Introduction to Clastic Sediments Flashcards
What are clastic sedimentary rocks?
Clastic (detrital) sediments are composed of clasts of pre-existing rocks.
What is the difference between matrix and cement?
Matrix: groundmass between large grains.
Cement: material binding grains together.
What are some processes of mechanical weathering (does not alter the chemistry)?
Freeze-thaw, abrasion and friction (wind, water, glaciers, particle collisions), exfoliation (release of pressure), thermal expansion/contraction, and vegetation.
What is one of the most common sources of clastic sediment?
High-relief hinterland areas; i.e. orogenic belts where rocks have been uplifted are mountains; rate of weathering/sediment production a function of climate/tectonics.
What aspects can be used to describe sedimentary texture?
Grain size, sorting, sphericity/grain shape, fabric (preferential grain orientation), framework, composition/mineralogy, and colour (not a great diagnostic property).
How is grain size classified on the Udden-Wentworth scale?
Phi units; -log base 2 of the grain diameter in mm, negative values for increasing grain size, positive values for decreasing grain size.
What methods can we use to measure grain size?
Grain size cards (hand samples, outcrops), mechanical sieving, laser diffraction (Coulter counter), and settling tubes (fines).
How does laser diffraction work?
A laser beam is passed through a sediment suspension, where light diffracts at large angles from small particles and small angles from large particles; only clay to sand size particles can be measured.
What can sorting tell us about a sample?
Transport processes, energy levels, degree of sediment cycling, and textural maturity (poor sorting = low textural maturity).
What are the four clast shapes on the Zingg Form Index?
Discoid/oblate, equant, bladed, rod/prolate.
What are the 4 main types of sandstone?
Quartz arenite, arkose (feldspathic arenite), lithic sandstone (litharenite), and greywacke
Quartz arenite
Composed almost entirely of quartz; well rounded, sorted, and cemented; mature to super-mature (polycyclic); forms in aeolian dunes, beaches, and shallow shelf environments.
Arkose
Poorly sorted; abundant in feldspar (>25%); lithic fragments; immature, limited transport and sediment cycling; forms in alluvian fan deposits and braided gravelly rivers in hinterland areas.
Litharenite
Poorly-sorted; contains abundant lithic fragments (>50%); angular to sub-angular grains, texturally sub-mature to mature; commonly fluvial, hinterland areas with active uplift; very limited transport and cycling.
Greywacke
Heterogeneous, poorly sorted sandstone; abundant lithic fragments, angular quartz grains; >15-75% silt/mud matrix by volume; submarine fans, slope, and abyssal plain deposits; low energy environments dominated by turbidity current deposition.
What is the Dott Classification?
AKA “Toblerone” plot expands upon QFL diagram by including % mud matrix: <15% mud in arenites, >15% mud in greywackes, and >75% mud in mudrock.
What’s the difference between an orthoconglomerate and a paraconglomerate?
Orthoconglomerates are clast supported and paraconglomerates are matrix supported.
