Geol 4110
Class 7 – Relative Age Dating and More Geologic Maps
3/1/07
3/8/07
In Class
- Overview of relative and absolute age dating of geologic events
- Geopuzzle exercise
- In class/homework exercise “virtual geologic mapping”
Primer on Relative Geologic Time
" All we need but do is call on unlimited drafts upon the bank of antiquity"
George Scrope, 1827
Relative Time Scales
• Principle of Superposition- Successively higher strata is progressively younger.
• Principle of Faunal Assemblages - rocks containing the same assemblage of organisms are
the same age.
• Principle of Faunal Succession - Populations of organisms always change and once
changed, will not reappear in their previous form.
Index Fossils - Widespread distribution
Restricted, known time of existence
Common to many environments
• Principle of Cross-Cutting Relations - Igneous rocks that cut across other rocks are younger
than the rocks they cut.
Global Time Markers
• Magnetic reversals - the polarity of the earth's magnetic field commonly reverses itself
which affect the alignment of magnetic mineral in many rock, especially crystallizing
igneous rocks.
• Explosive volcanic eruptions - Large-scale volcanic eruptions commonly spew ash into the
upper atmosphere where it then gets distributed globally, settles slowly to the surface
and becomes part of the rock record as a ash layer.
• Large meteorite impacts - Impacts of large (>200 m) meteorites generate the same
distribution of fine particulate matter into the atmosphere. Such a event can be
recognized by the unique compositions of meteoritic ash (e.g. Iridium)
• Climatic change - Changes in global climate which causes changes in the amount of water
locked up in glacial ice is recorded in the isotopic composition of water and ice. Ice
stores up heavy oxygen O18 thus causing the oceans to become depleted in O18
relative to O16 during times of global cooling. Limestone deposited from seawater
will reflect this change in seawater composition
Gaps in the Timescale
Unconformity - a substantial break in the stratigraphic section marking either a hiatus in
deposition or a period of erosion. Recognize three types:
1. Angular Unconformity - a surface separating the older strata below that were obviously
deformed then eroded before the younger stata above was deposited
2. Disconformity - irregular surface of erosion between two similar and parallel rock strata
3. Nonconformity - a surface separating strata of distinctly different origins
In Class/Homework Exercise - Virtual Geologic Mapping
This exercise is a modification of the MMEW exercise “Understanding and Making Bedrock
Geologic Maps”, which was handed out last week in class. The purpose of this exercise is to give
students a sense of what information geologists collect from field observations, how those
observations are recorded on base maps, what interpretations are then involved in creating a
geologic map from those observations, and what story can be read from the geologic map.
Procedure:
Choose to work on one of the four geologic field area (A, B, C, or D)
1) OUTCROPS - As accurately as possible, record outcrop locations and shapes to your gridded
base map using specific colors for specific rock types (though they need not be the same
color as the outcrops). Darkly color your outcrops or outline them in pencil.
2) STRUCTURE – Show the approximate strike and dip (direction and approx. value) of planar
geologic structures at at least five locations on your map. Use the following symbols for the
two main types of structures
45º
45º
Sedimentary
Bedding
Igneous Contact
- Inclined
- Vertical
3) GEOLOGIC CONTACTS AND UNITS- Keying off outcrops with exposed contacts, infer
where these contacts project through your map area by making dashed lines with a #2 pencil.
Use the strike and dip measurements to guide you in how to trend your contact lines (on the
horizontal surface of your map, geologic contact lines should approximately parallel the strike
of your measurements). If you find that the outcrop geology requires that the contact curves in
violation of the structure measurements, this may imply that a fault has offset the contact.
Another clue to the existence of faults is that rivers tend to follow these zones of weakness. In
the end, all outcrops of a certain rock type should be enclosed by geologic contacts or the
margins of the map. Using the same colors as the outcrops, lightly shade the map units defined
by the geologic contacts.
4) MAP LEGEND - Determine the number and relative ages of your geologic map units. On the
map legend sheet, color and label them in stratigraphic order (i.e., oldest at the bottom,
youngest at the top. Give them unique names if you like (e.g. the Bizzaro Formation), but
indicate the major rock type that composes it. CAUTION – realize that it is possible to find
the same rock type forming geologic units of different age; not all sandstones formed at the
same time in earth history. Units separated by and unconformity, should be noted by a zigzag line between the map unit boxes. Also on the legend, fill out the geologic structure and
contact symbols appropriate for your map.
5) CROSS SECTION – Trace a line across your map area that generally traverses a right angle to
the strike of your geologic units. On graph paper, construct a geologic cross section of that
traverse. Using the dip measurements you measured, project your units into the subsurface at
least two reallyfars deep (~2”). See the figure in the MMEW handout.
6) GEOLOGIC STORY – In an outline form, tell the geologic story of your area. Include
depositional events, deformational events, and erosional events