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. flow and granule to pebble conglomerate Bonavista Peninsula LII^ 0 Dark gr een brecciated mafic Medium to coarse trough volcanic flow U x-bedded sandstone
0 - Blue-gr een silicifed siltstone with LI Coarse massive sandstone with thin pink recrystalized tuff beds outsized granules and rare pebbles
150- Tuff derposit reworked with coarse Medium to coarse massive conglo nerate flow ---sandstone Thinly laaminated to massive Laminated siltstone with lenses of welded tuff with thin pink beds sand and granule conglomerate *OBJP-01 .~ - Mediurm to coarse sandstone with Green to blue-green laminated - sparse ravel-sized dropstones siltstone; rare x-beds and sand lenses Massive diamictite; clast-rich, purple matrix; mostly volcanic with minor Green to purple laminated siltstone; -- - sedime ntary clasts, including low angle x-beds and sand lenses ~onvolu te bedded siltstone
- Stratifle d diamictite; clast concen- Green laminated siltstone to tration decreases upsection. sandstone, ripples Medlur sandstone deposit; bounde d by glacial deposits
100- --H-100 B .Trinity Pond C. Cat Cove B 60, onavista Peninsula Bonavista Peninsula C: 0 54 01. 0
0 -- V-V-V-80.
(0 34 -- s-----m
7V V 7V V V V V OBJP-03 2( V V--V V7 77V V V V
50- 7 V 7 V V
*131552 77VV me mete
- 20.- %; Glacigenic diamictite ...... ---- AggIomerates, lithic tu s, and air-fall ash Sandstone and siltstone ...... Geochronology sample
0* - meters si I m c pbl meters si f m c pbl Figure 2: Bonavista stratigraphic sections. Measured stratigraphic sections of the Rocky Harbour Formation from Old Bonaventure (A), Trinity Pond (B), and Cat Cove (C) with geochronologic samples denoted by orange stars. Page 6 of 26 Stratigraphic sections of the Rocky Harbour Formation were measured to establish the geological context of the Trinity facies (Fig. 2). The three locations were picked based on the exposure of the Trinity facies mapped by Normore (2011) and continuity of sections, since much of the stratigraphy in the area is disrupted by faulting. Siliciclastic rocks ranging in grain size from siltstone to pebble conglomerate compose >60 m of the section beneath the first appearance of diamictite. The grain size distribution and the prevalence of bi-directional cross-stratification supports a shallow-marine origin for these facies and correlation with the Monk Bay and Cape
Bonavista facies described previously (O'Brien and King, 2002; Normore, 2011). The diamictite itself is massive at the base and transitions upwards into stratified diamictite with fewer and smaller clasts. Clasts vary in size from sand to cobbles and boulders and also in lithology, with a variety of sedimentary and volcanic protoliths. Beds immediately overlying the diamictite consist of fine- grained, laminated, silicified siltstone with coarser-grained sandstone lenses. These laminated beds were deposited below wave base possibly during a glacio-eustatic transgression, and the sandstone lenses could result from flows caused by occasional seismic or storm events. The beds observed above the Trinity sections in our sections most closely match Normore's (2011) descriptions of the Kings Cove Lighthouse and Kings Cove North facies and generally agree with his stratigraphic framework.
Section B 1552 was 163.0 m thick and was measured near Old Bonaventure (Fig. 2A). The base of the section consists of 25 m of coarse white to grey sandstone and laminated siltstone with maroon siltstone drapes and bi-directional crossbedding. For another -20 m above, granule to pebble conglomerate is interbedded with maroon, laminated siltstone and white cross-bedded sandstone. The pebble conglomerate contains volcanic clasts with feldspar phenocrysts. A series of volcanic tuffs and interbedded siltstone lie above and are overlain by approximately 10 m of
Page 7 of 26 cross-stratified coarse-grained sandstone to pebble conglomerate. Sample B1552 was taken from
42.2 m in the section from a massive light-yellow to pink bed interpreted to have formed as an air-
fall tuff. The massive diamictite of the Trinity facies starts at 53.3 m above the base of the section
and the matrix of the diamictite changes in color up-section from green to purple. It continues into
stratified diamictite after about 20 m. The diamictite contains a range of siliciclastic sand- to
cobble-sized clasts in addition to volcanic clasts that have feldspar phenocrysts, similar to
descriptions of the Bull Arm Formation. Sample OBJP-03 was taken at 60 m above the base level
of the measured section from purple diamictite that had a tuffaceous matrix. The diamictite in this
section is -36 m thick and is overlain by ~45 m of light green pistachio-colored, finely laminated,
silicified siltstone. Section B 1552 also contains another suite of volcanic agglomerates, flows, and tuffs above the diamictite that is at least 26 m thick. The tuffs are distinct peach-colored, silicified beds that are about 2-3 cm thick at most and interbedded with blue siltstone. Sample OBJP-01 was collected from one peach tuff bed at 140.0 m in the section. Above the interbedded tuffs and siltstone is a 15 m thick basaltic bed that contains lithic clasts. This is succeeded by at least another
10 m of volcaniclastic rocks and mafic flows that is interrupted roughly along bedding by a regional thrust fault.
Section B 1553 was measured along NL-230 by Trinity Pond and contains beds of cross- stratified coarse sand to pebble conglomerate with laminated siltstone for at least 60 m beneath the first appearance of diamictite in the section (Fig. 2B). Beds commonly have outsized clasts that vary in grain size from coarse sand to pebble and are gray to green in color, with minor purple to maroon interbedded siltstone. The cross-stratification in these beds is steep and a ripple surface can be seen at about 28.0 m above base level with the troughs infilled by coarse sandstone. The
Trinity facies exposed in section B1553 consists of a basal clast-supported pebble conglomerate
Page 8 of 26 overlain by massive green, matrix-supported diamictite. After ~7 m the diamictite becomes stratified and includes sandstone trains and fine siltstone laminations with fewer and smaller clasts moving up-section. Above the diamictite is green pistachio-colored siltstone, with wavy laminations and convolute bedding indicating dewatering.
Section B 1554 was measured in Cat Cove with faults bounding the beginning and end of the 52 m of stratigraphy (Fig. 2C). Beneath the diamictite are packages of pebble conglomerate fining upwards into coarse sandstone. Cross-stratification and outsized clasts are also present, as seen in the other two sections. At its base, the diamictite has a sandstone matrix and large clasts that reach boulder size. The matrix becomes finer and laminations develop in the overlying stratified diamictite. Clasts again decrease in size and number moving up-section. The diamictite is overlain by medium- to coarse-grained sandstone with siltstone drapes. These beds also show evidence of dewatering in the form of flame structures and convolute deformation. Overlying these beds is the fine-grained, pistachio-colored, silicified siltstone that we see above the Trinity facies in all of our sections. The pistachio-colored siltstone shows symmetric, large ripples that are several cm across in wavelength that could indicate deposition at storm wave base, suggesting a transition to deeper water.
Methods
Mineral separation
All samples were scrubbed thoroughly with a steel brush to remove any stray grains from their external surfaces. Samples were then broken into approximately 1 cm 3 pieces using a sledgehammer and ground to powder using a Spex Shatterbox in short, repeated runs of <5 s. The
sample was sieved through a 500 pim mesh after each Shatterbox run and the <500 pm fraction was handwashed in the initial density separation step. The washed fraction was dried under a heat
Page 9 of 26 lamp and run through the Frantz magnetic separator. The angle and amperage of the Frantz magnet was varied from 200 and 0.3 A depending on how magnetic the fraction was. Sample B1552 was run at 0.40 A with a tilt of 30' and sample OBJP-01 was run at 0.60 A at 20' because they were largely nonmagnetic. Sample OBJP-03 contained more magnetic material and was run twice, first with 0.3 A at 30' and then with 0.5 A at 30'. The non-magnetic fraction of each sample was put through a second density separation step using methylene iodide to isolate dense mineral grains.
The pure zircon fraction was then handpicked under a microscope with a preference for prismatic, elongate crystals with lengthwise melt inclusions to try and maximize the probability that the zircon was part of a magmatic population.
Chemical abrasion
The uranium in the crystal lattice of zircon undergoes alpha decay, which over time damages the crystal structure and can result in lead loss. The chemical abrasion technique outlined by Mattinson (2005) allows us to largely resolve the issue of lead loss by first preferentially leaching the areas that are damaged to leave behind zircon that should produce a concordant age, although older zircon crystals with more radiation damage can completely dissolve during the leaching step. The zircon fractions were annealed at 900'C for 60 h so that the damaged parts of the zircon (ZrSiO4) form baddeleyite (ZrO2) and silica (SiO 2), both of which will dissolve more readily in hydrofluoric acid (HF) than zircon. Zircon grains were loaded into Teflon microcapsules and about 75 ptL of 29M HF were added for the leach step. The samples were then placed in a Parr acid digestion vessel at 210 C for ~-12 h. Because the samples were Neoproterozoic in age, they were leached for 11.5 h in order to increase chances that the zircon grains would survive the leach step. All grains from samples B1552 and OBJP-01 survived the leach, but only half the grains
Page 10 of 26 from sample OBJP-03 were left after 11.5 h. In order to address a potential lead loss problem with sample OBJP-01, grains 8 through 12 were leached for 12 h.
Isotope dilution column chemistry
Following the leach, the samples were transferred to small plastic beakers that had been cleaned by fluxing on the hot plate in 6N hydrochloric acid (HCl) overnight. Approximately 250
p.L of Milli-Q@ (MQ) H2 0 and 100 pL of concentrated nitric acid (HN0 3) were also added to the
beaker. Aqua regia (25 ptL of concentrated HN0 3 and 75 ptL of 12N HCl) was added to each capsule. The beakers were left to flux on the hot plate for an hour, and then sonicated for an hour
as well before the mixture was pipetted off. MQ H2 0 was added to the beaker and pipetted off again as a rinsing step. 6.2N HC1 was added to the beakers for the second fluxing and sonicating step, during which the microcapsules also went on the hot plate to flux for about an hour before being emptied and rinsed with MQ H20. The samples in the beakers were rinsed again before being loaded into the microcapsules.
The EARTHTIME 2 15Pb- 23 3 U- 2 35U isotopic tracer (Condon et al., 2015) was used to spike the samples. The dropper bottle was weighed before and after spiking each sample to determine the spike weight in the sample. About 50 ptL of 29M HF was added to each capsule and the samples were left in an oven at 21 00 C for 48 h to completely dissolve. After dissolution, the samples were dried down and converted to chloride salts by adding 50 tL of 6.2N HCl to each capsule and leaving the vessel in the oven at 180'C for ~12 h before drying the samples down again. 50 PL of
3N HCl was added to each sample to condition them for column chemistry.
AG1-X8 anion resin, Cl- form, 200-400 mesh was used for the column chemistry following the procedure of Krogh (1973). The resin was cleaned by allowing a full reservoir of 6N HCI and
MQ H20 to drip through, and then about 400 pLL each of 6.2N HCl and 0. IN HCl were dripped
Page 11 of 26 through the columns. The resin was preconditioned for sample loading by adding 200 PL of 3N
HC1 to each column. After samples were loaded in the columns, a sample rinse using 25 ptL of 3N
HCl was done three times. Lead (Pb) was eluted from the columns first by adding 200 PL of 6.2N
HCI, and then uranium (U) was eluted by adding 250 ptL of 0.1N HCl. Beakers that had been cleaned by fluxing with 6N HC1, dilute HF, and then 6N HCl again for -12 h each round were
used to collect the samples. One drop (-25 pL) of 0.05M phosphoric acid (H 3 PO4) was added to each sample for drying down.
TIMS analyses
Samples were redissolved in silica gel and loaded onto outgassed rhenium filaments for
TIMS analyses. All measurements were made on the VG Sector 54 TIMS in the Massachusetts
Institute of Technology Radiogenic Isotopes Lab under Professor Samuel Bowring. Lead isotope ratios were measured using a Daly detector and corrected for fractionation based on analyses of the NBS 981 Pb standard. Uranium was measured using Faraday cups and corrected for fractionation using the 2 33U- 2 35U spike. All measured 204 Pb was assumed to come from laboratory blank, and a blank correction was applied to each measured ratio using 2 06Pb/ 2 04 Pb = 18.1458
0.4752, 2 07Pb/2 04 Pb = 15.3039 0.2955, and 208Pb/204Pb = 37.1078 0.8751 (Ic uncertainty).
These corrections were based on measurements of procedural blanks in the lab. A thorium ( 2 30Th) correction of Th/U = 2.8 1 was applied to each analysis as well to account for initial Th deficiency
2 3 8 2 3 in the zircon crystal (Machlus et al., 2015). The decay constants for U and 1U were taken from
Jaffey et al. (1971), and all data reduction was carried out using the U-PbRedux software package
(Bowring et al., 2011). Uncertainties on ages are reported in the format X/Y/Z, where X is the analytical uncertainty, Y is the analytical uncertainty and the uncertainty in the isotopic composition of the tracer solution, and Z encompasses the uncertainties in X and Y and includes
Page 12 of 26 the uncertainty in the 238U decay constant. Subsequent discussion of the same ages in this paper will only mention the analytical uncertainty (2cy) since the same isotopic tracer and decay constants were used.
Results
1 570.0 ,I B1552 OBJP-03 OBJP-01
I<
572.001- 579.63 579.35 579.24 0.15 Ma 0.33 Ma 0.17 Ma
MSWD = 0.018 MSWD = 0.73 MSWD = 1.3 n=6 n=4 n=9 574.00|-
3 576.00|-
578.001-
5 5111111 me
580.00-
z1 z2
582.001 42.2 m 60 m 140.0 m 0.0 m Distance above base level
Figure 3: CA-ID-TIMS zircon analyses. Each rectangle represents a CA-ID-TIMS analysis on a single zircon with 2a uncertainty. Mean ages were calculated from zircon grains that appeared to be part of a single population (MSWD < 1 or ~ 1) and give a maximum duration of 390 320 kyr for the glaciation. Two analyses from OBJP-03 yielded detrital zircon ages and are not shown in this figure, and two anomalously young dates from sample OBJP-0 I were attributed to pervasive lead loss.
Page 13 of 26 Analyses of six zircon grains from sample B 1552 at 42.2 m in the section gave a mean age of 579.63 0.15/0.29/0.68 Ma with a mean square weighted deviation (MSWD) of 0.018
(Fig. 3). Sample OBJP-03 from the diamictite yielded a few detrital grains that had ages >600
Ma, but also had a distinct population of zircon that produced a mean age of 579.35
0.33/0.42/0.75 Ma with an MSWD of 0.73 for four grains.
Several zircon grains from sample OBJP-0 1 yielded dates that appeared to be part of a single population, with some spread in crystallization age. The age was interpreted to be the mean of the youngest population, which was 579.24 0.17/0.30/0.69 Ma with an MSWD of 1.3 for nine grains. Two zircon grains from OBJP-01 yielded anomalously young dates with no overlap that were attributed to pervasive lead loss. Data from individual analyses is included in
Table I in the Appendix along with concordia plots for each sample.
Discussion
The maximum age constraint of 579.63 0.15 Ma on the Trinity diamictite indicates that the facies is a time-equivalent of the ca. 580 Ma Gaskiers diamictite. Dates produced in this study provide new, high-precision age constraints on the Trinity diamictite on Bonavista Peninsula,
Newfoundland and the correlative Gaskiers glaciation. The glaciation event is bracketed between
579.63 0.15 Ma and 579.24 0.17 Ma, giving a maximum duration of 390 320 kyr. In comparison to the >55 Ma duration of the Sturtian glaciation (Rooney et al., 2015), the Gaskiers glaciation is clearly a much shorter-lived event. The commonly accepted mechanism for escaping
a Snowball glaciation is catastrophic deglaciation due to buildup of atmospheric CO 2 from volcanic outgassing. Based on current outgassing rates, several millions of years would be needed to build up enough atmospheric C02to escape a Snowball (Hoffman and Schrag, 2002). The <1
Myr duration of the Gaskiers glaciation makes it unlikely to have been a Snowball event.
Page 14 of 26 Recent paleomagnetic data from the Bull Arm Formation underlying the Rocky Harbour
Formation (Fig. 1) suggests that the Avalon terrane had a paleolatitude of 19.1 11.10 in the mid-
Ediacaran (Pisarevsky et al., 2011), well within the threshold for the ice-albedo feedback effect to cause a Snowball glaciation (Budyko, 1969). The possibility of having ice at low latitudes with the demonstrated short-duration of glaciation raises several questions, including the precise age of the rocks that the paleomagnetic data was obtained from, since the Bull Arm Formation has been shown to include both volcanic flows and intrusive bodies, and whether there are other mechanisms for escaping a Snowball state that do not involve millions of years of C02 buildup.
An alternative view of the Gaskiers glaciation would be that it represents an aborted
Snowball event or a glacial maximum in a longer Ediacaran ice age. The distinction between these hypotheses is dependent on resolving the age and duration of other Ediacaran glacial deposits as well as their paleolatitudes. The Squantum Tillite in New England, Mortenses and Moelv formations in Norway, Serra Azul Formation in Brazil, and Hankalchough Formation in China have been correlated to the Gaskiers glaciation event (Normore, 2011; McGee et al., 2013), but lack the precise geochronological and paleomagnetic controls needed for meaningful comparisons to the Gaskiers glaciation.
Conclusions
Sparse paleomagnetic data and complex structural relationships following multiple orogenic events on the Avalon terrane have made it difficult to determine the exact spatial extent and duration of the Gaskiers glaciation. CA-ID-TIMS techniques allow us to establish correlations between stratigraphy across the Bonavista and Avalon peninsulas and produce precise geochronological controls on the Gaskiers glaciation event. The deposition of the Trinity diamictite has been bracketed to between 579.63 0.15 Ma and 579.24 + 0.17 Ma with a maximum
Page 15 of 26 duration of 390 320 kyr. These new age constraints on the Trinity diamictite offer the most precise geochronological controls on the age and duration of the Gaskiers glaciation available. In order to show whether the Gaskiers glaciation was a discrete event or a glacial maximum in a
longer ice age, further geochronologic work is needed to firmly establish the paleolatitude of the
Avalon terrane at the time of the Gaskiers glaciation and the relationships of other Ediacaran glacial formations to the Gaskiers event.
Page 16 of 26 Appendix
Table 1: CA-ID-TIMS single zircon grain analyses. OBJP-01 Dates (Ma) Composition Isotopic Ratios 206Pb/ 207Pb/ 207Pb/ 206Pb/ 208Pb/ 206Pb/ 207Pb/ 207Pb/ 238U 2a 235U 2a 206Pb 2y Th/U Pbc Pb*/ 204Pb 206Pb 238U 235U 206Pb Fraction Th> a abs b abs b abs d (pg) e Pbc f g h
OBJP-03 Dates (Ma) Composition Isotopic Ratios 206Pb/ 207Pb/ 207Pb/ Th/ 206Pb/ 208Pb/ 206Pb/ 207Pb/ 207Pb/ 238U 2o 235U 2o 206Pb 2a U Pbc Pb*/ 204Pb 206Pb 238U 235U 206Pb 2o Fraction
0.7696 0.33 0.059382 0.30 z3 579.34 0.37 579.5 1.4 580.4 6.6 0.79 0.24 66 3635 0.246 0.094033 0.067 1.0 0.059531 0.94 z4 579.5 2.0 580.8 4.5 586 20 0.95 0.20 23 1256 0.295 0.094062 0.36 0.7717 1.8 0.059494 1.7 z5 579.9 1.1 580.8 7.9 585 38 0.84 0.57 11 614 0.262 0.094127 0.20 0.772 1.2 0.059703 1.2 z6 578.7 1.1 581.4 5.5 592 26 0.82 0.30 18 981 0.254 0.093931 0.21 0.7729
Page 17 of 26 B1552
Dates (Ma) Composition Isotopic Ratios 206Pb/ 207Pb/ 207Pb/ Th/ 206Pb/ 208Pb/ 206Pb/ 207Pb/ 207Pb/ 238U 2o 235U 2y 206Pb 2o U Pbc Pb*/ 204Pb 206Pb 238U 235U 206Pb Pbc Fraction
All measurements were made on the VG Sector 54 TIMS in the MIT Radiogenic Isotopes Laboratory. a: Corrected for initial Th/U disequilibrium using radiogenic 20 Pb and Th/U[magma] 2.80000; b: Isotopic dates calculated using the decay constants k238 = 1.55125E-1 0 and k235 = 9.8485E- 2 06 2 38 2 07 2 06 2 08 10 (Jaffey et al. 1971); c: % discordance = 100 - (100 * ( Pb/ U date) / ( Pb/ Pb date)); d: Th contents calculated from radiogenic Pb and the 2 07 Pb/2 06Pb date of the sample, assuming concordance between U-Th and Pb systems; e: Total mass of common Pb; f: Ratio of radiogenic Pb (including 201Pb) to common Pb; g: Measured ratio corrected for fractionation and spike contribution only; h: Measured ratios corrected for fractionation, tracer and blank. * denotes grains thought to have issues with lead loss.
Page 18 of 26 Figure 4A: Concordia plot of sample OBJP-01.
Th-corrected
NBJ -01
wihden20GPb/23@U (Th-corzoctod 1 57.4i0. 17/0.30/0. 69 N .3, n=9
577
576 C
575 .
5733:2
207pb/235 0.750 0.755 0.760 0.765 0.770 0.775 0.780 0.71 Analyses for sample OBJP-01 from a silicified tuff overlying the Trinity diamictite all overlapped with concordia or within error. The highlighted error ellipses denote the grains used in the mean age calculation (MSWD = 1.3). Two analyses that produced younger dates (z2 and zl 1) were thought to be affected by lead loss.
Figure 4B: Concordia plot of sample OBJP-03.
Th- correctod 630'3
- OBJP-625
620 zircon .mighted &n 206Pb/238U (Th-corrcted) 615 579.35 1 0.33/0.42/0.75 XSID - 0.73, r 4
610
605
600
595
585
0.76 0 77 0-7e 0.79 0.80 0.81 0.82 0.83 0.84 0.85 0.86
Analyses for sample OBJP-03 from the Trinity diamictite produced concordant ages. The red ellipses show the analyses used in the mean age calculation which had an MSWD < 1. Two grains produced detrital dates of >600 Ma.
Page 19 of 26 Figure 4C: Concordia plot of sample B1552.
Th-corrected
~~ 580.4/
miron580.2(, "eighted m a 206Pb/238V (Th-morrected) 579.63 0.15/0.29/0.68 .. SND - 0.019, n 6
579.0 207b/235U 0.766 0.768 0.770 0.772 0.774 0.776 0.778
Analyses of zircon from sample B 1552 from an airfall tuff underlying the Trinity diamictite produced a tightly clustered group of dates with an MSWD < 1.
Page 20 of 26 Acknowledgements
I am grateful to Francis Macdonald for suggesting this project to me and helping me develop this thesis. His passion for geology and enthusiasm in teaching continue to be invaluable to me. I am also grateful to Kristin Bergmann for being willing to discuss ideas with me and for her encouragement throughout the process.
Additionally, thanks to Jahan Ramezani for taking the time to teach me CA-ID-TIMS techniques and Michael Eddy for his support and patience in answering my questions, Eben
Hodgin and Andrea Mills for their expertise and help with fieldwork and the geologic framework,
Graham Oxman for help with sample processing, and Jane Connor for her advice on writing.
And I am indebted to Sam Bowring, who convinced me in my freshman year to study geology and has shown me unconditional support throughout my time as his student.
References
Bowring, J. F., McLean, N. M., and Bowring, S. A., 2011, Engineering cyber infrastructure for
U-Pb geochronology: Tripoli and U-PbRedux: Geochemistry Geophysics Geosystems,
v. 12, p. QOAA19.
Bowring, S. A., Myrow, P. M., Landing, E., and Ramezani, J., 2003, Geochronological
constraints on terminal Neoproterozoic events and the rise of metazoans: Geophysical
Research Abstracts, v. 5, p. 219.
Budyko, M. I., 1969, The effect of solar radiation variations on the climate of the Earth: Tellus,
v. 21, p. 611-619.
Canfield, D., Poulton, S. W., and Narbonne, G. M., 2007, Late-Neoproterozoic Deep-Ocean
Oxygenation and the Rise of Animal Life: Science, v. 315, no. 5808, p. 92-95.
Carto, S. L., and Eyles, N., 2011, The deep-marine glaciogenic Gaskiers Formation,
Page 21 of 26 Newfoundland, Canada: Geological Society, London, Memoirs, v. 36, no. 1, p. 467-473.
Condon, D. J., Schoene, B., McLean, N. M., Bowring, S. A., and Parrish, R. R., 2015, Metrology
and traceability of U-Pb isotope dilution geochronology (EARTHTIME tracer calibration
Part I): Geochimica et Cosmochimica Acta, v. 164, p. 464-480. de Alvarenga, C. J. S., Figueiredo, M. F., Babinski, M., and Pinho, F. E. C., 2007, Glacial
diamictites of Serra Azul Formation (Ediacaran, Paraguay belt): Evidence of the Gaskiers
glacial event in Brazil: Journal of South American Earth Sciences, v. 23, p. 236-241.
Eyles, N., and Eyles, C. H., 1989, Glacially-influenced deep-marine sedimentation of the Late
Precambrian Gaskiers Formation, Newfoundland, Canada: Sedimentology, v. 36, no. 4, p.
601-620.
Gravenor, C., 1980, Heavy minerals and sedimentological studies on the glaciogenic Late
Precambrian Gaskiers Formation of Newfoundland: Canadian Journal of Earth Sciences,
v. 17, no. 10, p. 1331-1341.
Halverson, G. P., Hoffman, P. F., Schrag, D. P., Maloof, A. C., and Rice, A. H. N., 2005,
Toward a Neoproterzoic composite carbon-isotope record: GSA Bulletin, v. 117, no.
9/10, p. 1181-1207.
Hoffman, P. F., Kaufman, A. J., Halverson, G. P., and Schrag, D. P., 1998, A Neoproterozoic
Snowball Earth: Science, v. 281, p. 1342-1346.
Hoffman, P. F., and Li, Z.-X., 2009, A palaeogeographic context for Neoproterozoic glaciations:
Palaeogeography Palaeoclimatology Palaeoecology, v. 277, no. 3-4, p. 158-172.
Hoffman, P. F., and Schrag, D. P., 2002, The snowball Earth hypothesis; testing the limits of global
change: Terra Nova, v. 14, no. 3, p. 129-155.
Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C., and Essling, A. M., 1971, Precision
Page 22 of 26 measurement of half-lives and specific activities of 235U and 238U: Physical Review C,
Nuclear Physics, v. 4, p. 1889-1906.
Krogh, T. E., 1973, Low-contamination method for hydrothermal decomposition of zircon and
extraction of U and Pb for isotopic age determinations: Geochimica et Cosmochimica
Acta, v. 37, p. 485-494.
Krogh, T., Strong, D., O'Brien, S., and Papezik, V., 1988, Precise U-Pb zircon dates from the
Avalon Terrane in Newfoundland: Canadian Journal of Earth Sciences, v. 25, no. 3, p. 442-
453.
Machlus, M. L., Ramezani, J., Bowring, S. A., Hemming, S. R., Tsukui, K., and Clyde, W.C.,
2015, A strategy for cross-calibrating U-Pb chronology and astrochronology of
sedimentary sequences: An example from the Green River Formation, Wyoming, USA:
Earth and Planetary Sciences, v. 413, p. 70-78.
Mattinson, J. M., 2005, Zircon U-Pb chemical abrasion ("CA-TIMS") method: Combined
annealing and multi-step partial dissolution analysis for improved precision and accuracy
of zircon ages: Chemical Geology, v. 220, p. 47-66.
McGee, B., Collins, A. S., and Trindade, R. 1., 2013, A glacially incised canyon in Brazil: Further
evidence for mid-Ediacaran glaciation?: The Journal of Geology, v. 121, no. 3, p. 275-287.
Mills, A., Dunning, G., and Langille, A., 2016, New geochronological constraints on the
Connecting Point Group, Bonavista Peninsula, Avalon Zone, Newfoundland: Current
Research (2016), Newfoundland and Labrador Department of Natural Resources, v.
Geological Survey, Report 16-1, p. 1-19.
Myrow, P. M., 1995, Neoproterozoic rocks of the Newfoundland Avalon zone: Precambrian
Research, v. 73, no. 1, p. 123-136.
Page 23 of 26 Myrow, P. M., and Kaufman, A. J., 1999, A newly discovered cap carbonate above Varanger-
age glacial deposits in Newfoundland, Canada: Journal of Sedimentary Research, v. 69,
p. 784-793.
Normore, L. S., 2010, Geology of the Bonavista Map Area (NTS 2C/1 1), Newfoundland:
Current Research (2010) Newfoundland and Labrador Department of Natural Resources,
Geological Survey, Report 10-1, p. 281-301.
Normore, L. S., 2011, Preliminary findings on the geology of the Trinity map area (NTS 2C/06),
Newfoundland: Current Research (2011), Newfoundland and Labrador Department of
Natural Resources, v. Geological Survey, Report 11-1, p. 273-293.
O'Brien, S., Dunning, G., Knight, I., and Dec, T., 1989, Late Precambrian geology of the north
shore of Bonavista Bay (Clode Sound to Lockers Bay): Newfoundland Department of
Mines and Energy, Report of Activities, p. 49-50.
O'Brien, S. J. and King, A. F., 2002, Neoproterozoic stratigraphy of the Bonavista Peninsula:
preliminary results, regional correlations and implications for sediment-hosted stratiform
copper exploration in the Newfoundland Avalon Zone: Current research (2002).
Newfoundland Department of Mines and Energy, Geological Survey, Report 02-01, p. 229-
244.
O'Brien, S. J. and King, A. F., 2004, Late Neoproterozoic to earliest Paleozoic stratigraphy of
the Avalon Zone in the Bonavista Peninsula, Newfoundland: An update: Current
Research (2004) Newfoundland Department of Mines and Energy, Geological Survey,
Report 04-1, p. 213-224.
O'Brien, S. J. and King, A. F., 2005, Late Neoproterozoic (Ediacaran) stratigraphy of Avalon Zone
sedimentary rocks, Bonavista Peninsula, Newfoundland: Current research, Newfoundland
Page 24 of 26 and Labrador Department of Natural Resources, Geological Survey, Report 05-01, p. 101-
113.
O'Brien, S., Wardle, R., and King, A., 1983, The Avalon zone: a Pan-African terrane in the
Appalachian orogen of Canada: Geological Journal, v. 18, no. 3, p. 195-222.Thompson,
M., Barr, S., and Grunow, A., 2012, Avalonian perspectives on Neoproterozoic
paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon
geochronology in SE New England, USA: Geological Society of America Bulletin, v. 124,
no. 3-4, p. 517-531.
Pisarevsky, S. A., McCausland, P. J., Hodych, J. P., O'Brien, S. J., Tait, J. A., Murphy, J. B., and
Colpron, M., 2011, Paleomagnetic study of the late Neoproterozoic Bull Arm and Crown
Hill formations (Musgravetown Group) of eastern Newfoundland: implications for
Avalonia and West Gondwana paleogeography 1 1 This article is one of a series of
papers published in CJES Special Issue: In honour of Ward Neale on the theme of
Appalachian and Grenvillian geology: Canadian Journal of Earth Sciences, v. 49, no. 1,
p. 308-327.
Rast, N., O'Brien, B. H., and Wardle, R., 1976, Relationships between Precambrian and lower
Palaeozoic rocks of the 'Avalon Platform'in New Brunswick, the northeast Appalachians
and the British Isles: Tectonophysics, v. 30, no. 3, p. 315-338.
Rooney, A. D., Strauss, J. V., Brandon, A. D., and Macdonald, F. A., 2015, A Cryogenian
Chronology: Two long-lasting, synchronous Neoproterozoic Snowball Earth glaciations:
Geology, v. 43, no. 5, p. 459-462.
Page 25 of 26 Sparkes, G., O'Brien, S., Dunning, G., and Dub6, B., 2005, U-Pb geochronological constraints on
the timing of magmatism, epithermal alteration and low-sulphidation gold mineralization,
eastern Avalon Zone, Newfoundland: Current research, p. 05-01.
Williams, H., and King, A. F., 1979, Trepassey map area, Newfoundland, Geological Survey
Memoir 389, pp. 24.
Page 26 of 26