Geology Reference
In-Depth Information
Plate 132 Peritidal Limestones: A Case Study from the Devonian
The plate shows common facies-diagnostic criteria of cyclic peritidal and subtidal limestones forming shallow-
ing-upward cycles. Important criteria of the intertidal and supratidal conditions are laminites and laminated
microbial mats (-> 1), stromatolites (-> 5), fenestral fabrics (-> 6), reworked sediment and intraclasts (-> 4),
and desiccation structures, e.g. mud cracks (-> 1). Synsedimentary reworking by tidal currents and by storms is
a common feature (-> 2, 4). Intertidal mudstones and wackestones are frequently strongly burrowed (-> 3). The
subtidal parts of the cycle can be recognized by fossils: Green algae (-> 7), ostracods (-> 8) and gastropods (-> 9)
are common constituents of wackestones and packstones formed in the shallow subtidal environments. The
taxonomic composition of the brachiopod fauna characterizes somewhat deeper parts of the subtidal zone.
All samples are from the Eifelian Wojciechowice Formation of the Skaly quarry of the Holy Cross Mountains
in central Poland.
Supratidal and intertidal
1
Fine-laminated lime mudstone (bindstone)
with small vertical desiccation cracks (arrows) indicating the shallowest su-
pratidal parts of the cycle.The microfacies corresponds to Standard Microfacies Type SMF 19.
2
Intraformational breccia
from topmost parts of intertidal laminites. The mixing of biotic elements from different habitats
(ostracods in the micritic clasts, white arrow; brachiopod shells in the micritic matrix, black arrow) suggests a tempestite
origin rather than an interpretation as tidal-channel deposit.
3
Transgressive basal breccia
occurring at the sole of the subtidal part of the cycles. Micrite lithoclasts within a coarse-
grained, dolomitized gastropod packstone matrix. Arrows point to burrows. The breccia is interpreted as surf-zone deposit
formed during the initial phase of a transgressive episode.
4
Tidal-channel deposit.
Interfingering of an intraformational tidal-channel breccia (BR) with intertidal laminated sedi-
ments (LS). Topmost part of intertidal laminites.
5
Detail of a hemispheroidal
stromatolite
occurring in well-defined horizons. Note the crinkled laminae consisting of mi-
crite and peloids.
6
Fenestral algal bindstones
occur in the lowermost part of the tidal sequence. The bindstone exhibits subhorizontal spar-
filled voids bridged by microbial mats. The arrow points to planar fenestral structure. Note the oblique position of parts of
the mat indicating incipient reworking. Intertidal.
Shallow subtidal
7
Algal wackestone.
The tubular structures are palaeoberesellid green algae regarded as dasyclad algae or as an independent
green algal group (palaeosiphonoclads, see Fig. 10.17 ). This Devonian and Carboniferous group was usually restricted to
calm, lagoonal shelf environments.
8
Ostracod wackestone.
Non-disarticulated smooth-shelled ostracods (arrows) are abundant. Dissolution of the thin shells
would result in the formation of 'mold peloids' (see Fig. 4.11).
9
Gastropod wackestone,
slightly dolomitized. The shallower part of the subtidal unit of the cyclothem is dominated by
gastropods in association with amphiporids (stromatoporoids, see Pl. 80/2). Devonian amphiporid stromatoporoids are
abundant in black, bituminous limestones. The organisms were adapted to tranquil, reef-lagoon and back-reef habitats
with diminished oxygen in bottom waters.
Deeper subtidal
10
Brachiopod wackestone.
Some Devonian brachiopods (e.g.
Bornhardtina
, shown here) and also Mesozoic bivalves
(Fürsich 1993) were adapted to euryhaline conditions. Dense
packing and stacking of shells is caused by high-energy events.
Note the difference of the matrix type as compared with -> 9
(homogeneous micrite and calcisiltite). Deeper part of the
cyclothem.
-> 1-10: Skompski and Szulczewski 1994
Fig. 15.11.
Idealized shallowing-upward succession in Middle
Devonian platform interior carbonates showing main character-
istics of intertidal-supratidal and shallow and somewhat deeper
subtidal carbonates. See Pl. 132 for characteristic microfacies
types. After Skompski and Szulczewski (1994).
