to describe modern examples of primary sedimentary surface textures (microbial, neoichnological, and abiotic) co-occurring and actively forming in drying pools of water in the Murchison River of Western Australia (Table 1).
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