Assigned Reading: Sedimentary Rocks
Part I: Sedimentary Rocks
I. Three Rock Types: The dynamic evolution of the Earth's lithosphere has produced three basic rock types.
- Igneous rocks: Rocks that form from the cooling and crystalization of magma. These include volcanic rocks such as basalt which form when magma is erupted onto the Earth's surface, and plutonic rocks such as granite which form when magma cools deep within the Earth.
- Sedimentary rocks: These form when fragments of material are transported, deposited together, and lithified by the action of minerals dissolved in groundwater. They may be composed of the remains of living organisms (e.g. limestone) or fragments of pre-existing rocks (e.g. sandstone or shale). Only sedimentary rocks are likely to preserve the fossil remains of organisms.
- Metamorphic rocks: Rocks formed by the chemical transformation of pre-existing rocks caused by extreme temerature or pressure. Examples include marble (altered limestone), quartzite (altered sandstone), and many others.
II. Sedimentation: Whenever weathering attacks exposed, uplifted rock (such as in mountainous regions,) its products, fragments of the original rock, are transported downhill and eventually deposited in basins which become depositional environments. Gravity and water are the most important methods of sediment transport. We recognize two general types of depositional environments:
There are also three general types of sedimentary rock:
- Marine environments: the ocean basins.
- Non-marine environments: the depositional basins of the continents (river valleys, lakes, and deserts) and of the shoreline (river deltas, lagoons, and beaches.)
Erosion, transport and deposition have been going on for all of the Earth's history. The mountains not been worn flat and the ocean basins filled up because plate tectonics constantly generate new uplifts and basins. When continents collide, mountain ranges are created. The modern Himalayas, for example, are currently being thrust up by the ongoing collision of India with Asia. Stress on continents can also create continental basins. The Great Basin of western North America was formed by the stretching and thinning of the continent.
- Clastic sedimentary rock is made of the remains of pre-existing rock that has been transported into a depositional environment. Terrigenous sedimentary rocks are further classified according to the size of the fragments of which they are composed:
- Conglomerate: contains large quantities of large fragments: gravel-size and larger.
- Sandstone: is composed of sand-sized grains.
- Siltstone: is composed of small, silt-sized grains.
- Mudstone: is composed of very small, mud-sized grains. Note: silt and mud look the same, however silt grains are still large enough that silt feels gritty to the touch. Mud feels smooth.
Clastic sediments usually originate on land and can be deposited in either terrestrial or marine environments.
- Biogenic sedimentary rock is made primarily of the remains of living organisms. There are two radically different types:
- A huge veriety of sea creatures secrete shells of calcium carbonate. The remains of these creatures accumulate to form carbonate rocks. To get an idea of the volume of calcium carbonate secreted by living creatures, consider that todays coral reefs, such as the enormous Great Barrier Reef of Australia, are tomorrows carbonate rocks. The most common carbonate rock is limestone. Not surprisingly, carbonate rocks are deposited in marine environments. There are exceptions, however. The some limestone, like that which makes up the stalactites and stalagmites of cave formations precipitates directly from groundwater in terrestrial settings.
- The remains of terrestrial plants are sometimes buried so quickly that they cannot decompose. These give rise to rocks such as coal and lignite.
- Chemical sediments are rocks composed of material that precipitates from water. Rock salt and gypsum are common examples.
At the same time, old sea floor is constantly destroy, along with its accumulated sediments resulting in the rejuvenation of the ocean basins. In this manner, the Earth is constantly thrusting up new parts of itself to be eroded, and suplying basins in which sediments may accumulate and be preserved.
Part Two: Becoming Part of the
Paleontologists exploit this fact to locate fossils, locating places that were basins collecting sediment in the past, but are exposed and undergoing erosion today. These are surprisingly common. Nevertheless, to make it into the fossil record, the remains of an organism must navigate an obstacle course of prerequisites. It MUST do the following things.
I. Possess hard parts:
Fossils are the parts of organisms that become incorporated in sedimentary deposits (i.e. buried). For this to happen, they must survive transport, deposition, and lithifiction. Generally only the hard parts; shells, bones, and teeth; can do this. Soft parts are mechanically destroyed during transport or eaten by scavengers and bacteria. An oyster might make it into the fossil record while a jellyfish from the same environment doesn't have a chance.
II. Get buried:
Most organism are eaten. To reduce the chances of this, the organism should be buried (incorporated in sedimentary deposits) as quickly as possible. The best chance of fossilization exists when burial IS the cause of death.
III. Become preserved over the long term. Tectonic subsidence helps:
For fossils to be preserved for long periods of time, they must be buried at depth, where they are safe from being exhumed and destroyed by erosion. While buried they may remain unaltered, or be altered chemically by:
- Molding and casting
IV. Avoid dissolution:
Most of the minerals that form hard parts are soluble under some circumstances. The same goes for minerals involved in permineralization. To remain fossils, organismal remains must avoid dissolution.
V. Become exposed over the long term. Tectonic uplift essential:
Without tectonic uplift, burial is forever. With uplift, erosion will remove the sediments from above, exposing the fossil to the light.
VI. Get discovered:
Once exposed to the elements, a fossil will be destroyed by weathering and erosion almost instantly. To make it into the fossil RECORD, the fossil must be found and brought to the attenton of a paleontologist, so its existance can be recorded.
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