Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 21 Graphic Logs and Stratigraphic
  Correlation

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Graphic Logs

• prepared with information recorded directly in
  the field from -
• OUTCROPS - usually logged from the base to the
  top of the quarry, roadcut, etc...
• DRILL CORE/VIBRACORE - usually logged from the
  top down, continuous where complete
• WELL or AUGER - always logged from the top down,
  usually from samples at a fixed interval (often 2
  foot spoon every 5 feet)

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Driller Jargon

• T.D. - total depth of the core, well, auger,
  etc...
• Refusal - the auger was "refused" by hard rock
• N (Blow Count) - the number of times the "hammer"
  (weight on rope) had to land on the top of the
  drill stem to drive the sampling spoon through an
  interval
• Recovery - the percent of material covered from
  the volume estimated to occur within the sampling
  interval (e.g. 6" in 2' spoon 25)

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2 foot "split spoon"
"continuous" vibracore sample
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• Hollow Stem Auger
• drilling rig set up in suburban New Jersey
• routinely used for drilling and sampling in
  unconsolidated sediment
• geotechnical investigations
• environmental monitoring
• water table wells
• drillers are trained to log the stratigraphy with
  the Unified Soil Classification System

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"HAMMER"
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Boring Log

• limited information on sediment texture (USCS
  categories)
• assume the split spoon samples represent each
  interval penetrated
• little or no information on stratigraphic
  contacts, unconformities, etc...

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Graphic Log
(upper shoreface) Clastic Shallow Sea (lower
shoreface)

• log beds from base to top of outcrop
• show sediment texture by varying the width of the
  lithology drawings

Carbonate Ramp (limestone beds would have broken
shell, ooids)
Turbidite beds (Bouma sequence) Abyssal Plain
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Strata may "match" but not correlate
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Correlation

• procedure for demonstrating temporal
  correspondence between geographically separated
  parts of a geologic unit. NACOSN, p. 473
• correspondence in stratigraphic position, and
• "some defined property"
• lithocorrelation links by similar lithology
• biocorrelation links by similar fossil content
• chronocorrelation

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Hard Correlations are made when the same units
occur both above and below a contact. Soft
Correlations (dashed lines) when one unit pinches
out or occurs intermittently. Direct
Correlations are made by physically tracing a
contact in outcrop Most regional mapping relies
upon some Indirect Correlations
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Stratigraphy and Time

• Stratigraphers should make an effort to separate
  terms and judgments regarding
• Time (geochronology)
• Material Evidence (Rocks, Minerals, Fossils)

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Chronostratigraphic Terms

• System - equivalent of Period
• Series - equivalent of Epoch
• Stage - equivalent of Age