The Proceedings of the Eighth International Conference on Creationism (2018)

4. Southeastern Proterozoic provinces geology In striking contrast to most of North America’s older Proterozoic provinces, the lithosphere of southeastern North America has an overall northeast-southwest regional trend. This includes the Late Paleoproterozoic Yavapai and Mazatzal provinces, as well as the Mesoproterozoic Grenville Province (Fig. 1). These northeast-trending provinces are characterised by voluminous granitoid plutonism (Mints 2015; Whitmeyer and Karlstrom 2007). The principal Mesoproterozoic province of North America is the Grenville Province. In addition, there are significant Mesoproterozoic rift basins. These include the Midcontinent Rift Basin (also known as the Keweenawan Rift) which is largely basaltic, and the Belt-Purcell Basin which is mainly sedimentary (Fig. 1). The Grenville Province is characterized by extremely high grades of metamorphism (amphibolite and granulite facies) in both basement terranes and supracrustal rocks of the Grenville Supergroup. Rock types are ensialic with quartzofeldspathic gneisses and metasediments including marble (Davidson 1998). Regions of similar isotopic signatures and geochemistry have been found in other parts of the world, including Western Australia, and have been correlated with the Grenville Province (Van Kraendonk and Kirkland 2013). The Midcontinent Rift Basin contains thick Mesoproterozoic basalt lavas (15 to 20 km) and an overlying Neoproterozoic terrigenous sedimentary sequence (Allen et al. 2015; Hoffman, 1989) and has been extrapolated southwestward beneath Phanerozoic cover on the basis of gravity and magnetic anomalies (Davidson 1998). The basalts are tholeiitic and host world-class copper deposits (Presnell 2004). The Belt-Purcell Basin is found on the western margin of North America but is considered here along with the southeastern Proterozoic provinces because it has a similar “age”. It outcrops over an area of about 200,000 km 2 (Lydon 2007). This sedimentary basin is enormously thick, ranging from about 6 km in thickness along its eastern margin to a probable maximum of 20 km in the centre of the Belt Basin (King 1976). Rock types include clastics, carbonates and volcanics (Lydon 2007). Throughout much of their extent, these rocks have been subjected only to lower grades of metamorphism and still retain their primary sedimentary structures (Frazier and Schwimmer 1987). Belt-Purcell rocks are overlain across a slight unconformity by the Neoproterozoic Windermere Group – a westward-thickening group of mainly detrital sediments up to 6 km thick (Frazier and Schwimmer 1987). The Belt-Purcell Basin hosts the world-class Sullivan sedimentary-exhalative (SEDEX) stratiform lead-zinc deposit (Presnell 2004). Grenville-age detrital zircon comprises a significant proportion of most sedimentary successions in western and southeastern North America (Rainbird et al. 2012). In western North America this includes northwest Canada (Rainbird and Young 2009) and the central Cordillera of California and Nevada (Fedo et al. 2003). There is a large zircon “age” peak in the late Mesoproterozoic geology of North America and all continents (Bradley 2011; Condie 2018; O’Neill et al. 2013; Voice et al. 2011) (Fig. 2). 5. Neoproterozoic sedimentary cover geology Neoproterozoic sedimentary cover is found in areas such as the Appalachians, and the Cordillera (Fig. 1), as well as cratonic sequences (Frazier and Schwimmer 1987; Schermerhorn 1974). There is a significant 87 Sr/ 86 Sr ratio increase in Neoproterozoic successions in North America of “age” between 0.9 Ga and 0.5 Ga (Peters and Gaines 2012) (Fig. 3). The Neoproterozoic to Lower Paleozoic geology of North America is characterized by thick sedimentary successions in rift basins and extensional- margin sedimentary wedges (Miall 2008). In the Grand Canyon, the ‘Great Unconformity’ is the contact between underlying either Paleoproterozoic crystalline basement or tilted Neoproterozoic Chuar Group sediments, and overlying Paleozoic layered sediments. This unconformity is traceable across North America (Peters and Gaines 2012; Timmons et al. 1999). Lower Neoproterozoic sequences in North America contain braided-type well-sorted sheet sandstones. These sequences contain Grenville-age detrital zircons in regions such as northwestern Canada, southeastern North America and overlying basaltic rocks in the Midcontinent Rift Basin (Krabbendam et al. 2017; Rainbird et al. 1997). In contrast, mid-Neoproterozoic deposits are mainly poorly sorted immature clastic sediments, with lesser volcanic rocks, and are characterized by deposits commonly interpreted to be “glacial” (Hoffman 1989). The geochemistry of Neoproterozoic cap carbonates, which overlie “glacials”, carries a strong hydrothermal signal (Young 2013). The Rapitan Group of the lower Windermere Supergroup in Canada consists of mixtite, siltstone, shale, sandstone (including arkosic sandstone), volcanic ash and tuff, as well as banded iron formation (Baldwin et al. 2012; Frazier and Schwimmer 1987). High-pressure metamorphic/orogenic belts are restricted to rocks Dickens ◀ North American Precambrian geology ▶ 2018 ICC 392 Figure 3. Summary of major geochemical and sedimentary patterns derived from Neoproterozoic to Cambrian strata (modified from Peters and Gaines 2012). I infer that: 1) The observed increase in Neoproterozoic strontium isotope ratios 87 Sr/ 86 Sr can be explained by accelerated rates of erosion due to the impact of the early Flood’s rain on the supercontinent and to associated Pan-African Event tectonism., and 2) The subsequent decline in 87 Sr/ 86 Sr ratio in post-Cambrian strata may be due to the globe being totally covered with ocean so that the Flood’s rain no longer directly impacted the land.