CHAPTER 6 SEDIMENTARY ROCKS

Materials that make up sediments Weathered Rocks Organic Material Mineral Fragments

Ways sedimentary rocks can form Compaction and cementation of sediments Evaporation of a solution Precipitate from a solution

Kinds of sedimentary rocks 1) Clastic/Detrital Composed of fragments and small pieces of rock (clasts) sometimes show layering Make up more than 85% of all sedimentary rocks Ex) sandstone, siltstone, shale

Fragments Round Fragments = CONGLOMERATE Angular Fragments = BRECCIA (pronounced Brechia) (“cc” is “ch” like Gucci)

Types of Clasts The formation of clastic rocks begins with the movement and relocation of fragments. The majority of these are moved by running water. Larger pebbles and gravels are often the first to be dropped and settle in shallow water near the shore. Next to settle are the smaller sands. Finally, in calm water, the silts and clays.

Formation of Clastic Rocks Loose sediments become solid rock when sediments become cemented. Ocean water, lake water, and ground water all contain natural cements in the form of dissolved mineral. Silica (SiO2), calcite (CaCO3), iron oxide (Fe2O3), and clay minerals. When minerals fill in the spaces between sand grains, pebbles, or other rock particles, they bind the fragments together through cementation. The type of cement influences the rock’s color.

2) Organic Contains the cemented (lithified) remains of once living things Sometimes contain fossils Ex) coal

3) Chemical Form by precipitation or evaporation of a solution Often contain crystals or appear “dried up” Ex) rock salt

4) Bioclastic Composed of broken shell fragments and similar remains of living organisms Ex) limestone

Features of Sedimentary Rocks Stratification The arrangement of visible layers. The most characteristic feature of sedimentary rocks. Results from the change in the type of sediment being laid down in one place. Fossils The remains, impression, or any other evidence of a plant or animal preserved in rock. Occurs when a dead organism is buried by sediment that gradually turns into rock. The soft parts of the organism decay; the hard parts become rock.

Stratification and Fossils

Features of Sedimentary Rocks Ripple Marks Common feature of sedimentary rocks. Sand patterns formed by the action of winds, streams, waves, or currents. Ripple marks are generally preserved in sandstone. Mud Cracks Develop when deposits of wet clay dry and contract. The cracks are filled in with different solutions and fossilize. Generally form in shale.

Ripple Marks and Mud Cracks

Features of Sedimentary Rocks Nodules Hard lumps of fine-grained silica Found in limestone and chalk. Concretions Round solid masses of calcium carbonate. Found in shale. Both Nodules and Concretions form when minerals in a solution precipitate around a fragment in the clay sediment.

Nodules and Concretions

Geodes Spheres of silica rock. Generally found in limestones. Groundwater creates cavities in limestone and minerals in the groundwater concentrates in the cavities to form crystals.