A TALE OF TWO TERRANES IN MONGOLIA

Mongolia occupies a key position for unraveling the complex geologic history of central Asia, which amalgamated through the accretion of “exotic” crustal fragments in the late Paleozoic-early Mesozoic. Study of the paleontology, stratigraphy, and sedimentology of Ordovician-Silurian deposits exposed in two Mongolian terranes is adding new data that will help refine paleogeographic models of Mongolia's terranes. The Mandalovoo terrane comprises a nearly continuous Paleozoic island-arc sequence characterized by mildly deformed volcanic and sedimentary rocks overlain by an Upper Paleozoic post-accretionary complex. Upper Ordovician-Upper Silurian rocks exposed in the terrane belong to the Mandalovoo Formation, a volcanic-sedimentary sequence estimated to be 2500 m thick. Siliciclastic deposits and fossiliferous limestones are interstratified with pillow basalts, andesites, volcanic agglomerates, and tuffs. Well-preserved fossils indicate a spectrum of shallow- and deep-marine environments associated with the growth and expansion of island-fringing carbonate platforms. Higher in the section, limestones rhythmically interbedded with shales document the transition upward into turbidites that formed along a deep-marine slope, perhaps of the adjacent Gobi Altai terrane.

Ordovician-Silurian deposits exposed in the Gobi Altai terrane exceed 3000 m in thickness and comprise one of the least deformed, best preserved, and most continuous stratigraphic sections in Mongolia. The Lower Paleozoic deposits reveal abundant shallow-water indicators, including oncoidal limestones and cross-bedded sandstones that grade upward into biohermal, fossiliferous limestones and domal stromatolites. These carbonates are interbedded with shales and overlain by Lower Devonian breccias that comprise angular-subrounded limestone clasts; the entire sequence is capped by younger Devonian-Triassic volcanic-sedimentary deposits. Future research on the preponderance of shallow-marine facies in the Gobi Altai terrane will help ascertain its proximity to other terranes, specifically if it is a backarc basin or a dismembered portion of the Mandalovoo arc terrane.