Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons


Karlstrom, Karl
Hagadorn, James
Gehrels, George
Matthews, William
Schmitz, Mark; Madronich, Lauren; Mulder, Jacob; Pecha, Mark; Giesler, Dominique & Crossey; Laura

The Sauk transgression was one of the most dramatic global marine transgressions in Earth history. It is recorded by deposition of predominantly Cambrian non-marine to shallow marine sheet sandstones unconformably above basement rocks far into the interiors of many continents. Here we use dating of detrital zircons sampled from above and below the Great Unconformity in the Grand Canyon region to bracket the timing of the Sauk transgression at this classic location. We find that the Sixtymile Formation, long considered a Precambrian unit beneath the Great Unconformity, has maximum depositional ages that get younger up-section from 527 to 509 million years old. The unit contains angular unconformities and soft-sediment deformation that record a previously unknown period of intracratonic faulting and epeirogeny spanning four Cambrian stages. The overlying Tapeats Sandstone has youngest detrital zircon ages of 505 to 501 million years old. When linked to calibrated trilobite zone ages of greater than 500 million years old, these age constraints show that the marine transgression across a greater than 300-km-wide cratonic region took place during an interval 505 to 500 million years ago—more recently and more rapidly than previously thought. We redefine this onlap as the main Sauk transgression in the region. Mechanisms for this rapid flooding of the continent include thermal subsidence following the final breakup of Rodinia, combined with abrupt global eustatic changes driven by climate and/or mantle buoyancy modifications.

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Left, ages are plotted on the Cambrian chronostratigraphic chart; GSSP, global boundary stratotype section and point16. Estimates of the ages of the trilobite zones (in blue boxes) are based on global correlations. The stratigraphic column of the Cambrian system in the Grand Canyon region is shown on the right (note the breaks in the scale); purple, Sixtymile Formation; yellow, Tapeats Sandstone; blue, Bright Angel Shale; green, Muav Limestone; errors for IDTIMS ages are black and are smaller than the red dots. Right, normalized probability density plots (y axis is relative probability) show main age peaks; maximum depositional ages were calculated using the weighted mean LA-ICPMS age of the youngest grain population (see Supplementary Table 1); єzr = 4/317 = number of Cambrian grains/total number of grains dated. AZ, Arizona; NV, Nevada; GC, Grand Canyon; Fm, Formation.

Publication Listing

Nature Geoscience volume 11, pages 438–443 (2018)