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 SiO 2 minerals quartz, chalcedony, and opal + minor impurities Occur throughout the rock record Most common in Jurassic, Cretaceous, Paleogene rocks (180-40 Ma) Bedded Nodular Chert - microcrystalline quartz, w/minor calcedony/opal Grain sizes/shapes variable (1-50 µm) Biogenic Silica - amorphous Silica/Opal A (SiO 2 *H 2 O) Readily transforms to chert 1
Silica Geochemistry Amorphous SiO 2 - highly soluble Groundwater 100-200 ppm Source: feldspar to clay 2KAlSi 3 O 8 + 2H + + 9H 2 0 H 4 Al 2 Si 2 O 9 + 4H 4 SiO 4 + 2K + Solubility increases in Alkaline (hi ph)water Silica Geochemistry Amorphous SiO 2 - highly soluble Seawater (H 4 SiO 4 ) <1 to 11 ppm Highly undersaturated! Organic coatings preserve shell opal Accumulation occurs only where fluxes are high Diatom/radiolarian oozes 2
2 Types: 1. Biogenic Chert 2. Nonfossiliferous Chert Requirements: 1. Silica Source 2. Precipitation mechanism Supersaturation Origin of Chert Chlorophyll contents in the Pacific 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 Partial Silicification of calcite with the development of radially fibrous or botryoidal quartz (e.g., chalcedonyfibrous) oolitic limestone has been completely replaced by quartz 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% P 2 O 5, 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 - Ca 5 (PO 4 ) 3 F 5 Chlorapatite - Ca 5 (PO 4 ) 3 Cl Hydroxyapatite - Ca 5 (PO 4 ) 3 OH Carbonate hydroxyl fluorapatites (10% PO 4 is replaced by CO 3 ) 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 Varying thickness, interbedded, fish debris Phosphoria (Permian), Australia, N. Africa 2. Nodular Phosphorites Brownish to black, diameter (cm-m), layered (concentrically banded) Modern upwelling zones 3. Pebble-bed phosphorites Phosphatized fragments, fossils, nodules Florida (Miocene) 4. 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 PO 4 (µmol/kg) 12
Phosphorite Origin/Deposition 100-1000 m water depth (i.e., shelf, slope) 1. Upwelling of nutrient rich waters 2. Hi organic carbon flux, burial 3. Slow decay releases PO 4, consumes O 2 4. Pore waters - saturated 5. 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 (CaSO 4 ) Gypsum (CaSO 4 H 2 O) Calcite Non-Marine Evaporites: May include the above, but tend to have less Cl, more HCO 3 and Mg 19
Gypsum and Anhydrites Deposited mainly as Gypsum Rapid dehydration or during burial ( compaction) - Anhydrite Anhydrites - CaSO 4 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 Grayburg- San 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