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Feb 08 2014

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Non-skeletal Constituents-Carbonate Petrology

Coated grains

Ooids
– Size: 0.25 mm to 2 mm diameter
– Identification: clear nucleus or site of the nucleus. Several (not always) concentric cortical layers. Cortical layers are continuous (no overlapping). Generally larger than peloids but much smaller than oncoids. When the stage is rotated under XPL, they will undergo pseudo-uniaxial cross type extinction.
– Microfabric: tangential, radial and/or random crystals in each cortical layer.
– Rocks that predominantly made of ooids known as oolite.

Pisoids
– Size: > 2 mm to < 10 mm diameter - Identification: very similar (or rather same except the size) to ooids. Can the nucleus be observed???? - Rocks that predominantly made of pisoids are known as pisolite.

Note: Oooids and Pisoids both have nucleus. However, often pisoids nucleuses are harder to identify. Each layer of these two types of grains are known as cortical layers the boundaries of cortical layers are known as cortex.

Oncoids
– Size: > 2 mm; the outline of oncoids usually can be observed on thin sections with naked eye. Larger than peloids and ooids.
– Formed as a result of building layers on biological or lithological clastic nucleus.
– Found in all forms of water conditions (fresh/brackish/marine).
– Identification: irregular in shape with cortical layers of variable thickness. Overlapping discontinuous and irregular laminae. Alternating organic (dark) and microspar (light) layers. Overall shape can be described as rather a deformed circular one (asymmetric) as opposed to perfect circular patterns on ooids/pisoids.

Others

Pellets
– Size: 0.03 mm to 0.30 mm
– Identification: lacks internal structures but uniform shape and size.

Peloids
– Size: ~ 30 mm to 100 um; most commonly < 200 um - Modern peloids can be found in low energy environments such as shallow tidal and subtidal platforms. - Identification: lacks morphological structure (not always). In a sample, they are often widely varies in size and shape. - Rocks that predominantly made of peloids are peloidal packstones or wackestones. - Can be described as degraded pellets that have been moved causing mud rip-ups/broken tissues.

Aggregates

There are no universally accepted rules in the Geological community on how to separate grapestones from lumps. In fact, a grains aggregate can be classified as a grapestone by one petrologist and may be successfully disputed by another. However, generally grapestones are tightly packed together or touch each other. They often have the shape of the grains in their outer boundary. Lumps are generally full of “matrix” hence they lacks contact between the grains.

Clasts

Lithoclasts is a general term used to describe sedimentary clasts of pre-existing consolidated carbonate rock types (intra- or extraclasts). Intraclasts are fragmented lithofied or partially lithofied carbonate fragments. Extraclasts are fragmented carbonates particles.

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