Sequence Strat contd... Parasequences and Seismic Data

Question 1

What is a parasequence?

Answer 1

• Small scale m - 10s of m
• Conformable succession of genetically related beds bounded by flooding surfaces
• Identified by coarsening upwards prograding lobes into shallow marine settings

Question 2

Compare grain size and accommodation space/sedimentation ratios in parasequences moving seaward, landward or at the same position

Answer 2

Moving seaward - shallowing upwards, acommodation less than sedimentation, fining upwards
Moving landward - Deepening upwards, sedimentation more than accommodation space, coarsening upwards
Staying the same position - Similar grain size, accommodation space = sedimentation

Question 3

Limitations to parasequence approach

Answer 3

Bounding surfaces not clearly defined, e.g. hard to distinguish transgressive ravinement surface, maximum regressive surface and maximum flooding surface etc.

Question 4

What effect does rock composition have on seismic velocities?

Answer 4

Faster velocities in harder more compact rocks

Generally lower velocities in clastic rocks and high velocities in carbonates and evaporites

Question 5

How is seismic energy lost?

Answer 5

Lost through absorption as the wave travels deeper into the earth’s crust.