GY 402: Sedimentary Petrology

Lecture 20: Carbonate Petrography 3:

Skeletal Allochems

Instructor: Dr. Douglas W. Haywick

UNIVERSITY OF SOUTH ALABAMA

Last Time Carbonate Petrography 1

• Carbonate “grains” (skeletal versus non-skeletal)

• Non-skeletal allochems • Ooids • Peloids • Intraclasts • Grapestones

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”

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)

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

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

Grainstone Petrography

Mudstones are mostly composed of silt and clay-sized particles of calcite

PPL

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

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

Grainstone Petrography

2 mm PPL

Skeletal grainstones are full of beasties (even Dr. Clark would freak out!)

Today’s Agenda Carbonate Petrography 3

• Mineralogy issues (chalk board)

• Beasties (know your enemy!) • Corals • Molluscs (gastropods and bivalves) • Echinoderms • Brachiopods • Foraminifera • Bryozoans • Barnacles

Mineralogy Calcite vs Aragonite vs Magnesium Calcite

chalk board

Skeletal Allochems

8 mm PPL

Corals: common constituents in tropical limestones (especially rudstones/ floatstones). Rugose and Tabulate orders were calcitic. Scleractinian corals are aragonitic.

1 mm PPL

Skeletal Allochems Because corals were originally very porous, expect a wide variety of pore-filling cements…

Skeletal Allochems

0.5 mm PPL

1 mm PPL

Skeletal Allochems

Don’t ever assume that staining alone will reveal everything important about a thin section.

1 mm PPL 1 mm XN

Skeletal Allochems

Don’t ever assume that staining alone will reveal everything important about a thin section.

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

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.

Bivalves

Skeletal Allochems …but this may be lost during skeletal diagenesis.

4 mm PPL

“Aragonitic” Bivalves

Skeletal Allochems

Calcitic Bivalves

? 1 mm PPL

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

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

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

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)

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)

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

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

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

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

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!

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

GY 402: Sedimentary Petrology

Lecture 20: Skeletal Allochems

Instructor: Dr. Doug Haywick dhaywick@southalabama.edu

This is a free open access lecture, but not for commercial purposes. For personal use only.