Geology - Stratigraphy and Geologic Time
Geology 101 - Gale Martin - Class Notes
Studying individual rocks results in limited information: it's relationship to other rocks is far more important to understanding tectonic events. Rock outcrops contain information about the sequence of events evolved with its origin and the conditions under which the rocks formed. With some basic principles and assumptions, rocks can be used to interpret geologic history.
Geologic Principles and Relative Age Dating
Tectonic events are interpreted on a global scale, yet rock outcrops vary significantly even within a single valley. Stratigraphy is used to study, interpret and correlate the relationships between the outcrops to understand how they fit into "the big picture". The following basic principles of geologic processes, are used to decipher the relative ages of rocks.
Suppose a geologist watches a volcanic eruption and proceeds to sample the material created. Driving down the road in some distant land, he/she observes an outcrop of the same rock. The geologist will conclude that the rock was formed in the same type of volcanic eruption. That geologist is using the principle of uniformitarianism. Uniformitarianism assumes that geologic processes that are currently acting on the earth behaved the same way during previous geologic events. (There is no assumption of rate or intensity.) You can determine how a rock was formed by observing current erosion/tectonism and applying it to ancient rocks.
At an outcrop, there are several principles which can be used to understand the sequence in which the rocks formed. Ask yourself: Are the rocks in flat lying beds? Rock material is naturally deposited in horizontal layers (principle of original horizontality). If the rocks are tilted or folded, a tectonic event has moved the layers. Which rock is older: the top or bottom? Loose rock material is deposited on top of older existing rock (principle of superposition). Unless the rock is overturned (i.e. tectonics) the younger rock in a sequence is at the top and older material at the base. Is there evidence of tectonic/igneous activity? Any activity that will deform or displace a rock must be older than the rock it affected (cross-cutting relationships). Such activities include igneous intrusions (with contact metamorphism), folding, faults, weathering and metamorphic activity. Using these principles you can determine a chronological sequence of events for a single outcrop of rock.
Knowing their relative ages, however, does not give the relationship of this outcrop to the rocks further down the road or in another country. This outcrop must be correlated to other outcrops using additional basic principles. The easiest method would be to follow one of the formations to the new outcrop (physical continuity). Tracing a physical characteristic of a rock over great distances, either above ground or in the subsurface, is limited by the range/shape of the original environment (How big is a coastline? swamp? or lava flow?). Often you must refer to the similarity of rock types. Observe the facies: what distinguishing features make it unique? Note the sequence of rock types in the rock column: are they the same in different localities? Repeated assemblages of rocks in relatively close areas are probably developed under the exact same conditions.
The most dependable means is to use any fossils present in sedimentary rock. Fossils can be used in a number of ways. They are often good indicators of the environment of deposition. (Horses are not found in oceans and coral in arctic tundra!) This is especially true of fossil assemblages. One fossil may have been misplaced (washed in, moved by scavengers, etc.) but a group of similar individuals will usually result in a better interpretation. Index fossils, fossils limited in their time span of existence but extensive in their distribution, are commonly used to denote specific age dates. (T. Rex is an example: they existed in the Cretaceous period (brief) over large areas (N. America with relatives in other continents). Same holds true for Triceratops. So why did Crichton call it "Jurassic Park"?!?) Fossil occurrences and patterns of development are also consistent through time (faunal succession) and can be used to compare fossils between outcrops.
Using these basic geologic principles, rocks throughout the world have been correlated to produce a geologic time scale with a stratigraphic rock column for specific areas and different geologic ages. Keep in mind that not all areas have rock available to represent all time units. Erosion and tectonics (i.e. the rock cycle) will destroy some of the rock and create "gaps" in the rock column for that area. Such unconformities are common. (Many of the arguments that exist in geology ("missing links", origin of the earth, etc.) are due to the missing "evidence" in the rock "record".)
Geologic Time Scale
The geologic time scale is a chart used to represent the chronological sequence of events in geologic history. It is divided into various lengths of time based on the fossils in correlated rock units. Refer to your text for the chart. Note that the largest time division is eons. The oldest time unit, Precambrian, was originally referred to as theProterozoic (proteros - fore; zoe - life) and represented an ancient time when (it was believed) no fossils existed (this no longer holds true). The Precambrian is divided into smaller units (save this for historical geology). Phanerozoic (phaneros - evident) follows the Proterozoic and is the youngest eon. The rocks that represent this eon contain fossils with "hard parts" (shells, teeth, bones, etc.; they're easier to preserve in rock). Though a relatively short span of time (compared to the Precambrian) it contains the most extensive record of the earth's history (less time to destroy the evidence!).
The Phanerozoic is divided into three eras. Each era is represents a significant change in the fossil rock record usually bounded by a major extinction event. They are from oldest to youngest: Paleozoic (palaios - ancient), the "Age of Invertebrates, Fish and Amphibians"; Mesozoic (mesos - middle), the "Age of Reptiles"; and Cenozoic (kainos - new, recent), the "Age of Mammals".
Each era is divided further into smaller units known as geologic periods. The periods are named after specifi course usually does not refer to them (historical geology does).
Radiometric Dating and Absolute Ages
The principles of relative age dating result in a chronological sequence of geologic events but can not determine when that event occurred. Radiometric dating is used to determine the age of a rock. Many rocks contain radioactive elements incorporated into their minerals. These isotopes (let's thick of them as unstable elements) naturally break down through a process know as radioactive decay. During decay, an isotope is altered by chemical/physical means which gives off subatomic particles. (Historical geology covers the processes in more detail.) Each type of radioactive isotope produces a unique "daughter element" that accumulates in the rock (this may take several steps). This occurs at a predictable rate; measured in terms of an isotope's half-life. (See text for common half-lifes.) Because the decay rate for each isotope is unique and constant, it can be used to measure the age of rocks.
Radiometric dating has its limitation. Radiometric "clocks" are usually "set" by metamorphic and igneous events. To date sedimentary rocks and the fossils they contain (remember they are the basis of the geologic time scale), the ages must be bracketed between tectonic events with measured radiometric dates. For example, if you find a bone in a volcanic ash flow, you can date that bone using the igneous rock. After the fossil's age has been determined it can then be used to suggest dates for other rocks that contain that exact fossil. You might also be able to determine approximate dates for rocks immediately above or below the flow (assuming no unconformities!) or for rocks between two volcanic ash flows.
Through years of research, radiometric age dates have been applied to the geologic time scale. Absolute ages (in millions of years before present (mybp)) are commonly given for fossil, tectonic and erosional events. Refer to the text for important radiometric dates. Keep in mind that the interpretation of geologic history has been greatly modified over the centuries as better technology and new data (fossils, stratigraphy and absolute ages of rocks) brings new ideas and new theories.