Geologic History of the Santa Monica Mountains
Amanda D’Elia
ESS 1F Lab 1C
15 November 2010
The geologic history of the Santa Monica Mountains breaks up into many different phases of geologic activity. The area begins as a marine environment eventually becoming a terrestrial environment as a result of erosion, deposition, rifting, and subduction. The formations seen on the field trip provide supporting evidence to the tectonic theory of the Santa Monica Mountains. The theory breaks the geologic history into four main phases, but based on the formations eight smaller more precise phases are possible. However, the geologic history of the Santa Monica Mountains, based on the formations from the field, generally fits with the big picture plate tectonic theory synthesis.
The earliest rocks of the Santa Monica Mountains are from the Lat Cretaceous over sixty-five million years ago and were found in the Tuna Canyon formation. These rocks are marine in nature and provide evidence of the first major phase of the ancient terrain. The formation consisted of large conglomerate rocks with metamorphosed volcanic and granite clasts as well as bedded sandstones and graded shale. These findings shed interesting light on the type of marine environment of the ancient surroundings. The mass of large conglomerate boulders indicates that the current of the area was fast. The volcanic and granite composition of the conglomerates suggests that the rocks came from onshore volcanoes and were carried from the mountainous peaks via streams or rivers and dumped in the ocean. Other proof of this deposit is fossils found in this formation of both marine and terrestrial life forms. The bedded sandstone was the main channel of the canyon in which fast water flowed. The graded shale, which must have formed in calmer due to its graded nature, was on the outer margins of the marine canyon from a fan deposit. These characteristics point to an ancient turbidity current as well as volcanism most likely due to subduction. This phase is consistent with the plate tectonic synthesis of the subduction of the Farallon Plate under the North American Plate as it shows ancient marine characteristics and present onshore volcanism, which are consistent with subduction zones (Atwater, 1970).
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The rocks of the Paleocene to early Eocene show continuation of these trends with a greater percentage of conglomerates and small amounts of shallow-water algal limestone suggesting that the canyons were getting less deep and the coastline was expanding westward. These findings indicate that subduction was continuing but uplift was also occurring. This finding is not completely consistent with the materials on tectonic theory but very possible.
The next phase in the history of the Santa Monica Mountains occurred during the Middle Eocene about forty-nine to thirty-seven million years ago. Evidence of this time was found in the Llajas formation, which consisted of shale and siltstone with basal conglomerates overlain by rippled sands and rich in marine fossils. Because the shale and siltstone are somewhat bedded with small conglomerates on bottom and sand on top the current in this marine environment were low and sediment deposition was low. This explanation also accounts for the rippling of the sand. Low currents and sediment deposition tied with an increase in amount and variety of marine fossils implies the presence of a shallow bay. This form of marine environment suggests that the subduction of the Farallon plate must have become flat because the volcanism had stopped because no new sediment was being created and brought quickly to the shore. This assertion is consistent with the tectonic theory of the area, which states that the Farallon plate underwent flat subduction forming volcanism in the area that is now the Rocky Mountains (Ingersoll and Rumelhart1999).
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Following the shallow bay of the Middle Eocene, the history of the Santa Monica Mountains saw great tectonic change. In the Sespe formation of the Lat Eocene to Late Oligocene of thirty-seven to twenty-three million years ago there were medium- to coarse-bedded sandstones with evidence of oxidation, layers of claystone and some conglomerates of pebbles of igneous rocks. The bedded sandstone is the makeup of the base sands. The oxidization of these and some of the other rocks proves that this formation is not marine, because oxidation occurs in the presence of O2 in the atmosphere. The conglomerates of pebbles must have come from faster currents of water, leading to the assertion that the area was home to a river delta. The igneous composition of the rocks confirms that rivers must have carried the rocks, volcanic in nature, from the inland volcanic arc towards the shore. The water levels were also infrequent, changing abruptly. However, small amounts of purple ryholite found in the formation linking to ryholite found in Arizona and the Mojave Desert lead to a very interesting tectonic conclusion. The river delta found in the Santa Monica Mountains was once the delta of the Colorado and Gila Rivers, which deposit in Mexico. The rocks being an unconformity must have been moved from Mexico to the current position in Los Angeles via a transform fault. This conclusion is consistent with the plate tectonic theory that upon full subduction of the Farallon plate, the Pacific and North American plates met and formed a transform boundary known as the San Andreas Fault during the Middle Eocene (Atwater, 1970).
The next phase in the geologic history of the Santa Monica Mountains occurred in the Early Miocene about twenty-three to sixteen million years ago. The Lower Topanga formation of this time consisted of sandstone, shale, some white algal limestone, and many marine fossils. All of these indicate a return to a marine environment. Because of the fossils and white algal limestone the water must have been shallow and a lack of conglomerate explains a low current and low sediment deposition. This might account for a shallow bay. The change from earlier river deltas back to shallow seas shows that the area was undergoing subsidence, or a decrease in elevation. This evidence is consistent with the tectonic theory that after the Pacific Plate and North American Plate met, there was continental extension and rifting (Ingersoll and Rumelhart1999).
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Rifting in its early stages causes subsidence.
Following the subsidence of the Early Miocene, the rifting continued as seen in the Middle Miocene from fifteen to thirteen million years ago. The formation known as the Conejo Volcanics were composed of basaltic flows and breccias. The first set of rocks exhibited pillow basalts interbedded with shale meaning the eruption of the volcanic materials creating the basalt was occurring under water. There was also great fragmentation of the basalt probably due to phreatic explosions. The second set of basaltic rocks was gray andesite with tree fossils. These rocks and tree fossils suggest volcanism on terrestrial surfaces. The lack of order is due to the nature of eruption, which was a pyroclastic flow, possibly beginning on land and flowing into water. The reason for the volcanism during the Early Miocene is consistent with the tectonic theory stating that continental rifting was occurring forming the L.A. basin (Ingersoll and Rumelhart1999).
The rifting was occurring because the Pacific Plate was moving away from the North American Plate ripping it and taking parts of it. This rifting created partial melting and volcanism seen in the Conejo Volcanic formations.
During the Middle to Lat Miocene from thirteen to eleven million years ago, the Middle and Upper Topanga formations were created. These formations were made up of gray clay shale, sandstones, minor pebble conglomerates and organic rich sediments with fossil fish scales. The environment returned back to a marine environment as proven by the shale sandstone and fossil fish scales. This is due to subsidence relating to the late stages of rifting. The terrain lowers in elevation dropping below sea level to become a marine environment, because of cooling and thermal contraction at the end of the extension period. This formation is consistent with the tectonic theory of the area as it occurred after the major volcanism associated with continental rifting (Ingersoll and Rumelhart1999).
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After the rifting event completely ended, the geologic history of the Santa Monica Mountains brought evidence of more marine sediment during the lat Miocene from eleven to five million years ago. The Monterey formation was formed out of siliceous shale containing tests of plankton such as diatoms and radiolaria. This formation points to a marine environment, which was probably fairly deep due to the content of planktons. There proved to be no vertical movement. This is fairly consistent with the tectonic theory though it did not specifically reference this period of only the simple transform margin.
The final phase of the geologic history of the Santa Monica Mountains is fo und at Point Dume Beach and has been occurring since one point eight million years ago. There was lots of sand and steep cliffs. These clues lead to the assertion that deposition from mountain erosion is occurring along with uplift of the mountains. This uplift is seen in all the previous formations as most are greatly tilted. This uplift is also consistent with the tectonic theory because the theory states that the San Andreas Fault has moved inward towards the Gulf of California causing a bend with thrust faults in the Transverse Ranges (Ingersoll and Rumelhart1999).
The thrust faults are responsible for the continued uplift of the Santa Monica Mountains as well as all the formations seen on the field trip.
The geologic history of the Santa Monica Mountains is one of great change due to ever changing plate tectonics. The formations seen during the field trip gave ample evidence of the corresponding theory of plate tectonics in the Santa Monica Mountains. From ancient marine environments, volcanoes and river deltas, to current uplift and beach deposition, the area known today as the Santa Monica Mountains has been on a long road to uplift encountering erosion, deposition, rifting, and subduction along the way.
Sources
Atwater, T. (1970) Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, Geological Society of America Bulletin, Volume 81, pages 3513-3536.
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Ingersoll, R. V., and P. E. Rumelhart (1999) Three-stage evolution of the Los Angeles basin, southern California, Geology, Volume 27, pages 593-596.
