Sandstones and Conglomerates© Geoff Habiger
Mar 25, 2002
Sandstones and their coarser grained relatives, the conglomerates, have always received much of the attention of sedimentologists. Less abundant that the mudrocks, only 20-25% of the stratigraphic record is sandstone, they have features that are easily seen and quite diagnostic of transport and depositional environment. The shape of the grains, their size, how well sorted the grains are, the presence of cross beds or ripples all tell us something about the sandstones history. Such attention to detail is important since much of the world's petroleum reserves and most of the groundwater we drink is located in reservoirs and aquifers of sandstone.
Like the study of mudrocks, the identification of sandstones in the field depends on several factors, all of which must be recorded to make an accurate description of the rock. Grain size and shape, bedding, and sedimentary structures can tell the geologist a lot about the sandstones history, and knowing this history is important in making interpretations about the ancient environment that the sandstone was deposited in. Was it an alluvial fan located at the base of a mountain, or desert sand dunes blown by the wind? Sandstones can tell us where rivers and the edges of the continents were located and which way the water flowed. Grain size is an important textural feature of every sandstone and conglomerate. Unlike with mudrocks, grain size in sandstones can often be determined readily in the field with the aid of a ruler marked in millimeters. It is necessary to determine the average size of the grains present and the distribution of sizes, called sorting. The grain size chart at right shows the various groups of grain-sizes, their name and average size in millimeters. The phi number is commonly used in performing statistical calculations on grain size data. Sorting is important as a tool to measure the uniformity of currents or waves, often a valuable tool in interpreting the paleoenvironment. Sorting is usually estimated with the aid of a hand lens by determining the size of the majority (around 80%) of the grains. Many times geologist will carry sorting cards with them in the field. These cards have examples of the different degrees of sorting and can be used to compare the the rock at the outcrop. Grain shape, usually determined by the degree of rounding of grains, is a reflection of the length of transport for a grain. The degree of rounding is described as being from very angular to well rounded. Usually, the more angular a grain, the closer you are to the source material for that grain. A well rounded grain represents a long period of abrasion.
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