By Salah ud din Shabab

2. Submitted by:

3. Definition
Carbonate Shelf Environment.
 Inner shelf -nearshore, shallow water to
subaerial.
 Middle shelf - , subtidal zone out to shelf break.
 Outer shelf - narrow zone including shelf break.

4. Diagnostic Features of Inner Shelf.
 Tectonic setting.
 Restricted to clear, shallow, tropical to subtropical.
 Geometry.
 Thin, laterally continuous beds over 1000’s sq km of shelf.
 Sequence.
 Shallowing upword – lagoonal mud to beach sand.
 Sedimentology.
 Algal lamination, stromatolite, mudcrack, tidal channel, dolomitization,
evaporates.

5. Introduction
 Inner shelf environments can be well defined
geographically in modern settings.
 In the geologic record, the term is used for low
energy, shallow-water carbonates.

6. What is a restricted shelf?
 Defined as any part of a continental or island
shelf with slow water circulation resulting in:
 Abnormal salinity
 Oxygen depletion
 Nutrient depletion
 Temperature extremes

7.  Reduction of normal wave activity or
current energy results from physical
barriers (reefs, island, skeletal or oolitic
sand shoals).
In ancient settings, it may be inferred
from stratigraphic evidence similar to
those in modern settings.

8.  With less stratigraphic resolution, restriction may
be inferred from impoverishment of fauna or
lithofacies.
 Presence of muddy sediment containing:
 Organic matter
 Pyrite
 Evaporites

9. Diagnostic Criteria (SBM)
 No single criterion is diagnostic for such
environment.
 The most reliable guides of diagnosis are
sequences and transitions.
 Lateral transitions.
 Vertical sequences

11.  Vertical sequences
are used to infer
lateral relations

12. Biota is an important
clue to restricted
settings.
 Typically low faunal diversity.
 Dwarf fauna.
 Aberrant growth forms are clues
of adverse conditions.

13. Abundance of
burrows is
characteristic, but
not diagnostic.

14.  Non-skeletal grain
composition is not very
diagnostic, but fecal pellets
and peloids are characteristic
sedimentary particles.
 They may comprise the entire
sediment.

15.  Grapestone aggregates
and small intraclasts are
also common.

16. Minerology is not
indicative of
depositional
environment.
 Evaporites.
 Dolomites.

17. Diagnostic Criteria (scoffin)
 Bioturbated sand and silt substrates
supporting epifauna and infauna
 Mollusks, benthic forams and echinoids
 Contribute coarse and fine grains to a poorly
sorted, mixed terrigenous and calcareous
sediments
 Carbonates are 40 to 60% of sediments
 Sand and gravel size with no mud
 Dominant mineralogy: calcite (50%), aragonite
and Mg calcite (25% each)

18. Setting

Lateral Facies Relationships
 Most reliable clue in restricted shelf
recognition
 Seaward or windward deposits may
produce restrictions
 Landward or leeward deposits may record
the restricted conditions more dramatically

20.  Islands formed by
depositional or
erosional
topography of
contemporary or
earlier depositional
episodes may also
contribute to
restriction

21.  Restricted settings may also originate without
significant physiographic barriers
 Broad expanses of shallow water (epeiric
seas) may damp out tidal and wave energy
 Evaporation leading to hypersalinity
 Rainfall and runoff causing dilution
 Strong seasonal climate fluctuation influencing
salinity

22.  Restricted settings may also originate without
significant physiographic barriers
 Broad expanses of shallow water (epeiric
seas) may damp out tidal and wave energy
 Evaporation leading to hypersalinity
 Rainfall and runoff causing dilution
 Strong seasonal climate fluctuation influencing
salinity

23.  Shallow water, restricted environments
develop further leeward or landward
 Covering vast expanses
 Little facies differentiation
 Lateral transitions are very gradual and
fluctuate in position with time

24. Microfacies
 Characterized by a limited number of
grain types:
 Carbonate or terrigenous mud
 Fecal pellets
 Peloids
 Grapestones
 Limited range of skeletal components
 Benthic forams, ostracods, gastropods, oysters,
algal oncoids, serpulid worms and brachiopods

25.  Indicators of slow accumulation rates
are extensive:
 Micritization
 Boring
 Metal-oxide staining
 Encrustation

26.  Typical standard microfacies (SMF)
types:
 Grapestone-peloid-intraclast grainstone
 Laminated to bioturbated pellet mudstone
 Oncoid wackestone or floatstone
 Foraminiferal or dasyclad algal grainstone
with peloids

27. Sequences
 Shallowing upward sequences common
 Series of cycles
 Supratidal – unfossiliferous, laminated,
mudcracked, dolomitic, pelletal carbonated
mudstone
 Intertidal – sparsely fossiliferous, pelletal
carbonate mudstone interbedded with skeletal
calcarenite
 Subtidal – fossiliferous, pelletal carbonate
mudstones interbedded with stromatoporoids

28.  Sedimentary Structures
Burrow