Primary Structures Flashcards
what are primary vs secondary structures
Primary structures - Non-tectonic
-original features of sedimentary or igneous rocks, resulting from deposition or emplacement.
Secondary structures - Tectonic
- Brittle structures
- Ductile structures
What is the best marker
a marker is a feature or object that can be used to track, identify, or interpret the orientation, movement, or age of geological structures
Bedding
-> planar, horizontal to start
-> if bedding is no longer planar or horizontal, you know it has been deformed
primary beds are markers in structural analysis
what causes graded bedding
Graded bedding is when sediments are coarser grained at the base and finer grained at the top
-occurs due to turbidity currents
as current slows, coarse, heavy particles settle first
->finer grained stuff is lighter, can be carried for longer in a slower current
->progressively finer sediments settle atop coarse sediments
younging indicator
what is a turbidity current
rapid downhill flow of water due to increased density from high amounts of sediment
these form when sediment is loosened due to geological disturbances (eg., earthquakes, etc.)
bouma sequence
turbidity
a and b will be more preserved, upper layers less likely to be preserved
e = mud , hemipelagic and pelagic deposition
d = laminated silt
c = cross laminated sand , lower flow regime ripples
b = parallel laminated sand , upper flow regime plane bed
a = massive granules to sand , rapid deposition , upper flow regime
Cross bedding
the younger one always cuts the old one (cross cutting relationships)
Cross beds are surfaces within a thicker, master bed that are oblique to the overall bounding surfaces of the master bed.
flute casts
vortices (mini tornadoes) dig into unconsolidated substrate and form asymmetric troughs
form from turbidity currents
troughs are deeper at the upstream end as that is where the vortex is stronger (bulbous end of the scoop points upstream
sediment fills the troughs making a cast of the hollows (flute cast)
good facing indicator
ripple marks
angled ripple marks from currents are harder to tell what the top or bottom is bc it looks the same upside-down. mineral deposits can give us a clue to this
fluid flow causes ridges and valleys on the surface of a bed
symmetric if fluid flows back and forth (like on a beach), crests tend to be pointed and troughs tend to be rounded
-> good facing indicators
asymmetric if uniform current flow
-> not good facing indicators but show current direction
name the 2 load structures and how they form
load structures occur at the contact between a mud layer and a sand layer
sand deposit overlies water logged mud. sand is denser than mud so it will put pressure on a pre-existing weakness in the mud and sink into it.
flame structures:
mud injects up into the sand forming a flame structure
ball and pillow:
sand sinks into mud creating a ball, mud wraps around it as a flame structure, flame curls in making the ball and pillow
rounded structure is a load cast, pointed is the flame structure
dewatering structures
Sediments oversaturated with water
and under pressure
-Disturbed by an event (such as an
earthquake or slumping), water wants to get out
-water goes up
sand volcanoes
-water comes up, sand creates dome shape
clastic dyke
-water creates a vertical crack as water tries to escape to the surface
-the crack then fills with sediment
scour structure or channel-and-fill structure
a sedimentary deposit that fills a channel cut into bedrock by flowing water
(flute cast)
describe the 3 types of contacts
- depositional contacts
-rocks on ground, sediments deposit on top
types:
-conformable contact (continuous deposition, no substantial time gap)
-unconformable contact (not continuous deposition, time gap)
-hiatus (gap in time represented by an unconformity)
2.fault contact
contact caused by a fault
3.intrusive contact
-magma intrudes on rock
types of unconformities
disconformity
-major gap in time
-erosional surface
-bedding above and below are parallel
angular unconformity
-folded, often metamorphosed
-top is eroded, sediment deposited on top
nonconformity
-intrusive rock or high grade metamorphic rock
compaction and diagenesis structures
pinch-and-swell structures
-observed as a difference in thickness
weaker zones of the rock deform more easily than stronger ones
weaker zones are stretched (pinched) and cause compression (swell) in the stronger zones
- compression between each stretched spot
<–> <–> <–> <–> <–>
-looks like a rope braid or sausage
pressure solution and stylolites
compaction and diagenesis structure
pressure solution:
soluble grains dissolve along the faces at which stress is the greatest (solubility increases with increased pressure)
-> creates a serrated pattern
->these dissolved grains are removed by fluids in the rock
Insoluble minerals like clays are left behind in the serrated structure, making it visible
-> these are called stylolites
->happens in carbonates (limestones)
penecontemporaneous structures
folds/faults
how can you tell they are primary?
aka slump structures
sediment layers are loosely held on a steep slope
some trigger increases instability (eg, earthquake, undercutting by erosion) causes the layers to slide down the slope where the slump at the bottom
-original bedding maintains some cohesion (able to still see layers) but they are faulted/folded
-re-sedimentation occurs in the new position
in this case, the faulting/folding is primary (not due to tectonics)
-regular layers, but in between are chaotic structures that look like they’re sloping in one direction
-the bedding would be more regular if it was due to tectonics (moved after it was lithified)
explain dykes and intrusive foliations
how they form, if they are good younging indicators, intrusive vs extrusive
dykes: sheet intrusion, relatively vertical, cross cut stratified sequence
-their cross cutting nature is good for relative dating of a stratigraphic sequence
intrusive foliations
-alignment of minerals or textures within intrusive igneous rocks
-magma moves within a chamber, elongate or platy minerals (eg feldspar, biotite) align parallel to the flow direction
-As the magma cools, heavier crystals (e.g., olivine, pyroxene) form first and may settle at the bottom
-not a good younging indicator since its within the crust, ie there wont be youngest on top
explain pillow lava, flow foliation, columnar joints
how they form, if they are good younging indicators, intrusive vs extrusive
Pillow lavas/submarine (basalt)
-lava erupts underwater, rapidly cooling and solidifying into bulbous, pillow-shaped structures
-younging indicator: new pillows on top, older underneath
Flow foliation (banding)
-bands form as lava cools as it flows
-shear stress due to variations of flow speed within the lava
-some parts of the lava cool faster than others (layers of fine/coarse grained material)
-certain minerals form before others as it cools (form at different temps)
-younging indicator: The youngest part of the flow is typically on top, and older parts are towards the bottom of the flow
columnar joints
-magma cools, contracts as it solidifies, fractures due to tensile stress (stretching)
-cools from top down, fracture propagates down creating columns
-crack in a hexagonal shape as this is the most efficient way to distribute stress
-not good for younging, no clear top to bottom structure
