Sedimentary Rock Identification

1)  Your answer sheet asks you to DESCRIBE known sedimentary rocks, EXPLAIN sedimentary structures, and IDENTIFY unknown rocks.  You may use the following information (as well as the book and the Internet) to assist you.  ALSO, answer the essay questions at the end of the sheet.

There are two types of sedimentary rocks:

• CLASTIC rocks are composed of grains or fragments of rocks and minerals that have been eroded, transported, deposited, and cemented together.

• CHEMICAL rocks are composed of minerals that have been precipitated from water by inorganic processes or biological processes (shell formation).

 Identification of CLASTIC ROCKS requires that we observe grain size, grain shape, the degree of sorting of the grains, their composition, and other features that are key to the environment they formed in.

Grain Size:   Sediment and sedimentary rocks often have similar names.  Knowing the sediment grain size and name may help you when determining the name of a specific clastic sedimentary rock.  In terms of depositional environment, think about the strength of the water current that would be required to carry each of these (also consider wind and glaciers!).

 Grain Shape: This refers to the degree of rounding of the grains. Generally, the more rounded the grains, the further they have been transported from the source. All those rough edges have worn off.

well-rounded       rounded        sub-rounded         sub-angular         angular

Grain Sorting: Sorting refers to whether the particles are relatively uniform in size or are many sizes. A well-sorted sediment consists of grains of uniform size and a poorly-sorted sediment contains particles of many sizes.

poorly-sorted sediment           moderately-sorted sediment          well-sorted sediment

Composition: The grains in a sedimentary rock tend to be those things that are resistant to weathering. One of the most resistant minerals is quartz, thus many sandstones are made predominantly of quartz grains. Feldspar is less resistant than quartz, but still resistant enough to be the primary mineral grain in some rocks - these are called feldspathic sandstone or arkose. Some sedimentary rocks, such as conglomerates, are made of sediment that is simply fragments of other rocks - these are called lithic fragments.

Other Features: These include structures that are keys to the environment in which the rock formed. Here are a few examples:

·        Bedding - the layering of sediments as they are deposited; may be millimeters thick or many meters thick

·        Cross-bedding - sets of bedded sediments at various, on-lapping, inclined angles; reflects changing direction of wind or water currents

·        Ripple marks - very small dunes whose long axes are perpendicular to the current that formed them

·        Mudcracks - desiccation cracks formed by the drying-up of a muddy surface (then buried again and preserved)

·        Fossils - these occur in both clastic and chemical rocks

·        Bioturbation and Tracks - anything from a worm burrow to a dinosaur footprint

Environment of Deposition: This is your chance to be a geologic Sherlock Holmes. All of the preceding characteristics, grain size, shape, sorting, composition, and other features give us clues as to how and where the rock formed. Since sedimentary rocks are deposited at the surface of the earth, they tell us what the climatic conditions were at the time (although, like Sherlock Holmes, all clues are open to interpretation!) For example, the fact that we find limestone in Montana tells us that those areas of the state must have been inundated by a shallow sea, since that is one of the only environments where this type of limestone could form. Figure 13.8 in your text (page 335) is a good diagram showing depositional environments. Use this along with the table below to answer the following question.

Identification of CHEMICAL ROCKS requires identification of the primary mineral. These rocks can either be fragmental like clastic rocks, or they may have interlocking crystals typical of a chemical precipitate. The majority of chemical rocks are made of calcium carbonate (calcite) or calcium-magnesium carbonate (dolomite).

CHEMICAL SEDIMENTARY ROCK CLASSIFICATION