Paleoceanographic and tectonic controls on deposition of the Monterey formation and related siliceous rocks in California

Abstract

The timing of paleoceanographic and tectonic events that shaped the deposition of the Monterey Formation of California and related siliceous rocks has been determined by application of a refined biochronology. The base of the Monterey at 17.5 Ma coincides with rising global sea level and a switch in biogenous silica deposition from the Caribbean and low-latitude North Atlantic to the North Pacific. Major polar cooling, which began at 15 Ma, postdates the base of the Monterey by more than 2 Ma and cannot be invoked to cause the deposition of diatomaceous sediments occurring in the lowermost Monterey. Later polar cooling in the early late Miocene, however, apparently caused increased upwelling and deposition of purer diatomites in the upper Monterey. The top of the Monterey at about 6 Ma coincides with a major sea level drop and is commonly marked by an unconformity. Equivalent unconformities are widespread around the rim of the North Pacific and typically separate more pelagic sediments from overlying sediments with a greater terrigenous component. Above the Monterey, diatoms persist in California sediments to 4.5–4.0 m.y., where their decline coincides with increased deposition of diatoms in the Antarctic. Carbon isotope records in the Pacific and Indian Oceans record storage of 12 C in the Monterey Formation and equivalent organic-rich sediments around the rim of the North Pacific. A +1.0‰ excursion in δ 13 C beginning at 17.5 Ma coincides with rising sea level and probably reflects storage of organic material in Monterey-like marginal reservoirs. A reverse −1.0‰ shift at 6.2 Ma closely approximates the top of the Monterey and may represent erosion of these marginal reservoirs and reintroduction of stored organic carbon into the ocean—atmosphere system. Initiation of transform faulting and extension in the California margin in the latest Oligocene and early Miocene caused the subsidence of basins which later received Monterey sediments. A major tectonic event centered at 10–9 Ma, which included rotation of the Santa Barbara Basin and a change in Pacific plate motion, is recorded by an unconformity and/or interval of compressed sediments in many Monterey sections. Another change in plate motion at about 6 Ma may be expressed by the angular discordance observed in many sections across the unconformity at the top of the Monterey.