Geology Reference
In-Depth Information
for identifying and interpreting layering in carbonates
without internal cross-stratification was provided by
Demicco and Hardie (1994). The classification is based
on layer thickness (Fig. 3.2, layer geometry (planar:
Pl. 20/7; wavy: Pl. 18/2, 4, Pl. 123/1; and crinkled: Pl.
25/5, Pl. 123/3) and layer composition (using Dunham's
limestone classification). The authors present a very
useful discussion of the possible origins of laminated
and bedded carbonates. Layer geometry and layer com-
position can be evaluated by microfacies data.
Layers or laminae deposited at an angle to the bound-
ing surfaces are responsible for the formation of cross-
lamination. All types of cross-stratification as well as
flaser and lenticular bedding known from siliciclastic
rocks also occur in carbonate sediments: Small- and
large-scale cross-stratification, wave-ripple cross-strati-
fication, hummocky cross-stratification (Imbrie and
Buchanan 1965; Duke 1985). Planar lamination and
cross-lamination are common in shallow-marine car-
bonates, but cross-lamination is often obliterated. Par-
allel laminae form in a variety of environments (lacus-
trine; peritidal to deep sea) but are often destroyed by
burrowing organisms and therefore better preserved in
reducing or anaerobic environments. Heterolithic strati-
fication is characterized by irregular alternations of mud
and sand layers (Pl. 18/2) which result in flaser, wavy
or lenticular bedding structures.
(4) Vertical bed sequences can be recognized from
the differences in texture, fossil content or be thick-
ness. Thinning/fining-upward and coarsening/thicken-
ing-upward sequences related to rates of environmen-
tal change and depositional energy are the significant
patterns recording changes in sea level and accom-
odation.
(1995). Allen (1984) discussed the processes control-
ling specific sedimentary structures.
Various classifications of sedimentary and diagenetic
structures differ in their more genetic and process-ori-
ented or descriptive approaches. Demicco and Hardie
(1994) differentiate physical structures, biogenic struc-
tures, and chemical features. Physical structures include
stratification, breccias and conglomerates, mudcracks
and other disruption structures, soft deformation struc-
tures, mechanical compaction structures and miscella-
neous physical structures. Erosional structures, slump
structures, diagenetic deformational structures and dikes
are larger scale structures, commonly involving sev-
eral beds. Slumping and sliding structures need spe-
cific studies. Instructive field and sample illustrations
of sedimentary and diagenetic structures can be found
in Pettijohn and Potter (1964) and Demicco and Hardie
(1994).
STRATIFICATION
Lamination and bedding
planar
wavy
crinkled
CROSSBEDDING
hummocky
GRADED BEDDING
Fining upward (normal) Coarsening upward (inverse)
CRACKS
Mudcracks
Prism cracks
FENESTRAE
Fenestral structures
BRECCIAS, CONGLOMERATES
Breccia
Conglomerate
Sedimentary structures and diagenetic features: These
structures exhibit a wide variety of pre-, syn- and post-
depositional features which allow the processes and the
interpretation of paleoenvironments to be dismissed
(Fig. 3.5). A highly recommended introduction to sedi-
mentary structures and early diagenetic features of shal-
low-marine carbonates is the well-illustrated SEPM
Atlas prepared by Demicco and Hardie (1994). Many
sedimentary structures are identical with those of si-
liciclastic rocks (e.g. cross-bedding, ripple marks) but,
because of the strong impact of biogenic and diage-
netic factors on carbonate sedimentation, structures also
exist which are more common in or limited to lime-
stones (e.g. fenestral and growth cavities, tepees).
Excellent overviews of sedimentary structures of
non-carbonate and carbonate rocks were provided by
Potter and Pettijohn (1963), Collinson and Thompson
(1988), Reineck and Singh (1986), and Ricchi Lucchi
NEPTUNIAN DYKES
DEFORMATION STRUCTURE S
slumping
load casts
convolute bedding
BIOGENIC STRUCTURES
Stromatolites
Bioturbation
Bored surface
DIAGENETIC
Tepee
Solution pores
STRUCTURES
Gypsum
Anhydrite
Fig. 3.5. Standard Shell Legend symbols of abundant
sedimentary, biogenic and diagenetic structures occurring in
carbonate rocks. The symbols are useful for a uniform
schematic representation.
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