gy 402: sedimentary petrology - university of south alabama · paleozoic-aged). consist of sea...
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GY 402: Sedimentary Petrology
Lecture 20: Carbonate Petrography 3:
Skeletal Allochems
Instructor: Dr. Douglas W. Haywick
UNIVERSITY OF SOUTH ALABAMA
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Last Time Carbonate Petrography 1
• Carbonate “grains” (skeletal versus non-skeletal)
• Non-skeletal allochems • Ooids • Peloids • Intraclasts • Grapestones
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Allochems: Come in many different “flavors”, but we can group them into two broad divisions,
Skeletal – the remains of once living beasties (body fossils)
Non-Skeletal – chemically precipitated or inorganically produced components
Carbonate ”Grains”
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Non-skeletal Allochems
Peloids (or Pellets): a generic allochem composed of carbonate mud (micrite) irrespective of size, shape (but they are usually round to ovoid) or origin (they are commonly fossilized fecal pellets)
Intraclasts: an allochem composed of composite grains (equivalent to a rock fragment in siliciclastic terms; BUT implies on the sea floor cementation)
Grapestone: another allochem composed of composite grains, but these are restricted to multiply coated groups of ooids (also implies cementation on the sea floor)
Ooids: a concentrically coated, spherical allochem that forms through “inorganic” cementation on the sea floor in a wave agitated environment (0.25 to 2.0 mm)
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1 mm PPL
Non-skeletal Allochems Ooids: a concentrically coated, spherical allochem that forms through
“inorganic” cementation on the sea floor in a wave agitated environment (0.25 to 2.0 mm)
Small nucleus
1 mm
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2 mm PPL
Non-skeletal Allochems Ooids: a concentrically coated, spherical allochem that forms through
“inorganic” cementation on the sea floor in a wave agitated environment (0.25 to 2.0 mm)
1 mm
Large nucleus
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Grainstone Petrography
Mudstones are mostly composed of silt and clay-sized particles of calcite
PPL
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Grainstone Petrography
2 mm PPL
Grainstones contain a wide variety of sand-sized allochems, and even those classified as “non-skeletal” contain a lot of skeletal bits
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Grainstone Petrography
2 mm PPL
Grainstones contain a wide variety of sand-sized allochems, and even those classified as “non-skeletal” contain a lot of skeletal bits
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Grainstone Petrography
2 mm PPL
Skeletal grainstones are full of beasties (even Dr. Clark would freak out!)
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Today’s Agenda Carbonate Petrography 3
• Mineralogy issues (chalk board)
• Beasties (know your enemy!) • Corals • Molluscs (gastropods and bivalves) • Echinoderms • Brachiopods • Foraminifera • Bryozoans • Barnacles
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Mineralogy Calcite vs Aragonite vs Magnesium Calcite
chalk board
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Skeletal Allochems
8 mm PPL
Corals: common constituents in tropical limestones (especially rudstones/ floatstones). Rugose and Tabulate orders were calcitic. Scleractinian corals are aragonitic.
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1 mm PPL
Skeletal Allochems Because corals were originally very porous, expect a wide variety of pore-filling cements…
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Skeletal Allochems
0.5 mm PPL
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1 mm PPL
Skeletal Allochems
Don’t ever assume that staining alone will reveal everything important about a thin section.
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1 mm PPL 1 mm XN
Skeletal Allochems
Don’t ever assume that staining alone will reveal everything important about a thin section.
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Gastropod
Skeletal Allochems Molluscs: common constituents in all limestones. Come in many shapes and sizes. Snails are aragonitic. Bivalves come in aragonitic and calcitic forms. There is a wide variety of “skeletal fabrics” in this phylum.
0.5 mm PPL
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Skeletal Allochems Molluscs are characterized by several biogenic fabrics (e.g., cross lamellar, fibrous, foliated)….
From Ginsburg, R.G.C., 1975. Carbonate Sediments and Their Diagenesis. Elsevier, Amsterdam, 658p.
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Bivalves
Skeletal Allochems …but this may be lost during skeletal diagenesis.
4 mm PPL
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“Aragonitic” Bivalves
Skeletal Allochems
Calcitic Bivalves
? 1 mm PPL
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1 mm PPL (unstained)
Echinoderms: common constituents in all limestones (especially Paleozoic-aged). Consist of sea urchins, sea stars, sea bisquits, and especially crinoids. Composed of calcite or Mg-calcite.
Skeletal Allochems
Crinoid
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1 mm PPL
Echinoderms are all composed of “plates” each of which is essentially a single crystal of calcite Look for unit extinction.
Skeletal Allochems
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1 mm PPL XN 1 mm
Echinoderms are all composed of “plates” each of which is essentially a single crystal of calcite Look for unit extinction.
Skeletal Allochems
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1 mm PPL XN
1 mm
Brachiopods: common constituents in Paleozoic limestones. Rare in Mesozoic/Cenozoic limestones. Composed of calcite in either fibrous or prismatic textures.
Skeletal Allochems
Brachiopod (prismatic)
Brachiopod (fibrous?-vertical orientation)
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1 mm PPL XN
1 mm
Brachiopods: common constituents in Paleozoic limestones. Rare in Mesozoic/Cenozoic limestones. Composed of calcite in either fibrous or prismatic textures.
Skeletal Allochems
Brachiopod (fibrous-tangential orientation)
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0.125 mm PPL
Foraminifera: common constituents in all muddy limestones. Rarer in grainstones. Composed of calcite, look for planktonic and benthic forms.
Skeletal Allochems
Benthic foram
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3 mm PPL
Fusulinina forams were huge… you’ll see them in hand specimens during the New Mexico portion of your field school.
Skeletal Allochems
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2 mm PPL
Bryozoans: colonial beasties that are ubiquitous in Paleozoic limestones. Less common in Meso/Cenozoic limestones. Composed of calcite. First evolved during the Latest Cambrian or earliest Ordovician. They are your Gods.
4 mm
Skeletal Allochems
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Barnacles: annoying arthropods that stick to the bottom of your boat. Formed from plates of calcite, they are common carbonate constituents, but really only dominate sediment in cold water limestones.
2 mm PPL 3 mm
Skeletal Allochems
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Calcite Cements:
0.5 mm PPL (unstained)
For the ooid grainstone portion of this lab, determine the percentage of calcite cement, but you do not have to identify the various cement phases. That happy task will come soon!
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Upcoming Stuff Homework
Perdido section Due Friday March 24th
Lab this week:
Tidal Flat thin sections Due today by 5 PM
Coming Up Moscow Landing Trip March 31 to April 1
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GY 402: Sedimentary Petrology
Lecture 20: Skeletal Allochems
Instructor: Dr. Doug Haywick [email protected]
This is a free open access lecture, but not for commercial purposes. For personal use only.