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UC Berkeley UC Berkeley Previously Published Works UC Berkeley UC Berkeley Previously Published Works Title Neoproterozoic glacial origin of the Great Unconformity. Permalink https://escholarship.org/uc/item/5nn1z7m4 Journal Proceedings of the National Academy of Sciences of the United States of America, 116(4) ISSN 0027-8424 Authors Keller, C Brenhin Husson, Jon M Mitchell, Ross N et al. Publication Date 2019 DOI 10.1073/pnas.1804350116 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Neoproterozoic glacial origin of the Great Unconformity Preprint ∗ C. Brenhin Keller1,2, Jon M. Husson3, Ross N. Mitchell4, William F. Bottke5, Thomas M. Gernon6, Patrick Boehnke7, Elizabeth A. Bell8, Nicholas L. Swanson-Hysell2, and Shanan E. Peters9 1Berkeley Geochronology Center 2Department of Earth and Planetary Science, University of California, Berkeley 3School of Earth and Ocean Sciences, University of Victoria 4Department of Applied Geology, Curtin University 5Southwest Research Institute, Boulder 6Ocean and Earth Science, University of Southampton 7Department of the Geophysical Sciences, The University of Chicago 8Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles 9Department of Geoscience, University of Wisconsin, Madison Abstract The Great Unconformity, a profound gap in Earth’s stratigraphic record often evident below the base of the Cambrian system, has remained among the most enigmatic field observations in Earth science for over a cen- tury. While long associated directly or indirectly with the occurrence of the earliest complex animal fossils, a conclusive explanation for the formation and global extent of the Great Unconformity has remained elusive. Here we show that the Great Unconformity is associated with a set of large global oxygen and hafnium isotope excursions in magmatic zircon that suggest a late Neoproterozoic crustal erosion and sediment subduction event of unprecedented scale. These excursions, the Great Unconformity, preservational irregularities in the terres- trial bolide impact record, and the first-order pattern of Phanerozoic sedimentation can together be explained by spatially heterogeneous Neoproterozoic glacial erosion totaling a global average of three to five vertical kilometers, along with the subsequent thermal and isostatic consequences of this erosion for global continental freeboard. Significance Walcott [2]. Observing a dearth of conformable sections span- ning the lower boundary of the Cambrian, Walcott proposed a It has long been observed that the sequence of sedimentary rocks “Lipalian” interval of continental exposure and erosion, which deposited in the past half-billion years often sharply overlies older igneous or metamorphic basement at an erosional surface known would have restricted any fossil precursors of the Cambrian as the Great Unconformity. We provide evidence that this uncon- fauna to the deep ocean basins. Subsequent investigation has formity may record rapid erosion during Neoproterozoic “snow- revealed a more complete Proterozoic, including fossiliferous ball Earth” glaciations. We show that the extent of Phanerozoic strata and conformable boundary sections; yet the observation sedimentation in shallow continental seas can be accurately repro- of a profound and extensive (if discontinuous) pre-Cambrian EarthArXiV Preprint 10.31223/osf.io/4k6pd duced by modeling the accommodation space produced by the unconformity remains [4, 5, Dataset S1]. Here we attempt to Proc. Natl. Acad. Sci. 10.1073/pnas.1804350116 proposed glacial erosion, underlining the importance of glacia- unite disparate evidence including the zircon Hf and O isotope tion as a means for lowering erosional base level. These results records, the terrestrial bolide impact record, and the record of provide new constraints on the sedimentary and geochemical en- continental sediment coverage in the context of this widespread vironment in which the first multicellular animals evolved and unconformity. diversified in the “Cambrian explosion” following the unconfor- mity. ADiscontinuous Global Unconformity arth’s sedimentary cover necessarily rests at depth upon ig- E neous or metamorphic crystalline basement. This contact The extent and magnitude of secular variation in preserved sed- need not be abrupt, since accumulating sediments gradually re- iment abundance across the Proterozoic-Phanerozoic boundary crystallize and metamorphose under increasing heat and pres- was first quantified by Ronov [4, Dataset S2], estimating pre- sure. Where observed, however, this transition often takes the served sediment volume flux over the past 1.6 Gyr from mapped form of a spatially abrupt and temporally correlated exposure sedimentary basin areas and stratigraphic thicknesses. The re- surface known as the Great Unconformity, a lacuna of both time sulting temporal pattern has been subsequently refined in Lau- and mass [1, 2, 3, 4, 5]. While often deeply buried, the Great rentia by the Macrostrat database [7, 8, 9] which (within North Unconformity is exposed in areas of relief such as the Grand America) provides higher-resolution temporal and spatial con- Canyon of the southwestern United States, where it was recog- straints. Together these records corroborate the presence of a nized by Powell [1], most dramatically at the sharp nonconfor- large global shift in preserved continental sediment abundance mity between the Paleoproterozoic Vishnu Schist and Cambrian near the base of the Cambrian (Fig. 1a; Figs. S1-S3). Tapeats Sandstone [6]. The ubiquity of this pattern – unde- The observed increase from roughly 0.2 km3/yr of preserved formed clastic sediments deposited directly and unconformably sedimentary rock in the Proterozoic to ∼1 km3/yr in the atop Precambrian basement – was subsequently recognized by Phanerozoic (Fig. 1a) might be attributed in principle to either *correspondence: [email protected] Preprint –Neoproterozoic glacial origin of the Great Unconformity 2 Ronov,1980 (global) Alternatively, a purely constructive interpretation would require /yr) 3 a roughly five-fold increase in sediment supply and/or continen- 3 Macrostrat (N. Am.) scaled: tal accommodation space, sustained throughout the Phanero- fluxN.Am.∗area land / areaN.Am. zoic. However, the observed Great Unconformity is profoundly Proterozoic Phanerozoic erosional in nature, characteristically juxtaposing fluvial sedi- ment with crystalline basement that was formed at great depth 2 in the crust. For instance, as shown in Fig. 1b, the Cambrian Ig- nacio Quartzite is deposited directly upon the Mesoproterozoic Eolus Granite (Fig. 1), a pluton with an emplacement depth of approximately 10-15 km (3-4.5 kbar) [12], requiring the ero- sion of over a third of the nominal thickness of the continental 1 crust over some subset of the ∼0.9 Gyr of geologic history miss- ing from this section. Posing an additional conundrum in either scenario, the a Phanerozoic-Proterozoic boundary is rather unexceptional from Preserved sediment volume (km 0 a mantle perspective, with no major variation in mantle po- 2500 2000 1500 1000 500 0 tential temperature or tectonic style evident in the continental Age (Ma) record [13, 14, 15, 16]. Consequently, it is difficult to conceive of a model where tectonic sediment supply and basin forma- b tion increase profoundly as a result of Neoproterozoic solid- Earth processes alone, or one in which dramatically increased tectonic exhumation drives unprecedented erosion. Moreover, while the Rodinian supercontinent cycle features a number of Cambrian marine sandstone noteworthy irregularities – including extroverted superconti- nent assembly [17] and an unusual ore deposit profile [18, 19] Great Unconformity – it is unclear how such irregularities could contribute to the formation of the Great Unconformity and associated global pre- Mesoproterozoic granite served sediment abundance variations in the absence of signifi- cant excursions in mantle potential temperature. In either a constructive or destructive endmember scenario, if global sediment supply from tectonic uplift is held constant 50 m near Phanerozoic levels, then the depressed Proterozoic sedi- ment volume in Fig. 1a suggests that on the order of 109 km3 Figure 1: The Great Unconformity. (a) Global preserved sedimen- of sediment are absent from the continental crust and deposited tary rock volume increases by more than a factor of five across the Phanerozoic–Proterozoic boundary in both the estimate of Ronov [4] instead in the deep ocean basins – either gradually, throughout and a global scaling of North American units from the Macrostrat the Proterozoic due to a diminished sediment storage capacity database by the area ratio of global land area to North American land of the continents in a constructive model, or rapidly during an area (a factor of 6.1) after Husson and Peters [8], excluding recent allu- interval of enhanced erosion near the Proterozoic-Phanerozoic vium. (b) The Cambrian Ignacio Quartzite overlies the Mesoprotero- boundary in a destructive model. Indeed, prior to the plate zoic (∼1.43 Ga) Eolus Granite at a sharp peneplanar nonconformity in tectonic revolution, the missing sediments from Walcott’s “Li- the Needle Mountains, Colorado. palian interval” were generally expected to reside in the ocean basins [2, 20]; their absence, along with the young age of the ocean crust, was considered a significant point of evidence in fa- vor of seafloor spreading and plate tectonics [20]. In a plate tec- tonic
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