Geology Reference
In-Depth Information
Box 12.9.
Grain association types
of warm-water and cool-water shelf carbonates. The list records the name, author and
age of the material used, followed by the abundant and (separated by a hyphen) minor constituents, and the observed or
inferred setting. H (heterozoan) and P (photozoan) at the end of the paragraph indicate the position within the major
Grain Associations distinguished by James (1997). Percentage composition corresponds to average values for grains not
for bulk samples.
• Chlorosponge
(Beauchamp 1994; Permian). Green al-
gae, coralline calcareous sponges. - Benthic foraminifera,
oncoids. Warm-water. Low latitudes. P.
• Chlorozoan
(Lees and Buller 1972; modern). Calcare-
ous green algae, zooxanthellate corals. - Benthic foramin-
ifera, articulated coralline red algae, mollusks, non-skel-
etal grains. Tropical. Warm-water. P.
• Echinofor
(Hayton et al. 1995; Tertiary). Echinoderms
(about 40%; mainly echinoids), benthic foraminifera
(~35%; foraminifera may outspace the echinoderms). -
Bryozoans, planktonic foraminifera. Non-tropical. H.
• Foramol
(Lees and Buller 1972; modern). Benthic fora-
minifera, mollusks. - Bryozoans. The term is now some-
what outdated and replaced by new terms, including the
rhodalgal and molechfor associations. - Non-tropical. Tem-
perate and cool-water. H.
• Hyalosponge
(Beauchamp 1994; Permian). Siliceous
sponge spicules. The name is taken from modern examples
(Henrich et al. 1992). Deep-water sediments (carbonates
and cherts) at low latitudes, or shallow to deeper-water
settings at high latitudes. H.
• Molechfor
(Carannante et al. 1988; Tertiary). Mollusks,
benthic foraminifera. Non-tropical, warm-temperate envi-
ronments. H.
• Nannofor
(Hayton et al. 1995; Tertiary). Planktonic
foraminifera (~80%), nannofossils (coccolithophorids). -
Echinoderm spines, sponge spicules, benthic foraminifera,
bryozoans. Not a typical shelf assemblage. Usually in off-
shelf deeper waters, but also in shallow, partly enclosed
basins.
• Rhodalgal
(Carannante et al. 1988; Tertiary). Calcare-
ous crustose red algae (> 80%; commonly rhodoids), bryo-
zoans, benthic foraminifera, barnacles, bivalves, echino-
derms. - Common in non-tropical warm-temperate settings.
High-energetic photic zone. Transitional between chloro-
zoan-dominated tropical areas and cold-temperate ones.
Also in tropical zones where environmental conditions pre-
vent the development of a chlorozoan facies. H.
• Rhodechfor
(Hayton et al. 1995; Tertiary). Calcareous
red algae (up to 80%), echinoderms (mostly echinoids),
benthic foraminifera, bryozoans. - Barnacles, bivalves. -
Non-tropical temperate settings. H.
• Barnamol
(Hayton et al. 1995; Tertiary). Barnacles
(~60%), bivalve mollusks (~20%). - Bryozoans, echino-
derms, benthic foraminifera. Non-tropical coastal and open-
marine shelf environments. H.
• Bimol
(Hayton et al. 1995; Tertiary). Bivalve mollusks
(infaunal and/or epifaunal; > 60%, up to 80%). - Bryozo-
ans, benthic foraminifera, echinoderms, calcareous red al-
gae, barnacles. Non-tropical, but also tropical. H.
• Bryomol
(Nelson 1988; modern and Tertiary). Bryo-
zoans (> 50%), bivalve mollusks (infaunal and epifaunal).
- Benthic foraminifera, echinoderms, calcareous red al-
gae, barnacles. Abundant in non-tropical, mostly cool- and
cold-water environments, but also occurring in subtropi-
cal and some tropical environments. High-energy open
shelves and ramps. Analysis of bryozoan growth form as-
sists in distinguishing shallow and deeper settings (see over-
view by Smith 1995). H. See Pl. 106/2, 3.
• Bryonoderm
(Beauchamp 1994; Beauchamp and
Desrochers 1997; Permian). Bryozoans, echinoderms (usu-
ally crinoids). - Brachiopods; rare peloids. Regarded as
the equivalent of modern bryomol associations. Low-lati-
tude deeper-water shelves to high-latitude, deep- to shal-
low-water shelves. Shallow and deep cool-water carbon-
ates are biotically alike, but differ in depositional texture
(grainstone versus packstone). Cool-water and cold-water.
H. See Pl. 106/1.
• Bryonoderm extended
(Beauchamp 1994; Permian).
Bryozoans, echinoderms. - The main composition corre-
sponds to that of the bryonoderm association, but addi-
tional biota elements (solitary and colonial corals, benthic
smaller foraminifera, larger foraminifera, brachiopods) may
be common. Non-tropical and tropical environments. H.
• Chloralgal
(Lees 1975; modern). Calcareous green al-
gae. - Benthic foraminifera, branching coralline algae, mol-
lusks. Tropical and warm-temperate zones. Controlled by
salinity and water temperature. P. See Pl. 105/2.
• Chloroforam
(Beauchamp 1994; Permian).- Calcare-
ous green algae (dasyclads, udoteaceans), benthic foramin-
ifera (encrusting and free forms, smaller and larger fora-
minifera), non-skeletal grains (ooids, aggregate grains) and
oncoids. Warm-water. Low latitudes. P. See Pl. 105/1.
associations consider the most abundant skeletal con-
stituents in wackestones, packstones, and grainstones.
Some definitions are satisfied with the qualitative as-
pect, but most of them use quantitative data derived
from point-counter measurements. Hayton et al. (1995)
subjected the skeletal abundance data to cluster analy-
ses that indicate the dominant groups and their varia-
tions.
Grain association analysis is an essential element
in categorizing Standard Microfacies Types (Sect. 14.3)
and interpreting Standard Facies Models (Sect. 14.3.1).
12.2.2 Practical Advice, Examples and State
of Current Information
12.2.2.1 Distinguishing Grain Association Types
Attributing unknown samples to the associations sum-
marized in Box 12.9 requires the following steps:
• Check criteria that point to autochthonous grain as-
semblages or to mixing and resedimentation of envi-
ronmentally different grains.
