The great equatorial glacier

The boring-billion ended 825-765 Mya when pancontinental Rodinia split. One daughter-craton, we call "Laurentia". North America now.

Thomas M. Gernon and five coauthors argue that the split caused a rift. Rifting might dip a deep valley below but it also raises a highland escarpment at the rim. They have inherited the term Cordillerian, for a rim of (later) sedimentary fallout in the Ediacaran era. That forms the northern rim of the escarpment, which escarpment they propose bent south at its western edge following the then-new continent.

Between 765 Mya and the Ediacaran, at 717 Mya came a Snowball Earth episode: the Sturtian event of "Cryogenian". The paper says the escarpment had by then risen eight kilometers. Maybe this occidental himalaya averaged "only" seven above present sealevel but that still would be a great place to collect ocean moisture (tides were higher then too). This water froze up there, glaciating just about all Laurentia and lowering the ocean. Most the uplifted rock got scoured down to the "Vishnu Supercyst" laid down long before 1.6 Gya; although some intervening rock survives in gaps. New sediment won't be laid down to stay, until the narrow Cordillerian; the Grand Canyon region must wait longer, for the "Tonto" known to Brits as Cambrian. Thus, the Unconformity. (We're not missing much, we're assured.)

For those pondering if glaciers might end up halting their flow and ceasing erosion, allow me to supplement: the Sturtian was never a uniform freeze and it formally melted 660 Mya. The paper doesn't name the Sturtian but it does know the Marinoan when the glaciers returned 650-635 Mya. These climate shifts might not have mattered in higher latitudes but they assuredly mattered near the equator.

The ghost of this super-escarpment seems to have held. The Old Rocky Mountains perhaps guarded our flank in the Permian, although we don't get a map. We do get a map of postapocalyptic 65 Mya. By that point everything here is rotated and, of course, wholly northern. Here the then-new Rocky Mountains butt up against that once-north-now-west boundary, and the Mexican Peninsula's altiplano from the now-south. (Central America, then, was an archipelago, like the Greater Antilles; both also-then-new.)