Limestones (8) Flashcards
1
Q
Limestone Grains
A
- divided into skeletal (bioclasts) and non-skeletal
- Non-skeletal grains can have a biological contribution to formation but are not skeletal
2
Q
Matrix-supported
A
Held together by a matrix not grains either:
Micrite - Mud supported, a lime mud
Cement supported - crystals that form in-situ within the rock and hold it together - can form at any time in diagenisis
3
Q
Skeletal Grains
A
Bivalve mollusc - Bivalves have a plane of symmetry that cuts between their two valves. They have symmetry similar to our hands. If you were to separate their two halves (or your two hands), each side perfectly mirrors the other, even though the shell shape itself might not be symmetrical. Bivalves are often described as having left and right valves. e.g. mussels and clams Brachiopods - have a plane of symmetry that cuts across the two valves. This you can think of if someone to cut your body in half down the middle, each side would have an eye, arm, and leg that matches the other side. If you were to cut a closed brachiopod shell in half down the center, both halves would mirror each other but the valves themselves may be different shapes or sizes. Belemnite (cuttlefish bone) Echinoid (urchins) Crinoid stems Coral Bryozoa Gastropod
4
Q
Non-Skeletal grains
A
Coated grains (inc. Ooids) - Ooid large >2mm, spherical grains with a nucleus formed in shallow lagoons radially or concentrically
Uncoated grains - peloids, rounded spehrical particles of micrite around 0.5 mm
Intraclasts - Locally derived particles of calcium carbonate reworked by waves
Lithoclasts - reworked particles of limestone
5
Q
Dunham callisfication 6 categories
A
- CRYSTALLINE: depositional texture is not recognisable
- BOUNDSTONE: Bound in deposition together (by framework, baffles or binding)
- GRAINSTONE: No carbonate mud and is grain supported
- PACKSTONE: Grain supported with mud
- WACKESTONE: Mud supported with more than 10% grains
- MUDSTONE: Mud supported with less than 10% grains
