Practical skills

Question 1

Describing a sandstone in thin section:

Answer 1

Describing Sandstones in Thin-Section:
• Grains present (and sub-types) and %
o Quartz
o Feldspar
o Lithic grains
o Mica
o Bioclasts
o Others
• Texture
o Roundness, sorting, fabric, packing, preferred orientation of grains
• Cements
o Quartz, calcite, dolomite, hematitie, clays, anhydrite, cement geometry and timing
• Replacements
o Alteration, dissolution, feldspar preservation, calcite and calys replacing grains
• Evidence of compaction
o Broken and squashed grain
• Porosity
o Intergranular – reduced/enhanced, mouldic, fracture
• Sandstone type
o Arenite/wacke, Quartz arenite, arkose, litharenite, greywacke – lithic/feldspathic/quarzitic
• Depositional environment
o Marine/non-marine, fluvial/aeolian, shallow/deep, low/high energy
• Order of diagenetic events

Question 2

Concept of facies and Walthers Law?

Answer 2

• Facies: a distinctive packet of sedimentary rock with specific sedimentary features. The characteristics of the rock unit come from the depositional environment
• Lithofacies: facies type based on grain size and mineralogy e.g. shale facies; oolitic limestone facies. Can also have biofacies.
• Facies Relationships
• Walther’s Law: Vertical succession of facies reflects lateral changes in environment. Fundamental for understand transgressions and regressions
• Accommodation space is a primary control upon sedimentation and ultimately facies relationships

Question 3

Basics of sequence stratigraphy?

Answer 3

Sequence Stratigraphy: The study of the relationship between sea level change and sedimentation
• Baselevel or sea level rise and fall is a fundamental control on sedimentation and also known as a autocyclic control on sedimentation.
• Sea level controls nic points of rivers so that river needs to equilibriate as sea level changes
• Non-marine (fluvial) areas are not influenced by sea level so sequence stratigraphy cant be applied
• Fall in sea level = coarsening upwards
• Basic but sea level will vary too much for a solid change – more overall change with blips
• Can predict what facies will be where horizontally if the sea level change is known

Question 4

Transgression and its relation to facies (vice versa for regression)

Answer 4

Transgression - sea level rises - sediment accumulation cant keep up - leads to movement of facies inland
Regression - vice versa - the facies move seaward

Question 5

Order of sequence stratigraphy?

Answer 5

Sea level rises
Point at which the sea level rises fastest is the Transgressive Systems Tract - causes fining upwards as sedimentation cant keep up with sea level rise
Maximum Flooding surface
High Stand System Tract - highest sea level but not highest rate of sea level rise - Inflection as sea level can keep up - starts to coarsen upwards (MFS) and then an erosional surface as preservation is stopped as sea level falls quickly
Sea level falls - coarsens upwards
LST - sea level stopped falling - possible fining or coarsening depending on accommodation space but lowest sea level means it will be coarsest material in sequence

Question 6

What is a parasequence?

Answer 6

Parasequences:
• Relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces.
In addition to these defining characteristics, most parasequences are asymmetical shallowing-upward sedimentary cycles.

Question 7

Sandstone types and their depositional environment?

Answer 7

Quartz Arenite: Aeollian
Lithic Arenite: Fluvial, Delta
Arkose: Semi-arid/glacial - delta
Greywacke: Turbidity current