Page 238 - Laboratory Manual in Physical Geology

D
C
Inclusions
C
B

B
A
A

FIGURE 8.2 Law of superposition in horizontal strata. FIGURE 8.3 Inclusions on a disconformity. These strata
This is a sequence of strata that has maintained its original are all horizontal. Limestone A is older, because it is on the bottom
horizontality and does not seem to be disturbed. Therefore, Formation of a sequence of strata. Sandstone C is younger, because it is on top
A is the oldest, because it is on the bottom of a sedimentary sequence of the limestone and has inclusions (fragments of older rock) of the
of rocks. D is the youngest, because it is at the top of the sedimentary older limestone. Contact C is unconformable, because some of the
sequence. The sequence of events was deposition of A, B, C, and D, in limestone layers were eroded (making a gap in the rock layers) and
that order and stacked one atop the other. became inclusions in the overlying sandstone. An unconformity
between parallel strata, like this one, is a disconformity.

FIGURE 8.4 Angular unconformity. This is another sequence
of strata, some of which do not have their original horizontality.
Formation A is the oldest, because it is at the bottom of the
F E F sedimentary sequence. Formation F is youngest, because it forms the
top of the sequence. Tilting and erosion of the sequence occurred after
D but before deposition of Formation F. E is an angular unconformity.
D
The sequence of events began with deposition of A, B, C, and
D, in that order and stacked one atop the other. The sequence
C
of A–D was then tilted, and its top was eroded (E) . Siltstone F was
deposited horizontally on top of the erosional surface (E), which is
B
now an angular unconformity.
A

G
E
F
D
KEY TO SYMBOLS
H C
Sedimentary rocks B

A
Conglomerate Gravel Sandstone Siltstone

FIGURE 8.5 Law of cross-cutting. The body of igneous rock
Shale Clay Limestone Dolomite
H is the youngest rock unit, because it cuts across all of the others.
Igneous rocks Metamorphic rocks (When a narrow body of igneous rock cuts across strata in this way, it
is called a dike .) A is the oldest formation because it is at the bottom
of the sedimentary rock sequence that is cut by H. Folding and
erosion occurred after D was deposited, but before F was deposited. E
Granite Basalt Schist Gneiss
is an angular unconformity.
Other features The sequence of events began with deposition of formations
A through D in alphabetical order and one atop the other. That
sequence was folded, and the top of the fold was eroded. Formation
Zone of Unconformity Fault Contact F was deposited horizontally atop the folded sequence and the
contact
metamorphism erosional surface, which became angular unconformity E. G was
deposited atop F. Lastly, a magma intruded across all of the strata and
cooled to form basalt dike H.

210 ■ L ABOR ATORY 8