Geology Reference
In-Depth Information
to distinguish between intertidal and supratidal carbon-
ates if only a few facies criteria are used. You must try
to find as many criteria as possible. Table 15.1 sum-
marizes the main criteria of peritidal carbonates and
gives an idea of the relative abundance of these crite-
ria.
and fossils. Most peritidal carbonates are fine-grained
bedded dolomites, differing in their yellow to brown
rock color from the dark gray subtidal carbonates. This
difference is also known from recent tidal carbonates,
both in humid and arid settings. Facies-diagnostic cri-
teria are small-scale parallel and crinkled lamination,
hemispheroidal and isolated domal stromatolites, intra-
formational breccias, polygonal networks of up to
20 cm deep desiccation cracks, and small-scale chan-
nel-fill cross bedding.
15.5.2 Case Study: Middle Devonian
Peritidal Carbonates from Poland
Meter- to tens-of-meter-thick shallowing-upward cyclo-
thems are common features of platform carbonates.
Shallowing is indicated by changes in fossils, sedimen-
tary structures and microfacies types. The case study
from the Devonian of the Holy Cross Mountains in
Poland (Skompski and Szulczewski 1994), documented
in Pl. 132, describes a typical facies pattern showing
some of the diagnostic facies criteria that allow sub-
tidal and intertidal-supratidal carbonates to be differ-
entiated.
The Eifelian Wojciechowice Formation consists of
lagoonal dolomites and limestones. Cyclic sedimenta-
tion is evident both in the limestone and dolomite part
of the section studied (Fig. 15.11). The succession was
formed in low-energy environments, as seen in the
dominance of lime mudstones, wackestones and bind-
stones, and the rarity of grainstones and packstones.
Rare high-energy episodes are indicated only by a bra-
chiopod coquina (interpreted as a storm deposit), a basal
breccia of one of the cycles, and few intraformational
breccias at the top of intertidal laminites.
Subtidal environments
are derived from the compo-
sition and diversity of the biota comprising brachio-
pods, massive stromatoporoids, dendroid amphiporids,
gastropods, ostracods, dasyclad calcareous algae as well
as very rare trilobites and solitary rugose corals. Shal-
low subtidal and deeper subtidal assemblages have been
differentiated by looking at the inferred ecological re-
quirements of the biota. Shallow subtidal assemblages
are characterized by amphiporids, gastropods, ostra-
cods and calcareous algae. Deeper subtidal assemblages
consist of large brachiopods (
Bornhardtina, Emanuella
)
and massive stromatoporoid heads, and only subordi-
nate gastropods and amphiporids. Six microfacies types
can be distinguished based on the dominant fossils:
(1) Brachiopod-stromatoporoid mudstone/wackestone,
(2) brachiopod-peloid packstone, (3)
Amphipora
float-
stone, (4) gastropod wackestone/packstone, (5) algal
wackestone, and (6) ostracod wackestone. All the
microfacies types are strongly burrowed.
Carbonates interpreted as
intertidal-supratidal
are
distinguished from the subtidal limestones by texture
Basics: Peritidal carbonates
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