Chapter 4. Precambrian Paleontology of Grand Canyon National Park
Total Page：16
File Type：pdf, Size：1020Kb
Chapter 4. Precambrian Paleontology of Grand Canyon National Park By Justin Tweet1 1National Park Service 9149 79th Street S. Cottage Grove, Minnesota 55016 Introduction The Precambrian paleontology of Grand Canyon National Park (GRCA) is fundamentally unlike the paleontology of any other portion of the park’s substantial stratigraphic column. There are no shells, teeth, bones, footprints, leaves, or similar large fossils. The only fossils visible to the naked eye are layered structures left by microbial mats and the very largest of a diverse assemblage of microfossils. The tools of choice are not shovels and picks, but chemical treatments and powerful microscopes. Very few visitors would be able to spot a Precambrian fossil. Notwithstanding the humble, inconspicuous nature of these fossils, they offer important glimpses at two stages in the development of life, approximately 1,250 to 1,100 million years ago (Ma) and 780 to 730 Ma, long before the appearance of familiar multicellular organisms with hard parts. The Precambrian fossils of GRCA are the oldest fossils of the park and include type specimens for 18 taxa (Appendix 4-B), such as the organic-walled microfossil Chuaria circularis and species of Melanocyrillium, the first described vase-shaped microfossil, as well as the notable pseudofossil (resembling a fossil but inorganic in origin) Brooksella canyonensis, first described as a possible jellyfish. They also have a place in the history of Precambrian paleontology, from Charles Walcott’s early explorations to the present. History of Research An excellent summary of the early Precambrian paleontology of GRCA can be found in Spamer (1984). Although Powell (1876) and White (1876) briefly mentioned potential fossils in the Grand Canyon Supergroup (at that time defined as a group), the study of Precambrian fossils at GRCA was pioneered by Charles Doolittle Walcott in the 1880s and 1890s. The study of Precambrian fossils in general was just beginning, and Walcott made several misidentifications among his more lasting discoveries (see also “Notable Pseudofossils and Dubiofossils” below). For example, his erroneous identifications of several “fossils” led him to initially regard the Chuar Group as Cambrian in age (Walcott 1883). Of his initial assemblage, only the “stromatoporoids” (stromatolites) and some of the “brachiopods” (Chuaria) proved to be legitimate fossils, and he retreated on several of the identifications later (Walcott 1899). Walcott (1899) includes stratigraphic descriptions of the Grand Canyon Supergroup and the description of Chuaria circularis (Figure 4-1). Dawson (1897) also gave the name Cryptozoon occidentale to an example of Walcott’s “stromatoporoids” during this time frame. After the 1890s, Walcott’s attention turned to other topics, including the overlying Cambrian strata at GRCA, and little research on Precambrian fossils at GRCA was conducted until the 1920s. Toward the end of the 1920s, David White briefly mentioned some fossils and potential fossils from the 75 Precambrian rocks of GRCA alongside his research on later Paleozoic plant fossils from the canyon (White 1927, 1928a, 1928b, 1929). Because of White’s paleobotanical focus, the Precambrian fossils and pseudofossils he described were primarily stromatolites, due to their “algal” origin, and “fucoids”, a now-obsolete term for what were then thought to be seaweeds. “Fucoids” as a whole are now known to include various invertebrate burrows and burrow-like sedimentary features such as mud cracks, and the Precambrian “fucoids” appear to be inorganic features. Figure 4-1. Chuaria circularis depicted in Walcott (1899: Plate 27:12). C. circularis specimens are up to 5 mm (0.2 in) across, making them the largest of GRCA’s Precambrian body fossils. Between the 1920s and 1970s, most of the work on GRCA Precambrian paleontology focused on objects now generally regarded as pseudofossils. The most significant of these is the long- controversial Brooksella canyonensis from the Nankoweap Formation, which was first interpreted as a potential jellyfish (Van Gundy 1937, 1951; Bassler 1941). A running dialog on the interpretation of various enigmatic features took place in the literature (e.g., Seilacher 1956; Alf 1959; Cloud 1968; Glaessner 1969; Nitecki 1971). Toward the end of the 1960s, Ford et al. (1969) published initial work toward a complete stratigraphic re-evaluation of the Chuar Group, and Downie (1969) published a brief discussion of microfossils from these rocks, an area of research that would become increasingly significant. Beginning with the late 1960s publications, paleontological work in the Precambrian of GRCA has focused on the microfossils of the Kwagunt and Galeros formations of the Chuar Group. Many advances and discoveries were published during the 1970s, including: the formal division of the Chuar Group (Ford and Breed 1973a); the redescription of Chuaria circularis (Ford and Breed 1973b, 1977); the first report of GRCA filamentous fossils (Schopf et al. 1973); and the initial description of vase-shaped microfossils (Bloeser et al. 1977). Since the 1970s, Precambrian research at GRCA has documented fossils including acritarchs and other organic-walled microfossils (Vidal and Ford 1985; Nagy et al. 2009; Porter and Riedman 2016), filaments (Horodyski and Bloeser 1983), vase-shaped microfossils (Bloeser 1985; Porter and Knoll 2000; Porter et al. 2003), and apparent puncture traces (Porter 2016a). Chemical biomarkers have also been studied (Summons et al. 1988; Brocks et al. 2016). In recent years there have also been re-assessments of the Nankoweap Formation (Dehler et al. 2017) and Sixtymile Formation (Karlstrom et al. 2018), finding both to be younger than originally thought. In the case of the Sixtymile Formation, it is now known to be 76 Cambrian in age, but it will be included briefly in this chapter due to the recency of its reassessment and long historical placement in the Proterozoic. Stratigraphic Distribution of Fossils The Precambrian rocks of GRCA include crystalline Paleoproterozoic basement rocks and a sequence of Mesoproterozoic and Neoproterozoic sedimentary rocks known as the Grand Canyon Supergroup. The Supergroup is divided into the Mesoproterozoic Unkar Group and the Neoproterozoic Chuar Group, which in turn are both divided into several formations. The Unkar Group includes, in ascending order (oldest first), the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. The Chuar Group includes the Nankoweap, Galeros, and Kwagunt formations. As mentioned above, the Sixtymile Formation was previously thought to be a Precambrian formation, but is now known to be Cambrian in age. Although the crystalline rocks beneath the Grand Canyon Supergroup can be found near the western end of GRCA, the rocks of the Supergroup are exposed only in eastern GRCA, from roughly Nankoweap Canyon in the east to Deer Creek Falls in the west; all of their outcrops are limited to GRCA. The Unkar Group is more widely exposed, being found from a short distance below the Little Colorado River to Deer Creek Falls. The Chuar Group is limited to the stretch of the canyon between approximately Nankoweap Canyon and Unkar Creek, and except for outcrops of the Nankoweap Formation on the south/east side of the Colorado River within a few km or mi of the Tanner Trail, is only exposed west or north of the river (Figure 4-2). Outcrops are discontinuous, and it is not uncommon for rocks of the overlying Tonto Group to overlie the ancient crystalline rocks without the Supergroup between them, indicating erosion between the Paleoproterozoic and Cambrian. Fossils are confirmed from the Bass Formation, Dox Formation, Galeros Formation, and Kwagunt Formation. More controversial reports have come from the Hakatai Shale, Shinumo Quartzite, and Nankoweap Formation (Table 4-1). Figure 4-2. An aerial view showing several topographic features west of the Colorado River in eastern GRCA. The distant mesa is Nankoweap Mesa. Nankoweap Butte is to the left. Kwagunt Butte is near the center of the photo. Awatubi Crest is in the foreground (NPS). 77 Formations are discussed below in ascending order. See Table 4-1 for a brief summary. The following information is also included in tabular form in Appendix 4-A. Table 4-1. Summary of GRCA Precambrian stratigraphy, fossils, and depositional settings in descending order of age, from youngest to oldest; Sixtymile Formation included based on historical classification. Details and references can be found in the text Depositional Formation Age Fossils Within GRCA Environment Lacustrine, fluvial, and Sixtymile Formation early Cambrian Potential undetermined fragment shallow marine Stromatolites and other microbial features, acritarchs and colonial organic-walled microfossils, microbial Primarily subtidal to middle Neoproterozoic filaments, vase-shaped microfossils, Kwagunt Formation intertidal with active (late Tonian) various unspecified microfossils, tectonism “vampire traces” on microfossils, chemical evidence for possible sponges, possible meiofaunal traces Stromatolites and other microbial Primarily subtidal to features, acritarchs and colonial middle Neoproterozoic supratidal, Galeros Formation organic-walled microfossils, microbial (late Tonian) intermittently restricted filaments, various unspecified with active tectonism microfossils, “vampire traces” Primarily marine middle Neoproterozoic None to date, unless Brooksella Nankoweap Formation shoreface with active (late Tonian) canyonensis is