SEDIMENTARY PHOSPHORITES: GENESIS, FACIES AND OCCURRENCE. Peter Berger Samantha Dwyer Jessica Hellwig Eric Obrock Kristin Read

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1 SEDIMENTARY PHOSPHORITES: GENESIS, FACIES AND OCCURRENCE Peter Berger Samantha Dwyer Jessica Hellwig Eric Obrock Kristin Read

2 Introduction Phosphogenesis & Deposystems Facies Case Studies: Western U.S., Florida, Central Morocco Stratigraphic Position

3 PHOSPHOGENESIS authigenic and biological phosphate mineral precipitation Phosphorite Genesis transforming of in-situ phosphate fabric into clastic grains Reworking Physical destruction of lithified sediments Grain Formation Winnowing Sorting which removes non-lithified matrix and exposes phosphate fabric

4 DERIVATION OF PHOSPHATIC ALLOCHEM GRAINS Baturin Cycle Reworking and winowing of initial fabrics Phosphatization of allochem grains Are bioclasts supplied by phosphatic shells

5 CARBONATE DEPOSYSTEMS Siliciclastic Deposystems Usually phosphatic crusts and phosphatized erosional surfaces Three Physical Mechanisms Bioerosion Reworking of phosphatized carbonate mud and crusts Reworking of older phosphatized strata or grains Usually organic rich mudstones Variety of micro-fabrics Four Grain Formation Processes Winnowing of muds Reworking of phosphate fabrics Renewal of omission surfaces Destruction of phoscrusts

6 PHOSPHORITE CONCENTRATION Determined by relative apatite contents Concentration through matrix reduction by two processes - Concentration of apatite particles - Syn-sedimentary destruction of the matrix Removal of fines by winnowing

7 SEDIMENT TRANSPORT AND REDEPOSITION Downslope Sediment Transport Mass sediment transport directed downslope Gravity induced Deposits are characterized by isolated phosphorite beds that are inserted in sediments groups that do not contain phosphorites Vagabonding Sediment Transport Sediment is mobilized by agitated water Sediment moved upslope Occurs in depositional systems that develop in shallow epeiric basins or broad, platform-like shelf environments Granular phosphorites from these settings show evidence in sediment structures

8 PATHWAYS OF CONCENTRATION AND SEDIMENT TRANSPORT Regressive Stage Base level drop causes increase in water energy and increased storm mixing of sediments Deeper environments have increased bottom current activity Carbonate environments have grain trapping Transgressive Stage Low clastic sediment supply In environments with significant dip, sediments are drowned and buried In environments with small dip, sediments are transformed in landward-moving water to blanket sands

9 PATHWAYS OF PHOSPHORITE FORMATION The mechanical concentration, phosphatization, and transport of sediments leads to allochthonous and autochthronous phosphorite deposits Classified by amount of sediment displacement They differ in their facies and paleogeographical setting

10 FOUR METHODS OF CONCENTRATION

11 LITHOFACIES phosphate-black shale-chert phosphate-dolomite/magnesium clays-chert phosphate-glauconite phosphate-banded iron oxides-chert (just Precambrian) phosphate-manganese/ferromanganese oxides

12 CENTRAL MOROCCO Vagabonding Blanket Sand Marginal Basin Blanket Phosphorites Latest Cretaceous to Middle Eocene Intershelfal sag basin deposit Low order sea level fluctuation

13 CASE STUDY: UTAH, IDAHO, WYOMING, NW NEVADA Mid-Permian Phosphate Rocks Phosphoria formation = Black shale-chert Park City & Shedhorn = calcareous & sandstone Transgressive/Regressive sequence evident Processes: replacement, biogenesis, authogenesis

14 Possible causes of early transgressive Phosphoria stratification: a 1)Temperature stratification: Upwelling at continental margin cold, low O2, low saline oceanic water 2) Salinity stratification Nutrient loaded upwelling flooded basin, raising temp which caused mixing and increasing salinity of the water

15 Phosphates show distinct vertical successions The base lag concentrate (Fish Scale Bed) often has abundant vertebrate remains.

16 CASE STUDY: SOUTHEASTERN USA Mixed carbonate, siliciclastic, and phosphorites Phosphorite formation: intense reworking during sealevel changes Carbonates replaced or sandstone impregnated with phosphorites All clastic phosphorites represent abundantly reworked sediments

17 Summary Formation of phosphates Deposystem & Concentration Downslope, Vagabonding, Transport Pathways of Concentration and Formation Stratigraphic position Case Studies: Western U.S., Florida, Morocco

18 REFERENCES Trappe, J Phanerozoic phosphorite depositional systems, Springer Lecture Notes in Earth Sciences, Chapters 7, 8 and 9. Floridia Department of Environmental Protection. Florida Geological Survey - Geologic Topics. Mineral: Hardrock Phosphate. April 3, Available: tm Evelyn, Skepchick. Pink Rocks: Another Day at Sea Dredging. April 3, Available: Bourge, Le. Office de Tourisme de Limogne: Les Escaliers du Temps: Site Naturel du Cloup D Aural. April 3, Available: