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. How are these concepts used in Historical Geology?

3. Define transgression & regression.

4. How are these used in Historical Geology?

5. What are the roles of eustatic sealevel changes, sedimentation & tectonics in causing transgressions & regressions?

6. What are the causes of eustatic sealevel changes?

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