Page 76 - Carbonate Facies in Geologic History

Standard Microfacies Types 63

d) Are shells which were presumably originally aragonite preferentially replaced by do-
lomite or dissolved?
e) Are there coarse dolomite veins or patches?
f) Are dolomite rhombs particularly iron rich?
g) Is dolomitization pervasive and fine-grained and apparently in no way controlled by
original fabric of sediment?
9. Geopetal structure.
a) Does micrite exist as internal sediment inside of shells or cavities?
b) Is the internal sediment laminated, pure or silty?
c) Are pellet forms better developed in and under shells?
d) Is dolomitization better developed in and under shells? (This may be due to lack of
early compaction under the protection ofthe shells and hence more permeability, fluid
flow, and dolomitization during subsequent diagenesis.)
e) Are the levels of internal sediment horizontal or tilted affording a level bubble for
original depositional dip?
f) Was mud and silt which inftltered the original cavities deposited horizontally or is it
micro cross-laminated or slumped?
g) What is the crystal form of cement in the upper part ofthe geopetal cavity?
10. Micrite matrix.
a) Is it pure lime mudstone?
b) Is its grain size that of true micrite (4--5 microns) or is it of microspar range (10-
20 microns)?
c) Is the matrix vaguely pelleted or clotted, grumelous? This type of matrix commonly
occurs in grain-supported packstone or in interstices of boundstone.
d) Is the matrix entire or brecciated?
e) Does it have a fenestral or birdseye fabric with enclosed geopetals?
f) Is the matrix laminated or homogeneous?
g) Does the microcrystalline calcite consist purely of rhombs or platy crystals or are
remains of nannoplankton abundant?
11. Burrows.
a) What biological interpretation is possible for the burrows? What organisms caused
them?
b) Is there a mottling effect which outlines burrows, i.e., a color difference between
burrow fill and matrix sediment? Such differences may be caused by a different micro-
chemical environment due to organic decay of material in the burrow.
c) Is there a grain-size difference in burrow sediment and matrix? This might indicate a
later filling of the burrow.
d) Was the sediment hard, soft, or viscous when burrowed? Was matrix sediment soft
enough to be distorted by compaction after burrows were formed?
e) Is there evidence that lithoclasts could have been formed by collapse of burrows?
f) Can one see discoloration and sharp contacts at edges of the burrows, i.e., were
burrows lined by mineral matter or mucous?
g) Are most burrows vertical and straight?

Standard Microfacies Types

This Section lists 24 standard microfacies types which are considered of prime
sedimentological significance. The concept of depositional interpretation of mi-
crofacies may be credited to the French micropaleontologist 1. Cuvillier of the
Sorbonne in Paris in the early 1950's. See Fairbridge (1954) for an early review of
the importance of the concept. Many of the basic types have been categorized
by Erik Flugel (1972) who added sedimentological criteria to the basic paleonto-
logical approach used by many European researchers. In addition, some of the