Geology Reference
In-Depth Information
• Identify skeletal grains in thin sections.
• Determine their abundance, i.e. major and minor
constituents, using semi-quantitative or point-counter
techniques.
• Compare the results with the grain association types
listed in Box 12.9. Note that these types are rather
loosely defined and that sometimes 'minor' skeletal
grains will appear at a first rank position.
• Assign your grain associations to statistically based
assemblages, using cluster analysis or more sophisti-
cated methods (see Sect. 6.3).
• For Cenozoic carbonates consider larger foramin-
ifera for differentiating warm-temperate and tropical
zones (Betzler et al. 1997; Halfar et al. 2000).
carbonates will be discussed in Sect. 16.2.5.2. The
samples were selected from case studies whose
paleolatitudinal position (derived from the paleomaps
in Kiessling et al. 2002) is in fair accordance with the
attribution of the sample to tropical, temperate or cold-
water zones.
12.2.2.3 State of the Art
The composition of Cenozoic warm-water carbonates
is well known, although more studies are needed based
on thorough quantitative data. Nearly all the studies of
ancient temperate and cool-water carbonates deal with
Ordovician, Late Paleozoic, Late Cretaceous and Ter-
tiary examples (Box 12.10). Ordovician and Late Pa-
leozoic case studies reflect the influence of glaciation
on climate-dependent cold-water carbonates formed in
periods of deglaciation.
Regional climatic variations and the occurrence of
tropical cold-water grain associations in warm-water
environments are discussed in papers on Cretaceous
shelf carbonates (e.g. Gischler et al. 1994).
Tertiary examples from southern Australia and New
Zealand are of particular interest, because of the con-
tinuous record of temperate sedimentation in these re-
gions.
Of particular value in determining the skeletal grain
assemblage of Cenozoic shelf carbonates is the com-
posite triangular diagram developed by Hayton et al.
(1995).
12.2.2.2 Examples
Plates 105, 106 and 107 display Paleozoic, Cretaceous
and Tertiary grain association types used to evaluate
paleoclimatic conditions. The examples consider tropi-
cal and non-tropical shelf carbonates. Non-tropical reef
Plate 105 Skeletal Grain Association Analysis: Tropical WarmWater Shelf Carbonates
Modern low-latitude tropical and mid- to high-latitude non-tropical shelf carbonates differ in the overall compo-
sition of abundant and dominating skeletal grains. Common constituents in tropical warm-water settings are
calcareous green algae, benthic foraminifera, mollusks and hermatypic corals. This gross composition can also
be recognized in Mesozoic carbonates. Corals, larger foraminifera and some bivalves harbor endosymbionts that
require light and are therefore limited to water depths of only a few meters. Non-skeletal particles are ooids,
peloids and aggregate grains. The plate displays common warm-water grain assemblages of the Photozoan
Association type.
1
Chloroforam grain association . This association is typified by the abundance of calcareous green algae and benthic
foraminifera, often accompanied by non-skeletal grains. Characteristic chloroforam associations occur as early as the
Paleozoic and are common in the warm waters of tropical platforms and shallow ramps. The sample is from the inner part
of a Cretaceous extended ramp whose paleoposition was approximately 10° S. Algae are dasyclads (D; Cylindroporella
and others) and udoteaceans (U; Arabicodium ). Foraminifera are represented by miliolids (M) and biserial textulariids
(T). Other grains include cortoids and peloids (left corner). Note early marine and shallow burial interskeletal cements, a
common feature of warm-water carbonates. Middle Cretaceous: Subsurface, Arabia.
2
Chloralgal grain association characterized by the abundance of calcareous green algae. This association has been known
since the Cambrian. Modern examples occur in protected areas of tropical and subtropical platforms. The plate-like
fragments are Epimastopora Pia, an alga commonly assigned to the dasyclads. Some dasyclads are partly encrusted by
foraminifera. Additional biota in modern chloralgal associations are benthic foraminifera and mollusks. Late Paleozoic
associations commonly exhibit brachiopods (BR) instead of mollusks. The letter O indicates rare ostracods. The sample
comes from the inner part of a locally restricted carbonate platform with a paleolatitude of ~10° S. SMF 18-D ASY . Early
Permian: Carnic Alps, Austri a.
Search WWH ::




Custom Search