Geology Reference
In-Depth Information
Stromatolites
Spectacular stromatolites (Sect. 9.1.4; Fig. 9.2) con-
sisting of laterally linked hemispheroidal growth forms
and stacked hemispheroids are known from recent and
ancient intertidal settings, but the existence of stroma-
tolites does not necessarily indicate intertidal paleoen-
vironments. Many ancient stromatolites, particularly
those with vertically pronounced reliefs, were formed
in subtidal settings and are also known in modern sub-
tidal environments.
versity is generally high. Low faunal diversity may in-
dicate stressed environmental conditions. Most com-
mon invertebrates and many calcareous algae live in
the illuminated, shallow upper part of the subtidal zone.
Sea grass
Today, sea grass meadows are located in the inter-
tidal and the shallow subtidal zone. Since the vegeta-
tion determines the nature and distribution of carbon-
ate sediment types, each zone has its own distinctive
skeletal communities (Cann and Gostin 1985) and is
reflected by calcareous epiphytes and organisms liv-
ing in protected niches below and between the aquatic
plants.
• The subtidal
Posidonia
meadows occurring in shore-
line-attached platforms or offshore banks can be rec-
ognized by foraminifera, coralline algae, bivalves and
gastropods living on the blades. The corresponding
sediment is a poorly sorted skeletal packstone.
• Intertidal flats may be covered by meadows of the
sea grass
Zostera
, that grows in loose communities in
lower intertidal settings and in dense meadows in the
upper intertidal zone. The sediment is a poorly sorted
mixture of skeletal sand with molluscan infauna, and
terrigenous mud and sand. These sand flats merge land-
ward into the
• upper intertidal mangrove zone, characterized by
laminar mudstones, pseudobreccias and 'root rocks'
(Galli 1991). The latter exhibits a dense, irregular net-
work of circular voids and vertical cavities exhibiting
downward bifurcations.
Biogenic structures
Root structures
: Submillimeter- and millimeter-
sized, spar-filled branched tubular structures, different
from fenestral structures, may represent relicts of plant
roots (Pl. 20/6).
-> Note: Root molds of land plants are common in
supratidal environments, but morphologically similar
molds are produced by subtidal aquatic plants. Struc-
tures interpreted as molds of plant roots, are no defini-
tive characteristics for subaerial or subaquatic settings.
Burrows
(Pl. 19) are common in all marine settings,
but show differences with regard to shape, size and dis-
tribution patterns in different parts of shelf environ-
ments (Sect. 5.1.4).
-> Note: Variously arranged burrows are rare in su-
pratidal environments. The lower intertidal zone is com-
monly heavily burrowed and often characterized by
millimeter-sized vertical burrows. Note that strong
burrowing occurs also in shallow and deeper parts of
the subtidal zone.
Biota
The biotic diversity and the composition of benthic
assemblages offer the best criteria for distinguishing
and subdividing tidal and subtidal carbonates.
•
Supratidal environments
are characterized by very
low diversity, rare well-preserved fossils and the domi-
nance of only a few organism groups (cyanobacteria,
ostracods, some foraminifera, a few mollusks). Plant
litter may be common.
•
Intertidal environments
are characterized by low
diversity organisms that adapted to rapidly changing
stress conditions. More frequent organisms are
(cerithid) gastropods, ostracods, benthic foraminifera
and bivalves. Plant litter is common. Cyanobacteria are
abundant (coastal and interior algal/microbial marshes
in humid climate). Calcareous algae are represented
by dasyclads. Storm layers may contain transported
subtidal organisms.
•
Subtidal environments
are characterized by normal
marine biota including skeletal invertebrates and shell
debris as well as trace fossils, especially burrows. Di-
Diagenetic features
Cementation:
Some early diagenetic criteria occur
preferentially in tidal limestones (see Sect. 7.4.2 and
Sect. 7.4.3). These criteria include:
• Gravitational (microstalaktitic) cements, e.g. drip-
stone cements in intertidal birdseyes (Pl. 33/4).
• Meniscus cement between grains of supratidal
beachrock (Pl. 33/4).
• Meteoric-vadose pisoids (Pl. 14/1; Pl. 126/1, 2) .
• Calcite or aragonite sinter crusts formed on inter-
tidal and supratidal surfaces.
• Irregular carbonate encrustations on substrates af-
fected by surf sprays; associated with pisoids (conia-
tolites; Purser and Loreau 1973; Pl. 14/2).
Dolomitization:
Intertidal and supratidal carbonates
may be characterized by very early diagenetic fine-crys-
talline dolomicrite formed under conditions described
by the sabkha evaporation model and the mixing zone
model (see Sect. 7.8.2). Early diagenetic origin is indi-
cated by dolomite chips occurring in tidal breccias.
Often associated with fenestral voids (Pl. 39/2).
