Geology Reference
In-Depth Information
4.2.7 Aggregate Grains: Grapestones,
Lumps and Other Composite Grains
Gulf (Taylor and Illing 1969, Kendall and Skipwith
1969) and also in the Gulf of Batabano south of Cuba
(Bathurst 1975). The grains are common in platform
environments, but can also be associated with reefs
(Milliman 1967). Different names characterize differ-
ent shapes and the variable contribution of organisms
and carbonate cements to the formation of these com-
posite grains:
Grapestones:
Aggregates of spherical grains (often
micritized ooids) whose external shape resembles mi-
croscopic clusters of grapes. Typical grapestones ex-
hibit very little microcrystalline carbonate, but often
interparticle carbonate cements (Pl. 15/2). Micritiza-
tion of these cements may transform a grapestone grain
into a lump grain (Pl. 62/1).
Lumps:
Aggregates with a smoother outline than
grapestones; they commonly have hollow interiors and
are often strongly micritized (Pl. 15/3).
Botryoidal or oolitic lumps
: Grapestone or lumps
with a thin oolitic coating.
Microbial/algal aggregate grains:
Sedimentary par-
ticles are agglutinated by biofilms, cyanobacteria and
algae (Pl. 15/5). The grains are common in lagoonal
environments and often associated with microbial mats.
Encrusted aggregate grains:
These grains are char-
acterized by strong biogenic encrustations with sessile
foraminifera, calcareous algae, serpulids and other or-
ganisms (Pl. 15/4). The grains are common in protected
lagoonal environments.
Originally separated ooids, bioclasts and other grains
can be bound together by organic films, encrusting or-
ganisms and aragonite or Mg-calcite carbonate cements,
forming composite grains. The best known modern ex-
amples are the sand-sized 'grapestones' of the Bahama
Banks which constitute up to eighty volume percent of
the total number of particles in a very large transitional
area from agitated ooid shoals to the protected mud
and peloid mud environments. Strong micritization of
the component particles often makes it difficult to dis-
tinguish between aggregate grains and resediments,
which are reworked chunks of partly consolidated sedi-
ment. Most aggregate grains form in very shallow ar-
eas where currents and waves are sufficient to remove
fine-grained sediment, but not sand.
Recent Bahamian-type aggregate grains occur in
subtidal and intertidal shallow-marine environments
with restricted or changing circulation. Grapestones,
consisting of peloids, ooids and oncoids, and cemented
by aragonite or cryptocrystalline Mg-calcite are also
known from sea-marginal hypersaline pools (e.g. Red
Sea: Friedman 1978). In hypersaline coastal ponds of
the Black Sea, cementation within grain aggregates oc-
curs by means of bacterial degradation of calcium sul-
phate (Baltres 1975).
Grapestones and other varieties of aggregate grains
are characteristic constituents of many Precambrian and
Phanerozoic platform carbonates and valuable paleoen-
vironmental proxies.
A typical feature of aggregate grains is the irregular
shape and the typically lobate outline (Pl. 15/1, 8). The
size ranges between 0.5 and several millimeters, but
lies commonly below 1 mm.
Although many transitions exist between these va-
rieties (Halley et al. 1983), the more common types are
Terminology
Aggregate grains were first observed in the Bahama
Banks (Illing 1954; Purdy 1963), then in the Persian
Fig. 4.28.
Multi-stage formation of aggregate grains (grapestones and lumps).
Stage 1: Ooids and skeletal grains are bound together by microbial filaments, organic biofilms and encrusting organisms, e.g.
foraminifera.
Stage 2: Increasing calcification of microbiota contributes to the formation of a porous, cemented aggregate (grapestone)
with a lobate outline. The grains are affected by microborers leading to micritization.
Stage 3: Increased cementation (black) creates a smoother relief and reduces the intergranular porosity.
Stage 4: Continuous micritization and cementation form a matrix-rich aggregate grain whose outline becomes successively
more rounded. After Tucker (1990).
