Depositional Environments Current Environment Mud Cracks in Rock

What is a Depositional Environment? All rocks form in specific environments. An example of this that we talked about earlier are igneous rocks which are going to form in a volcanic area. So it you find a layer of igneous rock, you know that area once experienced volcanism. Sedimentary rocks do a great job of preserving evidence of ancient environments for a variety of reasons. You will learn about many of these reasons throughout the rest of the power point.

Depositional Environments – Clastic Sedimentary Rocks Clastic sedimentary rocks are classified by the size of the sediment embedded in the rock. After rocks is weathered the sediment that is created tends to move around by wind, glaciers, rock falls, or rivers/streams. Most sediment is moved downstream by rivers. As rivers slow down they are going to deposit the sediment they are carrying. The bigger sediment is going to be deposited first because the water is not able to move it anymore. Figure 1 shows two examples of what was just mentioned. Rivers will sort sediment by size. This process is referred to as horizontal sorting. a b gravel sand Figure 1. The diagram (a) and the picture of the stream table (b) from our class both show how streams sort sediment by size. silt and clay

Depositional Environments – Breccia Breccia is made up of angular pebble-size sediment. The fact that they are angular tells us that these sediments didn’t travel far from their source region. We can infer this because if the sediment did travel far from the rock that it was weathered from then the pebbles would become rounded. Therefore breccia’s (Figure 2) tend to form at the base of rocks falls referred to as talus slopes (Figure 3). Figure 2. Above is a sample of breccia. Figure 3. The picture above is known as talus slopes. A zoomed in picture of talus is in the red box. These rocks formed though a process called frost wedging. Frost wedging occurs when temperatures drop below freezing, causing water inside the rock to expand, breaking the rock apart.

Depositional Environments – Conglomerate Like a breccia, conglomerates are made up of pebble-size sediment, however unlike a breccia these pebbles are rounded. The fact that they are rounded tells us that these sediments did travel far from their source region. We can infer this because as sediments move along the surface of the earth the sharp edges break off leaving a rounded shape (Figure 4). Because rivers tend to move most of the sediment on earth, conglomerates, like the one seen in Figure 5, represent an ancient river bed. Pebbles and cobbles are also deposited along some beaches were the waves are more intense. We call both of these environments, high-energy environments. a b c Figure 5. Above is a sample of conglomerate. Figure 4. The riverbeds (a & b) seen above are lined with boulders, cobbles, and pebbles. The streams in these locations are moving very fast (high-energy) and therefore all of the smaller sediment, like sand, silt, and clay, are carried down stream, leaving behind the larger sediment. The beach in the picture above (c) is in an area where the waves are so intense that all of the sand is carried away, leaving behind the larger sediment.

Depositional Environments – Sandstone Sandstone (Figure 6) will form anywhere sand is deposited. Sand can be deposited in slower moving rivers or deltas, beaches, and deserts. Therefore if you find a layer of sandstone infer what the environment was like when that layer of rock formed (Figure 7). a b Figure 7. Deserts (a) and beaches (b) are two environments were large amounts of sand is deposited. Figure 6. Above is a sample of sandstone taken the bedrock on the left. This sandstone is from Utah.

Depositional Environments – Siltstones and Shales Siltstone and shale (Figure 8) are made up of silt and clay, respectively. These sediments are deposited in low-energy environments like lakes and more commonly distances further from the coastline (Figure 9). This sediment won’t be deposited along the coast because the water there is moving too fast for these smaller sediments to settle. The water must be very calm in order for silt and clay to be deposited. b a Figure 8. Above is a sample of siltstone (a) and shale (b). Notice the siltstone contains fossils of marine shells, which can commonly be found in siltstone and shales in NY state. Figure 9. The diagram above shows rivers on the left carrying sediment into an ocean, and the locations in which you would expect to find deposits of silt and clay.

Depositional Environments – Chemically and/or Organically Formed Sedimentary Rocks As you can see from Figure 10, chemically formed sedimentary rocks like rock salt, rock gypsum and dolostone form by precipitation, which happens when the water is saturated, which is to say it contains so much of the dissolved mineral, it can’t hold anymore. The mineral then forms, and precipitates and falls to form layers on the bottom. This is very much like snow flakes falling to the ground and piling up. This occurs faster in arid (dry) environments when evaporation rates are very high. The great salt flats (Figure 11) of the western United States provide a good example of these deposits, which were left over after the evaporation of an inland sea. Figure 10. Under the comments section, highlighted in blue, in your reference tables you can see that these types of sedimentary rocks form the precipitation of a solution, like sea water. Figure 11. Great Salt Lake in Utah is located in an arid climate resulting in the formation of rock salt and gypsum.

Depositional Environments – Limestone As Figure 12 shows, limestone primarily forms from the deposition of marine organisms that make their shells out of calcium carbonate (calcite). These organism, like plankton, coral, and foraminifera (tiny see creatures) typically live in warm shallow oceans, so the presence of limestone infers that an area was once a warm tropical ocean (Figure 13). a c e b d Figure 12. Under the texture and comments section you can see that limestone forms from the remains of organic (bioclastic) materials. Figure 13. Foraminifera (a) turns to chalk (b), shells (c) turn to coquina (d), and most forms of limestone when buried deep in the earth convert to the crystalline limestone shown on the right (e).

Depositional Environments – Limestone About 400 million years ago the only plants that existed on Earth were moss-like plants that didn’t grow above a few feet tall. It was around this time period that some plants evolved the ability grow into the large size that we see today. Bacteria and fungi weren’t able to decompose this material so the organic material built up into thick layers. This organic material was later buried and pushed deep into the earth were heat and pressure converted it into coal. Therefore, coal only formed in tropical swamps, so if a layer of coal is found it indicates that area was once a tropical swamp. The presence of large deposits of coal in Pennsylvania infer that this area once resided at the equator and was a lush tropical swamp like the one in Figure 14. Figure 14. This swamp in Mississippi is similar to the coal-producing swamps from the past.