Geology Reference
In-Depth Information
analysis. Relative sea-level changes will affect the com-
position and distribution of microfacies types both of
platform and ramp carbonates as well as on slopes and
in basins. Box 16.3 lists papers dealing with the study
of carbonate rocks using both the sequence-stratigraphic
and the microfacies approach.
Detailed bed-per-bed microfacies analysis allows
the complex reactions taking place during the forma-
tion of parasequences to be demonstrated. The present
section includes examples exhibiting the use of micro-
facies types in recognizing sequence boundaries and
systems tracts.
Cretaceous carbonate ramps and platforms of the Sinai
(Egypt) demonstrate the differences in the composi-
tion and diversity of microfacies types in different sys-
tems tracts.
Early Jurassic shelf carbonates of central Greece
Pl. 141 and Pl. 142 display microfacies types of a
section composed of bedded limestones. The micro-
facies types are defined using grain composition and
texture. Most microfacies types can be attributed to
Standard Microfacies Types (SMF) that characterize
major depositional facies belts (Standard Facies Zones,
FZ). Microfacies criteria reflect changes in depositional
water depths and the existence of subaerial and sub-
marine sequence boundaries.
The example from Early Jurassic carbonates from
Greece shown on Pl. 141 and Pl. 142 illustrates the
recognition of sequence boundaries and systems tracts
on a thin-section scale. The case studies dealing with
Plate 142 Microfacies, SeaLevel Fluctuations and SequenceStratigraphic Analysis: Case Study Part 2
(continued from Pl. 141)
Microfacies types of an Early Jurassic succession of the central Peloponnesus in Greece. The topmost sample of
this section is -> 20.
The oncoid limestone of -> 11 (as well as the sample taken from the overlying bed, not shown in the plate)
corresponding to SMF 13 was deposited in a shallow platform interior environment (FZ 2). The sample mani-
fests sea-level rise and TST conditions. A sequence boundary is recorded by the lag deposit (SMF 14) shown in
-> 12. The sample indicates reduced or no sedimentation. LST conditions are indicated by the fenestral lime-
stones of tidal lagoons shown in -> 13 to 15. The coral/microbial boundstone of -> 16 and a wackestone with
megalodontid bivalves in the overlying sample (not shown in the plate) point to changes from intertidal/su-
pratidal to subtidal environments.
A sequence boundary is recorded by the microbreccia of -> 17. This erosional boundary may record a trans-
gressive surface on the top of a drowned platform. The samples displayed in -> 18-20 indicate sea-level rise and
TST/HST conditions. The samples correspond to SMF 9 and the Standard Facies Zone FZ 2 (deep shelf), the
same depositional environment as inferred for the samples from the lower part of the section (-> 4-6 on Pl. 141).
11
Oncoid rudstone.
The oncoids consist of tubular porostromate cyanobacteria. Nuclei are shells. Oncoids are bound
together by a network consisting of peloids containing a few foraminifera. Interparticle pores are filled with calcite
cement. SMF 13 (defined by large cyanoids and porostromate oncoids). Sample B 20.
12
Lithoclastic floatstone.
Large grains are rounded calcitic lithoclasts and pebbles (reworked fossils), smaller grains are
lithoclasts. The arrows point to a blackened and iron-stained crust with quartz grains. SMF 14. Lag deposit. Sequence
boundary. Sample B 22.
13
Laminar fenestral bindstone made of densely packed peloids.
Note the downward extensions of the roofs of the fenestrae
(indicating microbial contribution; black arrow) and the infilling with gray silt (indicating vadose diagenesis; white
arrows). SMF 21-F
EN
(defined as fenestral peloid bindstone with birdseyes and stromactoid voids). Sample B 23.
14
Fenestral bindstone with coprolites.
Note decapod coprolite grain (
Favreina
; arrows) and thin isopachous cement rims.
SMF 21. Sample B 25.
15
Fenestral rudstone/grainstone.
The arrow points to a reworked vadoid grain. SMF 21-F
EN
. Sample B 26E.
16
Coral/microbial boundstone/floatstone.
Reworked corals (C) connected by microbial crusts (M). SMF 5. Sample B 27.
17
Microbreccia
consisting of mudstone and packstone clasts exhibiting small voids. Interspaces between the clasts are
partly infilled with fine-grained peloidal sediment (arrows). Sequence boundary. Sample B 29.
18
Bioclastic wackestone
with gastropods, algal debris (A) and a few foraminifera. SMF 9. Sample B 31.
19
Burrowed fine-bioclastic wackestone
with shell-echinoderm debris. SMF 9. Sample B 32.
20
Burrowed fine-bioclastic wackestone.
Grains: Echinoderm debris (E), ostracods (O), small gastropods (G), foraminifera
(F) and peloids. SMF 9. Sample B 33.
