Geology Reference
In-Depth Information
ebb ooid tidal deltas are developed seaward of tidal
channels (Fig.
21.2a
) that cut through the wave-formed
oolitic barrier island complex (Purser and Evans
1973
) .
These sediments generally contain microbial pisoids
known as oncoids (see below), peloids, low diversity
skeletal components including gastropods, ostracods,
green algae, and benthic foraminifera or abundant
numbers of a certain individual biota depending on
climate and salinity (Fig.
21.5c, d
). Open marine and
restricted subtidal lagoon sediments may comprise
microbial structures known as microbialites.
Microbialites are organosedimentary deposits formed
by interactions between biological, environmental and
diagenetic processes as a result of benthic microbial
organisms that trap and bind sediment and/or form the
locus of calcium carbonate precipitation (Burne and
Moore
1987
). Trapping and binding mechanisms and
early diagenetic calcifi cation and/or precipitation by
bacteria appear to be the major processes for the forma-
tion and preservation of microbialites in modern marine
subtidal and tidal fl at environments (e.g., Riding
2000
;
Reid et al.
2000
; Dupraz et al.
2009
) . Marine microbi-
alites include the non-laminated structures with macro-
scopic, dark-colored clotted fabric (Fig.
21.6a
) referred
to as thrombolites and laminated forms (Fig.
21.6b, f
)
known as stromatolites (e.g., Pratt and James
1982
;
Riding
1999,
2000
) commonly constructed by fi la-
mentous calcified (Fig.
21.6c
) and non-calcified
(Fig.
21.6d, f
) bacteria. Microbialites represent arid
upper intertidal (Fig.
21.2a
), humid supratidal marsh
(Fig.
21.2b
), hypersaline subtidal- to intertidal and
normal marine platform margin environments.
Stromatolite pisoidal (Figs.
21.5d
and
21.6c, d
) and
columnar microbialite structures (Fig.
21.6b, e
) forms
represent both modern and ancient open marine and
restricted subtidal sediments as old as 3.5 Ga (e.g.
Grotzinger
1989
; Riding
2000
) , but thrombolites are
normally found in Phanerozoic subtidal deposits.
Sediment of a high energy restricted lagoon may con-
tain columnar stromatolite and ooids/skeletal grains
derived from the nearby platform margin (see The
Precambrian and Middle Cambrian tidalites in
Sects.
21.7.1
and
21.7.2
). Laterally extensive interl-
ayered gypsum/anhydrite-bearing dolomudstone/lime-
stone and layered nodular anhydrite with no evidence
of subaerial exposure (Fig.
21.7
) may record deposition
in a subtidal lagoon environment where the water
became increasingly saline due to poor circulation.
The upper subtidal sediment in high energy exposed
coasts typically consists of beach facies characterized
by fl at-laminated to cross-bedded grainstone facies.
These strata may contain skeletal grains and/or ooids,
21.5
Facies Belts and Their Diagnostic
Features
Periodic changes in the direction and speed of tidal
currents, differences in environmental conditions
within the inner shelf, biogenic activity of organisms,
and intermittent exposure of the proximal areas of a
tropical carbonate platform result in various facies in
different environments of a tidal system. These facies
are characterized by a variety of constituents and
diagnostic features formed during deposition and early
diagenesis. Tidal fl at deposits commonly consist of thin
bedded and laminated lime mudstones/dolomudstones
or microbial laminae known as laminites (Fig.
21.3a-c
).
They are tan to light brownish gray in the fi eld as
opposed to gray-colored subtidal facies formed under
a relatively reducing condition (Fig.
21.3d
). Some tidal
fl at deposits consist of mixed carbonate and clay- to
sand-sized siliciclastics indicating proximity to aeolian
or fl uvial/deltaic systems or intermittent advance of
siliciclastics from a nearby upland area during fl oods
(see the Middle Cambrian tidalites in Sect.
21.7.2
).
Tidal fl at and subtidal facies are interlayered with
various erosive-based, graded intraclastic storm depo-
sits (“tempestites”) in some stratigraphic intervals
(e.g., Wignall and Twitchett
1999
, Y. Lasemi et al.
2008
) recording deposition in a storm dominated
platform (see the Middle Cambrian and the Lower
Triassic tidalites in Sects.
21.7.2
and
21.7.5.1
).
Carbonate storm beds commonly consist of intraclasts
(edgewise conglomerate) of various facies (Fig.
21.4a, b
)
and/or ooids, peloids and bioclasts of mixed fauna and
normally display hummocky cross-stratifi cation, lami-
nation, and gutter casts (Fig.
21.4c, d
). Repeated storm
events may partially or completely remove the previ-
ously deposited tidal sediments leading to stacked
storm deposits (Fig.
21.4 c, d
).
21.5.1 Subtidal Belt
The quiet water, back-barrier lagoon facies generally are
muddy and consist of characteristic gray and bioturbated
mudstone to packstone texture (Figs.
21.3d
and
21.5a, b
).