Stratigraphic Terms & Concepts
FACIES
- LATERAL GRADATION OF SEDIMENTARY UNITS HAVING
DISTINCTIVE SET OF PHYSICAL, CHEMICAL, & BIOLOGICAL ATTRIBUTES DUE
TO GRADATION IN ENVIRONMENTS OF DEPOSITION DURING A GIVEN TIME
WALTHER'S LAW
- FACIES ADJACENT IN A CONTINUOUS VERTICAL SEQUENCE
ARE ADJACENT LATERALLY AS WELL
TRANSGRESSION
& REGRESSION
- MOVEMENT OF SHORELINE LANDWARD = TRANSGRESSION
- MOVEMENT OF SHORELINE SEAWARD = REGRESSION
- PRODUCE DISTINCT VERTICAL SEQUENCES: TRANSGRESSION
- NEARSHORE FACIES OVERLAIN BY OFFSHORE FACIES; REGRESSION - OFFSHORE
FACIES OVERLAIN BY NEARSHORE FACIES
- TRANSGRESSIONS ALSO PRODUCE INDIVIDUAL
ROCK UNITS THAT BECOME YOUNGER IN A LANDWARD DIRECTION
(THAT IS, THEY ARE TIME TRANSGRESSIVE)
SEA LEVEL CHANGES
- ULTIMATE CAUSE OF TRANSGRESSIONS & REGRESSIONS
& MOVEMENT OF FACIES
- EUSTATIC CHANGES
OF SEA LEVEL ARE WORLDWIDE & RESULT FROM SEAFLOOR SPREADING
RATE CHANGES OR CONTINENTAL GLACIATION
- RELATIVE CHANGES ARE LOCAL, &
MAY BE CAUSED BY EUSTATIC CHANGES OF SEA LEVEL, OR BY UPLIFT
OR SUBSIDENCE, OR CHANGES IN SEDIMENTATION RATE, OR A COMBINATION
MODERN
STRATIGRAPHIC TERMINOLOGY
- CHRONOSTRATIGRAPHIC (TIME-STRATIGRAPHIC) UNITS
- System: Fundamental time-stratigraphic
unit
- rocks deposited during a certain time interval
- defined by rocks of a particular area
(stratotype) & recognized elsewhere by fossil content
- modified into smaller units by upper, middle &
lower
- other chronostratigraphic units: eonothem, erathem,
series, stage
- GEOCHRONOLOGIC (TIME) UNITS
- Period: Fundamental time unit
- abstract units
- modified into smaller units by late, middle &
early
- other geochronologic units: eon, era, epoch, age
- EQUIVALENCE OF CHRONOSTRATIGRAPHIC & GEOCHRONOLOGIC
UNIT TERMS
| Chronostratigraphic |
Geochronologic |
Examples |
| Eonathem |
Eon |
Phanerozoic |
| Erathem |
Era |
Mesozoic |
| System |
Period |
Triassic |
| Series |
Epoch |
Upper/Late |
| Stage |
Age |
Norian |
LITHOSTRATIGRAPHIC UNITS
- Formation: Fundamental lithostratigraphic
unit
- distinctive lithology with recognizable
contacts with underlying & overlying units that CAN BE TRACED
LATERALLY (that is can be correlated) with no regard
for time boundaries
- other lithostratigraphic units: supergroups, groups,
members, beds
- formations can be & often
are time transgressive
- BIOSTRATIGRAPHIC UNITS
- Biozone: Fundamental biostratigraphic
unit
- recognizable assemblage of distinct fossils
- Biozones are often nearly time synchronous
& are used to establish chronostratigraphic units
- Index fossils: fossil species that
are geographically widespread, occur in many types of sediments, and are
restricted to a narrow time interval
- MAGNETOSTRATIGRAPHIC UNITS
- Have aspects of biostratigraphic, chronostratigraphic
& geochronologic units
- Polarity zone, Polarity chronozone & Polarity
chron: Fundamental magnetostratigraphic units
- Magnetopolarity zones are essentially time
synchronous
CORRELATION
- PROCESS OF DEMONSTRATING EQUIVALENCY
- LITHOSTRATIGRAPHIC CORRELATION - generally
not time equivalent
- However, volcanic ash beds, glacial
tills, coal beds, or evaporites = key,
or marker beds - widespread & instantaneous
& are time equivalent
- DRILLING LOGS
- SEISMIC PROFILES
- BIOSTRATIGRAPHIC CORRELATION - approximates
time synchroneity
- MAGNETOPOLARITY CORRELATION - time
synchronous
- GEOCHEMICAL CORRELATION - may be time
synchronous; for example, Strontium or Oxygen Isotopes
Study Questions
1. Define facies & Walther's
Law.
2. Define transgression
& regression.
3. What are the roles
of eustatic sealevel changes, sedimentation & tectonics in causing transgressions
& regressions?
4. What are the causes
of eustatic sealevel changes?
5. How
are lithostratigraphic, biostratigraphic, time-stratigraphic and time units
defined & what are their fundamental unit names?
6. How are various stratigraphic
units correlated?