Geology Reference
In-Depth Information
whose fine-grained matrix is developed as microspar.
Sometimes the term is used simultaneously with mi-
crospar.
Folk (1959) described a gap in crystal sizes some-
where between 3 and 4 m between micrite and micro-
spar. The data of Lasemi and Sandberg (1993) suggest
that the 'micrite curtain' (Folk 1965) is an artifact of
lumping two mineralogically different groups of mi-
crites. Munnecke (1997), studying Silurian fine-grained
carbonates, also found no gap.
Origin: Several explanations for the formation of
microsparitic limestones have been suggested (Fig. 4.5):
(1) Recrystallization (aggrading neomorphism: Folk
1965) of micrite calcite subsequent to the removal of
Mg 2+ ions which form a 'micrite curtain' and are re-
sponsible for the gap in crystal size somewhere between
3 and 4 m between micrite and microsparite (Folk
1974). Breaking through the 'Mg-cage', which in turn
initiates recrystallization, is triggered by freshwater in-
put related to freshwater aquifers, the influence of rain
water, and weathering near the surface (Longman 1977).
Interbedded clay could attract magnesium ions from
adjacent micrites to facilitate microspar formation
through aggrading neomorphism.
(2) One-step neomorphic process of cementation and
replacement (calcitization) of aragonite-dominated pre-
cursors to microspar without a micrite stage, also con-
nected with the infiltration of meteoric waters. Poros-
ity reduction occurs without compaction. Examples:
Holocene and Pleistocene (Steinen 1978, 1982; Lasemi
and Sandberg 1984; 1993), Pennsylvanian (Wiggins
1989) and Silurian (Munnecke and Samtleben 1995;
Munnecke 1997).
(3) Neomorphic growth from deep surface fluids
(Brand and Veizer 1981; Suchecki and Hubert 1984).
Wiggins (1989) demonstrated that saturation with re-
spect to calcite appears to be the only requirement for
microspar formation. This situation can also be met in-
dependently of freshwater if organic acids react with
aragonite in marine environments (Baker et al. 1982).
(4) Recrystallization of silt-sized carbonate grains
(Duringer and Vecsei 1998).
Fig. 4.6. Polygenetic carbonate mud production in a shallow-
marine platform environment. Lime mud budget for the
Bight of Abaco, Little Bahama Bank. Numbers indicate
possible modes of origin (see Fig. 4.1). The Bight of Abaco
is a semi-closed, 7 m deep shallow carbonate lagoon.
Temperature and salinity vary seasonally from 20 °C to
30 °C, and from 34 to 39‰, respectively. Skeletal aragonite
and Mg-calcite muds and sands are the predominant
sediments. The main source of fine-grained carbonates are
sub-microscopical elements of calcareous green algae.
Additional sources include fine-grained material produced
by the bioerosion, abrasion and breakdown of carbonate
grains, and by bacterially-induced and possible chemical
precipitation of aragonite and Mg-calcite. An important
source is the import of inter- and supratidal muds to the
lagoon. Lagoonal pellets disaggregate and return to the fined
fraction; the disintegration of cemented pellets formed near
the bank edge provides a source for lagoonal muds. The
export of lagoonal muds to tidal flats and onshore mud
transport are triggered by storms. Some lagoonal carbonate
is exported in solution. The Abaco example demonstrates
that more carbonate mud is produced by algal disintegration
and other sources than can be accommodated on the platform
top, and that mud transport moves on to adjacent tidal flats
and into deeper waters around the platform. Based on
Neumann and Land (1975).
carbonate rocks has favored the formation of microspar
fabrics. Global chemical abd biochemical controls have
been suggested for the abundance of microspar in the
enigmatic Proterozoic 'molar tooth structures' (Long
2003)
Significance: Depending on the model used, the oc-
currence of microsparitic limestones has been regarded
as evidence for meteoric diagenesis, for the influence
of low-salinity fluids or for subaerial exposure.
Honjo (1969) pointed to the frequency of micro-
sparitic limestones in the Paleozoic and the abundance
of micritic limestones in the Mesozoic. The author dis-
cusses the possibility of controls by changes in global
sea-water chemistry. A more likely explanation is that
the longer time involved in the diagenesis of Paleozoic
4.1.4 Calcisiltite
Definition: The name was proposed by Kay (1951)
for limestones consisting predominantly of detrital cal-
cite particles of silt size (2-63 m). Calcisiltite em-
braces the coarser part of 'calcilutite' (Grabau 1904)
which designates a fine-grained rock composed of clay-
and silt-sized carbonate particles. Later Lindholm
Search WWH ::




Custom Search