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Time & Geological Record
Associate Professor John Worden
DEC
University of Southern Qld
Time & Geological Record
 Geology’s greatest contribution- immensity of TIME.
Earth is 4.56 Billion years old.
Earth processes slow & occur mostly over millions of years.
Have to expand our conception of time to study Earth
processes.
Time approached in two ways by geoscientists:
RELATIVE TIME & ABSOLUTE TIMERelative time identifies the oldest/ first event
followed by progressive events in a sequence,
until the last/ youngest event.
Absolute time measured in years B.P
(before present) since an event .
Time & Geological Record
 Relative Geological Time:
 James Hutton in 1788, first to appreciate concept when he recognised an
Angular Unconformity at Siccar Point, Scotland.
 He realised that underlying rocks had to be sourced from yet older preexisting rocks by weathering, erosion, transport, & deposition. They had
then been buried, lithified, tilted, uplifted, exposed, eroded, & later had the
overlying rocks deposited on top of them.
 Hutton was first to grasp the significance of the ‘Rock Cycle’, it’s slow
steady progression & huge amounts of time for
it’s completion.
 He advanced the “Principle of Uniformitarianism”
• “Rates of Geological Processes do not change with
time.”
 Today , we know that :
• Rates of processes do change with time within limits.
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 Relative Geological Time:
 Hutton recognised that the rock (geological) record held many major
discontinuities, when time was not recorded by rock sequences (these are
termed ‘Unconformities’ & there are three distinct types).
 Later sedimentary rocks overlying igneous rocks = ‘Nonconformity’.
 Much younger sediments overlying older sediments without apparent break
= ‘Disconformity’;(confirmed by different fossil assemblages).
 Horizontal younger sediments overlying inclined strata =‘Angular
Unconformity’.
 Hutton’s work led Charles Lyell to define five
principles for determining relative time.
 Law of Original Horizontality states that:
• “Water-laid sediments are deposited horizontally.”
Time & Geological Record
 Principle of Superposition states that- “in any sequence of sedimentary
strata, the oldest strata are at the base and the youngest at the top.”
 Principle of Cross-cutting Relationships- “Igneous intrusions & faults are
younger than the rocks they cut.”
 Principle of Faunal Succession- “groups of fossils (animals + plants)
occur in the geological record in a definite and determinable order & that a
geological period can be recognised by it’s characteristic fossils.”
• English Surveyor Smith used this to predict location & properties of sub-surface
rocks during canal construction, before Darwin’s
Theory of natural selection.
 Principle of Inclusion• “Any fragment of rock incorporated or included in
another is older than its host rock.”
 These then used to construct the Geological Column.
Time & Geological Record
 Geological Column:
 Places fossil-controlled sequences in relative chronological order from
oldest to youngest.
 As rock formations named for localities where they are best exposed type
localities), they became standard names for portions of column.
 These names ranked as ‘EONS’, ‘ERAS’, ‘PERIODS’ etc, in declining
order. Four Eons- Hadean, Archean, Proterozoic & Phanerozoic.
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Proterozoic Eon is a large time interval with only trace fossils (< 1400 Ma),
Paleozoic Era- “ancient life”.
Mesozoic Era- “middle life”.
Cainozoic Era- “recent life’.
Jurassic Period - part of the Mesozoic Era.
 Fossils permit correlation of areas worldwide.
Time & Geological Record
 Relative geological time permitted development of a worldwide time scale
which placed all rock formations in their correct chronological sequence.
 However, it is still desirable to know the ‘Absolute Age’ of the Earth, that
of any Eon, Era, Period, and of any individual geological event.
 Absolute Time:
 Early attempts to measure length of geologic time were indirect.
 All failed due to incorrect assumptions:
• Thickness of sedimentary strata- estimates
between 3 Ma  1.5 B yrs.
• Sea salt concept - 90 Ma.
• Cooling Earth model - 100 Ma
 Only measured by an independent process that:
• is constant, unidirectional, & independent of T & P.
Time & Geological Record
 Thickness of Sedimentary Strata Concept:
 Wide-ranging estimates obtained depending on “average sedimentation
rate”, ( 0.3 m/ 1000 years);
 Problem- Gaps in Sedimentary Record yield a minimum estimate.
 Sea-Salt Concept:
 Basic premise- Oceans initially fresh water;
 Progressively polluted by common salt from weathering of Continents;
 Estimate total river runoff & contained salt content
 Age of Earth.
 Problems- Ignored evaporite deposits that lock
away vast quantities of Salt, and
 Failed to consider ‘Cyclic Salt’ (removal of salt
from Oceans to Continents by prevailing winds).
Time & Geological Record
 Cooling Earth Model:
 Assumption- Planet formed in molten state & cooled rapidly ever since.
 Radioactivity unknown when concept used to estimate the Age of Earth, so:
an important source of heat overlooked,
 Radioactive decay of elements releases heat that has contributed to
lowering the actual cooling rate for the Earth.
 Hence concept flawed and the estimate was too low.
 Time is measured by any regularly recurring
event, provided the event is measurable.
 At least three natural events satisfy these terms:
- Tree rings,
- Varves, and
- Ice sheets of Antarctica, Greenland, etc.
Time & Geological Record
 Tree rings (Dendrochronology):
 Early in growing season, trees produce cells with thin walls that appear light
in color, along the outermost circumference of bole.
 Later in growing season, cells are small with thick walls and appear dark in
color.
 Thus two growth rates appear as varying width bands or rings, and
 Each couplet represents a year.
 Oldest trees date back < 5000 years B.P.  Little use geologically!
 Varves:
 Geological equivalent of tree rings.
 Seasonally -based mountain lake deposits, formed
from glacial stream- transported detritus.
 During winter, stream runoff reduced & only
transports fine ground “rock flour”.
Time & Geological Record
 Summer Thaw with Melt-water runoff. Transports larger quantities of
coarser sediment into lake.
 Results in sediment couplet representing each year.
 By coring lake bottom sediments, varves can be counted, & used to date
events. Important in recent Climate studies, but limited use in Geology.
 Ice Sheets:
 Seasonally -controlled snowfalls compact to different thickness layers.
 Annual couplets measured in ice cores from Ice
Sheets.
 Below particular depth, ice deforms, recrystallises
& flows destroying the record.
 Time resolution to >200,000 years B.P .
 Vital to Climate studies; very limited geological
application.
Time & Geological Record
 Absolute Time:
 Radioactivity is a constant, unidirectional & independent process.
 Vast majority of Isotopes are stable, but a few are unstable and
spontaneously decay to lighter isotopes = ‘Radioactive Decay’.
 All radioactive decay follows an exponential curve.
 Decay rates unaffected by changes in chemical & physical environment.
 Totally independent of geological processes.
 Decay rates expressed as decay constants () , or
 As half lives. (T= 0.6931/  )
• The time needed for the number of parent atoms to be
reduced by one half.
 Basic equation: t (age)= loge ( 1+ Nd/Np)/ 
• Where Np= Amount of Radioactive parent now present: Nd= Daughter product.
Time & Geological Record
 Absolute Time:
 Equation assumes two conditions;
• that the Decay Constant () is constant, and
• the system being determined has remained closed, ie no addition or loss of
parent or daughter atoms in response to internal/external factors.
 Strength- reflects nuclear processes independent of T& P & geological
processes.
 Which isotopic systems are useful for isotopic dating?
 Carbon (C14) Dating:
• C14 forms in upper atmosphere by cosmic ray
bombardment of N14 (slow thermal neutron capture) ie
N714 + n C614 + p. Subsequently, C614  N714 + .
• Production & decay of C14 in equilibrium, & rapid
mixing as CO2 ,ensures constant in all carbon reservoirs.
Time & Geological Record
 Absolute time:
 C14 has a half life of 5730 +/- 30 years.
 Too short for significant geological use.
 Used for time range 100 -80,000 years only.
 Potassium/Argon (K/Ar) dating:
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K1940 +   Ar1840 by K-electron shell capture ( ie p +   n).
This decay scheme is important for terrestrial heat generation in the Earth..
Since Argon (Ar) is a noble gas, it does not bond with other elements!
When K-bearing minerals crystallise, they include K40.
At high T, Ar40 diffuses out of mineral & is lost.
Retention of Ar40 is an important assumption.
Therefore, prefer K/Ar ages to be measured onrandomly -orientated materials like Basalt, Hornblende.
Time & Geological Record
 Absolute Time:
 Rubidium-Strontium (Rb/Sr) dating:
• Rb3787  Sr3887 + ie n  p + 
• Decay constant () = 1.41 x 10-11 yr –1 (recently reviewed & modified).
• Rb like K, is widely distributed in rocks and minerals & offers many dating
possibilities.
• Use suites of rocks from same formation (to obtain variable Rb/Sr ratio spread).
• Best-suited to igneous rocks, both intrusive & extrusive. Will give ‘cooling
ages’ for metamorphic rocks, or time when contained
minerals cooled through their ‘closure temperatures’.
• Least-suitable for detrital sedimentary rocks.
• Has advantage that does not involve gas decay product.
Time & Geological Record
 Absolute time:
 Uranium-Thorium-Lead (U-Th-Pb):
 Both Uranium & Thorium decay to Lead with very long half lives.
 Three decay schemes- three potentially discrete dating methods + fourth
internal check by comparing U235 & U238 decay as Pb207/Pb206 ratio.
 If the Pb207/Pb206 age differs from the two U-Pb ages, then the mineral or
rock has experienced a later geological event.
 Use:
 U238  Pb206 + 8 + 6 , t ½ = 4.51 x109 yrs;
 U235  Pb207 + 7 + 4 , t ½ = 0.71 x109 yrs;
 Th232  Pb208 + 6 + 4 , t ½= 13.9 x109 yrs.
Time & Geological Record
 Paleomagnetism:
 Earth’s liquid outer core generates a magnetic field;
 As a result, planet behaves like a giant ‘bar magnet’;
 Iron-containing minerals in forming sediments/igneous rocks align to
prevailing magnetic field (like a compass needle);
 But periodically, Earth’s magnetic poles switch (at irregular intervals);
 Any rock sequence forming over time preserves a record of polarities:
 Dated rock sequences used to reveal history of magnetic reversals:
 Can be used to determine age of other sequences
by matching magnetic reversals pattern;
 Measure rock’s residual magnetism;
 Used worldwide to date Mesozoic-Cainozoic rocks.
Time & Geological Record
 Absolute time & Geological time scale:
 Radiometric dating best applied to igneous rocks, but sedimentary rocks
with their contained fossils define the geological time scale.
 With careful & critical examination of intrusive & extrusive igneous rocks
and their related sedimentary rock hosts, it is possible to assign absolute
ages to various Eras, Periods, etc of the geological time scale.
 Careful & often tedious work of 19th Century geologists proven correct by
absolute time radiometric dating, as well as length of time that the rock
cycle has been active.
 Oldest Earth age is 4.1-4.2 B yrs for zircon from:
• Narryer metamorphic gneisses, Western Australia.
• Oldest rocks are Greenland gneisses (3.8-3.9 B yrs).
• Hadean Eon not represented on Earth, but in Lunar
rocks and Meteorites. As Earth & Moon formed at
same time, Age = 4.56 B yrs.
Time & Geological Record
 Age of the Earth:
 If Meteorites have remained closed systems since condensation of the solar
system, then their contained Pb isotope ratios will yield the age of planet
formation.
 Iron meteorites contain extremely small quantities of U & Th and their Pb
ratios are essentially primeval Lead.
 Stony meteorites have higher U & Th producing variable mixtures of
primeval & radiogenic Pb. (Stony meteorites contain silicate minerals).
 Yield age of formation of meteorites = 4.56 B yrs.
 Lunar Highlands samples plot on same isochron.
 Deep ocean sediments also plot on same isochron.
• They are best ‘average’ samples of Earth’s Pb comp.
• Therefore the Earth developed it’s Pb isotopic comp
at 4.56 B yrs, and is the same age as the GEOCHRON.