Page 24 - Carbonate Facies in Geologic History

Possibility of Grain Support Inferred from Estimates of Degree of Particle Packing 11

was able to remain at the site of deposition. This emphasis on what might be called currents
of removal seems advisable if we wish to characterize carbonate sediment systematically in
terms of hydraulic environments.
"Inasmuch as calm water is characterized by mud being able to settle to the bottom
and remain there, it seems that the muddy rocks deserve to be contrasted with mud-free
rocks, regardless of the amount and size of included coarse material" (DUNHAM, 1962).

2. The Possibility of Grain Support Inferred from Estimates of Degree of Particle
Packing

Did the sediment consist of a self-supporting framework of grains or do grains
float in a micrite matrix which was originally lime mud? The variability of grain
to micrite ratios in shallow water carbonate sediments, as well as the extremely
variable shapes of particles, makes the packing-framework concept of Dunham
fully as useful and about as accurate in application as grain to micrite ratios
employed by other classifications. Naturally, the development of grain framework
depends not only on the number, but also on the shape of the grains. Solid spheres
will form a self-supporting pack with about 60% of the volume being grain bulk.
Dense branched twigs and arcuate shells will form a framework with only 20-
30% grain bulk (Dunham, 1962, Plate II). Because the grain-supported fabric may
lead to important diagenetic modification (e.g., easier early solution or coarser
crystallization because of lack of compaction of intergranular mud, and greater
grain to grain solution penetration), the concept of grain-supported fabric may be
very useful. The carbonate textural spectrum of Folk (Fig. 1-4) reflects the packing
concept fully as well as does the original Dunham classification (Fig. 1-5).

Pwcenl
~~~----+-----~-----+------~~==+-~~~~~~~~~

Rock
T,rml

T"'i9lnoUI
Anol~IIL-____________ ~~~~~~~~~~~~~~~~~~~~Ull~

• LIME MUD MATRIX
~ SPARRY CALCITE CEMENT

Fig. 1-4. Carbonate textural spectrum from Folk (1962, Fig.4), with permission of American
Association of Petroleum Geologists