L14 - Facies/Walther's Law and Mechanisms of Sea Level Change

Define a facies.

• “part of a rock body that has characteristics form which we can infer the depositional environment”

• This means that sediments reflect the environment (Physical Conditions) in which they are deposited.

Define Uniformitarianism.

• “The present is the key to the past“

• Present day process of erosion and deposition can be used to interpret facies in the rock record in terms of their original environment of deposition

How do Sea Facies change laterally over time?

Sand → Silt → Clay → Carbonates

• decreasing sediment size

• decreasing energy

At what times were the sea levels much higher than today?

• Ordovician

• Middle to Late Cretaceous

At what times were the sea levels lower than today?

• The last ice age

What does Transgression mean?

Sea level is rising

What does regression mean?

Sea level dropping

What does Diachronous mean?

Sediments that are deposited during periods of changing sea level

What does Chronostratigraphic mean?

Sediments that are deposited at the same time. (sand, silts, clays, carbonates)

What is the order of facies in drill cores when sea level is transgressing?

Sands → Silts → Clays → Carbonates

Oldest → Youngest

What is the order of facies in drill cores when sea level is regressing?

Carbonates→ Silts → Clays → Sand

Oldest to youngest

What is the order of facies in drill cores when sea level is at a steady state?

Boundaries don’t migrate and therefore sediment remains the same overtime.

What is Walther’s Law?

1. Facies represent different environments of deposition.

2. As conditions change, facies will migrate following the conditions that favor their deposition

3. Facies will migrate over one another.

4. Overlapping facies will become stacked on top of each other over time

5. Vertical successions of facies can be used to track changes in environmental conditions over time.

What is Walther’s Law in Summary?

Facies that occur in a conformable vertical succession of strata were deposited in laterally adjacent depositional environments

What are Eustatic Changes?

Impact sea level on a global scale.

What are Localized changes?

Regional effects - relative to sea level changes

What are some Eustatic Sea level changes?

• Glacial periods cause lower sea levels

• Emergence of land bridge between Asia and North America (Beringia)

• Emergence of land bridge between Britain and Europe (Doggerland)

What are some Mechanisms of Sea level change?

• Spreading ridge volume

• Crustal deformation

• Isostatic Changes

How do Spreading Ridge Rates effect sea level change?

• High spreading rates: Broad, Hot, Buoyant ridges - displace large volumes of ocean onto continents

• Low spreading rates: Narrow, cool, dense ridges - displace less volumes of ocean onto continents

How does Crustal deformation effect sea level?

• Mountain building

• Continent is elevated

• local (relative) sea level change

What is isostasy?

Describes how the crust “floats” on top of the mantle.

How does isostatic changes effect sea level?

• Object with a lot of mass physically depress (“push down“) of the crust into the mantle.

• After mass is removed, the mantle flows back (rebounds).

• Takes thousands to tens of thousands of years before rebound is complete

• Temporary decrease (Glacier present) then increase (during rebound) in sea level