A formal inquest into the death in jail of South African black leader Steve Biko opened today but was immediately adjourned until Nov. 14. The maneuver effectively silenced the Biko family and delayed release of crucial documents.

The presiding judge said Biko apparently did not die of natural causes and one prosecutor suggested criminal charges might be filed, but that step cannot be taken while the inquest is in progress.

Pretoria's chief magistrate, M. J. Prins, opened the inquest unexpectedly and immediately adjourned it until Nov. 14.

Sources close to the Biko family said the inquest was originally intended to open Friday, but Pretoria's chief magistrate, M. J. Prins, moved it up one day to put the affair under jurisdiction of the courts, thus banning public statements.

Prins said that from "certain information" provided by Transvaal attorney general Jacobus Nothling, it appeared Biko's death was not natural.

The decision to hold an inquest is significant because it provides the Biko family lawyers the opportunity to summon witnesses.

Biko was arrested in Port Elizabeth in eastern Cape Province Aug. 5, but was transferred to Pretoria the night before his death.

Sources close to the Biko family said the autopsy - a joint report by a state pathologist and one appointed by Biko's widow, Ntsieki - showed that Biko died of severe brain damage impaired blood circulation and acute kidney failure.

In another celebrated case, the trial of black militant Winnie Mandela on charges of violating a government banning order was adjourned today until Jan. 17. Her attorney requested the adjournment because a defense witness was taken ill.

The wife of jailed nationalist leader Nelson Mandela earlier pleaded innocent to charges involving visits and "unlawful social gatherings" while under banning orders.

Mrs. Mandela, 43, was allowed to return to her home in Brandfort until the trial resumes.