Download Earth Structure

Primary and Nontectonic Structures
Differential erosion of bedding surfaces in the Wasatch Formation, Bryce Canyon, Utah.
http://www.nps.gov/brca/photo_gallery.html
Sedimentary Structures
Terminology of Stratification
Bedding: Primary layering in a sedimentary rock, formed
during deposition, manifested by changes in texture, color,
and/or composition; may be emphasized by the presence
of parting.
Compaction: Squeezing unlithified sedimentary in response
to pressure exerted by the weight of overlying layers.
Overturned beds: Beds that have been rotated past vertical in
an Earth-surface frame of reference; as a consequence,
facing is down.
Parting: The tendency of sedimentary layers to split or
fracture along planes parallel to bedding; parting may be
due to weak bonds between beds of difference
composition, or may be due to a preference for bedparallel orientation of clay.
Strata: A sequence composed of layers of sedimentary rock.
Stratigraphic facing or younging direction: The direction to
younger strata, or, in other words, the direction to the
depositional top of beds.
Bedding Parallel Parting
Fissility: Parting forms when beds are unroofed,
and uplifted to shallower depths in the crust.
Consequently, the load pushing down on the
strata decreases and the strata expand slightly.
During this expansion, fractures from along
weak bedding plane and define the parting.
This fracturing reflects the weaker bonds
between contrasting lithologies of adjacent beds,
or the occurrence of a preferred orientation of
sedimentary grains (e.g., mica).
The Use of Bedding in Structural
Analysis
Law of Original Horizontality (Bedding is labeled as
S0)
Depositional environment: the setting in which the
sediment was originally deposited.
Stratigraphic facing or younging direction: the
direction in which in a sequence are
progressively younger.
Current direction: the direction in which fluid was
flowing during depositions.
Facing Indicators
Facing indicators allow you to determine whether a bed is right-side-up
(facing up) or overturned (facing down) with respect to the Earth’s surface.
Graded bedding in a turbidite
sequences (Bouma sequence)
Bouma sequence
Graded Bedding
Cross Beds 交錯層
Surface Markings
Local environment phenomena, such as rain, desiccation
(drying), current traction, and the movement of organisms,
affect the surface of a bed of a sediment. If the sediment is
unlithified, these phenomena leave an imprint.
Animal tracks
Clast imbrication
Flute casts
Mudcracks
Raindrop impression
Ripple Marks
Traction lineation
Worm burrows
Mudcracks
Asymmetric ripple marks
Load Cast (Ball-and- pillow structure):
荷重鑄型
Clastic Dikes
Disrupted bedding
Conformable and Unconformable
Contacts
Three basic type of contacts:
1. Depositional contacts
2. Fault contacts
3. Intrusive contacts
The principal types of
unconformities
Disconformity(平行不整合或假整合): At a discomformity, beds
of rock sequence above and below the unconformity are parallel
to one another, but there is a measurable age difference between
the two sequences. The disconformity surface represents a period
of nondeposition and/or erosion.
Angular Unconformity (交角不整合)
Strata below the unconformity have a different attitude than
strata above the unconformity. Beds below the
unconformity are truncated at the unconformity, while
beds above the unconformity roughly parallel the
unconformity surface.
Angular unconformity
Caledonides at Siccar Point, Scotland
Nonconformity (非整合)
Nonconformity is used for unconformities which
strata were deposited on a basement of older
crystalline rocks. The crystalline rock may be
either plutonic or metamorphic.
Buttress unconformity(拱壁不整合):
onlap unconformity (超覆不整合)
Occurs where beds of the younger sequence where
deposited in a region of significant predepositional
topography. Note that a buttress unconformity differs from
an angular unconformity in that the younger layer are
truncated at the unconformity surface.
Some Features to Identify Unconformities:
Scour channels in sediments
Unconformable contact between mid-Proterozoic
Grenville gneiss (dark gray)and Cambrain
sandstone and Pleistocene soils
Pitted Pebble
Stylolites (壓溶縫合線)
In limestones and sandstones that contains some clay, the clay
enhances the pressure solution process, Specifically, pressure
solution occurs faster where the initial clay concentration is
higher. Distinct seams of clay residue develop in the rock.
Penecontemporaneous Structures
Penecontemporaneous folds in the Maranosa Arenaci (Italian Apennines)
Penecontemporaneous Structures
http://www.geo.cornell.edu/geology/classes/Geo101/structure/GEOL101/index.htm
Thrust fault related folds and
folding
Detachment fold: Folds developed above a detachment or
thrust that is bedding parallel. Detachment folds require a
ductile decollement layer.
Idealized Thrust Fault
Idealized thrust fault showing kink band folding in the
hangingwall and no deformation in the footwall
Ramp-flat trajectories
Fault-bend fold: fold generated by thrust sheet over
ramp
Thin-skin Tectonics
Thin-skin: Amount of shortening that occurred in the
wedge was large and below thrust wedge, basement is
not involved in deformation.
Decollement (detachment): defines boundary between
thrust wedge and basement, occurring along weak
horizon.
Thick-skin tectonics: basement
involved tectonics
Courtesy from Lin Nina Yunong
Salt Structure: Halokinesis
Salt is a sedimentary rock that forms by the
precipitation of evaporite minerals (halite,
gypsum, anhydrite, calcium sulfates) from saline
water.
Salt differs from other sedimentary rocks in that it is
much weaker and, is able to flow like a viscous
fluid under conditions in which other sedimentary
rocks behave in a brittle fashion. In some case,
deformation of salt is due to tectonic faulting or
folding, because salt is so weak, it may deform
solely in response to gravity, and thereby cause
deformation of surrounding sedimentary rock.
Why Halokinesis Occurs?
All of these factors occur in a passive-margin
setting
(1) The development of density inversion:
Salt is a nonporous and essentially incompressible material.
When it gets buried deeply in a sedimentary pile, it doesn’t
become denser. In fact, salt actually get less dense with
depth, because at greater depths it becomes warmer and
expands. Other sedimentary rocks, in contrast, form from
sediments that originally had high porosity and thus become
denser with depth because pressure caused by overburden
make them compact.
At depths greater than 6 km, salt density is about 2200
kg/m3, whereas the density of sedimentary rock is about
2500 kg/m3.
Positive buoyancy: forces in a gravity field cause lower density
material to try to rise above higher density material.
Negative buoyancy: force that causes a denser material to sink
through a less dense material.
Why Halokinesis Occurs?
(2)
A salt layer takes places when the downward force on the salt
layer caused by the weight of overlying strata varies laterally. This
may occur where there are primary variations in the thickness or
composition of overlying strata, primary variations in the original
surface topography of the salt layer, or changes in the thickness
of the overlying due to faulting.
Neutral buoyancy: depth at which it is no longer buoyant. At this level,
salt has the same density as surrounding strata. The density of
mildly compacted clastic strata equals that of salt at depths
around 500-1500 m below the surface of the basin, depending on
the composition. At the level of neutral buoyancy, salt may begin
to flow laterally.
(3) Existence of a slope at the base of a salt layer
Stages in the Formation of Salt
Structures
Cross section illustrating a normal fault
array over the top of a salt dome in Texas
Salt Dome in Zagros Mountains,
Southwestern Iran
http://earth.jsc.nasa.gov/EarthObservatory/
Zagros Mountains
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•
•
The Zagros Mountains in southwestern Iran present an impressive landscape of
long linear ridges and valleys. Formed by collision of the Eurasian and Arabian
tectonic plates, the ridges and valleys extend hundreds of kilometers. Stresses
induced in the Earth’s crust by the collision caused extensive folding of the
preexisting layered sedimentary rocks. Subsequent erosion removed softer rocks,
such as mudstone and siltstone while leaving harder rocks, such as limestone and
dolomite This differential erosion formed the linear ridges of the Zagros Mountains.
The depositional environment and tectonic history of the rocks were conducive to
the formation and trapping of petroleum, and the Zagros region is an important
part of Persian Gulf production.
This astronaut photograph of the southwestern edge of the Zagros mountain belt
includes another common feature of the region—a salt dome (Kuh-e-Namak or
“mountain of salt” in Farsi). Thick layers of minerals such as halite typically
accumulate in closed basins during alternating wet and dry climatic conditions.
Over geologic time, these layers of salt are buried under younger layers of rock.
The pressure from overlying rock layers causes the lower-density salt to flow
upwards, bending the overlying rock layers and creating a dome-like structure.
Erosion has spectacularly revealed the uplifted tan and brown rock layers
surrounding the white Kuh-e-Namak to the northwest and southeast (center of
image). Radial drainage patterns indicate another salt dome is located to the
southwest (image left center). If the rising plug of salt (called a salt diapir)
breaches the surface, it can become a flowing salt glacier. Salt domes are an
important target for oil exploration, as the impermeable salt frequently traps
petroleum beneath other rock layers.
Astronaut photograph ISS012-E-18774 was acquired February 28, 2006, with a
Kodak 760C digital camera using a 180 mm lens, and is provided by the ISS Crew
Earth Observations experiment and the Image Science & Analysis Group
Gravity-Drived Faulting and Folding
Types of Sheet Intrusion around a
Volcano
Cooling Fractures: Columnar
Jointing
Columnar jointing in the Massif Central, France
Barringer Meteor Crater of Arizona
Barringer Meteor Crater near Winslow, Arizona. The crater is believed
to have formed from the impact of a large (30-50 m diameter) iron
meteorite 50,000 years ago. The crater is about 1.2 km in diameter.
Photo by D. Roddy and K. Zeller, USGS.
Barringer Meteor Crater
Barringer Crater, also known as
'Meteor Crater', is a 1,300-meter (0.8
mile) diameter, 174-meter (570-feet)
deep hole in the flat-lying desert
sandstones 30 kilometers (18.6 miles)
west of Winslow, Arizona. Since the
1890s geologic studies here played a
leading role in developing an
understanding of impact processes on
the Earth, the moon and elsewhere in
the solar system.
This view was acquired by the Landsat
4 satellite on December 14, 1982.
http://visibleearth.nasa.gov/
Geological Cross Section of Impact
Structures
Shatter cone：岩石爆開成為碎塊，在碎塊表面形
成扇形分布的裂痕。表面具有扇形裂痕的岩石碎
塊。
Pseudotachylite or Pseudotachylyte
• Tachylite is a volcanic rock. Early geologists in Vredefort
identified something a bit like it, and called it
Pseudotachylite. It is formed largely by frictional melting
along faults or after the impact event. Clasts of the country
rock (here granite) are found within. In these images, the
Pseudotachylitic breccia is the black stuff.
Homework and Assignment
• Table 2.3: Common Surface Markings
• Facing indicators
• Table 2.5: Terminology of Salt Structures