Clastic rock types in detail: Sandstone


Sandstone is the indurated equivalent of unconsolidated sand containing clasts between 2 and 1/16 mm. They make up between 10% and 20% of sedimentary rocks. Interesting to sedimentologists for the information they contain about the provenance of sediment and depositional environments. For the practical person, they are important as fluid reservoirs (aquifers and petroleum reservoir rock). Indeed, sandstones hold over 1/2 of the worlds oil reserves.

Sandstone classification:

There are MANY different classification schemes for sandstones, but the two defining parameters are: These are refined by consideration of texture, including: Taken together, these factors can be encompassed in the broad concept of maturity (see below).

Framework composition:

Framework grains can consist of any rock or mineral, but for practical purposes, we reduce these to the common ones:

Matrix: Fine grained matrix (<30μm), can be detrital or products of in situ feldspar diagenesis. Depending on the percentage of matrix, a sandstone can be a:

Any more matrix and the rock is regarded as a mudrock.

We use the classification system of Dott, 1964 that can nicely be summarized by a ternary diagram showing relative volume of quartz, feldspar, and rock fragments, with an extra dimension for matrix volume.

Depending on the predominant framework composition, arenites are broken down into:

Wackes, similarly break down into: We will return to these. First...

Quantifying texture

Textural maturity

Much of the foregoing can only be assessed meaningfully in thin section. Maturity , however, is a convenient general description that encompasses proportion of matrix, sorting, and rounding and can be approximated in the field:
Maturity has implications for depositional environment.

With all of this in hand, we can describe the major sandstone types.

Quartz arenites: These sediments are most likely the product of multiple recycling of quartz grains (polycyclic). If they are first generation it suggests an intense weathering environment.

Feldspathic arenite: A medium to coarse grained arenite with >25% feldspar is also known as an arkose. These sandstones form from the rapid disentigration of granite to form grus, which is rapidly deposited, most often in alluvial fan deposits. They originate either: