Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth
Rebecca M. Flowers, Francis A. Macdonald, View ORCID ProfileChristine S. Siddoway, and Rachel Havranek
PNAS May 12, 2020 117 (19) 10172-10180; first published April 27, 2020 https://doi.org/10.1073/pnas.1913131117
Edited by Paul F. Hoffman, University of Victoria, Victoria, Canada, and approved March 6, 2020 (received for review July 30, 2019)
Significance
Erosion below the Great Unconformity has been interpreted as a global phenomenon associated with Snowball Earth. Geological relationships and thermochronologic data provide evidence that the bulk of erosion below the Great Unconformity in Colorado occurred prior to Cryogenian glaciation. We suggest that there are multiple, regionally diachronous Great Unconformities that are tectonic in origin.
Abstract
The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.
Great Unconformity Snowball Earth thermochronology zircon (U-Th)/Heinjectites
Footnotes
↵1To whom correspondence may be addressed. Email: rebecca.flowers@colorado.edu.
Author contributions: R.M.F. and F.A.M. designed research; R.M.F., F.A.M., C.S.S., and R.H. performed research; and R.M.F., F.A.M., and C.S.S. wrote the paper.
Competing interest statement: P.F.H. and F.A.M. are coauthors on three papers, most recently in 2017.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1913131117/-/DCSupplemental.
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