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Primary Structures
Geologic Structure
A definable shape or fabric in a rock
Primary Structure: A structure formed during or
shortly after deposition (sedimentary) or formation
(igneous) of rocks
Secondary Structure: A structure formed after its
host rock is formed
Tectonic Structure: A structure formed as a result
of strain due to tectonic deformation
Primary Sedimentary Structures
Bedding: The primary surface in a sedimentary rock, separating
beds with different composition, texture, color, cement (make
sure you recognize beds based on these criteria!)
Different beds represent different source, sedimentary processes,
and environments of deposition
Emphasized in outcrop by parting and differential weathering
and erosion
A plane of separation, along which the rock has a tendency to
split or fracture parallel to bedding (don’t confuse with fracture!)
Commonly due to the weak bonds between different beds, or
preferred orientation of clays
Commonly, there is a bedding-parallel fracture which forms
due to unloading or rocks
Closely-spaced parting is called fissility (e.g., in shale).
Bedding between inter-bedded sandstone
and conglomerate
Reasons Why Clays or Clasts are
Preferrably-Oriented?
1.
Sedimentary settling of elongate or planar flakes in the gravity
field (syn-depositional)
2.
Rotation and reorientation of flakes in a flowing fluid (syndepositional). Flakes reorient so that the traction is minimized
This may lead to imbrication (grains overlap like roof
singles) which may be used for paleo-current analysis
(finding past flow direction and regime) e.g.,
Pebble Imbrication where shingled flat pebbles indicate current
direction
3.
Reorientation (rotation) due to post-depositional compaction
(squeezing of unlithified sediment due to the weight of the
overlying rocks).
Bedding is Important in Structural Analysis
Bedding is used as a paleo-horizontal, or nearly horizontal
reference frame (recall the principle of original horizontality)
Bedding as a primary structure (S0, or original surface) is the
first object that becomes deformed. The subsequent
deformation surfaces created (S1, S2, S3) are compared relative
to the S0
Structures, textures, fossils, etc, in beds provide clues as to the:
Depositional environment
Stratigraphic facing (younging direction) to identify rightside-up or overturned beds
Current direction
Beds help us to better map stratigraphic contacts, and identify
large structures such as folds, faults, and unconformities.
Bedding between sandstone & conglomerate
Graded Beds
Graded beds: Progressive fining of clast grain size,
from the base to the top of a bed; form as a consequence
of deposition by turbidity currents (e.g., in turbidite)
Can indicate which way is up provided the bed is not
inversely graded
Provide information for stratigraphic facing and
possibly current direction, e.g., if cross-beds are present
Must know what kind of depositional environment
deposited the bed – example:
debris flows - deposit inverse graded beds,
storm deposits (tempestites) & turbidites are typically graded
beds
Graded Bedding
Cross Beds provide information for facing and
possibly current direction
Cross beds: Are surfaces within a thicker, master bed that are
oblique to the bedding in the master bed
Defined by subtle parting or concentration of grains
Form when grains move from the windward or upstream side of
a dune ripple, toward the leeward or downstream side
Topset: thin, usually concave upward, laminations parallel to the
upper master bedding.
Foreset: inclined, curved, laminations or beds deposited parallel
to the slip face. These merge with the topset and bottomset
beds. Foresets define the cross beds. Current direction is
perpendicular to the strike of the foreset
Bottomset: thin laminations parallel to the bottom master
bedding
Cross Bedding
Cross Beds
Erosion truncates the topset and upper part of the
foreset, juxtaposing younger bottomsets on the older
foreset; this forms higher foreset angles at the upper
bedding compared to the tangential angles below
(used for facing).
The foreset beds are inclined at an angle to the main
planes of stratification.
- Truncated at top
- Tangential at bottom .
- Dip direction indicates transport direction
Ripple Marks
Ridges and valleys on the surface of a bed, formed due to
current flow. Cross stratification with wave amplitude < 6“
(1)
(2)
Oscillation or Symmetric Ripple Marks
Oscillation wave produced ripples (current moving in two
opposite directions)
• Crests are pointed and troughs are curved
Symmetrical concave up small scale (amplitude < 6") cross
stratification.
• Good facing indicator
Current or Asymmetric Ripple Marks
Asymmetric cross stratification produced by current
moving in one direction; i.e., uniformly flowing current
• Good current direction indicator
Ripple Marks
Mud Cracks
Polygon shape in map view.
Result from desiccation into an array of polygons
separated by mud cracks.
Thin (typically sand filled) fractures that taper
down in cross section because each polygon curls
upwards along its margin.
Good facing indicator (individual cracks taper
downward.
Mud Cracks
Other Casts
Erosion or scraping, filling, subsequent erosion produce positive
relief casts. Good indicators of current direction
Groove casts - Elongate nearly straight ridges
Bounce, Brush, Skip marks
All are discontinuous type of groove cast
Flute Cast – Asymmetric troughs formed by fluid vortices or
eddies (mini-tornadoes) that dig into unconsolidated
sediment
Stronger vortex at the upstream end cuts deeper and
narrower than the downstream part which is shallower and
wider. Thus, flute casts taper down-stream!
Sole Marks - Load Casts
Bulbous protrusions of denser sand into less dense mud
layers
Forms due to density instability when sediment is still
soft (i.e., still unlithified)
The sinking is triggered by the disturbance during
earthquake, storm, or slump
At greater depths, partially consolidated mud breaks
into pieces and sink into underlying sand, forming
disrupted bedding
Contacts
Contact: Boundary between two geologic units of
any kind.
1. Depositional contact: a sedimentary unit is
deposited on top of another.
2. Fault contact: two units are juxtaposed by a fault.
3. Intrusive contact: an igneous cuts across another
rock body.
Unconformities
Conformable contact: The boundary between adjacent beds
or units does not represent substantial gap in time
A succession of beds of nearly the same age that represent
nearly continuous deposition
1.
Diastem
Erosion surfaces within a conformable succession of strata
Unconformable contact (unconformity):
Represents an interruption in sedimentation, such that there
is a substantial gap in time (called hiatus), few years to
billions of years, across the contact
Contact represents erosion or non-deposition of strata
Unconformity
Four Types of Unconformity
Angular unconformity - Beds below and above the
unconformity have different attitudes.
Beds below are truncated by the unconformity.
Buttress (onlap) unconformity – New beds lie on areas
with significant pre-depositional topography.
The younger layers are truncated by the rugged unconformity
(difference with angular unconformity).
Beds above and below the unconformity may or may not
parallel the unconformity.
There is an angular discordance between the beds above and
below the unconformity
Types of Unconformity
Disconformities – Beds above and below the
unconformity are parallel, but there is a hiatus,
created by non-deposition or erosion.
A disconformity is hard to recognize in the field
Fossils, paleosols, or scour features help!
Nonconformities – Strata deposited on older,
crystalline (igneous or metamorphic) basement
rocks
Identifying Unconformities
Basal conglomerates, rest on unconformable
surface and contain fragments (clasts) of
underlying rocks
Topographic relief
Paleosols - Ancient soils, weathered zone just
below the unconformity
Recognized by color change, and soil structures
Soft Sediment
Penecontemporaneous Structures
Sediments may be deposited with a gentle initial dip.
In this case, gravity may pull them down during
storm or earthquake. The downslope movement is
helped by fluid pressure
If sediments that move down the slope are soft, they may
produce a slurry of clasts suspended in a matrix called
debris flow. When the debris flow comes to rest, it
forms a poorly-sorted conglomerate
If the sediments are compacted sufficiently before they
are dislodged by gravity, they maintain their cohesion,
and produce what is called slumping
Penecontemporaneous structures
The folds and faults formed during slumping are called
penecontemporaneous
Penecontemporaneous means that they formed
almost (hence “pene”) at the same time as the
original deposition of the layers
Penecontemporaneous folds and faults are
characteristically chaotic
They are intra-formational, i.e., bounded above and
below by relatively undeformed strata
Growth Faults
Synsedimentary faulting - fault displacement
continues as sediment is deposited on top of
the fault blocks
Thickness of sedimentary units varies across
the fault
Volcanic Structures
Flow Layering
Layers of volcanic flows defined by color, texture
and weathering.
Flow structures
Pahoehoe; Ropy lava - Good flow direction indicator
Pillow Structures
Flat bottomed, curved top basalt encased in thin
obsidian cover
Good facing indicator
Volcanic Structures, cont’d
Vesicles
Voids formed by gas bubbles typically more
numerous at the top of the flow
Good facing indicator
Columnar Jointing
Fractures formed in basaltic lava due cooling and
shrinkage
Polygonal columns
Product of slow cooling, top of flow does not have
as well defined columnar joints as base of flow.
Good facing indicator
Intrusive - Plutonic Structures
Flow Foliation
Aligned minerals in intrusive igneous rocks
occurs while rocks are still melted or partially
melted and flowing.
Indicates flow direction