Shallowing-upward cycles in the Middle Proterozoic Altyn Formation

Phanerozoic carbonate rocks commonly contain cycles, a few metres thick, which were deposited during progressive shallowing of their oceanic environment¹. Each cycle represents a deepening of water in a nearshore area, followed by the accumulation of sediments up to sea level. The causes of the water deepening which begins each cycle are rarely known. Possibilities include tectonic activity that affected the depositional basin and changes in ocean volume due to the expansion and contraction of ice sheets. I report here that similar cycles occur in the Middle Proterozoic Altyn Formation, part of the Belt Supergroup². The cycles are probably related to vertical tectonic activity which influenced sedimentation in the Belt basin. The presence of shallowing-upward cycles in the Altyn Formation shows that such cycles are not confined to the Phanerozoic. Other Precambrian sequences should be examined to see if they too contain cycles. Where such cycles are found they may give important information about Precambrian tectonism and glaciation.