Do you mean your DBF has a column with geologic unit and a column with quadrangle (name/number?) in one row?
If so, to retrieve required data you can query table with something similar to this:
If you realy have to do it with Python, you may need additional module like http://pypi.python.org/pypi/dbf/
SELECT DISTINCT 'Quadrangle'
WHERE 'GeologicUnit' = "user provided name"
From your problem description, it appears that no spatial operation is required (actually it can't be as .dbf do not store spatial type).
In this case maybe its better to import file into Access of Excell and create some form?