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Stratigrafi Analisis

(8)

STRATIGRAPHIC CONTACTS

Hadi Nugroho

Geological Department

Diponegoro University

2010


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Stratigraphic Contacts

Different lithologic units are separatedfrom each other by contacts, which areplane or irregular surfaces between

different types of rocks.

Vertically superposed strata are said to beeither conformable or unconformable.

Conformable strata are characterized byunbroken depositional sequences,

generally deposited in parallel order,under the same general conditions.


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The surface that separates conformablestrata; a surface that separates youngerstrata from older rocks strata, but alongwhich there is no physical evidence of

nondeposition. A conformable contact indicates that no

significant break, or hiatus, in depositionhas occured.

A hiatus is defined as the total intervalof geologic time represented by missing

strata at a specific position along a

stratigraphic surface.

Conformity


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Unconformity

Is a surface of erosion or nondeposition,separating younger strata from older rocks

strata, that represent a significant hiatus.

Unconformable strata are strata, in

vertical sequence, that do not succeed

underlaying rocks in immediateorderof

ageordo not fit togetherwith themas

part ofa continuous whole. Thecontact betweensuch strata are

called unconformity.


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Unconformity

Indicate a lack of continuity in deposition

and correspond to periods of non

deposition, weathering, or erosion, either

subaerial or subaqueous, prior to

deposition of younger beds. Represent a substantial break in the

geologic record that may correspond to

periods of erosion or nondepositionlasting millions or even hundered of

millions of years.


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(A). Contact between Conformable Strata

(1) Abrupt Contact Occur as a result of sudden, distinct

changes in lithology.

Most abrupt contacts coincide with

primary depositional bedding planes thatformed as a result of changes in local

depositional conditions.

In general, bedding planes representminor interruption in depositionalconditions.


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Abrupt contacts may be caused also by

post depositional chemical alteration of

beds, producing changes in color owing

to oxidation or reduction of iron-bearing

minerals, changes in grain size owing to

recrystallization or dolomitization, orchanges in resistance to weathering

owing to cementation by silica or

carbonate minerals.


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(2) Gradational Contact

If the change from one lithology to another isgradual, reflecting gradual change in

depositional conditions with time.

Gradational contacts may be of either the

progressive gradual type or the

intercalated type.


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(3) Progressive Gradual Contact

Occur where one lithology grades into anotherby progressive, more or less uniform changes in

grain size, mineral composition, or othercharacteristic.

Example: an sandstone units that becomeprogressively finer grained upward

until they change to mudstone.


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(4) Intercalated Contacts

Gradational contacts that occur owing to

an increasing number of interbeds of

another lithology that appear upward.


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Types of Gradational Vertical Contacts

A. Progressive Gradual Contact. B. Intercalated Contact.

Source: Boggs (1987)


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(B) Contact between Unconformable Strata

(1) Angular Unconformity Is a type of unconformity in which

younger sediments rest upon the

eroded surface of tilted or folded olderrocks.

That is the older rocks dip at a different,

commonly steeper angle than do the

younger rocks.


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Angular Unconformity


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(2) Disconformity

The bedding planes above and below theunconformables surface are essentially parallel

and the contact between younger and older

beds is marked by visible, irregular, or unevenerosional surface.

Disconformity surfaces may be marked by

fossil soil zones or may include lag-gravel

deposits lying immediately above the

unconformable surface and containing pebbles

of the same lithology as the underlying unit.


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Disconformities are presumed to form as a

result of a significant period of erosionduring which older rocks remained

essentially horizontal during

nearly vertical uplift andsubsequent downwarping.


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Disconformity


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(3) Paraconformity

Is an obscure unconformity in which the

beds above and below the unconformity

contact are parallel and in which no

erosional surcafe or other physical

evidence of unconformity is discernable.

The zone of contact may even appear to

be a simple bedding plane.


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Paraconformities are not easily

recognized and must be identifiedon the basis of missing strata

as determined from paleontologic evidence,

such as absence of faunal zones orabrupt faunal changes.


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Paraconformity


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(4) Nonconformity

An unconformity developed between

sedimentary rock and older igneous or

massive metamorphic rock that has been

exposed to erosion prior to being covered

by new sediments.


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Nonconformity


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Many stratigraphic sequences are

bounded by unconformities, indicating

that these sequences are incomplete

records of past sedimentation process.

Showed that some part of the

stratigraphic record is missing.

Indicated that an important geologic

event took place during the time period

an episode of uplift and erosion, or anextended period of nondeposition.


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Unconformities

(a) Time

An unconformity developed during a

period of time in which no sediment is

deposited.

This concept equates deposition and time,

and an unconformity represent

unrecorded time.


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(b) Deposition

Any interruption of deposition process, or

nondeposition whether large or small inextent, is an unconformity.

(c) Structure Structurally, an unconformity may beregarded as planar structures separating

older beds below from younger bedsabove, representing the break as

defined in (a) and (b).


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Plane of Unconformity

A plane of unconformity may be asurface of weathering, erosion or

denudation, or a surface ofnondeposition, or possibly somecombination of these factors.


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Contact between Laterally

Adjacent Lithosome

Wheeler and Mallory (1956) introducedthe term lithosome to refer to masses of

rock essentially uniform character and

having intertonguing relationships withadjacent masses of different lithology.


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(1) Pinchout

Lateral changes may be accompanied

by progressive thinning of units

to extinction.


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(2) Intertonguing

lateral splitting of a lithologic unit into

many thin units that pinch out

independently.


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(3) Lateral Gradation

similar to progressive vertical gradation, if

the change from one lithology to another

is gradual.


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Lateral Relationship of Sedimentary Units

(A) Pinchout

(B) Intertonguing

(C) Lateral Gradation


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