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ABSTRACT

The Meganos channel, a fossil submarine channel, is in the central part of the Sacramento Valley of California. This channel was cut and filled in a marine environment during a relatively short interval of late Paleocene time. It has been traced in the subsurface more than 50 miles and has a maximum width of 6 miles. The greatest known thickness of sediments filling the channel is 2,015 feet. Formations truncated by the channel range in age from latest Paleocene through Late Cretaceous.

The rocks eroded by the channel are principally arenaceous; however, sediments filling the channel are composed of more than 95 per cent shale. Paleontologic studies indicate that the channel deposits accumulated in water depths ranging from neritic to upper bathyal.

A major factor contributing to the formation of the Meganos channel is thought to be regional faulting. Before the channel existed, the Midland fault, a major north-south-striking feature, began to form. It is the writers’ opinion that the declivity created by this down-to-the-west normal fault set up conditions favorable for extensive slumping and turbidity currents which caused erosion of the sea floor and development of the channel. Subsequently, fine-grained terrigenous elastics filled this erosional feature and, at the same time, were deposited in a thin layer in the areas outside of the channel.

The channel shale contained an original large volume of interstitial water. Burial and overburden pressures compacted the shale to 35-60 per cent of its original volume.

Comparison among ancient subsurface and existing submarine canyons shows that features comparable with the Meganos channel have been formed in the past and are being eroded today, all under differing geologic settings.

Truncation of underlying formations by the channel shale combined with the local structure formed commercial accumulations of hydrocarbons at the Brentwood oil and gas field and the Dutch Slough, River Break, and West Thornton gas fields.

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