Robert Brody is an expert on this, from the close of the talmud (including the compilation of it etc) to the period of the early gaonim is very blurry, we know about the Savoranim but we do not know who they were andhave very little in terms of Sheilot and Teshuvot though we do know that the savoranim were the last in dictating the oral law interpretations and were sort of in between the Amoraim and the Horaa of the Gaonim.