Lithostratigraphic units Geology By Misson Choudhury

Misson Choudhury, Budding Geologists ,graduated from utkal university odisha,now pursuing M.sc applied geology in Bangalore university ..love to geologic mapping,drawing ,climbing ,tracking..

1. Thepurposeoflithostratigraphicclassificationisto
organizesystematicallyrockstrataoftheearthinto
namedunitsthatwillrepresentthevariationsof
theserocksinlithologiccharacter.
Allstratigraphicunitsarecomposedofrockand
thus have rock characters,but only
lithostratigraphicunitsaredifferentiatedonthe
basisofkindofrock(lithologiccharacter);
limestone,sandstone,sand,tuff,claystone,basalt,
marbleandsoon.
Therecognitionofsuchunitsisusefulin
visualizingthephysicalpictureoftheEarth’sstrata
Indetermininglocalandregionalstructure
Ininvestigatinganddevelopingmineralresources,
Indeterminingtheoriginofrockstrata

2. Definitions
Lithostratigraphy
The element of stratigraphy that deals with the
lithology of strata and with their organization into
units based on lithologic character.
Lithostratigraphic classification
The organization of rock strata into units on the
basis of their lithologic character.
Lithostratigraphic units
A body of rock strata that is unified by consisting
dominantly of certain lithologic type or
combination of lithologic types
A lithostratigraphic unit may consist of
sedimentary or igneous, or metamorphic rocks or
of an association of two or more of these.

3. Lithostratigraphicunitsarerecognizedanddefined
byobservablephysicalfeaturesandnotby
inferrredgeologichistoryormodeofgenesis.
Fossilsmaybeimportantintherecognitionofa
lithostratigraphicunit.
Onlymajorlithologicfeaturesreadilyrecognizable
inthefieldshouldserveasthebasisforlithologic
units.
LithostratigraphicZone(Lithozone)
Aninformallithostratigraphicunitusedtoindicate
abodyofstratathatisunifiedinageneralwayby
lithologicfeatures

4. Lithostratigraphic horizon (lithohorizon)
Asurfaceoflithostratigraphicchangeorof
distinctivelithostratigraphiccharacter.
Kindsoflithostratigraphicunits
1.HierarchyofFormalLithostratigraphicunits
Formallithostratigraphicunitsarethosethatare
definedandnamedinaccordancewithan
establishedorconventionallyagreedschemeof
classificationandnomenclature.
The conventional hierarchyofformal
lithostrtigraphictermsisasfollows.
Group–twoormoreformations
Formation–primaryunitoflithostratigraphy
Member–namedlithologicentitywithina
formation
Bed–nameddistinctivelayerinamemberor
formation

5. 2. Formation
Theformationistheprimaryformalunitof
lithostratigraphicclassification
Itisabodyofrockstrataofintermediaterankin
thehierarchyoflithostratigraphicunits.
Formationsaretheonlyformallithostratigraphic
unitsintowhichthestratigraphiccolumn
everywhereshouldbedividedcompletelyonthe
basisoflithology.
Formationsmaybecomposedofsedimentary
rocks,orextrusiveorintrusiveigneousrocksor
metamorphicrocksorundersomecircumstances
associationsoftwoormoreofthesetypesofrock.

6. Member
Amemberistheformallithostratigraphicunitnextin
rankbelowaformationandisalwaysapartofa
formation.
Itisrecognizedasanamedentitywithintheformation
becauseitpossesslithologiccharactersdistinguishingit
fromadjacentpartsoftheformation.
Nofixedstandardisrequiredfortheextendor
thicknessofamember.
Aformationneednotbedividedintomembersunless
usefulpurposeisserved.
Someformationsmaybecompletelydividedinto
members, othersmayhaveonlycertainparts
designatedasmembers;stillothersmayhavenot
members.
Amembermayextendfromoneformationtoanother.

7. Bed
Abedisthesmallestformalunitinthehierarchy
oflithostratigraphicunits.
Itisaunitlayerinastratifiedsequenceofrocks
whichislithologicallydistinguishablefromother
layersaboveandbelow.
Theterm“bed”isappliedtolayersacentimeterto
afewmetersinthickness.
Thoseoflessthicknessarecalledlaminae
Asinglenamedbedornamedsequenceofbeds
mayconstituteamemberoraformation,andin
suchacase,theunittermbedisreplacedby
memberorformation,forexample“Bracklesham
Beds”=BrackleshamFormation.

8. Group
Agroupistheformallithostratigraphicunitnextin
rankaboveaformation.
Thetermisappliedmostcommonlytoasequence
oftwoormore associatedformationswith
significantunifyinglithologicfeaturesincommon.
Formationsneednotbeaggregatedintogroups,
butthetermgroupisgenerallyusedforan
assemblageofformations
Thenameofagroupshouldpreferablybederived
fromanappropriategeographicalfeaturesnearthe
typeareasofitscomponentformation.Eg.,Garwal
Group.
Agroupmaybeextendedlaterallybeyondthearea
whereitisdividedintoformations

9. Subgroup or Supergroup
Agroupmayonoccasionbedividedintosubgroups.
Thetermsupergroupmaybeusedforseveralassociated
groupsorforassociatedformationsandgroupswith
significantfeaturesincommon.
Complex
Acomplexislithostratigraphicunitcomposedofdiverse
typesofanyclassorclassesofrock(Sedimentary,igneous,
metamorphic)andcharacterizedbyhighlycomplicated
structuretotheextendthattheoriginalsequenceofthe
componentrocksmaybeobscured
Thetermmaybeusedasapartofformalnameintheplace
ofalithologicorrankterm,forex.,Akkajaurecomplex,
LewisianComplex.
Therankofacomplexmaybeequivalenttoagroup,a
formation,oramember.
Theterm‘basementcomplex’isappliedinformallytorocks
ofcomplicatedstructure(commonlyigneousormetamorphic)
underlyingalessdeformed,dominantlysedimentary
sequenceinanyregion.

10. Informal LithostratigraphicUnits
Theterm“measures”hasbeenused
informallyforasequenceofcoalbeds(eg.
TheMoracoalmeasures),but“TheCoal
Measures”hasalsobeenusedhistorically
foraseriesoftheCarboniferousSystem.
Wheninformalunitssuchasthose
mentionedabovearegivengeographic
names,theunit-termshouldnotbe
capitalized.
Geographicnamesshouldbecombined
withtheterms“Formation”and“Group”
onlyinformalnomenclature.

11. Tongue and Lentil
Tongueandlentilhavebeenusedfrequentlyas
independentlithostratigraphicrankterms,but
morecorrectlytheyareonlyspeciallyshapedforms
ofmembers.
Alentilisalens-shapedbodyofrockofdifferent
lithologythantheunit
Aprojectingpartofaunitextendingoutbeyongits
mainbodymaybecalledatongue.
Lentilmightbepartofanarrowtongue.

12. Reef
Reefisatermcommonlyappliedtoamassoforganic
carbonatebuiltinplacebycorals,algaeorothersedentary
organisms.
Reefsareofvariablesizeandshapeandmayoccuraslimited
isolatedmasses.
GreatBarrierReef
TheGreatBarrierReefistheworld'slargestcoralreef
systemcomposedofover
2,900individualreefsand
900islandsstretchingforover2,300kilometres(1,400mi)
overanareaofapproximately344,400squarekilometres
(133,000sqmi).
ThereefislocatedintheCoralSea,offthecoastof
Queensland,Australia.