Stromatolite research in the Shark Bay World Heritage Area

2014·15.6 MB·English

by  L B Collins

WA Science—Journal of the Royal Society of Western Australia, 97:189-219, 2014
Stromatolite research in the Shark Bay World Heritage Area
L B COLLINS & R J JAHNERT2
1Department of Applied Geology, Curtin University, GPO Box U1987, Perth 6845, Australia.
2Petrobras Company, Avenida Chile 65, Rio de Janeiro, 20031912, Brazil.
* Corresponding author: LE3 [email protected]
Three decades after declaration of World Heritage status for Shark Bay new research findings are
being reported on the specialised microbial habitats that characterise its hypersaline settings, the
composition of microbial communities, tidal flat evolution, stromatolite geochronology and subtidal
microbial systems. In the stable, semiarid and evaporative setting within the intertidal-subtidal
environment the microbial ecosystem is trapping, binding and biologically inducing carbonate
precipitation within laminated stromatolites, non-laminated thrombolitic forms and
cryptomicrobial non-laminated forms. Filamentous microbes constitute the dominant group in the
blister, tufted and smooth mat types, and coccoid microbes dominate the pustular, colloform and
microbial pavement deposit types. Detailed georeferenced substrate mapping has revealed
extensive subtidal microbial deposits occupying -300 km2 of the total Holocene 1400 km2 area of
Hamelin Pool. The microbial pavement covers 227 km2 of the subtidal substrate, which together
with columnar structures reveals a subtidal microbial habitat that occupies an area several times
larger than the area of the intertidal deposits. Oldest dated stromatolite heads are 1915 l4C years
BP, and the overall system was deposited in two stages: the first between 2000 and 1200 and the
last from 900 years BP to the present. Slow accretion rates vary from less than 0.1 to 0.5 mm/year.
Different internal fabrics were constructed according to their position in relation to the littoral zone
by distinct microbial communities, and lateral fabric relations have been established. Evidence of
shallowing-upward fabric sequences of microbial origin reflects relative falling sea levels during
the late Holocene and is likely useful in ancient environmental interpretation. A new substrate
map and depositional history for this distinctive microbial habitat has established the significance
of subtidal structures and emphasises the geoscientific importance of Hamelin Pool, especially
with respect to early life studies and ancient analogues for understanding microbial activity,
deposit characteristics, fenestral fabrics and distribution.
KEYWORDS: fabrics, geochronology, Holocene, microbial mats, stromatolites, subtidal
microbialites.
INTRODUCTION recently published detailed research accounts,
particularly Jahnert & Collins (2011, 2012, 2013) and
Since the existence of stromatolites in Shark Bay first Jahnert et al. (2012).
became known in the 1950s there has been a developing
interest by researchers in characterising the systems in
the unique hypersaline conditions of Hamelin Pool and METHODS
adjacent embayments (Figure 1), by marine geoscientists,
Regional mapping using remote sensing and ground-
biologists and others. The progress of this work is briefly
truth transects, shallow coring and sampling, and
summarised by Jahnert & Collins (2012 pp. 1-3), Burne &
laboratory analysis were employed to map tidal flats, and
Johnson (2012) and Playford et al. (2013).
marine embayments in order to document microbial
The purpose of this paper is to summarise the key habitats (Jahnert & Collins 2012, 2013).
findings of geoscientific investigations during 2008-2012
Georeferenced maps were created using ESRI's
that concentrated on several aspects considered
ArcGIS, high resolution (50 cm/pixel) Shark Bay
important among the wide opportunities for
orthophotos and aerial photos (1:25 000 scale). A
investigations of what is a remarkable occurrence of
Multibeam survey was undertaken with the Department
hypersaline microbial systems. The research was partly
of Transport (DoT), Western Australia, focusing on the
driven by a need to broaden the database required for
measurement of the depth and physical characteristics of
ongoing management of the microbial and related assets
the substrate along pre-defined transects. Submarine
of the Hamelin Pool Marine Nature Reserve and nearby
videos were recorded during marine investigations, and
areas in the Shark Bay embayments. Some of the
subtidal samples of microbial structures were collected in
questions addressed concern the nature of microbial tidal
partnership with the Department of Conservation (now
flats in areas of reduced salinity; the distribution of
Department of Parks and Wildlife) which managed
microbial habitats, substrates and fabrics on the tidal flats
vessels and divers. A Differential Global Positioning
of Hamelin Pool; the chronology of stromatolite
System (DGPS) was used to record transect positions and
development; and investigations of the subtidal microbial
high-resolution elevations (±5 cm) across tidal flats.
habitats in Hamelin Pool. This summary draws on
Underwater videos were produced using drop-down
video-camera, by diving or with cameras attached to the
© Royal Society of Western Australia 2014 side of the vessel.
189
WA Science—Journal of the Royal Society of Western Australia, 97(1), June 2014
Monkey SHARK BAY MARINE PARK
Peninsula r
Multi-boam and (cid:9632)(cid:9632)(cid:9632)(cid:9632)' ~~
Submarine —(cid:9632)>
Kocky Location
Point
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Hutchinson
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' V-.Nilemah /Telegraph Figure 1 Hamelin Pool, L'Haridon
26°25'0"S W — j/ Station Bight and Henri Freycinet
embayment at Shark Bay, Western
Australia. The image (from
Geoscience Australia) shows the
liySO'tTE 114°0'0"E 114°10'0"E locations of Nilemah, Garden Point
and Rocky Point.
Sampling involved limited collection of microbial ENVIRONMENTAL AND GEOLOGICAL
heads from shallow depths by snorkelling and by scuba
SETTING
diving in the deeper portions. A taxonomic study of
cyanobacteria was performed in the Microbiology
The Shark Bay embayments (Figure 1) are situated on the
Department at the Federal University of Rio de Janeiro,
central west coast of Australia, and are dominated by a
Brazil. Chemical analysis of microbial sediment and
semiarid setting and subtropical conditions which are
water samples involved X-ray fluorescence spectroscopy
favourable to carbonate secreting organisms and
(XRF), inductively coupled plasma (ICP) optical emission
communities. Shark Bay Marine Park is separated into
and mass spectrometry, produced by Ultra Trace
three major embayments: Freycinet, L'Haridon Bight and
Analytical Laboratories, WA, and Petrobras S.A.,
Hamelin Pool. Freycinet embayment in western Shark
Research Centre. I4C ages were obtained by the
Bay maintains the best connection with northerly oceanic
Radiocarbon Dating Centre of the Australian National
waters so that, despite the high evaporation, salinity is
University. Carbon and oxygen isotopes from sediment
metahaline (40-56: Logan & Cebulski 1970). Hamelin
samples were analysed by the Federal University of Sao
Pool, the easterly embayment, has restricted oceanic
Paulo (USP). X-ray diffraction (XRD) techniques were
water influx and hypersaline waters (56-70), and
used to characterise the crystallographic structure and
progressively decreases in salinity to the north towards
recognise mineralogical constituents of sediment, and
the tidal exchange channels crossing Faure Barrier Bank.
were performed at Curtin University. Scanning electron
L'Haridon Bight embayment contains metahaline
microscopy (SEM) was conducted at Curtin University.
conditions in tire north and hypersaline waters to the
190
Collins & Jahnert: Stromatolite research, Shark Bay
south. Tidal flats bordering the Shark Bay embayments stromatolite occurrences seen on intertidal surfaces in
have low substrate gradients (20-150 cm/km) with coastal settings have largely been stranded as sea level
shallow and restricted water circulation, resulting in has declined, so that subtidal stromatolites (at depths
hypersaline conditions and microbial deposits that are 0-7 m below LWL) comprise much of the present-day
widespread as mats or small elongate structures and microbial habitat.
discrete columns.
Shark Bay has three distinct geomorphic provinces; a
western limestone terrain (Edel Province) which includes RESULTS
Dirk Hartog Island; a central Peron Province with
characteristic red dune terrain, and an eastern limestone
Tidal flats in contrasting salinity settings
terrain which comprises the hinterland, the Toolonga
In a comparative study of tidal flat evolution in Shark
Province. The geographic features of Shark Bay are
Bay, three tidal flats (Figure 1) from contrasting salinity
controlled tectonically by a regional normal fault system
settings were evaluated (Jahnert & Collins 2013) to assess
of north-south orientation which intersects a north-
the role that salinity has played in their evolution, and to
northwest-south-southeast oriented fold system
describe any contrasting characteristics in microbial mat
responsible for confining the bays to the subsiding tidal-
and stromatolite occurrence and distribution. Hamelin
channel, tidal-flat, storm-beach and beach-ridge
Pool is a hypersaline embayment surrounded by
environments (Hocking et al. 1987). Two pre-Holocene
extensive tidal flats: Gladstone, Hutchinson and the
marine transgressions are recorded. The Bibra Limestone
study site at Nilemah. Other nearby tidal flats located in
('Bibra Formation' of Logan et al. 1970; amended by van
L'Haridon Bight (transitional between hypersaline and
de Graaff et al. 1983), formed during the Bibra marine
metahaline conditions) and Henri Freycinet embayment
phase, is estimated to have been deposited during the
(metahaline) are Rocky Point and Garden Point,
last Pleistocene interglacial (MIS 5e) high sea-level stand
respectively; these were also colonised by microbial
at 120-130ka (van de Graaff et al. 1983; Hocking et al.
communities, but under slightly different environmental
1987).
conditions and timing. Whereas Nilemah is a north¬
The Holocene sedimentary sequence was subdivided facing embayment marginal to the Hamelin basin, both
into five sedimentary units, based on their lithological Garden and Rocky Point tidal flats are semi-isolated from
variation, vertical and lateral relations and mapability: (i) their embayment waters by coquina barrier ridges.
Hamelin Coquina; (ii) Intertidal Veneer; (hi) Sublittoral
Falling sea levels during the late Holocene led to
Sheet; (iv) Bank Unit; and (v) Basal Sheet, which is
establishment of coquina barriers and development of
located on the embayment plain (Logan et al. 1974b),
small, semi-barred tidal flats at various sites in the Shark
where water depth is 10 m maximum. These sediments
Bay embayments, and establishment of areas suitable for
are dominantly organosedimentary in character and
microbial communities (Figures 1, 2). Such spit ridge
lithotypes in the peritidal zone can be broadly
accretion is usually controlled by south to north wind-
categorised into either microbialites or coquinites. The
driven longshore currents for north-south oriented
microbialites have been examined in many studies
shorelines which dominate the embayments, but tidally
(Logan 1961, 1968; Davies 1970a, b; Playford & Cockbain
constructed ridges often develop with north to south
1976; Playford 1979,1990; Logan et al. 1970, 1974b; Burne
orientation, depending on coastal facing. The north¬
& James 1986; Burne & Moore 1987; Burne 1992; Burns et
facing Nilemah embayment is partially enclosed by
al. 2004; Dupraz & Visscher 2005; Reid et al.2003;
storm ridges with southwest and southeast transport
Papineau et al. 2005; Burne & Johnson 2012; Jahnert &
directions on opposite shorelines. A combination of
Collins 2011, 2012, 2013). The most recent review of the
shallow conditions, low gradients (frequently 20 cm/km)
Shark Bay microbialites demonstrates that they exist in
and microtidal conditions controls the development of
the form of subtidal pavements, subtidal microbial
elevated salinities favourable for microbial colonisation
lithoherms of varying morphotypes and intertidal mats
in these settings.
(Jahnert & Collins 2011, 2012, 2013). Their distribution is
extensive, occupying nearly half the benthic substrate of The tidal flat environment has been colonised by
Hamelin Pool (Jahnert & Collins 2011, 2012, 2013). microbial communities specialised in surviving at specific
water depths, where a delicate balance between tidal
Shark Bay's recent geological past is characterised by
energy, waves, exposure time and water depth results in
three distinct marine transgressions, during the last part
accretion or erosion (Logan et al. 1974a, b) (Figure 3).
of the Quaternary as part of glaciation/deglaciation
Low water energy associated with high evaporation
climate changes (Logan et al. 1974b; Jahnert & Collins
rates, sediment pattern and space creation are the key
2011, 2012). These events are preserved in marine
elements for sediment accretion in Shark Bay. Bacteria
carbonate sequences that outcrop along the Shark Bay
take advantage of diurnal tidal currents and waves that
shoreline (O'Leary el al. 2008). The three drowning events
slowly cover the very flat area, supplying sediments and
correlate well with the polar ice core records and
a habitat for microbes that are still adapting and
represent a 100 000 year frequency signal of advance and
expanding, producing carbonate by trapping and
retreat of the sea. Slowly falling sea level (regression of 2
binding particles, biologically inducing carbonate
m) over the last 6000 years has been an important control
precipitation and being lithified by aragonite cement.
on salinity increase, decline of seagrass banks (with the
Cyclic tidal fluctuations also play a part in microbial
exception of the Faure Sill in the metahaline northern
distribution.
Hamelin Pool), stromatolite and tidal flat development,
as well as basinward growth (progradation) of coquina Hot summer temperatures (around 40°C) and strong
coastal ridge systems (Jahnert et al. 2012). The spectacular southerly winds (40 km/h) force water out of the tidal
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WA Science—Journal of the Royal Society of Western Australia, 97(1), June 2014
TRANSGRESSIVE MAXIMUM FLOODING
STAGE HIGHSTAND STAGE _
POST 8000 y BP/ ca 6000 y BP
GARDEN POINT
EMBAYMENT
REGRESSION EMERGENCE
PROGRADATION 4 LONGSHORE TRANSPOR'
POST 4000 y BP/ present • '
GARDEN POIN-
V \ SUBLITTORAL ',t EMBAYMENT
/, • • PLATFORM 'iSUBLITTORA
/J ’ PROGRADATIOF H PLATFORM
EMBAYMENT
V PLAIN
Figure 2 Morphological evolution proposed for the Garden Point embayment. Storm-wave activity was very important
uring te initin phase, while longshore currents, winds and tides were the morphological drivers during the late
stages. Microbial activity started near 2300 years during the fall in sea level. (Jahnert & Collins 2013 figure 5).
flats, causing large areal exposure and desiccation of
surfaces exposed to the sun for long periods of time and
microbial deposits (pustular mainly), which shed
belong to a novel type of halophilic archaeon, Halococcus
globules of dead microbial mats. The distribution of
hamelinensis sp. (Goh et al. 2006; Goh 2007).
sediments and microbial types on tidal flats such as at
Garden Point reveals a sediment veneer made of Tufted mats occur in the upper intertidal zone,
carbonate, reflecting recent microbial activity within an growing in scallops due to long filaments made by
environment that is frequently adapting to new Lyngbya (Hoffman 1976) that exploit the ability to avoid
conditions. direct contact with the ground and may block water and
sediment inside the created relief. This mat normally
Six different microbial deposit types were recognised
develops over shallow muddy substrate where sediment
(Figure 4), mapped and sampled. Principal development
maintains humidity landward of the pustular mat type.
and occurrence of mats is concentrated in the intertidal
The intertidal zone is the growth domain of pustular mat
zone at Garden and Rocky Point and as additional
spread as brown dark sheets of small bushes, inhabiting
microbial structures at Nilemah in the subtidal zone
the upper intertidal to the upper subtidal zone. To
(Jahnert & Collins 2013). The supratidal zone does not
construct a detailed map (Jahnert & Collins 2013), the
preserve or generate extensive mats due to strong
term mini-pustular was introduced to refer to small
erosional processes, constant sediment movement and
bushes or pustules <1 cm in height and diameter.
adverse conditions of temperature. Where bacteria
Pustular mats in the intertidal zone normally reach 3 cm
survive it is in detached sites receiving a sporadic high-
high and >1 cm in bush diameter. In the upper subtidal
wind-driven water supply or abnormal tides. Microbial
zone, the high rate of peloid deposition discourages
deposits in those sites are film and blister mats. Film
pustular growth that is still small in size (<1 cm high)
mats are a black veneer normally covering lithified
and sparse, such as mini-pustular mat.
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Collins & Jahnert: Stromatolite research, Shark Bay
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WA Science—Journal of the Royal Society of Western Australia, 97(1), June 2014

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Collins & Jahnert: Stromatolite research. Shark Bay
In the upper subtidal zone, different bacteria such as communities are responsible for the external and internal
Schizothrix friesii and Microcoleus produce a smooth mat colours and morphologies of organosedimentary
composed of fine carbonate grains placed between deposits. Sixteen species of cyanobacteria were identified.
vertical bacterial filaments that are able to permeate and Ten species that belong to the Class Cyanophyceae,
trap sediments and produce laminar stromatolite fabrics. Order Chroococcales, live in coccoid colonies and have
Garden Point includes a proximal pond where, in very small spherical to oval forms arranged in envelopes of
calm water rich in sediment particles, a set of coarse jelly-like mucilage, normally yellow to dark orange in
laminar smooth microbial mats has developed. However, colour. Another six species belong to the Class
Garden Point, in contrast to the other tidal flats, is in the Hormogonae, Order Oscillatoriales; these filamentous
initial phases of establishing bacteria, and only the bacteria with elongate formats are often surrounded by a
proximal substrate portion of the subtidal zone is sheath that contains many individual cells with colours
colonised. ranging from dark green to light green and blue.
Filamentous bacteria are the dominant group, producing
In the subtidal zone of Nilemah tidal flat, seaward of
blister mats (Microcoleus chthonoplastes), tufted mats
the smooth mat terrain, colloform microbial deposits (-0.5
(Lyngbya aestuarii, L. fragilis and Phormidium willei) and
to -1.5 m) develop as elongate structures followed by a
smooth mats (Schizothrix friesii and Microcoleus
tabular microbial carbonate pavement extending to deep
chthonoplastes).
subtidal zones (-1.0 to -6.0 m).
Coccoid bacteria dominate the pustular mats
The improved knowledge of the nature and
(Gloeocapsa punctata, Chroococcus minimus, Entophysalis
distribution of the tidal flat microbial deposits is
granulosa), colloform deposits (Entophysalis granulosa,
documented in georeferenced maps (Jahnert & Collins
Chroococcus turgidus, Gloeothece vibrio) and microbial
2013) of the sediments and organodeposits of Garden and
pavement (Cyanosarcina lhalassia, Chroococcus
Rocky Points. These are characterised by relatively
microscopicus, Entophysalis conferta). Diatoms including
extensive and prolific microbial activity during the last
Navicula were identified in samples from smooth,
2300 years, producing microbialites that are exposed in
colloform and microbial pavement, but despite the thick
the supratidal zone. These are now subject to erosion,
mucilage around the diatom cells, colonies of bacteria
and are progressively colonising the subtidal zone as a
have been seen inside the extracellular polymeric
consequence of sea level fall, although observations of
secretions (EPS), and the process of organomineralisation
recolonisation in the intertidal zone provides evidence of
appears to be driven by the bacteria even in diatom
a recent short marine transgression (Jahnert & Collins
domains.
2013).
COMPARATIVE EVOLUTION
TAXONOMIC STUDIES Table 1 is a summary of the contrasting properties of
A taxonomic and phylogenic grouping was established microbial mats and sediments within the three tidal flats
based on microscopic characteristics of the dominant studied (Jahnert & Collins 2013). While the intertidal
cyanobacteria on the surface of microbial mats or microbial systems are similar, the destructive effects of
structures (Figure 5) (Jahnert & Collins 2013). Bacterial bioturbation are more evident at Garden and Rocky Point
Table 1 Comparison between tidal flats, water salinity and the contrasting properties of microbial mats and sediments
within the littoral zones (Jahnert & Collins 2013 table 1).
Tidal flat Water salinity Contrasting properties
Garden Point Metahaline to Microbial mats: pustular mat dominance; smooth mat only in restricted pond.
hypersaline Microbial sediments: carbonate veneer (30 cm max.) with significant influx of quartz sand.
Subtidal zone: bioclastic-quartz sand sheets (proximal) and bioclastic seagrass banks (distal).
Bioturbation: disturbs and reworks microbial mats.
Sediment isotopes: concentrate in less positive values of 8'-'C (+3.4 to +5.2) and 8,80 (+2.0 to +3.6)
Onset of hypersalinity: coquina storm deposits dated UC 2050-2150 (±35 years)
Rocky Point Metahaline to Microbial mats: pustular mat domain intertidal zone and smooth mat in subtidal zone.
hypersaline Microbial sediments: carbonate veneer (50 cm max.) with influx of quartz sand.
Subtidal zone: smooth mat (proximal) and bioclastic seagrass banks (distal).
Bioturbation: disturbs and reworks microbial mats.
Sediment isotopes: concentrate in intermediate values of 813C (+3.6 to +5.3) and 8,80 (+2.5 to +3.8)
Onset of hypersalinity: coquina storm deposits dated 14C 2420, 2830 and 3160 (±35 years)
Nilemah Hypersaline Microbial mats: pustular mat domain intertidal and smooth, colloform and pavement in subtidal zone.
Microbial sediments: carbonate layer (1.30 m max.) and low influx of quartz sand.
Subtidal zone: smooth, colloform structures and microbial pavement widespread.
Bioturbation: limited by hypersalinity.
Sediment isotopes: concentrate in more positive values of 8”C (+4.0 to +5.9) and 8,80 (+3.0 to +3.9)
Onset of hypersalinity: coquina storm deposits dated 4630 (±35 years)
195
BLISTER ! .-I . TUFTED <l| PUSTULAR K SMOOTH I^COLLOFORM I PAVEMENT
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Figure 5 Principal microbial deposits and dominant microbial species. Filamentous bacteria dominate the blister, tufted and smooth mat environment and coccoid bacteria
dominate the pustular, colloform and microbial pavement. (Jahnert & Collins 2013 figure 17).
Collins & Jahnert: Stromatolite research. Shark Bay
than at Nilemah. Also, the subtidal smooth mats give Hamelin Pool substrates and microbial distribution
way to seagrass banks offshore, whereas in the Whilst previous research has documented specific tidal
hypersaline Nilemah subtidal zone, colloform flats and localised features, the opportunity for a wider
stromatolites and lithified bioclastic microbial pavement mapping of microbial substrates arose from the need for
are widespread. The onset and duration of elevated regional data, for World Heritage management. The
salinities appears to be the driving mechanism for these microbial systems and related sediments of Hamelin Pool
differences. There is a chronological progression in the were mapped (Figure 6) using high-resolution
,4C age of coquina beach ridges from the least saline to orthophotos, GIS and supporting terrestrial and
the most saline conditions such that Garden and Rocky submarine ground truth information, and classified
Point are relatively youthful, which likely explains the according to their regional occurrence and distribution
retention of seagrass banks, frequency of bioturbation, within the hinterland, supratidal, intertidal and subtidal
and lack of subtidal microbialites relative to the older, domains (Jahnert & Collins 2012) (Figure 7). More
more hypersaline Nilemah embayment.
113°50'0"E 114°0'0"E 114°10'0"E
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Quaiz anl BcdasU Said. Organs Deposits, Seagrass Wack *
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[ kAcrobiai Becky Pwemaril (Cryptomlcroblal Skud)* Serpifld Encrusted. Bioclastic. Shety OoUic Sand
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113°50'0"E 114°0'0"E 114°10'0"E
Figure 6 Hamelin Pool and L'Haridon Bight georeferenced sedimentary and organosubstrate map based on numerous
coastal and marine ground truth traverses, video transects, aerial surveys, samples and interpretation of digital
orthophotos.
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WA Science—Journal of the Royal Society of Western Australia, 97(1), June 2014
Aerial View
ri ' • --~(~
Shell Ridges * - -.
Shells and Bio SS Breccia an
Colloform build-ups
Cerebroid build-ups
Blocky Pavement
Hamelin Pool
Tabular Pavement
Figure 7 Hamelin Pool microbial substrates.
recently mapping has been extended to cover L'Haridon zone in Hamelin Pool occupies nearly 80 km2, and
Bight to the west of Hamelin Pool. contains two organosedimentary units which are exposed
Hinterland deposits are composed of quartz sandstone and prograding seaward, as described below.
(Peron Sandstone) that comprises most of the embayment The Hamelin Coquina, the upper unit of the
margins. The Peron Sandstone is an eolian Pliocene- Holocene system, is a supratidal beach ridge system
Pleistocene deposit generated during glacial phases which overlies thin Pleistocene units and the Pleistocene
(Butcher et al. 1984). Interdune depressions contain Bibra Formation and, as a consequence of sea level fall
spherical and ellipsoidal depositional basins consisting (Logan et al. 1974b), progrades toward the centre of the
of granular gypsum and quartz sediment fills, termed embayment over Holocene supratidal microbial
birridas. The eastern Hamelin embayment hinterland is deposits, as shore-parallel ridges above the normal
dominated by limestone of the Toolonga Calcilutite spring high tide level. Bioclastic-oolitic/quartz sand and
(Cretaceous) and superficial Quaternary cover that breccia occupy extensive areas between the coquina
includes calcrete, quartz sand, laterite, alluvial and deposits and the area reached by normal tides. Breccia
colluvial deposits. The eastern flank also contains pavements (Figure 8H) occur as lithified crusts that
emergent shoals composed of oolitic limestones of the developed over older microbial pavements and heads,
Carbla Oolite (Pleistocene) expressed as elongate bodies generated by processes that include desiccation,
parallel to the paleoshoreline which is to landward of the cementation and disruption by gypsum crystallisation
present shoreline. (Logan et al. 1974a, b).
The supratidal zone is influenced by storms and Supratidal areas are the domain of film and blister
abnormal tides and thus often exposed to erosional microbial mats. Film mat refers to a black veneer that
processes. Microbes survive in topographic lows and covers breccia clasts and lithified exposed material in
local depressions as detached sites of ephemeral mats sites that may have a connection with underground
which are only sporadically wet. These microbes are water or sites that receive any kind of water spray. Blister
adapted to survive in high substrate temperatures and mats develop in flat muddy substrates that receive
grow in blister, tuft or pinnacle forms. The supratidal sporadic flooding and maintain humidity.
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