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Terri Mathews




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Geology 112




















Correlation 

Learning Objectives


Rock Units (Lithostratigraphic Unit): bodies of rock identified by distinctive lithologic and
structural features without regard to time.

Formation: a formation is a mappable rock unit that is lithologically distinctive, has
recognizable upper and lower contacts and can be traced over large distances.   Formations do
not always have the same age throughout the unit.  Formations are named for their lithology and
their locale: St. Louis Limestone is a limestone that can be recognized in St. Louis.

Member: a distinctive, recognizable area within a formation that may or may not be present
throughout the formation.

Group: a series of formations that are related, example: transgressive sequence.
 

Facies: the term facies relates a rock unit or a member of a rock unit to a particular environment.
It is based on characteristics found within a rock that relate it to a particular environment.

Lithofacies: if the characteristics are based on lithology.

Biofacies: if the characteristics are based on fossils within the rock.

Facies change: a change in environment laterally.  This is an important concept.  Imagine the
area around Sandbridge and Pungo.  Stream drained upland (Pungo) slopes down to marshy
lowlands (shore of Back Bay), Back Bay is a restricted tidal lagoon, this gives way to Sandbridge
which is  a barrier bar.  Four distinct environments with four distinct types of sedimentation.
Each change in successive environment represents  a facies change. Deposition in each of these
four environments will continue as long as the environment remains the same. Eventually when
this sediment lithifies and becomes rock there will be four distinct rock types blending into one
another.  Each distinct rock type represents a facies because each rock possesses characteristics
that represent a particular environment.  Because these four facies all existed at the same time
they are considered synchronous facies.
 

The Earth changes, these changes are slow and we may not notice them.  As the Earth changes
environments change.
 
One of the most noticeable changes in the geologic record is sea level change.  Over geologic time
sea level has changed many times.

Eustatic Sea Level Change: sea level changes that occur world wide.  Caused by:
 

    1) Glacial activity: when more water is locked into glacial ice sea level is lower, when glaciers
    melt sea level rises.

    2) Sea Floor Spreading Rates: during active periods of sea floor spreading large undersea ridges
    form that act to push sea level higher.  When sea floor spreading slows down sea level drops.
     

    3) Climate change: Temperature on Earth has not been constant, it has been warmer than at
    present and colder.  During warm periods ocean water heats and expands rising sea level.
     

Transgressions (rising sea level) have created large shallow inland seas (epeiric seas) during the
geologic past.  Regression refers to the retreat or lowering of sea level.

**** The book refers to transgression/regression as onlap and offlap.

As sea level changes environments shift.  A shoreline consists of beach sand grading out to
nearshore silts and clays, finally grading into offshore carbonates.  When these sediments lithify
they consist of sandstone, shale limestone.

As sea level changes the beach migrates (the environment of deposition migrates); resulting in an
extensive deposition of sand (sandstone) that is not synchronous throughout its extent.  In fact a
larger sandstone formation may differ in age from one end to the other by millions of years.

Vertically transgressions and regressions are very distinctive in the rock record.

Transgression:

 limestone
 shale
 sandstone

Regression:

 sandstone
 shale
 limestone
 

Correlation:

Rock units can be traced from one locale to another.  Using natural exposures, roadcuts and well
samples geologists can trace the extent of a rock unit under the surface.  Geologists examine
rocks collected from two different locales to determine if rock units are continuous.

Geologic Columns: a slice at one locale through the Earth showing the arrangement of rock
formations from the surface downward.  Unless there is evidence that the units are overturned it
is assumed that the lower formations are oldest.  Data for geologic columns comes from drilling
or surface exposure.   In the subsurface time increases with depth.

Geologic Cross Sections: several geologic columns are compared to prepare a continuous view
of the subsurface.  Cross sections show the underground extent of rock formations.  In the
subsurface the lateral extent of a formation represents a continuous facies but not necessarily
synchronous time.
 

Lithostratigraphic correlation: correlating rocks based on lithology.
 

Biostratigraphic Correlation: correlating rocks based on the fossils they contain.
 

Unconformities:

The rock record is not always continuous temporally.  Sometimes rocks are missing from the
record due to erosion of formations or nondeposition.  This gap in the rock record represents
missing time.
 

  • Angular Unconformity:  Tilted rocks overlain by horizontal rock units.
  • Nonconformity: Igneous or metamorphic rocks overlain by horizontal rock units.
  • Disconformity: Horizontal rock units overlain by horizontal rock units.
 
Maps:

A variety of map types are used by geologists to help them interpret Earth's historical record.

Contour Maps: show surface topography through lines of equal elevation.

Geologic Maps: show the distribution of rock units (and their ages) that lie just below the loose
soil and vegetation covering of an area.  They also show the structure present: strike and dip,
faults.

Paleogeographic Maps: shows the location of geographic features (oceans, mountains ect.) Of
an area during a previous time.

Isopachous Map: similar to a contour map but shows the thicknesses of formations.  Important
for interpretation of depositional basins.

Lithofacies Map: shows the distribution of rock facies in an area, often added to
paleogeographic maps, shows subsurface facies.