Chronocorrelation and Seismic Stratigraphy
Seismic Section of Lake Cayuga
Seismic Units and interpretation:
• I, Valley Heads Moraine
(Not present in this lake)
• II-IV, Glaciolacustrine
• V, Alluvial
• VI, Lacustrine
(From GSA Special paper 311)
Chronocorrelation,
Application of radiometric dating
Short timescales
–
210Pb
dating or 14C dating
Longer timescale
– Three major methods of assessing Period/Epoch boundaries
• Direct radiometric dating of volcanics
• Bracketed igneous intrusions
• Interbedded volcanic and sedimentary rocks
Direct radiometric dating of sedimentary rocks?
– Limited possibilities
– Usually provides information on source or material, not time of sedimentary rock formation
Dating bracketed igneous intrusions
Dating interbedded Volcanics
Modern seismic surveying methods
• Seismic reflection
• Seismic refraction
• Sonar
 Echo sounding (PDR)
 Side-Scan
(From Garrison, Intro Oceanography)
Seismic Waves: "P" and "S"
•
•
"P" (primary) wave -compression
"S" (secondary) wave - shear
•
Distance can be inferred from wave velocity and time:
Velocity = distance/time
Thus
Distance = velocity x time
Graphic: Garrison, Fig. 3.5, pg 62.
Seismic "P" Waves in a Layered Earth
•
P waves:
-travel through both solids and liquids
- are bent as the travel through the Earth
•
This bending creates a small shadow zone where P waves are not detected at the surface
•
Graphic: Plumer et al., Physical Geology 1999 ed. , Fig 17.8
•
•
•
or Garrison, Fig. 3.6d, pg 63.
Constraints on Seismic Velocity, 1
Velocity in Earth Materials varies with composition
Seismic Velocities (V)
SS: 5,486-5,944 m/s
LS: 6,401-7,010 m/s
DS: 7,010-7,315 m/s
From AAPG Memoir 26
Constraints on Seismic Velocity, 2
V varies with porosity (Density),
From AAPG Memoir 26
Constraints on Seismic Velocity, 3
• Velocity varies with
pressure
From AAPG Memoir 26
Constraints on Seismic Velocity, 4
• Composition
• Porosity/Density
• Pressure
Question:
Does seismic velocity vary with formation age?
From AAPG Memoir 26
Fundamental Seismic Property
Impedance provides the useful measure of seismic visibility that accounts for the controls on seismic waveform
Impedance = velocity x density
From AAPG Memoir 26
Constraints on seismic imaging
Seismic “Visibility”
increases with
• Thickness
• Impedance contrast
From AAPG Memoir 26
Impact of impedance contrast on acoustic
waveforms
Greater the impedance contrast, the stronger the seismic layer or
reflector
From AAPG Memoir 26
Simple Seismic Reflection Patterns From AAPG Memoir 26
Types of Seismic boundaries
From AAPG Memoir 26
Erosional Truncation From AAPG Memoir 26
Top Lap
From AAPG Memoir 26
Onlap
From AAPG Memoir 26
Downlap From AAPG Memoir 26
Hypothetical form of Seismic Stratigraphic
structures
From AAPG Memoir 26
Complex Seismic Reflection Patterns
From AAPG Memoir 26
Prograding Clinoforms
A: Sigmoid
B: Tangential Oblique to Sigmoid
C: Mostly Parallel Oblique
D: Complex Sigmoid-Oblique From AAPG Memoir 26
Seismic Sections of Carbonate Buildups From AAPG Memoir 26
Pinnicle Reef Platform
From Emery & Meyers, Sequence stratigraphy