The Bayan Shireh Formation (also known as Bayanshiree Svita or the Baynshirenskaya Formation) is a geological formation in Burkhant, Mongolia, that dates to the late Cretaceous period. The exact age is uncertain, with two competing hypotheses; based on comparisons with other formations, the Bayan Shireh fauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago. However, examination of the magnetostratigraphy of the formation seems to confirm that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a possible Cenomanian through Santonian age, or between 98 and 83 million years ago. The fauna of this formation is similar to the nearby Iren Dabasu Formation.
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