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Lecture 11: Non-Carbonate Biogenic and Chemical Sedimentary Rocks • • • • Siliceous Sediments & Chert Phosphorites Evaporites Banded Iron Siliceous Sedimentary Rocks Fine-grained, dense, hard rocks composed predominantly of SiO2 minerals quartz, chalcedony, and opal + minor impurities • Occur throughout the rock record – Most common in Jurassic, Cretaceous, Paleogene rocks (18040 Ma) – Bedded – Nodular • Chert - microcrystalline quartz, w/minor calcedony/opal – Grain sizes/shapes variable (1-50 µm) • Biogenic Silica - amorphous Silica/Opal A (SiO2*H2O) – Readily transforms to chert 1 Silica Geochemistry Amorphous SiO2 - highly soluble • Groundwater – 100-200 ppm – Source: feldspar to clay 2KAlSi3O8 + 2H+ + 9H20 ⇒ H4Al2Si2O 9 + 4H4SiO4 + 2K+ – Solubility increases in Alkaline (hi pH)water Silica Geochemistry Amorphous SiO2 - highly soluble • Seawater (H4SiO4) – <1 to 11 ppm – Highly undersaturated! – Organic coatings preserve shell opal – Accumulation occurs only where fluxes are high – Diatom/radiolarian oozes 2 Origin of Chert 2 Types: 1. Biogenic Chert 2. Nonfossiliferous Chert • Requirements: Chlorophyll contents in the Pacific 1. Silica Source 2. Precipitation mechanism • Supersaturation Origin of Biogenic Chert 1. Silica Source: Upwelling zones-high productivity (diatoms) Chlorophyll contents in the Pacific ODP Leg 199 3 Leg 199 Sites: Si & Ca Wt% & mass accumulation rates (MARs) Si mass accumulation rates (MARs) in the mid- Cenozoic Meridional Pattern 4 Biogenic Opal to Chert Transformation • • • • • Rapid accumulation of diatom/rad ooze Compaction dissolution of opal frustules (unprotected) Rate of dissolution >> rate of diffusion Pore waters - Si saturation ~1000 µM Pore Water Chemsitry from Site 1218. Biogenic Opal to Chert Transformation Solution-Reprecipitation Process • Opal A - amorphous • Opal Ct - cristobalite (metastable phase) • Chert (microcrystalline) Transformation from A to Ct can occur at low temperatures <45°C, and burial depths (~50 m) – Absence of detrital impurities speeds up the process Bedded Chert 5 Biogenic Opal to Chert Transformation • Chert replaces limestone chert limestone oolitic limestone has been completely replaced by quartz Partial Silicification of calcite with the development of radially fibrous or botryoidal quartz (e.g., chalcedonyfibrous) Silicification of calcite with complete replacement of the limestone fabric with quartz 6 Cretaceous Cherts 7 Nodular Chert • Typical of shallow water environments – Continental shelves – Especially in carbonates (replacement) Bedded Chert • Typical of clastic starved basins – Pelagic setting (deep sea) – Shelf edge (upwelling) Tropical Radiolaria Red and green chert in the Marin Headlands Terrane of the Franciscan Complex 8 Highly Contorted Bedded Chert Marin Headlands, Franciscan Red Bedded Chert Mount Diablo 9 Chert (Radiolarites) Cretaceous Hawasina Group, Oman A: radiolarite. B.spiculite, C. lutecite, D. chalcedony (fiberous microquartz replacement) Phosphorites • Rocks that are significantly enriched in phosphorus – >15% P2O5, or 6.5%P – Average sediments <0.5% • If <15%, ~ “phosphatic” • Small fraction of the sedimentary rocks • Economically important – 80% of the worlds phosphate • Occur in rocks of all ages – Concentrated in certain regions (ie., central, SE Asia; eastern Europe, N Africa, SE US (florida) • Modern: – Coastal Peru, Chile, Baja, SW Africa 10 Phosphorites: Composition • Ca phosphate minerals (apatite) – – – – – Fluorapatite - Ca5(PO4)3F5 Chlorapatite - Ca5(PO4)3Cl Hydroxyapatite - Ca5(PO4)3OH Carbonate hydroxyl fluorapatites (10% PO4 is replaced by CO3) Accessory components - Detrital qtz, authigenic chert, opal-ct, dolomite, glauconite, zeolites Phosphorite Deposits • mm scale laminae to meter scale beds – • • Phosphoria Formation, ID & WY - several hundred meters thick Interbedded with shales, cherts, limestones, dolomites Textures: – – ooids, peloids, fossils (bioclasts), clasts or nodules sand size most common 4 types of deposits: 1. Bedded Phosphorus – – 2. Nodular Phosphorites – – 3. Brownish to black, diameter (cm-m), layered (concentrically banded) Modern upwelling zones Pebble-bed phosphorites – – 4. Varying thickness, interbedded, fish debris Phosphoria (Permian), Australia, N. Africa Phosphatized fragments, fossils, nodules Florida (Miocene) Guano deposits – – Bird and bat excrement - leached to form insoluble Ca phosphate Eastern Pacific 11 Permian Phosphoria Formation • Bedded Phosphorites (420 m thick) Phosphorite Origin/Deposition Pacific Ocean (150°W) : Dissolved PO4 (µmol/kg) 12 Phosphorite Origin/Deposition • 100-1000 m water depth (i.e., shelf, slope) 1. 2. 3. 4. 5. Upwelling of nutrient rich waters Hi organic carbon flux, burial Slow decay releases PO4, consumes O2 Pore waters - saturated Phosphorite precipitates on grains Peru Margin, ODP Leg 201 13 Deep Sea Core - Pore water chemistry (interstitial water) 14 Lake Neosho Shale Member, St. Louis Missouri, Middle Pennsylvanian Limestone lens with phosphate nodules (from bioclastic shale bed) Lake Neosho Shale Member, St. Louis Missouri, Middle Pennsylvanian 15 Miocene Phosphorites (Southeastern US) • Early Miocene (18 to 25 Ma) Paleocene (55-60 Ma) 16 Evaporites • Sediments (rocks) composed of minerals (salts) precipitated from saline solutions concentrated by evaporation • All ages – Common in Cambrian, Permian, Jurassic, and Miocene • Marine and non-Marine – Marine - thicker and more extensive • Semi-enclosed Basins & Arid climate 17 Salt Pan Sabkha (Playa), Death Valley Peritidal carbonate environments Peritidal marsh Tidal flat 18 Peritidal carbonate environments Stromatoliths in peritidal zone (Hamling Pool, Western Australia) Sabkha environment (Persian Gulf) Evaporites: Composition • Marine Evaporites: – – – – Halite (NaCl) Anhydrite (CaSO4) Gypsum (CaSO4•H2O) Calcite • Non-Marine Evaporites: – May include the above, but tend to have less Cl, more HCO3 and Mg 19 Gypsum and Anhydrites • Deposited mainly as Gypsum – Rapid dehydration or during burial ( compaction) Anhydrite • Anhydrites - CaSO4 – Nodular Anhydrites • Lumps in halite, clay, or carbonate matrix • Carbonate or clayey sediments - growth of gypsum • Sabkha environment – Laminated Anhydrites • Thin layers - alternate w/dark laminae of dolomite/organic matter (seasonal varves, Permian Formation) – Massive Anhydrites • Semi-enclosed Marine Basin (Mediterranean) Evaporite origin and deposition Evaporation Sequence • 50% remaining – Carbonate • 20% – Gypsum • 10% – Halite – Dolomite • <5% – MgCl, KCl Evaporation of 1000m SW will produce 15 m salt – Some evaporite deposits >2 km thick!? 20 Permian Basin Reef Guadalupe Mounts, Capitan, etc. geologic map of the Guadalupe Mounts 21 Castille Formation (Ochoan) Laminated and nodular (secondary) evaporites Laminated basin evaporites (annual) nodular anhydrite (dark gray) in GrayburgSan Andres dolomite 22 Messinian Crisis (late Miocene) between 5.96 and 5.33 m.y. laminated gypsum with soft-sediment deformation, Villadoro, Corvillo Basin Messinian Proposed Mechanisms for Isolation 1) a 60 m global drop in sea level due to glaciation, 2) horizontal squeezing, and 3) tectonic uplift 4) ???? 23 24