The Annalista Saxo records "Thietmarum marchionem et Geronem Coloniensem archiepiscopum" as sons of "Soror eorum [Gero marchiorum et Sigefridus] Hidda nominee", and their joint construction of an abbey "in predio suo [Hidda] iuxta Salam fluvium". His birth date range is estimated from his first mention as count in 951.
Graf im Gau Serimunt 951: "Otto…rex" confirmed the donation by his son Liudolf of property "in pago Serimunt in comitatu Thetmari comitis" on the intervention of "marchionis Geronis" to the church of Magdeburg by charter dated 28 Jul 951. Graf im Schwabengau 951/978. "Otto…imperator augustus" granted property "in pago Derlingon in comitatu Thiatmari comitis" to Magdeburg St Moritz by undated charter, placed in the compilation with other charters dated mid-965. "Otto…imperator augustus" gave property "in villa Drogobuli quæ Salam et Fonam fluvios interiacet in comitatu eiusdem comitis Thietmari" to "comiti nostro Thietmaro" by charter dated 29 Nov 965. "Otto…imperator augustus" donated property "in pago Hardegouue et in comitatu Thiatmari" to Magdeburg St Moritz by charter dated 23 Sep 967. Graf der Mark Merseburg und der Mark Meissen [Ostmark] 976. Markgraf.
The death and place of burial of "Thietmarus marchio frater Geronis Coloniensis archiepiscopi" is recorded in the Annalista Saxo. The necrology of Lüneburg records the death "3 Aug" of "Thietmarus marchio". 1
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