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summing-up
1Stratigraphy: rocks as the
pages in a book
• Stratigraphy provides the tools to
originated in an environment that
was stable over a long period of time.
A strata is the smallest unit of
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sedimentary rock formation: it is, in
general, of modest thickness but
large expanse.
Facies indicates the set of a rock’s
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lithological characteristics and
depends on the formation
environment. By determining the
facies of a rock, it is possible to trace
back to the environment in which it
was formed.
Amongst sedimentary facies there
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are common continental facies
(glacial, fluvial, desert etc.),
transition facies (marsh, estuary,
coastal, etc.), and marine facies
(coastal, neritic, pelagic, etc.).
2Principles of Stratigraphy
Stratigraphy has developed some
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water from previously flooded areas,
and ingression (or transgression),
the advance of the sea on continental
areas.
Transgression gives rise to an
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unconformity which may be:
–
simple, if the strata or layers prior to
the transgression and successive ones
are all parallel;
–
angular, if the layers prior to the
transgression form a certain angle
with successive ones.
For an unconformity, which
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describes a geometric aspect, there is
a corresponding sedimentation gap.
The presence, inside a series of
rocks, of an unconformity with a
sedimentation gap indicates that the
following events occurred in
succession in the area:
–deformation and uplift of the crust,
resulting in the emergence of a new
land mass (regression);
–erosion of the new land mass; and
–return of the sea with resumption of
sedimentation (transgression).
3Tectonics: why rocks
deform
Tectonics is the discipline that
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consequence of movements in the
Earth’s crust.
Rock formations appear more or less
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deformed (translated and/or
changed in form).
Deformations are successive to rock
•
formation and are due to the forces
acting within the Earth’s crust.
4The results of deformation:
faults and folds
Faults are lacerations of the crust
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There are different types of faults:
•
–
normal faults, which result in an
enlargement (extension) of the crust;
–
reverse faults, which on the contrary,
lead to a shortening of the crust.
If rocks, with an original planar
•
structure, such as sedimentary beds,
has been bent, without breaking,
they form folds (anticlines, fold
upward, or synclines, fold
downward). The folds may be
upright, inclined, or overturned.
5Complex deformation:
systems of faults and slabs
A set of normal faults may cause a local
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rift, in which the elevated areas of
crust that remain are called pillars.
If a sector of the Earth’s crust,
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sliding along a fault, thrusts over a
neighbouring sector thrust sheet
takes place.
In an override, the part overridden is
•
called the slab or covering blanket.
6The Geological cycle
The stratigraphic sequence of an
•
area describes the vertical sequence
of the formations involved.
The reading and interpretation of
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stratigraphic series reveals any
cyclical repetition of events.
The geological cycle (or Hutton
•
cycle) typically includes:
–
rock formation, usually on the
bottom of the sea;
–
tectonic deformation of these rocks
due to movements of the crust, with
the development of magmatism and
metamorphism, and with the
uplifting of mountain ranges; and
–the demolition and erosion of
mountains produced by tectonic
deformation, up to the landscape
evolves into lowlands and plains.
Afterwards, in the same area, a new
•
geological cycle may develop,
beginning with a marine
transgression and the formation of
new sedimentary rocks,
unconformally on the older ones.
Over billions of years, the traces of
numerous cycles, similar to that
described, can be seen in the rocks in
the Earth’s crust.
trace the original spatial arrangement
of rocks and the order in which they
were formed.
• The object of study in Stratigraphy is
the geological formation. A
geological formation is a rocky body,
of a generally uniform nature, that
principles for establishing, based on
field observations, the relative
chronology according to which
different rocky structures were
formed:
–The principle of original horizontality;
–The principle of stratigraphic
superposition;
–The principle of intersection.
The study of successive facies over
•
time sheds light on the phenomena
of regression, the withdrawal of
studies the deformation that affects
rocks after their formation, as a
along which some rock formations
glide slide with respect to adjacent
ones.
collapse of the crust, called tectonic
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
unità 2•La giacitura e le deformazioni delle rocce
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