5 -Reading the Stratigraphic Record in Carbonate

How does Walther’s Law impact carbonate dynamics?

Walther’s Law: “Facies occurring in a conformable vertical sequence were formed in laterally adjacent environments.” -

• Lateral migration of facies belts due to extrinsic or intrinsic forcing leads to vertical superposition of facies with time

What factors control eustasy in relation to sea level change?

Eustatic sea level change is controlled at long time scales >106 years) by:

• Volume of water in oceans – especially driven by glacial ice volumes

• Volume of ocean basins – related to spreading rates/plate tectonics

What are the features of peritidal carbonates shallowing-up successions?

• Most modern and ancient carbonates have stacked vertical successions of increasingly shallow water facies

• Evidence of seaward facies migration

• Controls may be related to sedimentary processes and/or eustatic sea level fluctuations

What are the affects of transgressions on ocean chemistry

Transgressions:

• Advect deeper-water nutrients to shelf: increased productivity

• Cause retention of nutrients in shallow seas: increased productivity

• Promote stratification and dys/anoxia: increased preservation

• Cause retention of clastics nearshore: less dilution

What are the affects of clastic input carbonate production?

• Input of clastic material inhibits carbonate production

• Suspended mud increases turbidity and decreases light levels (‘clastic poisoning’)

• Mixed carbonate-clastic deposition can occur as a result of delta lobe ‘switching’

How did reef ecology impact carbonate production during the Oxfordian?

• Callovian-Early Oxfordian: Wetter climate, increased siliciclastic input, transgressive anoxia do not favor carbonate productivity

• Middle-Late Oxfordian: Drier climate, low siliciclastic input favor carbonate productivity and build up development

Name some of the controls on carbonate sedimentation?

• Tectonics

• Topography

• Sea level

• Water depth

• Clastic sediment input

• Turbidity

• Light

• Climate

• Temperature

• Salinity

• Water circulation

What is the influence of evolution on carbonates through time?

Changes in biota through time mean that uniformitarianism does not strictly apply

What is the influence of plate tectonics on carbonates through time?

• Rates of sea floor spreading influences the abundance of atmospheric CO2 (and thus precipitation/dissolution)

• Regional and local tectonism may control platform geometry

• Plate tectonics controls the distribution of clastic sediment: influencing hinterland topography and river drainage

What is the influence of ocean chemistry on carbonates through time?

• Stratigraphic variation in ooid mineralogy (Sandberg, 1983)

• Recognition of ”aragonite seas” and “calcite seas” denoting dominant primary marine carbonate mineralogy

• Link to global sea level and greenhouse/icehouse cyclicity

• Precipitation of low magnesium calcite is favoured by high pCO2 and low Mg/Ca ratios

• Precipitation of aragonite and high magnesium calcite is favoured by low pCO2 and high Mg/Ca ratios (>5)

What is the influence of a greenhouse world on carbonates through time?

• High spreading rates

• Dispersed continents

• Relatively high sea level - large continental areas flooded

• Low latitudinal temperature gradient- warm at the poles

• No continental ice

• Increased pCO2 by production of CO2 from processes at subduction zones and MOR

• Lower Mg/Ca ratio of seawater via hydrothermal alteration of basalts at MOR

• Dominantly calcite seas

What is the influence of an icehouse world on carbonates through time?

• Low spreading rates

• Assembled continents

• Relatively low sea level - little continental areas flooded

• High latitudinal temperature gradient - cold at the poles

• Continental ice sheets at the poles

• Decreased pCO2 through increased photosynthesis

• Increased Mg/Ca ratio: increased Mg- supply via continental weathering, removal of Ca via evaporite precipitation

• Dominantly aragonitic seas