Online Lab 8 Sedimentaray Rocks.docx

Laboratory 4: Sedimentary Rocks Geology 101 Laboratory 4 - Sedimentary Rocks Objectives:  Be able to describe and interpret textural features of sedimentary rocks  Be able to describe the compositional features of sedimentary to classify them as detrital (siliciclastic), organiclastic (bioclastic) or chemical  Be able to determine the name of common sedimentary rocks based on texture and composition  Be able to identify and interpret common sedimentary structures  Infer the origin of a sedimentary rock based on texture and structure Back ground Sedimentary rocks or their unconsolidated equivalents are composed of sediment, but what is sediment? Sediment is solid particles that originate from the (1) Weathering, mass wasting, and erosion of previously existing Earth material, (2) minerals precipitated from a solution, or (3) the remains or traces of once living organisms (plant or animal) or material derived from their activities. If a sedimentary rock is dominated by component (1) above, then it is a detrital or siliciclastic sedimentary rock. If instead it is dominated by component (2), then it is a chemical sedimentary rock, and if it is dominated by (3), then it is bio-chemical or organic sedimentary rock. Clastic Sediments and Rocks Sediment normally accumulates in layers of cohesion-less particles. Such layers are termed beds (Figure 1). With time beds are buried beneath younger beds as more and more sediment is deposited. Eventually, this process results in the cohesion less particles being cemented together to form rock as fluids migrating through the deposit become supersaturated with CaCO 3 or SiO 2 , or pressures and temperatures due to burial are sufficient to cause minor recrystallization. Clastic sedimentary rocks or their unlithified (uncemented) deposits are composed of minerals and fragments of previously existing Earth material. Such rocks or deposits are commonly classified based on the size of the included minerals and fragments. These are called grain sizes and are illustrated in Figure 2. 1

Laboratory 4: Sedimentary Rocks Figure 1. Beds in an ancient alluvial succession. Each bed resembles the pages in a book. In the photograph different colored layers represent individual beds (e.g., white versus brownish red) Figure 2. Some common sediments and their sizes . Gravel or granules are a term used to refer to clastic particles that are greater than 2 mm in size. The term sand refers to particles between 0.0625 mm and 2.0 mm in size. Silt is smaller than sand and includes grains between 0.0035 mm and 0.0625 mm. Clay is the smallest and includes all particles less than 0.0035 mm. Some geologists do not subdivide the smaller grains into silt and clay categorizing all particles less than 0.0625 mm in size into mud (Some pictures follow). 2

Laboratory 4: Sedimentary Rocks 3

Laboratory 4: Sedimentary Rocks Part I After reading, the information below you will use the sedimentary identification chart to identify some common sedimentary rocks. Obtain these samples from your instructor. If clastic material is cemented or lithified to form a rock, then the following names would apply to the various sizes of clastic grains illustrated or discussed above: gravel = conglomerate (or breccia) sand = sandstone, silt =siltstone, clay = claystone, silt + clay = mudstone To apply the term conglomerate the included gravel must be rounded. In contrast, if gravel is angular or blocky in form, then the term breccia is more applicable. Sand and sandstones are ubiquitous features of the continents. For example, sand is common on the world’s beaches, coastal areas, and rivers. As a result of its ubiquity geologists have further subdivided it into the grain size categories shown below. The chart below gives more detailed information of sedimentary particle sizes used by geologists in the U.S. 4

Laboratory 4: Sedimentary Rocks 5

Laboratory 4: Sedimentary Rocks 6 Conglomerate Sandstone Siltstone Claystone

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