Palaeoecological, sedimentological and stratigraphical insights into microbially induced sedimentary structures of the lower Cambrian successions of Iran

Abstract

Micro‐organisms producing microbially induced sedimentary structures, particularly epibenthic cyanobacteria, are not facies‐dependent and could flourish in any environment if appropriate ecological conditions were provided. Hence, the changes in environmental parameters are the controlling factors on ecological tolerance of the producers. This study on the lower Cambrian successions of the Lalun Formation in Central Iran shows that paralic environments reacted differently to changes in parameters such as river and tide energy, palaeo‐topography, the rate of sediment supply and fluctuations in sea‐level, even though all were characterized by sandy substrates suitable for the development of microbially induced sedimentary structures. Therefore, the abundance and preservation of microbially induced sedimentary structures varied in the different paralic environments. From a sequence stratigraphic viewpoint, this study demonstrates that erosional discontinuities lacked the conditions required for the substrate stabilization by microbial communities. The distribution, size and type of microbially induced sedimentary structures within high frequency cycles generally follow the trends of changes in vertical facies stacking patterns. Within systems tracts, the pattern, morphological diversity and size of microbially induced sedimentary structures are not dependent on the type of systems tract, but on the type of depositional system developed such as delta, incised valley, coastal plain, estuaries and shoreline to shelf systems. Generally, estuarine and peritidal carbonates record an increase in the development of mat colonization during the transgressive systems tract, owing to decreased sedimentation rate as well as extended shallow water habitats. In contrast, the existence of microbially induced sedimentary structures depends on the pattern of shoreline shift in depositional systems developed during the highstand systems tract, such as open coast tidal flat and delta environments. If a shoreline regression was continuous (depositional trend and stacking pattern are a set of high frequency cycles), a greater increase in the aggradational component than the progradational component would cause intensified destructive processes hindering the development of microbial communities.

The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation

“…These recent developments are in line with current observations from both modern and ancient microbial mats of various environments suggesting that MISS‐producing microorganisms are not facies‐dependent, being instead found in any environment where the optimum ecological and taphonomic conditions are attained (e.g. Bailey et al, 2006; Bayet‐Goll & Daraei, 2020; Bayet‐Goll, Daraei, et al, 2021; Davies et al, 2006, 2016; Schieber et al, 2007). In fact, a recent compilation of ancient and modern examples of MISS documented formation of microbial mats in a wide variety of settings, including storm‐ and tide‐influenced shallow‐marine, deep‐marine, and fluvial environments (Davies et al, 2016).…”

“…The distribution pattern of similar MISS in modern and ancient microbial mats in the coastal to shelf environments (e.g. Bayet‐Goll & Daraei, 2020; Bayet‐Goll, Daraei, et al, 2021; Bose & Chafetz, 2009; Buatois & Mángano, 2012b; Buatois et al, 2013; Duperron & Scasso, 2020; Laflamme et al, 2011, 2012; Pan et al, 2013; Shepard & Sumner, 2010) shows that bioturbation plays a major role in the development and preservation of microbial matground‐dominated ecosystems. Most modern microbial mats from shallow marine habitats are preferably found in areas with low amount of sediment mixing and bioirrigation by tracemakers, where due to the presence of harsh environmental stresses, disruption of mat‐fabrics by burrowing and grazing is significantly diminished (Bose & Chafetz, 2009; Gerdes et al, 2000; Schieber et al, 2007; Shepard & Sumner, 2010).…”

“…This change in their behavioral strategy allowed them to avoid quick sedimentation and burial (Savrda, 2002). In deltaic environments, especially in distal tidal bars and prodelta (D1–D2), the occurrence of fluid muds and flocculation of clay minerals resulted in the development of soupgrounds, which were highly inhospitable to infaunal organisms (Bayet‐Goll & Daraei, 2020; Bayet‐Goll, Daraei, et al, 2021; Bhattacharya et al, 2020; MacEachern et al, 2005). In particular, fluid muds in prodelta facies may have excluded most suspension feeders, instead leading to dominance of surface grazing and detritus feeding trace fossils ( Monomorphichnus , Helminthopsis , and Gyrochorte ), shallow‐tier structures of deposit feeders ( Teichichnus) , and morphologically simple forms and nonspecialized structures ( Planolites and Palaeophycus ).…”

“…In particular, fluid muds in prodelta facies may have excluded most suspension feeders, instead leading to dominance of surface grazing and detritus feeding trace fossils ( Monomorphichnus , Helminthopsis , and Gyrochorte ), shallow‐tier structures of deposit feeders ( Teichichnus) , and morphologically simple forms and nonspecialized structures ( Planolites and Palaeophycus ). On the other hand, however, it seems that soupgrounds with their highly adhesive surfaces prevent cyanobacteria from moving freely, thus suppressing the formation of microbial mats (Bayet‐Goll & Daraei, 2020).…”

“…For the growth and preservation of microbial mats, such as wrinkle structures, these filamentous, mat‐forming bacteria required extreme slowdown or long‐lasting break in deposition (Leonowicz et al, 2021). The sum of these requirements explains why wrinkle structures formed in deep‐marine turbidity systems are preferably found in facies with heterolithic strata of turbidites with intermittent sediment reworking and deposition, similar to their shallow‐water counterparts (see Bayet‐Goll & Daraei, 2020; Davies et al, 2016; Mariotti et al, 2014; Mata & Bottjer, 2012). Similarly, in nonamalgamated packages of E2 and E3, the presence of appropriate facies (heterolithic strata of turbidites) including thin‐bedded sandstone turbidites (low‐density turbidity currents) with mudstone intercalations provided ideal conditions for the development and preservation of both wrinkle structures and predepositional elements.…”