Chugwater formation

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The Chugwater Formation is named for the town of Chugwater, Wyoming, in the western United States. It dates from the Triassic and is easily recognizable by its brick-red color. However, due to its lack of fossils and its presence underneath the highly-studied Morrison Formation, the Chugwater receives little attention. Its average stratigraphic dip is about twelve to fourteen degrees.

[edit] Characteristics

The most noticeable feature on a large scale is the brick-red color, caused by oxidation of iron minerals in the rock. This color is periodically interrupted by streaks and spots of reduced iron, a light bluish-gray shade. Near the top of the formation is a thick layer of gypsum, soft and easily carvable, of very high quality. The whole rock is interrupted by gypsum veins and floaters, as well as having a disrupted texture because of the precipitation of gypsum crystals after deposition of the rock.

The Chugwater is mainly siltstone or very fine-grained sandstone of a very durable nature; it is quite hard and does not break easily. Though most of the structures are not visible due to the gypsum interruption, there are a few examples of ripple marks and occasional crossbedding. Examples of infilled mud cracks, typically about a yard by half a yard and of irregular shape, are common.

[edit] Composition

The main constituent of the Chugwater rocks is quartz, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are hematite and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as biotite. The minerals are cemented by a non-carbonate matrix, probably siliceous.

The Chugwater is a prime example of red beds, commonly deposited during the Permian and Triassic periods. It is divided into three parts, the lowest of which is the Red Peak member, which constitutes most of the formation. The Alcova Limestone, a very thin layer of dolomitic limestone, overlies it with a suspected unconformity between the two.