Question

Construct a diagram that illustrates a cross section of six rock units that would account for the features listed below (not in order). Clearly label your units. Remember, these events are not in order-you must determine the order of events based on the descriptions. a) Rhyolite cross-cuts and covers all units except sandstone. b) Dark, fine-grained igneous rock cross-cuts and covers conglomerate and older units. c) Oldest rocks are made of black, biochemical layers that were later tilted. d) Coarse-grained clastic rock is deposited immediately over coal. e) Opaque chemical sedimentary rock is deposited directly over basalt. f) River cuts partially into limestone. g) Medium-grained clastic rock is deposited over small-grained, high-silica extrusive rock.

Short answer

Sequence: Biochemical rock, basalt, chemical sedimentary rock, coal, clastic rock layers, conglomerate; cut by igneous rock; then, rhyolite; sandstone on top.

Step-by-step solution

Identify Cross-cutting Relationships

Since rhyolite and dark fine-grained igneous rock cross-cut some of the other units, they must be younger than the units they cut through. Rhyolite is mentioned first; it cross-cuts all units except sandstone, meaning sandstone is likely the youngest or least affected. The dark igneous rock cross-cuts conglomerate and older units, but not sandstone or rhyolite, placing it after the deposition of coal and under rhyolite.

Sequence Layered Units

The oldest rocks are black, biochemical layers, which were later tilted, placing them at the bottom. Next, basalt is mentioned, which suggests its position immediately above this black biochemical layer. Opaque chemical sedimentary rock is deposited directly over basalt, and thus over it.

Include Clastic Deposits in Sequence

The coarsest clastic rock is mentioned as being directly above the coal, which suggests the presence of coal first. Medium-grained clastic rock is noted to be deposited over small-grained, high-silica extrusive rock, indicating the fine-grained rock precedes it.

Identify River Cut

The river cut affects only the limestone layer, which means it should lie over the layers but is older than rhyolite and younger than any chemical or clastic sequence that it can cut.

Determine Chronological Sequence

Compile the sequence: Black biochemical rock (oldest), basalt, opaque chemical sedimentary rock over basalt, coal, coarse-grained clastic rock over coal, fine-grained high-silica extrusive rock under medium-grained clastic rock, followed by conglomerate. Then comes the dark, fine-grained igneous rock cutting some of these sequences and then rhyolite cross-cutting except sandstone. Finally, sandstone is potentially youngest, affecting least other structures.

Key concepts

Rock Units

Rock units are the distinct layers of rock that can be identified in a geological cross-section. Each unit represents a different type of rock formation that has occurred over a period of time. In geology, a cross-section allows us to visualize the sequence of these rock layers and understand the history they tell about Earth's past.
In our exercise, we identify multiple rock units such as sandstone, conglomerate, coal, basalt, and others. Each of these units has specific characteristics that help geologists determine their age and formation process:

• Sandstone: Typically formed from cemented sand grains and is among the most common sedimentary rocks found.
• Conglomerate: A clastic rock composed mainly of pebbles and boulders cemented together.
• Coal: Formed from ancient plant material that has been compacted and lithified.
• Basalt: A fine-grained igneous rock formed from the rapid cooling of lava.

By analyzing these rock units, geologists piece together the environmental and geological conditions that existed during each rock formation period.

Cross-cutting Relationships

Cross-cutting relationships help geologists determine the relative ages of rock layers. When a rock or fault cuts through another, it is considered younger than the rock it cuts across. This principle is crucial in establishing the chronological sequence of geological events.
In our exercise, understanding cross-cutting relationships clarifies the order of events in the rock sequence. For example:

• The rhyolite crosses all units except sandstone, indicating that it is younger than all the units it intersects.
• The dark fine-grained igneous rock intersects the conglomerate and older layers, hence it's younger than these rocks.

Such analyses allow geologists to create a timeline of events, showing which geological processes occurred first and which followed later on.

Clastic Deposits

Clastic deposits are layers of rock that are formed from fragments, or clasts, of pre-existing rocks and minerals. These clasts have been transported, deposited, and lithified over time. Such deposits are integral to understanding sedimentary environments and geological history.
In our scenario, different clastic rocks, like conglomerate and sandstone, were deposited in a sequence. They tell us about the ancient environments where these rocks formed:

• Conglomerate: Typically forms in environments with strong water currents, like rivers, which can carry large grains.
• Sandstone: Usually forms in slower-moving environments like deltas, beaches, or sand dunes.

Each clastic deposit tells a unique story about the transportation and deposition processes that took place during its formation.

Chemical Sedimentary Rock

Chemical sedimentary rocks are formed from minerals precipitating out of a solution, typically seawater. These rocks are valuable as they provide information on the chemistry of the waters they formed in.
In our given series of rock units, we examine the opaque chemical sedimentary rock, which lies directly over basalt. This sequence tells us that there was a period when chemical sedimentation occurred, depositing layers above previously formed basalt. The characteristics of these rocks help to reconstruct ancient climatic and environmental conditions.
Such deposits often include rocks like limestone or evaporites, which hint at ancient lakes or marine environments.

Igneous Rock

Igneous rocks are formed from the solidification of molten magma or lava. They come in two main types - intrusive (below Earth's surface) and extrusive (on or above Earth's surface). These rocks carry crucial information about volcanic and tectonic activity.
In the exercise, igneous rock types present include basalt and rhyolite:

• Basalt: A dark, fine-grained extrusive igneous rock. It indicates volcanic activity that once occurred at or near the surface.
• Rhyolite: Known for its high silica content, it's an extrusive rock that often results from specific, silica-rich volcanic eruptions.

Igneous rocks in a cross-section help to interpret volcanic episodes and their impact on the existing geography of that era.