notes 3 geological time

PgDip/MSc Energy Programme/Subsurface Geological Time

The Robert Gordon University 2006 1 campus.rgu.ac.uk

Geological Time

Review

In this topic the student is introduced to how stratigraphicstructures can be related to geological time scales.

Content

Stratigraphy and Relative TimeGeologists deal with two types of time scales, relative and absolute.Relative time is related to the order in which a specific sequence ofevents has occurred, whereas absolute time is the actual time in yearsfrom a specific point that such an event occurred. The concept of relativetime and its importance in geology was realised in the nineteenth centurywhen European geologists started to piece together series of events fromfossil records contained within rock strata. Although these geologistscould not measure absolute time, as no reliable methods existed, theycould relate events to one another.

Due to erosive and tectonic forces on Earth, the majority of rocks we seeare sedimentary. A sequence of sediments kilometres thick hasaccumulated World wide over a length of time outwith normalcomprehension. Stratigraphy is the study of rock strata as a record of thegeological history of an area. The geological history can be interpreted toshow how an area evolved in terms of its plate tectonic setting throughtime. The sorts of time scales geologists work with can be demonstrated.For example, an accumulation of 0.1mm of sediment in one year wouldamount to 1km of sediment in 1 million years. Until the 1960s and thedevelopment of proper radioactive dating methods, the ages of rockswere expressed in terms of named intervals of relative time, based onrelationships between these layers of sediments. Relative dating is basedon several basic principles:

law of superposition - a particular layer is younger then the onebeneath it and older than the one on top;

law of original horizontality water laid sediments are laid in stratathat are horizontal or near horizontal (even cross bedding is laid ina horizontal series);

biostratigraphy - a bed can be identified by the characteristicfossils it contains;

Layers of rocks can thus be mapped as formations and contain materialsthat have the same physical appearance and properties (lithology). Inthis way an individual bed may be recognised in widely separatedlocalities. This is however an imprecise science for several reasons:

sediments are not laid at uniform rates even locally, let alonearound the globe;



the rock record does not tell us how many years have passedbetween periods of deposition;

there is no way of knowing the relative ages of two similar butwidely separated beds.

The study of fossils (palaeontology) and the identification ofdistinguishable fossil assemblages or sequences, known as faunalsuccession, yields extremely useful information in stratigraphy. Caremust be taken as individual species may be present in differentformations and fossils present will also vary with the environment,meaning that rocks in different places may therefore yield quite differentfaunas. However, paleontology remains the most useful tool for relativetime scaling. Petroleum exploration commonly uses microscopic animalfossils (micropalaeontology) or the spore and pollen from plants(palynology). Table 1 shows the relative time scale.

Table 1. Relative Time.

Eon Era Period Epoch Origin of Name

Cenozoic

Greek,Recent life

Quaternary *

Tertiary *

HolocenePleistocene

PlioceneMioceneOligoceneEocenePaleocene

Greek, Wholly recentMost recent

RecentLess recentSlightly recentDawn of the recentEarly dawn of therecent

Mesozoic

Greek,Middle life

Cretaceous

JurassicTriassic

Latin, chalk, after chalk cliffs ofsouthern England and FranceJura Mountains, Switzerland andFranceThreefold division of rocks in Germany

Phanerozoic

Greek,Visible life

Paleozoic

Greek, Oldlife

PermianCarboniferousDevonianSilurianOrdovicianCambrian

Province of Perm, RussiaCoal-producing strataDevonshire, English CountySilures, ancient Welsh Celtic tribeOrdovices, ancient Welsh Celtic tribeCambria, Roman name for Wales

Proterozoic Greek, Earlier life

Archean Greek, Ancient

Hadean Greek, Beneath the Earth. No rock record known on Earth

* Derived from eighteenth and nineteenth century geologic time scale that separatedcrustal rocks into a fourfold division of Primary, Secondary, Tertiary and Quaternary,based largely on relative degree of lithification and deformation.

Radiometric Dating and Absolute TimeAlthough early geologists attempted to age the Earth, all methodsdevised were based on flawed assumptions, such as uniform andconstant deposition rates. It was not until the the development of



radioactive dating that accurate figures could be applied to geologic time.Radioactivity was discovered in 1896, and it was only 1905 when thephysicist Ernest Rutherford suggested that rock could be aged by thelevels of radioactive decay within them. This was the start of radiometricdating.

If rocks contain suitable radioactive material, absolute ages can bedetermined. Unstable radioactive parent isotopes decay to stabledaughter isotopes over a fixed average time (half life). For example, K-Ar dating measures the proportion of argon derived from the radioactivebreakdown of potassium. If we know the average rate of the decay(which is constant with temperature and pressure), the amount ofpotassium and amount of argon, we can work back in time and calculatethe age of the rock, or more exactly, the time at which pottassium wasfirst trapped in newly formed minerals. Table 2 lists the major radioactiveseries used in geology today.

Table 2. Major Elements Used in Radiometric Dating.

IsotopesParent Daughter

Half Life ofParent (yrs)

Effective DatingRange (yrs)

Minerals and Other Materialsthat can be Dated

Uranium 238

Lead 206

4.5 billion 10 million 4.6billion

zircon, uraninite

Potassium 40

Argon 40

1.3 billion 50,000 4.6billion

muscovite, biotite,hornblende, whole volcanic

rock

Rubidium 87

Strontium87

47 billion 10 million 4.6billion

muscovite, biotite,potassium feldspar, wholemetamorphic or igneous

rock

Carbon 14

Nitrogen 14

5730 100 - 70,000 wood, charcoal, peat, bone& tissue, shells and othercalcium carbonates, water

and ice containing dissolvedcarbon dioxide

Through a combination of examination of relative geologic time andradiometric dating methods, scientists have been able to add numericaltime to the relative column (Figure 1). These dates are continuouslyrefined. For example, Precambrian time has recently been placed before544 Ma rather than the more accepted 570 Ma. This is importantgeologically as it relates to the most significant developments of hardbodied multi-cellular organisms.

The oldest radiometric date found is in a sedimentary rock ofapproximately 4.1 billion years ago. Therefore the rock cycle, and hencethe Earth, must have existed before this. Geological evidence suggeststhat the Earth was formed at the same time as the Moon, other planetsand meteorites found within the solar system. Through radiometric datingit has been possible to age both meteorites and dust brought back fromthe Moon to approximately 4.6 billion years. By inference, the Earth isthought to be this old. The oldest oil has been dated to around 3.2 billion



years, indicating that single celled organisms at least were around at thattime.

Rocks nearly 700 million years old all over world show substantial glacialdeposits indicating that there was a huge glaciation 700-750 million yearsago. Paleomagnetism studies show correlation meaning that the entireplanet was covered by ice. It is thought that volcanic action is responsiblefor the atmospheric changes needed for warming of the planet, a naturalgreen house effect, over a period of 50 million years or so. After thisperiod, evidence has been found to suggest that soft bodied animalsappeared subsequent to the thaw. Preservation of soft parts is extremelyrare so evidence is scant. Due to the development of hard bodied animalsabout 544 Ma however, it is this relatively recent period in geologicalterms that we know most about (Figure 2).

Figure 1. Geologic Column with Absolute Dates. (From THE DYNAMICEARTH by B.J. Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. Thismaterial is used by permission of John Wiley and Sons, Inc.)

Magnetic Polarity Time ScaleAs mentioned previously, the direction of magnetic polarity of the Earth isrecorded in lavas as they solidify and crystallise. Sequences can be seen



either side of diverging faults as the Earths polarity has reversed manytimes throughout geologic time. The reason for these reversals are notclear, but the fact that they happen provides us with a useful means ofcorrelation when dating rocks. Periods of normal polarity (with referenceto today) and periods of reversed polarity are termed magnetic chrons.During each chron, several sub chrons, or small periods of reversal, mayoccur (Figure 3).

Some sedimentary rocks can also form in a directional manner. If thesedimentary grains are small enough and formed from magnetite forexample, which will have its own magnetic direction, they will alignthemselves with the Earths magnetic field as they settle. The resultantlithified rock will therefore have a weak magnetic alignment.

Magnetic reversal cannot be used alone to date rocks, as one reversallooks like another, so additional information is required. Once acontinuous sequence is discovered however it is simply a matter ofcounting backwards a technique used for dating oceanic crust forexample. In sedimentary rocks this technique has proved to be veryimportant due to its sensitivity. Once an approximate age is found basedon fossil records, magnetic reversal sequences can be used for muchmore precise ageing. So good in fact is this correlation technique thataccurate sedimentation rates on the ocean floor can be determined.



Figure 2. Geological Time. (From UNDERSTANDING EARTH by Frank Press andRaymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)



Figure 3. Magnetic Chrons (last 20 million years) and Layering inRock Records. (From THE DYNAMIC EARTH by B.J. Skinner and S.C. Porter,copyright 2000 John Wiley and Sons. This material is used by permission of John Wileyand Sons, Inc.)

notes 3 geological time

Documents

absolute time

actual time

geological time scales

length of time

intervals of relative

types of time scales

relative ages

relative dating