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LAB # 2 - STRATIGRAPHY
Stratigraphy is the study of rock layers, termed strata. Most stratified rocks are
sedimentary, although extrusive igneous, metamorphosed sedimentary and metamorphosed
volcanic rocks also commonly exhibit stratification. Stratigraphy involves three branches of
study; lithostratigraphy, biostratigraphy and chronostratigraphy.
Lithostratigraphy
Lithostratigraphy defines rock units on the basis of their physical, or lithologic, features.
Nicolas Steno, a Danish physician and scientist (1638-1686), founded the basic principles of
stratigraphy including the principle of superposition, the principle of original horizontality and
the principle of original lateral continuity. The Principle of Superposition states that in a
sequence of undisturbed strata the oldest layer is on the bottom, the youngest is on top. Note that
this assumes that the strata have not been overturned. If the strata have been deformed, there
may be recognizable sedimentary features that indicate the original position of the strata. The
Principle of Original Horizontality states that sedimentary layers are originally deposited
horizontally and parallel to the surface. Therefore, if sedimentary layers are in a non-horizontal
position they have been deformed. The Principle of Original Lateral Continuity observes that
strata extend in all directions and either thin or abruptly end at the edge of a sedimentary basin.
Thus, if portions of the originally continuous strata are eroded and no longer form a continuous
unit, they may still be correlated. Correlation is the connecting together of equivalent rock
units. This may be through comparison of lithologies (rock types), fossils, or facies
relationships (rocks deposited at the same time in adjacent depositional environments).
Charles Lyell, an English geology professor (1797-1875), further refined and defined
several stratigraphic principles. These include the principle of cross-cutting relationships and the
principle of inclusions. The Principle of Cross-cutting Relationships states that a geologic
feature that cuts across another rock is younger than the rock it cuts across. Cross-cutting
features often include igneous dikes, sedimentary structures and faults. Lyell also observed that
fragments within larger rock masses are older than the rock masses in which they are enclosed.
This is termed the Principle of Inclusions.
Lithostratigraphic units are bodies of rock that are distinguished and defined by their
lithologic (rock) characteristics and their stratigraphic position relative to other bodies of rock.
They are usually layered sedimentary rocks that are separated from other lithostratigraphic units
by contacts. Lithostratigraphic units include both formal and informal units. Formal rock unit
names are always capitalized, and are named with an appropriate local geographic term
combined with either a unit-term indicating its rank (Ex. = Group, Formation, Member) or a
simple field lithologic term indicating the predominant rock type (granite, schist, gneiss, chalk,
sandstone) or both. Formal rock units include (in ascending rank) members, formations,
subgroups, groups and supergroups. The basic mapping unit in lithostratigraphy is the
formation. In order to represent a valid formal unit, a formation must have mappability and
lithologic constancy. That is, they must be easily recognized for mapping (usually from aerial
photographs) and the rock types must represent logical subdivisions. As formal units, formations
are designated by local geographic names and capitalized. A member is a horizontal or vertical
subdivision of at least one formation. They must also have lithologic constancy, but members
typically are localized than formations and are mappable at smaller scales. A couple of other
rock units roughly equivalent to a member are lentils and tongues. A lentil (lens, lenticle) is an
isolated body of rock that terminates on all sides within a formation. A tongue extends outward
beyondthe main body of a formation. An assemblage of two or more successive formations is
termed a group. Groups may be related by lithology or by their position with reference to
unconformities. Groups may be divided into subgroups or combined to form supergroups.
Beds are the smallest rock-stratigraphic units. They are often informal and therefore their names
not capitalized. Beds may have economic significance (Coal bed, oil sand, etc.) or used in
mapping (key beds or marker beds).
Correlation is demonstrating the equivalency of stratigraphic units. A simple and accurate
method of correlation is to "walk out" a rock unit to determine its relationship to rocks in
surrounding geographic areas. Finding a rock unit that is of unique lithology, sedimentary
structures or fossils (a key bed) will certainly aid in correlation. If the stratigrapher has
difficulty correlating a unit, its position in the sequence may sometimes be utilized. For
example, the unit may be sandwiched between two marker beds, such as distinctive limestones,
tuffs, etc. Another possibility is that the unit lies between unconformities, stratigraphic
discontinuities that may be used to delineate the stratum. In situations where an outcrop is not
accessible, as in an oil well, subsurface methods of correlation are necessary. Obtaining
sections of rocks (cores) from the well or studying the small fragments (chips) removed from the
well and carried to the surface in the driller's "mud" will be useful for correlation. Microfossils,
such as foraminifera, radiolaria, pollen, spores and algae may be found in these samples and used
in correlation and paleoecologic interpretation. Logging techniques, such as use of electric logs
(where the rock's electrical conductivity is measured) or radioactive logs (in which the rock's
natural radioactivity or reaction to bombardment by neutrons is determined) may be run within
the well to determine the rock types present. This "doodlebugging" is also useful in determining
depositional environments and for correlation.
Biostratigraphy (Stratigraphic Paleontology)
Biostratigraphy utilizes fossils for correlation. It has traditionally provided the most
precise and accurate means of correlation. A biostratigraphic unit is a body of rocks delimited
from adjacent rocks by their fossil content. The basic unit of biostratigraphic classification is the
zone or biozone. These are defined by the distribution of index fossils, fossils characteristic of
key formations. The best index fossils should have lived during a short time span, had a wide
geographic range, be as independent as possible of facies (i.e. lived in a wide range of
environments), and they should have been abundant, rapidly changing and with distinctive
morphology (form) and therefore easily recognized. Pelagic organisms, either planktonic
(floating) or nektonic (swimming)] forms, are most often utilized as index fossils. Macrofossils
(large fossils) that have proven useful for correlation of marine units include trilobites (for the
Cambrian), graptolites (Ordovician-Silurian) and the ammonites (Upper Paleozoic, Mesozoic).
Most terrestrial correlations using macrofossils have involved studies of land mammals and
plants (Cenozoic). Microfossils have proven especially useful for biostratigraphic correlation, as
they are very abundant and may be recovered intact during well drilling, etc. The most important
microfossils include foraminiferans, radiolarians, palynomorphs (including organic-walled
microfossils such as pollen, spores, dinoflagellates, acritarchs), calcareous
nannoplankton/coccoliths, and conodonts.
The most common biozones used for stratigraphic correlation are taxon and concurrent
range zones, marked by the first and last appearance of a type or types of fossil. A taxon range
zone represents the total horizontal and vertical range of a single type of fossil; it is named after
the organism upon which it is based. A concurrent range zone is defined by the overlapping
ranges of specified fossil organisms; it is named after two or more diagnostic organisms present
within the particular biozone. Another common biozone used in correlation is the assemblage
zone, in which the age of the rock unit is determined by the suite of fossils it contains.
Chronostratigraphy
In chronostratigraphy rocks are divided, classified and correlated and geologic events
defined on the basis of time intervals that are isochronous (the same age everywhere) and time
planes, surfaces or boundaries that are synchronous (formed everywhere simultaneously).
Chronostratigraphy is utilized for rock correlation and for placing rocks of the Earth's crust in a
geochronologic sequence. A chronostratigraphic unit is a body of strata formed during a
specific time span. The highest ranking chronostratigraphic unit is the eonothem, which
includes the Proterozoic, Archean and Phanerozoic. A subdivision of an eonothem is termed an
erathem, which may be further subdivided into systems. Systems are the fundamental units of
world-wide time-stratigraphic classification. They are usually based on local sections and then
correlated world-wide on the basis of fossils. Examples of systems include the Cambrian,
Cretaceous, Jurassic, and Tertiary. Next in rank below the systems are series. Most of these are
of worldwide extent, others are provincial. Series are commonly known by geographic names
(Comanchean, Gulfian) or may divide systems into lower, middle and upper (Lower Cretaceous,
Upper Cretaceous). The basic working unit of intraregional and intra- or intercontinental
chronostratigraphy is the stage. Stages are often groupings of biozones, and are often of
worldwide extent. Stages are often divided into substages.
The time, or geochronologic equivalent of an eonathem is the eon. The equivalent of an
erathem is the era, systems correspond to periods, series to epochs, and stages to ages. Lab
students are often confused in differentiating chronostratigraphic units from geochronologic
units. On field trips, we visit the Cretaceous System; we do not visit the Cretaceous Period. If
we visited the Cretaceous Period we would need a time machine and we could get eaten by those
mean theropod dinosaurs! See the difference between a system and a period? For this course,
you will be expected to learn the eons, eras, periods and several epochs of Earth history.
Absolute Dating Techniques
Absolute, or actual dating techniques establish geologic time in terms of years. One of
the most common absolute dating techniques is radiometric, or radioactive dating. This utilizes
rates of change from one form of an element (isotope) to another to establish the age of rocks.
Depending upon the type of dating technique, the absolute date of a geologic object or stratum
may be dated to within hundreds, thousands or millions of years. The radioactive dates have a
standard notation which may be summarized as follows:
Kiloannum (plural = Kiloannum; kilo an) = Ka = thousands of years in the radioisotopic time
scale
Megannum (plural = Meganna; mega an) = Ma = millions of years in the radioisotopic time
scale; M.Y. (or m.y) = millions of years, without reference to the radioisotopic time scale
Gigannum (plural = Giganna; giga an) = Ga = billions of years in the radioisotopic time scale
Another common dating technique is geomagnetic or paleomagnetic dating. The
intensity and magnetic polarity of the Earth's magnetic field changes, and these changes are
recorded in iron-rich rocks. As iron is a common component in many rock types, paleomagnetic
dating has good potential for dating many types of rocks. Periods of normal and reversed
polarity are dated by independent means, often by radiometric techniques, and then these polarity
events are used for dating rocks worldwide. Magnetostratigraphy is used in correlation,
geochronology, determination of paleoclimatology, definition of stratigraphic boundaries, and
for plotting movement of major plates or microcontinents.
Lab Exercise - Sequencing Geologic Events
For each of the following sections, number the units from oldest (# 1) to youngest. Then, to
the right of the illustrations, sequence the geologic events and state the reasons for your
diagnoses utilizing the principles of superposition, original horizontality, cross-cutting
relationships, etc. Also, list the unconformity types present within the sections.