Question

While on a field trip with your geology class, you stop at an outcrop of sandstone. An examination with a hand lens shows that the sandstone is poorly sorted and rich in feldspar and quartz. Your instructor tells you that the sediment was derived from one of two sites in the area. Select the most likely site and explain your choice. What name is given to this type of sandstone? Site #1 A nearby exposure of weathered basaltic lava flows. Site #2 An outcrop of granite at the previous field trip stop up the road.

Short answer

The sandstone likely originated from Site #2 (granite) and is called arkose.

Step-by-step solution

Understand the mineral composition of the sandstone

The sandstone is described as having a high content of feldspar and quartz. Feldspar, particularly in significant amounts alongside quartz, suggests the source material is rich in these minerals.

Consider the mineral composition of each site

Site #1, the basaltic lava flows, primarily consists of minerals like pyroxene and olivine, but typically not high in quartz and feldspar. Site #2, the granite outcrop, is composed mainly of quartz and feldspar, which are abundant in granitic rocks.

Match the sandstone composition with the potential source

Since the sandstone contains high levels of feldspar and quartz, it closely matches the mineral composition found in Site #2 (granite), not basalt from Site #1.

Identify the type of sandstone

A sandstone that is rich in feldspar (and not well-sorted) is referred to as arkose, typically indicating a minimal transport from a granitic source.

Key concepts

Feldspar

Feldspar is a group of minerals that is very common in the Earth's crust. It's an important component in many igneous, metamorphic, and sedimentary rocks. Feldspar is generally recognized by its glassy luster and hardness, making it durable. In the context of sandstone, feldspar is significant because it indicates the original source of the sediment wasn't far away.

• If a sandstone is rich in feldspar, it has likely been derived from a source rock like granite.
• This is because feldspar weathers relatively quickly compared to other minerals like quartz.

Finding a high concentration of feldspar in sandstone usually suggests a short transport distance from the source due to feldspar's susceptibility to weathering. This sediment is then quickly buried, preventing it from breaking down further.

Quartz

Quartz is another mineral that frequently appears in many types of rocks. Known for its hardness and resistance to weathering, quartz is often one of the last minerals to erode away during the transport of sediment.

• This makes it one of the most abundant minerals in sandstone.
• Quartz grains in sandstones are often rounded, a sign of their durability and extensive transport history.

In our example of arkose sandstone, however, the presence of quartz along with feldspar indicates that the source is more likely a granitic rock.
This is because granite contains abundant amounts of both feldspar and quartz, providing the composition we see in the sandstone.

Granite Outcrop

A granite outcrop refers to a visible exposure of granite rock at the Earth's surface. Granite is a coarse-grained igneous rock composed primarily of quartz and feldspar, with a portion of mica.

• Its composition makes it a key source for arkose sandstone when it breaks down.
• Weathering of granite typically results in sediments rich in both feldspar and quartz.

Such outcrops are crucial in geology for determining sediment provenance. They tell us about the original landscape and weathering processes.
In our scenario, stopping by a granite outcrop provides clear evidence that feldspar and quartz can be traced back to this source, solidifying the connection between granite and arkose sandstone.

Poor Sorting

Poor sorting in geology refers to sedimentary deposits that consist of a wide range of grain sizes. When sediments are poorly sorted, it indicates that they have not been subject to extensive transport, which would have otherwise sifted and separated grains by size.

• Poor sorting is common in deposits that are rapidly laid down, such as those found near a source rock.
• In the case of arkose sandstone, poor sorting is consistent with its composition, corroborating the minimal transport from granite to create the deposit.

This concept highlights how sediment characteristics can reveal their geological history, pointing back to the granite source. Poor sorting ensures that both feldspar and quartz, often differing in size, remain together in the deposit.