Page and figure numbers refer to the textbook: Stanley, Earth System History
formations (p. 161)
Particular rock units, especially sedimentary rocks, can often be
recognized over large areas. The smallest scale rock unit or
sedimentary unit that can be mapped is called a
formation. The formation is the basic division for
identifying and correlating sedimentary strata. A formation may
consist almost entirely of one rock type, for example limestone, or
may be composed of multiple rock types, for example limestone, shale,
and sandstone, that all formed in one related environment, such as a
coastal environment with rising and falling sea level. Formations can
be subdivided into members which are a set of
recognizable strata within a formation that are found in only a
portion of the areal extent of the formation. The least subdivision
of a formation is the bed which is recognized by visual
changes in color, grain size, or composition. Formations vary in
thickness from a few meters to a few thousand meters. Beds vary from
a few millimeters to a few meters in thickness. In some cases,
adjacent formations may be combined into a group of
related formations, all having been deposited in similar depositional
environments.
changes of sedimentation resulting from
changes in sea level
Changing sea level exerts a strong influence on sedimentation. In
the coastal environment rising sea level causes a
transgression or shoreward migration of the coastline
(Fig. 6-6). The beach moves inland. Clays are deposited on top
of sands that were deposited in the former beach and nearshore
environment. In warm, clear water, limestone may be deposited on top
of shales. A sea level drop causes a regression or oceanward
migration of the shoreline. Coarser sediments are deposited on top of
fine as water depth decreases. Sea level also affects terrestrial
deposition.
correlation (p. 156-163)
Correlation trying to fit together sedimentary strata found in
different places. Geologists try to determine the relative age of
widely separated strata. They are especially interested in
determining layers in separate strata that are exactly the same age.
This task is complicated as distance increases and sedimentary
environments change. One method for correlation is looking for
similarity of rock type and characteristics (grain
size, composition, cements, sorting and rounding) (p. 156).
Since there are a multitude of similar looking sandstones, for
example, more information than that is usually needed. An important
tool is to compare the sequence of beds (Fig.
6-15). As sea level rises and falls, or as the climate changes in
a region, all deposited strata will experience the same changes of
conditions. All might record a transgression followed by a
regression. This sequence allows for confident correlation. Some
strata may contain very distinct, unusual layers. These can serve as
helpful marker beds (p. 170, Fig. 6-13, 6-14).
Fossils are very important for doing long distance,
even global, correlation of sedimentary strata, especially if the
strata contain fossils of organisms that had wide geographic ranges
(p. 154, 157). Certain geophysical methods are also very
useful for correlation. Various electrical methods (see
lab) can be used to characterize sedimentary units in a well for
which lithologic information is unavailable (oil drillers normally
don't core a well and bring up pieces of the strata intact because
that is very expensive; it is easier to grind the rock and flush it
out).
Alongside biostratigraphy using fossils, magnetostratigraphy (p. 158) is extremely important for correlating strata from distant continents and to the standard timescale. Everyone knows that the Earth has a magnetic field. Most rocks contain small amounts of magnetic minerals such as magnetite and hematite. The magnetism in these magnetic mineral grains are permanently aligned with the Earth's magnetic field at the time the rock forms. The Earth's magnetic field has reversed at odd intervals throughout geologic time (Fig. 6-4). Sequences of sedimentary strata record the reversals of the Earth's magnetic field. Since all points on the Earth's surface experience a field reversal at the same time, a particular reversal recorded in sediments on opposite sides of the globe allows an easy exact correlation. Biostratigraphy and magnetostratigraphy are commonly used together to forge the best correlation possible.
unconformities (p. 10, 172, Fig.
1-10, 1-11)
Unconformities are erosional surfaces within a sedimentary
sequence. They represent missing time in the rock record. There are
three kinds of unconformities. In nonconformities,
sedimentary strata lie above plutonic igneous rocks or metamorphic
rocks. Since the crystalline rock form at depth in the Earth's crust
and sedimentary rocks are deposited at the Earth's surface, erosion
must have removed a great thickness of rock from above the
crystalline rock before the sediments were deposited. At
angular unconformities, non-parallel sets of
sedimentary strata meet. One set of strata (the upper) cuts across
the ends of the tilted sedimentary strata. Erosion must have occurred
to expose these "ends" because sediments are deposited as continuous,
horizontal layers. Disconformities are subtle time gap
between parallel sedimentary strata. They may be recognized by
recognizing that certain fossils are missing from the expected fossil
succession known from other locations, a large-scale hill and valley
topography on the unconformity, a basal conglomerate in the unit
above the unconformity containing clasts (inclusions) of the unit
from below the unconformity, or unrelated strata superimposed, for
example sandstone directly above limestone (shale would normally be
deposited between these two as sea level fell - Walther's Law
[p. 134]).
unconformity-bounded sequences (p.
173-177, Fig. 6-22)
Most unconformities are local, resulting from changing
sedimentation, local tectonic/structural events, etc. Some, however
are global events caused by sea level fall. Major stratigraphic
sequences on the continents have resulted from major transgressions
that flooded the continents, yielding thick marine strata. These
major sequences are separated from one another by major
unconformities resulting from sea level fall.