Sediments and Sedimentary Rocks Chapter 8 Sediments Cover most of the land surface and seafloor sedimentary rock cycleYour physical geography determines the dominant process at work in the sedimentary rock cycle weathering and/or erosione.g. (1) Mountains => weathering and/or erosion e.g., (2) Southern Louisiana => e.g., (3) offshore Louisiana => Sediments Cover most of the land surface and seafloor Your physical geography determines the dominant process at work in the sedimentary rock cycle e.g. (1) Mountains => weathering and/or erosion transportation or deposition or erosione.g., (2) Southern Louisiana => transportation or deposition or erosion e.g., (3) offshore Louisiana => Sediments Cover most of the land surface and seafloor Your physical geography determines the dominant process at work in the sedimentary rock cycle e.g. (1) Mountains => weathering and/or erosion e.g., (2) Southern Louisiana => transportation or deposition or erosion erosion or sedimentatione.g., (3) offshore Louisiana => erosion or sedimentation erosion weathering transportation Erosion includes BOTH weathering and transportation Sedimentary rocks are typically layered, (although layering is not diagnostic of only sedimentary rocks) Where do you live? What dominant sedimentary process is at work where you live? If deposition is the dominant process, e.g., offshore Lousiana then rocks are in the process of being formed: Diagnesis Diagnesis includes compaction (1) compaction = volume loss (mechanical squeezing) dewatering and is accompanied by dewatering (= water loss) (by chemical or physical means) (2) changes in mineral composition (chemical process with heat and or fluids) cementation (3) cementation (physical) lithified If a sediment eventually becomes a rock we say it is lithified. compaction DIAGENESIS compaction dewatering DIAGENESIS compaction dewatering Cementation & mineral changes DIAGENESIS degree of weathering and transport Geological FUZZY rules for determining degree of weathering and transport a rock or sediment has experienced respectively (1) Product composition (2) Degree of sorting Sortinghow similarSorting is a measure of how similar grain sizes are within a sediment or rock and tells us about the relative strength of the current before it dropped (deposited) it cargo. size settling velocityIn a current of water or air, the larger and denser grains fall faster than the smaller grains. That is, for the same density, size determines settling velocity. (3) Angularity or roundness (antonym) is a measure of the distance of transportation Cross-bedding Cross-bedding: Cross-bedding: sets of bedded material within rock layers that are inclined at angles as large as 35 degrees from the horizontal. These latter indicate wind- blown conditions in either a desert or a beach. 2 directions of fluid movement Fossil example of the past activity of organisms mixing sediment --- an example of fossil BIOTURBATION A sedimentary environment is a geographic location that has a peculiar combination of geological processes Walthers Rule (1894) The different (sedimentary) rocks (types) were formed beside each other in space, but in a crustal profile we see them lying on top of each other. Bedding sequences--- are successions of rock ( in a vertical profile) that help geologists work out the past environment Three types of sedimentary Rocks Clastic Biochemical Chemical Major Classes of sediments and sedimentary rocks For clastic sedimentary rocks there is a classification scheme based on the SIZE of their clasts, (or rock fragments) that comprise them. Clst size indicates ancient relative current velocity >=1.8 km/hr (strong currents)weakmoderate The End of Chapter 8 Understanding Earth Chapter 8: Sediments and Sedimentary Rocks Copyright 2004 by W. H. Freeman & Company Frank Press Raymond Siever John Grotzinger Tom Jordan Fourth Edition