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Certified by Signature Redacted Accepted by Signature Redacted I Geochronological constraints on the Trinity diamictite in Newfoundland: Implications for Ediacaran glaciation by Judy Pu Submitted to the Department of Earth, Atmospheric and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology June 2016 Copyright June 2016 Judy Pu. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author_ Signature redacted Department of Earth, Atmospheric and Planetary Sciences May 24, 2016 Certified by Signature redacted Kristin Bergmann Thesis Supervisor Accepted by Signature redacted Tanja Bosak I Chair, Committee on Und ergraduate Program ARCHIVES MASSAC-HUSETTS INSTITUTE OF TECHNOLOGY SEP 2 8 2017 LIBRARIES Abstract The Avalon terrane in Newfoundland includes the Ediacaran Gaskiers Formation, which has been associated with a Snowball glaciation event. The complicated regional stratigraphy and lack of precise geochronological constraints has made it difficult to determine the spatial and temporal extent of the Gaskiers glaciation. Recent recognition of a diamictite facies on the nearby Bonavista Peninsula correlative with the Gaskiers diamictite has allowed for new, high- precision geochronological constraints on the Gaskiers glaciation and constrains the duration of the event to less than 390 320 kyr. The Snowball Earth hypothesis requires that glaciation continued for several millions of years so that CO 2 could build up to high enough levels in the atmosphere for catastrophic deglaciation; the short duration of the Gaskiers event makes it unlikely to have been a Snowball event. Further geochronological studies are needed to determine whether the Gaskiers glaciation was a discrete event or if it was a glacial maximum in a longer Ediacaran ice age. Introduction The Avalon terrane and its tectono-stratigraphic equivalents extend along the east coast of North America from Newfoundland to New England, and on the other side of the Atlantic Ocean from the British Caledonides to northwest Africa (Rast et al., 1976; O'Brien et al., 1983). The Precambrian rocks of West Avalon are part of a Gondwana- or Baltica-derived terrane that collided with the Laurentian margin during the Devonian Acadian orogeny (O'Brien et al., 1983; Thompson et al., 2012), resulting in structural folding and displacement that have complicated stratigraphic correlations across the terrane. Avalon is primarily composed of Neoproterozoic volcanic, plutonic, and sedimentary rocks overlain by Cambrian-Ordovician sediments (Myrow, 1995, Carto and Eyles, 2011). In particular, the terrane includes the Ediacaran Gaskiers Page 1 of 26 although this age has only been mentioned in abstracts and was never published. The Gaskiers Formation can be up to 300 m thick and includes thinly bedded, fine-grained turbidites and mudstone, indicating a deep marine setting when it was deposited (Myrow, 1995; Carto and Eyles, 2011). Chatter-marked garnets, striated clasts, and dropstones in the diamictite provide evidence for a glacial origin (Williams and King, 1979; Gravenor, 1980; Eyles and Eyles, 1989). The presence of a thin (<50 cm) carbonate bed at the top of the unit in some localities with a negative carbon-isotope excursion down to -7 .8 %o (Myrow and Kaufman, 1999), in conjunction with recent paleomagnetic studies that suggest that Avalon was at low- to mid-latitudes around the mid-Ediacaran (Pisarevsky et al., 2011), has led to speculation about whether the Gaskiers Formation could represent a global Snowball event and/or could be related in some way to the Shuram carbon isotope excursion (e.g. Halverson et al., 2005). The Snowball Earth hypothesis requires that glaciation continued for several millions of years so that C02 could build up to high enough levels in the atmosphere to produce extreme greenhouse conditions and catastrophic deglaciation (Hoffman et al., 1998; Hoffman and Schrag, 2002), a hypothesis easily testable by obtaining geochronological constraints on the duration of the Gaskiers glaciation. An oxygenation event and the rise of Ediacaran fauna have also been associated with the Gaskiers glaciation (Canfield et al., 2007), and the attribution of other formations around the world with the Gaskiers Formation (de Alvarenga et al., 2007; McGee et al., 2013) without many reliable age constraints has made it clear that a precise geochronological study of the formation is needed. The correlation of Neoproterozoic volcanics and tuffs of the nearby Bonavista Peninsula to the rocks of the Avalon Peninsula proposed by O'Brien and King (2002) offers the opportunity to provide better age constraints on the Gaskiers glacial event. Mapping of the peninsula by Normore (2010, 2011) showed that the Rocky Harbour Formation of the Musgravetown Group Page 2 of 26 contains its own diamictite that he called the Trinity facies, which could be time-equivalent to the Gaskiers diamictite. Stratigraphic sections of the Rocky Harbour Formation were measured where the Trinity facies were exposed in Old Bonaventure, near Trinity Pond, and in Cat Cove. Geochronology samples were collected in measured stratigraphic sections from ash beds below and above the Trinity facies and from the diamictite itself. Analyses done using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) demonstrate that the Trinity facies is time-equivalent to the Gaskiers diamictite and provide new, high-precision age constraints on the glaciation event. Geologic background A. B. Bonavista Peninsula Figure 1: Generaliz ed BONAVISTA 100km geologic map and PENINSULA stratigraphic colum n of Newfoundlan Bonavista Peninsula , .... Newfoundland. A E geologic map of the Bonavista Peninsula Tnnity . modified from O'Bri en 0 and King (2002) sho wvs the aL U study area by the town of Trinity, denoted by t ie yellow star, and the 0 geologic context of t he > M0 4measured stratigraph ic V 'F1 IFig. 2 sections (shown in more I" detail in Fig. 2). The AN stratigraphic column displays the placeme at of _U the Trinity facies in the E V V Yv Rocky Harbour Formation of the Musgravetown Group (modified fro n Normore, 2011). Cvv Unconformity C Shale & siltstone .0.Siltstone & sandstone -[- Sandstone . Conglomerate Glaciogenic diamictite U Volcanic rocks 0> 0 100 km 2 Page 3 of 26 Ediacaran strata and volcanic rocks on the Bonavista Peninsula include the Love Cove, Connecting Point, and Musgravetown groups (Fig. 1). The Love Cove Group consists of submarine and subaerial volcanics ranging in composition from basalt to rhyolite. An age of 620 2 Ma was reported from the Love Cove Group (O'Brien et al., 1989), but the stratigraphic position and details of the geochronological analyses were not provided. Conformably overlying the Love Cove Group is the > 3 km Connecting Point Group, which consists of graded beds of volcaniclastic sandstone, siltstone, and shale interpreted as turbidites (Mills et al., 2016). U-Pb analyses of zircon fractions involving one or more grains from tuff beds of the middle and upper Connecting Point Group give ages ranging from 613-600 Ma (Mills et al., 2016). The volcanic deposits and available geochronology for the Love Cove and Connecting Point groups support correlation with volcanic rocks in the Harbour Main Group of the Avalon Peninsula that underlie the Gaskiers Formation, which have yielded air abrasion U-Pb zircon ages ranging from 632-608 Ma (Krogh et al., 1988). The Musgravetown Group unconformably overlies the Connecting Point Group and is divided into, in ascending order, the Bull Arm, Big Head, Rocky Harbour, and Crown Hill formations (O'Brien and King, 2005; Normore, 2011). The Bull Arm Formation consists of calc- alkaline, porphyritic volcanic rocks that range from intermediate to felsic in composition (Normore, 2011). A date of 570 +5/-3 Ma (O'Brien et al., 1989) on volcanic rocks correlated with the Bull Arm Formation that were previously thought to be stratigraphically below the Rocky Harbour Formation has been used to suggest that all units above the Connecting Point Group on the Bonavista Peninsula postdate the Gaskiers Formation on the Avalon Peninsula (O'Brien and King, 2004; Normore, 2011); however, more recent work has shown that there are complex intrusive relationships within the volcanics and with the surrounding siliciclastics of the Big Head and Page 4 of 26 Rocky Harbour formations (Sparkes and Dunning, 2014), and the stratigraphic placement of the 570 +5/-3 Ma age should be re-evaluated. The Big Head Formation of the Musgravetown Group consists of interbedded siltstone and sandstone and is thought to be unconformably overlain by the units of the Rocky Harbour Formation (O'Brien and King, 2004; Normore, 2010), but it has also previously been assigned to the base of the Rocky Harbour Formation as another facies (O'Brien and King, 2005), which is supported by mapping and our stratigraphy. The Rocky Harbour Formation is over 2.5 km thick and largely comprised of shallow-marine siltstone, sandstone, and pebble conglomerate that preserve cross-bedding and ripples. The facies of the Rocky Harbour Formation vary laterally and have interfingering relationships, which have complicated efforts to correlate stratigraphy across the peninsula. The Trinity diamictite facies of the Rocky Harbour Formation can be 10s of m thick and is generally green in color with rounded clasts. Dropstones and faceted clasts with striations provide evidence of a glacial origin (Normore, 2011). Conformably overlying the Rocky Harbour Formation is approximately 1 km of primarily terrestrial sandstone and conglomerate facies of the Crown Hill Formation (O'Brien and King, 2002; Normore, 2010), which have been correlated with the facies of the Signal Hill Group on the Avalon Peninsula (O'Brien and King, 2005). Page 5 of 26 Stratigraphy and sample description LEGEND Rework ed and partially rounded Trough x-bedded coarse sandstone A. Old Bonaventure conglo nerates of mafic volc.
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