Page 68 - Atlas of Sedimentary Rocks Under The Microscope

146, 147 Carbonate rocks
Limestone

classif i c a t i o n
(continued)

146 illustrates a grainstone. The rock is grain-supported
with a spar cement. The sediment is loosely-packed,
suggesting that cementation occurred before significant
compaction. The allochems arc a mixture of ooids (some
arc superficial ooids, see p. 35) and bioclasts. It is
therefore an oosparite according to Folk. Since the
allochems are rounded it would be a rounded oosparite,
using Folk's textural spectrum. 147 shows a packstone.
The rock shows two sizes of grains, having large and small
peloids. The f o rmer have a trace of oolitic structure in
places and may be micritized ooids (p. 54). The latter arc
probably faecal pellets. The sediment contains some
fcrroan calcite cement but also much carbonate mud
sediment in the matrix. It is nevertheless grain-supported
and thus a packstone. According to Folk's classification it
is a poorly-washed oosparite.

146: Unstained thin section . .!uras.tic. unknown locality.
Enl!,land; maf!.nification x 23. PPL.
i
/117: Stained acetate peel, In f erior Oolite, M d d /e Jurassic,
Cooper's Hill. Gloucestersllire. Enl!,land; ma�:n[fic­
ation x 13, PPL.
Other grainstones are shown (Folk classification in
brackets) in 73 (oosparite). 14 (oosparite). 15 (sorted
p e l s parite). 77 (unsorted intrasparite). 87 (unsorted bio­
sptlrite) and 124 (unsorted biosparite).
Other packs tones are shown in 72 (poorly-washed oos­
parite). 79 (packed intramicrite) 96 (poorly-washed
biosparite) anc/ 115 (packed biomicrite).

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