Seminal U.S. documents are the ones that influence the country and provide a basis for its further development. It can be a speech or a constitutional amendment, treaties or public laws and even Supreme Court decisions.
The Bill of Rights guarantees rights like freedom of speech, assembly, worship. So, it is a seminal document.
The Gettysburg Address given by President Lincoln is considered extremely important in history as he talked about equality and principles of liberty on which U.S. government is found.
Letter from Birmingham Jail is also a seminal document as it is based on moral grounds for the non-violent civil rights of Birmingham campaign.
The Declaration of Independence
declared "Thirteen Colonies" in North America as "Free and Independent".