Page 237 - Laboratory Manual in Physical Geology

Disconformity Angular unconformity Nonconformity
In a succession of rock layers (sedimentary An angular unconformity is an erosional A nonconformity is an erosional surface
strata or lava flows) parallel to one another, surface between two bodies of layered between older igneous and/or metamorphic
the disconformity surface is a gap in the sedimentary strata or lava flows that are not rocks and younger rock layers (sedimentary
layering. The gap may be a non-depositional parallel. The gap is because the older body strata or lava flows). The gap is because
surface where some layers never formed for of layered rock was tilted and partly eroded some of the older igneous and/or meta-
a while, or the gap may be an erosional (rock was removed) before a younger body morphic rocks were partly eroded (rock was
surface where some layers were removed of horizontal rock layers covered the eroded removed) before the younger rock layers
before younger layers covered up the surface. surface. covered the eroded surface.

FIGURE 8.1 Three kinds of unconformities. Unconformities are surfaces that represent gaps (missing layers) in the geologic
record; analogous to a gap (place where pages are missing) in a book. Red arrows point to the unconformity surface (bold black line) in
each block diagram.

Laws for Determining Relative Age Unconformities
Geologists use six basic laws for determining relative age Surfaces called unconformities represent gaps in the
relationships among bodies of rock based on their physical geologic record that formed wherever layers were not
relationships. They are as follows: deposited for a time or else layers were removed by erosion .
Most contacts between adjacent strata or formations are
■ Law of Original Horizontality — Sedimentary layers conformities, meaning that rocks on both sides of them

( strata ) and lava flows were originally deposited as rela- formed at about the same time. An unconformity is a rock
tively horizontal sheets, like a layer cake. If they are no surface that represents a gap in the geologic record. It is
longer horizontal or flat, it is because they have been like the place where pages are missing from a book. An
displaced by subsequent movements of Earth’s crust.
unconformity can be a buried surface where there was a
■ Law of Lateral Continuity — Lava flows and strata pause in sedimentation, a time between two lava flows,
extend laterally in all directions until they thin to nothing or a surface that was eroded before more sediment was
(pinch out) or reach the edge of their basin of deposition . deposited on top of it.
There are three kinds ( FIGURE 8.1 ). A disconformity is
■ Law of Superposition — In an undisturbed sequence of an unconformity between parallel strata or lava flows. Most

strata or lava flows, the oldest layer is at the bottom of the disconformities are very irregular surfaces, and pieces of
sequence and the youngest is at the top .
the underlying rock are often included in the strata above
■ Law of Inclusions — Any piece of rock (clast) that has them. An angular unconformity is an unconformity
become included in another rock or body of sediment between two sets of strata that are not parallel to one
must be older than the rock or sediment into which it another. It forms when new horizontal layers cover up
has been incorporated . Such a clast (usually a rock older layers folded by mountain-building processes and
fragment, crystal, or fossil) is called an inclusion . eroded down to a nearly level surface. A nonconformity is

The surrounding body of rock is called the matrix an unconformity between younger sedimentary rocks and
(or groundmass). Thus, an inclusion is older than its subjacent metamorphic or igneous rocks. It forms when
surrounding matrix. stratified sedimentary rocks or lava flows are deposited on
eroded igneous or metamorphic rocks.
■ Law of Cross Cutting — Any feature that cuts across a
rock or body of sediment must be younger than the rock or Relative Age Dating Examples
sediment that it cuts across . Such cross cutting features
include fractures (cracks in rock), faults (fractures Analyze and evaluate FIGURES 8.2 – 8.9 to learn how
along which movement has occurred), or masses of the above laws of relative age dating are applied in
magma ( igneous intrusions ) that cut across preexist- cross sections of Earth’s crust. These are the kinds of
ing rocks before they cooled. When a body of magma two-dimensional cross sections of Earth’s crust that
intrudes preexisting rocks, a narrow zone of contact are exposed in road cuts, quarry walls, and mountain
metamorphism usually forms in the preexisting rocks sides. Be sure that you consider all of these examples
adjacent to the intrusion. before proceeding .

Dating of Rocks, Fossils, and Geologic Events ■ 209