The Miocene Monterey Formation Sedimentology, Diagenesis, & Paleoceanographic Significance By Lisa White

Miocene Monterey Formation 16-6 MYA Lithology & Composition PetroleumDiagenesis Background Paleo-oceanography Geol & Geog. Setting

Geologic Setting Deposited during the Miocene Period(16-6 MYA )Deposited during the Miocene Period(16-6 MYA ) Deposited in pull-apart or wrench- faulted basinsDeposited in pull-apart or wrench- faulted basins Middle bathyal depths (ocean environment m) of accumulationMiddle bathyal depths (ocean environment m) of accumulation

Geographic Extent LOCAL: Discontinuous North-South extension of 1700kmDiscontinuous North-South extension of 1700km Typically m thickTypically m thick Upper facies is siliceous (diatomaceous) sedimentsUpper facies is siliceous (diatomaceous) sediments

Neogene Basins

Deposition Range:Range:  lower middle bathyal ( m) to upper middle bathyal ( m) Oxygen-deficient environmentOxygen-deficient environment Basin plains, slopes, banktops, shelf edgesBasin plains, slopes, banktops, shelf edges mid-water oxygen minimum zonemid-water oxygen minimum zone

Lithology & Composition Calcareous facies:Calcareous facies: –16-15MYA ; lower/basal layer –Carbonate: coccoliths & foraminifers Phosphatic facies:Phosphatic facies: –15-14MYA ; middle/transitional layer –phosphate, shale, & mudstones Siliceous facies:Siliceous facies: –14-6MYA; upper member/layer –diatoms & radiolarians

Geographic Extent GLOBAL: Discontinuous belt throughout North Pacific Rim (and South)Discontinuous belt throughout North Pacific Rim (and South) Onshore and offshore equivalents (ODP and DSDP sites)Onshore and offshore equivalents (ODP and DSDP sites)

Lithology & Composition Diatomaceous UnitsDiatomaceous Units

Lithology & Composition High diatomaceous and organic rich depositsHigh diatomaceous and organic rich deposits  plankton productivity =  upwelling;  plankton productivity =  upwelling;  terrigenous input =  sediment starvation; =>  relative proportions of Silica, OM, Phosphate, or Carbonate

Lithology & Composition

Silica Diagenesis

Diagenesis: Silica 2-Step Alteration:2-Step Alteration: Opal-A  Opal-CT Opal CT  Diagenetic Quartz Process of Alteration:Process of Alteration: –Complete dissolution –Re-precipitation

Diagenesis: Silica Transformation controlled by temp. & burial depth, bulk composition & rock properties (porosity & permeability)Transformation controlled by temp. & burial depth, bulk composition & rock properties (porosity & permeability) 2 narrow temp. ranges & burial depth:2 narrow temp. ranges & burial depth: 40-50ºC & 0.5-2km (Opal-A  Opal-CT) 65-80ºC & 1.5-3km (Opal CT  Quartz) Purest sediments alter early: hard, brittle Opal-CT (2-33ºC) & quartz cherts (36-76ºC)Purest sediments alter early: hard, brittle Opal-CT (2-33ºC) & quartz cherts (36-76ºC)

Silica Diagenesis

Paleoceanographic Significance

Paleo- Significance Deposited within important transition in:Deposited within important transition in:  Cenozoic cooling associated with Antartic Ice Sheet expansion  Increased upwelling associated with thermohaline circulation reorganization  Accelerated CO 2 drawdown associated with Himalayan uplift

Paleo- Significance Major changes: ocean circulation, global climate, tectonic transitionMajor changes: ocean circulation, global climate, tectonic transition Upwelling & Nutrients  (Diatom Blooms) Cooling  (Antarctic Ice Sheet) Burial & Preservation  (Organic Matter) CO 2 Drawdown  (Weathering)

Miocene Monterey Formation 16-6 MYA Lithology & Composition Petroleum Diagenesis Background Paleo-Environment Geol Setting & Geog Extent

Petroleum Resources Source Significance: both oil source & reservoirSignificance: both oil source & reservoir 1º source rock of hydrocarbons in Cal1º source rock of hydrocarbons in Cal Oil source generated in immature or marginally mature rocksOil source generated in immature or marginally mature rocks May largely come from organic-rich carbonates and phosphatesMay largely come from organic-rich carbonates and phosphates TOC: can be as much as 23%, but averages at 2% - 5%TOC: can be as much as 23%, but averages at 2% - 5%

Petroleum Resource Reservoirs Non-commercial reservoirs - lack effective porosity (n e ) & permeability (k) in composition of fine-grained, organic-rich rockNon-commercial reservoirs - lack effective porosity (n e ) & permeability (k) in composition of fine-grained, organic-rich rock Reservoirs generally consist of:Reservoirs generally consist of: 1.Adjacent or interfringed sandstone beds, members or formations 2.Naturally fractured, brittle diagenetic siliceous & dolomitic rocks

Summary Vast extent, deposited at different times and stages.Vast extent, deposited at different times and stages. Diagenesis stages relate to original rock composition and depth of burial.Diagenesis stages relate to original rock composition and depth of burial. Petroleum source and reservoir.Petroleum source and reservoir. Formation of carbonate, phosphate, and silica facies reflect global oceanic conditions.Formation of carbonate, phosphate, and silica facies reflect global oceanic conditions.

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