Page 74 - Carbonate Facies in Geologic History

Microfacies Interpretations 61

2. Character and amounts of different types of bioclastic grains.
a) Are many different organisms represented or only a few monotonous types? Diversity
indicates generally open marine conditions.
b) Are open marine types abundant or is the biota of restricted marine type? E.g.,
brachiopods, cephalopods, echinoderm, red algae indicate open marine conditions
whereas oysters, clams, snails, many foraminifera, ostracods, and types of green algae
indicate more restricted circulation.
c) What types of algae are present? See Ginsburg et ai., (1971) for environmental descrip-
tion. These are reviewed below in some detail (Fig. III-3).
3. Preservation of grains.
a) How sharp or ragged are the boundaries of bioclasts?
b) Are rinds on them micritized (shell structure altered to microcrystalline calcite)?
c) Are internal pores filled with mud or spar-cemented?
d) Is there pervasive internal micritization or does one observe well-preserved internal
shell structure?
e) Are original aragonite grains preferentially dissolved?
f) Are the grains rounded or angular?
g) How much rounding can be attributed to micritization?
4. Non-bioclastic grains.
a) Are ooids present or absent?
(1) Is oolite pure or are ooids mixed with other particles?
(2) Is there mere superficial coating of particles or complete ooids?
(3) What is the relative size range of the nuclei?
(4) Is the grain size of ooids uniform despite disparity in size of nuclei?
(5) How regular are the oolite coatings? Do they show extensive boring by ftlamen-
tous blue-green algae?
(6) What types of particles make up the nuclei?
(7) What is the packing of the ooids?
(8) If overpacked could this be caused by early solution compaction rather than by
later stylolitization?
b) Peloids (fecal pellets) and indeterminate pelletoids (rounded homogeneous micritic
grains).
(1) Is there any size variation in grains?
(2) Do isolated nests oflarge pellets occur?
(3) Are there many small well-sorted round ones?
(4) Do those in the shells have well-preserved form and are those in the matrix outside
of the shells squashed?
(5) Is there evidence of agglutination of peloid particles?
(6) Does peloidal micrite include silt grains as well as finer particles?
(7) Is there evidence in some grains of transition from rotten bioclasts to structureless
microcrystalline rounded grains, i.e., making peloids by micritizing other grains?
This is a process occurring only in very shallow water.
c) Lithoclasts are derived locally and penecontemporaneously (intraclasts) or from out-
side the basin. Allochthonous lithoclasts are eroded fragments of previously lithified
rock derived from some distance away from the site of deposition, i.e., pebbles in a
conglomerate.
(1) What is their external shape and size?
(2) Are they flat chips or equidimensional?
(3) Are they notably larger than associated peloids?
(4) Do they possess rinds?
(5) Is there a color difference relative to micrite matrix?
(6) What is their internal composition-similar or different from the local matrix?
(7) Is the arrangement of clasts parallel to bedding or crosswise?
d) Onkoids are large particles formed by coating of algae and other organisms.
(1) What is their size range?
(2) What type of matrix, calcarenite or lime mudstone?
(3) Are their laminae crinkled or even?