Stratigraphic Contacts
I. Intro
A. Litho. unit separated by contacts (planar or
irregular surface)
B. Contacts conformable or unconformable
D. Conformable= unbroken depositional
sequence--mostly layers deposited under
similar conditions
E. Surface separating conformable
strata=conformity; surface shows no
evidence of non-deposition
F. Therefore, conformable contact= no significant
break or hiatus in deposition
Conformable Contact
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Upper part makes a gradational transition from St.
Peter Sandstone into overlying Joachim Dolomite (thru
dolomitic sandstone, sandy dolomite) suggesting it is a
conformable formation contact
www.msstate.edu/dept/ge
osciences
Conformable Contact
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Silurian Tuscarora Sandstone and darker
Fm.,Virginia
Intro Continued
F. Hiatus = interval of geol. time represented by
missing strata
G. Unconformable strata do not succeed rks in
age or do not fit to form continuous
sequence
H. Contacts between unconformable strata=
unconformities
I. Unconformity = surface of erosion or nondeposition; separate younger rks from older
rks
1. unconformity represents significant hiatus
J. Unconformity= lack of continuity in deposition;
corresponds to periods of non-deposition,
weathering or erosion prior to deposition of
younger beds
II. Contacts between conformable
strata
A. Contacts between strata abrupt or gradational
B. abrupt/sharp= sudden distinct changes in
lithology
C. bedding planes are abrupt contacts= short
depositional breaks= diastems
D. abrupt contacts also formed by diagenetic
changes e.g. color, cement
E. gradational contact - gradual change form one
lithology to the other= gradual change in
depositional conditions with time
F. progressive gradual contact, one lithology
grades into the other
G. intercalated = gradational contact
w/increasing interbeds of upsection lithology
Beds are enclosed or bounded by sharply defined
upper and lower surfaces or bedding planes. These
surfaces are probably the easiest physical features of
sedimentary rocks to identify in outcrop. They are
used to subdivide successions of sedimentary rock into
their beds and are traditionally used to determine the
relative order and timing of the accumulation of the
sediments forming the beds. Concurrently the
character of the bedding planes be they eroded,
cemented, bored, bioturbated, or depositional surfaces
is used to aid in the interpretation of these
sedimentary rocks. To this end Allen (1983) has
described, using fluviatile sediments as an example,
that there at least four kinds of bounding sufaces:
concordant non-erosional (normal bedding) ;
disconcordant non-erosional ; concordant erosional;
and disconcordant erosional contacts.
Never the less, the origin of the bedding plane can be
enigmatic. The hypothesis presented here is that most
of these bedding planes are probably surfaces formed
by the erosion of unconsolidated sediment that
collected at the sediment surface. The weight of the
sediment, just beneath the sediment surface, causes
this sediment to dewater, compact and become
cohesive.
If this sediment surface is subjected to the erosive
force of:
• Storm waves
• Fast flowing currents of water (say in tidal or fluvial
channels)
• Turbid flow of a density current
then the bedding plane surface will cut down into
sediment, truncating the less cohesive sediment of the
surface and exposing a surface of the firmer cohesive
sediment below.
strata.geol.sc.edu/terminology/beddingplane.html
www.homepage.montana.edu
Sharp Contacts
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Exer[ted from: Did Modern Coal
Seams Form in a Peat Swamp?
by John D. Morris, Ph.D.
The accompanying photograph shows
two coal seams visible in a road cut
near Price, Utah. Observe the sharp
contacts between the coal and the
adjacent layers. If a swamp existed
above sea level, but then was
inundated by the sea to receive
overlying marine sediments, then
uplifted to become a swamp again,
and the cycle repeated, one would
think there would be some erosional
channels or variation in peat
thickness? How could they be so flat
and of constant thickness, and how
could there be such precise contacts
above and below?
Modern-day peats do not seem to be
of the same character as modern-day
coals. Perhaps the creation model of
decaying plant material collecting
under a large floating mat during the
Great Flood of Noah's day provides a
better answer.
This article was originally published
August, 2003. "Did Modern Coal
Seams Form in a Peat Swamp?",
Institute for Creation Research,
http://www.icr.org/article/didmodern-coal-seams-form-peatswamp (accessed April 06, 2009).

Rushford Fm., sharp, erosive contact between the
transgressive lag (displaying steeply dipping
cross-beds) and the finer-grained shoreface
sandstones at roadcut

Knife-sharp contact between Olentangy (below)
and Huron Shale (above). Stream bed,
Columbus/Ohio
www.uta.edu/.../Olentangy_Huron.jpg

Well-developed flooding surface in carbonate strata. Note the sharp contact
between dissimilar facies at the hammer head, with shallow subtidal facies
below the hammer and deep subtidal above. This photography was taken in the
Middle Ordovician Chickamauga Group near Fort Oglethorpe, Georgia.
www.uga.edu
www.ronrubin.net/Franciscan_Complex
Interbedding of chert and sandstone, observed in the study area as well as in other parts of the Franciscan,
requires special depositional processes and environments. For example, chert in the Marin Headlands terrane
accumulated at rates of about 0.3 mm/ka to 2.5 mm/ka (Murchey and Jones, 1984; Hagstrum and Murchey, 1993).
In contrast, turbidites on the modern Madeira Abyssal Plain can accumulate at rates of about 1.7-250 mm/ka (Stow
et al., 1996). Even the fastest rate of chert sedimentation would require 60 ka to produce the 15-cm-thick chert
bed described in the study area. It is highly unlikely that so much time passed between turbidity currents in the
thickly sedimented Franciscan subduction zone. Thus, the interbedding probably occurred at the most distal edge
of a fan near the abyssal plain, in a high-productivity zone such as a marginal basin, or due to pelagic turbidity
currents generated by instability upon a bathymetric high.

Sharp contact at the top and
gradational contact at the base
of Facies II-4 clay (interval
188-1167A-5H-3, 25-50 cm).
wwwodp.tamu.edu
Gradational?

Rushford � Caneadea
contact at outcrop
along Hillcrest Rd.,
Houghton quadrangle.
III. Contacts between unconformable
contacts
A. 4 types of unconformities
1.
2.
3.
4.
Angular unconformity
Disconformity
Paraconformity
Nonconformity
III. Contacts between unconformable
contacts
B. Angular unconformity= younger sed on eroded surface of
tilted or folded rks
1. have local and regional angular unconformities
C. Disconformity= parallel bedding above & below irregular
& uneven erosional surface
1.disconformity may be identified by gravel lag or soil
horizon
2.disconformity = period of erosion in which older rks
remained parallel during uplift
D. Paraconformity = beds above & below unconformity
contact are parallel--don’t see erosional or other
physical evidence of unconfomity
1. paraunconfomity identified through fossil data
C. Nonconformity-unconformity between sed rks & older ign
or meta rks that were exposed prior to being covered
by sediments
E. Unconformity indicate major geologic event took place

Horizontal fluvial deposits on truncated
dipping beds call attention to this angular
unconformity at Cody Wyoming.
Copyright © Bruce Molnia, Terra Photographics

Copyright © Marli Miller, University of Oregon
Copyright © Marli Miller, University of Oregon
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Angular unconformity at Siccar Point. It is here, at this
famous unconformity, that James Hutton made many of his
observations leading to modern thinking in the geosciences.
This spot is also knows as Hutton's Unconformity.
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Disconformity near Canmore, Alberta Devonian rocks lie
directly on rocks of the Cambrian period.
Copyright © Bruce Molnia, Terra Photographics
Disconformity - unconformity, or gap in geologic time,
caused by the process of erosion
Copyright © Marli Miller, University of Oregon
Nonconformity between the Cambrian
Tintic Quartzite on Precambrian basement.
Copyright © Marli Miller, University of Oregon
Unconformity with crystalline granite in contact with
Mesozoic sandstone occurs near Colorado Springs.
Copyright © Bruce Molnia, Terra Photographics
IV. Vertical Succession of Beds
A. beds succeed each other vertically in
ways such as
B. lithologic uniformity
C. lithologic heterogeneity
D. cyclic successions
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Buttress angular unconformity in northern Arizona. A buttress
unconformity is one in which the younger, overlying rocks are
cut by the contact. This relationship occurs because the
younger sediments are deposited against the older rocks as
they stood out in topographic relief
Gravel Lag?
Unconformity on the Langness peninsula, IOM
The lower part of the conglomerate contained angular and poorly sorted
pebbles from the Manx Group.
www.mangeolassoc.org.uk/iomunconformity.jpg
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Contacts between laterally
adjacent lithosomes
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pinchouts
intertonguing
progressive lateral gradation