1. A geologic record exists with erosional, depositional, and deformational
processes, but a complete record does not exist. In order for a complete geologic
record to exist, a basin would have had to have been continuously receiving
sediment since the beginning of the earth, uninterrupted, and there is no where
that this occurs. The geologic record tells the ‘story’ of the earth’s history and
includes not only the depositional periods, but from erosional surfaces certain
processes can be inferred and from deformation such as folding and faulting the
‘story’ of plate motion can be inferred. One reason the complete record does not
exist, besides the fact that there isn’t a basin that has received sediment
uninterrupted, is that the layers of rock are exposed to processes such as
weathering and erosion, and destruction from plate tectonic processes such as
convergence.
2. There are six relative dating principles and they include: the law of superposition,
the principle of original horizontality, the principle of lateral continuity, the law of
cross-cutting relationships, the principle of inclusions, and the principle of faunal
succession. The strengths of the principle of faunal succession is that rocks of
similar age can be matched up on a large regional scale by using index fossils.
The strengths of the principles of original horizontality and superposition are that
this gives a ‘starting point’ where the sediments were deposited in a flat layer
where the oldest layer is on the bottom. In addition, if rocks in two different areas
are determined to be the same age, the ages of the beds above and below it must
be the same. These same principles also come into play for the identification of
key bed like volcanic ash that is spread across a wide area, in one layer,
horizontally.
3. Starting from the bottom with the oldest events:
a. The sandstone and limestone (sedimentary units) were deposited. Older
than 400 m.y.
b. The sedimentary units were folded.
c. The batholith intruded 400 m.y.
d. Erosion. Angular disconformity.
e. Deposition of shale bed with igneous inclusions.
f. Dike intrusion 100 m.y.
g. Erosion. Disconformity.
h. Deposition of sandstone. 75 m.y.
i. Sill deposited. 25 m.y.
j. Deposition of top layer with contemporary erosion.
The figure illustrates a basin where sedimentary units were deposited first. Sandstone
and then limestone were deposited more than 400 million years ago and then went
through a period of deformation where the beds were folded. Then the igneous batholith
intruded 400 million years ago and resulted in contact metamorphism between the
batholith and the sedimentary units. There was then a period of uplift and erosion that
resulted in an angular disconformity where the dip of the lower units does not match the
horizontal units above the surface of erosion. Inclusions from the batholith are found in
the shale bed deposited above the disconformity. Following the deposition of the shale
bed, an igneous dike intruded 100 million years ago.
Following the intrusion of the dike, there was another period of uplift and erosion. This
is recognized by the abrupt termination of the dike. After the sandstone layer was
deposited (100-25 million years ago), a sill intruded 25 million years ago. The uppermost
unit is a sedimentary layer that has undergone uplift and is displaying contemporary
erosion.