Geology Reference
In-Depth Information
Box 4.3.
Check your sample!
A) Seafloor micrite or internal micrite?
Check your sample to discover whether you are looking at a sea-floor micrite formed at the bottom/water interface (the
more common case), or at an internal micrite formed within the cavities and intergranular voids of the sediment. This
distinction is of major importance in the paleoenvironmental evaluation of micritic rocks, because internally precipi-
tated micrite alters the mud/grain ratio and may invalidate interpretations that relate the amount of micrite matrix to
water energy levels (Reid et al. 1990).
Internal micrites form (a) in the water column or on the sea floor, the skeletal or non-skeletal grains are transported
into the cavities. Or, (b) the internal micrite forms within the cavities. The sediment consists of skeletal debris or
unattached precipitates, or of attached non-skeletal precipitates. The last two cases correspond to the autochthonous
'container organomicrites' (Fig. 4.2) and are characterized by common peloidal fabrics (Pl. 6/4). The micrite within
cavities differs in color, microtexture, crystal size and included grains from the micrite of the host rock. Infilling of
micrite may be totally or restricted to the bottom part of the cavity, resulting in a geopetal structure (e.g. in 'stromatactis',
Pl. 20/1, 8). Alternation of micrite and grains may result in normal and reverse gradation structures. Internal micrite
should not be confused with the diagenetically formed
internal vadose silt
(see Sect. 7.4.2.1).
B) Describe the overall microfabric!
Microfabrics give indications of the possible origin of the fine-grained matrix and assist in attributing your sample to
one (or several) of the groups summarized in Fig. 4.1.
Homogeneous/massive matrix
(Pl. 6/1; Pl. 90/1): Appears dense in thin sections, dark in transmitted light, commonly
without any or with only a few grains, often without fossils, and with no or only vague lamination. May point to group
1 of Fig. 4.1, but also to a diagenetic pseudomicrite matrix (modes 11 and 12).
Inhomogeneous matrix
(Pl. 19/1, 6; Pl. 25/4): Characterized by irregularly distributed micrite areas differing in color
(dark and light gray), microfabric (with or without peloids and bioclasts) and the degree of reworking, which is often
caused by burrowing and bioturbation.
Peloidal matrix
(Pl. 8/6): Fine-grained matrix with abundant very small peloids (no fecal peloids), commonly densely
packed, sometimes exhibiting a clotted fabric, appearing in various shades of gray in transmitted light. Often associated
with sponges. Common fabric of internal micrites and organomicrites. Points to modes 2 and 3.
Fine-bioclastic matrix:
(Pl. 6/1, Pl. 87/8-9): Micrite containing abundant very small bioclasts, which appear white in
transmitted light. Bioclasts originate from the disintegration of various invertebrates, and often cannot be identified in
thin sections. Points to the modes 6 to 8.
Laminated matrix:
Fine, curved or plane laminae, consisting of homogeneous and/or peloidal micrite, sometimes alter-
nating with spar- and grain-bearing zones. Are laminae boundaries sharp or gradual? Lamination may reflect (a) differ-
ences in automicrite production (autochthonous growth of microbialites; modes 2, 3 and 4; Pl. 18/4; Pl. 50/2, 4, Pl. 123/
3), (b) differences in sediment deposition (e.g. caused by tides or bottom currents; Pl. 18/2, Pl. 20/8), or (c) pressure
solution seams (see Sect. 7.5.2; Pl. 37/6).
Siltitic matrix:
Is the bulk of the matrix within the silt-sized range, and do small angular lithoclasts occur? This might
point to the abrasion of former carbonate rocks (mode 10).
C) Are there conspicuous textural differences in your sample?
Marked differences in texture, rock color and the type of common fossils (e.g. encrusting and boring biota) may indicate
changes or breaks in sedimentation associated with hardgrounds (common in micrites of mode 9).
D) Try to evaluate crystal size and crystal fabric!
Petrographical thin sections allow the measurement of crystal sizes as well as the recognition of crystal shapes (an-, sub-
or euhedral) and fabrics which may be equant (consisting of nearly equidimensional crystals) or non-equant (composed
of different sized crystals). Grain size is the conventional measure in distinguishing micrite and microspar. Fabrics made
of non-equant crystals may point to recrystallization (Pl. 38/3; mode 12).
E) Take a look at the fossils and biogenic textures occurring within the micrite matrix!
Fossils should be differentiated according to their life habits (planktonic, benthic; soft or hard bottom dwellers), fre-
quency and preservation. Borings and fragmentation of calcareous green and red algae may indicate mud production
related to the disintegration of benthic algae living on the sea bottom or on seagrass and soft macro-algae (modes 5 and
6). Strong fragmentation and intensive borings of invertebrate skeletons points to the modes 7 and 8. An abundance of
calcareous plankton suggests mud formation from the accumulation of pelagic organisms (mode 9).
F) Noncarbonate material
Many microcrystalline limestones contain varying amounts of clay, quartz and other non-carbonate minerals. Care
should be taken in differentiating the mineralogical composition and distribution (random, in zones) of these constitu-
ents. The percentage of clay in micritic limestones is a major control on recrystallization. Interbedded clay could attract
magnesium ions from adjacent micrites and facilitate microspar formation through aggrading neomorphism (Sect. 7.6).
