SHELF ENVIRONMENT

SALAH UD DIN WAZIR

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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.

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.

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.

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

With less stratigraphic resolution, restriction may be inferred from impoverishment of fauna or lithofacies.

Presence of muddy sediment containing: Organic matter Pyrite Evaporites

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

Vertical sequences are used to infer lateral relations

Biota is an important clue to restricted settings.

Typically low faunal diversity.

Dwarf fauna.

Aberrant growth forms are clues of adverse conditions.

Abundance of burrows is characteristic, but not diagnostic.

Non-skeletal grain composition is not very diagnostic, but fecal pellets and peloids are characteristic sedimentary particles.

They may comprise the entire sediment.

Grapestone aggregates and small intraclasts are also common.

Minerology is not indicative of depositional environment.

Evaporites. Dolomites.

Diagnostic Criteria (scoffin) Bioturbated sand and silt substrates

supporting epifauna and infauna

Mollusks, benthic forams and echinoids

Contribute coarse and fine grains to a poorlysorted, mixed terrigenous and calcareoussediments

Carbonates are 40 to 60% of sediments

Sand and gravel size with no mud

Dominant mineralogy: calcite (50%), aragoniteand Mg calcite (25% each)

Setting

Lateral Facies Relationships

Most reliable clue in restricted shelfrecognition

Seaward or windward deposits mayproduce restrictions

Landward or leeward deposits may recordthe restricted conditions more dramatically

Islands formed bydepositional orerosionaltopography ofcontemporary orearlier depositionalepisodes may alsocontribute torestriction

Restricted settings may also originate withoutsignificant physiographic barriers

Broad expanses of shallow water (epeiricseas) may damp out tidal and wave energy

Evaporation leading to hypersalinity

Rainfall and runoff causing dilution

Strong seasonal climate fluctuation influencingsalinity

Restricted settings may also originate withoutsignificant physiographic barriers

Broad expanses of shallow water (epeiricseas) may damp out tidal and wave energy

Evaporation leading to hypersalinity

Rainfall and runoff causing dilution

Strong seasonal climate fluctuation influencingsalinity

Shallow water, restricted environmentsdevelop further leeward or landward

Covering vast expanses

Little facies differentiation

Lateral transitions are very gradual andfluctuate in position with time

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

Indicators of slow accumulation ratesare extensive:

Micritization

Boring

Metal-oxide staining

Encrustation

Typical standard microfacies (SMF)types:

Grapestone-peloid-intraclast grainstone

Laminated to bioturbated pellet mudstone

Oncoid wackestone or floatstone

Foraminiferal or dasyclad algal grainstonewith peloids

Sequences Shallowing upward sequences

common Series of cycles Supratidal – unfossiliferous, laminated,

mudcracked, dolomitic, pelletal carbonatedmudstone

Intertidal – sparsely fossiliferous, pelletalcarbonate mudstone interbedded with skeletalcalcarenite

Subtidal – fossiliferous, pelletal carbonatemudstones interbedded with stromatoporoids

Sedimentary StructuresBurrow