Geology Reference
In-Depth Information
Plate 123 Standard Microfacies Types: SMF 19, SMF 20 and SMF 21
SMF 19. Densely laminated bindstone
Criteria:
Bindstone with a laminated fabric consisting of millimeter thick laminae that differ in internal
composition. Finer laminae correspond to micrite layers. Somewhat coarser laminae consist of densely spaced,
very small peloids or sparry calcite. The biofabric is characterized by couplets with irregular, sometimes wavy
boundaries. Fossils are rare except ostracods and some foraminifera, gastropods and a few algae.
Occurrence:
Near-coast platform interior (FZ 8 and FZ 9).
Tidal flats of attached and isolated platforms and inner ramps.
Additional picture:
Pl. 132/1.
SMF 20. Laminated stromatolitic bindstones/mudstones
Criteria:
Planar or variously dome-shaped laminated bindstones composed of fine- or coarse-grained lami-
nae sometimes exhibiting microbial or algal structures (see Sect. 9.1.4; Pl. 50).
Occurrence:
Very common in the
intertidal zone, but also in supratidal and shallow subtidal environments. Open platforms (subtidal, FZ 7), tidal
zones of restricted lagoons (FZ 8) and arid coasts (FZ 9A, supratidal).
Additional picture:
Pl. 132/5.
SMF 21-F
EN
. Fenestral packstones and bindstones
SMF Type 21 was originally defined by a 'spongiostrome mudstone fabric' with algal tufts. Relicts of poro-
stromate algae or calcimicrobes may be preserved (SMF 21-P
ORO
; see Pl. 124/1) or not. More useful criteria in
distinguishing this microfacies are fenestral fabrics (SMF 21-F
EN
) typically developed in FZ 8 and FZ 9A.
Pl. 123/3 and Pl. 124/1 display two common variants of SMF 21 occurring in association or separated
.
Criteria:
Bindstone characterized by variously sized fenestral cavities within a framework formed by sedi-
mentary and biogenic grains. Smaller fenestrae correspond to birdseyes, larger structures may have stromatactoid
shapes (compare Sect. 5.1.5 and Pl. 20).
Occurrence:
Supratidal and intertidal environments (restricted lagoons,
FZ 8) and evaporitic lagoons (FZ 9A).
Additional pictures:
Pl. 20/2, Pl. 41/2, Pl. 46/2, Pl. 50/5, Pl. 132/6.
SMF 19, SMF 20 and SMF 21 are common and in facies analyses often used as indicators for intertidal and
supratidal environments. The main criteria are lamination textures caused by microbial/algal mats and fenestral
fabrics. Differences in the morphology of the mats (-> 1 and 3) in combination with other sedimentary, organic
and mineralogical features are used in estimating the 'exposure index' within the peritidal zone. This index
reflects the percentage of time during which the area where the laminated sediment was formed was exposed
(Ginsburg et al. 1977). This approach also can be applied to ancient peritidal carbonates and allows microfacies
samples to be allocated to predominantly intertidal, mostly supratidal but with some intertidal, or only supratidal
subenvironments (Smosna and Warshauer 1981).
1
SMF 19.
Laminated bindstone
consisting of alternating couplets of micritic and peloidal layers (FZ 8). This microfacies
may grade into bindstones with fenestral fabrics. Late Permian (Main Dolomite, Zechstein): Subsurface, Miloszewo well,
Northern Poland.
2
SMF 20.
Laminated stromatolite bindstone.
The sample shows finely wrinkled laminae arranged in dense and closely-
spaced growth patterns exhibiting vertically stacked protuberances (arrows). This fabric meets the definition of stromato-
lites, favored by Riding (1999): 'A stromatolite is a laminated benthic deposit'. The irregular extended structure marked
by SP probably represents soft non-rigid sponges. The irregularly-shaped white calcite-filled structures indicate burrow-
ing and disintegration of the microbial mats by grazing organisms. The sample comes from cyclic bedded limestones
(member B of the 'Lofer cycle'). Intertidal environment (FZ 8). Late Triassic (Dachstein limestone, Norian): Zelenica,
Begunjscica Mountains, northern Slovenia.
3
SMF 21F
EN
.
Fenestral packstone
characterized by small spar-filled cavities (birdseyes, white arrows) and larger irregu-
lar voids some of which are filled with fine-grained sediment at the bottom (stromatactoid fabric, black arrow). The
sedimentary framework formed by peloids and cyanobacteria tufts (CB) encloses transported dasyclads (D). Porostro-
mate cyanobacteria tufts are characteristic constituents of micritic, non-laminated intertidal carbonates. The sample comes
from the intertidal member of a Lofer cycle (see Sect. 16.1.2). Early Jurassic: Korfu Island, Greece.
-> 1: Peryt 1986
