By Paul Rincon

Science reporter, BBC News, Houston



A seismic survey peels away the sediments to reveal the structure

The 5.5km-wide bowl is buried under shale sediments west of Stockton, in San Joaquin County, and is thought to be between 37 and 49 million years old.

Researchers are continuing to analyse cuttings from oil exploration wells drilled in the structure.

Details of the discovery were presented at the Lunar and Planetary Science Conference in Houston.

Data from a 3D seismic survey of an ancient sea bed clearly shows a circular structure buried 1,490-1,600m (4,890-5,250ft) below sea level.

The Victoria Island structure, as it has been named, has a concentric rim surrounding a "central uplift" - a peak at the centre - which are both characteristic of impact craters.

Co-investigator Professor Jared Morrow, from San Diego State University, said the context of the crater in relation to surrounding rock layers suggested it dated from the middle Eocene Epoch.

Crater pair

Preliminary estimates for the age of the crater suggest it is just too early for an episode of multiple impacts on Earth known as the late Eocene bombardment, which occurred about 35 million years ago.

One of the largest impact craters in the world - the 90km-wide (56 miles) Chesapeake Bay structure on the eastern shore of North America - is from this period.

The team plans to look for characteristic geological signatures of impact sites in cuttings drilled from wells in and around the structure.

These include a type of quartz deformed under intense pressure - known as shocked quartz - as well as glass and melt particles, and an enhancement of the element iridium.

Victoria Island is not the first crater proposed for the Central Valley. A 1.3km-wide (0.8 miles) feature to the north known as the Cowell structure, dating to the Miocene (5 to 24 million years ago), has also been put forward as the location of a space impact.

Dr Morrow co-authored the work with Samuel Spevack from Grossmont Middle College High School, in California, and Bennett Spevack from ABA Energy.

Paul.Rincon-INTERNET@bbc.co.uk