Geology Reference
In-Depth Information
Table 4.2
Detailed descriptions of carbonate microfacies
Lithofacies/
microfacies
Occurence and thickness
Sedimentary structures
Components and microfacies
Environment
MF1: shale
Thickness: decametric to metric
beds interbedded of carbonates;
top gradual to MF2
Submillimetric laminated
structures
Fine-grained matrix chlorite-illite
clay minerals, often dolomitic, and
rich in organic matter
Basinal to lower
subtidal zone
MF2:
homogenous
dolomudstone
Lowermost carbonate unit of the
BII Group;
thickness
Clean structureless dolomudstone;
rare lamination; rare erosion
surface
Fine to medium crystalline
dolomite (5-15 cm); xenotopic to
hypidiotopic mosaic texture; rare
rounded silt-fine dolomite
(
Lower subtidal zone
10-60 cm; top
gradual to MF3
15
μ
m); peloidal texture
>
MF3: planar
dololaminite
Thickness
10-30 cm; to
gradual to MF4; near base
intercalations of MF2 with sharp
or gradual contacts
Millimetric laminated dark and
light dolomite; sharp or gradual
contacts
Alternation of submillimetric-
thick flat laminated dolomudstone
with (i) dark fine-grained
crystalline organic-rich dolomite
(5-10
μ
m) and (ii) light medium-
grained organic-poor dolomite
(10-15
μ
m); (i) equigranular
xentopic to hypidiotopic texture;
(ii) hypidiotopic texture; between
the laminations, pseudomorphs of
sulphate
Lower subtidal zone
MF4: wavy
dololaminite
Thickness: decimetric bedsets
often associated with MF2-3;
gradual contact to MF5
Submillimetric to millilmetric
smooth to crinckly laminated
dolomite; in-situ brecciation
Submillimetric-scale couplets
with (i) dark organic-rich and
(ii) light dolomites; (i) smooth to
crinckly laminae of equigranular
xenotopic to hypidiptopic texture
(5-10
Shallow subtidal
zone
m); (ii) hypidiotopic
dolomite crystals (10-15
μ
m);
poorly preserved dolomicritic
films in (i); interlaminated
'
μ
dolomitic crusts with
reminescent pseudomorphs of
sulphate; crystalline and diffuse
pseudomorphs of sulphate
pallisadic
'
Middle ramp
MF5:
columnar
stromatolite
Plurimetric-scale bioherms
(
LLH- and SH-type stromatolites;
millimetric to metric-sized domes;
unbranched columns like type-
Conophyton
stromatolite or
branching columnar stromatolite
like
Kasaia-Baicalia
and
Tungussia;
transitional to
dololaminites (MF3 and MF4);
often syn-sedimentary or early
diagenetic breccia; rare
desiccation cracks
Submillimetric-scale streaky
structure with (i) dolomicritic
films; (ii) clots and (iii) selvages
and grumeaux; (i) xenotopic to
hypidiotopic mosaic structure;
(ii) coalescent spindle-shaped and
patchy dolomicritic matrix coated
by fibrous and radial cements;
(iii) coalescent irregular patches of
dolomicrite; replacive evaporites;
strongly recrystallized dolomite;
dolomitized herringbone cements
Bioherms on an
upper subtidal zone
100 m thick); interbeds of MF1
to MF4
>
Lithofacies/
microfacies
Occurence and thickness
Sedimentary structures
Components and microfacies
Environment
Evaporative
environment
Inner ramp
MF6:
evaporative
massive
dolomudstone
Basal unit of inner ramp; to
gradual to MF5; centimeter- to
decimeter-thick beds
Clean structureless dolomudstone;
replacive evaporite crystals; rare
lamination; rare erosion surface
Fine crystalline dolomite
(5-10
Restricted subtidal
zone
m); equigranular
xenotopic to hypidiotopic mosaic
texture; replacive evaporites
μ
MF7:
evaporative
laminated
dolomudstone
Centimetric to decimetric thick;
top gradual to MF6
Similar to MF3 and MF4;
millimetric-scale lamination, even
planar or wavy, often crinckly;
replacive evaporites; gradual
contacts
Alternation of submillimetric-
thick (i) dark fine-grained
crystalline organic-rich dolomite
(5-10
Restricted shallow
subtidal to intertidal
zone
m) and (ii) light medium-
grained organic-poor dolomite
(10-15
μ
m); (i) equigranular
xenotopic to hypidiotopic texture,
μ
(
continued
)
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