GEOL_0800_Geologic_Time - Copy

GEOL_0800_Geologic_Time - Copy - Geologic Time Absolute and...

Info iconThis preview shows pages 1–11. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Geologic Time Absolute and Relative Dating 2 Two ways to measure time: Relative ages establish the order in which events occurred, but do not give numerical dates. Absolute ages fix numerical dates on events. Absolute dates obviously also establish relative ages, but as you’ll see, relative ages are fast, easy, cheap, and abundant! 3 Where are the oldest beds? This question considers the idea of superpositioning ! Canyonlands, UT 4 2. Original Horizontality 2. The Principle of Original Horizontality: Gravity causes sediments to accumulate in generally horizontal layers. Photo shows horizontal sheets of beach sands exposed at low tide (Australia). 5 Horizontality seems trivial, but it’s good to be clear in our assumptions: If the beds are not horizontal, we may generally infer that some sort of geologic event tilted the beds from their originally horizontal position. Haymond Formation, Marathon Basin, west Texas 6 3. Lateral continuity 3. The Principle of Lateral Continuity: Sedimentary layers generally accumulate in sheets that cover broad areas. They tend to gradually change in thickness and end only when they hit the edge of a basin or as the rock type (facies) gradually changes (e.g., from beach sand to shelf shale). For some reason, facies shifts are not shown. 7 Allows us to trace distinctive rock layers over considerable distances and to recognize faults as faults(because faults cause beds to abruptly end). Gooseneck along the San Juan River, UT Lateral continuity 8 4. The Principle of Cross-Cutting Relations: If one geologic feature cuts across another, the cut feature is older (i.e. it had to exist first in order to be cut). In the example above, the sedimentary layers must be older than the dike because they had to exist in order for the dike to have cut through them. 4. Cross-Cutting Relations 9 Sedimentary beds were (1) originally horizontal and (2) originally laterally continuous. (3) Oldest beds are on bottom. (4) Faults younger than beds Four principles: 10 5. The Principle of Inclusion: If a body of rock contains fragments of another rock, the included rock fragments must be older than the enclosing rock body. (The fragments had to exist first in order to get incorporated.) The sill and the lava flow look very similar. Note how the principle of inclusion here overrides the principle of superposition when it comes to relative age of the sill....
View Full Document

This note was uploaded on 02/26/2012 for the course GEOL 0800 taught by Professor Charlesjones during the Fall '08 term at Pittsburgh.

Page1 / 49

GEOL_0800_Geologic_Time - Copy - Geologic Time Absolute and...

This preview shows document pages 1 - 11. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online