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Lecture 8a: Stratigraphy, Paleomagnetism

• Questions– How is stratigraphy related to analysis of sedimentary

environments?– What happens when sea-level varies?– How do variations in the terrestrial magnetic field get

recorded in rocks and used by geologists to reconstruct history?

• Reading– Grotzinger & Jordan, chapters 8 (again) and 14

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Principles of Stratigraphy (revisited)

• Recall the fundamental principles of stratigraphy: original horizontality, superposition, cross-cuttting

• A more detailed study brings up three major themes:– Uniformitarianism: the interpretation of ancient deposits by analogy to

modern, observable environments– Cyclicity: climate, sea-level, annual, tidal variations, etc., all generate

repeating cycles of sedimentation– Hierarchy: basic stratigraphic principles apply across a wide range of

space and time scales

• Definitions of stratigraphic elements: Rock units are organized into a hierarchy of classifications

There are also supergroups and subgroups, used when original group definitions later prove inadequate to describe important associations.

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• The boundaries between rock units can be conformable or unconformable.– Conformable describes continuous deposition with no major

breaks in time or erosional episodes. This definition is scale-dependent – just how long or large a gap is an unconformity depends on the size duration of the units being divided.

• A vertical succession of strata represents progressive passage of time, either continuously at the scale of observation (conformable) or discontinuously (unconformable).

• A lateral succession of strata represents changing environments of deposition at the time of sedimentation or diagenesis.– Each recognizable environment in a lateral succession is

called a facies.

Stratigraphy:definitions

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• Disconformity is used when beds above and below are parallel but a well-developed erosional surface can be recognized, by irregular incision, soil development, or basal gravel deposits on top.

• Paraconformity is used for obscure unconformities where correlation with time markers elsewhere indicates missing strata, even though no evidence of a gap is present locally.

• Nonconformity is used for deposition of bedded strata on unbedded (usually igneous or metamorphic) basement.

Stratigraphy:definitions• Unconformities are usually divided into four types:• Angular unconformity is used when layers below are clearly tilted or folded

and then eroded before deposition continues on the eroded surface

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Stratigraphy:definitions• Any package of sedimentary strata bounded above and

below by an unconformity (of any kind) is a sequence.– Traditional stratigraphy uses formations as the fundamental

units of the rock record, withinterpretation of sedimentary environments as the essential product of stratigraphic studies.

– Sequence stratigraphy makes sequences the fundamental units of the rock record and emphasizes periods of deposition and nondeposition (episodes of rising and falling sea level?) as essential information. Sequence stratigraphy grew out of seismic stratigraphy; unconformities are easily distinguished in seismic records, but lithology is often unknown.

• Sedimentary accumulation (hence the boundaries of sequences) is controlled by changes in base level, the elevation to which sediments will accumulate if the local land surface is too low, or erode is the local land surface is too high.

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Stratigraphy: Base Level• On land, base level is set by the equilibrium profile of

river systems.• In marginal marine settings, base level is often the same

as sea level• In the deep sea there is no base level and sedimentation

is controlled only by sediment supply.• Changes in base level allow the sedimentary record to

preserve evidence of geological events:• Relative sea level change is the most important determinant of

changes in base level.• Local tectonic uplift or subsidence changes base level and leads to

erosion or accumulation.• Changes in water supply or sediment load affect the equilibrium

profile of a river and therefore the base level downstream.

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Stratigraphy: Base Level

The parameters of the curve for each river are different. Changes in these parameters will cause the river to aggrade or incise to reach a new base level.

Parameters include the elevation of the headwaters, which may change by uplift or erosion; the elevation of the mouth, which may change up uplift or sea-level change; the sediment supply, the water discharge, the type of rock being cut.

• On land, base level is set by the equilibrium longitudinal profile of river systems, which evolve to a characteristic shape:

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Stratigraphy: Base Level

The placing of an artificial knickpoint in a river by building a dam has curious consequences, both upstream and downstream.

A waterfall must retreat because it is steeper than the equilibrium gradient for the reach of the river below the falls.

A sudden drop in base-level leads to the formation of river terraces

A knickpoint (resistant bed or lake) where the form of the river is interrupted leads to a nested set of river profiles.

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Stratigraphy: Relative Sea Level• Relative sea level is the depth of water relative to the

local land surface. – Relative sea level can change due to local vertical tectonic motions or due

to eustatic sea level variations (i.e. global changes in the volume of ocean water or of the ocean basins).

• In both sequence and traditional stratigraphy, the critical events that determine the locations of environments and unconformities are transgressions and regressions.– A transgression is a landward shift in the coastline, and hence

a landward shift in all marginal marine environments. A regression is a seaward shift in the coastline.

– A drop in relative sea level always causes a regression. A transgression hence requires rising relative sea level. However, rising sea-level can result in transgression, stationary shorelines, or regression depending on sediment supply.

– This asymmetry results because sediment flux from land is always positive, and because transgression during sea-level fall would create unstable, over-steepened long-valley profiles.

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Stratigraphy: Relative Sea Level

• Whether transgression or regression occurs controls the preservation potential and vertical succession of environments like barrier islands

• rising sea-level can result in transgression, stationary shorelines, or regression depending on sediment supply.

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Stratigraphy: Walther’s Law• We are now ready to state the third fundamental tenet of

traditional stratigraphy, lateral continuity, which is expressed by Walther’s Law:– In a conformable vertical succession, only those facies that can

be observed laterally adjacent to one another can be superimposed vertically

– That is, if the lateral shifting of sedimentary environments is controlled by continuous changes in base-level, each point accumulating sediments vertically passes through all intermediate environments continuously.

– Thus, e.g., deep-sea sediments directly overlying a terrestrial flood-plain facies demands an unconformity in between.

• Consider again the vertical succession of beach facies, which maps the lateral succession of beach facies onto a single point as the beach progrades outwards during a regressive relative sea-level rise.

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Stratigraphy: Transgression and Regression• In vertical succession,

transgression is recognized by progression from inland towards deep water sediments moving up section; regression, if preserved, is recognized by progressively shallower water facies moving towards continental settings as you go up section.

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Stratigraphy: Transgression and Regression

The ideal sequence consists of a transgressive clastic formation, a carbonate formation deposited when essentially the whole continent was flooded, and a regressive clastic formation (less often preserved after erosion).

On a regional-continental scale, transgression is recognized by lateral migration of environments with time, from the coast towards the interior, and regression by migration of environments towards the coast.

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Sequence StratigraphyOn a continental scale in

North America, there are recognized six major transgressions and regressions, bounded by five major regional unconformities. These sequences were named in North America by Sloss (1963), but they correlate fairly well with patterns seen on other continents. They are therefore interpreted as major changes in eustatic sea level, not as continental-scale uplift and subsidence.

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Sequence Stratigraphy• Superimposed on the major Sloss sequences are second-

order cycles of transgression and regression usually called Vail curves, and superimposed on these are third-order cycles that are correlated with individual reflectors in seismic sections of marine strata. Tracing and correlating these sequences is the main project of sequence stratigraphy.

• Repeated transgressions and regressions, presumably related to cyclic rises and falls of sea level, lead to cyclic sedimentation episodes in sedimentary basins. In particular, the Pennsylvanian strata of the eastern U.S. show at least 50 distinct cylcothems consisting of the triplet of deposits: marine-fluvial-coal. Each is a regression, probably caused by withdrawal of water from the oceans during a glacial advance.

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Causes of sea-level change• Relative sea level can change due to local or regional tectonics,

which cause vertical motions (uplift and subsidence). Global sea level can only change by altering either the volume of sea water or the volume of the ocean basins themselves.

• On time scales of 103–105 years, glaciation can quickly tie up and release enough water to change global sea level by ~200 m. But Sloss cycles have time scales of 108 years and amplitudes of 1000 m!

• Changes in the global configuration of continents and the working of plate tectonics can affect global sea level by changing the volume of the oceans:– when continents are assembled into supercontinents, the area of shallow

shelves is greatly decreased and the mean age of the ocean crust is a maximum, because there are few small oceans and one big one. This should lead to a big fall in sea level (Permian through Jurassic regression?).

– when continents rift, a new, shallow ocean is created at the expense somewhere of an old, deep ocean. Sea level should rise.

– an increase in spreading rate of the global ridge system leads with time to increase in the volume of water displaced by the mid-ocean ridges and a sea-level rise (cause of Cretaceous transgression?).

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Paleomagnetism

– A magnetic mineral crystallized above the Curie point and then cooled through it acquires a thermal remanent magnetism (TRM) in the same direction as and with intensity proportional to the applied field.

• As we have already discussed, the Earth’s magnetic field varies with time and records of the paleomagnetic field are preserved in rocks. Let’s look in more detail.

• Magnetization of rocks:– At high temperatures, all materials are paramagnetic, meaning

their magnetization is proportional to the applied field, and zero in the absence of an applied field

– Materials with unpaired electron spins can undergo a phase transition to ferromagnetic behavior at a temperature called the Curie Point.

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Paleomagnetism

– If a magnetic mineral is formed by chemical alteration or metamorphism at temperatures below its Curie Point, it acquires a chemical remanent magnetism. If a given rock cooled at one time with some magnetic minerals and was altered later to grow new magnetic minerals, the TRM and CRM may point in different directions.

• They can be separately measured by progressive demagnetization of a sample with increasing temperature.

– If magnetic particles are eroded from a source, transported, and deposited in a new rock under appropriate conditions, all below the Curie Point, they will have a preferred orientation governed by the magnetic field at the time of sedimentation, a depositional remanent magnetism. This will typically be ~1000 times weaker than the magnetic moment in a lava where each little dipole is perfectly aligned, but it is measurable.

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Paleomagnetism• Measurement of the vector remanent magnetic field in a

rock sample gives the declination and inclination of the field at the time and location of acquisition.

• If the terrestrial magnetic field was a simple dipole at the time of acquisition, this measurement gives a virtual magnetic pole:– The declination gives the orientation of the great circle on

which the pole lies, and the inclination gives the magnetic latitude of the sample.

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Paleomagnetism• A measured virtual

magnetic pole reveals several facts:– Magnetic polarity at

time of magnetization, assuming you know which hemisphere the sample was in and have some rough idea of horizontal

– Intensity of the field at the time of magnetization, if you correct for the susceptibility of the particular sample.

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Paleomagnetism– The apparent latitude of the sample at the time of

magnetization. If it does not match the present latitude, you can infer that the sample has moved north or south.

• There are terranes on the west coast of North America whose magnetic inclinations imply motions of thousands of kilometers.

• You get no information on longitude, which is a limitation in the reconstruction of positions of continents in the past; this is particularly serious before the Mesozoic, when there are no marine magnetic anomalies to go by.

– Tectonic rotations about a vertical axis show up through anomalies in the measured declination.

– A sequence of virtual magnetic poles from a series of rocks of different ages attached to one stable continent defines an apparent polar wander (APW) path.

• “Apparent” because it is not clear without a fixed frame of reference whether it is the continent or the pole that has wandered.

• However, the difference between APW paths for two different continents gives an accurate measurement of the relative motion between the two continents.