Geology Reference
In-Depth Information
Are grainstone fabrics infallible indicators of high-
energy conditions near the depositional interface?
Many grainstones represent deposition in current or
wave-influenced settings where mud-grade sediment
does not accumulate. It should be kept in mind that not
all grainstones indicate the existence of high-energy
conditions and that some 'grainstones' may be the re-
sult of specific depositional or diagenetic processes:
• Some grainstone fabrics are the product of a local
accumulation of grains that have settled out of a flow
of a mud-grain mixture while the mud is washed away.
• Other grainstones develop from the winnowing of
mud from previously deposited grainy/muddy sediment.
• A too rapid production and accumulation of grains
will prevent the contamination by mud.
• The desiccation of subaerially exposed carbonate
muds can produce 'diagenetic grainstones'. This pro-
cess, called 'grainification' (Mazzullo and Birdwell
1989; Wright 1990) is common in peritidal, paludal,
and pedogenic carbonates.
• Diagenetic grainstones are also produced by pres-
sure solution during burial diagenesis, resulting in 'con-
densed grainstones' and 'fitted grainstones' (Wright
1992).
Floatstone and rudstone fabrics may be pitfalls in
environmental interpretation:
• A coarse-grained texture can be better seen in larger
rock samples and thin sections than in small ones, due
to the variability in grain sizes of transported bioclasts.
Plate 45 Dunham's Classification of Carbonate Rocks: Grainstone, Floatstone and Rudstone
The terms floatstone and rudstone were proposed for equivalents of wackestones, packstones and grainstones
differing from these rock types in their larger mm- to cm-sized grains. Distinguishing floatstone/wackestone and
rudstone/grainstone is particularly meaningful for investigating reef limestones and allochthonous limestones
deposited on slopes.
Grainstones
are grain-supported and mud-free carbonate rocks and consist of skeletal and non-skeletal car-
bonate grains. The absence of mud has various causes: Deposition of grains in high-energy environments (e.g. in
intertidal and shallow subtidal environments), rapid accumulation of grains allowing no coeval mud sedimenta-
tion (e.g. turbidites), deposition in current-controlled environments, or winnowing of mud from previously de-
posited grain/mud mixtures. Grainstones are highly variable with regard to grain type, shape, size, and sorting.
Significance: Grainstones are common rocks in platform and ramp carbonates. They are usually related to the
locus of wave energy absorption such as shorelines, shoals or shelf breaks. Here, grainstones form thick accumu-
lations at the outer shelf margins or in inner ramp settings. 'Bank-margin sands' occur in tidal bar belts, tidal
deltas, marine sand belts, back-reef areas, on beaches and in subaerial dunes. Note that accumulations of grain-
stones can also originate below the wave sweep base owing to current effects. Owing to their high interparticle
porosities and the possibility of additional secondary moldic porosity caused by dissolution of grains (Pl. 29/3,
4), grainstones are of particular interest as reservoir rocks.
Floatstones
are matrix-supported carbonate rocks yielding more than 10% grains larger than 2 mm. This
2 mm boundary creates difficulties in applying the term to fossiliferous limestones or limestones with oncoids
because the size of skeletal grains and oncoids can only reflect growth stages. The use of a strict terminology
would put a gastropod limestone yielding adult snails into the floatstone category and limestones containing the
same but juvenile snails into the wackestone category. Note that floatstone and rudstone were originally pro-
posed for reef carbonates and carbonate breccias. The 'matrix' of floatstones does not necessarily correspond to
micrite, but often exhibits fine-grained textures that must be described separately (-> 2).
Rudstones
are grain-supported carbonates rocks containing more than 10% grains larger than 2 mm. Many
breccias belong in this category. Rudstones and floatstones must be further characterized by compositional and
textural criteria (-> 3). Formation of rudstones needs erosion and transport. Erosion can be triggered by shallow-
water settings allowing destruction by storms. Slopes are common depositional settings of rudstones.
1
Grainstone
. Medium-sorted oolitic grainstone. Note the difference in ooid sizes, indicating some transport. SMF 15-C.
Late Triassic (Carnian): Obir, Carinthia/ Austria.
2
Floatstone
. Bioclastic coral (C)-gastropod (G) floatstone with a fine-grained calcimudstone matrix. The originally arago-
nitic corals are strongly recrystallized. Arrow points to still preserved septae. SMF 8. Late Triassic (Kössen Formation):
Allgäu, southern Germany.
3
Rudstone
. Poorly sorted lithoclastic rudstone. The large coated bioclasts are sclerosponges (S). Lithoclasts (L) corre-
spond to reef debris deposited on reef slopes and redeposited as part of a megabreccia. Interparticle pores are filled with
submarine and burial carbonate cements. SMF 6. Late Permian: Sosio, western Sicily, Italy.
