Geology Reference
In-Depth Information
a matter of days and infest a grain in a few weeks;
(b) Death of the microborers and vacation of tubes;
(c) Emplacement of micritic aragonite or High-Mg
calcite cements within vacant tubes. The formation
of cements can be triggered by organic acids pro-
duced during the bacterial decomposition of organic
compounds. Multiple repetition of boring and fill-
ing destroys the peripheral zone of the grains and
finally results in the formation of a circumgranular
micritic rim recognized in thin section. The surface
of the rim exhibits shallow pits; the boundary be-
tween the envelope and the skeletal core is irregular
and may preserve microtube structures penetrating
into the skeletal grain (Pl. 10/4). Destructive cor-
toids are abundant in shallow-marine environments
(Lloyd 1971; Margolis and Rex 1971; Harris et al.
1979), but also occur in transitional and non-marine
(e.g. caliche: James 1972; lacustrine environments:
Schneider et al. 1983; Pl. 130/3) and vadose settings
(Pl. 33/4; Pl. 35/2).
laginous coating of the grains (Kendall and Skipwith
1969). Modified versions of these interpretations are
gaining more and more acceptance.
Constructive micrite envelopes are formed both in
marine and in continental and lacustrine environments
(e.g. karst: Jones and Kahle 1995; eolianites: Calvet
1982; lake carbonates: Schneider et al. 1983). Terres-
trial vadose cortoids are caused by calcified filaments
of fungi and actinomycetes that collapse and coalesce
forming an intertwined organic meshwork that in turn
triggers the precipitation of microcrystalline cements.
Recognizing different modes of origin: Destructive
and constructive micrite envelopes are difficult to dis-
tinguish in thin sections. Constructive cortoids may
show irregularities on the outer surface of the grains
and the micritic rim may be of variable thickness,
whereas destructive cortoids exhibit irregularities to-
wards the grain interior and often reveal relicts of micro-
borings. Because primary mineralogy controls the sus-
ceptibility to the effects of microborings in carbonate
grains (Perry 1998), aragonitic bioclasts often are more
frequently bored than primary calcitic bioclasts. Mi-
critization is selective, affecting some shells more than
others. Shells with homogeneous prismatic microstruc-
tures exhibit micritic envelopes more often than shells
with fibrous or foliated microstructures.
A special mode of destructive micritization has been
proposed by Reid and Macintyre (2000). This model
focuses on concurrent microboring and filling of bore
holes (Fig. 4.13B). In contrast to the Bathurst model,
micritization occurs in microborings of cyanobacteria
penetrating just beneath the grain surface and produc-
ing tunnels. The tunnels are filled by radial-fibrous ara-
gonite cements that are precipitated as the organism
advances. Extensive multiple repetition of the process
results in the micritization of just the peripheries of the
grains or the whole grain. As opposed to micritized by
processes corresponding to the Bathurst model, the
grain margins here are almost completely preserved.
(3)
Partial dissolution and recrystallization:
Partial dis-
solution of skeletal grains associated with recrystalli-
zation and crystal diminution are other processes known
to lead to the formation of micrite envelopes. Mollusk
shells composed both of aragonite and Mg-calcite are
affected differently by submarine early diagenetic se-
lective dissolution. This may result in a peripheral rim
consisting of residual micrite (Winland 1968; Alexan-
dersson 1972; Pl. 52/6).
The micritic preservation of echinoderms in the chalk
of Texas is an outcome of partial dissolution called 'in-
organic micritization' (Neugebauer and Ruhrmann
1978). High-Mg calcite echinoderm fragments may lose
Mg and convert to Low-Mg calcite without a dissolu-
tion stage. Inorganic micritization and 'shell-residue
micrite' envelopes are widespread at the bottom of
modern seas in temperate regions. Micritization also
can be the result of decomposition of indigenous or-
ganic matter within shells, producing the simultaneous
growth of microcrystalline carbonate (Purdy 1968; Bur-
gess 1979).
(2)
Constructive micrite envelopes related to epilithic
organisms
(Kobluk and Risk 1977a, 1977b): These cor-
toids result from the precipitation of microcrystalline
calcite around and between dense populations of ex-
posed filaments of endo- and epilithic algae and cyan-
obacteria (Fig. 4.13C). The precipitation occurs pre-
dominantly upon dead filaments. The outer boundary
of the micrite rim appears irregular. Contrary to mode
1, the micrite envelope is formed without destruction
or alteration of the grain periphery. The process involves
addition of carbonate to the exterior of the grain and
requires grain stabilization for the duration of the en-
velope formation. This occurs in low-energy environ-
ments or shallow burial.
Another possibility for the formation of construc-
tive micrite envelopes is the growth of nannocrystals
within biofilms coating carbonate grains (Loreau 1970)
or the dissolution and reprecipitation within a muci-
Significance of cortoids
Preservation potential:
The susceptibility of skel-
etal grains to micritization differs with regard to the
