PHOSPHORITES

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Intro A. Phosphorites = sed rks > 15-20% P 2 O 5 ; < 15% = phosphatic rk (e.g. phosphatic shale) 1. shales = P 2 O 5, ss P 2 O 5, limestone P 2 O 5 B. Phosphorites = small volume of sed rk 1. important phosphate resource--agricultural purposes 2. important deposits--Miocene-Pliocene, L.Cret-Eoc, Perm 3. present from PreCam to Holocene C. Occurrence--nodules, marlstone, pellets D. Considerations: 1. ocean water=70ppb phosphorus; phosphorites to 30-40% P 2 O 5 2. what special conditions led to concentration of "P“? 3. how was concentration accomplished? 4. why are they more volumetrically present in certain periods ?

II) Occurrence & Distribution A. Mostly in marine deposits 1. in U.S. mostly Tertiary B. Phosphorite nodules/pellets along margins of continents 1. at depths < 400 m C. Associated with cherts, & organic-rich seds p D. Beds lenticular to uniform over 100's km E. Phosphoria Fm- Permian, Idaho--best studied F. Monterey Fm- Miocene, California, well studied

Monterey Laminated Phosphatic Rock

Phosphatic Rocks--Distribution

III) Composition of Phosphates A. Most are varieties of apatite 1. fluorapatite [Ca 5 (PO 4 ) 3 F], chlorapatite [Ca 5 (PO 4 ) 3 Cl], hydoxyapatite [Ca 5 (PO 4 ) 3 OH] 2. most are carbonate hydroxyl flourapatite Ca 10 (PO 4,CO 3 )F 2-3 commonly called francolite 3. francolite = only phosphate mineral in unweathered marine phosphorites

IV) Grain Types A. Pellets/ooids/peloids 1. contain inclusions of detrital grains, fossils-- Implications? 2. Peloids--may be fecal pellets; ooids--core with concentric laminae 3. fossils may be important constituent 4. nodule--singular or composite (formation, exhumation, precipitation 5. most grains are sand size

V) Phosphate Deposits--Classification A. Bedded 1. distinct beds--assoc. with mudrocks, chert, and carbonates 2. phosphorites = pellets, oolites, pisolites, phosphotized fossils B. Bioclastic 1. vertebrae skeletal fragments e.g. fish bones, shark teeth, fish scales C. Nodular Phosphorites 1. brwnish-blk, cm-m, structureless to concentric 2. form in upwelling zones 3. some lie on unconformities D. Pebble bed 1. reworked phosphatic nodules E. Guano Deposits 1. Bird excrement--bat also? 2. found in eastern Pacific islands and West Indies

Typical phosphate nodule horizons in the middle Triassic, Bravaisberget Formation, here at Festningen, outer Isfjorden, Svalbard. Photo: A. Mørk

Guano, Chincha Is, Peru

VI) Conditions for Deposition A. Principal mineral = carbonate apatite 1. carbonate precipitates at pH 7.5 and greater B. Must concentrate phosphorus 1. 'P' removed in water by carbonate mineral 2. in tissues of organism--then buried 3. by forming apatite minerals-- How is 'P' concentrated C. x-beds & laminations imply phosphate seds are primary deposits

VI) Conditions for Deposition Continued D. Phosphorites assoc. with upwelling 1. upwelled waters = decreased pressure, increased Temp & photosynthesis, decreased CO2, therefore increase pH 2. increase pH favors carbonate apatite precipitation 3. but Mg2+ inhibits growth of carbonate apatite xls 4. also 'P' used by organisms--prevents 'P' saturation E. Nodules form in upwelling areas 1. organic material concentrates on seafloor 2. have reducing conditions & preservation of organic matter after burial 3. slow decay of tissue releases 'P' i. modern interstitial waters show [P] of ppb ii. phosphate begins to precipitate on grains, bones, organic matter 4. Some apatite precipitate from pore water in organic-rich muds--Peru i. suggest prec. linked to dissolution of fish debris & presence of microbial mats ii. Mg not depleted, therefore not a requirement for 'P' precipitation iii. filamentous bacteria may play a role

The Monterey Fm Example A. Phosphatic rocks common in Monterey 1. Contain up to20-30% organic matter 2. Form the source rock for Monterey Oil B. Requirements for preservation of phosphatic sediments 1. Basin to capture sediments 2. Abundant organic matter(o.m.) 3. Low oxygen conditions for o.m. preservation

Miocene California Borderland A. Series of basins and ridges on margin B. Upwelling on California margin Present day borderland.

Cross section through Borderland Basins capture o.m. Need to preserve o.m. Preservation tied to oxygen in ocean waters

V) Sed Structures A. How get sed structures in phosphatic sediments 1. formation of phosphorite in reducing basins 2. reworking during low stands of sea--high energy, oxidizing environment B. But why significant volumes of phosphorites during certain periods in geologic time periods