Lesson 3: Grain Shape and Fabric Flashcards
encompasses all aspects of the external morphology of particles
grain shape
what are the 3 aspects of a particles external morphology
- form
- roundedness
- surface texture
refers to the overall morphology of the of the particles, 3D measure
form
refers to the absence of sharp edges of the individual grains, commonly measured in 2D
roundedness
refers to the overall shape of the particles approaching the shape of a sphere
sphericity
refers to microrelief features on grains
surface texture
example of surface texture features
scratches, pits, rills
explain the implication of surface texture
these microrelief features usually tell much about the transportation history of sediments.
what can change the texture of grains
- abrasion during transport
- cementation or diagenesis can change the form and roundness which ultimately affect surface texture
explain the hierarchy of the 3 aspects of grain shape
- form is first order
- roundedness is superimposed on top making it 2nd order bc changes on form makes changes on roundedness
- lastly surface texture is third order because it gets affected by both form and roundedness, but of course keeping in mind that they are independent parameters
significance of grain shape analysis
provenance studies and environmental studies
what are the 4 factors to be considered when interpreting the provenance of sediments based on shape
- source of the parent rock
- the erosion and transport mechanisms
- the depositional setting and process (which affects the fabric)
- the physical and chemical conditions of burial environment
what are the 2 things to be considered when trying to study the parent rock when studying sediment sources
- mineralogy
- petrology
when talking about grain shape, what type of sedimentary particles is this usually applied to?
siliciclastic particles
significance of grain shapes that examines where it came from as well as the transport history
provenance studies
significance of grain shape that examines the history of the end depositional environment
environmental studies
explain how the depositional setting and processes affect fabric
even in depositional environments, there are still a lot of processes that could rework it, ultimately affecting the fabric of sedimentary rocks
what are the physical and chemical conditions during burial diagenesis
this is when lithification occurs, the rocks are exposed in a half-baked metamorphic environment. still high pressure with fluids
explain how sediment sources and parent rocks affect sediments
different tributaries have different types of rocks that produce different kinds of sediments, even the same mineral, but itf different environments, it could still be characterized
that’s why if they end up in the same depositional environment (mixing). meaning by examining particles in the same dep envi, you can still possibly identify the location of the parent rocks.
what are the 3 main influences of erosional and transportation processes
- sediment size and density
- abrasion during transport
- transportation agent
what do we mean by depositional setting and processes
we mean the residual velocity from the transport agent orr the recurrence of a transport agent
-sorting
-reworking
what do we mean by environmental studies of grain shape analysis
we mean the post lithification and diagenesis, because sedimentary rocks can be used for fingerprinting paleoenvironments
how does sediment size affect transportation processes
well actually, depending on the current, the type of transportation agent and also the mineralogy and petrology of the rocks itself, but generally the heavier sediments are transported by stronger currents
how does abrasion affect transport process
abrasion in suspended vs saltation, esp if hard minerals bounce, abrasion could round them out
explain how type of transport agent can influence erosional and transportational process
the stronger agent can carry heavier sediments. ex. wind vs water in desert environments, water carries larger sized.
what is the more significant agent in desert environments, water vs wind?
water, wind is prevalent but weak
explain how depositional setting and processes affect sediments
-larger sediments upstream can get reworked because of recurrence of transpo agents, thus they become oriented to the direction of the current
-in beaches, finer sediments can get reworked bc of backwash and swash, rounding the sediments
thus reworking bc of transpo agent and depositional environment setting as well as also interaction with other sediments
explain how sorting can tell the depositional environment
river
-upstream, high energy so bigger sized boulders
-downstream: finer
-delta: mixed because of meeting sa sea and river
-beach-finer bc of sea reworking
explain environmental studies for grain shape analysis
tracing features tell much about the paleoenvironment, like fossils, and features that can reveal things about the depositional environement. ex. shell fossils, thus probs marine envi
refers to the utilization of space among sediments, as well as the manner of arrangement or spacing of the solid particles
grain packing
what are the 4 stages of maturity
immature, submature, mature, supermature
describe immature grains based on clay content, sorting, roundedness and transport+reworking
more than 5% of clay, poorly sorted, poorly rounded, not undergone sufficient sediment transport and reworking
describe submature grains based on clay content, sorting, roundedness and transport+reworking
low clay, not well sorted, not well rounded, undergone more transport and reworking
describe mature grains based on clay content, sorting and roundedness
low clay, well sorted, not well rounded
describe supermature grains based on clay content, sorting and roundedness
no clay, well sorted, well rounded
3 things to consider about grain packing
- Sediment size and sorting distribution
- Particle shape and orientation
- Degree of Diagenesis
particle orientation may be a factor of 5 variables, what are they
- nature of flow
- bed slopes
- flow velocity
- particle size and shape
- packing velocity
explain the relationship of sorting and reworking
if the particles have been sufficiently reworked, they’d get well sorted
5 kinds of packing
- floating
- point
- long
- concavo-convex
- sutured
what is the difference between tight packing in igneous, sedimentary and metamorphic rocks
sedimentary -in contact, no transition
metamorphic -with transition
igneous -individual crystals
relationship of roundedness to transport velocity
the more angular, the more surface area, faster to travel
can reworking happen in quieter depositional areas
yes, biological tho.
