Identifying the Different Non-clastic Sedimentary Rocks

Carbonate Sedimentary Rocks

Carbonates are one of the most important groups of sedimentary rocks. They can result in distinctive landscapes, called karst topography (more on this in Groundwater) and human hazards like sinkholes.

Limestone, one of the most abundant sedimentary rocks, is composed of the carbonate mineral calcite. If you remember our mineral unit, calcite undergoes dissolution in acids, and will bubble, fizz or effervesce, when dilute HCl is applied (Figure 11.21). There are many varieties of limestone and all vary greatly in appearance; however, all varieties will fizz or bubble when dilute HCl is applied. Limestone can form inorganically from a supersaturation of calcium and carbonate ions in water in a range of environments from caves to tropical beaches, although most limestone is marine in origin.

A close-up of a mineral showing bubbles. — **Figure 11.21:** Calcite reacting (bubbling) with the addition of dilute (1M) hydrochloric acid (HCl). (CC-BY 4.0, Chloe Branciforte, own work)

Crystalline limestone (micrite) is typically inorganic in nature and consists of crystals of calcite or microcrystalline masses of calcite (Figure 11.22). It can be light gray to darker gray in color and the calcite crystals will sparkle when the sample is moved around under a light. Typically, crystalline limestone is indicative of a deep marine environment, but there are also lacustrine crystalline limestones.

A close-up of a dark gray sedimentary rock composed entirely calcite. — **Figure 11.22:** Micrite limestone is composed entirely of crystalline calcite. (CC-BY 2.0; James St. John)

Coquina often has a biologic component and is composed exclusively of shell or coral fragments (sometimes referred to as shell hash). Coquina looks a bit like dried oatmeal or a granola bar (Figure 11.23).

A close-up of a sedimentary rock composed entirely of broken shells. — **Figure 11.23:** Coquina is composed entirely of shell hash (finely-busted up seashells). (CC-BY 2.0; James St. John)

Fossiliferous limestone will contain fossils, but unlike coquina will not be entirely made of fossils. Oftentimes, between the fossils there will be calcite-rich mud (Figure 11.24). Typically, the fossils are marine in origin and include shells, crinoids, or other invertebrate marine life.

A close-up of a sedimentary rock containing fossils. — **Figure 11.24:** Fossiliferous limestone contains an abundance of fossils but is not entirely composed of them. Typically, the areas between the fossils are composed of limey muds, which also react with HCl. (CC-BY 2.0; James St. John)

Oolitic limestone is composed of ooids (a fun geology word!), small calcite spheres that typically form in shallow, warm, highly agitated marine environments (Figure 11.25).

A close-up of a sedimentary rock containing rounded spheres of calcite. Inset image is a close-up of the rounded spheres. — **Figure 11.25:** Oolitic limestone with ooids, rounded spheres of calcite. (CC-BY 2.0, James St. John; modified by Chloe Branciforte)

Chalk also has a biochemical origin and is made entirely of the small tests (shells) of marine organisms called coccolithophores, or coccoliths for short. These organisms are incredibly small, and can only be viewed under extreme magnification (Figure 11.26). Remember writing with chalk on a chalkboard? This rock will be powdery and leave a mess on your hands.

A close-up of a white, powdery sedimentary rock. Inset image is a close-up of a coccolith. — **Figure 11.26:** Chalk limestone, with a close-up of a single coccolithopore. (CC-BY 2.0, James St. John; inset image, CC-BY 2.5, Alison R. Taylor/University of North Carolina Wilmington Microscopy Facility; modified by Chloe Branciforte)

Travertine and tufa are both unique limestone varieties as they form in terrestrial (continental) environments. Travertine is commonly associated with hot spring environments, where supersaturated alkaline waters are geothermally heated (Figure 11.27).

A close-up of a calcite-rich sedimentary rock with many openings. — **Figure 11.27:** Travertine limestone with vugs (openings or holes). (CC-BY 2.0, James St. John)

The most famous hot spring travertine deposits are from Italy. Here in the US, the largest deposits are located in Mammoth Hot Springs in Yellowstone National Park. In California, Lassen Volcanic National Park has smaller deposits of travertine. Travertine can also be associated with caves and speleothems. Tufa commonly forms when carbonate minerals precipitate out of ambient temperature water, usually water that has not been geothermally heated. Here in California, there are beautiful tufa spires in Mono Lake and the Trona Pinnacles (Figure 11.28).

A panorama image of the Trona Pinnacles in the Searles Dry Lake basin. — **Figure 11.28:** The Trona Pinnacles, located San Bernardino county, are a collection of 500 tufa spires rising from the bed of the Searles Dry Lake basin. These tufa spires, some as high as 140 feet, were formed underwater, 10,000 to 100,000 years ago, when Searles Lake, and other Pleistocene lakes existed between Mono Lake and Death Valley. (Public Domain; Bob Wick/BLM)

Dolostone is made from microscopic crystals of the mineral dolomite (Figure 11.29). Although also a carbonate mineral, dolomite only weakly reacts to dilute HCl. You must scratch and powder dolostone to increase the surface area to see the reaction with acid. Even after all that work, the reaction is muted (and underwhelming) when compared with calcite’s reaction. Dolostone typically forms through the inorganic chemical alteration of a limestone and is therefore a chemical rather than biochemical sedimentary rock.

A close-up of a gray, non-descript sedimentary rock. — **Figure 11.29:** Dolostone which contains the mineral dolomite. Most dolostones are chemically altered limestones. (CC-BY 2.0; James St. John)

Siliceous Sedimentary Rocks

Chert is composed of microcrystalline varieties of quartz, and thus it has properties that are associated with quartz itself, such as conchoidal fracturing and hardness greater than glass (Figure 11.30). Typically, cherts are identified based on their appearance; for example, jasper is often red chert, flint is gray chert, and agate are banded cherts. Chert typically forms in deep marine environments from siliceous material that is either inorganic (silica clay) or biologic, a siliceous ooze formed from the skeletons of sponges and single-celled organisms like diatoms and radiolarians. Chert may also form petrified woods; these cherts form due to chemical changes during the heat and pressure of burial (diagenesis). Deep marine chert can be bedded or found as nodules in limestone layers. In prehistoric times, chert was often used for the construction of tools, like blades, where obsidian (volcanic glass) was unavailable.

Top left, a close-up of a gray rock with sharp edges. Bottom left, a mottled red, white and orange rock with sharp edges. Right, a close up of a radiolarian. Far right, a close up of a marine diatom. — **Figure 11.30:** Chert, a hard sedimentary rock, will often show conchoidal fracture or sharp edges. Most deep marine cherts will be composed of siliceous single-celled organisms, like radiolarians and diatoms. (Top left, CC-BY 2.0, James St. John; Bottom right, CC-BY 2.0, James St. John; Right, CC-BY 2.0, Picturepest; Far Right, CC-BY-SA 3.0; Mogana Das Murtey and Patchamuthu Ramasamy; modified by Chloe Branciforte )

Evaporite Sedimentary Rocks

Evaporites are formed via the precipitation of minerals from the evaporation of water, in either restricted marine basins, playa lakes, or salt flats, like those of Death Valley, CA (Figure 11.31). Many of the evaporite deposits in California are geologically young. Rock salt, an aggregate of the mineral halite, may retain its cubic form and will vary in color. Rock gypsum (gypstone), an aggregate of the mineral gypsum, will typically be soft and may be massive or fibrous in appearance. Many of the evaporites contain significant economic minerals; for example, salt domes can be important in the search for petroleum.

A map-view of white salt-flats in Badwater Basin. — **Figure 11.31:** Evaporite minerals compose the salt flats of Badwater Basin, Death Valley. (CC-BY 4.0; Chloe Branciforte, via Google Earth)

Organic Sedimentary Rocks

Organic sedimentary rocks are rocks that consist mostly of organic carbon and are associated with significant biological activity. Other sedimentary rocks, such as limestone and shale (like oil shale) can contain carbon, but at much lower concentration. The most common organic sedimentary rock is coal. Coal is formed from the preservation and compaction of abundant plant material, buried in areas where oxygen is lacking (anoxic environments), such as swamps, wetlands, and bogs. All coal is combustible, and therefore can be ignited and burned; however, not all coal burns the same. In fact, there are grades of coal, which reflect increasing compaction or lithification.

Peat, typically dark brown (like potting soil), will contain obvious pieces of vegetation, and is typically the precursor to coal (Figure 11.32). In Ireland and Scotland there are peat bogs, from which the peat is mined, cut into bricks, dried, and used for the cooking and heating of homes.

Lignite coal is brittle with a low density (light heft) and has a matte brown or black color. It will also leave distinctive sooty marks on your fingers and hands. Typically, lignite coal contains 25%–35% carbon and has the lowest energy content of all coal ranks (Figure 11.32). Lignite coal deposits tend to be relatively young and were not subjected to extreme heat or pressure. Lignite accounts for about 10% of total U.S. coal production, most from North Dakota and Texas.

Bituminous coal will also be brittle with a low density (light heft), but its black color has a bit more shine to it. Typically, bituminous coal contains 45%–86% carbon (Figure 11.32). Here in the United States, bituminous coal is the most abundant of the coal varieties and accounts for almost 50% of our coal usage. Significant deposits are located in Wyoming, West Virginia, Pennsylvania, Kentucky, and Indiana. Much of the bituminous coal worldwide is from the Carboniferous period of the late Paleozoic (about 300 million years ago). Much of the bituminous coal mined here in the US is used to generate electricity and for processing iron to make steel.

A cross-sectional view of how coal develops over time and through the addition of compaction and heat. — **Figure 11.32:** The stages of sedimentary coal formation. (CC-BY 4.0; Chloe Branciforte, own work)

Anthracite coal is much harder than the previously outlined coal varieties. It is black in color with a submetallic luster. Anthracite accounts for <1% of coal mined in the US, with most mined from northeastern Pennsylvania (the “Anthracite Belt”). Anthracite typically contains 86%–97% carbon and is the highest grade of coal. Technically, anthracite has undergone so much heat and pressure that it is considered a metamorphic rock. We will discuss this rock further in the metamorphic rock chapter.