Geology Reference
In-Depth Information
Plate 112 Standard Microfacies Types: SMF 1 and SMF 2
This plate and the following plates up to Pl. 126 start with a
general text
explaining the diagnostic criteria and the
occurrence of the SMF Type in the different depositional settings. The
description of thin
sections
is focused on
characteristics of the SMF Types and indicates their more frequent distribution in generalized facies zones.
SMF 1: Spiculite wackestone or packstone, often with a calcisiltite matrix
Criteria:
Dark, bedded limestones (mudstone, wackestone), commonly argillaceous and organic-rich. Abun-
dant siliceous (or calcified) sponge spicules, often oriented. Parallel orientation of spicules and clay laminae
may cause a very fine depositional 'lamination' pattern (Wilson 1969). In addition pelagic microfossils (e.g.
radiolarians) may occur. The microfossils may be densely packed (-> 1) or sparsely distributed due to intensive
burrowing (SMF 1-B
URROWED
: -> 2).
Occurrence:
Basinal deep-water environment with slow sedimentation.
This microfacies is common in Paleozoic and Mesozoic basinal carbonates (FZ 1), deeper shelf carbonates (FZ
2) as well as in mid-ramp and outer ramp settings. Spiculites are common in deep cold-water settings, but also
occur in shallow-marine environments (Gammon and James 2001).
Additional pictures:
Pl. 44/1, Pl. 139/1.
SMF 1-B
URROWED
:
Burrowed bioclastic wackestone with abundant fine pelagic and benthic biodetritus.
Criteria:
Very small bioclasts, commonly shell debris, scattered within a dense, strongly burrowed matrix.
SMF 1-B
URROWED
differs from SMF 1 in the dominance of sparsely distributed skeletal grains representing a
mixture of benthic and planktonic elements.
Occurrence:
Basin (FZ 1), open sea shelf (FZ 2), and outer ramp.
SMF 2: Microbioclastic peloidal calcisiltite.
Criteria:
Thin- to medium bedded limestones. Very fine-grained packstones or grainstones consisting of pe-
loids and small litho- and bioclasts. The grains are a mixture of off-bank debris and reworked sediment. Com-
mon bioclasts are echinoderms and mollusks. Small-scale ripple cross-lamination and laminae exhibiting milli-
meter-scale gradation are common.
Occurrence:
Common in deeper basins (FZ 1) and open-marine shelf (FZ 2)
as well as in deep-shelf toe-of-slope position (FZ 3). Abundant in outer ramp settings.
1
SMF 1.
Radiolaria-bearing spiculite packstone.
The bioclastic packstone consists predominantly of accumulations of
monaxon and other hexactinellid sponge spicules. Some of the spicules are partly calcified. Larger circular structures are
cross-sections of radiolarians (arrows). Note parallel orientation of spicules indicating bottom currents. The matrix is
organic-rich micrite. Water energy was sufficient to orientate the spicules, but not to winnow the carbonate mud. Deep
basin (FZ 1A). Jurassic of the Northern Calcareous Alps (Roman mosaic stone): Kraiburg, southern Bavaria, Germany.
SMF 1
B
URROWED
.
Burrowed wackestone.
Skeletal grains are conspicuously small and comprise shell debris, sponge
spicules (SP), radiolarians, and small echinoderm fragments (E).
Burrowing (B) is abundant, corresponding to the biotur-
bation grade 4 of Taylor and Goldring (1993). This index describes burrow abundance, amount of burrow overlap and
sharpness of primary sedimentary fabric. Grade 4 is character-
ized by high burrow density with common overlap. Burrowing
contributes to the diminution of skeletal grains. Deep open shelf
(FZ 2). Early Jurassic (Sinemurian): Korfu Island, Greece.
2
3
SMF 2
.
Fine-grained peloid grainstone (FPG) and peloid-bio-
clast packstone (PBP).
Most bioclasts are echinoderms with
syntaxial rim cements. The sediment consists of peloids and skel-
etal debris. White arrow points to a shell fragment, the black
arrow to serpulid tubes. The currents that deposited the silt-sized
grains also winnowed most of the lime mud. Off-bank facies of
an outer ramp. Cretaceous: Switzerland.
Fig. 14.14.
SMF 2
:
Cross-bedded fine-grained quartz-peloid
packstone.
Dark layers consist of densely packed micritic peloids
(redeposited intraclasts). Light layers contain angular, silt-sized
terrigenous quartz grains. Note the brachiopod shell (arrow). The
sample represents a basinal facies. The sediment was deposited in
deep-water canyons (see Buchroithner et al. 1980). Note the vertical
calcite-filled microcracks displacing an earlier oblique calcite vein.
Standard Facies Zone (FZ 2). Late Devonian: Ferragut Vell, Menorca
Island, Spain. Scale is 5 mm.
