Page 73 - Carbonate Facies in Geologic History

60 Outline of Carbonate Petrography

back-scattered electrons. Depth of field is at least 20 times that of a light micro-
scope at comparable magnifications. Magnifications of X500-10000-20000 are
commonly employed with SEM. This instrument, along with the microprobe,
furnishes abundant new information about such things as the constituents of lime
mud, e.g., the character of calcitic plates and aragonite needles and the abundance
of submicroscopic algae, coccolithophores, comparative studies of the diagenesis
of lime mud and its alteration to limestone, investigation of trace-element chemis-
try and crystal structure of carbonate cements, studies of microscopic structural
deformation of fabric and micron-size surface coatings of carbonate particles.

Microfacies Interpretations

Microscopic study is the most important of the various levels of observation
possible in the broad field of carbonate petrograpphy. But, despite the advantages
that identifiable carbonate particles offer toward environmental interpretation,
detailed petrographic study of limestones and dolomites may be difficult because
of their susceptibility to diagenetic alteration. Prevailing neomorphic crystalliza-
tion, cementation, and mineralogical replacement of the original carbonate sedi-
ment result in a mat of dense crystal fabric. Ordinarily, forms seen in a thin
section or peel are caused by color and impurities (dust lines) and by different
crystal shapes and sizes rather than the original particles and fine, grained matrix.
Carbonate rocks are in a sense metamorphic-at least metasomatic, the replace-
ment occurring through the passage of time and water rather than through the
agencies of heat and pressure.
Nonetheless, petrographic study lies at the heart of the considerable success
geologists have had in depositional interpretation of carbonate strata, particu-
larly when petrography is combined with detailed stratigraphic control and when
Holocene sediments are studied to offer depositional models. Many authors have
treated the subject of depositional interpretation in part, particularly relying on
interpretations of organic particles and on large numbers of photomicrographs.
See especially Horowitz and Potter (1971), the International Sedimentary Petrog-
raphy Series of Cuvillier and Schiirmann (1951-1969), and Carozzi et al. (1972).
The following check list of observations and pertinent questions should aid in
the study of limestones as they are observed in polished and oiled plaquettes,
peels, and thin sections. Several other such lists are available (e.g., Klovan's list
given in Horowitz and Potter, 1971, p. 10). The present list is adapted particularly
for depositional interpretation. It is followed by an outline on diagenesis and an
example of actual thin-section interpretation.

1. What are the relative amounts of the major components?
a) Carbonate grains
b) Silt and quartz sand grains
c) Micrite matrix Oime mudstone)
d) Cement (sparry calcite)
e) Clay
f) Authigenic minerals including dolomite