Geology Reference
In-Depth Information
Reviews summarizing the state of the art of the do-
lomite problem and the enigma concerning the uneven
distribution of dolomite rocks during the Precambrian
and the Phanerozoic were given by Zenger et al. (1980),
Land (1983), Machel and Mountjoy (1986), Hardie
(1987), Given and Wilkinson (1987), Shukla and Baker
(1988), Morrow (1990a, 1990b), Purser et al. (1994),
Sun (1994), Burns et al. (2000), and Warren (2000).
lomite crystals as well as their mutual relationships are
important for classifying those dolomite rocks whose
original textures have been obscured or obliterated
(Friedman and Sander 1967).
Most dolomite nomenclature systems are descrip-
tive and based on compositional grouping modified by
textural terms (Folk 1959; Leighton and Pendexter
1962; Powers 1962; Bissell and Chilingar 1967). A
coarsely crystalline oolitic dolomite (Folk 1959), for
example, is a dolomite characterized by ooids within a
matrix consisting of dolomite crystals 0.25 to 1 mm in
size.
7.8.1 Descriptive Criteria and Terminology of
Dolomite Fabrics
Similar to limestones, dolomite rocks can be grouped
according to
mineralogical and chemical composition
,
and to
textural criteria
.
The basic terminology used in describing dolomite
rocks was developed by Friedman (1965; see Box 7.12)
and expanded by Randazzo and Zachos (1983) and
Sibley and Gregg (1987). The use of crystal size scales
allows more extensive differentiation of dolomite fab-
rics. Different crystal size limits are used by Friedman
(1965), Randazzo and Zachos (1984) and Wright
(1992). The latter coined the terms
dolomicrostone
(crystal size < 4
m),
dolomicrosparstone
(crystal size
4-10
m) and
dolosparstone
(crystal size > 10
m).
Crystal size distributions carry information on nucle-
ation and growth (Sibley et al. 1993).
Various terms are in use for describing the amount
of dolomite contents in carbonate rocks. The definition
of these terms is not uniform and sometimes vague:
• The use of the term
dolomitized
may (a) simply in-
dicate that dolomitization has converted limestones
wholly or partly to dolomite rocks or dolomitic lime-
stone, or more specifically, (b) that the rock contains
more than 10% replacement dolomite (Folk 1959).
•
Dolomitic
refers to (a) a dolomite-bearing or dolo-
mite-containing rock that contains different amounts
of the mineral dolomite.
Dolomitic limestones
yield a
conspicuous amount of the mineral dolomite, but cal-
cite is more important. The opposite is a
calcareous
dolomite
(see Fig. 3.1). The ill-defined term
magne-
sian limestone
designates a rock composed of mixtures
of calcite and dolomite or a limestone with some MgO
but no dolomite.
The
nomenclature proposed by Randazzo and
Zachos
(Fig. 7.22A) offers a possibility for recogniz-
ing dolostone types originating from homogeneous
and heterogeneous dolomitization.
Practical use
of this classification demands
answers
to the following questions:
•
Crystal size:
Very fine, fine, medium, or coarse crys-
talline?
•
Crystal size texture:
Equigranular, inequigranular
(unimodal, polymodal) or extremely fine (aphano-
topic)?
•
Fabric:
peloidal, mosaic, porphyrotopic, poikilo-
topic?
•
Crystal distribution:
Tightly packed, loosely packed,
isolated patches, crystals isolated or floating?
•
Crystal shape:
Anhedral, euhedral or subhedral?
Dolostones
are classified genetically into (1)
synge-
netic dolostone
(penecontemporaneously formed within
the depositional environment), (2)
diagenetic dolostone
(formed by replacement of carbonate sediments or lime-
stones during of following consolidation), and (3)
epi-
genetic dolostone
(formed by localized replacement
along postdepositional faults and fractures).
7.8.1.1 Thin-Section Description and
Terminology of Dolomite Rocks
Following the Randazzo and Zachos classification
an inequigranular, idiotopic, floating-rhomb porphyro-
topic dolomite fabric is a fabric composed of predomi-
nantly euhedral, isolated crystals of different size, float-
ing within a fine-grained matrix. An equigranular, micro-
xenotopic, sutured mosaic fabric is a fabric consisting
of fine, tightly packed, predominantly anhedral crys-
tals of equant size. Further examples for the use of both
classifications are shown on Pl. 39 and Pl. 40.
In thin sections, dolomites may or may not exhibit
their original depositional and compositional textures.
Calcareous dolostones and dolostones having preserved
the original depositional components and textures are
classified according to textural composition, crystal tex-
tures, and crystal fabrics. The shape and the size of do-
