Geology of Southern California.pdf - Grossmont College

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26 Geology of Southern California � Figure SC.42 The light brown exposures of 17-million-year-old lava and ash flows in the western Transverse Ranges. organic-rich marine sediments were subjected to elevated temperatures and pressures. The original organic residues were converted to hydrocarbon compounds that comprise oil and natural gas. Once formed, the fluid and gaseous hydrocarbons can migrate through rock bodies in response to subsurface pressures. The formation of a hydrocarbon reservoir in the subsurface requires some mechanism to interrupt the subsurface flow oil and gas and the presence of porous or fractured rock that can absorbed the fluids and gases. Porous Cenozoic sandstone is an important reservoir rock in many southern California oil fields. In addition, the many faults and folds in the region create structural traps (Figure SC.43) that block the flow of hydrocarbons and allow them to saturate the underground reservoir rock. The combination of organic-rich source rock, porous reservoir rock, and structural traps occurs in many areas in the Transverse Ranges and adjacent interior basins. Examples include the Los Angeles basin, the Long Beach area, and the offshore oil fields in the Santa Barbara area (Figure SC.44). In addition to being historically important producers of petroleum and natural gas, the Cenozoic rocks of southern California still contain substantial reserves of petroleum in the subsurface reservoirs and offshore fields. � Figure SC.43 Faults and folds can block the migration of oil and gas in the subsurface, trapping it in layers of porous rock. Frank DeCourten � Figure SC.44 Offshore oil platform west of the southern California coast. Some of the subsurface oil in southern California naturally rises to the surface through fractured rock, within confined layers of porous rock reaching the surface, or along faults. Natural oil seeps occur in many places, both on land and on the seafloor (Figure SC.45). Oil seeping to the surface tends to lose the volatile and more fluid components to the atmosphere, leaving behind thick and viscous tar deposits. People commonly notice tar globs on their shoes or feet after strolling on some of the southern California beaches. This tar generally originates from oil seeps on the neighboring seafloor, and arrives on the beach as a sticky residue after the more fluid components have escaped. In some places, oil flows to the surface in large enough volume for a pool of viscous hydrocarbons to form. The LaBrea tar pits, famous for the remains of ice age animals trapped in the sticky tar, is the best-known example of such a natural oil pool (Figure SC.46). The Channel Islands: Offshore from southern California are several prominent islands including Santa Catalina and � Figure SC.45 Natural oil seep near Ojai in the western Transverse Ranges. Dick Hilton Dick Hilton
Sue Monroe Image courtesy of NASA/Earth Observatory � Figure SC.46 The LaBrea Tar Pits in Los Angeles. San Clemente Islands south of the Los Angeles basin. North of these, the east-west oriented Channel Islands are comprised of San Miguel, Santa Rosa, Santa Cruz, and Anacapa Islands (Figure SC.47). The four Channel Islands were united in the form of an offshore mountain range until about 12,000 years ago when rising sea level isolated the tallest summits as islands separated by ocean passages. The alignment and bedrock geology of the Channel Islands is very similar to the Santa Monica Mountains, a component � Figure SC.47 Offshore islands of southern California. Southern California.6 The Transverse Ranges, Offshore Islands, and Interior Basins 27 Dick Hilton Image courtesy of National Oceanographic and Atmospheric Agency � Figure SC.48 Flat surface in the foreground on Santa Rosa Island, and on San Miguel Island in the distance, are marine terraces eroded at sea level. of the western Transverse Ranges. In particular the mid- Cenozoic volcanic rocks of the Channel Islands are almost identical to those of the Santa Monica Mountains. In addition, many of the geological structures such as faults and folds in the Transverse Ranges can be traced offshore from land onto the Channel Islands. This suggests that the islands represent the same mountain system as the Santa Monica Mountains, except that the westernmost portion was submerged when sea level rose at the end of the last glacial epoch. Like the rest of the Transverse Ranges, the islands offshore of southern California are still rising under the influence of compressive forces generated along the transform plate boundary. Marine terraces are welldeveloped on many of the Channel Islands and reflect as much as 600 meters (2000 feet) of uplift over the past 2 to 3 million years (Figure SC.48). Between the offshore islands of southern California are deep basins and channels such as the Santa Barbara Channel (Figure SC.49), � Figure SC.49 Digital elevation model of the Santa Barbara Channel.
Page 1 and 2: Southern California Geology of Sout
Page 3 and 4: ESSENTIAL QUESTIONS TO ASK Southern
Page 5 and 6: southern portion. For the purposes
Page 7 and 8: in southern California (Figure SC.7
Page 9 and 10: Frank DeCourten directly from the w
Page 11 and 12: The collision between North America
Page 13 and 14: Frank DeCourten � Figure SC.15 Th
Page 15 and 16: state since that time. Much of this
Page 17 and 18: � Figure SC.21 Location of the th
Page 19 and 20: � Figure SC.24 Groundwater flow i
Page 21 and 22: Courtesy of the U.S. Geological Sur
Page 23 and 24: � Figure SC.32 The Salinian block
Page 25 and 26: Courtesy of the U.S. Geological Sur
Page 27: Dick Hilton � Figure SC.39 Precam
Page 31 and 32: Dick Hilton � Figure SC.51 Light-
Page 33 and 34: contain many elements such as boron
Page 35 and 36: Reed Wicander Dick Hilton � Figur
Page 37 and 38: Dick Hilton Marli Bryant Miller �
Page 39 and 40: Ted Soqui/Sygma/Corbis historic eve
Page 41 and 42: John S. Shelton Nick Ut/AP/Wide Wor
Page 43 and 44: closer to the edge of the retreatin
Page 45 and 46: Over-pumping of groundwater in the
Page 47 and 48: cinder cone - a small steep-sided v

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