402-pp22 - UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary...

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Unformatted text preview: UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 22: Carbonate Petrography 3: Skeletal Allochems Last Time Carbonate Petrography 2 Carbonate "grains" (skeletal versus non-skeletal) Non-skeletal allochems Ooids Peloids Intraclasts Grapestones Carbonate "Grains" 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 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) 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) 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 PPL 1 mm 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) Large nucleus 1 mm PPL 2 mm Grainstone Petrography Grainstones contain a wide variety of sandsized allochems, and even those classified as "non-skeletal" contain a lot of skeletal bits PPL 2 mm Grainstone Petrography Grainstones contain a wide variety of sandsized allochems, and even those classified as "non-skeletal" contain a lot of skeletal bits PPL 2 mm Grainstone Petrography Skeletal grainstones are full of beasties (even Dr. Clark would freak out!) PPL 2 mm 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 Corals: common constituents in tropical limestones (especially rudstones/ floatstones). Rugose and Tabulate orders were calcitic. Scleractinian corals are aragonitic. PPL 8 mm Skeletal Allochems Because corals were originally very porous, expect a wide variety of porefilling cements... PPL 1 mm Skeletal Allochems PPL 0.5 mm Skeletal Allochems PPL 1 mm Don't ever assume that staining alone will reveal everything important about a thin section. Skeletal Allochems PPL 1 mm XN 1 mm Don't ever assume that staining alone will reveal everything important about a thin section. 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. Gastropod PPL 0.5 mm 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. Diagenesis. Skeletal Allochems ...but this may be lost during skeletal diagenesis. Bivalves PPL 4 mm Skeletal Allochems "Aragonitic" Bivalves Calcitic Bivalves PPL ? 1 mm Skeletal Allochems 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. Crinoid PPL (unstained) 1 mm Skeletal Allochems Echinoderms are all composed of "plates" each of which is essentially a single crystal of calcite Look for unit extinction. PPL 1 mm Skeletal Allochems Echinoderms are all composed of "plates" each of which is essentially a single crystal of calcite Look for unit extinction. PPL 1 mm XN 1 mm Skeletal Allochems Brachiopods: common constituents in Paleozoic limestones. Rare in Mesozoic/Cenozoic limestones. Composed of calcite in either fibrous or prismatic textures. Brachiopod (prismatic?) PPL XN 1 mm Brachiopod (prismatic) 1 mm Skeletal Allochems Brachiopods: common constituents in Paleozoic limestones. Rare in Mesozoic/Cenozoic limestones. Composed of calcite in either fibrous or prismatic textures. Brachiopod (fibrous) PPL XN 1 mm 1 mm Skeletal Allochems Foraminifera: common constituents in all muddy limestones. Rarer in grainstones. Composed of calcite, look for planktonic and benthic forms. Benthic foram PPL 0.125 mm Skeletal Allochems Fusulinina forams were huge... you'll see them in hand specimens during the New Mexico portion of your field school. PPL 3 mm Skeletal Allochems 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. 2 mm PPL 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. 3 mm PPL 2 mm Calcite Cements: 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! PPL (unstained) 0.5 mm Upcoming Stuff Homework 1) Non skeletal thin section lab due Thursday 2) Moscow Landing Field trip departs Friday at 4PM 3) Moscow Landing introduction/methods due Friday April 22 Lab this Week Non-skeletal Allochems Next Lecture: Carbonate Shelves ...
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