Sedimentary Rocks, our most Valuable Rocks. Or, what you will probably find when you are outdoors exploring.

Transcription

1 Sedimentary Rocks, our most Valuable Rocks Or, what you will probably find when you are outdoors exploring.

2 Sedimentary rocks give us evidence to earth s earlier history. We look at processes happening today and compare them with what we see in older rocks. Many of our valuable geologic resources are found by understanding this principle.

3 Remember from an older slide: Weathering Erosion Transportation Deposition Preservation Lithification Soils were in here somewhere.

4 Grain size Big particles are harder to carry than smaller particles. The further a particle is transported, the smaller it becomes, usually (continued weathering)

5 Transportation After a particle is eroded, it can be transported by various agents, for various distances Rounding

6 Sorting Certain environments separate different sizes, others don t The beach The cliff Think about the waves on a beach and compare that to dumping a bucket of particles off the side of a cliff. How would the two deposits be sorted?

7 After any rock is ❶weathered, ❸transported and ❷eroded, ❹deposited, it may become ❺lithified into a new rock type sedimentary. (That is, if it can be preserved from further erosion.) - about 5% of the crust is composed of sedimentary rocks, but almost all of them are on the surface. That means about 75% of the continents are covered by sedimentary rocks.

8 Four major categories of sedimentary rocks: 1. Most (85%) sedimentary rocks are Clastic fragments of rocks/minerals lithified into a new rock. Clastic rocks are named based on the size of the clasts. This tells a lot about their origin. Size Rock Gravel > 2 mm diameter Conglomerate and breccias. Sand 2 to 1/16 mm Sandstone Silt 1/16 1/256 mm Siltstone Clay <1/256 mm Shale Most gravel is of mixed mineralogy Most sands and silts are quartz Most clay size particles are clay minerals.

9 Lots of money has been made in sedimentary rocks: Coal, Petroleum, Natural gas, Fresh water, Building stones, etc. The economic value of many clastic rocks is due to the holes or pores between the individual grains.

10 Shales have lots of holes, often more than sandstones, but they are very, very tiny, so you can t move oil or water through them fast enough to make a profit.

11 Clastic rocks

12 2. Chemical sedimentary rocks or Evaporites form when crystals precipitate out of solution. These form from evaporating water, or water that is holding too many ions in solution - supersaturated. This is the reverse of Dissolution. Limestones are calcium carbonate (the rocks that dissolve so easily to form caves) Halite is table salt, NaCl Gypsum (wall board) is calcium sulfate.

13 The third type of Sed. Rx. 3. Bioclastic sedimentary rocks are formed from fragments of things made by organisms. Physically they are just like clastic rocks (with holes and all). Many limestones and dolomites can be classified as bioclastic because they contain shells. Chalk is a soft, bioclastic limestone made up of very tiny microscopic shells

14 Marine depositional environments

15 Where do we get our drinking water besides Lake Erie? In Waterville, Swanton and other towns west of Toledo, wells are drilled into a crystalline limestone. Do you see any problems with this? Yes, most crystalline limestones are non-porous.

16 4. Organic sedimentary rocks are lithified plant and animal remains: Some Chert is pure silica, the remains of tiny marine organisms. Other chert is a chemical sedimentary rock. We know it as flint. Coal is the accumulated remains of plant material that was buried in swamps or marshes.

17 Just a few economic uses of sedimentary rocks Limestone Sandstone Phosphates Sand Gypsum Gravel Clay Dolomite Sandstone, limestone Borates Building stone Building stone Fertilizers Glass Wall board, plaster Roads Aluminum cans, pottery Vitamins Fresh water, petroleum, natural gas soap

18 A dinosaur skeleton is found in a sandstone. The shape of the sandstone is long and linear. To the north the sandstone grades into a conglomerate, made up of granite pebbles. Further to the north is a granitic batholith outcrop. To the south, east and west is a shale deposit. And further to the south is a limestone. The sandstone shows cross-bedding and the shale to the east and west has mudcracks. All of these rocks are 100 million years old and are now in the desert. What did the area look like back then? The fossils, cross bedding and mudcracks are sedimentary structures and give us clues as to the environment when the sediments were deposited.