Trevor D. Ford

Microbiotas from the late Proterozoic Chuar Group (Northern Arizona) and Uinta Mountain Group (Utah) and their chronostratigraphic implications

Abstract Samples of shales from the dominantly clastic upper Proterozoic Chuar and Uinta Mountain Groups in northern Arizona and Utah yielded relatively abundant assemblages of acritarchs. These are taxonomically varied and are interpreted as consisting of openwater, cosmopolitan eukaryotic plankton. The assemblages are compared with late Proterozoic (late “Riphean” and early Vendian; c. 800-700 Ma) sequences in the southern Urals of the USSR, the Russian Platform, northern and southern Scandinavia, Svalbard, and East and North West Greenland. In all instances both the assemblages and discrete form-taxa predate the Varangerian (Vendian) glacial event of the north Atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western North America. One new acritarch species; Vandalosphaeridium walcottii is erected and the partly megascopic species Chuaria circularis is emended.

Late Precambrian Chuar Group, Grand Canyon, Arizona

The Chuar Group is exposed in tributary canyons to the Colorado River over an area some 15 mi (24 km) long and 4 mi (6 km) wide. The rocks are faulted against Paleozoic rocks by the Butte fault on the east, and unconformably overlain by Paleozoic rocks to the west. The group is 6,610 ft (2,013 m) thick and has been divided into three formations and seven members. The lower two formations, Galeros (below) and Kwagunt (above) are predominantly argillaceous with subordinate thin limestone beds, while the highest, Sixty Mile Formation, is mostly coarse breccia. Stromatolites are present at three horizons, one of them biohermal. The form-genera Inzeria , Baicalia , and Boxonia indicate an upper Riphean age. The mega-planktonic fossil Chuaria occurs near the top of the Kwagunt Formation. The Chuar rocks are probably younger than any other Precambrian rocks in Arizona. They may be contemporary with rocks below the Cambrian in eastern California, and with the Windermere Formation of the northern Cordillera.

Microorganisms from the Late Precambrian of the Grand Canyon, Arizona

An assemblage of cellularly well-preserved, filamentous and spheroidal plant microfossils has been detected in a cherty pisolite bed of the late Precambrian Chuar Group from the eastern Grand Canyon of the Colorado River. This newly discovered microflora, probably among the youngest Precambrian biological communities now known, appears to be of both evolutionary and biostratigraphic significance.

U-Pb geochronology and global context of the Charnian Supergroup, UK: Constraints on the age of key Ediacaran fossil assemblages

U-Pb (zircon) ages for key stratigraphic volcanic horizons within the ∼3200-m-thick Ediacaran-age Charnian Supergroup provide an improved age model for the included Avalonian assemblage macrofossils and, hence, temporal constraints essential for intercomparisons of the Charnian fossils with other Ediacaran fossil assemblages globally. The Ives Head Formation (Blackbrook Group), the oldest exposed part of the volcaniclastic Charnian Supergroup of the late Neoproterozoic Avalonian volcanic arc system of southern Britain, contains a bedding plane with an impoverished assemblage of ivesheadiomorphs that is constrained to between ca. 611 Ma and 569.1 ± 0.9 Ma (total uncertainty). Higher-diversity biotas, including the holotypes of Charnia, Charniodiscus, and Bradgatia, occupy the upper part of the volcaniclastic succession (Maplewell Group) and are dated at 561.9 ± 0.9 Ma (total uncertainty) and younger by zircons interpreted as coeval with eruption and deposition of the Park Breccia, Bradgate Formation. An ashy volcanic-pebble conglomerate in the Hanging Rocks Formation at the very top of the supergroup yielded two U-Pb zircon populations: an older detrital one at ca. 604 Ma, and a younger population at ca. 557 Ma, which is interpreted as the approximate depositional age. The temporal association of the fossiliferous Charnian Supergroup with comparable fossiliferous deep-water successions in Newfoundland, and the probable temporal overlap of the youngest Charnwood macrofossils with those from different paleoenvironmental settings, such as the Ediacaran White Sea macrofossils, indicate a primary role for ecological sensitivity in determining the composition of these late Neoproterozoic communities.

The Ediacaran fossils of Charnwood Forest, Leicestershire

The current status of the recorded fossils, dubiofossils and pseudofossils in Charnwood forest is reviewed. They belong to two Phyla, Coelenterata and Arthropoda, and represent life forms living in planktonic and benthonic environments during the period between 570 and 700 million years ago. Comparable fossils are known in rocks of this age in Australia, Namibia, Russia and Newfoundland, generally referred to the Ediacaran or Vendian stage of Precambrian time.

White watson (1760–1835) and his geological sections

Abstract Overlooked by most works on the history of geology, White Watson is shown to be a pioneer in the study of Derbyshire stratigraphy, although relatively little of his researches appeared in print. Much more was publicised in the form of inlaid marble «Sections of Strata’, or in lectures and discussions in his museum-shop in Bakewell. His extensive collections were the basis of much early palaeontological work. His outlook on geology and his influence on contemporary geological thought are assessed.

Tadpole holes formed during desiccation of overbank pools

ABSTRACT An example of tadpole holes formed during the desiccation of a silt-floored pool in the overbank part of a stream course in Nankoweap Canyon, Grand Canyon, is described. The origin as well as the significance of these structures in the interpretation of past sedimentary features is discussed.