The Miocene Monterey
Formation
Sedimentology, Diagenesis, &
Paleoceanographic
Significance
By Lisa White
Miocene Monterey Formation
16-6MYA
Background
Geol & Geog.
Setting
Lithology &
Composition
Diagenesis
Paleooceanography
Petroleum
Geologic Setting
• Deposited during the Miocene
Period(16-6MYA)
• Deposited in pull-apart or wrenchfaulted basins
• Middle bathyal depths (ocean
environment 200-4000m) of
accumulation
Geographic Extent
LOCAL:
• Discontinuous North-South
extension of 1700km
• Typically 300-500m thick
• Upper facies is siliceous
(diatomaceous) sediments
Neogene Basins
Deposition
• Range:
 lower middle bathyal (1500-2300m) to
upper middle bathyal (500-1500m)
• Oxygen-deficient environment
• Basin plains, slopes, banktops, shelf edges
• mid-water oxygen minimum zone
Lithology & Composition
• Calcareous facies:
– 16-15MYA ; lower/basal layer
– Carbonate: coccoliths & foraminifers
• Phosphatic facies:
– 15-14MYA ; middle/transitional layer
– phosphate, shale, & mudstones
• Siliceous facies:
– 14-6MYA; upper member/layer
– diatoms & radiolarians
Geographic Extent
GLOBAL:
• Discontinuous belt throughout North
Pacific Rim (and South)
• Onshore and offshore equivalents
(ODP and DSDP sites)
Lithology & Composition
• Diatomaceous
Units
Lithology & Composition
• High diatomaceous and organic rich
deposits
•  plankton productivity =  upwelling;
 terrigenous input =  sediment
starvation;
=>  relative proportions of Silica, OM,
Phosphate, or Carbonate
Lithology & Composition
Lithology & Composition
Silica
Diagenesis
Diagenesis: Silica
• 2-Step Alteration:
Opal-A  Opal-CT
Opal CT  Diagenetic Quartz
• Process of Alteration:
– Complete dissolution
– Re-precipitation
Diagenesis: Silica
• Transformation controlled by temp. &
burial depth, bulk composition & rock
properties (porosity & permeability)
• 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)
Silica Diagenesis
Paleoceanographic
Significance
Paleo- Significance
• Deposited within important transition in:
 Cenozoic cooling associated with
Antartic Ice Sheet expansion
 Increased upwelling associated with
thermohaline circulation
reorganization
 Accelerated CO2 drawdown associated
with Himalayan uplift
Paleo- Significance
• Major changes: ocean circulation, global
climate, tectonic transition
Upwelling &
Cooling 
Nutrients 
(Antarctic Ice Sheet)
(Diatom Blooms)
CO2 Drawdown 
Burial &
Preservation 
(Weathering)
(Organic Matter)
Miocene Monterey Formation
16-6MYA
Background
Geol Setting &
Geog Extent
Lithology &
Composition
Diagenesis
PaleoEnvironment
Petroleum
Petroleum Resources
Source
• Significance: both oil source & reservoir
• 1º source rock of hydrocarbons in Cal
• Oil source generated in immature or
marginally mature rocks
• May largely come from organic-rich
carbonates and phosphates
• TOC: can be as much as 23%, but
averages at 2% - 5%
Petroleum Resource
Reservoirs
•
•
Non-commercial reservoirs - lack
effective porosity (ne) & permeability
(k) in composition of fine-grained,
organic-rich rock
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.
• Diagenesis stages relate to original rock
composition and depth of burial.
• Petroleum source and reservoir.
• Formation of carbonate, phosphate, and
silica facies reflect global oceanic
conditions.
Internet References
• http://www.sci.sdsu.edu/pacsepm/SEPMpas
tfldtrips.htm
• http://www.ucmp.berkeley.edu/tertiary/mio/
monterey.html
• http://www.seis.natsci.csulb.edu/rbehl/Mont
.htm