Stratigraphy

Question
• Group Question: discuss the following
• What is stratigraphy and why is it important?

Stratigraphy
• What is stratigraphy?
• Facies in Time and Space
• Why?
– Earth History
– Economics and Environment
– Put modern problems in context

Lithostratigraphy
• Lithology - lithostratigraphic units
• The Formation
– Members
– Beds
– Groups
• Stratotype
– Type section

Contacts
• Conformity versus unconformity
• Types of conformable contacts
• Vertical
– Sharp
– Gradational
• Lateral - continuous versus discontinuous
– Gradational
– Intertonguing
– Pinchout

Contacts
• Unconformable contacts - missing time
• Angular unconformity
• Disconformity
• Paraconformity
• Nonconformity

Vertical Successions
• Lithologic uniformity - monotonous sections
• Lithologic heterogeneity
• Cyclic successions - varying scales and mechanisms
– Cyclic sedimentation - recurring migration of environments
• Autocyclic versus allocyclic mechanisms

Allocyclic Succesions
• Scales of Cycles
– Changes in S/L, climate, and tectonism
• Hierarchy of cycle orders
• First-order cycles
• 100’s of millions of years
• Supercontinent cycles
– Wilson cycles

Allocyclic Succesions
• Second-order cycles
• 10’s - 100’s millions of years
• Sloss/megasequences
• Rates of seafloor spreading

Cycles
• Third-order cycles
• 1-10 million years
• Sequences - unconformity bound
• Mechanisms?
• Global correlation?

Cycles
• Fourth- and fifth-order
• 10’s - 100’s ky
• Milankovich cycles
• Orbital climate forcing
– Eccentricity
– Obliquity
– precession
• Cyclothems
• Parasequences
• Climate Setting
– Amplitude
– Duration

Facies Patterns - Walther’s Law
• Vertically stacked facies were once adjacent environments
• Only applies within conformable intervals of rock
• Vertical facies patterns = cycle recognition

T & R and Facies Patterns
• Facies shift positions through time
• Transgression - landward shift
• Regression - seaward shift
• Eustatic versus relative
• Predicable facies patterns!!!!

Classification
• Stratigraphic Classification - North American Stratigraphic Code
• Physical and Time Stratigraphic Units
• Why are time and rock considered separately?
• Lithostrat. Units
• Allostrat. Units
– discontinuities

Correlation
• Correlation = demonstration of equivalency of stratigraphic units
• Equivalence in time
• Lithocorrelation - similar rock types and stratigraphic position
• Biocorrelation - fossil content and biostratigraphic position
• Chronocorrelation - correspondence in age

Correlation
• Lithostratigraphic correlation
– Chronostratigraphic - locally
– Diachronous - regionally
• Correlation does not equal matching
• Direct versus indirect
• Stratigraphic Position
– Key beds or marker beds
• Bentonites, epiboles, tempestites
– Discontinuities