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GL3521 Notes

Lecture 1 (19/1/17) – Sedimentary Environments (Carbonates):

• Clastics (or siliciclastics) are formed through erosion and transportation

• Autochthonous: generated in situ

o Carbonates tend to reflect the environment they were deposited in

• Most carbonates exist where there is little or no clastic input

• Distribution of proved OIL reserves:

o At least 60% of the world’s oil reserves are in carbonates

o There has been a spike in Reserves to Production (R/P) ratios in South and Central American due to

oil in Brazil

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• Oil Reserves to Production (R/P) Ratios

• Distribution of proved GAS reserves:

o Carbonates not quite as important when looking at gas due to Russia clastic sediments

• Oil Reserves to Production (R/P) ratios:

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• Major controls on carbonate deposition

o Geotectonics (Major control)

▪ Lack of siliciclastic material

▪ Depositional setting

• Rimmed shelf, ramp, isolated platform

▪ Global sea level

o Climate

▪ Highest productivity in the tropics

• Corals, blue-green algae only in warm waters

• Molluscs and calcareous red algae can exist up to latitudes of 76°N (Arctic Norway)

o Temperature and Salinity

▪ Determines skeletal grain associations

• Chlorozoan (temps > 15°C, Sal. 32-40°/oo), Chloralgal (elevated salinity)

o Tropical

• Foramol (<15°C)

o Temperate

▪ Can see what fauna there is in a location and then think of the environment in which it

would have needed to grow

o Water Depth

▪ Most productivity in water depths <15m = Carbonate Factory

▪ Photic Zone

• Required for photosynthetic organisms such as algae and marine grasses

o Turbidity

▪ Lime mud in suspension reduces the amount of light

• Has an impact on the photic zone

▪ Corals fixed to certain point so need to live in an environment where water reaches them

▪ If too much turbidity, certain organisms can’t tolerate that

o Water circulation and current regime

▪ Impact on skeletal type e.g. corals flourish in turbulent areas but other organisms prefer

quieter areas

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o Tropical Environment

▪ Ooids, peloids, and lime mud

• Mud not seen North/South due to saturation state of sea water

o Sea water much more saturated with Calcium Carbonate in the tropics that’s

why inorganic carbonates precipitate here rather than higher latitudes

• When sea level rises/lowers, there is a big impact on the environment

o Corals can be above sea level/too deep for what they require

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o Antecedent Surface

▪ Relatively high area of land in the ocean body itself

▪ Potentially faulting or volcanic hotspot sitting underneath which would raise the sea floor

o Start-up is just about inhabitable

o Accommodation Space

▪ Amount of physical space in the water column for sediments to accumulate

• The water above the surface

o Carbonates will keep on accumulating as long as environment remains pretty much the same

▪ No water warming/cooling, change of environment

o Keep up

▪ Accommodation space is reduced to nothing, so carbonates build up laterally

• Pro-grade movement

o Platform Drowning

▪ Sea level increased so transgressed over the coastal system

▪ Sea level dropping is Regression

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• Carbonate Faces and Sequences

o Facies Description

▪ Lithofacies (sedimentary characteristics)

▪ Biofacies

• Important to look at what type of fauna it was and where these fauna would have

lived

• Ooids very specific high energy environments

o Specific environmental indicators

▪ E.g. Algal lamination, fenestral porosity, rootlets, ooids

▪ Often need to look at the vertical sequence of facies to identify the larger scale processes

• To see how the sub-environments relate to each other

o Has sea level been increasing/decreasing and how the facies change through

succession (walking through time and what has happened during those

environments)

o Cyclicity

▪ Short term (frequent)

• E.g. fining up sequences in shelf carbonates (storm deposits)

o Large rocks drop out first then later finer grained

▪ Long term

• Systematic changes in e.g. bed thickness, facies, grainsize etc. reflecting much

longer-term changes in sea level

▪ Nature of bed-boundaries very important e.g. marine hardgrounds, exposure surfaces

• Evidence of marine cementation, exposed to fresh water?

• Sediment Composition

o Mineralogy (CaCo3)

▪ Modern carbonate sediments composed mainly of two minerals forms (aragonite and

calcite)

• Dominance has changed through time

o Grain Types

▪ Limestones in outcrop/core often reveal little information unlike clastics

• Significant petrographic study required

▪ Non-skeletal

• Coated grains e.g. ooids, peloids

▪ Skeletal

• E.g. bivalves, corals, forams

o Biostratigraphy

▪ Matrix (lime mud)

• Significant constituent of most limestones except very clean grainstones

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o Grainstone: