Geologic History of the Santa Monica Mountains
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 ...view middle of the document...
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). 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).
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...