3.4.1 GeoBiology: Examples of Modern Microbially Induced
Sedimentary Structures (MISS) and Their Fossil
Counterparts
It has been previously stated that GeoBiology has largely benefited from actualistic
approaches, interpreting ancient processes and fossil structures in tidal sandstones of
all Earth ages, from the study of modern, analogous MISS (Noffke et al. 2008). In
this section, a few examples of modern MISS derived from microbial activity will be
presented comparatively with their fossil and sedimentary sequence counterparts.
All three examples are drawn from the siliciclastic basin at Paso Seco, for which the
structure of a hypersaline microbial mat has been described in the previous section.
Gas domes arise from post-burial decay of buried mats and the accumulation of
archaea-produced methane in deep anoxic layers, which diffuses upward and gets
trapped underneath a levelled, cohesive, plastic mat (Gerdes 2007). Eventually, the
mechanical stress created by the accumulation of gas acting upon a biostabilized
surface leads to the formation of gas domes that may or may not become encrusted
by gypsum (Gerdes et al. 2000). A gas dome is a somehow labile, transient structure
that evolves into another type of MISS termed pustular structure (Bohacs and
Junium 2007) or tepees (Gerdes et al. 2000; Gerdes 2007). This is achieved through
cycles of desiccation and re-wetting, gypsum encrusting, and ecological succession
of the microbial community, and it may happen in a relatively short period of time
(months). Pustular structures known as tepees have been recognized in modern
(Fig. 3.4a) (Horodyski et al. 1977; Gerdes 2007) and fossil environments (Schieber
2004; Bohacs and Junium 2007). Figure 3.4b illustrates Neoproterozoic pustular
structures in section, embedded within a biolaminite sequence (Bouougri and Porada
2002).
The modern “pinnacles” and protruding tufts shown in cross section in Fig. 3.4c
are three-dimensional MISS that form from regular reticulate patterns created by
highly motile cyanobacteria, at their junctional encounter points over a levelled
plane. The process implies the secretion of copious amounts of EPS by the microbial
community when submerged under stagnant seawater and subjected to repeated
cycles of desiccation and rehydration (Cuadrado and Pan 2018). Ultimately, these
“pinnacles” become a perennial feature with an active role in sediment baffling and
trapping that can withstand high-energy hydrodynamic regimes. Their fossil counterparts replicating their morphology and morphometrics (Fig. 3.4d) have been
described for 3.22-Ga-old Archaean coastal deposits from the Barberton belt in
South Africa (Homann et al. 2015).
Mat folds are a type of MISS that are formed by a series of hydrodynamic process
acting on a levelled microbially colonized sediment. First, microbial mats subject to
pulsating inundation get thickened and hardened by cycles of desiccation and
rehydration, as the underlying sandy sediment is liquefied (Cuadrado et al. 2014).
Subsequently a tear (that acts as a weakness line) may be formed due to solar
radiation, and then, under hydrodynamic shear stress acting on the flexible and
re-wetted mat surface, folds are formed and detached from the liquefied sandy
72
J. Pan
Sedimentary Structures (MISS) and Their Fossil
Counterparts
It has been previously stated that GeoBiology has largely benefited from actualistic
approaches, interpreting ancient processes and fossil structures in tidal sandstones of
all Earth ages, from the study of modern, analogous MISS (Noffke et al. 2008). In
this section, a few examples of modern MISS derived from microbial activity will be
presented comparatively with their fossil and sedimentary sequence counterparts.
All three examples are drawn from the siliciclastic basin at Paso Seco, for which the
structure of a hypersaline microbial mat has been described in the previous section.
Gas domes arise from post-burial decay of buried mats and the accumulation of
archaea-produced methane in deep anoxic layers, which diffuses upward and gets
trapped underneath a levelled, cohesive, plastic mat (Gerdes 2007). Eventually, the
mechanical stress created by the accumulation of gas acting upon a biostabilized
surface leads to the formation of gas domes that may or may not become encrusted
by gypsum (Gerdes et al. 2000). A gas dome is a somehow labile, transient structure
that evolves into another type of MISS termed pustular structure (Bohacs and
Junium 2007) or tepees (Gerdes et al. 2000; Gerdes 2007). This is achieved through
cycles of desiccation and re-wetting, gypsum encrusting, and ecological succession
of the microbial community, and it may happen in a relatively short period of time
(months). Pustular structures known as tepees have been recognized in modern
(Fig. 3.4a) (Horodyski et al. 1977; Gerdes 2007) and fossil environments (Schieber
2004; Bohacs and Junium 2007). Figure 3.4b illustrates Neoproterozoic pustular
structures in section, embedded within a biolaminite sequence (Bouougri and Porada
2002).
The modern “pinnacles” and protruding tufts shown in cross section in Fig. 3.4c
are three-dimensional MISS that form from regular reticulate patterns created by
highly motile cyanobacteria, at their junctional encounter points over a levelled
plane. The process implies the secretion of copious amounts of EPS by the microbial
community when submerged under stagnant seawater and subjected to repeated
cycles of desiccation and rehydration (Cuadrado and Pan 2018). Ultimately, these
“pinnacles” become a perennial feature with an active role in sediment baffling and
trapping that can withstand high-energy hydrodynamic regimes. Their fossil counterparts replicating their morphology and morphometrics (Fig. 3.4d) have been
described for 3.22-Ga-old Archaean coastal deposits from the Barberton belt in
South Africa (Homann et al. 2015).
Mat folds are a type of MISS that are formed by a series of hydrodynamic process
acting on a levelled microbially colonized sediment. First, microbial mats subject to
pulsating inundation get thickened and hardened by cycles of desiccation and
rehydration, as the underlying sandy sediment is liquefied (Cuadrado et al. 2014).
Subsequently a tear (that acts as a weakness line) may be formed due to solar
radiation, and then, under hydrodynamic shear stress acting on the flexible and
re-wetted mat surface, folds are formed and detached from the liquefied sandy
72
J. Pan