Page 73 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 157, 158
L i m e s t o n e porosity
(continued)

Fenestrae is the name used for pores in a carbonate
sediment which are larger than grain-supported spaces.
They usually become infilled with internal sediment or
cement, or a combination of the two. Fenestrae can be
different shapes and sizes depending on their mode of
origin.
157 shows spar-filled f e nestrae in a micrite. Most are
irregular in shape and probably formed as a result of the
entrapment of fluid in a sediment during desiccation,
although the elongated fe nestra in the centre may have
been a burrow. Fenestrae of this type are sometimes called
birds-eye slmctures. The sediment contains a few micrite­
walled f o raminifera. Fenestral micrites were called dis­
micrile by Folk (see Table 4).
158 shows f e nestrae in a fine pellet grainstone. They
show a tendency to be elongate parallel to the bedding.
Fencst rae oft his type arc known as laminoidfenestrae and
may form from the decay of organic matter associated
with algal stromatolites (p. 53).

157: Swined thin section , Lower Jurassic. Central H ig h
Alias. Moroao: magn(/icalion x 14. PPL.
158: Stained ace/ate peel, Woo Dale Limestone. Loll'er
CarboniFerous. Derbyshire. England: magnification x 7.
PPL

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