Geology Reference
In-Depth Information
Bahamian aggregate grains (Pl. 15/1, 3) demon-
strate (a) a strong control of grapestone formation by
microorganisms living in the interstices between the
composite grains and in microborings (Fabricius 1977),
and (b) continuous successions of increasing binding,
agglutination and induration. Cyanobacteria, algae and
fungi are effective in connecting and binding the con-
stituent grains, trapping loose silt material (Cros 1979),
destructively alterating the composite grains, and trig-
gering cementation within intergranular voids.
The formation of grapestones requires a sequence
of depositional conditions, starting with the supply of
the constituent grains (Fig. 4.28). After an environmen-
tal change, a subsequent set of depositional conditions
must allow binding and cementation of grains. These
changes correspond to periods of bottom mobility al-
ternating with periods of bottom stability. Grapestone
sites have been regarded as areas of nondeposition or
at least reduced deposition. Marine cementation is
strong because the process has millennia to operate on
a limited amount of sedimentary material (Winland and
Matthews 1974).
Not all ancient limestones with abundant aggregate
grains were formed by the processes outlined above:
Constituents of the well-sorted grapestone sands of the
Bahamas, may also correspond to sand and granule-
sized lithoclast fragments derived by erosion of firm-
ground crusts (Demicco and Hardie 1994). Firmgrounds
are sediments in the initial stages of hardground for-
mation that can be easily broken up and transported as
cohesive intraclasts (Jindrich 1969; Hardie and Garrett
1977; Dravis 1979). Carbonate aggregates, similar in
morphology to 'grapestones' are common in lower slope
fore-reef sediments, off Barbados. They were formed
in situ by submarine lithification processes (Stentoft
1994).
behind and adjacent to shelf-margin reefs (e.g. Pl. 121/
4), and areas which are transitional on the seaward side
to ooid shoals and green algal-foraminiferal sands on
the landward side (Pl. 61/1).
Significance of aggregate grains
Aggregate grains, which can be interpreted as ana-
logs of Bahamian grapestones, indicate low to moder-
ate and changing water energy levels, tropical and sub-
tropical warm-water conditions, low-nutrient environ-
ments and low sedimentation rates. The depositional
setting of bedded limestones composed predominantly
of aggregate grains usually corresponds to attached or
isolated platforms. Grainstones predominantly consist-
ing of aggregate grains are characteristic parts of plat-
forms formed during sea level highstand phases.
Economic potential:
Carbonates with aggregate
grains occurring in association with oolitic limestones
are reservoir rocks (e.g. in the Cretaceous of the Near
East). Because most grapestones originate in environ-
ments with no or very low terrigenous input, the non-
carbonate residues of many limestones with abundant
aggregate grains are conspicuously low. These lime-
stones are important resources of chemically high-qual-
ity carbonate rocks (Flügel and Haditsch 1977).
4.2.8 Resediments: Intra-, Extra- and
Lithoclasts - Insiders and Foreigners
The redeposition of material derived from preexisting
carbonate sediments and rocks is a common process in
marine and non-marine environments. Reworking and
redeposition can take place early and contemporane-
ous with sedimentation, or late and post-depositional.
Syn- and postsedimentary reworking and erosion pro-
duce carbonate clasts. These clasts reflect environmen-
tal conditions within the depositional basins as well as
the input of material brought into the basins. Constitu-
ent clast analysis of extrabasinal material has become
a major tool of microfacies studies providing an excel-
lent basis for reconstructing 'lost carbonate platforms'
which are recorded only by their relics preserved in
small clasts in limestones or abundant clasts of brec-
cias (Sect. 16.3.2.1).
Ancient aggregate grains
Carbonate grains which are similar to modern shal-
low-marine aggregate grains with respect to shape, size,
composition and internal cements are already known
from Precambrian limestones. Phanerozoic records of
limestones with abundant and dominating aggregate
grains are common in platform carbonates, but appear
to be concentrated in specific time intervals including
the Devonian, the Late Permian (Pl. 15/4, Pl. 34/7), the
Middle and Late Triassic (Pl. 15/5, 7, 8), Jurassic (Pl.
15/6, Pl. 62/1) and the Cretaceous (Pl. 29/9, Pl. 43/1,
Pl. 127/4). Limestones with abundant aggregate grains,
peloids and only a few special skeletal grains are com-
mon members of platform carbonates formed in open-
marine settings. Two common paleogeographic settings
of larger grapestone depositional areas were the areas
Terminology
The two major types of millimeter- to centimeter-
sized redeposited grains are intraclasts and extraclasts
(Folk 1959). An
intraclast
is a carbonate fragment of
lithified or partly lithified sediment, derived from the
erosion of nearby penecontemporaneous sediment from
