Search for: All records

Abstract The Cambrian Explosion saw the widespread development of mineralized skeletons. At this time, nearly every major animal phylum independently evolved strategies to build skeletons through either agglutination or biomineralization. Although most organisms settled on a single strategy,SalterellaBillings, 1865 employed both strategies by secreting a biocalcitic exterior shell that is lined with layers of agglutinated sediments surrounding a central hollow tube. The slightly older fossil,VolborthellaSchmidt, 1888, shares a similar construction with agglutinated grains encompassing a central tube but lacks a biomineralized exterior shell. Together these fossils have been grouped in the phylum Agmata Yochelson, 1977, although no phylogenetic relationship has been suggested to link them with the broader metazoan tree, which limits their contribution to our understanding of the evolution of shells in early animals. To understand their ecology and place them in a phylogenetic context, we investigatedSalterellaandVolborthellafossils from the Wood Canyon and Harkless formations of Nevada, USA, the Illtyd Formation of Yukon, Canada, and the Shady Formation of Virginia, USA. Thin-section petrography, acid maceration, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray tomographic microscopy were used to provide new insights into these enigmatic faunas. First, morphological similarities in the aperture divergence angle and ratio of central tube diameter to agglutinated layer thickness suggestSalterellaandVolborthellaare related. Second, both fossils exhibit agglutinated grain compositions that are distinctive from their surrounding environments and demonstrate selectivity on the part of their producers. Finally, the calcitic shell composition and simple layers of blocky prismatic shell microstructure inSalterellasuggest a possible cnidarian affinity. Together these data point to these organisms being sessile, semi-infaunal filter or deposit feeders and an early experimentation in cnidarian biomineralization chronicling a hypothesized transition from an organic sheath inVolborthellato a biomineralized shell inSalterella. 

Here, we present a comprehensive overview of available paleomagnetic studies and datasets from northern Alaska, USA, and Yukon, Canada. Most studies in this region were conducted when our understanding of best-practices paleomagnetism was still developing; as a result, many do not meet modern standards of data quality and many interpretations of the paleomagnetic data—though valid at the time—are now also outdated. In this review, we assess what data are reliable, what interpretations have stood the test of time, and what the existing data can constrain about the tectonic history of this region. We find that although a middle to Late Cretaceous (Aptian–Turonian) overprint pervades much of this area, many sites still retain primary remanence directions, some dating as far back as the Neoproterozoic. Studies that found complete overprinting in the Cretaceous also typically analyzed poor lithologic recorders of paleomagnetic directions, such as carbonates. Based on our assessment of the most reliable data from the study region, relative motion between the examined outboard terranes and Laurentia was not yet complete by the early Late Cretaceous. We also find that “high latitude” dinosaur fossil sites were more northerly than today, confirming previous assumptions about the paleolatitudes at these sites. Finally, we discuss how the widely cited Jurassic–Cretaceous counterclockwise rotation hypothesis for the Arctic Alaska terrane is no longer supported by the existing paleomagnetic data, and the validity of this hypothesis should be critically re-examined. 

Abstract Low‐lying islands in tropical regions are vulnerable to near‐term sea‐level rise and hurricane‐induced flooding, with substantial human impact. These risks motivate researchers to elucidate the processes and timescales involved in the formation, growth and stabilization of coastlines through the study of Holocene shoreline dynamics. Little Ambergris Cay (Turks and Caicos Islands) is a low‐lying carbonate island that provides a case study in the nucleation and growth of such islands. This study investigates the sedimentology and radiocarbon chronology of the island's lithified sediments to develop a model for its history. The island's lithified rim encloses a tidal swamp populated by microbial mats and mangroves. Preliminary radiocarbon data supported a long‐standing inference that the island is Holocene in age. This study integrates petrographic, sedimentological and new radiocarbon data to quantify the age of the island and develop a model for its evolution. Results indicate that the ages of most lithified sediments on the island are <1000 cal yrbp, and the generation and lithification of carbonate sediment in this system supports coastline growth of at least 5 cm/year. The lithification of anthropogenic detritus was documented, consistent with other evidence that in recent centuries the lithified rim has grown by rates up to tens of centimetres per year. A unit of mid‐Holocene age was identified and correlated with a similar unit of early transgressive aeolianite described from San Salvador, The Bahamas. It is proposed that this antecedent feature played an important role in the nucleation and formation of the modern island. Results extend an established Bahamian stratigraphic framework to the south‐western extreme of the Lucayan archipelago, and highlight the dynamism of carbonate shorelines, which should inform forward‐looking mitigation strategies to increase coastal resiliency to sea‐level rise. These results inform interpretation of the palaeoenvironmental record of carbonate environments, underscoring their geologically rapid pace of lithification. 

The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE.