Lesson 5: Sedimentary Structures Flashcards
why do we need to study sedimentary structures?
- textures of the sediments can give us a clue on:
- depositional environment
- provenance - sedimentary structures can further elaborate or describe the manner of the transportation media:
* Energy
* Depth
* Flow directions
* Post-depositional events
9 kinds of bedding and bedforms
- bed
- laminae
- varves
- laminated beds
- graded bedding: normal, reverse
- massive beddings
- cross-bedding: tabular, trough
- flaser cross bedding
- lenticular bedding
- hummocky cross stratification
At least 1 cm thick, lenticular/tabular
beds
refers to those with lenses
lenticular beds
examples of resources commonly found with lenticular bedding
petroleum and coal resources (in swamps)
beds that are less than 1 cm thick
laminae
contact between beds is of erosional in nature
alamgamation surface, the beds (above and belown) between the surface are called amalgamation beds
what are amalgamation beds comparable to?
unconformities, diastem (period of nondeposition/erosion)
beds or laminae which features seasonal variation
varves
how to tell seasonal changes in varves
darker shades: organic matter; rainy season, deposited in wet conditions
lighter shades: dry season
For fine grained clastics and evaporites, results from suspension settling
laminated beds
explain laminated beds
more of settling action. if water carrying suspended solids and ions (esp in evaporites) becomes stagnant, the load will get deposited sa floor.
for larger particles like sandstones, mostly result of traction (bedload or bouncing)
“finering” upwards indicates:
- a sequence of a “sinking” basin
- turbidity currents
normal grading
explain normal grading for sinking basins
the floor of the basin is going down = particulate matter is slowly becoming finer
“coarsening” upwards (mud to conglomerate upwards) indicates:
- basin being filled with sediments, transitioning deep to shallow to subaerial environments (becoming shallower)
reverse grading
explain normal grading bc of turbidity currents
occurs between continental rise and slope, indicative of submarine landslides/canyons/currents
example of stratigraphic column that has a reverse grading
cagayan valley stratigraphic column (deep marine: finer; sands: fine, well-sorted sa beach depth; floodplains
bedding that has no internal structures, poor sorting, rare with coarser sediments, only really for finer sediments
massive beddings
what kind of deposits are mostly in massive beddings
finer deposits like sandstorm deposits or “loess”
structureless clay-sized silts (windblown sediments)
loess
the type of cross bedding depends on?
the initial surface
cross bedding is applicable to what other structures?
dunes and ripple marks
explain how cross bedding is formed
abrupt changes in wind/water direction and velocity causes carried sediments to get deposited in the lee side (steeper side) creating cross beds, while the stoss side (gentler slope) is getting eroded
flaser and lenticular bedding is related to what?
ripple marks
fine sediments that partially or fully covers along the apex of a ripple marks, favoring sand being deposited over mud (high sand supply)
flaser cross beddings
are more or less continuous beds, in favor of mud being deposited over sands (high mud supply)
lenticular bedding
for flaser bedding, where does clay get deposited?
at the trough of sands
Indicates continental shelf, and shoreface environments; continuous
hummocky cross stratification
common thickness of hummocky cross stratification
15-20 cm
what sometimes triggers hummocky bedding
sole marks
most of the time forms transverse (parallel) to the laminar flowing current
ripple marks and dunes
ripple marks or oscillation ripple is dependent on what 2 things?
- flow velocity
- flow direction
explain the difference between current ripple and oscillation ripples
current ripples
- asymmetrical ripples (partial to one side bc of leeside and stoss side)
- oriented to one direction of current, one side stronger than other
oscillation ripples
- more symmetrical
- because of swash and backwash motion = oscillate
poorly understood, however authors agree that these are fluid rich sediment beds undergoing deformation
convolute structures
convolute structures are what kind of stratification?
irregular stratification
explain formation of convolute structures
related to syn (or sine?) sedimentary structures, result of deformation during sediments still (mud or sands), not yet rocks
3 types of convoluted structures based on time of formation
syn-depositional, meta-depositional, post-depositional
type of convoluted structure in which the deposition and tectonic deformation occurred at the same time, that is why it’s chaotic
syn-depositional
type of convoluted structure in which the tectonic deformation occurred right after deposition. there was not much settling that happened yet, then boom deformation
meta-depositional
type of convoluted structure in which the tectonic deformation occurred after beds have been deposited and settled, ready to be lithified but then suddenly tectonism occurred.
post-depositional
Commonly occurring on when mud is overlain by sandstone, and heavier sediments, where the mud sediments are reaching upwards owing to the weight of the heavier overlying sediments
flame structures
do ball and pillow structures always occur with flame structures?
commonly, but not always, bc individual sand grains can form ball and pillow structures, but not necessarily flame structures
Commonly are water escape structures
dish and pillar structures
laterally extensive, some are impermeable, around 1 to 50cm
dish
vertical to near vertical cross cutting columns, saturated with water
pillar
related to sand boils, events of liquefaction
dish and pillar structures
2 kinds Erosional Structures
channels
Scour and fill/cut and fill
Sediment filled, U shaped or V shaped cross section that cut across previously formed beds or lamination,
channels
resemble channels, however short and discontinuous
when river dries up and sediments fill the channel
Scour and fill/cut and fill
example of Scour and fill/cut and fill
part of oxbow lake, meander scars, former dried up streams
examples of biogenic structures
fossils -stromatolites
organosedimentary structures formed largely by the trapping and binding activities of cyanobacteria
stromatolites
blue green algae
cyanobacteria
examples of bedding plane markings
- sole markings: bounce, brush, prod, roll, skip
- flute casts, groove casts
- current crescents
- rain drop imprints
- mudcracks
- rill marks
casts of sediments covering bedding plane marks
flute casts
larger, heavier sediments, obstruct the
path of fluid flow, protecting the sediments behind it
current crescents
6 structures with secondary origin
- sand dikes and sills
- concretions
- nodules
- color banding or liesagang banding
- stylolites
- cone-in-cone structure
refers to those cementing materials in between clasts
concretions
rhythmic layering resulting from the oxidation of metals and fluid saturated form thin, closely spaced, commonly curved
color banding or liesagang banding
