GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 236-3
Presentation Time: 8:40 AM


MACDONALD, Francis A., Department of Earth Science, UCSB, 2111 Webb Hall, Santa Barbara, CA 93109 and FLOWERS, Rebecca, Department of Geological Sciences, University of Colorado, Campus Box 399, 2200 Colorado Ave, Boulder, CO 80309

The Great Unconformity (GU) marks a major time gap in North America, locally separating Precambrian basement rocks from Cambrian sedimentary rocks. Recently it was proposed that erosion across the GU was primarily associated with Snowball Earth glaciation, with a global average of ~4 km Late Neoproterozoic denudation1. This hypothetical pulse of erosion has been suggested to have lubricated subduction zones, leading to changes in tectonic styles2 and the chemistry of igneous rocks1. These interpretation are based on: 1) Macrostrat data, 2) zircon Hf and O isotopes, 3) analogs with Quaternary glacial erosion, 4) the impact cratering record, 5) a Neoproterozoic increase in passive margins, 6) seawater Sr isotope data, and 7) consistency with extant thermochronology data. We address these interpretations: 1) The Macrostrat record is North America craton-centric, does not account for tectonic basin formation such as Late Neoproterozoic to Cambrian rifting, and misses Late Paleozoic and Mesozoic erosion revealed by thermochronology. 2) The Hf and O isotope record in zircon is complicated by sampling bias, metamorphism, crustal melting, and contamination of individual measurements from large Hf isotope spot sizes, which results in erroneous εHf values and model ages. 3) Erosion of Paleozoic cover from the Canadian and Fennoscandian shields predated glaciation. Erosion below the Laurentide, Fennoscandian, and Antarctic ice sheets was minimal. 4) The relative lack of Proterozoic impact craters is due in part to screening of poorly dated Proterozoic craters, and is not distinct from predictions of exponentially decreasing preservation potential through time. 5) Active margins with topography, not passive margins, have the highest erosion rates and send more sediment to the deep oceans. 6) The Sr isotope composition of seawater is driven as much by the composition of what is being weathered as the amount of weathering. 7) Existing theromochronology data are consistent with most of the erosion below the GU being pre-Cryogenian in age and unrelated to Snowball Earth. Tonian to Cryogenian stratigraphy of Australia, Congo, Kalahari, and Laurentia indicates 10s-100s of meters of Cryogenian erosion, with local channel incision of >1 km, not a global average of ~4 km.

1Keller et al., 2019, PNAS.

2Sobolev & Brown, 2019, Nature.