8/6/2019 MK STRATANAS-8
1/31
Stratigrafi Analisis
(8)
STRATIGRAPHIC CONTACTS
Hadi Nugroho
Geological Department
Diponegoro University
2010
8/6/2019 MK STRATANAS-8
2/31
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.
8/6/2019 MK STRATANAS-8
3/31
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
8/6/2019 MK STRATANAS-8
4/31
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.
8/6/2019 MK STRATANAS-8
5/31
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.
8/6/2019 MK STRATANAS-8
6/31
(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.
8/6/2019 MK STRATANAS-8
7/31
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.
8/6/2019 MK STRATANAS-8
8/31
(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.
8/6/2019 MK STRATANAS-8
9/31
(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.
8/6/2019 MK STRATANAS-8
10/31
(4) Intercalated Contacts
Gradational contacts that occur owing to
an increasing number of interbeds of
another lithology that appear upward.
8/6/2019 MK STRATANAS-8
11/31
Types of Gradational Vertical Contacts
A. Progressive Gradual Contact. B. Intercalated Contact.
Source: Boggs (1987)
8/6/2019 MK STRATANAS-8
12/31
(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.
8/6/2019 MK STRATANAS-8
13/31
Angular Unconformity
8/6/2019 MK STRATANAS-8
14/31
(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.
8/6/2019 MK STRATANAS-8
15/31
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.
8/6/2019 MK STRATANAS-8
16/31
Disconformity
8/6/2019 MK STRATANAS-8
17/31
(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.
8/6/2019 MK STRATANAS-8
18/31
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.
8/6/2019 MK STRATANAS-8
19/31
Paraconformity
8/6/2019 MK STRATANAS-8
20/31
(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.
8/6/2019 MK STRATANAS-8
21/31
Nonconformity
8/6/2019 MK STRATANAS-8
22/31
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.
8/6/2019 MK STRATANAS-8
23/31
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.
8/6/2019 MK STRATANAS-8
24/31
(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).
8/6/2019 MK STRATANAS-8
25/31
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.
8/6/2019 MK STRATANAS-8
26/31
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.
8/6/2019 MK STRATANAS-8
27/31
(1) Pinchout
Lateral changes may be accompanied
by progressive thinning of units
to extinction.
8/6/2019 MK STRATANAS-8
28/31
(2) Intertonguing
lateral splitting of a lithologic unit into
many thin units that pinch out
independently.
8/6/2019 MK STRATANAS-8
29/31
(3) Lateral Gradation
similar to progressive vertical gradation, if
the change from one lithology to another
is gradual.
8/6/2019 MK STRATANAS-8
30/31
Lateral Relationship of Sedimentary Units
(A) Pinchout
(B) Intertonguing
(C) Lateral Gradation
8/6/2019 MK STRATANAS-8
31/31
Selamat Belajar
danSemoga Sukses