Page 75 - Carbonate Facies in Geologic History

62 Outline of Carbonate Petrography

(4) Do they show more or less equal concentric growth or periodic growth stages on
different sides?
(5) Internally do they contain algal fIlaments (microtubules of Girvanella) or other
encrusting organisms such as foraminifera, sponges, spirorbids, or stromatopo-
roids?
(6) Were the onkoids hard or soft when emplaced in the sediment?
(7) Are they deformed in any way?
e) Grapestone lumps are a compound of agglutinated or aggregated particles.
(1) How much larger are such particles than peloids or lithoclasts?
(2) Are they coated or not?
(3) Does intergrain material show any organic structure?
(4) Does fibrous druse exist between grains?
(5) Are grains on the edge ofthe lump truncated or whole?
(6) Are there internal algal fIlaments?
(7) What kinds of particles are agglutinated?
(8) Are they exclusively peloids?
5. Textural considerations.
a) Is there evidence of any systematic size-sorting of grains?
b) Is there evidence of any systematic shape-sorting of grains?
c) Are elongate axes of grains and structures parallel with bedding or is fabric homoge-
nized by bioturbation (burrowing)?
d) Is there evidence of textural inversion, i.e., grains of a given size and shape indicative of
higher energy environment deposited in a micritic matrix?
e) Is there infIltering of mud between grains? Are there bridging and umbrella effects, i.e.,
grains caught above flat fragments or flat fragments acting as protectors for underly-
ing pore space which became filled with spar? This would indicate original deposition
of a grain-supported fabric. Use Dunham or Folk textural criteria to determine
whether the fabric is grain or mud-supported as a background to further observations
on grain packing.
6. Compaction history.
a) Is there evidence of early (syndepositional) solution compaction? Examine types of
grain contacts: points, facial, or sutured contacts. In sands with spherical grains,
more than 0.7 point contacts per grain constitutes overpacking.
b) Is there any evidence of multiple (horsetail) microstylolites or large individual ones?
Stylolites are always very late diagenetic features.
7. Types of cement (see this chapter's outline on diagenesis and Horowitz and Potter (1971,
Fig. 3).
a) What is the form of cement? Isopachous rims, dogtooth spar, fibrous druse, palisade
crystals, micrite?
b) Are there one, two or three generations of cement? Does any blocky calcite cement
appear to have existed from the beginning of cementation or is it purely a late feature?
c) Is the blocky calcite mosaic of evenly uniform texture or irregular in size? Are crystals
filling void space equidimensional or do they enlarge centripetally?
d) Is the later generation of cement ferroan compared to earlier cement?
e) Is earliest cement cloudy and later cement clear, perhaps indicating respectively ma-
rine and meteoric origin?
f) Are there overgrowths on echinoderm grains? On other grains as well ?
g) Are there enfacial angles (Bathurst, 1971)?
h) Are there enclaves of irregular-sized calcite crystals?
i) Is the cement inside hollow grains like that outside?
j) What is the age relation of cement types to compaction?
8. Dolomite content.
a) Are the rhombs of uniform size?
b) Do the rhombs possess clear rims and cloudy centers?
c) Are the rhombs located in such positions to infer that early permeability or fluid
content controlled replacement? E.g., are they preferentially in micrite or mud pellets?
Do rhombs avoid what were originally dense calcitic bioclasts?