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: 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. Select the most likely site and explain your choice. What name is given to this type of sandstone?

Short answer

The sandstone likely came from Site 2 (granite outcrop) and is called arkose.

Step-by-step solution

Identify and Characterize the Key Minerals

Examine the mineral composition of the sandstone mentioned in the problem. The sandstone is described as rich in feldspar and quartz. Note these key minerals as they indicate a granitic source, as both minerals are commonly found in granite.

Review the Characteristics of Possible Sites

Consider the geological compositions of the two potential source sites. Site 1 features weathered basaltic lava flows, which typically contain minerals like pyroxenes and olivines, but not high amounts of feldspar and quartz. Site 2 is an outcrop of granite, which commonly includes feldspar and quartz, aligning with the mineralogy of the sandstone.

Analyze the Sorting and Composition Factors

The sandstone is poorly sorted and rich in feldspar, indicating minimal transportation. Feldspar undergoes chemical weathering relatively quickly and breaks down into clays, but its abundance suggests the source was nearby and transported minimally, characteristic of granite from Site 2.

Conclude the Most Likely Source

Based on the mineral composition and sorting quality, conclude that Site 2, the granite outcrop, is the most likely source of the sandstone. This matches the mineralogical characteristics and the poor sorting of the sediment.

Identify the Type of Sandstone

Sandstone rich in feldspar is typically called "arkose." Recognize that this composition and mineralogy define the sandstone as arkose, which is associated with granitic sources and often poorly sorted due to feldspar preservation.

Key concepts

Arkose Sandstone

Arkose sandstone is a type of sedimentary rock that is characterized by its significant content of feldspar, often accompanied by quartz. This type of sandstone typically forms from the rapid breakdown of granite, which is rich in both feldspar and quartz minerals. Arkose is easily recognized by its generally pinkish or reddish color, although sometimes it can appear gray.

If you encounter sandstone that is both rich in feldspar and quartz, with poor sorting, it is often classified as arkose. This is because these minerals are commonly found together in granite, and their presence indicates minimal weathering and transportation from the source rock. When identifying arkose sandstone in the field, its rough texture and granular appearance are key distinguishing features.

Understanding arkose can provide important clues to geologists about past environments and geological processes, particularly the proximity of sedimentary deposits to their granitic source.

Feldspar

Feldspar is one of the most abundant minerals in the Earth’s crust, forming about 41% of it. It's essential in identifying the geological history of rocks. Feldspar minerals are vital indicators in sedimentary rocks like arkose sandstone, as they suggest a specific origin.

In the context of arkose sandstone, the presence of feldspar suggests minimal transport from the source rock. This means that the sedimentary grains didn't travel far, preserving the feldspar before it could chemically weather to more stable minerals like clay.

Characteristics of feldspar include:

• Colors: Prospectively pink, white, or gray.
• Hardness: Feldspar has a Mohs hardness scale rating of about 6.
• Cleavage: It shows two cleavage planes meeting at right angles.

These attributes make feldspar relatively easy to identify, even in rock samples.

Quartz

Quartz is another key mineral frequently found in sedimentary rocks, especially in arkose sandstone. Known for its hardiness and resistance to weathering, quartz can remain unaltered even in harsh conditions.

It is composed of silicon dioxide ( ext{SiO}_2) and is notable for its glass-like appearance. Quartz grains in sandstone often appear rounded due to natural processes of erosion and weathering. Its presence, paired with feldspar, reaffirms the likelihood that the rock's origin is granitic.

Key properties of quartz include:

• Hardness: Quartz ranks 7 on the Mohs hardness scale, indicating significant toughness.
• Optical: It's usually clear, white, or can present shades like pink or purple, depending on impurities.
• Chemical Stability: Quartz's stable chemical structure is crucial for its perseveration in various sedimentary environments.

Quartz's durability makes it a common component in sediments and sedimentary rocks.

Granite

Granite is an igneous rock, widely recognized for its coarse-grained texture and composition predominantly of quartz, feldspar, and mica. These minerals give granite a distinctive speckled appearance, often seen in countertops and monuments.

In terms of geological significance, granite serves as a primary source for arkose sandstone. Its minerals resist degradation and allow them to survive as sediments over time, especially feldspar and quartz. When granite weathers, it can break down into less altered pieces that include these specific minerals.

Granite's composition includes:

• Quartz: Up to 70%.
• Feldspar: Often potassium-rich, accounts for about 10-65%.
• Mica or Hornblende: Adds to the remaining composition.

The breakdown and erosion of granite directly influence the mineral composition found within arkose sandstone, making it a target rock for geologists studying sediment sources.

Poorly Sorted Sediment

Poorly sorted sediment refers to sedimentary material where grains of differing sizes and shapes are mixed together. This condition usually results from minimal transportation, where sediments didn't travel far from their source.

In the context of arkose sandstone, poor sorting signifies that the sediment came from close by, such as a nearby granite outcrop. The absence of sorting suggests that the forces transporting the sediment—wind, water, or ice—were not strong enough to separate the particles into distinct sizes.

Characteristics of poorly sorted sediment include:

• Mixed grain sizes ranging from fine to coarse.
• Varied shapes, from angular to rounded.
• Short transport distance usually indicated.

Poorly sorted sediments thus provide insights into the transportation dynamics and depositional environment of sedimentary rocks like arkose sandstone.