Archean To N E O P R O T E Ro Z Oi C

The Canyon's Unkar Group of strata, forming the lower part of the Grand Canyon Supergroup and ranging in age from 1.2-1.05 billion years, record the situation when the Rodinian super continent was assembled. But the story is far from complete. There is probably a significant time gap between the top of the Unkar strata and the base of the overlying Chuar strata, which may represent as much as 250 million years (between 1050 and 800 million years ago).

Analysis of the Chuar strata shows that a major change in tectonic style was underway and that Rodinia was in the process of being broken apart as the crust was stretched and broken into fault blocks. But even the superb Canyon record is incomplete here and does not fully represent late Proterozoic time. The top of the Chuar, with the last and youngest preserved strata being somewhat more than 700 million years old, leaves another gap of around 150 million years before the beginning of Cambrian time. Fortunately, this gap is filled by strata preserved not too far away in southern California.

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