LOWER SILURIAN CONODONTS AND TRILOBITES FROM THE GOBI ALTAI-MANDALOVOO TERRANE, SOUTHERN MONGOLIA

Mongolia is a collage of 44 terranes that accreted throughout the Paleozoic and Mesozoic. This complex coalition of distinct geologic units has made it challenging to decipher the geologic history of the region. To better understand the geologic events that formed Mongolia this study focused on constraining the ages of two formations in the Gobi Altai-Mandalovoo Terrane near Yamaan Us in the Shine Jinst area of southwestern Mongolia. Conodonts and trilobites were recovered from the upper Zalaa and lower Sharchuluut formations from a 60 meter interval across the contact. The Zalaa Formation consists largely of shale with numerous thin fossiliferous trilobite-rich limestone beds in the uppermost portion. The Sharchuluut Formation is mainly crinoidal limestone and siltstones considered early to mid-Silurian based on brachiopod and coral remains. 34 samples horizons were examined for macrofossils and five were found to contain Proetidae trilobites. 20 samples were processed for conodonts and eleven produced identifiable elements. The conodonts Aspelundia expansa, Distomodus kentuckyensis, and Ozarkodina hassi indicate the upper Zalaa Formation and the lower Sharchuluut Formation are lowest Silurian assignable to the expansa Zone. These cosmopolitan conodonts indicate placement of the Gobi Altai – Mandalovoo Terrane in a tropical realm with open sea circulation during the Early Silurian.