Microbial Mat Related Features In The Makgabeng Interdune Deposits

In addition to the "normal" sedimentary structures (e.g., cross-bedding, planar bedding, ripple marks, evaporite crystal imprints etc. - see Eriksson et al., 2000; Simpson et al., 2004) identified within the interdune facies at Makgabeng, four main groups of inferred microbial mat-related features were also observed:

1. Wrinkle structures (Fig. 3)

2. Sand cracks - from simple to complex-curved forms (Fig. 4)

3. Sand chips and desiccated larger fragments (Fig. 5)

3.3.1. Wrinkle Structures

These features were observed on a single siltstone bed upper surface, and comprise sinuous flat-topped ridges, which bifurcate and show a measure of parallelism of crest orientations within an overall more honeycomb-like pattern (Fig. 3). Ridge widths are up to about 5 mm, with spacings between ridges up to about 10 mm, and ridge heights are a few millimetres; ridges tend to be steep-sided (Fig. 3).

Figure 3. Wrinkle structures preserved on the upper surface of a siltstone bed in the Makgabeng Formation; lens cap for scale. Note bifurcations (splitting) of sinuous flat-topped ridges, and the measure of parallelism of ridge crests within an overall honeycomb-like pattern.

Figure 4. A variety of sand crack types observed within sandstone beds of the Makgabeng Formation. (A) Short, isolated, spindle-shaped cracks (hand lens for scale). (B) Apparent joining up of spindle-shaped cracks to form triradiate ("triple junction") crack patterns (pen for scale). (C) Longer and more sinuous sand crack patterns, to which shorter, triradiate cracks appear to be joined - note the triradiate crack at the pen-tip, which appears to be a separate crack system, adjacent to rather than connected to the longer, curved features. (D) Triradiate crack set, between modern crack in sandstone bed and middle of pen (scale), which appears to be incompletely linked to sinuous, longer crack features. (E) As longer cracks develop, they become even more sinuous, even developing circular patterns (pen for scale). (F) Larger-scale view of rippled upper sandstone bed surface in the Makgabeng Formation, covered by a variety of sand cracks, mainly of higher sinuosity and circular geometry.

Figure 4. A variety of sand crack types observed within sandstone beds of the Makgabeng Formation. (A) Short, isolated, spindle-shaped cracks (hand lens for scale). (B) Apparent joining up of spindle-shaped cracks to form triradiate ("triple junction") crack patterns (pen for scale). (C) Longer and more sinuous sand crack patterns, to which shorter, triradiate cracks appear to be joined - note the triradiate crack at the pen-tip, which appears to be a separate crack system, adjacent to rather than connected to the longer, curved features. (D) Triradiate crack set, between modern crack in sandstone bed and middle of pen (scale), which appears to be incompletely linked to sinuous, longer crack features. (E) As longer cracks develop, they become even more sinuous, even developing circular patterns (pen for scale). (F) Larger-scale view of rippled upper sandstone bed surface in the Makgabeng Formation, covered by a variety of sand cracks, mainly of higher sinuosity and circular geometry.

Figure 5. Fragments of detached microbial mat in the sandstone beds of the Makgabeng Formation. (A) Sand chips (several are marked with arrows), reflecting microbially-bound sand still attached to mat fragments which were then transported and partially reworked by flash flood currents before deposition (pen for scale). (B) A set of varied mat fragments, from relatively rounded sand chips to partially rolled-up mat-bound sand fragments, to larger, more irregularly shaped "microbial clasts" (pen for scale). This mixture reflects the irregular energy levels concomitant with desert flash floods. (C) Large (dark and segmented) muddy fragment (below point of hammer; scale) which appears to have suffered desiccation only after physical mat destruction and flash flood deposition had occurred.

Figure 5. Fragments of detached microbial mat in the sandstone beds of the Makgabeng Formation. (A) Sand chips (several are marked with arrows), reflecting microbially-bound sand still attached to mat fragments which were then transported and partially reworked by flash flood currents before deposition (pen for scale). (B) A set of varied mat fragments, from relatively rounded sand chips to partially rolled-up mat-bound sand fragments, to larger, more irregularly shaped "microbial clasts" (pen for scale). This mixture reflects the irregular energy levels concomitant with desert flash floods. (C) Large (dark and segmented) muddy fragment (below point of hammer; scale) which appears to have suffered desiccation only after physical mat destruction and flash flood deposition had occurred.

These characteristics closely resemble those of "Kinneyia" (Martinson, 1965; Bloos, 1976), which refer to wrinkle structures likely formed beneath a microbial mat and which were then preserved on the upper surface of the underlying, flat siltstone or sandstone beds upon which the mat grew (cf. Porada and Bouougri, 2007). Such structures are known only from the ancient rock record, without modern examples yet found, and are most common within intercalated siltstone/ sandstone beds or heterolithic siltstone/mudstone successions deposited within inferred shallow subtidal, intertidal to lower supratidal paleoenvironments (e.g., Hantzschel and Reineck, 1968; Bouougri and Porada, 2002; Noffke et al., 2002; Porada and Bouougri, 2007). Their association to a microbial mat origin is commonly reinforced by the occurrence of what has been called a "microbial mat

Figure 6. Various roll-up features within the sandstones of the Makgabeng Formation. (A) Thin mudstone layers which have been rolled up for more than a complete 360° revolution, with the example shown having apparently rolled up from both ends (pen for scale). (B) Longitudinal view of roll-up, showing an elongated concentric feature (pen for scale); note partially broken concentric layers, resembling a cigar, near pen-tip. (C) Larger, irregular fragment of mat-bound mud (now mudstone) which became torn and subsequently rolled up along two angles at almost 90° to each other (pen for scale).

Figure 6. Various roll-up features within the sandstones of the Makgabeng Formation. (A) Thin mudstone layers which have been rolled up for more than a complete 360° revolution, with the example shown having apparently rolled up from both ends (pen for scale). (B) Longitudinal view of roll-up, showing an elongated concentric feature (pen for scale); note partially broken concentric layers, resembling a cigar, near pen-tip. (C) Larger, irregular fragment of mat-bound mud (now mudstone) which became torn and subsequently rolled up along two angles at almost 90° to each other (pen for scale).

facies" comprising a set of associated microbially induced structures, commonly made up of spindle-shaped cracks, longer curved cracks which become sinuous to even circular, and sand chips (e.g., Pfluger and Gresse, 1996; Pfluger, 1999; Porada and Loffler, 2000; Bouougri and Porada, 2002). A very similar association is noted in the Makgabeng interdune beds, with the addition of roll-up structures, as will be described below.

3.3.2. Sand Cracks

A large variety of such sand cracks is found within the Makgabeng interdune facies (Fig. 4), varying from short, isolated spindle-shaped cracks (Fig. 4A), which can then apparently lengthen and join up, forming "triple junction"-like patterns (Fig. 4B), which are sometimes connected to more curved and longer cracks (Fig. 4C). It should be emphasized that not all spindle-shaped cracks join up to form "triple junctions" and that not all of the latter become connected to the longer, sinuous variety. Some of the "triple junction" features appear to connect naturally with the longer sinuous cracks, but others almost appear to reflect separate sand crack systems (compare Fig. 4C, D). As the cracks become more sinuous, even circular patterns may result (Fig. 4E). Many of the thin sandstone beds which predominate within the inferred interdune facies have complex sand crack patterns developed across their upper surfaces, commonly formed on primary current ripples (Fig. 4F).

Seeing that silt and sand lack any cohesion to enable such sediments to undergo cracking, it is necessary that either mud was present to provide the necessary cohesiveness, or that a microbial mat growing on top of such clastic sedimentary beds gave enough cohesion for underlying sand within which the mat was embedded, to crack (e.g., Schieber, 1998; Eriksson et al., 2007). In the absence of any observed mudstone interbeds (see for example in Fig. 4F, that the sand-cracked sandstone bed surface is immediately overlain by a succeeding sandstone bed - at left) or muddy sediment within sandstones (as confirmed by thin section studies) in the Makgabeng examples, a microbial mat binding influence to provide cohesiveness to the sand, is inferred by us.

3.3.3. Sand Chips and Desiccated Larger Fragments

Sand chips (Fig. 5A) reflect eroded mat fragments which can be considered as an end-member in a series of mat destruction features comprising curled crack margins - flipped-over mat edges - rolled-up mat fragments (cf. roll-ups) (Schieber, 2004). They are commonly a few centimetres across and have rounded and plastically deformed shapes, as also observed in Fig. 5A, and can be ascribed to mats providing cohesion for sand beneath the mat, and during high energy erosive events, the mat and its sublayer of sand are broken up into variously-sized fragments (Schieber et al., 2007). These microbially-bound sand clasts can then be transported like other sedimentary particles, and may then display current alignment or even imbrication (Pfluger and Gresse, 1996; Bouougri and Porada, 2002). They are often found associated with larger mat fragments of irregular shapes and sizes as well as rolled-up mat fragments (also comprising microbially bound sand, mud or silt), reflecting mat-breakup during flash flood events in the present examples (Fig. 5B). Some of these fragments observed in the Makgabeng Formation comprise muddy rather than sandy sediment, and some of the larger such fragments exhibit cracking due to desiccation, which must logically have occurred after deposition of the flat, microbially-bound mud clast (Fig. 5C), as otherwise, such a flat muddy particle would have fragmented along pre-transpor-tational cracks.

3.3.4. Roll-Up Structures

These structures are quite common in the inferred interdune facies of the Makgabeng Formation, and comprise thin mudstone layers which have been rolled up through several full revolutions (Fig. 6A) - it is common knowledge that drying mud cracks and the separate pieces of mud then become curled around their edges. However, for more than a full revolution of such mud through 360°, there must be another factor present to provide the necessary cohesiveness and lateral binding (Schieber et al., 2007) to allow such muddy rolled-up fragments to form - microbial mats provide an obvious answer to this (Eriksson et al., 2000 for discussion of Makgabeng examples). These rolled-up mat fragments with thin (several millimetres) underlying mud layers still attached are broken off mat-bound sediment layers during erosive sedimentation events, analogously to the mat chips and larger desiccated fragments discussed above. The roll-ups commonly form somewhat larger fragments than the sand chips, and vary from long, thin features similar to cigars (Fig. 6B), to more complex features comprising more than one rolled-up fragment at high angles to each other (Fig. 6C).

3.3.5. The Microbial Mat Facies of the Makgabeng Formation

The microbial mat facies of the Makgabeng Formation comprises wrinkle structures, complex patterns of sand cracks, sand chips and larger mat fragments, and rolled-up mat fragments, as discussed above. The wrinkle structures are uncommon and show no observable spatial association with the other features, which occur as two groups, found in separate sandstone-siltstone bed successions within the interdune facies of the study area. The first group comprises the sand cracks, reflecting mat-bound sandstone bed upper surfaces that were preserved in situ after desiccation of the mat, and which were preserved in the underlying clastic beds due to cohesiveness supplied to the sand by the living mats. The second group consists of an association of sand chips, larger inferred mat fragments (some desiccated after mat breakup and transport had occurred) and roll-ups.

With the exception of the uncommon wrinkle structures, the other mat-related features are all associated with mat destruction, but of two distinct types: (1) relatively slow mat destruction through exposure and desiccation, to form the sand cracks of various types; (2) relatively rapid mat destruction through flash floods which break up growing mats and their immediate cohesive clastic granular sediment substrate and locally rework them before their deposition within the flood deposits. In the second group, presumably, desiccation and crack formation would have aided the rapid destruction through flash-flooding. It is hardly surprising that products related to desiccation rather than aqueous settings should preferentially survive within a paleodesert environment. Analogously, aqueous deposits preserved in the Makgabeng Formation preferentially favour those formed through high energy flash flood processes and dune-front collapse due to extreme rainfall events (cf. Simpson et al., 2002, 2004) rather than the low energy playa and interdune pond deposits. Both of the latter would tend to be reworked by either extreme rainfall events or, over a longer period, by desiccation and eolian processes. As expected, eolian dune deposits totally dominate in three dimensions within the preserved Makgabeng Formation.

The microbial mat-related features discussed here thus reinforce the interpretations of the Makgabeng paleodesert made on the basis of the "normal" clastic sedimentary structures (cf. Eriksson et al., 2000; Simpson et al., 2002, 2004); this is in accord with such mat-related features being seen as an additional set of sedimentary structures rather than a fully unique group of features set apart from the physical processes of sedimentation (cf. Noffke et al., 2001). Even though the two extreme silty-sandy environments discussed in this paper, namely paleodeserts and modern supratidal flats, have a significant measure of common influences (viz. desiccation of small ponds, high energy aqueous events such as flash floods in deserts and spring and high tides in suptratidal settings), the microbial mat facies of the two environments exhibit significant differences, as will be seen in the next example discussed below.

4. Microbial Mat Features on Modern Supratidal Flats

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