Page 61 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 1 2 8, 129, 130
Ca r b o n ate cements
(continued)

/\n early phase of cementation may occur in the vadose
lone (above the water table), where pores in the sediment
arc not completely water-filled. Water, and hence calcite
cement, occur around grain contacts in the form of a
meniscus.
128 shows a sediment largely made up of segments of
the codiaccan alga Halimeda (1 12). The rock is highly
porous, and although impregnated (the brownish-gr e y
background material is the impregnating resin), it has
been difficult to take a peel and hence there are numerous
air bubbles. The algal segments have been cemented by a
small volume of pink-stained fine calcite spa rite at grain
contacts. This is characteristic of cementation from
meteoric waters in the vadose zone. Note the meniscus
clfcct leading to the rounding of pore spaces, well seen to
the left of centre of the photograph.
Another feature which can occur in the vadose :zone is a
drip.Holle or microstalactitic cement. In this case water
droplets and hence cements are concentrated on the
undersurfaces of grains. 129 shows a sediment in which
the first generation of cement occurs only on the lower
surfaces of some grains. In the photograph it is very pale,
brownish-coloured and never more than a millimetre
thick. Vadose cements can form from marine pore waters
in the intertidal and supratidal zones as well as from
meteoric waters. In the former case the cement will have a
radial-fibrous fa bric. l n the example here, the cement is
too fine-grained for its fabric to be resolved at the
magnification shown. A later generation of coarse sparitc
fills the pores.
Cements, especially those deposited in a marine envir­
onment, may be micrite. In ancient limestones where
pore-spaces are completely filled, it is difficult to distingu­
ish micrite cements, which have nucleated on grain
surfaces and grown outwards to fill or partially fill pore­
spaces, from carbonate mud sediment deposited with the
grains. 130 illustrates a sediment comprising fragments of
algae and micrite peloids having a matrix which is a
mixture of micrite (greenish-brown) and spa rite (colour­
less). The micrite coats some grains and forms 'bridges'
between adjacent grains and it may therefore be a cement.
llowcver, it is possible that micrite sediment, deposited
along with the grains, became partially lithified and was
then subject to erosion by a through flow of water which
removed unlithified material.
128: Stained acetate peel, Quaternary, Mombasa, Kenya,
magnification x 9, PPL.
129: Unstained thin section, Woo Dale Limestone, Loll'er
Carboniferous, Long Dale, Derbyshire, England; magnific­
ation x 22, PPL.
130: Unstained thin section, Coal Measures, Upper Carbo­
niferous, Meta/lie T il e ries, Chesterton, Staffordshire,
£ngla11d; mag11ijication x 20, PPL.