Transcript Chapter 2: Non-tectonic Structures

Chapter 2
Nontectonic Structures
Nontectonic Structures
► Nontectonic
structures help to determine
the original orientation of strata
 Primary structures such as mud cracks, ripples,
sole marks, and vesicles help to determine
original orientation
 These features help us determine the facing
direction of a sequence of rocks.
An overturned fold in Scotland
Nontectonic Structures
► Structures
that formed in primary
depositional environments may mimic
tectonic structures
 Examples ductile flow in water-saturated silt,
glacial ice, or evaporites
► Primary
structures are usually older than
tectonic structures and cross-cutting
relationships can help differentiate between
them
Tectonic vs Nontectonic
Structures
Soft-sediment folds
Slumping
Tectonic folds
Primary Sedimentary
Structures
► Bedding
 Bedding Planes
 Graded Beds
 Cross Bedding
Cross Bedding
Cross bedding structures
Primary Sedimentary Structures
► Mudcracks
 Polygonal blocks formed by desiccation and
shrinkage of saturated sediments
 Taper down from the surface until they
terminate
 The surface layer may also separate and curl
upward
Mud cracks
Primary Sedimentary
Structures
► Ripple
Marks
 Current (translational) ripple marks form
under a prevailing direction of fluid flow
►Asymmetrical
with the steep side toward
downstream
►Not useful to determine facing because they
have the same shape whether they are upright
or overturned
 Oscillatory ripple marks form with backand-forth fluid flow
►Symmetrical
with pointed peaks showing
original facing direction
Ripple Marks
Primary Sedimentary Structures
► Rain
Imprints
 Can determine facing direction
► Tracks
and Trails
 May be used to determine facing direction
► Sole
Marks, Scour Marks, and Flute Casts
 Are later filled with sediment and can be used
to determine facing direction
Rain Imprints
Sole Marks and Flute Casts
Primary Sedimentary Structures
► Load
Casts
 Form from dewatering of the underlying
sediment from the weight of the newly
deposited sediment
 Can be used to determine the facing direction
 The broadly convex bases of the load casts
show the original bottom of the structure
Load Casts
Fossils
► Fossils
 Provide relative ages
 May be used to determine facing direction
based on fossil sequences or growing conditions
in life assemblages
 May also be used as strain indicators
Sedimentary Environments
► Sedimentary
Facies
 Sedimentary rock units vary laterally and
vertically as paleoenvironments change.
 Facies are separated by composition, texture,
sorting, physical and biogenic sedimentary
structures.
 Walther’s principle - Only those facies that once
existed side-by-side can be observed vertically
juxtaposed in outcrop.
Sedimentary Facies
Unconformities
► Unconformity
– A break in the sedimentary
record where part of a stratigraphic
succession is missing.




Produced by erosion or nondeposition
Disconformities
Angular unconformities
Nonconformities
Unconformities
Cross section through the
Grand Canyon
Primary Igneous Structures
► Igneous
plutons and lava flows may form in
shapes that resemble sedimentary features
or may tectonically alter previously
emplaced country rock
Geologic Map of the Sierra Nevada
Primary Igneous Structures
► Compositional
banding
 Occurs in igneous rocks and may result from
crystal settling, differentiation, fractional
crystallization, and multiple parallel intrusions or
flows.
 Study of these may reveal differentiation or
fractional crystallization sequences
Primary Igneous Structures
► Vesicles
 Cavities left by gas bubbles that form in magma
due to pressure release
 Vesicles generally accumulate at the top of the
magma chamber providing the facing direction
Vesicle
Primary Igneous Structures
► Pillow
Structures
 Form from lava flows into or below water
 Vesicular glassy curved tops and V-shaped
nonvesicular bases indicate facing direction
Pillow Lavas
Pillow Lava
Primary Igneous Structures
► Contact
flows
Metamorphic Zones from lava
 The metamorphic aureole forms beneath the
lava flow and can be used to determine facing
direction
Contact Metamorphism
Gravity-Related Structures
► Landslide
and Submarine Flows
 May resemble tectonic structures because of the
flow patterns of these sediments
►Olistostromes
are matrix supported bedded
sediments
►Diamictites are matrix supported sediments with no
bedding
►Turbidites are unsorted sedimentary deposits
produced from rapid flow of sediment
Gravity-Related Structures
► Salt
Structures
 Occur in evaporite sequences of
anhydrite, gypsum, or halite
 These rocks flow more readily than any
other common rock type and can form
folded structures under surface
temperature and pressure
 These structures that move upward and
gravitationally intrude overlying sediments
are called diapirs.
Eroded salt dome in Iran and diapir evolution
Grand Saline Salt Dome in Texas
Experimentally produced diapirs
Impact Structures
► Structures
with circular or elliptical outlines
can be formed from meteorite impacts as
apposed to tectonic forces
 Shatter cones are produced from brittle
deformation and can be good evidence of
impacts
Geologic Map of Wells Creek Tennessee
Shatter Cone
Reconstruction of the impact at
Wells Creek
Nontectonic Structures
► Nontectonic
structures can appear to be
tectonic features
► Nontectonic structures can also be used to
determine facing direction, which can aid in
the interpretation of tectonic features