Part B
Relative Geologic Dating Principles
To understand the geologic history from a map or cross section, we must determine the age of rock units, geologic structures, and geologic events. Ages can also be absolute, where a certain number value represents the age (e.g., the age of the Cardenas Lavas in the eastern Grand Canyon is 1,103 Ma). Absolute ages are determined by a number of different methods that typically involve measurement of radioactive isotopes present in certain minerals. Ages can also be given in a relative order, where the number age is not determined (e.g., the Cardenas Lavas are older than the Tapeats Sandstone). A few important relative dating principles are critical for interpreting geologic cross sections, including Superposition, Original Horizontality, Cross-cutting Relations, and Inclusions. Relative and absolute age dating can be used together to create an ordered history of geologic events that may or may not have precise number dates attached to it.
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Figure 3-4. Apollo Temple and the Basalt Cliffs in the eastern Grand Canyon. The Dox Formation forms the lower slopes, the Cardenas Basalt the dark gray slopes, followed by the the Nankoweap and Galeros Formations, with the Tapeats Sandstone forming the overlying cliff. Redwall Limestone forms the upper part of the temple. |
Superposition
Sedimentary and volcanic layers are originally deposited at the surface on top of one another (Figure 3-5), so that the rocks on the bottom of a sequence are always the older than those at the top of the sequence. This also applies to tilted layers as long as the sequence has not been overturned.
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Figure 3-5. A cross section showing the relative dating principle of Superposition and Original Horizontality. Units and structures with lower numbers are older than units with higher numbers. |
Original Horizontality
Sedimentary and volcanic units are also typically deposited on the Earth's surface in horizontal or nearly horizontal layers (see Figure 3-5). Thus, if these rocks are tilted or dipping, then the layers must have been disturbed by fault rotation or folding sometime after the rocks were deposited. Figure 3-6 shows a view of the relatively flat-lying sedimentary layers in the Grand Canyon.
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Figure 3-6. Field relations showing the relative dating principle of Superposition and Original Horizontality in the Grand Canyon. View is to the south from the North Rim across Bright Angel Canyon of Brahma Temple (left) and Zoroaster Temple (right). The light-colored cliffs are mostly Coconino Sandstone, whereas the reddish rocks below belong to the Hermit Shale and Supai Groups. Kendrick Mountain is on the skyline. Click on the picture to see the annotated image. |
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Example 3 |
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Which one of the labeled sedimentary units is the youngest? |
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Pt (Toroweap Formation) The Toroweap Formation overlies the others, and the Principle of Superposition says that younger sedimentary layers overlie older sedimentary layers. |
Cross-cutting Relations
Where a fault cuts across other rock layers, the fault is younger than the layers it cuts (see Figures 3-7 and 3-8). Additionally, where an igneous intrusion cuts across or bakes other rock units, the intrusion is younger than the units it cuts across or bakes. In other words, the older rocks need to exist before they can be faulted, intruded, or baked.
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Figure 3-7. A cross section displaying the relative dating principles of cross-cutting relations and inclusions. Units and structures with lower numbers are older than units with higher numbers. |
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Figure 3-8. Field relations showing the relative dating principle of cross-cutting relations in Tuna Creek, Colorado River mile 99.1, Grand Canyon. On the left, the light-colored granite intrusion is offset by a small fault. The fault that must have occurred after the granite intrusion had cooled enough to respond to the stress by breaking (brittle deformation) instead of folding (ductile deformation). On the right, a light-colored intrusion and a quartz vein crosscut the darker diorite host rock. Click on the pictures to see the annotated images. |
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Example 4 |
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True or False: The thin pink intrusion (quartz vein) F is younger than the thin gray granodiorite intrusion G. |
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True |
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Example 5 |
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True or False: the fault is younger than the light-colored volcanic tuff. |
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True The fault crosscuts the light-colored tuff, and the Principle of Cross-cutting Relations says that anything that cuts across another feature is younger than whatever it cuts. |
Inclusions
Inclusions (pieces of another rock) within a larger host rock are older than the host rock. So, the inclusions must have formed first and then were later included within the host rock. Igneous intrusions may contain pieces of older wall rock that become inclusions in the magma (see Figures 3-9 and 3-10). When the inclusions are a different rock than the host, they are called xenoliths ("foreign rock"). Many sedimentary rocks (like conglomerate and breccia) also contain pieces or pre-existing rock. These pieces are called clasts.
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Figure 3-9. Field relations showing the relative dating principle of inclusions in Tuna Creek, Colorado River mile 99.1, Grand Canyon. The darker amphibolite inclusions (xenoliths) are surrounded by lighter granodiorite, thus the amphibolite is older than the granodiorite. The angularity of the xenoliths suggest short exposure time to the host magma. Rounded xenoliths typically represent higher degrees of thermal-mechanical erosion from longer exposure to assimilation processes. Swiss army knife for scale. Click on the picture to see the annotated image. |
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Figure 3-10. Field relations showing the relative dating principle of inclusions in Titus Canyon, Death Valley NP. The darker limestone inclusions are surrounded by a calcite-rich matrix, thus the limestone blocks are older than the sedimentary mega-breccia unit that contains them. The big limestone clast at left is about 2 meters wide. Click on the picture to see the annotated image. |
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Example 6 |
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True or False: The amphibolite is younger than granodiorite. |
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FALSE The Principle of Inclusions states that the inclusions must be younger than the host rock. Thus, the amphibolite inclusions must be older than the granodiorite host rock. |
Geologic Sequence Diagrams
A geologic sequence diagram (Figure 3-11) is basically a geologic cross section where the relative order of units/events has been determined using the common sense relative dating principles. Although sequence diagrams are simplifications of what you would see in nature and represent geologic relations that may not be visible in a single location, their real utility is in summarizing the geology of an area.
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Figure 3-11. A geologic cross section is referred to as a sequence diagram when used to illustrate the order of geologic events. Unit R is an igneous intrusion with inclusions and a "bake zone". |
Units in sequence diagrams, geologic cross sections, and geologic maps can be given pattern fills that represent certain rock types. A key showing the fill patterns used for diagrams in this class is shown in Figure 3-12.
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Figure 3-12. A key for the fill patterns used in this class. Click HERE for a PDF version of this chart. |
Quiz Me! question B11 refers to the fill pattern key in Figure 3-12.
Using relative dating principles and knowledge of the rock types from the fill patterns, further interpretations can be made about the geologic history.
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Example 7 |
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What is the most probable sequence of events created this geologic cross section that we see today? |
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Using Superposition and Original Horizontality, we determine that sandstone unit D was deposited first (1). |
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This was followed by deposition of mudstone unit M (2). |
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A period of uplift, tilting, and erosion must have followed (3). |
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Conglomerate unit B was then deposited last (4). |
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We now have established an order of events (a geologic history) for this cross section, making it a geologic sequence diagram. In a geologic history sequence, we list the rock units from left to right with the oldest units on the left. D - M - B The period of uplift and erosion between M and B is implied, but not listed in the sequence. |
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Unconformities
Unconformities are contacts between adjacent rock units that are significantly different in age. Understanding how to recognize and interpret these features is important in unraveling the geologic history of an area. There are three distinct types of unconformities that are defined by the type of rock ABOVE and BELOW the unconformity and/or their ORIENTATION: nonconformities, angular unconformities, and disconformities.
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Nonconformity |
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Plutonic or metamorphic below the unconformity; sedimentary or volcanic rocks above. |
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Look for igneous or metamorphic rock types; know their fill patterns in the sequence diagrams. |
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Angular Unconformity |
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The layered rocks above and below the unconformity are at an angle to each other. |
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Look for two sets of sedimentary layering with an angular difference in orientation. |
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Disconformity |
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The layered sedimentary rocks above and below the unconformity are parallel to each other. |
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Look for all sedimentary layering that all has the same orientation. |
Nonconformities
A nonconformity is a contact between rocks that form at the Earth's surface and rocks that formed at depth within the Earth (see Figures 3-13 and 3-14). Older metamorphic and plutonic igneous rocks (units A and B) can be exposed at the surface due to tectonic uplift (possibly with tilting), and erosion . Subsequent deposition of sedimentary or volcanic rocks (unit C) directly on top of the older rocks (units A and B) produces the nonconformity, which represents the period of uplift, tilting and erosion. The age difference between unit C and units A & B below may be significant (millions or billions of years).
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Figure 3-13. A geologic cross section showing a nonconformity. |
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Figure 3-14. A nonconformity at Colorado River river mile 119.5 in the Grand Canyon (near Blacktail Canyon). The layered sedimentary rocks of the Cambrian Tapeats Sandstone overlie the purplish-gray Paleoproterozoic metamorphic rocks of the Brahma Schist to form this nonconformity, also known as "the Great Unconformity". The unconformity here spans the period between 1750 Ma and 520 Ma - that's 1230 million years! Click on the picture to see the annotated image. |
Angular Unconformities
An angular unconformity is relatively easy to recognize because it is a contact between rocks layers that are at an angle to each other (see Figures 3-15 and 3-16). It consists of tilted or folded sedimentary rocks that are overlain by younger layers. Angular unconformities suggest that uplift, tilting, folding, and erosion occurred between the time of deposition of the layers above and below the contact. The tilting and folding likely represents a deformational (mountain-building) event.
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Figure 3-15. A geologic cross section showing an angular unconformity. |
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Figure 3-16. Field relations showing an angular unconformity in the Grand Canyon. View is to the north from Desert View of the Basalt Cliffs angular unconformity formed by relatively flat-lying Cambrian sedimentary rocks and the gently tilted Proterozoic rocks. Click on the picture to see the annotated image. |
Disconformities
A disconformity is a contact between parallel (but not necessarily horizontal) sedimentary layers that are significantly different in age (see Figures 3-17 and 3-18). Rocks above and below a disconformity may look similar, so it may be difficult to identify without any additional information. Helpful observations that suggest significant time difference between the two units include the presence of lower unit inclusions in the upper unit, large differences in the abundance and types of fossils across the contact, stream channels carved into the lower unit, etc. Disconformities suggest a period of erosion occurred with little or no tilting or folding.
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Figure 3-17. A geologic cross section showing a disconformity. |
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Figure 3-18. Field relations showing a disconformity in the Grand Canyon. View is to the northeast from Yavapai Point toward O'Neill Butte. Click on the picture to see the annotated image. |
Interpreting a Sequence Diagram
Figure 3-19 shows a sequence diagram with the unconformities labeled. In this course, only the geologic units and faults are included in the geologic answer sequence; unconformities are not included.
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Figure 3-19. A sequence diagram with unconformities X, Y, and Z labeled. Y actually represents two types of unconformities. |
Quiz Me! questions B15 through B30 refer to Figure 3-19.