A dispute between the Brazilian Navy and an American marine archeologist has led Brazil to bar the diver from entering the country and to place a ban on all underwater exploration. The dispute involves Robert Marx, a Florida author and treasure hunter, who asserts that the Brazilian Navy dumped a thick layer of silt on the remains of a Roman vessel that he discovered inside Rio de Janeiro's bay. The reason he gave for the Navy's action was that proof of a Roman presence would require Brazil to rewrite its recorded history, which has the Portuguese navigator Pedro Alvares Cabral discovering the country in 1500. All ... permits for underwater exploration and digging, a prolific field in Brazil, have been canceled as a result of the Marx controversy ... Navy officials said. The story goes back to 1976 when lobster divers first found potsherds studded with barnacles. Then a Brazilian diver brought up two complete jars with twin handles, tapering at the bottom, the kind that ancient Mediterranean peoples widely used for storage and are known as amphoras. According to Elizabeth Will, a professor of classics and specialist in ancient Roman amphoras at the University of Massachusetts at Amherst, the jars are very similar to the ones produced at Kouass, a Roman Empire colony that was a center for amphora-making on the Atlantic coast of Morocco. Reached by telephone, Professor Will said of the fragments she had studied: ''They look to be ancient and because of the profile, the thin-walled fabric and the shape of the rims I suggested they belong to the third century A.D..''