Geologic Time ScaleGeologic Time ScaleCVE 3205CVE
3205Engineering GeologyEngineering GeologyWong Jee KhaiWong Jee
KhaiIntroduction Introduction The concept that most geologic
processes happen very slowly was propose !y James "#tton
$%&2'(%&)&*+Geologists sort Earth,s history into a
se-#ence o. events+/0osition in that se-#ence ienti.ies relative
age. / Numerical age can !e etermine thro#gh analysis o. the
pro#cts o. raioactive ecayRecord of Layered RocksRecord of Layered
Rocks1ayere seimentary or volcanic roc2s contain important cl#es
a!o#t past environments at an near Earth,s s#r.ace+Their se-#ence
an relative ages provie the !asis .or reconstr#cting m#ch o.
Earth,s history+The st#y o. strata is calle stratigraphy+The Laws
of Stratigraphy The Laws of Stratigraphy 3ost seiment is lai own in
the sea4 generally in relatively shallow waters4 or !y streams on
the lan+Each new layer is lai own hori5ontally over oler ones+The
law of original horizontality states that water(lai seiments are
eposite in strata that are hori5ontal or nearly
hori5ontal+Stratigraphic SuperpositionStratigraphic
SuperpositionThe principle o. stratigraphic superposition states
that any se-#ence o. seimentary strata was eposite .rom !ottom to
top+Charles 1yell an other geologists o. the nineteenth cent#ry
spec#late that it might !e possi!le to etermine n#merical ages !y
#sing stratigraphic recor+Stratigraphic SuperpositionStratigraphic
SuperpositionTwo ass#mptions m#st !e correct .or the metho to
wor26/7t m#st !e ass#me that the rate o. seimentation was constant
thro#gho#t the time o. seiment acc#m#lation+/7t m#st !e ass#me that
all strata e8hi!it con.ormity4 meaning they have !een eposite layer
a.ter layer witho#t interr#ption+Stratigraphic
SuperpositionStratigraphic SuperpositionThe .irst ass#mption is
.alse !eca#se it can !e o!serve toay that seimentation rates vary
wiely .rom place to place an time to time+The secon an even more
important ass#mption is .alse !eca#se seimentation can !e isr#pte
perioically !y ma9or environmental changes4 s#ch as sea level
changes an tectonic activity that lea to intervals o. erosion or
non eposition+Kinds of UnconformitiesKinds of Unconformities:n
#ncon.ormity is a s#!stantial !rea2 or gap in a stratigraphic
se-#ence+Three important 2ins o. #ncon.ormities are .o#n in
seimentary roc2s6/ Angular unconformity+The oler strata were e.orme
an then c#t o.. !y erosion !e.ore the yo#nger layers were eposite
across them+Kinds of UnconformitiesKinds of
Unconformities/;iscon.ormity+7t is an irreg#lar s#r.ace o. erosion
!etween parallel strata+: iscon.ormity implies a cessation o.
seimentation an erosion4 !#t not tilting+7t is o.ten har to
recogni5e4 !eca#se the strata a!ove an !elow are parallel+/
