TITLE INTRODUCTION DETRITAL ROCKS CHEMICAL ROCKS
ENVIRONMENTAL CLUES IN
SEDIMENTARY ROCKS
graded bedding cross bedding ripples mud cracks
raindrop impressions bioturbation tracks and trails fossils
Geologists carefully study rocks to learn about planet Earth. Igneous rocks contain mineral crystals formed due to the cooling of molten rock from Earth's interior, so they provide information about the chemical composition of Earth. Metamorphic rocks are composed of a variety of minerals formed or introduced by processes occurring within Earth, so they tell us about physical and chemical conditions within Earth. Sedimentary rocks tend to form at or near Earth's surface, so they often contain information about natural environments that have existed on Earth over the past few billion years.
Transportation of Sediment Sediment is moved about on Earth's surface by a variety of means, primarily by streams, wind, and ocean currents. Streams and ocean currents are fluids capable of moving all sizes of sediment grains, from clay to sand, and even large boulders if current velocity is swift and energetic enough. Wind is generally capable of moving only clay to sand-sized grains unless it is blowing with hurricane force.
Scott in a stream. A Death Valley wind storm. The Pacific Ocean at Palos Verdes Peninsula.
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Deposition of Sediment Sediment will settle from streams, ocean currents, or wind as flow velocity decreases. Typically, sediment will accumulate as horizontal layers, one atop the other. As this process occurs, certain distinctive depositional features may develop within the sediment layers, providing information about the method of deposition, and therefore clues as to the natural environment in which the sediment was deposited.
graded bedding This feature forms as a current of water (stream or ocean) slows gradually. Larger sediment grains will settle first, followed by smaller grains as the current's energy decreases. So, a typical graded bed has larger grains at the bottom, with progressively smaller grains toward the top. Graded beds are characteristic of stream deposits, but they can also form where turbulent ocean currents move rapidly from the continental shelf or continental slope out into deeper water.
A single graded bed (note ID card for scale). A single and a series of graded beds, with Scott for scale.
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cross bedding Cross beds form as high-velocity currents (wind or water) transport sediment grains over an obstacle such as a boulder or ridge. As this occurs, gravity pulls the sediment grains downward, behind the obstacle. As a result, tilted sediment layers are deposited behind the obstacle, forming cross beds. Cross beds formed from water currents tend to be relatively small in scale because their development is greatly modified or impeded by complex current flow within the current's channel. Wind-formed cross beds, in comparison, can be quite large because air flow can be constant for hours or even days, producing sand dunes composed of sets of cross beds up to 100 feet thick. Since the angle at which cross bedding tilts is in the direction of current flow, cross beds within ancient rocks can tell geologists the direction that a stream flowed millions of years ago, or the direction of prevailing winds in the distant past. Excellent bedding diagrams and animations are presented in the United States Geological Survey's website at http://walrus.wr.usgs.gov/eseds/ .
Bob surveys a modern, active dune. The cross beds are tilted to the
right, the direction of wind flow.
Wayne hiking across ancient sand dunes preserved within sandstone of
northern Arizona.
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ripples Ripples form as wind or water move over a surface composed of loose, fine-grained sediment. Friction from the wind or water drags sediment grains a short distance before they begin to pile up on each other in rows that are perpendicular to the direction of current flow. These ripples form sinuous, beautiful patterns on sand dunes, along beaches, and on the bottom of stream channels and the ocean floor.
Like cross bedding, a ripple can indicate the direction of current flow because its steep side points in the direction of current flow. Such ripples are asymmetrical in shape, versus the less-common symmetrical ripples that form as waves rock back and forth in shallow water.
asymmetrical ripples - both with current direction toward the right
New ripples with footprints. Ancient ripples on rock slab next to ski pole.
symmetrical ripples - current direction is variable
Bruce pointing to new ripples formed by waves. Ancient ripples formed by small waves.
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Other Environmental Clues Within Sedimentary Rocks Once sediment is deposited on Earth's surface, natural events such as changing weather conditions or biological activity may alter the primary depositional features that formed during deposition. These features also provide geologists with important clues about the ancient natural environment that existed soon after the sediment was deposited, before it turned into rock.
mud cracks Mud cracks form within wet clay-rich sediment as it dries and shrinks. Mud cracks are common features along the shores of lakes or streams that dry up during a drought or toward the end of a hot summer season. Mud cracks are readily preserved if they become covered with sediment that protects them from erosion.
Diane next to modern mud cracks. Ancient mud cracks on bottom (sole) of a rock layer.
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raindrop impressions Raindrops striking unconsolidated sediment can form miniature impact craters. Once the sediment dries, these impressions can be preserved if they become covered with sediment soon after they were formed. Below are numerous raindrop impressions on modern sediment, with a shoe for scale.
bioturbation Bioturbation, that is, the disturbance of sediment layers by biological activity, is a significant process on the ocean floor. In that environment, numerous animals such as worms exist by consuming organic matter trapped between sediment grains. Animals like clams burrow through sediment to hide from predators swimming or crawling above the ocean floor. Either activity requires the animals to burrow through the sediment, destroying some of the pre-existing sedimentary features such as cross bedding or ripples.
Burrows within sandstone. Traces of burrowing animals in limestone. Disruption of layers within limestone.
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tracks and trails As animals move across unconsolidated they may leave a trace of their passage. Their tracks and trails are most readily preserved in wet sediment that becomes buried under more sediment soon after the animals pass by.
The trail on the left was made recently by a snail moving along the surface of a mud puddle. For scale, the stick is one inch in diameter. Snails methodically make a spiral pattern on the sediment as they consume organic matter on top of the mud surface. The image to the right shows similar patterns on sandstone surface that is approximately 400 million years old. Comparison of these trails illustrates the principle of uniformitarianism, the foundation of geology, that states that the present is the key to the past. So, the spiral patterns on the ancient rock were probably made by an animal similar to modern snails, in a similar natural environment.
The images below show a dinosaur pathway on steeply tilted sedimentary rock in Bolivia, just outside of Sucre. On the left is a distant view of the outcrop. The rounded tracks shown in the second and third images were made by a small herd of sauropods over 100 million years ago. The fourth image is of a smaller three-toed track. The image to the far right is of the same pathway, here showing the smaller three-toed tracks intersecting the larger tracks.
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If you don't want to journey to Bolivia to see dinosaur tracks, visit the Navajo Indian reservation in northern Arizona. A local guide can lead you to excellent tracks preserved in sandstone. Water poured onto the tracks highlights their features, but this will accelerate chemical weathering which may eventually ruin the tracks. The relatively small tracks on the left were made by plant-eating dinosaurs, but the large track to the right has been attributed to the king of carnivores, Tyrannosaurus rex. Note the size 11 sandals for scale.
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fossils Fossils are the remains of once-living organisms that became preserved within sediment and sedimentary rock. Both plants and animals can form fossils, but usually only their hard, skeletal components are preserved. Scavengers and bacteria typically destroy softer tissues. Plants and animals that die on land are not readily preserved unless they are rapidly buried in sediment, perhaps during a flash flood or some other disasterous event. Plants and animals that live and die in the ocean are far more likely to be preserved because the ocean is an environment where sediment is constantly settling to the ocean floor, covering and preserving the remains of plankton, clams, fish and whales.
There are several modes of preservation, including preservation of the fossil in an unaltered condition, replacement or recrystallization of the original skeletal material, or dissolution of the original skeletal material which can lead to the formation of a mold or cast of the fossil. These different modes of preservation result in varying qualities of detail preservation of the fossil organism's surface shape and internal and external characteristics. The more details that are preserved, the more geologists can learn from the fossil about its behavior and habitat. With regard to the modes of preservation and the quality of detail preservation, unaltered fossils show the finest details, followed by replacement, recrystallization, and finally by typically crude preservation of molds and casts.
The following images show a variety of fossils in different modes of preservation.
LAND FOSSILS Image 1 shows a dinosaur leg bone, Dinosaur National Monument, Colorado. The second image is a fossilized turtle shell. The third image is a fern leaf fossil cast. The image 4 is a leaf fossil impression in the limestone wall rock of the Getty Museum, Los Angeles.
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OCEAN FOSSILS
row A The first image is fossiliferous limestone containing a variety of recrystallized fragments of brachiopods, trilobites and bryozoans. Image 2 is a microscopic image of a similar limestone displaying closeup views of the fossil fragments. The third image is also a fossiliferous limestone, but the dominant fossils are brachiopod valves that are recrystallized. The fourth image is a fossiliferous limestone rich in recrystallized gastropod fossils.
row A 
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row B The first image is an urchin minus its spines, which tend to fall off soon after death. This specimen is partially recrystallized, and is filled with sedimentary rock forming an internal mold within. The second image is a large recrystallized coral. This specimen was photographed in the desert of Morocco, a very different environment from the shallow ocean in which the coral colony once lived. The image 3 shows a trilobite whose exoskeleton has been replaced by a carbon film. The fourth image is of a recrystallized trilobite.
row B 
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row C The first image shows a cluster of ammonites whose original aragonite shells have been completely replaced by pyrite. The second image is of small gastropods that are partially replaced by pyrite. The image 3 contains an ammonite and a gastropod from the previous images. Note the metallic luster from the pyrite. (The fossils in this row were collected by Ms. Julie Draper.)
row C
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row D These fossilized fish skeletons were collected from ancient lakebed deposits in Wyoming. Note the fine details preserved by the replacement of the original skeletal material with hematite.
