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Sedimentology, Sea-Level Hlstory, and Tectonic Context of a Mesoproterozoic Carbonate Ramp, Baffin Island, Nunavut

Anne Geneviève Sherman

A thesis submitted to the Department of Geological Sciences and Geological Engineering in conformity with the requirements for the degree of Doctor of Philosophy

Queen's University Kingston, Ontario, Canada March, 2001

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Name(s) of coauthar(s) Signatures of CO-author(s) Yiews ofthe Vicror Bay Formation

Cllrs of sîrongiy cyclic shallow-waier limestone overlying recessive deep-wuter shale in the Ector Bay Formation. Field of view is 1.5 kilo~etmwide and contains part of the eastern wu11 ofPmgo Valley, Sirnirlik National Purk nwthem B@n Island.

DarA limestone cl* of the 450-metmthick Yictor Bay Fornatron merlyingpale Society CZ#i dolosones, togetherfonnihgpart of the Pmcipitopls Mountains alung the western shore of Tremblay Sm4 notthm BeIdand ABSTRACT

The -1.2 Ga Victor Bay Formation is a -300-600-m-thick succession of predominantly carbonate rocks within the Mesoproterozoic Borden Basin of northem Baffin

Island in Arctic Canada. These sediments are dominated by lime mudstone facies and, given

their Precambrian age, contain an anomalously Iow proportion of shallow-water stromatolitic

rocks. They fom one of the first examples of a Phanerozoic-style carbonate ramp, and

reflect the interplay between sea level and tectonics during a critical period in the evolution

of the Borden Basin aulacogen.

Of the wide spectrum of carbonate depositional systems that had evolved by the late

Mesoproterozoic, the Victor Bay ramp succession represents a muddy endmember wherein

the carbonate factory produced lime mud in far greater amounts than ooids, microbialites, or

benthic precipitates combined. Lime rnud precipitated from the water column and grains

fomed by early lithification of the muddy sediments. Episodic storms generated a variety of

intraclast particles and redistributed mud and grains on this otherwise Iow-energy microtidal

ramp. Stromatolites were deposited only when accommodation space increased rapidly, but

during such times flourished and accumulated as large reefs. The ramp carbonates are demonstrably cyclic at several scales. Most are packaged in

decametre-scale cycles of 2û-50 rn, thicknesses that exceed those of classic shallowing-

upward cycles by a hctor of ten. Three types of cycles are recognized: (1) deep-subtidal

cycles on the outer-ramp, with black dolomitic shale at the base overlain by ribbon, nodular,

and carbonaceous carbonate facies, al1 of which exhibit signs of synsedimentary dimption;

(2) shallow-subtidal cycles with basal deepwater facies and an upper layer of subtidal rnolar-

tooth limestone tempestite interbedded with microspar calcarenite facies; and (3) pentidal

cycles similar to shallow-subtidal cycles and capped by dolomitic tidai-flat microbia1 Iarninite, red shale sabkha facies and sandy polymictic conglomerate. Maximum progradation of Uuier- ramp tidal Bats over outer- and mid-mp facies during shoaling coincides with a zone of slope failure that may have created accommodation space and therefore promoted the development of the stromatolitic reefs. Consistently thick and cornplete decametre-scale cycles are compatible with the rapid creation of accommodation space during high-amplitude, high-frequency eustasy. A glacio-eustatic interpretation of these cycles supports the global climatic supercycle theory that predicts icehouse conditions in the Iate Mesoproterozoic. At the hectometre scale, the eustatic signal was overprinted by pulses of tectonism that defined major sequence-bounding unconfonnities and flooding surfaces.

Tectonic effects bdamentally altered patterns of sedimentation in the Borden Basin during Victor Bay the. Regional correlations indicate that during deposition of the carbonate ramp, the overall deepening trend within the basin was reversed, leading to karsting in the west and drowning in the east. This sequence of events and the disruption of the paleogeography across the structural grain of the aulacogen are best reconciled with reactivation of basement faults in a foreland basin setting. This new interpretation is consistent with observations of late Mesoproterozoic deformation in the northwestern

Arctic. CO-AUTHORSHIP

The following work is my own, but owes its final form to input from Dr. Noel James and Dr. Guy Narbonne. niey are CO-authorson the three manuscripts and contributed in both a scientific and editorial role. The two published manuscripts were improved both conceptually and stylistically by J.P. Groizinger, P.N. Southgate, M.E. Tucker, J.F. Read, J.B.

Southard, and D. Pettyjohn. The third paper benefited greatly fiom recommendations by

H.T. Helmstaedt.

ACKNOWLEDGEMENTS

This tome is the distillation of several years of work, and it presents new information

and ideas. Iust as importantly, it represents the tremendous gift of knowledge, guidance,

patience, and support contributed by Noel lames and Guy Narbonne in thrir role as advisors.

Their example helped me understand the real work behind the science: how to ask questions,

how to approach problems, and how to communicate results. Al1 my thanks to you both for

fostenng an ideal learning environment with good advice and an open door. Whenever we

"taked shop", your dynamism and perspective unfailingly renewed my enthusiasm and

promoted new ways of thinking. 1 consider myself extremely fortunate to have had two

exceptional advison during my time here. Thank you also for accepting the spasmodic Pace

of my progress through many work stints and the usual complement of persona1

complications that corne with being Homo supiens. A special note of appreciation, Guy, for

showing me how a real geologist crosses a Stream; and Noel, for teaching us that lunchtime

means Merlot.

A big thanks to Jason Harrington for Kraft Suppers, mernorable Lindsay stories, and

excellent field assistance. 1 was fortunate to find someone masochistic enough to put up with

me for one summer, let alone two. This project would not have possible without logistical

support from the Polar Continental Shelf Project, with special thanks to Resolute base managers Ji.Godden and Dave Maloley, and crews from BradleyEirst Air and Canadian

Helicopters. Staff at Nanisivik Mine, especially Gary Dyck, Jack Haynes, Noel Hedges, and

Ron Sutherland, are thanked for generously providing storage space, a mine tour, and many truck rides up the hill. The communities of Arctic Bay and Pond Inlet are thanked for permission to conduct research on their land. Financial support for the project was provided by the Natural Sciences and Engineering Research CounciI, the Northern Studies Training

Program, a GSA Research Grant, and a Queen's Doctoral Travel Grant. The Department of

Geological Sciences and Geological Engineering and the School of Graduate Studies are thanked for financial assistance through scholarships, gants, and teaching assistantships.

Technical and administrative assistance from staff membea is much appreciated, especially

Rob Renaud, Greg Hounsell, Ela Rusak, Jerzy Adwent, Roger I~es,Dianne Hyde, Joan

Charbonneau, Linda Anderson, Ellen Mulder, Hanne Sherbonneau, and Larke Zarichny.

Thanks to Bill Macfarlane for his help with photographie prints.

One of the pleasures of pduate school is the opportunity to meet and interact with exceptional people. To leff Lukasik, Dave Scott, and Liz Turner, my sincere gratitude for many years of fkiendship, advice, and support. 1 can't possibly summarize it al1 here, so 1 wonlt even try! Thanks go out to Tom Ullrich for aesthetic counsel of al1 kinds, Jeny Grant for improving the student body through hockey, Sara Ryan for coffee & chick-talk, Kasper

Frederiksen for sharing many ideas during his short visit, fellow Borden Babe Linda Kah for fruitfil discussions and collaboration, also Vicki Bannister, Frank Brunton, Cathy Corrigan,

Dave Kerr, Annukka Lipponen, Dave Love, Annette MacIlroy, Sarah Palmer, Amelia

Rainbow, Clare Robinson, Ian Russell, Gord Stretch, Marian Warren, Don Wood, Karen

Wright, and a string of tolerant officemates. Merci aussi à Doug Archibald, Herb Helmsatedt,

Colin Thomson, and Mike Doggett (Br Dee Thiessen). Venerable and wise "former graduate students" Clint Cowan, Mike Guaning, Eric Hiatt, Rob Harrap, and Rob MacNaughton were generous with advice that helped me focus on the "Ph" part of "Ph.D.lt A wam thank-you to

Grad Club staff (Connie, James, Virginia, Paul et al.) for the brain food and "smart drinks". Hey, Kyle, you get your own page! I saved the best for last, of coune. lhank you fiom the bottom of my heart for your support and encouragement over the past two-and-a- half years. Even must have noticed a drain lately on your near-endless supply of humour and patience, as you were strapped in with me on the blood-sugar roller-coaster. 1 consider myself a very lucky penon. I even promise to stick by you if you lose your arms and can't cook for me anymore. Now that's love! CONTENTS .. ABSTRACT...... 11

.. CONTENTS...... ,.VI I

*. . LIST OF TABLES ...... -...... VIII .. LIST OF FIGURES...... VI11 DEDICATION...... x

CHAPTER 1 General Introduction and Setting ...... ,...... 1 CHAPTER 2 Sedimentoiogy of a Late Mesoproterozoic Mudày Carbonate Ramp, Northern BaBn Island, Arctic Canada...... 1 1 Anne G. Sherman, Noel P. James, and Guy M. Narbonne SEPM Special Publication 67, p. 2 75-2 94.

CHAPTER 3 Anatomy of a Cyclically-Packaged Mesoproterozoic Carbonate Ramp in Northern Canada ...... ,., ...... 5 3 Anne G. Sherman, Guy M. Narbonne, and Noei P. James Sedimentary Geology, v. 13 9, p. 1 71-203,

CHAPTER 4 Tectonic Inversion and Carbonate Sedirnentation: Mesoproterozoic Borden Basin. BaBn Island. Arctic Canada...... 9 7

Anne G, Sherman, Noel P. James, and Guy M. Narbonne Canadian Journal of Earth Sciences, in review.

CHAE'TER 5 GeneraZ Conclurions and Synthesh of Research...... 1 3 3

REFERENCES ...... -13 9

APPENDICES Appendrjr A: Section Logs ...... 1 62 Appendrjr B.- ûriented Feutures...... *...... *....1 73

vii LIST OF TABLES

CHAPTER 2

Table 2.1 : Summary of Victor Bay lithofacies...... 22

CHAPTER 3 Table 3.1 : Surnmary of Victor Bay facies and interpreted paleoenvironments...... 62

CWTER 4 Table 4.1. History of the Milne Inlet Trough ...... 113 Table 4.2. Structural Interpretations of the Borden Basin ...... 126

LIST OF FIGURES

CHAPTER 1 1. 1 Timeline...... 2 1.2 Map of the Bylot basins...... 4 1.3 Geological map of Borden Peninsula ...... 5 1.4 Bylot Supergroup stratigraphy ...... 6

CHAPTER 2 Map of Borden Peninsula and Victor Bay outcrop...... 13 Bylot Supergroup stratigraphy ...... 15 Regional correlations in MiIne Inlet Trough ...... I9 Cyclic packaging in the upper Victor Bay member ...... 2 1 Detailed correlations in upper Victor Bay strata...... 21 Dololaminite facies ...... 25 Molar-tooth mudstone facies ...... 27 Molar-tooth calcarenite facies and poIymicitc conglomerate facies ...... 30

Deep-water and stromatolitic facies...... ,., ...... 3 2 .. Depositional environments...... 40 Geological map of Borden Peninsula ...... 56 Geological map of study area ...... 57 Bylot Supergroup stratigraphy ...... 59 Pentidal. . cycle...... 66 Regional correlations in eastem Milne Iniet Trough ...... 68 Oriented features at Phgo Valley ...... 69 Erosion surfaces...... 70 Pingo Valley Photomosaic...... 72 Lithocorrelations along Pingo Valley ...... 74 Parasequences and sequence tracts...... -75 Synsedimentary deformation and resedimentation ...... 78 Stromatolitic features at Pingo Valley ...... 80 Schematic parasequence architecture...... 84 Sequence development fiom Victor Bay to Athole Point time ...... 87

Map of the Bylot basins ...... 100 Geological map of the Borden Peninsula ...... 101 Stratigraphy of the Bylot Supergroup...... 103 Upper Victor Bay reefs ...... *...... 108 Maps of reef disaibution...... 109 Mid- to inner-ramp transition in Mala River area ...... II Mid- to outer-ramp transition on the west coast of Tremblay Sound...... 112 Map of section locations ...... 114 Correlations dong northem margin of Milne Inlet Trough ...... 115 Correlations across central Milne Inlet Trough ...... 116 Correlations across Milne Inlet Trough West of Tremblay Sound...... IL7 Reef development and sequence stratigraphy ...... *...*...120 Tectonic evolution of Milne Inlet Trough ...... 121

CWAPTER 5 5.1 Mesoproterozoic sediment dynamics ...... 137 To Kyle and my family GENERAL INTRODUCTION

Carbonate platfonns, structures formed by prolific marine carbonate sediment accumulation, are profoundly sensitive indicaton of past environments. They contain distinctive fabrics and facies that are some of the best records of evolving ocean chemistry, atmospheric composition, and biotic influence through geological time. Their growth during the long span of Precambnan time is particularly important because it provides a record through a critical yet poorly understood period in Earth history. If we are to understand how our world evolved before the appearance of organisms with skeletons, then these platforms must be documented, interpreted and viewed against both the older and younger geological world. The vast Precambrian shield of northem Canada contains some of the best preserved such platforms on the globe. Of these, the succession in the Borden Basin of northem Baffin

Island stands out because of its undeformed nature and well-preserved character. The topic of this thesis is a critical Iimestone unit within the Basin, the Victor Bay Formation, a carbonate

that accumulated in a variety of paleoenvironments under an evolving tectonic regime.

Rocks of the Victor Bay Formation outcrop in the central northem part of Baffin

Island. Ranging From shales to sandstone to carbonates, they are flat-lying and dissected by

numerous fiords and river valleys. Lack of vegetation, active erosion, and simpIe tectonic

expression has led to superb cliff exposures that show continuous bedding and facies

transitions across many tens of kilometres (see fiontispiece). Stratigraphie problems can be

solved by simple walking out of lithological transitions or mcing over long distances using

helicoptea. Most of the carbonate is limestone, with excellent fabric preservation for rocks

so old. The stratigraphie packaging is relatively simple, allowing the succession to be easily

placed in a paleosealevel context. Yet these carbonates are also unlike many other middie Proterozoic rocks and their facies nlationships are much more complex than would be expected for such an epicratonic succession.

THESTS OBJECTIVES

The purpose of this thesis is to describe a late Precambrian, -1.2 billion-year-old (late

Mesoproterozoic, Fig. 1.1) carbonate depositional system composed of pristine litnestones with excellent preservation of sedimentary fabrics, spectacular cliff exposures of shallowing- upward cycles, and good regional outcrop control. Of interest are intriguing sedimentary structures, a paucity of stromatolites within cycles in apparent contradiction with local development of large stromatolitic reefs, and a tectonic regime of uncertain afinity.

Addressing these problems is crucial to interpreting the tectonostratigraphy of Borden Basin in particular and understanding the evolution of carbonate platforms throughout geological time. Integration of facies analysis, sequence stratigraphy, and tectonostratigraphy were utilized in order to understand the interplay of sedimentation and tectonism that controls the

Iife cycle of a Mesoproterozoic carbonate ramp.

Specific objectives are (1) to decipher facies, describe their sedimentological composition, and evaluate their inter-relationships, (2) to interpret depositional environments and reconsmict the particular type of carbonate platform present, (3) to

Churchill BYLOT Province Admirdty busement SUPERGROUP Grwp

TM(Gu) 2.5 i .6 f!27 f.0 0.5 t

Fig. 1.1 - Time scale iflustrating the relative ages of geological elements on Borden Peninsula. Chapter I-Genmal Introduction discuss controls on the sedimentology of the shelf, (4) to describe stratigraphic packaging, (5) to interpret the sea-level history of the shelf and propose a mechanism for sea-level change,

(6) to present the basin-scale stratigraphy and propose a new mode1 for the tectonic evolution of the basin.

METHODS AND APPROACH

Fieldwork

Regional mapping, synthesis, and stratigraphie-sedimentological studies on the

Borden Basin (Fig. 1.2) by Blackadar (1 956, 1WO), Lemon and Blackadar (1963), Geldsetzer

(1973), Jackson et al. (1978, 1980), lannelli (1979, 1992), Jackson and Iamelli (1981),

Jackson and Sangster (1987), and Knight and Jackson (1994) provided the stratigraphic and tectonic context for this study. Previous work on the Victor Bay Formation concentrated on bioherms at the top of the formation in the White Bay area in the east (Geldsetzer, 1973;

Jackson and Iannelli, 1989) and Strathcona Sound area in the West (Jackson and Iannelli,

1989; Narbonne and James, 1996). The bulk of the formation has been studied only at the reconnaissance level, thus the overall nature of the Victor Bay system and the context of the reefs in relation to the rest of the carbonate platform is unclear.

Field investigation in 1994 afforded a regional overview of the Victor Bay Formation across the Borden Peninsula (Fig. 1.3). Study at five key localities indicated that shallow- water environments were restricted to the north aqd east portion of the Milne Inlet Trough, whereas the wea and south were dominated by slope and basin facies. A second field season in 1995 targeted the shallow-water succession close to the northem margin of the Milne Inlet

Trough at three localities in order to specifically delineate the shallow- to deep-water transition and identify the lithofacies occurring at the transition. Resulting facies analysis is based on data colleaed fiom measured sections, with facies descriptions supplemented by samples and field photographs. Areal photograph analysis was critical in documenting three- dimensional cycle and facies relationships in sea-cliff and canyon-wall exposures. Chapter I-GeneraI Introduction

Fig. 1.2 - Map of the Arctic Archipelago and c1.27 Ga Mesoproteroroic Byiot basim (Fuhrîg et al., 198 1). Modt~edfiomTreltn (1 989). Chapter I-General Introduction

GEOLOGY OF THE STUDY AREA

Regionoi Structural SeMing of the Bylot Supergtuup

The Bylot bains.-

The Victor Bay Formation belongs to the Bylot Supergroup (Jackson and Iannelli, 1981) which consists of a - 6 kiIometre thickness of volcanic, siIiciclastic, and carbonate

LEGEND BP BRODNR PENINSULA 0tce D WVlNlNLEf O Pimneroroic secîirnentary rocks fl Zone M BAILiARGEBAY % Downthrownbtock SS STRATHCONA SûUND Victor 6ay Formation AS ADAMS SOUND Mesoproteromfc sedlmentary Study locality: u LEVASSEURINW rocks, undivideci V 1994 fl FLEMlNG IN~ -O Cryrtalline basement R FABRlCIUS FIORD TS IREMôLAY SOUND

Fig. 2.3 - Geologicol map of the Borden Peninnila and Bylot Island showing distribution of the Victor Bqy Formation and major structurul elements of the Borden Basin. including fdzones, grabens (iroughs), and horsts (highs). Afier Jackson and lannelii (1981) and Jackson and Sungster (1987). Chapter I-Generuf Introduction rocks (Fig. 1.4). It was deposited in the Borden Basin (Christie et al., 1972), a rift developed in Archean and Paleoproterozoic crystalline rocks of the Churchill structural province

(Jackson and Iannelli, 1981). Extensional tectonics also created similar-aged basins in northeastem Laurentia (Jackson and Iannelli, 1981; Galley et al., 1983; Fahrig, 1987), now collectively referred to as the Bylot basins (Fahrig et al., 1981) (Fig. 1.2). The age of the

Bylot Supergroup is bracketed by 1.27 Ga basalts at the base of the succession (U-Pb baddeleyite age; LeCheminant and Hearnan, 1989), and by 0.72 Ga cross-cutting dykes

(Pehnson and Buchan, 1999). The age of the succession is believed to be closer to the maximum age based on paleomagnetic data (see discussion in Knight and Jackson. 1994), and

PbPb isotopic analysis of Uluksan and Nunatsiaq carbonates (Kah in Samuelsson et al.,

1999).

Uluksan/ Lower Nunatsiaq carbonates calcareous O sandstone and ' -1.2 Ga shale siltstone Lirnestone a conglomerate dolostone 0 basalt shale diabase

k2 1,267 Ga

Fig. 1.4 - SirnpItj7ed stratigraphy of the Bylot Supergroup. Modtfied fiom Hofmmn and Jackson (1991).

6 Chapter I-General Inh*oducrion

The Borden Basin.-

The Borden Basin is interpreted by Olson (1977) and Fahrig (1987) to be an aulacogen, and by Jackson and IanneIIi (198 1) as either an aulacogen or, along with the other Bylot basins, as preserved remnants of a pericontinental succession along the postulated Poseidon Ocean. Borden Basin occupies most of Borden Peninsula of northem Baffin Island, contains the thickest succession, and has the largest preserved surface area among the Bylot basins. The depression is composed of three principal grabens defined by major fault zones: the Miine Inlet, Eclipse, and North Bylot troughs (Jackson et al., 1975). The Milne Inlet

Trough is the largest of the three with a preserved length of 320 km and width of 20 to 100 km, Most of the Bylot Supergroup is flat-lying. Deformation is limited to synsedimentary extensional faulting, isolated late reverse faulting, and some post-depositional folding on northem By lot Island.

The Bylot Supergroup.-

The Mesoproterozoic rocks of the basin belong to the Bylot Supergroup, which is subdivided into three groups (Jackson and Iannelli, 198 1) (Fig. 1.3). The sandstones, shales,

and continental tholeiites of the Eqalulik Group reflect initial nfting (Galley et ai., 1983;

Dostal et al., 1989), and are deposited directly on Archean and Proterozoic crystalline basement. Carbonates of the overlying Uluksan Group accumulated during an interval of

tectonic quiescence (Jackson and Iannelli, 198 1, 1989). Succeeding sandstones, siltstones,

shales, and minor carbonate rocks of the Nunatsiaq Group represent sedimentation in fluvial

and marine environments during a period of renewed tectonisrn (Geldsetzer, 1973; Jackson

and Iannelli, 198 1, 1989). The succession was later intruded by Neoproterozoic Franklin

dykes, which are in turn tnincated below an angular unconformity covered by lower Palaeozoic siliciclastic and carbonate rocks of the Admiralty Group. Chapter 1-General Introduction

The Victor Bay Formution.-

The Victor Bay Formation is a -300400-m-thick unit of limestone, dolostone and minor siliciclastic rocks. It was deposited under basinai to inner rmp conditions and fonns the upper portion of the carbonate-dominated Uluksan Group. The lower part is a 50-370

(typically 100-200) m-thick lower member of siliciclastic shale, dolomitic shale, and nodular limestone. Gradua1 contact with the upper member is marked by an increase in carbonate content, bed thickness, and appearance of shallow-water facies. The upper member consists of limestone, dolostone, and minor siliciclastic rocks, attaining a maximum thickness of 702 m in eastem Borden Basin (Jackson and Iannelli, 198 1). In southwestern Milne Inlet Trough, upper-member facies are predominantly deep-water, consisting of dolomitic shale, nodular limestone, and nbbon limestone. In the northeast, shallow-water facies dominate the succession, and include peritidal carbonate mudstone, coarse inîraclast limestone, and stromatolitic carbonate. The upper member also hosts stromatolite buildups up to 275 m thick and up to 1 km wide (Geldsetzer, 1973; Jackson and iannelli, 1989; Narbonne and

James, 1996).

The Victor Bay Formation is unusual among Proterozoic carbonate platforms, where stromatolites and ooids are ubiquitous. Other roughly coeval Mesoproterozoic ramps exhibit

abundant shallow-water stromatolites, including the underlying Society Cliffs Formation

where microbialites and benthic cements are common in lagoonal and tidal-flat environrnents

or associated with ooid shoals (Kah, 1997). Instead, shallow-water Victor Bay facies are

dominated by dolomitic microbial laminite, intraclast grainstone, and molar-tooth lime

mudstone. Molar-tooth structure is a network of ptygmatically-folded cracks filled with early

microspar cement fint described in the late 19' cenhiry (Baueman, 1885) but only recently

recognized as a hallmark of shallow subtidal deposition on muddy carbonate platforms (James

et al., 1998). Chapter I-GeneraI Introdiiction

STRUCTURE OF THE THESIS

Nature of the carbonate platform

Chapter 2 is a description of sedimentary cornponents and facies, and interprets depositional environments within a carbonate platform architecture. Facies characterization illustrates the role of lime mud as both the principal component of subtidal facies and as the indirect source of grains through early lithification and reworking by storms or slope instability. Such lime rnudstone is lithified early and reworked by storms across the mid- and inner rarnp to form grainy sediments. Facies distribution and sedimentary structures indicate that deposition occurred on a storm-dominated, distally steepened microtidal ramp. The predominance of lime mud, paucity of microbialites and ooids suggest that this ramp formed under the influence of intense highstand water-column precipitation of mud, punctuated by benthic carbonate precipitation as micrcbialites and ooids during periods of abrupt sea-level rise.

Cyclicity and sea-level history

Chapter 3 describes and interprets stratipphic packaging of the rocks. Regular successions of deep subtidal to supratidal facies occur in 20-50 m-thick units. These units are anomalously thick shallowing-upward cycles, or parasequences. Superlative exposure along vegetation-free cliff faces at one exceptional locality allows direct observation of the parasequences perpendicular to depositional mike. Outboard thiming of peritidal cycle caps coincides with a zone of mas-wasting, supporting the interpretation of a steepened ramp architecture. The scale of the parasequences suggests high-magnitude sea-level excursions, which has repercussions for interpretation of Mesoproterozoic within the global icehouse-greenhouse cycle.

Tectonic context of the Victor Bay Formation and impkotions for barin evolution

Chapter 4 incorporates observations fiom outcrop scale to. basin scale to explain bathymetric reversal that lead to exposure of deep-water reefs in the west, and contemporaneous drowning of shallow-water environments to the east. Correlation of facies, identification of flooding and karst surfaces, and delineation of reef tracts reveal a transition within the "passive" Uluksan carbonate phase to a tectonically active regime. This chapter presents a new tectonic interpretation of the Borden Basin that expIains regionai facies trends in the Victor Bay Formation. The resulting model offen an alternative to the strict aulacogen model and has strong implications for the evolution of northem Laurentia and the

configuration of Rodinia in the late Mesoproterozoic. CHAPTER 2:

SEDIMENTOLOGY OF A LATE MESOPROTEROZOIC lMUDDY CARBONATE

W,NORTHERN BAFFIN ISLAND, ARCTIC CANADA'

ABSTRACT The - 1.2 Ga Victor Bay Formation is a muddy, predominantly subtidal carbonate ramp succession that crops out in the Borden Basin of northern Baffin Island. The - 400-m- thick upper member is dominated by a variety of lime mudstone facies representing supratidal to deep subtidal environments. The lateral transition between shallow-water and deep-water environments is best exposed in cliff sections along the northeast margin of the Milne Inlet

Trough, the major graben in the Borden Basin.

The principal controls on the style of deposition on the Victor Bay ramp are (1) production of lime mud, interpreted to have taken place in the water column, and (2) redistribution of the mud and mudstone-denved grains during storms. Grainstones in the

shallow subtidal and nearshore environments are composed of subtidal molar-tooth crack411

grains and intraclasts of peritidal dololaminite facies, with on1y minor amounts of ooids.

Microbialites form large stromatolitic buildups that accreted during times of rapid increase in

accommodation space andor times of decreased time mud production.

Comparison with other Proteromic carbonate platforms indicates that the Victor

Bay ramp represents an end member where production of clastic lime mud far exceeded

seafloor cementation by inorganic precipitates or growth of microbialites. It is therefore

more similar to mud- and grain-dominated Phanerozoic ramps than to cement- and

stromatolite-dominated Paleoproterozoic carbonate systems. The close temporal association

of the muddy Victor Bay ramp with the underlying Society Cliffs Formation, which has

From: Sherman et ai. (2000), SEPM Speciai Publication 67, p. 275-294. Chapter 2-Muddy carbonate rarnp attributes more typical of early Paleoproterozoic carbonates, attests to the wide spectrum of carbonate depositional systerns that had evolved by the late Mesoproterozoic. tNTRODUCTION Although different in detail, the main themes of carbonate sedimentation have remained roughly similar throughout much of geologic history (Wilson, 1975). There appears, however, to be an evolution in the style of carbonate shelves and ramps as well as in their constituent particle composition though Proterozoic time (Grotzinger, 1989). Older platforms are dominated by various kinds of benthic precipitates, whereas late

Mesoproterozoic and Neoproterozoic ramps have many of the attributes ascribed to skeletal- nch Phanerozoic successions. Nevertheless, these younger Proterozoic carbonate platforms are consûucted by elements in some ways peculiar to this geological interval, a time of rapid and dramatic change in tectonic plate configuration and ocean composition (Grotzinger,

1990; Knoll and Swett, 1990). Sedimentary rocks of the tate Mesoproterozoic Victor Bay Formation in Arctic

Canada record carbonate sedimentation in a rarnp-to-basin setting within the Borden Basin on northem Bafin Island (Fig. 2.1). The strata are unmetamorphosed, flat-lying, and well- exposed along extensive cliff faces. Shallow-water to deep-water facies transitions can be walked out or traced laterally over comparatively short, kilometre-scale distances, allowing temporal and spatial facies relationships to be discerned both parallel and perpendicular to depositional strike. The rocks are mainly limestone and have excellent fabric preservation.

Thus, these strata are an exceptional example of a late Proterozoic carbonate depositional system.

The purpose of this paper is (1) to describe the facies attributes of the rocks, (2) to determine the origin of the sediments, and (3) to interpret the depositional nature of the system. The sediments are then compared to those on other carbonate ramps to determine their place in the carbonate depositional spectnrm. Chupter 2-Muddy carbonate ramp

Fig. 2.I - A) Mup of Borden Peninncla showing outcrop of the Victor Bay Formation and location of the major grabens (troughs) in the Borden Busin. The stuc& area is outlined and enlarged in 0.After Jackson and Iannelfi (1981) and Jackson and Sangster (1987). B) Locution of memred sections in the stuc& area: 1, Adams River; 2, Pingo Valley South; 3, P ingo Valley North; 4. Mala River; 5, Camp Prozac; 6. AIfied Point; 7, Trembloy Sound. Lines A and B refr to cross sections in Figure 2.3. GEOLOGICAL SETTING

Age und Strtictural Setting of the Bylot Supergroup

Rifting of Archean and Paleoproterozoic crystalline rocks in northem Laurentia ca.

1.27 Ga created several Mesoproterozoic basins exposed in the present-day Arctic (Jackson and Iannelli, 1981; Galley et al., 1983; Fahrig, 1987). The Borden Basin rocks (Christie et al., 1972) are broadly contemporaneous to strata of the Fury and Hecla Basin of northwest

B&n Island (Chandler, 1988), the ThuIe Basin of Ellesmere Island and Western Greenland

(Dawes, 1976, 1997), and the Aston-Hunting succession of Somerset Island (Blackadar,

1967; Reinson et al., 1976; Stewart, 1987). Lithostnitigraphic correlation between Bylot,

Thule, and Aston-Hunting strata is reinforced by recent WCchemostratigraphy (Kah et al.,

1999).

Of these successions, the Bylot Supergroup in the Borden Basin is the thickest

(Jackson and Iannelli, 1981). Most of the 6 km of strata (Fig. 2.2) is flat-lying and undefomed Basal sedimentary rocks of the Bylot Supergroup locally reach sub-greenschist facies (Jackson and Morgan, 1978; Galley et al., 1983; Dostal et al., 1989), but al1 overlying units including the rocks of this study are unmetamorphosed (Knight and Jackson, 1994).

The Bylot Supergroup is divided into three groups (Blackadar, 1956). The sandstones, shales, and subaerially deposited basalts of the Eqalulik Group reflect initial rifiing (Galley et al.,

1983; Dostal et al., 1989). Limestones and dolostones of the overlying Uluksan Group

accurnulated during a subsequent interval of relative tectonic quiescence (Jackson and Iannelli,

1981, 1989). Overlying sandstones, siltstones, shales, and minor carbonate rocks of the Nunatsiaq Group represent sedirnentation in a variety of fluvial and marine environments

that developed during renewed rifiing and eventual post-rifi subsidence (Jackson and Iannelli,

1981, 1989). An unconformity separates the Bylot Supergroup from overlying lower

Paleozoic Admiralty Group strata. Chapter 2-Muddy carbonaie rump

Rocks of the Bylot Supergroup are exposed in three principal grabens (Fig. 2.1A): the Milne Met, Eclipse, and North Bylot troughs (Jackson et al., 1975). The graben margins are defined by major fault zones that separate the sedimentary rocks fiom horsts of crystalline basement (Jackson et al., 1975). The snidy area is located in the central to southeastern part of the NW-SE-oriented Milne Inlet Trough, the largest of the grabens (Fig.

2.18). Strata on either side of the trough dip gently towards the center, forming a broad

caicateaus a sandstone and shale süktone EJlimestone eanglmerate dolostom basait C3stiale 'diabase crystailine rocks

Fig. 2.2 - Generalized stratigraphie section of the Bylot Supergroup. Modified from HoMann and Jackson (1991) and Iannelli (1992). Nauyat basalt uge from LeCheminant and Hemn (1989); ütuksan Group age fiom Kah (in Samuelsson et al, 1999); and Franklin +ke age fimn Heoman et al. (1992) and Pehrsson and Buchan (1994). Chapter 2-Muddy carbonate rump synclinal structure that plunges to the W.

Bylot Supergroup acritarchs cannot be used for high-resolution biostratigraphy, but are consistent with late Precambrian assemblages (Hohann and Jackson, 199 1, 1994). A maximum late Mesoproterozoic age for the succession was obtained hmpaleomagnetic studies and isotope geochemistry of Nauyat Formation basal&. These are paleomagnetically indistinguishable (Fahrig et al., 198 1) from the 1267 * 2 Ma Mackenzie dyke swarm (U-Pb baddeleyite age; LeCheminant and Heaman, 1989). Minimum U-Pb baddeleyite ages for the basin of 723 +4/-2 Ma (Heaman et al., 1992) and ca. 725 Ma (Pehrsson and Buchan, 1994) have been obtained fiom analyses of Franklin diabase dykes on Baffin Island that intmde atl formations of the Bylot Supergroup. Paleomagnetic data suggest that the entire Bylot

Supergroup was deposited prior to 1.2 1-1.19 Ga (sec discussion in Knight and Jackson, 1994).

A late Mesoproterozoic age for the succession is further implied by two recent PbPb dates

nom carbonates of the Uluksan Group and lowermost Nunatsiaq Group (Kah in Samuelsson et al., 1999). Samples fiom the Society Cliffs Formation and lowermost Victor Bay Formation yield an age of 1199 k 24 Ma, and an age of 1204 * 22 Ma has been obtained for a suite taken across the entire Uluksan group and into the lowermost Athole Point Formation

(Nunatsiaq Group). Paleomagnetic studies suggest that the paleolatitude of the Borden Basin

was equatonal at the time of deposition of the lower Bylot Supergroup, and had reached

-lOaNwhen the upper Bylot strata were deposited (Fahrig et al., 1981).

The Uluksan Group

The carbonate piatfom phase, represented by the Uluksan Group (Fig. 24, is divided

into two major packages: (1) dolostones, minor limestones, and siliciclastic rocks of the

Society Cliffs Formation (Kah, 2000) and, along the southem margin of the basin, laterally

equivalent siliciclastic conglomerates and breccias of the Fabricius Fiord Formation; and (2)

the Victor Bay Formation, comprising a lower member of dolomitic shales (VB,of Jackson

and IanneIli, 1981) and an upper member of limestones, dolostones, and silicicIastic rocks Chapter 2-Mut& carbonate ramp

(VB2 of Jackson and Iannelli, 1981). In most of the Milne Inlet Trough the contact between the peritidal to shaIlow subtidal Society Cliffs Formation and basinal shales of the lower

Victor Bay member is confonnable and either abrupt or gradational (Jackson and Iannelli,

1981). In the Eclipse Trough and northwestern Milne lnlet Trough, the lower Victor Bay member is absent and the Society Ciiffs Formation is directly overlain by upper Victor Bay

member carbonates. Limestones and dolostones of the upper member include tidal flat, shallow to deep subtidal, and inner to outer slope facies. Deep-water settings host extensive

stromatolitic reef complexes (Geldsetzer, 1973; Jackson and Iannelli, 1981; Narbonne and

James, 1996). In central and eastern Milne Inlet Trough the upper member is in conformable

contact with overlying calcareous shales of the Athole Point Formation. In the Eclipse

Trough and in the western part of the Milne Inlet Trough, the Athole Point Formation is

absent and the Victor Bay Formation is unconformably overlain by siltstones, sandstones,

conglomerates, and minor dolostones of the lower Strathcona Sound Formation (Jackson and

Iannelli, 198 1; Narbonne and James, 1996).

METHODS

This study is based on cliff sections and Stream sections measured in the Victor Bay

Formation east of the communities of Arctic Bay and Nanisivik (Fig. 2.1A). Stratigraphie

sections at Nanisivik and seven other localities (Fig. 2.1 B) were examined during two six-

week field seasons in 1994 and 1995. Facies descriptions are based on tield observations,

photographs, slabbed lithological samples, and thin sections. Cyclic packaging of these facies and regional correlations of Victor Bay strata wilI be addressed in fiiture work.

FACES OF THE VICTOR BAY FORMATION

Lower Vicîor Bay Member

Brown, black, and dark grey dolomitic shaIe and minor dolosiltite-shale turbidites overlie Society Cliffs Formation dolostones conformably to locally disconformabIy (Jackson

and Iannelli, 1981). Shale is a maximum of 370 m thick on central Borden Peninsula Chapter 2-Muddy carbonate ramp

(Iannelli, 1992) but averages 100 to 150 m thick in the study area (Fig. 2.3). The basal 15 to

25 m are black, bitminous shales containing nodular and disseminated pyrite. At intervals of

0.5 to 1 m, the shale contains centimetre-thick beds of buff-weathering microcrystalIine dolostone. There is an upward increase in bed thickness, abundance of Td- (Bouma, 1962) calciturbidites, and number of coarse conglomerates with nodular and ribbon limestone clasts through the lower member. Transition into carbonates of the upper Victor Bay is marked by

15 to 25 m of interlayered nodular limestone and small bioherms of columnar stromatolites. The sharp transition fiom shallow-water carbonate rocks of the Society Cliffs

Formation to deep-water shales of the Victor Bay Formation has been interpreted as a rapid

drowning event, possibly tectonically driven (Jackson and Iannelli, 1981; Iannelli, 1992).

The gradua1 upward increase in carbonate content, more numerous carbonate turbidites, and the transition into upper carbonate member rock types is interpreted as reflecting

progradation and restoration of the Uluksan carbonate platform (Jackson and Iannelli, 1981; Iannelli 1992).

Upper Victor Bay Member

In contrast to the lower member, which is 1ithologicaIly uniform at the regional scale, the limestones, dolostones, and minor siliciclastic rocks of the upper Victor Bay member

suggest a significant shallowing fiom SW to NE. Where measured along the southwestern

edge of the study area, upper-member facies xe predominantly deep-water, consisting of

dolomitic shale, nodular limestone, and ribbon limestone (Fig. 2.3). In the northeast, however, shallow-water facies dominate the succession, and include petitidal carbonate mudstone, coarse intraclast limestone, and stromatolitic carbonate,

Stromatolite buildups up to 275 m thick and up to 1 km wide occur in the upper

member of the Victor Bay Formation (Geldsetzer, 1973; Jackson and tanneIli, 1989;

Narbonne and James, 1996). In the eastem part of the Mihe Inlet Trough, including the Chapter 2-Muddy carbonate ramp

sw & NE Athole Pt. Fm.

shallow subtidal cycle

Fig. 2.3 - Regional correlation of sections in the sttidy ore Rom southwest tu northeast. Section numbers refer to locutions in Figure LIB. Niwtheastern sections (2, 3, 4, 7) are dominated by shallow-subtidal to supratidal cycles, whereas cycles ore erclusiveiy deep- water in the southwestern sections (1, 6). ldeaiized cycles in the iegend iiiushate lateral facies changes fiom shaliow to deep water. me outlined port of cross section A k enlarged in Figure 2.5. See Egures 4. IO and 4. I 1 for more iietailed correZations. Chapter 2-Mud& carbonate ramp study area, they are present only on the northem margin of the Trough (Fig. 2.3). To the

West, where the Mihe Inlet Trough and the Eclipse Trough meet, the reef outcrop stretches almost to the southem margin. The upper member attains a maximum thickness of 702 m in eastem Borden Basin (Jackson and Iannelli, 1981) and ranges between 350 and 450 m in the study area. Late dolomitization of the upper member in North Bylot Trough, Eclipse

Trough, and southeastem Milne Inlet Trough has resulted in finely crystalline, pink, grey, or white dolospar that locally contains vugs filled with saddle dolomite. With the exception of the largest stromatolite reef (Section 2, Fig. 2.LB) and almost al1 facies at the eastemmost locality (Section 7, Fig. 2.1B), limestone in the study area did not undergo this late dolomitization and retains excellent fabric detail.

Upper Victor Bay rocks are packaged into shallowing-upwards cycles 20 to 50 m thick (Fig. 2.3). Where they are exposed on the northeast margin of the Milne Inlet Trough

(Fig. 2.4), shallow-water facies thin to the southwest and are replaced by deeper-water equivalents (Fig. 2.5). This transition can be ûaced visually in continuous outcrop over the

6.5 km distance separating Sections 2 and 3 (Fig. 2.5). Upper Victor Bay lithofacies are

described and interpreted below, and summarized in Table 1.

Red Shale Facies.-

Weakly larninated red shale forms units less than 1 m thick with desiccation cracks,

rare discoid sulfate rnolds up to a centimetre long, and thin Iayers of scattered gypsum

nodules. Evaporite minerals and molds make up an estimated 10% of the rock volume. Thin

Iayers of fine to coane sand-size particles consist of rounded microcline, quartz, and granitic

gneiss. These shales are commonly interlayered with dolomitic mudstone, and locally with

green shale, carbonaceou shale, and thinly bedded limestone.

Desiccation cracks, sulfate molds, and nodular gypsurn indicate an environment that

was subjected to subaerial exposure in a hypersaline setting (Kendall and Skipwith, 1969).

Laminated to massive red mudstones with layers of coarse sand and evaporites are known

fkom modem inud fiats associated with playas, salinas, or coastal sabkhas (Wright, 1984; Chapter 2-Mude carbonate ramp

Fig. 2.4 - Cyclicallj packaged carbonates in the upper member of the Victor Bay Formation 2.5 km NNE of Section 2. Recessive deep-water facies alternate with resistant shallow-water molar-tooth mudstone, molar-tooth calearenite, and pale-weafhering peritiduf dolostone. Dolomitic shales of the lower Victor Bay member form the recessive dope below the cl~rs.Scale bar = 50 m.

Fig. 2.5 - Correlation of parts of upper Victor Bay strata in Sections 2, 3, and 4 hung on the marker sirontaro fite biostmme, showing peritidal dololaminite facies (shodet$ thinning to the south. See figures 2. IB and 2.3 for regional context.

2 1 Chapter 2-Muddj carbonate ramp

Table 2.1 - Summary of lithofacies and interpreted environments. 0. Facies Constitucnts Bedding Sedimentary Structures Interpretation 2

Deep subtidal Ribbon Couplets of lime mudstone and Thin, 2-4 cm couplets, Slump folds, synsedimentary (below stom I limestone sub-equal layers (ribbon microlamination in lime breccias wave base: inter- 3 limestone) to thin seams (parted mudstone limestone) of argillaceous to dolomitic mudstone; debrites and breccias of ribbon limestone. Deep subtidal Dolospar/celcispar-~arbonaceous Laminae 1-2 mm thick, beds Doming and warping of beds by (below storm shale couplets, bitumen-rich 2-20 cm, tocally undulose creep, synsedimentary breccias wave base; intermediate outer WP) Deep subtidal Dolomitic Fissile black and dark grey shale, Thin, with weak lamination (below storrn shale rare thin limestone flake breccia wave base; distal outer ramp to bas in)

Shallow subtidal Stromatolite Columnar limestone 0.5-1.5 m thick biostromcs with Elongation and inclination of (inner ramp to biostrome stromatolites 1-5 cm wide, up to planar to undulose upper surface, columns mid-ramp) 50 cm tall, with gently to steeply mm-scale stromatolitic laminac, convex lam-inae; inter-colurnnar synoptic relief 1-4 cm sediment of stromaclasts, lime mud, quartzose sand, mudstone intraclasts; green shale cap

Stromatolitc Initial develop- Columnar limestone to dolostone Bioherms up to 275 n~ thick, 1-2 Elongation of columns on upper Id' ment in deep sub- stromatolites 3-10 cm wide, up km wide with flat IO sloping suriace; talus blocks up to 40 m in tidal (usually to 1 m taIl with linked upper surfaces height adjacent to southern outer ramp). hemisplierical to conicof laminae; (basinward) mrirgin Growrh of largest domal stromatolites 1-1 0 rn in reefs is main- diameter tained into shallow subtidal. Chapter 2-Muddy carbonate ramp

Kendall, 1992). Interlayering with laminated dolomitic mudstones of marine character

(dololaminite facies; see below) suggests proximity to tidewater, in the form of coastal salina or sabkha environments. The nodular and matrix-dominated style of the evaporite-bearing layers more specifically suggests a high supratidal, coastal sabkha mud flat environment

(Pratt et al., 1992). The discoid evaporite molds are typical of displacive intra-sediment growth (Kendall, 1992), and the preservation of nodular sulfate testifies to the aridity and high temperature at the tirne of deposition (Kinsman, 1976). Corne siliciclastic interlayea of gneiss pebbles and other siliciclastic material indicate a nearby temgenous source, possibly crystalline rocks from uplifted rift blocks of Paleoproterozoic basement.

Doloiam inite Facies.-

The dololaminite facies includes two principal rock types: (1) thin- to medium- bedded laminated dolomitic mudstone and (2) intraclast packstone to rudstone with temgenous sand. The mudstone outcrops recessively and is buff in colour, whereas fiesh surfaces range frorn medium to dark grey with increasing organic content. Millimetre-scale

laminae are predominantly srnooth, locally tufted and crinkled, and bounded by thin, black

organic-rich layes. Small stromatolitic domes under 10 cm in height, microbial mat roll-ups,

wave ripples, fenestral pores, desiccation cracks, and tepee structures (Fig. 2.6A) are present

locally. Lenses of blue to black displacive nodular chert 3 to 10 cm wide and 1 to 2 cm thick

are relatively rare. Massive to mdely layered packstone to rudstone beds are 10 to 50 cm

thick, with basal lags of dolomitic mudstone intraclasts up to 10 cm long (Fig. 2.6B).

Laminated and cross-stratified medium to coarse quartrose sand constitutes up to 20% of the

intraclast beds, and locally forms cross-bedded layers up to 0.5 m thick.

Although Precambrian stratiform microbial fabrics are interpreted to have formed in

quiet environments in both shallow and deep water (Serebryakov and Semikhatov, 1974;

Southgate, 1989; Sarni and James, 1993) a quiet subtidal origin for this facies is discounted

because of associated mudcracks and tepee structures (Shinn, 1986; Kendall and Warren,

1987) as well as abundant intraclasts and cross-bedded sandstone. Furthemore, smooth and Chapter 2-Muddy carbonate ramp

Nfted cryptomicrobial laminites are abundant in modem carbonate tidal flats (Shinn, 1986) and commonly ncognized in ancient peritidal successions (Hardie, 1986a; Fischer, 1964;

Grotzinger, 1986a; Pratt et al., 1992). The association of evapontes, dololaminites, and dolomudstone intraclasts is similar to that in Holocene shallowing-upwards sequences of the

Persian Gulf (merand Evans, 1973). A low supratidal to high intertidal environment is

Fig. 2.6 - Field photographs of dololaminite facies, upper member of the Victor Bay Formation A) Laminuted dolomitic mudstone with tepee cracks overlain by bed of carbonate mudstone inlraclrct rudstone. Lens cap = 5.5 cm in diameter. B) Quarke lag of dolomudstone intraclasts (dark with light clasts) overluin by laminated dolomitic mudstone (tight). Lens cap = 5.5 cm in diameter.

25 Chapter 2-Mud& carbonate rarnp therefore inferred for the dololarninite facies. Furthemore, the altemating microbial layers and thick, graded beds of mud and intniclasts resemble facies on unprotected tidal levees and beach ridges where microbial mats briefly reestablish themselves between episodes of storm deposition in modem supratidal environments (Shim, 1986; Wadess et al., 1988).

The interpretation of a low supratidal to high intertidal environment is also supported by the interbedding of dololaminites with calcitic mudstones and grainstones, implying early fabric-specific or facies-specific dolomitization of carbonate mud in the supratidal subsurface (e.g., Persian Gulf; Patterson and Kinsman, 1982). Preservation of primary sedimentary structures is good, and coarse dolospar associated with later burial dolomitization is absent. The abundance of quartz and microcline sand Mersuggests proximity to a terrestrial source.

Molar-Tooth Mudstone Facies.-

The term "molar-tooth limestone" was fiat coined by Bauerman (1885) for lime mudstone in the Mesoproterozoic Belt-Purcell Supergroup, which contains sinuous vertical and horizontal sheet cracks and bubble-like pods, al1 filled with fine calcite spar. Molar-tooth structure in the upper Victor Bay Formation is dominated by ptygmatically folded synsedirnentary fractures but pods are relatively rare (Fig. 2.7A-C). White- to medium-grey- weathering microspar crack-fil1 dissects a matrix of pale yellow to brownish-grey, argilIaceous to dolomitic lime mudstone. Most cracks are subvertical, 1 to 10 mm wide, up to 50 cm deep, and commonly reach the top of the bed. SubhorizontaI, microspar-filled sheet cracks are less cornmon, and tend to be resaicted to the muddy tops of graded beds. They locally account for up to 10% of beds by volume. As much as 75% by volume of beds can be composed of densely packed convoluted cracks; at the other end of the spectnim, short and simple spinde-shaped cracks make up less than 5% of some layers. Cracks occur in beds that are either (1) uniformly muddy, or (2) graded fiom a rudstone or packstone base to a mudstone top. Basal rudstone and packstone are composed of reworked particles of microspar identical to the molar-tooth crack-fill, with ma11 amounts of quartzose sand and Chapter 2-Muddy carbonate romp bestow a "welded" appearance to the coarser layen. Layen of finer, more equant fine dololaminite clasts. Imbrication, compaction, and undulose shape of the microspar clasts hgments are locally cross-stratified. Bed bases range f?om planar to locally undulose where scours and small calcarenite dunes create reIief on the order of 2 to 5 cm, Crack-fiIIs can be tnincated at scour sdaces, or protrude into the basal lag of the overlying bed. Bedding-plane expression of the cracks includes evenly spaced subparallel arrangements (Fig. 2.7D) to pseudo-polygonal networks. Polygonal networks are most common at the bases of molar- tooth mudstone packages, whereas subparallel cracks tend to occur towards the tops of the

Fig. 2.7 - Molar-tooth mudstone facies, upper member of the Yictor Bay Formation. A) Sparse inillimetre-scale crenulated cracks and sntall pods in mudstone bed Note basal lag of crack-fili intraclast rudstone above sharp erosional m@ce (lower lefi at fingeriip). Scafe bor = 5 cm. B) Cross-sectional view of large molar-tooth fractures creating a dense pattern in the mudstone matrix. Some cracks reach 50 cm in depth. Hammer = 35 cm long. C) Thin- to medium-bedded molar-tooth mudstone-to-rudstone ternpestites. Note angulm, scoured bases and numerous graded beds. Hammer (in vertical fiucture at leji) = 35 cm long. D) Bedding-plane view of molar-tooth cracks in do[omitic hemudstone. A set of well-developed parallel &actures is associated with smuller perpendcular spinde-shaped cracks. Hammer = 35 cm long. Chapter 2-Muddy carbonate ramp units. Where measured at Sections 2 and 3, subparallel cracks have a consistent NW-SE orientation, parallel to the interpreted depositional strüce. In the Victor Bay Formation, molar-tooth mudstone is consistently underlain by nodular Iimestone facies and overlain by molar-tooth calcarenite facies above (James et al., 1998; Sherman et al., 2000).

The depositional environment of molar-tooth mudstone is interpreted to be subtidal

(James et al., 1998) on the basis of the shape of the cracks and their depositional context within the graded beds. Inclusion of crack-fil1 intraclasts in the molar-tooth storm beds, and tnincated crack-fill, point to early precipitation of microspar in the fissures. Despite a variety of fortns, molar-tooth structure never defines closed polygons on the surface of beds as do desiccation cracks. They are not diastasis cracks (Cowan and James, 1992), because they do not (1) have a brinle or fragmented appearance or (2) involve the overlying bed.

Although molar-tooth cracks are morphologically similar to spindle-shaped synaeresis cracks, they do not contain detntal material as do fissures created by desiccation, diastasis, or spaeresis. Absence of cryptomicrobial fabncs and fenestrae, which are common in intertidal and supratidal facies, Mersupports an interpretation of subtidal deposition. Rudstone lags

and scoured bases of graded beds are typical of tempestites and point to deposition above

stom wave base (Aigner, 1982). Predominance of mud in the tops of tempestite layers

suggests that they are distal storm beds deposited below fair-weather wave base (Aigner, 1982,

1985; Dott and Bourgeois, 1982).

Much debate has surrounded the origin of molar-tooth structure, and proposed

mechanisrns have included subaqueous shrinkage (Horodyski, 1976; Knoll and Swett, 1990),

microbial action (Smith, 1968; O'Connor, 1972; Funiiss et al., 1998). evaponte replacement

(Eby, 1975), and seismicity (Fairchild et ai., 1997; Pratt, 1998). Experimentai work by

Fumiss et al. (1998) strongly suggests that coupled dewatering and gas escape in lime mud

could account for vertical and horizontal sheet cracks in molar-tooth carbonates. In a S"C

study of molar-tooth microspar, Frank and Lyons (1998) identi& COdegassing as a possible

mechanimi for seafloor precipitation of the microspar, and suggest that changes in salinity, Chapter 2-MÙddy carbonate ramp temperature, water depth, and microbial uptake of CO2 could have been responsible (See Fig.

5.1 for interpretation of sediment dynamics and rnolar-tooth formation).

Mdar-Tooth Calcarenite Facies.-

This facies consists of thin- to medium-bedded, massive to fining-upward molar- tooth-fiagrnent rudstone, packstone, and grainstone with grain size ranging from 0.5 mm to

4 cm and averaging 1 to 2 mm. It is similar to the coarse lags at the bases of molar-tooth

mudstone tempestites, but grains are better sorted and generally finer. Weathering colour is

medium to dark grey and fiesh surfaces are dark grey. Particles are equant to platy microspar fragments with minor quartz sand and dolomitic mudstone intraclasts. Thin interlayers and

lenses of lime mudstone (Fig. 2.8A) are similar to those associated with modem storm-

current reactivation of subaqueous dunes (Shinn et al., 1993). Although generally rare, ooids

are locally important and can represent from 5 to 35% of some beds (Hamngton, 1995).

Physical sedimentary structures are planar lamination, wave ripples (Fig. 2.8B), dune and

ripple cross-lamination, and rare hummoclq cross-stratification (HCS; Harms et al., 1975;

Harms, 1979). Diastasis cracks developed in the thin mudstone interlayea (Fig. 2.8A) otten

have a curlicue shape in plan view (Fig. 2.8C; cf. Fig. 6D in Cowan and lames, 1992). In rare

cases, early cementation of calcarenite beds was followed by erosion, resulting in irregulariy

scoured surfaces with relief of up to 30 cm. These erosional surfaces bear a thin, centimetre- thick layer of laminated cernent which is microstalactitic where it encrusts the underside of overhangs.

Wave- and storm-generated bedforms, coarse grain size, and Iow mud content of this

lithofacies suggest that the sediments accumulated as proximal tempestites well above fair-

weather wave base (Aigner, 1982; Don and Bourgeois, 1982). Beds maintain constant thickness laterally and do not have lenticular shapes. large subaqueous dunes, or herringbone

cross-bedding, features associated with tidal channels or migrating shoals. Grains have a

mixed provenance, some transported hm nibtidal molar-tooth mudstone facies and the rest

fiom intertidal to supraticla1 quartz-rich dololaminite facies. Abundant planar lamination is consistent with hi&-energy regime of modem storm-dominated shoreface settings

(Greenwood and Sherman, 1986). Rare beds of early cemented, eroded, and subsequentiy encnisted calcarenite resemble beachrock (Ginsburg, 1953; Stoddart and Cam, 1965; and others) and suggest episodic subaerial exposure at the strandline.

San& Polymictic Conglomerate Facies.-

This conglomerate consists of pebble- to cobble-size dololarninite intraclasts, fragments of columnar stromatolites, and pieces of molar-tooth microspar (Fig. 2.8D). The matrix is (1) medium to coarse sand-size quartz and microcline, and (2) sand-size carbonate

Fig. 2.8 - Molar-tooth culcurenite facies and sun& polymictic conglomerate facies, upper meinber, Victor Bay Formution A) Dimtmis cracks in lime mudslone (light) between thin beds of rnolar-tooth calcarenite (durk). Scale bar = 5 cm. B) Thick package of wave ripples in molar-tooth calcurenite. Scale bar = 5 cm. C) Bedding-phne view of ripped molar-tooth iniraclat graimtone tu packslone. Mud dmpes in ripple troughs display curlicue diustasis crack Lens cap = 5.5 cm in diameter. D) Sun& polymictic conglomerate facies with large columnar stromatolite fragments. Lens cap = 5.5 cm in diameter. Chapter 2-Muddy carbonate ramp particles of molar-tooth microspar, dolomitic mudstone, and minor ooids. Beds are up to 50 cm thick, have planar to scalloped bases, and exhibit varying degrees of cnide gnding, planar lamination, cross-stratification, and convolute bedding.

The wide range of sources represented by different clast types suggests that the conglomeratic material was derived fiom erosion of al1 of the shallow-water facies described herein. The absence of mud and the high proportion of quartz and microcline suggest a hi@- energy depositional environment close to a terrestrial source of siliciclastic material. The conglomerate exhibits characteristics of both shoreface beach deposition (cross-stratification. planar lamination) and proximal ternpestite accumulation (convolute bed tops, grading). The interpreted environment for this facies is a series of nearshore channels with rapid deposition of material.

Nodular Limesrone Facies.-

This facies consists of dark-grey-weathering limestone nodules in a matrix of buff to brownish-grey dolomite, dolomitic limestone, or, rarely, dolomitic shale. Nodules are either

(1) large and smooth-sided or (2) small and irregularly shaped (Fig. 2.9A). In cross section, smooth nodules have subcircular to elliptical shapes with sharply defined margins. They attain diameten of several tens of centimetres and have cross-sectional aspect ratios of up to

MO. lrregular nodules are much smaller, with diameters of 1 to 2 cm and sharp to diffuse

rnargins, giving the limestone a mottled appearance. Srnooth-sided nodules are present

everywhere, but irregular nodules occur only in shallow-water cycles. Nodules are included in

muddy debntes up to 1 m thick, attesting to an early diagenetic origin for the nodules prior to

transport. Slumped and dislocated nodular beds are common in the north of the sîudy area

and ubiquitous in the south.

It is generally agreed that suspension settling of lime mud swept offshore by storm

wave action mates fine-grained carbonate deposits in deep subtidal waters on the proximal

outer dope, below storm wave base (Bathurst, 1975; MuIIins et al., 1980; M6Iler and

Kvingan, 1988). Early cementation of this lime mud results in partial to complete layer Chapter 2-Mud& carbonate ramp

Flg. 2.9 - Deep-water and stromaiolitic facies, upper mentber of the Victor Bay Formation. A) Irregular noduiar limestone interbedded with thin ribbon limestone and beds of smooth, elongate nodules. Hammer = 35 cm. B) Ribbon and parted limestone beds showing synredimentary dumping and brecciation of some beds (right side of photograph). Lem cap = 5.5 cm in diameter. 9 Carbonaceuus rhythmite showing sep- sediment deformation and brinle synsedimentary fiocturing. Scale graduated in miilimetres. D-F) Sîromatolitic facies. D) Small upright to inclined columnur stromatolites with gently tu steeply conver luminue. Lens cap = 5.5 cm in dicimeter. E) Stromatolites in D huve devehped on a bed of sanày intraclast rudstone tu form a three- tiered bioshoome 1.5 m thick and with a lateral extent of at least 20 km. Person for scale (circled ut right). F) Stromatolite reef facies in the lower I7Q-rn-thick biostrome (top and buse indicated by mows) at Section 2 (see Figure 2. IB), composed of laminated domes I-10 nt in diameter, and columnar stromatolites 3-10 cm in diameter with conical to convex laminae, Scale bar = 100 m. Chapm 2-Muddy carbonate rump lithification. The preponderance of mud to the exclusion of other grain sizes and lack of storm-generated sedimentary structures in this Victor Bay lithofacies also suggests subtidal deposition below stom wave base. Slumping and accumulation of nodule-bearing debrite sheets indicate a slope and the accommodation afforded by deep subtidal dope environments, as in Holocene slope nodular periplatform ooze (Mullins et al., 1980) and ancient slope nodular limestones (Cook and Mullins, 1983; M8ller and Kvingan, 1988).

Ribbon and Parted Limestone Facies.-

A ribbon-like appearance is imparted to this facies by subequal couplets of limestone and argillaceous carbonate. Thin, continuous layers of dark grey lime mudstone 1 to 2 cm thick are separated by equal thicknesses of less cesistant buff-weathering argillaceous to dolomitic limestone. Also included in this facies is parted limestone (Coniglio and Dix, 1992) in which lime mudstone layers are thicker than the argillaceous Iayen by a factor of 2: 1.

Locally beds can be traced into slurnp fol& ancilor mosaic to rubble floatbreccia (Morrow,

1982) with clasts of lime mud derived fiom the ribbon limestone (Fig. 2.9B). Tc+ and Tde calciturbidites (Bouma, 1962) occur as rare interbeds, and dolosiltite with HCS fabric is

present at one locality. This facies is also associated with columnar stromatolite bioherms,

and can be traced laterally into the flanks of these carbonate buildups.

Alternation of limy and argillaceous beds as in ribbon or parted limestone arises from

the combination of the primary sedimentation signal and late cementation of carbonate

layen (Hallam, 1986; Coniglio and James, 1990). Plate-like ribbon limestone clasts in upper

Victor Bay debrites attest to early cemmtation of lime mud layers in an environment prone

to resedimentation, as do the turbidites. Influence of stonn waves is not evident. This ribbon

limestone is likely the deeper, more completely cemented equivalent of the nodular

limestone facies (Bathurst, 1975; Mullins et al., 1980). Similar rock types in modem and

ancient deposits are interpreted as hemipelagic periplatform deposits, common in dope Chapter 2-Muddy carbonate rmp enviromnents below storm wave base and which represent fallout deposits of distal slope origin (Cook and Mullins, 1983; Coniglio and Dix, 1992).

Carbonaceous Rhyfhmite Facies.-

A "pinstriped" appearance is imparted to bis lithofacies by alternating millimetre- scale layers of limestone/dolostone and carbonaceous shale. Weathering colours are dark brown and bue fiesh surfaces are black and grey and have a strong bituminous odor. Brinle and sofi-sediment deformation is apparent in the fom of abundant slump features and pinch- and-swell structures (Fig. 2.9C). Carbonaceous rhythmite units are generally less than 1 m thick and usually intercalated with nibon and nodular limestone. These rhythmites occur almost excIusively in the southem part of the study area.

Synsedimentary deformation and fracturing of the laminae in the carbonaceous rhythmite indicate the presence of a dope, whereas the absence of wave- or current-formed sedimentary structures suggests deposition in a zone of deep water. The millimetre-scale laminae record periodic alternation between organic-matter-dominated and carbonate-mud- dominated sedimentation, and likely represent higher-fiequency events (Wetzel, 199 1) than the centimetre-scale nbbon limestone couplets. It is inferred that, as a whole, the carbonaceous rhythmite facies represents deposition during a time of hi& organic productivity. Periodic, perhaps seasonal, decreases in productivity are defined by laminae of

relatively organic-poor carbonate mud derived fiom suspension settling of stonn-borne

platformal mud (Bathurst, 1975; Mullins et al., 1980; Moller and Kvingan, 1988).

Dolomitic Shole Facies.-

This greenish-grey to buff-grey weathering dolomitic shale crops out as units less than

a few metres thick and is a minor constituent by volume of the upper member. It is fissile to

platy and recessively weathering, with a weak lamination visible on black to dark grey fkesh

surfaces. It is interIayered with nodular and nibon carbonate, and at some localities with thin

debntes containing millimetre-thick nibon limestone and shale clasts. Chupter 2-Mud@ carbonate ramp

The environment of deposition was quiet, and either received or accwnulated very

Iittle carbonate sediment. Unlike the carbonaceous rhythmites, no regular millimetre-scale laminae are discernible and the relative proportion of organic material is much lower. A basinal to distal slope environment is suggested by the lack of shallow-water sedimentary structures, and by its association with other deep-water facies (Potter et al., 1980). This interpretation is supported by the high degree of similarity bebveen this facies and the deep- water shales of the lower Victor Bay member (Jackson and Iannelli, 1981; Iannelli, 1992).

Stromatolitic Facies.-

Apart from smooth laminae in the dololaminite facies, microbial structures are relatively rare in the upper member. They occur in two principal guises: (1) as columnar forms in thin, metre-scale biostromes and (2) as columns and large domal stromatolites in

large decarnette- to hectometre-scale reefs.

Mete-Seale Biostrome Facies.-These limestone biostromes are 0.5 to 1.5 m thick

and composed of columnar stromatolites with gently to steeply convex laminae (Fig. 2.9D).

Columns, typically 5 to 15 cm tall, are nucleated on shallow-water deposits of dololaminite

intraclast conglomerate, sandy polyrnictic conglomerate facies, or molar-tooth calcarenite

facies. Carbonate mudstone inüaclasts, quartzose sand, and molar-tooth microspar clasts are

present as corne intercolumnar sediment. Rarely, stromatolite biostromes cm be traced

laterally into coarsc polymictic conglomerate composed of cobbles of columnar stromatolites

and other shallow-water lithoclasts. The most extensive and thickest biostrome is a

distinctive three-tiered marker unit up to 3 m thick (Fig. 2.9E). It is composed of inclined to

vertical non branching columnar stromatolites 3 to 5 cm wide and 5 to 20 cm long. In the

basal tier, the stromatolites define domes 1 to 1.5 m wide with Surface relief of 0.3 to 0.5 m.

The stromatolite columns become narrower and more uniformly vertical in the upper tiers,

although they collectively preserve the domal shape. In plan view, stromatolite columns at

the top of the biostrorne are elongated roughly perpendicular to the infemd paleostrike of

the ramp. Elongation is most pronounced on the flanks of domes and less distinct on the Chapter 2-Muddy carbonate ramp crests. Coarse intercolumnar sediment consists of stromaclasts (sensu Sami and James,

1994). molar-tooth microspar clasts, and fine quartzose sand. Unlarninated green dolomitic shale is present as thin drapes on some biostrome tops where it is locally interlayered with nodular limestone andor dolomitic shale facies.

Modem stromatolites accrete in the photic zone, and this is also assumed to be the case in most ancient successions. Coarse intercolumnar sediment in metre-scale biostromes and elongation of columns suggest presence of a persistent current (Hofian, 1976;

Horodyski, 1976). A shallow subtidal environment is therefore interpreted for the stromatolites. The abrupt transition into the overlying deep-water facies indicates that deepening to below storm wave base had occurred by the time shate and nodular limestone facies were deposited.

Stromotolite Reefs.Atrornatolite buildups up to 275 m in thickness (Geldsetzer,

1973; Jackson and Iannelli, 1989; Narbonne and James, 1996) occur over several tens of kilometres of outcrop length. In the study area, intervals of reef growth are associated with two deepening events, and defme the tops of the two stratigraphic packages (Fig. 2.3A). In

the northwest of the study area (Section 2), the lower buildup is 170 m thick (Fig. 2.9F) and

the upper buildup is 60 m thick. The composition of the reefs varies £tom dolostone to

limestone, and two principal fabrics are recognized: domes 1-10 m in diameter with smooth

to crinkled stratiform laminae, and columnar foms 3-10 cm in width with hemispherical to

conical Iaminae. Colurnnar stromatolites on the uppermost surface of the Iower reef have a

strongly eIongate shape in transverse section, suggesting that they were constructed under the

influence of currents that flowed roughly perpendicular to the inferred ramp paleostrike. In

the study area, these reefs occur in a narrow, 1-2 km strip where they separate shallow-water

strata to the NE nom deep-water strata to the SW. This ree&tract zone contains evidence

of dope instability: an ~Iistostromeseveral tens of metres thick extends for at least a

kilometre at the base of the reefs and contains blocks several tens of cubic metres in volume.

The reef appears to have nucleated on the blocks. and the stromatolites at the base of the Chapter 2-Mid& carbonate ramp reef can be followed laterally into nibon limestone facies. in the east of the study area

(Section 7; Fig. 2.18). the dolomitized upper buildup is flanked by ovemimed, decarnetre- scale reef blocks, suggesting that the reef had developed substantial vertical relief before it was mantled by deep-water strata of the overlying Athole Point Formation.

It appears that the stromatolite reefs developed at least initially in deep water, coevally with, and Iocally on blocks of, deep subtidal ribbon limestone facies. Current- elongation of stromatolites at the top of the reef at Section 2, however, suggests that it must have eventually built up into shallow water (Hohan, 1976; Horodyski, 1976).

DEPOSITIONAL ATTFüBUTES OF TEFE VICTOR BAY FORMATION

Regional Stratigraphy and Correlations within the Upper Member

Correlation of Victor Bay strata in the study area indicates thinning of the carbonate upper member to the southwest together with slight thickening of the shaly lower member

(Fig. 2.3). Upper member lithofacies are packaged into shallowing-upwards cycles 20 to 50

m thick that are classified on the basis of the capping lithofacies (idealized cycles of Fig. 2.3):

(1) peritidal cycles, capped by either dololaminite or red shale, and containing molar-tooth calcarenite and molar-tooth mudstone; (2) shallow subtidal cycles topped by molar-tooth

calcarenite and dominated by molar-tooth mudstone; and (3) deep-water cycles dominated by

nodular and ribbon limestone facies. The upper member is divided into two main packages

that can be correlated across the axis of the Milne Inlet Trough (Fig. 2.3). The base of the

lower package contains peritidal cycles in the north that gradually deepen southward into

subtidal cycles (Fig. 2.5) and eventually deep-water cycles in the south. In the upper half of

the package, the 170-m-thick stromatolite bioherm occupies the transition between peritidal

and subtidal cycles (see also Fig. 3.9A). This buildup is capped by current-elongated

stromatolites that pass laterally into a sandy polymictic conglomerate above the pentidal

cycIes to the north. Deep-water dotornitic shale overlies the reef and the conglomerate,

defining a floodiig event at the base of the second package. This upper package consists Chapter 2-Mud& carbonate ramp everywhere of peritidal cycles, and is capped by a 60-m-thick stromatolite reef and its laterdly equivalent deep-water strata.

The lower package is analogous to sequences identified in the upper Victor Bay Formation by Narbonne and James (1996) in the Strathcona River reef complex, located - 60 km NW of the study area. There, a mid-rarnp, subreef sequence is overlain by a wedge- shaped stromatolite biostrome (first reefal sequence). This biostrome is capped by a karst surface and overlain by a second reefal sequence. The morphology of the wedge-shaped reef and the abundance of pinstones in the subreef sequence are sirnilar to the lower package of

Section 2 (Fig. 2.3). The first reefal sequence at Strathcona River, however, is coeval with deep-water lithofacies and ends at a karst surface. The lack of shallow-water lithofacies in off-reef strata, together with kanting of the Strathcona reefs, point to a tectonic mechanism for rapid exposure and flooding events (Narbonne and James, 1996). The regional differences in the character of sequences across the Borden Peninsula suggest that local block tectonics might have been important (Jackson and Iannelli, 198 1; IameIli, 1992).

Carbonate Plaîjorm Architecture

The distribution and geometry of upper Victor Bay lithofacies indicate deposition on a ramp rather than a shelf, because there is no sharp declivity or shelf break, and shalIow- water facies pass gradually offshore into outer-dope and basinal strata (Ahr, 1973; Burchette and Wright, 1992). Gradua1 thiming of shallow-water strata can be walked out and traced visually in outcrop. This exceptional tmsect from NE to SW, through the transition fiom

imer ramp to mid-ramp, clearly shows wedges of peritidal doIolaminite facies passing

basinward into laterally equivalent subtidal molar-tooth calcarenite facies (Fig. 2.5).

Muddy, tempestite-bearing ramps are considered to be Iow-wave-energy, microtidal,

storm-dominated settings (Burchette and Wright, 1992). Because the influence of tides and

waves is negligible, waters on these ramps are caim, except during episodic storrn events that

remobilize fine material and fiagrnent lithified units into intraciasts. As a whole, the upper

Victor Bay carbonate ramp consists by volume of 60 to 90% lime mudstone facies. Virtually Chapter 2-Muddy carbonate rmp mud-fiee molar-tooth calcarenite facies, however, attest to efficient wimowing in the inner ramp, perhaps by breaking waves in the littoral zone. AI1 Victor Bay peritidal cycles contain at least a few metres of molar-tooth calcarenite facies, which inevitably separate subtidal molar-tooth mudstone tempestites fiom the peritidal dololaminites. The slight topographic relief afforded by sheets of brecciated carbonate might have protected an inner-ramp tidal flat (e.g., Fairchild et al., 1997). In the study area, however, such a thin bank or accumulation of molar-tooth calcarenite facies was insufficient to protect the intertidal and supratidal zones against stom waves (Fig. 2.10).

The relationship between reefs and platformal strata in the study area gives insight into the eflect of the reefs on inshore sedimentation. Storm bedding is no less abundant in shallow-water strata that are coeval with reef growth than it is in those strata deposited at a tirne when no reefs were present. This suggests that these bioherms did not form a banier complex and that they were ineffective in damping waves or storms to the shallow inshore waters. These buildups are nucleated on slumps and deep-water strata in a context similar to that of the reefs in the Strathcona River area (Narbonne and James, 1996).

Ramps are traditionally subdivided into inner, middle, and outer ramp, where the mid- ramp is bracketed by fair-weather wave base inshore and stom wave base offshore (Burchette and Wright, 1992). On the Victor Bay ramp, interpreted imer-ramp facies include molar- tooth calcarenite, doloiaminite, and red shale facies. Molar-tooth mudstone distal

tempestites occupy the mid-camp, whereas nodular limestone, t-ibbon and parted limestone,

and carbonaceous rhythmite occur on the sloping outer ramp. Deposition of dolomitic shale

is interpreted to have occurred at the transition from distal outer ramp to basin. Geographic

distribution of these facies suggests a minimum width for the imer ramp of 10 km, a mid-

ramp of approximately 5 km, and an outer ramp more than 20 km wide (Fig. 2.10).

Clasts in thick debrites of the upper Victor Bay geaeraily belong to outer-ramp facies,

suggesting distal steepening in the outer ramp (Read, 1982, 1985). The inflection might

have been (1) created by rapid aggradation of the upper Victor Bay ramp, (2) partly inherited Chapter 2-Mud. carbomte ramp

LEGEND

molar-tooth coated grains r]lime mud microspar

microbialite nodular and ribbon . quarkose sand mudstone clasts dololaminite intraclasts

Fig. 2. IO - Depositional environments interpreted for the Victor Boy ramp. Pie diograms illusrrae the distribution of sediment components on the inner, middle, and outer ramp. Abundance of lime mud remains roughfy the same in the inner and mid-romp. but increases below stonn wave base in the outer ramp. Lime mud and mud-generated intraclasts together form no less than 75% and as much as 100% of sediment components across the ramp. An exception is the locally important accumulation of microbial ites in large deep-water stromatol ite buildups. A bundances of sedintentaty particles, represented in the pie diagrams, are based on a visual estimate of the composition of each lithofacies; this estirnate was used to derive an average abundunce, which was weighted on (1) the total lithofacies thickness in the srudy area, and (2) ifs volumetric importance in each of the three rarnp zones. FWWB =fair-weuther wuve base, SWB = storm wave base. Chapter 2-Muddy carbonate rump fiom the underlying Society Cliffs Formation, or (3) caused by synsedimentary block faulting at the transition fiom mid-ramp to outer ramp. Previous workers (Jackson and Iannelli,

198 1; Jackson and Sangster, 1987; Ianelli, 1992) mapped a zone of southwest-dipping normal faults which coincides with the position of the reef tract and which might have been the locus of synsedimentary faulting. Alternatively, instability of the outer ramp could have been related to rapid accumulation of lime mud exported to deep water by storm currents, especially during times of high carbonate productivity (Droxler and Schlager, 1985; Milliman et al., 1993).

Controls on the Distribution of Carbonate Sediment Purticles

The proportion and type of sedimentary components in the inner-ramp, mid-ramp, and outer-ramp environments of the Victor Bay Formation reflect hydrodynamic conditions

(Fig. 2.10). In tidal flats and calcarenite banks, the imer ramp contains an overall volume of mud (- 50%) similar to mid-ramp molar-tooth tempestites, but the types of grains are quite different. Molar-tooth microspar intraclasts are the only significant component in the mid- ramp, whereas dololaminite intraclasts, temgenous sand, and, to a Iesser degree, molar-tooth spar intraclasts and ooids are a11 significant in the coarse fraction of inner-ramp lithofacies. The molar-tooth calcarenite facies, as a subset of the imer-camp environment, contains

little mud, usually in the form of thin drapes, and is interpreted to represent high-energy

shoreface and shallow subtidal deposition above fair-weather wave base. Below fair-weather

wave base, the proportion of gravel- and sand-size grains decreases rapidly. With the

exception of the stromatolite bioherm facies, lime mud and debrites of early lithified Iime mudstone make up almost al1 outer-ramp facies.

Lime Mud-

Distribution.-Lime mud is the principal carbonate sediment in al1 ramp lithofacies

except molar-tooth calcarenite. The high interticid-low supratidal zone of the imer rarnp

(doloiaminite facies), and the shallow to deep subtidal mid- to outer ramp are at least 50% Chapter 2-Mu* carbonate rantp lime rnud On the inner-ramp tidal flats, the fine sediment settled out after storm events. In the mid-ramp, degassing, dewatering, and stiffening of muds (Furniss et al., 1998) might have prevented to some degree the erosion of fine-grained carbonate by waves or storms. Storm suspension and offshore transport into deep water probably contnbuted mud to the outer ramp.

Origine-Production of lime rnud in the Proterozoic most likely arises nom direct precipitation of CaC03 in the water column (Grotzinger, 1989, 1990; Knoll and Swett, 1990) in a manner analogous to modem-day marine whitings (Shim et al., 1989; Robbins and

Blackwelder, 1992). Metabolic and degradational processes involving microbes and organic matter in the water column cm promote micrite precipitation (Robbins and Blackwelder,

1992), and this rnechanism has been postulated to generate vast amounts of lime rnud in today's oceans. On Great Bahama Bank, the volume of rnud suspended in modem whitings can account for almost three times the bank-top accumulation (Robbins et al., 1997).

Furthemore, as whitings occur today in seas that are believed to be less sahirated in CaCOj than Proterozoic oceans, a similar source of micrite would have been of great importance in

Precambrian carbonate systems (Grotzinger, 1989; Knoll and Swett, 1990; Grotzinger and

Kasting, 1993). Once formed, this detrital micrite was deposited in iow-energy pentidal areas or was swept offshore by storms, accurnulating in slope and basinal environrnents (e.g., Sami and James, 1996). It is conceivabie that at a time of relatively high carbonate saturation in marine waters (Grotzinger, 1989; Hemngton and Fairchild, 1989) this mechanisrn could have supplied most of the lime rnud to al1 upper Victor Bay rarnp facies, from inner ramp to distal outer ramp. The inner-ramp and mid-ramp waters adjacent to an aria evaporitic margin at

low latitude must have been a zone of high precipitation of lime mud, in view of the warm

water temperatures and the added effect of prolific organic productivity associated with those

wam waters. In contras& presumably colder offshore water above the outer ramp might not

have fostered as high a rate of precipitation. The rhythmic packaging of carbonate rnud in

those deeper-water lithofacies (e.g., nodular and ribbon limestone) might therefore reflect Chapfer2-Muddy carbunute ramp episodic stom influx of hemipelagic mud to the outer ramp, rather than fluctuations in the rate of carbonate mud production on the ramp. ut shallow subtidal environments, lime mud was deposited in the zone of carbonate production as caps on molar-tooth tempestites, perhaps locally bound by microbial films. Mudstone intraclasts are not found in tempestite lags, suggesting that in the molar-tooth mudstone facies early lithification of microspar crack-fil1 and did not extend to the muddy matrix.

Grains.- Distribution.-The grainiest deposits in the Victor Bay ramp occur in inner rarnp facies that were most affected by fair-weather and storm wave conditions. Grain types are diverse, including dololaminite intraclasts, molar-tooth microspar, quartzose sand, and rare ooids. The diveaity and abundance of coarse particles tapen off towards the mid-ramp.

There, grains are almost exclusively molar-tooth microspar and are restricted to the bases of molar-tooth tempestites. Coane slope-derived conglomerates of remobilized, early lithified nodular, ribbon and parted limestone facies occw well beiow storm wave base on the outer

ramp.

0rigin.-The shallow-water agitated zone acted as a repository for clasts produced

and brought shoreward by storms, but was not a zone of production of grains such as ooids.

Al1 three types of coarse carbonate material-molar-tooth grains, dololaminite intraclasts

and slope conglomerate hgments-are derived kom reworked muddy lithofacies. Early

lithified dololaminite tidal flat facies provided the material for storm-generated intraclasts

(c.f. Shinn, 1983). Whereas these clasts were formed by stonn erosion of existing bedding-

parallel cnists, rnolar-tooth grains were released kom a casing of unlithified mud during storm

events. This is analogous to wimowed coane skeletai grains from a packstone or muddy

mdstone matrix (James et al., 1998). According to Furniss et al. (1998), molar-tooth-like

cracks are produced in modem sediments, and early microspar precipitation in the

synsedimentary cracks is the key to their preservation in late Proterozoic strata (James et

al., 1998). Thus, were it not for microspar precipitation with such shailow subtidal molar- Chapter 2-Me carbonate ramp tooth mudstone facies, carbonate grains on the Victor Bay ramp would be limited tu mostly dololaminite intraclasts and deepwater debrite clasts, with some ooids.

Ooids are rare on the Victor Bay ramp, despite an ample supply of suitable nuclei such as molar-tooth calcarenite grains. Interestingly, they are Iocally abundant in calcarenite uni& underlying equally rare stromatolitic intervals, or in units of polymictic conglomerate that also contain stromatolite clasts. The association of ooids and stromatolites in shallow-water

facies of the Victor Bay ramp indicates that they require a cornmon set of environmental

conditions to develop. Experimental work points to the significant influence of microbial

films on the formation and continued growth of ooids (Davies et al., 1978; Ferguson et al.,

1978). Perhaps episodic decreases in micnte production in the shallow subtidal environment

of the Victor Bay ramp fostered the development of rnicrobial films and led to an increase in

benthic precipitates in the form of ooids and stromatolites.

Microbialites and Cements.- Distribution.-Microbial structures on the Victor Bay ramp are neither abundant nor

consistently present in shallow-water cycles. Crinkly stratifon microbialites of the

dololaminite facies are an important exception, but they are usually overlain by thick beds of

inhliclast- and sand-rich tempestites. Columnar and domal stromatolites account for less

than 1% of shallow-water facies, and where present, are associated with rapid creation of

accommodation space and with a hard or grainy substrate suitable for nucleation. Srrial1

columnar stromatolites nucleated on shallow-water grainstones or mdstones at the tops of a

few cycles, and grew as space was made available by rapid nses in sea level. The extensive

mid-ramp bioherms are also an anomalous facies, because they are the by-product of slope

failure at the mid- to outer-nunp slope break. The deep amphitheaters of slump scarç

provided growth space, and lithified slump blocks were suitable for nucleation of

stromatolites. The only other thick bioherms in the upper member developed as water depth

increased rapidly at the contact with the deep-water shale of the Athole Point Formation. In Chapter 2-Mude carbonare ramp either case, whether by slope failure or by relative sea-level rise, stromatolite bioherms grew . to signifiant thicknesses only when accommodation space was rapidly increasing.

0rigin.-Strornatolites, lithified supratidal mats, and inorganically precipitated cmts and fans are classic components of early Paleoproterozoic platformal successions. The decline of stromatolite abundance throughout the Proterozoic (Awamik, 1971; Walter and

Heys, 1985; Grotzinger, 1990) was well underway by Mesoproterozoic time, as the conditions favorable to microbial benthic precipitation becarne less widespread. In the Victor Bay

Formation these conditions were rare in shallow environments but did occur in deeper water where they fostered the development of impressive stromatolitic reefs.

Growth of Victor Bay stromatolites always coincided with a rapid increase in depth, and accumulated on hard or grainy substrates. For example, the three-tiered marker biostrome (Fig. 2.9E) is nucleated on planar-bedded shallow-water conglomerates and achieved synoptic relief of a few decimetres. It is directly overlain by subtidal facies, either calcareous shale and nodular limestone, or by argillaceous molar-tooth mudstone. Similarly, the largest strornatolitic reefs accumulated on lithified slump blocks of deep-water ribbon limestone and were mantled by doIomitic shale or more ribbon limestone. In this instance, . there may also have been a link between tectonic activity and reef growth, whereby the slumping and creation of new substrate was a direct cause of seismicity. Accommodation space would have been further magnified if the seismic event was related to tectonic subsidence. Judging fiom the development of thick reefs in the upper member, once stromatolites gained a toehold, they had the potential to maintain their growth and coexist with rnuddy mid- to inner-ramp facies, as demonstrated by the interfhgering of mid-ramp reefs with moIar-tooth mudstone, molar-tooth grainstone, and dololaminite facies. The reef complex might have, by virtue of its sheer size, provided patches of fim substrate for Mergrowth, even at times of abundant mud production. Chapter 2-Mu@ carbonate rmp

In shallower water, the generally high rate of water-colurnn rnud production might have limited benthic microbial cementation either by blocking the penetration of light ont0 the seafloor or by maintainhg a soft, inhospitable substrate, thus preventing the nucleation of columnar stromatolites. In such a scenario, the benthic microbialites would therefore tend to occur when water-colurnn rnud production decreased across the entire ramp during transgressive episodes. Influx of carbonate-starved bottom waters (Berger et al., 198 1) with higher CO2 content might have increased carbonate solubility, thus decreasing turbidity in the water column. This could in tum have promoted sunlight penetration to the seafloor and

fostered blooms of benthic photosynthetic microbes. Altematively, a differing chemistq of the transgressive waters could have initiated a shift fiom water-colurnn to benthic precipitation. For example, the presence of inhibitors such as ~e'+could have impeded

water-column precipitation of calcite and favoured the growth of benthic carbonate, as

Sumner and Grotzinger (1996) have proposed for the Archean and early Proterozoic.

A Mid-Dominated Carbonate Fact0t-y.-

Primary and secondary carbonate sedimentation on the Victor Bay ramp was

controlled by the production and redistribution of lime rnud on the ramp. Most lithofacies

were mud-dominated, and by vimie of its stiffening and early lithification in the i~erramp

and mid-ramp, lime rnud was the source of essentially al1 carbonate grains, including outer-

ramp resedimented limestones. The only carbonate lithofacies not dominated by either rnud

or grains are stromatolitic, and these are volumetrically minor. Abundance of clastic

carbonate rnud and development of shallow-water strornatolit~oidassociations appear to be

mutually exclusive. This suggests that if carbonate rnud production had been much less

important on the Victor Bay ramp, not only would molar-tooth mudstones and molar-tooth

calcarenite facies have been absent, but subtidal stromatolites and ooids would have been

ubiquitous. Chapter 2-Muddy carbonate mp

DISCUSSION

Cornparison wilh Ancient and Modern Carbonate PIarfrms

Archean and Pa1eoproterozoic.-

The record of Archean carbonate deposition is relatively sparse, but evidence from preserved remnants suggests that extensive plaLloms had evolved by late Archean time

(Grotzinger, 1989). Paleoproterozoic carbonate platforms deveioped a range of profiles, from ramps to nmmed shelves (Grotzinger, 1989). What characterizes both Archean and

Paleoproterozoic carbonate platfoms of al1 types is an overail abundance of in situ cement textures and stromatolites. Microbially mediated and inoqanically precipitated cements are ubiquitous, occur in various foms (Grotzinger and Read, 1983; Hohann and Jackson, 1987;

Sami and James, 1996), and span the entire facies spectrum across both ramps and shelves.

Carbonate mud derived fiom the water column was a significant sedimentary component only below storm wave base or in peritidal complexes in the Paleoproterozoic (e.g., Pethei

Platform; Sarni and lames, 1994, 1996), and was deposited contemporaneously with microbialites. Carbonate mud is the principal sedimentary component across the Victor Bay ramp (Fig. 2.10), but its occurrence is antithetical to that of stromatolitic facies. Extrinsic factors such as the postulated secular decrease in seawater carbonate concentration during the

Proterozoic (Grotzinger, 1990; Knoll and Swett, 1990) might have Merimpeded the development of microbialites on the Victor Bay ramp in combination with local factors such as substrate, accommodation space, turbidity, and water temperature.

Mesaproteroroic und Neoproterozoic.-

The apparent global decline in stromatolites in the Mesoproterozoic and

Neoproterozoic (Awramik, 197 1; Grotzinger, 1990) has been explained as resulting from lower concentrations of carbonate in sea water. The relegation of benthic carbonate precipitates to restricted intertidal to supratidal envirooments was well under way by late

Mesoproterozoic time (Kah and Knoll, 1996). Low nunp profile, abundance of lime mud, Chapter 2-Muddj carbonate ramp and presence of molar-tooth mudstone facies are features shared by other mid- and late

Proterozoic carbonate platforms. Neoproterozoic carbonate platforms with abundant molar- tooth limestone and ramp tempestites, such as the Shaler Group (Young, 1974, 1981),

Akademikerbreen Group (Knoll and Swett, 1990); Canyon Formation (Fairchild, 1989;

Fairchild and Herrington, 1989; Hemngton and Fairchild, 1989), Xinmincun Formation of

China (Fairchild et al., 1997), and Wonoka Formation of Australia (Haines, 1988; James et al., 1998) al1 attest to the overall shift fiom cement-nch rimmed shelves to muddy storm- dominated ramps, a trend that began in the latest Paleoproterozoic and early

Mesoproterozoic. Stromatolite-poor cyclic successions in the Wallace (Grotzinger, 1986b) and correlative Helena-Siyeh formations of the Belt-Purcell Supergroup (O'Connor, 1972;

Winston and Lyons, 1993) indicate that by 1.4-1.3 Ga the conditions on muddy ramps rich

in molar-tooth lithofacies were not conducive to extensive stromatolite accumulation in

shallow subtidal waters.

Society CII~SFormation.-

A great contrast in facies exists even within the Uluksan Group, between the shallow-

water Society Cliffs Formation and the upper Victor Bay member. Both ramp types

encompass roughly similar marine environrnents and are interpreted to have developed in

arid climates (Jackson and Iannelli, 1981; Kah, 1997), but the proportion of sedimentary

components is quite different (Kah, 1997; Sherman et al., 2000).

Society Cliffs facies associations range from marine shoreline to mid-ramp

environrnents (Kah, 1997; Kah, 2000). Microbialites and isopachous cements dominate the

imer ramp, which is separated fiom laminated and nodular dolostones of the mid-ramp by

ooid and intraclast shoals. Imer-ramp facies interfinger shoreward with gypsum-bearing

redbeds. In the study area, sirnilm redbeds in the Victor Bay Formation (the red shale facies)

interfinger with dololaminite facies and are not traceable into isopachous cernent facies

typical of the Society Cliffs inner ramp. Stromatolite-bearing ooid shoals in the Society

Cliffs Formation were deposited under higher energy than the molar-tooth calcarenite banks Chapter 2-Muddy carbonate ranp or sand sheets of the Victor Bay ramp (Kah, 1997). The cornmon presence of small columnar sîromatolite bioherms in the Society Cliffs mid-ramp and inner ramp contrasts with their rarity in the same zones on the Victor Bay ramp. Conversely, the large deep-water reefs of the Victor Bay Formation have no equivalent in the Society Cliffs rarnp.

In both Society Cliffs and upper Victor Bay Formations, stromatolites are associated with ooids, and formed at the bases of cycles at a time when the rate of increase in accommodation space was high. The nlatively high abundance of stromatolites in the

Society Cliffs Formation, however, suggests that conditions wen favorable for the developrnent and preservation of subtidal benthic precipitates (Kah and Knoll, 1996).

Tellingly, Society Cliffs mid-ramp facies do not exhibit significant storm-reworking, perhaps because of a higher degree of seafloor cementation than on the Victor Bay rarnp. In this sense, the Society Cliffs ramp is Paleoproterozoic in style, as fiirther evidenced by the early-

Proterozoic-style taphonomic conditions (Kah and Knoll, 1996) that dominated the hi@- intertidal to supratidal environments in the Society Cliffs ramp. That these two rarnps were deposited in succession and under similar paleogeographic conditions and yet differ so strongly in carbonate constituents, suggests that the late Mesoproterozoic was a time of carbonate platfonn diveaity, where the spectrum had broadened to include both

Paleoproterozoic-style and Phanerozoic-style carbonate depositional systems.

Phanerozoic. - The absence of molar-tooth facies in the Phanerozoic is attributed to the lack of early microspar precipitation in voids created by dewatering and degassing of mu&, and to the destruction of cracks by burrowing macrobiota (Fumiss et al., 1998). Nonetheless, sediment dynarnics in the upper member of the Victor Bay are more similar to those of muddy

Phanerozoic mps than they are to those of most Paleoproterozoic carbonate plaûorms.

Despite the absence of shelly macrofossils in the late froterozoic, molar-tooth crack-fi11 grains cm be considered analogous to skeletal allochems in that they form the coarse fraction of molar-tooth tempestites (James et al., 1998). Low-energy, stom-affected, and muddy, Chapter 2-Mudi& carbonate ramp the Victor Bay Formation shares features with carbonate ramps that accreted when development of skeletal reefs was at an ebb in the Phanerozoic (see review in Burchette and

Wright, 1992).

SYNTHESIS The distribution of Victor Bay carbonate facies and their principal components hinges on the abundance of lime mud in the water column and its early diagenesis on the sedoor. Turbidity, substrate availability and consistency, and formation of carbonate grains are al1 govemed by the presence or absence of micrite in the water column. The microtidal nature of the Victor Bay ramp and the strong influence of storms on sedimentation in the middle to inner ramp ultimately dictated the distribution of mud and therefore the production of pins. The transfer of most carbonate production fiom the seafloor to the water column in the Mesoproterozoic and Neoproterozoic drastically reduced the ability of microbial benthos to contribute to early cementation of the substnite. The surplus CaC03 eventually precipitated as the distinctive molar-tooth crack fil1 (James et ai., 1998; Frank and Lyons,

1998).

The influence of water-column carbonate precipitation is understated in Phanerozoic carbonate systems, where it is overshadowed by metaphyte and metazoan carbonate production. In today's oceans, a significant volume of micrite forms in the water column

(Robbins et al., 1997), but this contribution to the overall carbonate sediment budget pales in comparison to that of the Iate Precambrîan water-colurnn factory. Better understanding of the mechanisms that drive carbonate precipitation in the modem oceans will yield greater

insight into the relationship between water-column micrite and the decline of microbialites in

the Mesoproterozoic and Neoproterozoic. Chapter 2-Mudày carbonate ramp

SUMMARY

1) Abundance of mudstone-based lithofacies, and the distribution of tempestites and resedimented deepwater facies in the Victor Bay Formation, suggest that deposition occurred on a low-energy, microtidal, storrn-dominated, distally steepened rarnp.

2) Lime mud constitutes 50% to 90% by volume of upper Victor Bay lithofacies and is by far the primary sediment component from inner to outer ramp. It accumulated on the imer ramp as peritidal dololaminite facies, on the mid-ramp as molar-tooth mudstone facies, and on the outer ramp as nodular, ribbon, and parted limestone facies.

3) Storm-reworked muddy lithofacies were the principal source of grains in the imer ramp and mid-ramp, such as dololaminite intraclasts and molar-tooth microspar grains. Early cementation of mud in nodular and ribbon lirnestones provided clasts that were incorporated into slope debrites on the outer ramp.

4) The correlation of stromatolites to periods of decreased carbonate production and rapid creation of accommodation space points to an antithetical relationship between deposition of benthic microbial carbonate and precipitation of lime mud in the water column.

Increased turbidity related to mud suspended in the water column, and the predominance of soft, muddy substrates, might have been inimical to nucleation and development of stromatolites.

5) The distribution of lithofacies in the Victor Bay ramp is recognized in rocks of similar age, which are also interpreted as low-energy, storm-dominated ramps with strong similarities to Phanerozoic ramps. Whereas the Victor Bay Formation lacks abundant ooids

and shallow-water stromatolites, the underlying, lithologically contrasting Society Cliffs

Formation is similar to older cernent-, stromatolite-, and ooid-rich carbonate platforas of

the Paleoproterozoic. This provides an example of the broadening spectrum of carbonate

platform types in the late Mesoproterozoic. Chapter 2-Mi carbonate ramp

ACKNOWLEDGEMENTS

We tbenk P.N. Southgate, J.P. Grotzinger, E.C. Turner, RB. MacNaughton, and an anonymous reviewer for constructive comments that greatly improved this manuscript.

Excellent field assistance was provided by LEM. Harrington in 1994 and 1995. Financial support was provided through research grants to N.P. James and G.M. Narbonne from the Naniral Sciences and Engineering Research Council of Canada, a Geological Society of

America Research Grant and a Queen's Doctoral Travel Grant to A.G. Sherman, as well as

Northern Studies Training Program grants to A.G. Sherman and I.E.M.Harrington.

Logistical support fiom Breakwater Resources Ltd.-Nanisivik Mines and the Polar

Continental Shelf Project was essential to the success of this research (PCSP publication no.

00299). ANATOMY OF A CYCLICALLY PACKAGED MESOPROTEROZOIC CARBONATE

RAMP IN NORTHERN CANADA*

ABSTRACT

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 2û-50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of ten. Stratigraphic and sedimentological evidence suggests high-amplitude, high-fiequency glacio-eustatic cyclicity, and thus a cool global climate Ca. I .2 Ga.

The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proponions of lime mud, carbonate grains, and microbialites are more typical of younger

Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distaIly steepened rarnp, Outer-ramp facies are hemipelagic lime mudstone, shaIe, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-rarnp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.

Deepsubtidal, outer-ramp cycles occur in the southwestern part of the study area.

Black dolomitic shaIe at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, a11 of which exhibit signs of synsedimentary disniption. Cycles in the northeast are shallow-subtidal and peritidai in character. Shallow-subtidai cycles consist of basal deep-water

From: Sherman et ai. (2001) Sedimentaiy Geology, v. 139 p. 171-203. Chapter 34yclically-packaged carbonate ramp

facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except

that they contain a cap of doIomitic tidal-flat microbial laminite, and rarely of red shale

sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley

extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp

tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward

progradation of peritidal facies coincides with a zone of slope failure that may have

promoted the development of the stromatolitic reefs.

The sea-Ievel history of the Victor Bay Formation is represented by three

hectometre-scale sequences. An initial flooding event resulted in deposition of the lower

Victor Bay shale member. Upper member carbonate cycles were then deposited during

highstand. Mid-ramp slumping was followed by late-highstand reef development. The second

sequence began with development of an inner-camp lowstand unconforrnity and a thick mid-

ramp lowstand wedge. A second transgression promoted a more modest phase of reef

development at the mid-ramp and shallow-water deposition continued inboard. A third and

final transgressive episode eventually led to flooding of the backstepping ramp.

Overall consistent cycle thickness and absence of truncated cycles, as well as the high

rate and amount of creation of accommodation space, suggest that the penodicity and

amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than

tectonic mechanisrn of relative sea-levei change. High-amplitude, high-fiequency eustatic

sea-level change is characteristic of icehouse worlds in which short-tenn, large-scale sea-level

fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital

forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis

of a late Mesoproterozoic glacial period, as proposed by previous worken.

INTRODUCTlON

On wide pla$orms, shallowing-upward carbonate cycles have been correlated over

hundreds of kilometres (e.g., Groainger, 1986a). Correlation of individual cycles outboard of Chapter 3-CyclicaIiy-puckaged carbonate ramp the shelf edge becomes more difficult as drarnatic changes in gradient accur across a narrow zone (e.g. Playford, 1980; James and Mountjoy, 1983). In contras& the gentle gradient of carbonate ramps allows the correlation of shallow-water cycles with deeper-water slope and basin equivalents (Read, 1982, 1985; Burchette and Wright, 1992; Koenchner and Read,

1989; Marke110 and Read, 1981). Evaluation of vertical successions and Lateral relationships is well documented in Phanerozoic ramps (Aigner, 1984, 1985; Osleger and Read, 199 1;

Koerschner and Read, 1989; Fauher, 1988; Calvet and Tucker, 1988). Relatively few studies have documented the lateral relationships of Proterozoic cycles and their relationship to overall ramp architecture (e.g., Sami et al., 2000). The Mesoproterozoic carbonate rocks

of the Victor Bay Formation of Baffin Island's Borden Basin provide a superb opportunity to

examine the critical imer- to outer-ramp transition in almost Bat-lying, unmetarnorphosed

strata where the preservation of the original limestone facies is excellent. The foundation of

this interpretation is an 8.5-km-long continuous cross section perpendicular to depositional

strike, from inner- to mid-rarnp, through 400 m of cyclically packaged carbonates and minor

siliciclastic rocks. The extent of these strata throughout most of the Borden Basin allows a

regional perspective on ramp development at the onset of tectonism that terminated the

stable platform phase.

The purpose of this paper is to (1) describe the lateral changes in facies fiom i~er

camp to outer ramp, (2) interpret the depositional history of the Victor Bay ramp, (3) discusç

the effect of sea-level dynamics on the evolving ramp architecture, and (4) propose possible

mechanisms for the development of decametre-scale cycles and larger-scale sequences in the

context of the tectonosedimentary and climatic influences on the Borden Basin.

METBODS

Facies descriptions and regional correlation of Victor Bay strata are based on 22

sections at eight localities: one at Nanisivik (Fig. 3.1), and seven West of Tremblay Sound

(Fig. 32A). The area depicted in Figure 3.2B was studiied in detail, and had previously yielded

an interpretation of facies relationships (Sherman et al., 2000). Correlations are based on Chapter 3-CycIicaZZ'packaged carbonate ramp sections supplemented by senal photography of cliff-faces utilking low-level helicopter flights. Sequence-stratigraphie tenninology foilows Van Wagoner et al. (1988) and

Posamentier and James (1993).

REGIONAL SETTING AND STRATIGRAPHY

The Borden Basin

Late Mesoproterozoic crusta1 extension along the northern margin of Laurentia resulted in a system of aulacogens, oriented perpendicular to the new continental margin

(Olson, 1977; Jackson and Iannelli, 198 1). These Bylot basins (Fahrig et al., 198 1) contain

Mesoproterozoic platform carbonates together with siliciclastic rocks derived from sunounding basement highs. The most areally extensive of these depressions is the Borden

Basin, located on the Borden Peninsula of northern Baffin Island (Fig. 3. l), which contains

Fig. 3.1 - Geologîcal map of Borden Peninsula showing distribution of Victor Bay Formation outcrop and major depositional grabens (iroughs) in the Borden Basin. A@ Jackson and Iunnelli (1981) und Juchon and Sangster (1987). Chapter 3-CyclicaIJy-packaged carbonate ramp

Fig. 3.2 - A) Geological map of the stu& mea, showing location of Pingo Valley (PV. Section locations are mmbered: I, Adams River; 2, Pingo Valley South; 3, Pingo Valley North; 4, Malu River; 5, Camp Prozac; 6, Tremblay Sound; 7, Alfied Point. Cross sections A and B are represented in Figure 3.5. Boxed area is enlarged below. B) Detailed geologicaf mop of the trmect area. Romp carbonates examined in thk shcdy me exposed ahg the vallq walk of a NE-flowing tributory of the Malu River. Black lines indicate measured sections. Chupter 3ycIicaZij+packaged carbonate romp the Bylot Supergroup (Lemon and Blackadar, 1963). Although geographically distinct, the

By lot bains-including the Thule Basin of southeastem Ellesmere Island and no rthwestem

Greenland, the Aston-Hunting Basin of Somerset and Prince of Wales islands, and the Fury and Hecla Basin of Baffin Islanbhave been correlated on the basis of paleomagnetics

(Fahrig et al., 1981) and 6"~chemostratigraphy (Kah et al., 1999).

The Bylot Supergroup is the thickest of these Mesoproterozoic successions with more than 6 km of strata (Fig. 3.3). It was deposited on rified Archean and Proterozoic crystalline basement, and was mcated at an angular unconformity with overlying lower Paleozoic siliciclastic rocks. The basal Eqalulik Group represents rifting and associated volcanism, the middle Uluksan Group is a passive ma@ carbonate succession, and the Nunatsiaq Group sificiclastics accumulated during subsequent rift reactivation (Geldsetzer, 1973; Jackson and

Iannelli, 1981). Within the carbonate Uluksan Group, cyclically packaged dolostones and minor siliciclastic rocks of the Society Cliffs Formation are overlain by the predominantly limestone Victor Bay Format-ion. Erosion of crystalline basement horsts adjacent to fault zones at the southern margin of the basin produced conglomerates of the Fabricius Fiord

Formation coeval to development of the Society Cliffs carbonate farther north.

Age

A maximum late Mesoproterozoic age Ca. 1.27 Ga for the Bylot Supergroup is provided by paleomagnetic comlation of the Nauyat rift basalts at the base of the Bylot Supergroup (Fig. 3.3) with the 1267 * 2 Ma Mackenzie dyke swm(U-Pb baddeleyite age,

LeCheminant and Heaman, 1989). A minimum Neoproterozoic age of ca. 720 Ma for the basin has been obtained fiom Wbbaddeleyite geochronology of Franklin dykes that cross- cut al1 Precambrian rocks (Heaman et al., 1992; Pehrsson and Buchan, 1999). Isotope chemostratigmphy of the üluksan Group and lower Nunatsiaq Group suggests an age of ca.

1.2 Ga for the Victor Bay Formation, a view supported by PbPb dates of 1199 i 24 Ma and

1204 22 Ma (Kah in Samuelsson et al., 1999). Chapter 3-Cyclicall'packaged carbonate ramp

Regionaf stratigraphy of the Victor Bay Formation

nie Victor Bay Formation was deposited duriag the waning of the Uluksan stable platfonn phase (Jackson and Iannelli, 198 1; Iannelli, 1992). In the Milne Inlet Trough (Fig.

3.1), the largest of three principal grabens that constitute the preserved portion of the

Borden Basin, the 800-m-thick Society Cliffs Formation is overlain by 100 to 250 m of the lower member of the Victor Bay Formation. The shallow-water Society Cliffs stromatolitic dolostones are buried by 15-20 m of black pyritiferous shale that passes upward into dark grey dolomitic shale bearing thin carbonate mudstone interbeds. The lower member grades upward into carbonates of the upper member, which in the study area are predominantly limestone, lesser syndepositional dolostone, minor shale, and rare sandstone.

Fig. 3.3 - GeneralUed stratigraphy of the Borden Basin modified fiom Hofmann and Jackson (199 1) und Iannelli (1992). The Mesoproterozoic Bylot Supergrotip is divided into three groups representing emly rift sedimentation (Eqalulk)), stable plawrn development (UIuksan), and siliciclarric ni$-to-drfl phase (Nunatsiae). U-Pb dates: Nauyat basalts fiom LeCheminant and Heaman (1989), and Franklin &ka jPom Keuman et al. (1992) and Pehrsson and Buchan (1999). Pb-Pb dates for the Uluksan and lower Nunatsiaq groups fiom Kah (in Somuelsson et al., 1999). Chapter 3ycficafly-packagedcarbonate ramp

The absence of the lower Victor Bay shale rnernber and Athole Point Formation in

Eclipse and North Bylot Troughs (Jackson and Iannelli, 1981 ; Iannelli, 1992) suggests that the deepest water conditions existed in the Miine Inlet Trough, and that its history was distinct during Uluksan and early Nunatsiaq tirne. Within the Milne Inlet Trough, the lower

Victor Bay rnernber thickens to the west, both toward the axis of the graben and toward the junction of the aulacogen with the postulated Poseidon Ocean (Jackson and Iannelli, 1981).

Shallower-water facies exist across the SE portion of the Milne Inlet Trough and along the

NE margin north of Milne Inlet. At the end of Uluksan time, the western-deepening trend was reversed. In the west, Upper Victor Bay strata of the Strathcona River reef complex

(Fig. 3.1) underwent kanting prior to deposition of the Strathcona Sound Formation

(Narbonne and James, 1996). North of this location, Victor Bay rocks underlie an angular unconformity covered by carbonate boufder conglomerate, sandstone and siltstone of the

Strathcona Sound Formation (Jackson et al., 1978; Jackson and Iannelli, 1981; Iannelli,

1992). In contrast, the eastem half of the basin including the study area (Fig. 3.2A, B) experienced a higher relative rate of subsidence that resulted in an expanded stratigraphy and the deposition of the Athole Point Formation, a unit of fine-grained carbonaceous carbonate

that passes upward into mixed siiiciclastic-carbonate turbidites and sandstones. The acme of

reef development in the Bylot Supergroup occurred during mid-to-late Victor Bay tirne,

immediately preceding this fundamental change in basin bathymetry. During this time,

buildups accumulated throughout the Milne Inlet Trough, fiom the Strathcona River complex

in the West (Jackson and Iannelli, 1989; Narbonne and James, 1996) to White Bay in the east

(Geldsetzer, 1973; Jackson and Iannelli, 1989), reaching almost to the easternmost limit of

outcrop (Fig. 3.1).

Facies

The Victor Bay Formation shares facies associations with muddy storm-dominated

ramps of sirnilar age (Grotzinger, 1986b; Fairchild and Herringon, 1989; Fairchild et al., Chapter 7ycIicaIIj+packaged carbonate ramp

1997; Fredenksen et al., 1998, 1999). Facies types and interpreted environments are described in detail in Sherman et al. (2000) and are summarized in Table 3.1.

Ribbon Iimestone, nodular limestone, carbonaceous carbonate, and dolomitic shale predominate in upper Victor Bay strata in southwestern Borden Basin. Ribbon limestone consists of cm-scale couplets of lime mud and argillaceous partings. Nodular limestone is similar to ribbon limestone but cementation in the calcareous zones is discontinuous. Fetid carbonaceous lime mudstone is laminated on the mm-scale with rhythmic altemations of carbonate and organic material. Black to dark grey finely laminated dolomitic shale occun in lesser amounts. Commonly, beds of the above facies contain brecciated or deformed zones, and are interlayered with debrites.

In the northeastern portion of the study area, lime mudstone with molar-tooth structure is cornmon. An enigmatic trademark of middle to late Proterozoic carbonates, molar-tooth structure is a network of vertical and horizontal cracks in lime mudstone cemented with microspar. In the study area, the microspar-filled cracks are predominantly vertical and range fiom smooth and sinuous to irregular and ptygmatic. Lags of pebble- to sand-size crack-fil1 Gagments grade upward into muddy molar-tooth mudstone, fonning beds several centimeires thick. Finer grains of molar-tooth microspar also Corn thick beds of cross-laminated calcarenite which are interbedded with carbonate mudstone bearing diastasis cracks.

Dolomitic carbonate mudstone with crinkiy cryptomicrobial lamination

(dololaminite) is most abundant adjacent to the northeastern fault-bounded margin of the

Milne Inlet Trough. Desiccation cracks and tepee structures are common features of dololaminite, which is interbedded with intraclast rudstone. Closest to the &ou& margin, it is locally interlayered with red dolomitic shale bearing desiccation cracks and sulphate moulds.

Rare polymictic conglomerate with ftagments of molar-tooth crack-fiII, dololaminite intraclasts, and stromatolites has a matrix of mixed carbonate-siIiciclastic sand, and also occurs closest to the northeast margin. Interpretation Facies Constituents Sedirnentary Structures 3

OUTER RAMP f Distal Deep subtidal Dolornitic shale Fissile, weakly laminated black to da& grey, rare Slump folds, synsedirnentary thin lirnestone flake breccias bmias @ ?ru If O84 Intermediate Deep subtidal Carbonaceous Dolospar / calcispar-carbonaceous shale Deformation of beds by +s rhyihmite couplets, laminae 1-2 mm thick, fetid, creep, synsedimen& bituminous breccias

Proximal Deep subtidal Ribbon limestone Couplets of lime mudstone and subequal2-4 cm Slump folds, synsedirnentary layers (ribbon limestone) to thin seams (parted breccias limestone) of argillaceous to dolomitic mudstone

Proximal Deep subtidal Nodular limestone Calcite nodules in an argillaceous to dolomitic Slurnps, deformed beds limestone matrix, debrites

TRANSITlONAL Initial development in Stromatolite reef Colurnnar limestone to dolostone stromatolites Elongation of columns on deep subtidal, I 3-1 0 cm wide, up to 1 m tail with hernispherical upper swface; decametre- growth into shallow to conical laminae; domal stromatolites 1-1 0 m scale talus at basinward subtidal in diameter. Biohens up to 275 m thick, 1-2 margin km wide

MID-RAMP Shallow subtidal Molar-100th Lime mudstonc to dolornitic lime mudstone Scows, rare cross- mudstone with ptygmatically folded microspar-filled cracks, stratification and imbrication rudstone ta packstone of reworked crack-fill. of intraclasts in storm beds Graded bedding in rudstone-to-mudstone couplets Interpretation Facies Constituents Sedimentary Structures iS 9 c, INNER RAMP Shallow subtidal (inner Stromatolite Columnar limestone stromatolites 1-5 cm wide, Elongation and inclination ramp ro mid-mmp) biostrome up to 50 cm tdl, with convex laminae; inter- of columns 3 =r columnar sediment of stromaclasts, lime mud, 3 quartzose sand, mudstone intmdasts; fonn 2 biostromes 0.5-1.5 m thick, with green shale cap %

Shallow subtidal Molar-tooth Microspar intraclasts of fine sand- to pebble-size, Planar lamination, scours, (foreshore io sho~face; calcanmite fine to coarse quartzose sand, rare ooids, sparry wave and ripple cmss- inner ramp) cement lamination, HCS

Shallow subtidal Sandy polymictic Medium to coarse sand-size mairix of quartzose Cross-stratification, (shoreface; inner mmp) conglomerate sand and calcarenite, with dololaminite scours, crude grading of intraclasis, stromatolite fragments, and molar- clam tooth microspar grains Low supratidal to Dololarnioite Laminated dolomitic mudstone, sandy intraclast Tepee rurd desiccation interticid packstone to rudstone, layers of fine to corne cracks, small domal qmsand stromatolites, chert nodules, fenestml pores, roll-ups, wave ripples, cross-stratification in smdy

SABKHA High supratidai Red shaie Red argillûceous mudstone, cmquartz- Desiccation cracks, micmline sand gypsum nodules, sulfate molds Chapter 1ycIicaIly-pachged carbonate ramp

Stromatolites occur as (a) thin biostromes of columnar to hemispherical forms with grainy to muddy intercolumnar sediment, or (b) large buildups composed of alternating layers of metre-scale domes and centimetre-scale columns. Elongated columns occur on the upper surfaces of the thin biostromes, and in one instance, on the top of a large buildup. In the study area, the small biostromes occur close to the northeastem margin of the Milne Inlet

Trough, whereas the large buildups occur closer to the axis of the Trough.

The high ratio of lime mud to carbonate grains and the preponderance of tempestites and stonn-affected lithofacies, with the virtual exclusion of wave or tidally influenced facies, points to a storm-dominated carbonate system (Burchette and Wright, 1992). The distribution of lithofacies suggests that this muddy, microtidal, storm-dominated ramp deepened to the southwest (Sherman et al., 2000). Supratidal characteristics such as desiccation cracks, tepees and sulphate moulds occur in dololaminite and red shale found only in the northeast. On the imer-ramp tidal flat, graded dololaminite intraclast rudstone interbeds are interpreted as tempestites. The southwestern increase in graded beds of molar- tooth mudstone and microspar lags is interpreted as the transition into subtidal conditions between stom- and fair-weather wave base (James et al., 1998). Subtidal gas escape and

dewatering of lime mudstone created molar-tooth structure (Furniss et al., 1998), and storms

transported cemented crack-fil1 grains to the distal inner ramp where they accumulated as

molar-tooth calcarenite. Ribbon limestone and nodular limestone and interbedded debrites are

essentially fkee of shallow-water structures and are interpreted as variably disrupted

hemipelagic deposits on a deep-water slope. Periods of low carbonate supply to offshore

waters resulted in deposition of dolomitic shale and carbonaceous lime mudstone. Rarely,

small stromatolitic biostromes fonned on the i~erramp under conditions likely related to

substrate and accommodation space (Sherman et al., 2000). Similar conditions led to

nucleation of large mimbial buildups on the middle and outer ramp, but the elongations on

upper daces indicate that some accumulated into shailow water. Chapter 3ycIicapacged carbonate ramp

The Victor Bay sediments were supplied by a resilient lime-mud carbonate factory which rarely favoured the accumulation of microbialites. The overall scarcity of stromatolites and ooids is atypical of late Proterozoic carbonate successions (Sherman et al.,

2000). With the exception of quartzose sand, local stromaclasts and rare ooids, al1 grains have an intraclastic origin, and al1 originated fiom lime mud: molar-tooth crack-fill, dololaminite mud clasts, early lithified deepwater nodules, and flat clasts of ribbon limestone

(Table 3.1). In essence, clastic mud and its early lithified equivalents dominated the spectnim of depositional environments. Spontaneous precipitation of carbonate in the water-column, analogous to modem-day whitings and the result of metabolic and degradational processes of photosynthetic microbes (Robbins and Blackwelder, 1992; Milliman et al., 1993; Robbins et al., 1997), is interpreted as the source of the abundant lime mud in mid-to-late Proterozoic carbonate systems (Grotzinger, 1989; Knoll and Swen, 1990).

CYCLICITY WITHIN THE VICTOR BAY FORMATION

Victor Bay cycles are atypical of classic 1-10 m shoaling-upward cycles described from peritidal, shallow-subtidal and deep-subtidal successions of (Fischer, 1964; Wilson, 1975;

Aigner, 1984, 1985; James, 1984; Wright, 1984; Grotzinger, l986a; Calvet and Tucker,

1988; Osleger, 1991; Pratt et al., 1992; Montafiez and Osleger, 1993). They exhibit a wider bathymetric range, as interpreted fiom the constituent facies, and are considerabiy thicker, fiom 20 to 50 m (Fig. 3.4). Correlation of peritidal cycles and shallow-subtidal cycles is facilitated by continuous outcrop in the study area. To the southwest, however, the shallow- water rocks along the northem margin of the Trough dip under the Iower Nunatsiaq Group.

Where the Victor Bay Formation re-emerges in the south, the cycles coasist exclusively of deep-subtidal, hemipelagic outer-ramp facies. Ctropter 3f-CycIicaliypackaged carbonate ramp

DoIolaminite facies (peritidal, inner ramp)

Ribbon limestone facies (deep subtidal, outer ramp)

Fig. 3.4 - Cycle I is a typical peritidal cycle fiom the upper Victor Bay Formution at Section 2a. The entire spectrum of outer ramp to inner romp facies h preserved within the 2 5-m - thick cycle: outer-rarnp shale and ribbon limestone form the base, are overlain by clir- fonning midrump molar-tooth limestone facies, and are capped by inner-ramp peritidal dololaminite. Chapter 3-Cyciicai&-packuged carbonate ramp

Cycle types across the rarnp

Deep-subtida I cycles. - Deep-subtidal cycles occur in the central southem limb of the Milne Inlet Trough

(Fig. 3.5). Ribbon (see inset photograph in Fig. 3.4), nodular and carbonaceous carbonate facies consecutively overlie basal black doIomitic shale. No microbial features are evident.

Debris flows, turbidites, slump folds, and creeped beds are common, and intrafonnational tmcations in ribbon limestone locally involve several metres of strata. Fine dolarenite with

HCS and molar-tooth mudstone with thin spidery cracks occur as very rare, thin interbeds.

ALI lithofacies in these cycles were deposited entirely in calm water, below storm wave base, with the exception of the HCS-bearing dolarenite, The thin molar-tooth beds lack the crack- fil1 lags that are usually associated with storm-reworking. Abundant evidence of resedimentation also suggests proximal to dista1 outer-ramp slope environments (Calvet and

Tucker, 1988; Coniglio and Dix, 1992).

Shallow-subtidal (to shoreface) cycles.-

Cycles are capped by molar-tooth calcarenite with shallow-subtidal to shoreface characteristics. The shallow-subtidal cycles consist of two parts: a thin basal layer of ribbon limestone and nodular limestone facies, rarely containing columnar stmmatolites; and an upper 60-80% composed of interbedded shallow-subtidal molar-tooth mudstone (see inset photograph in Fig. 3.4) and molar-tooth calcarenite facies with shallow-water sedimentary structures. In bedding-plane view, molar-tooth structure in mudstone forms pseudo-polygonal networks of cracks at the base of cycles. The networks graduaIIy acquire a predominant direction of alignrnent, such that, by the top of the cycle, the networks have becorne arrays of subparallel cracks with small, sinuous offshoots (Fig. 2.7D). The overall NW-SE orientation of the army (Fig. 3.6) coincides with inferred paleostrike. The basal Iayer is interpreted as an outer-ramp deposit, and the upper portion represents deposition above stom wave base on the middle and imer ramp. Oriented molar-tooth cracks have been Chopter 3-CyclicaI~pacKoged carbonaie romp

4 NE Athole Point Formatton

sw A i Athole Pt. Fm.

Fig, 3.5 - Regional correlcztions of Victor Bay strata in eastern MiIne Inlet Trough. Profiles - A and B-correspond to lines of section in Figure 3.2A. Refr to terit jbr detailed descriptions of cycle types illustrated in legend. observed in Proterozoic rocks (Adshead, 1963; Eby, 1977, Fairchild and Herrington, 1989), and may have developed paralle1 to wave crests (Wells and Coleman, 198 1; Smith and

Winston, 1997), which would explain their orientation along depositional strike. A rare example of beachrock in cross-stratified molar-tooth calcarenite occm at Section 5 (Fig.

3.7A). Relief of several tens of centimetres was developed along an erosional surface that is encrusted by laminated, locally microstalactitic cernent. Erosion and beachrock development at the top of this cycle suggests that molar-tooth calcarenite locally accurnulated to the strandline. Generally, however, the upper portion of the cycles do not exhibit erosional features. They are similar to subtidal cycles of Osleger (1991) that do not accumulate to sea level owing to wave-sweeping in shallow water.

Peritidul cycles.-

Pentidal cycles (Fig. 3.4) are identical to shallow-subtidal cycles except that they are capped by tidal-flat and sabkha facies. Approximately 50-80% of a typical peritidal cycle

Fig. 3.6 - Uriented fiataues at Pingo VaIIey (location on inset map). Stromatolite elungatiom correspond to the long of elliptical transverse sections of stromatolite colwnns, mecrnrredfrom bedding planes. Molor-tooth cracks were meczsured on beddirtg planes where a predominant orientation war observed Chapter 1ycIicaI~pacRagedcarbonate ramp was deposited in shallow water. Capping facies of dololarninite (Fig. 3.4, inset) and interbedded intraclast nidstone are locally topped by thh evapontic red shale facies. Beds of quartzose polymictic conglomerate with planar to scalloped lower surfaces are rare and serve as a nucleation site for small columnar stromatolites that mark the base of an overlying cycle

(Fig. 3.7B). Transition to the overlying cycle is generally abrupt and appears conforniable.

Locally, the relationship between pentidal facies and with the overlying bed is erosional. The

scoured surface attains relief of a few centimetres and locally provides a substrate for

stromatolite growth (Fig. 3.7C). Some peritidal cycles display a more gradational boundq,

where the overlying cycle contains at its base thin layers of molar-tooth calcarenite or

dololarninite intraclast rudstone. In these instances, it is also possible to find a thin layer of

Fig. 3.7 - Field photographs of encrusted erosion su.faces. A) Molor-tooth calcarenite beachrock devdoped on eroded molar-tooth calcarenite at top of a shallow-subtidal cycle. Note the Iaminated crut (Ic) on the knob of molor-tooth calcmenite und microstaIactitic cement (ms) devehped under the overhmg. Scale bar = 5 cm. B) Small stromatolilei accumuhted on irregulm erosion mrface of coarse poljmictic conglomerate. Scale bar = 5 cm. 9 SrnaII columnor stromatolites sirnilm to (B) mcleated on scoured early lithifed dololaminite. Scale = IS cm. Chqter ~yc1icall)~-pachgedcarbonate ramp columnar stromatolite with a veneer of green shale, overlain in him by the deep-water noddar or nibon limestone facies, Overall upward trends document the transition fiom outer ramp to inner rarnp, as captured in each peritidal cycle: (1) an Uicrease in shallow-water sedimentary structures, (2) a pater degree of reworbg and deposition atûibuted to stom and waves, (3) an increase in coane siliciclastic material, and (4) the appearance of subaeriai exposure features. The supraticid to intertidal nature of facies in cycle tops points to Suier-rarnp conditions during final stages of deposition of the cycle. Absence of capping tufas and microdigitate stromatolites, however, sets Victor Bay inner-ramp cycles apart fiom archetypical

Paleoproterozoic peritidal cycles (Grotzinger, 1986a, 1989; Pratt et al., 1992).

Pingo Yailey transect The relationship between shallow-subtidal and peritidal cycles is clearly displayed at a Iocality infonnally dubbed Pingo Valley on the northeast margin of the Milne Inlet Trough (Fig. 3.2B). Steep cliff walls on both sides of the NNE-SSW valley afEord continuous exposure perpendicular to depositional strike, revealing undeformed shallow-water facies dominated by pristine limestones and fabric-retentive syndepositional dolostones. These provide a unique opportunity to analyse the basinward transition of facies, cycles and ramp architecture. Strata are generally horizontal except in the southernmost third of the valley where they dip gentiy southward. Cycle linkage was reinforced by examination of complementary sections (Fig. 3.2A) located inboard (Section 4) and along depositional strike (Section 5) fiom Section 3. Photographs taken fiom airdwere merged into a photo-illustration depicting continuous cliff exposure along the 85-km-Long northwest valley wall of Pingo Valley (Fig.

3.8). Society CWs Formation occurs in the vdey floor where alluvium obscures al1 but the uppermost few metres, and the thick talus at the base of the resistant clBs is the recesively weathering Iower shale member of the Victor Bay Formation. The upper cl=foming member is strikingly cyclic, and exhibits a large stromatolite biostrome at its southwestern sw Fia. 3A1C

LEGEND

Fig. 3.8 - Photo-illlrstrutîon d mterprettve Iine rli'ogram of the western wdof Pingo Valley, repesenîing a fiansecf pependifcum ro depostposttiom1srrike phite marks idcate edges of meshed photograpk). The tolw siope represenîs the recessivelyweathering lower Victor Bq member. Cycles m the he (nght) me ccpiped Q penti&l facies. These C~JN thin and disappem to the SM(lep) in the themer sequeme that co~uimthe large reef: Imet Photograpb: 1, SW (bmimumd) appearmce of reef; with thin layer of &&rites m bare; 2. view of intemediate portion ofreex with a cornplex msociatiomof olistolitk (O). ribbon lfmestone (r& and sbamtoiite bioherms (s); 3, thinner ALNE (Jhorewmd)portion of reewte the trtmcctted basinward exb.emity of Cycle D;4, mid-point of the iransect where reef b absent and cm1 cl@$onning ingles E and F overlfeA-D (note abundme of pule-weathering peritidd shaa); 5, cycles aî the Nmextremity of the transect.

VALLEY TRANSECT NE

Chqper 3-CycIicaI&p~ckaged carbonate ramp mid-ramp extremity. Appreciable vertical offset of strata occun dong two north-dipping normal faults with stratigraphie separations of - 60 m and - 150 m. Stratigraphie units, however, are easily correlated across the breaks. Detailed meamernent and description of strata at bath ends of the northwest wail (Sections 2% Sb, 3) provided lithological control for facies analysis and corretations.

Carbonate packaging in Pingo Valley

Decametre-scale (20-50 nt) parasepences.-

Cycles are entuely pentidal at the northem end of Pingo Valley (Section 3) and are shallow-subtidal and peritidal at the southem end (Sections 2% 2b). The correlation of cycles between sections (Fig. 3.9A) defines parasequences that change in character bom proximal imer ramp to distal inner ramp and which can be grouped into chree main packages: (1) four parasequences (A-D) with evidence of gradua1 deepening towards the southwest, (2) two parasequences (E, F) laterally equivalent to an outboard reef, and (3) an upper package of parasequences with peritidal caps exhibiting no significant deepening across the transect. In the lowerrnost four parasequences A-D, distinctive pale-yellow-weathenng dololaminite caps of individual peritidal cycles @ale strata in Figure 3.8, inset 4) can be traced visually fiom

NNE to SSW in the valley wall for 6.5 km, until they gradually thin and disappear (inset 3).

The massive reef at the basinward edge of the transect is 170 m thick and overlies the shallow-subtidal portion of the parasequences (insets 1 and 3). Evidence of disniption under the reef indicates that it nucleated within a slump scar (inset 2). Initial slurnping propagated

from the base of Parasequence D in the southwest and reached as fat upsection as the base of Parasequence E, up to two kilometres inboard.

Outboard attmuation of pentidal facies (Fig. 3.8, 3.9) is typical of carbonate ramps

and interpreted as a graduai transition into subtidal conditions (Ahr, 1973; Burchette and

Wright, 1992). In the study ana, evidence of erosion or non-deposition at the top of

shallow-subtidal cycles is limited to local occurrences of beachrock. Basmward thmning of Chapter 3-CycIicaII~pc~carbonate ramp

Fig. 3.9 - A) Lithocorrelution of the upper Victor Bay rnember and adjocent siruta aiong Pingo Va1IeyeyFacies between sections 2 and 3 were interpreted with the aid of jied photogmphs.

Chapter 3-Cyclicaipacged ctzrbon~teramp dololaminite therefore appears to be pnmary and not an erosional feature. MoIar-tooth calcarenite facies capping shallow-subtidal cycles to the southwest are interpreted as tirne- equivalent to dololarninite facies to the northeast, The transition of shallow-subtidal into deepsubtidai cycles is not well exposed in the study ana and is obscured by the development of the reef biostrome. It is nonetheless reasonable to correlate peritidal and shallow-subtidal cycles to deep-subtidal cycles and assume that molar-tooth mudstone tempestites thin and disappear below stonn-wave base, and are replaced by entirely muddy outer-ramp facies such as are found at Section 1 (3 1 km SW of Section 3 reefs) and at Section 6 (45 km SSE of

Section 3 reefs; 15 km WSW of upper Victor Bay reefs at Section 7).

Hectomeire-scale (- 150-300m) depositional cycles.-

Decametre-scale parasequences are grouped into three hectometre-scale depositional cycles contained within the Victor Bay and lower Athole Point Formations. The thickness of these cycles corresponds to that of third-order sequences of Vail et al. (1977).

The lowermost sequence (Sl) comprises 50-100 m of lower Victor Bay member shales and the first - 250 m of the upper member, including parasequences A-F and the stromatolitic reef (Fig. 3.98). The lower sequence boundary is the transgressive surface

between shallow-water Society Cliffs dolostones and deep-water lower Victor Bay shales.

Upper-member parasequences A-D contain the best exarnple of peritidal cycles passing into

shallow-subtidal cycles basinward (Fig. 3.9A). At the southem end of the transect,

Parasequence D is laterally equivalent to slump blocks, debntes and the lower portion of the

stromatolitic reef. The principal mass of the reef overlies D and its equivalents, and thins

shoreward, passing into parasequences E and F.

The middle sequence (S2) consists of 110-160 m of peritidal cycles. Debrites of

various lithofacies are common at Sections 2a and 3. $2 is on the whole shallower in

character than S 1, suggesting that peritidal ber-ramp facies had prograded at least several

kilometres basinward. Slumping of shallow-water facies is cornmon and impiies that this Chopter 3-Cyclically-packaged carbonate ramp portion of the ramp was gravitationally unstable during progradation of inner-ramp strata into deeper water.

In eastern Borden Basin, stromatolite reefs, thinly bedded carbonaceous lime mudstones, and ribbon-limestone debrites typify the base of S3. This abrupt transition into deeper-water facies contains the contact between the Victor Bay and Athole Point

Formations, and includes the upper Victor Bay reefs and Athole Point Fonnation carbonaceous lime mudstone. In western Borden Basin, fine-grained siliciclastic rocks of the

Strathcona Sound Formation overlie the Victor Bay Formation, whereas in the east. The

Victor Bay Formation is overlain by the Athole Point Formation. For this reason, the reefs at the Uluksan-Nunatsiaq contact have been altematively assigned to the Strathcona Sound

Fonnation (Jackson et al., 1978; Jackson and Iannelli, 1989), Athole Point Formation

(Jackson and Iannelli, 1989), and the Victor Bay Formation (Narbonne and lames, 1996). In this study, reefs and associated debrites at Pingo Valley are clearly within the Victor Bay

Fonnation.

Ramp architecture.-

Regional paleocumnt patterns (Jackson and Iannelli, 198 l), consistent bedding-plane orientation of molar-tooth cracks (Fig. 3.6), slump folds in deep-water facies at Pingo Valley and in the southern sections, orientation of the reef tract, orientation of structures on bioherm surfaces, shallowing of facies, and elongation of columnar stromatolites (Hofian,

1976) al1 support a depositional strike of SE-NW. The ramp profile is inferred to be distally steepened, as resedimentation features are comrnon in deep-water facies throughout the study area (Fig. 3.1OA). On the berrarnp, peritidal facies taper seaward along the Pingo Valley

transect on the gentle imer ramp slope. An abrupt increase in the gradient at the mid-tamp

is interpreted where local synsedimentary slope failure involving shallow-subtidal and peritidal

facies, even entire decametre-scale cycles (Fig. 3.10B-D). A flexure on the distal inner ramp

could explain these features as well as the disappearance of paîtidal caps at this location.

Distal steepening can theoretically develop in ramps as higher rates of deposition in the inner Chopter 3-Cyciically-packaged carbonate ramp ramp lead to thickening relative to the outer rarnp (Read, 1982, 1985). Gravity flows in the lowermost parasequences (A-D) of the upper mernber indicate, however, that even during initial development of the ramp, the gradient was high and the sediment pile unstable.

Progradation of upper Victor Bay strata into the basin may have resulted in deposition of shallow-water facies in a gravitatioaally unstable location (cf. Pedley, 1998; Pedley et al.,

1992). The break in slope could be the byproduct of antecedent topography of the underlying Society Cliffs Formation. Altematively, vertical movement associated with a zone of normal faulting near the south end of Pingo Valley (Jackson and Sangster, 1987) could have created the inflection and triggered slope failure.

Fig. 3.10 - Field photographs of synredimentary deformation and resedimented facies. A) Slump fol& and breccia in deep nrbtidal ribbon limestone. Lem cap = 5.5. cm. B) Skie block at the base of Cycle D. The coherent block of molar-tooth limestone was dkpiaced and rotated along a glide plane parallel to the underlying bedding. Hammer = 35 cm. C) Several beds of ribbon limestone debrites at the southern end of the strornotolite reef: Outlined area enlarged in (Il).Jacob's st#= 1.5 m. D) Enlarged view of slumped ribbon limestone breccia in (C). Lens cap = 5.5. cm. Chapter 3-Cyclicall'packaged carbonate ramp

The most striking feature of the steepened mid-ramp zone at Pingo Valley is a thick stromatolite buildup that occupies the proximal mid-ramp (Fig. 3.8). Whereas stromatolitic bioherms are of modest size in sub-reef strata (c2 rn thick, e.g., Fig. 3.1 1A, B), the rnid-ramp reef is at least 2 km long perpendicular to depositional strike and 170 rn thick. It belongs to a discontinuous tract more than 20 km in length that follows the strike of the ramp, and along which the reefs protrude fkom the surrounding strata, in some cases exhibiting growth topography on their upper surfaces (Fig. 3.1 1C). Three units within the dolomitic reef are interpreted as distinct growth phases: (1) domal stromatolites 1-10 m in diameter nucleated on blocks of ribbon limestone coeval with early lithified deep-water facies of D2 (Fig. 3.1 ID), and laterally traceable fiom the DIslide block (Fig. 3.108); (2) domal foms 1-1 0 m diameter (Fig. 3.1 lE, F) interlayered with columnar forms 5-10 cm in diameter, forrning the bulk of the reef and corresponding to "back-reef" Parasequence E; and (3) a thinner unit of domal and columnar stromatolites equivalent to Parasequence F. Temination of reef growth occurred following the deposition of a 1.3 m cap of limestone stromatolites (Fig. 3.1 lG, H) which are columnar and elongate in plan view (Fig. 3.1 11). The cyclic alternation of domal and columnar foms throughout the middle and upper reef nits suggesu growth under fluctuating sea level, where dornal foms represent accumulation in deep water, and columnar foms grew under higher-energy, shallower-water conditions (Horodyski, 1993; Southgate,

1989). The columnar cap with its elongated foms suggests growth into shallow water. The distribution of stonn-affected facies at Section 3 reveals that the downslope position of the reef offered the imer rarnp little or no protection from storm waves. Peritidal facies inboard of the buildup show no lesser degree of storm influence than those inboard of shallow-subtidal cycles below the reef. The reef may have had little effect on the hydrodynarnic regime in the inner rarnp, but it nmetheless attests to the growth potential of microbiaiites once they become established on a suitable substrate.

A second phase of reef growth in S2, correlative to Parasequence K, resulted in pinnacle-shaped bioherms that acquired tens of metres of synoptic relief (Fig. 3.1 LI). These Chapter 3yclicafl'pockagedcarbonate ramp

Fig. 3.11 (caption p. 81) Chapter 3-C'ycIicalIy-packaged carbonate mp

Fig. 3.1 1 (cont'd) - Field photographs of stmmatolitic features at Pingo Valley. A) Small bioherm of columnar stromatolites, approximately 75 cm thick overiying a unit of ribbon limestone. Scale bar = 50 cm. B) Top view of a smoll stromatolitic bioherm showing elongation of columns roughly perpendieular to paleostriRe. C) Aerial view toward the NE, onto the upper &ce of the lower reef Scale bar = 50 m. D) Laminated microbialite encrusîing tilted block of ribbon limestone. The block belongs to an olistostrome lateralb equivalent to the slide block in Figure 3.108. Jacob's stg = 1.5 m. E) Domal microbial f&c in the upperrnost 50 m of the reef: Hammer = 35 cm. F) Ine domal stromatolites form in hrrn larger structures at least 15 m in diameter, with synoptic relief of severai metres. Person fbr scale. G) Cross section through the upper 1 0 m of the reef: The topmost 3.5 m is the unit of columnar stromatolites in (H). which is underlain by stratr$iorm stromatolites. Person (circled)for scale. 8)Cap of stromatolitic limestone on the predominantly dolomitic reef: Columns are up tu 1 m ta/[ and consist of hemisphencal laminue. Hammer = 35 cm. 9 Bedding-plane view of elongate siromatoiites in 0. Elongation direction ik roughly perpendieular to inferred depositionaf shike. The hmmer LP 35 cm long and oriented approximately NS. J) Yiew to the WSWcapturing 60th the lower dolomitic reef and the upper limestone reeJ nie îwo buiidups are separated by 100 m of strata representingfour peritidal cycles. Chapter 3-Cyclicaipacged carbonde ramp reefs are developed on debrites containing lithoclasts representing successively deeper facies upward. Reefal talus shed at Pingo Valley forms a thick layer of arnalgamated debrites that pinches out latedly to the southwest. At another locality (Section 7) the debns includes a

40-m ta11 block of the reef containing coniform columnar stromatolites.

Facies response to seu-tevel change and development of parasequences

Transgressive, highstand and lowstand depositional phases are identified within the parasequences, based on facies analysis and the lateral relationship of cycles at the transect locality and in the wider study area (Fig. 3.12).

Transgression.-

On the imer ramp, the flooding surface is usually sharply defined, and dololaminite is overlain by deep-subtidal nbbon or nodular limestone of the overlying cycle. This sharp boundary is interpreted as an abrupt transition to a quiet, deep-water depositional environment. Less commonly, a gradational contact between shallow-water and deep-water

facies is interpreted as impingement of shallow-subtidal conditions on the tidal Bats during

initial flooding prier to drowning, and may reflect a lesser rate of sea-level nse. The small

columnar stromatolites encrusting the peritida1 facies accumulated during early transgression,

as accommodation space was created under decreasing hydrodynamic energy.

Flooding on the distal inner rarnp and proximal mid-ramp is represented by molar-

tooth calcarenite overlain by transgressive ribbon andlor nodular limestone, or rarely by a

thin biostrome of columnar stromatolites. Relative sea level rise on the outer ramp is

marked by the deposition of black shale, riibon limestone, and carbonaceous limestone

rhythmite.

Maximum flooding within the parasequence is recorded by green shale and nibon

lirnestone in the mid- and inner ramp and carbonaceous carbonate rhytbmite in the outer

rarnp. These lithofacies an interpreted as background sedimentation under conditions of Chuptw 3-CycIically-packagedcarbonate ramp reduced carbonate production. The relative hcrease in organic sedirnentation rnight be attributable to an increase in pelagic productivity associated with nutrient-nch transgressive waters (HaIlock and Schlager, 1986) and magnified by sequestration of organic matter owing to encroachment of anoxic bottom waters. Altematively, carbonaceous rhythmites at

maximum flooding could be explained by influx of waters less saturated in CaC03 during

transgression. Such waters would decrease micrite precipitation in the water-column and thus

reduce turbidity. The association of stromatolites with flooding could be a side-effect of increased light penetration to the shallow sea floor that encourages precipitation of

microbialty mediated benthic carbonates such as stromatolites and ooids (Sherman et al.,

2000).

Highstand. -

Once the inhibiting effects of sea-leveI rise waned, the site of carbonate production shifted from seafloor to water-column. Whereas transgression signified a clear, stratified

water colurnn, highstand would be characterized by mixed waters made turbid by whitings. Unlike modem whitings where spontaneous precipitation is sporadic, higher concentrations

of carbonate in Mesoproterozoic and Neoproterozoic oceans promoted abundant production

of lime mud in the water column (Grotzinger, 1989). As the rate of relative sea-level rise

began to decrease, accumuIation of mud wouId drive imer-rarnp facies to aggrade and prograde into deeper water (Fig. 3.12). Middle highstand deposition of molar-tooth

tempestite in shallow-subtidal and peritidal cycles would correspond to nodular limestone

facies in deep- subtidal outer-ramp cycles. As aggradation and progradation continued, storm-

wave base encroached on the accumulating pile of molar-tooth mudstone. The anisotropy in bedding-plane orientation of molar-tooth cracks (Fig. 3.6) could be linked to shoaling either

(1) by degassing under conditions of increasing energy of the oscillating current, or (2) by the increasing mechanical contrast between upward-thinning mudstone beds and increasingly

common grainy beds. Chupter 3Cyclicully-packaged curbonate ramp

Lo wstand.- Unequivocal evidence of sea-level fa11 is generally more difficult to assess on ramps, where deposition can continue in deeper water even as the proximal portion of the ramp is exposed during fourth-order sea-level fluctuations (Burchette and Wright, 1992; Tucker et al., 1993). For this reason it is sometimes difficult to distinguish lowstand fiom highstand on

Shallow subtidal peritiàal cydes (mici- b cydes (inner inner ramp) ram~)

Fig. 3.12 - Schernatic architecture of on upper Victor Bay ramp parosequence. Shallowing-upward on the innet rarnp is recorded as a peritidal cycle and on the outer rump os a deepwater cycle. Exposure surjiaces above lowstand deposits are rare. Inboard unconfomities ore poshrloted, but me not preserved in the shrdy area. Proximal and dista1 outer-ramp debrites occur outboard of the steep mid-romp. Chapter 3-Cyclicallppackaged carbonate rump ramps. On the outer Victor Bay rarnp, rare examples of HCS-bearing dolosiltite and minor beds of molar-tooth limestone could be the diaal signature of lowstand. Further inshore, beachrock at the top of shallow-subtidal cycles required falling sea-level and exposure of the distal inner ramp. Fourth-order sea-level fa11 would also lead to deposition of dololaminite and inboard correlative evaporitic red shale facies evident in peritidal cycle caps. Greater- magnitude fluctuations could be responsible for the deposition of quartzose polymictic conglomerate during late lowstand / early transgression, when low sea level led to erosion of pentidal carbonates and transport of siliciclastic grains onto the imer ramp.

Sea-IeveI history of the Victor Bay ramp

Ramp response to sea-level change.-

Ramps react to sea-level change differently from shelves, and their geometry is

conducive to the preservation of a more continuous sedimentation record, as the carbonate

factory remains active throughout the deposition of the transgressive systems tract (TST)

and highstand systems tract (HST)(Burchette and Wright, 1992). Condensed horizons are

therefore of minimal importance. Facies belts and carbonate production simply shif't

offshore during highstand and lowstand (Handford and Loucks, 1993; Tucker et al., 1993),

thus exposure and kanting of the proximal imer ramp do not necessarily result in a complete

cessation of sedimentation. Consequently, the lowstand systems tract (LST) is difficult to

identify, and highstand facies in parasequences appear to be directly overlain by transgressive

facies of the next parasequence (Burchette and Wright, 1992; Tucker et al., 1993). Shelf-

margin wedges do not develop during lowstand, lmless the ramp is distally steepened (see

models in Tucker et al., 1993). In contrast to rimmed shelves, ramps usually develop a

retrogradational pattern of parasequences during TST and a prograding pattern during HST

(Burchette and Wright, 1992) owing to their generally slower rates of accumulation (Sarg,

1988). Chapter Wyclicallppackaged carbonate ramp

Accordingly, the Victor Bay ramp hosts few unconformities, even between sequences, and within parasequences the tmsition nom highstand to lowstand is subtle. Condensed horizons above flooding surfaces are absent. It diffen nom typical rarnps, however, in its

LST deposits and aggradational highstand parasequences. The LST at the base of 52, accumulated outboard of the steepened portion of the ramp, is correlated to the unconfomity between Parasequences F and H.

There appears to be a certain degree of stasis with respect to component facies from one parasequence to the next. tndividual parasequences within systems tracts on ramps tend to track sea-level and follow either a retrogradational or progradational pattem (Burchette

and Wright, 1992). Victor Bay parasequences are uniform in character within systems tracts, i.e. there is no change in the spectnim of facies preserved fiom one parasequence to the next.

Even in the most shoreward exposures (Section 3), al1 parasequences, Save one, contain a complete range of facies fiom outer-ramp ribbon limestone to inner-ramp dololarninite. This suggests that the parasequences follow an aggradational pattem within systems tracts,

and inner-camp facies neither "gain ground" or "lose gmund" to an appreciable degree with

each successive parasequence. This results in abrupt transitions between sequences and

systems tracts that are commonly confined to, or even occur within, a single parasequence. The only exception is the lower Victor Bay member which shows a classic TST unidirectional

upward shifi nom basinal cycles to distal outer slope cycles. This aggradational pattern is

similar to that of carbonate platforms that "keep up" with sea-level rise (Kendall and

Schlager, 1981; Sarg, 1988). It would appear that once highstand sedimentation was

established, the Victor Bay nimp did not prograde significantly during S1 or S2.

Sea-bel history during SI.-

The Society Cliffs Formation ramp was drowned during a major transgressive episode

marking the base of the lower member of the Victor Bay Formation (Fig. 3.13). The maximum Booding daceis likely within the black pyritiferous shale imrnediately above the

Society Cliffs contact. The overlying fissile dolomitic shale and carbonate mudstone grade Chapter 3-Cyclically-packagedcarbonate ramp

Fig. 3-13 - Cartoons depicting the development of sequences at P ingo Valley from eady Victor Bay time to eudy Athole Point tirne. Chapter 3-Cyclically-pucb-ged carbonate ramp into thin nodular and ribbon limestone interbeds at the top of the shale mernber, signaling the progradation of the ramp into deeper water during early HST. Parasequences A-D (Fig. 9A, B) were deposited when the transition fiom shallow-subtidal to peritidal facies during early to middle HST was located at the southem end of the transect. The accumulation of muddy subtidal strata in a zone of inflection might have been susceptible to failure leading to the mass-wasting event that tnuicated Parasequence Dl and produced the olistostrome.

Stromatolitic mounds then nucleated on the resultant blocks of early cemented ribbon limestone in the mid-camp; meanwhile, D2 was deposited on the unaffected inner ramp.

Accumulation of nodular and ribbon limestone during transgressions at the bases of parasequences E and F translated into phases of reef growth offshore.

Sea-bel history during S2.-

Falling sea level resulted in exposure of peritidal smta proximally and a lowstand wedge distally (Parasequence G). Accumulation resumed inboard during flooding in early

Parasequence H tirne, indicated by the deposition of polymictic conglomerate containing a wide range of shallow-water lithoclasts at the base of the cycle (Fig. 3.9A). LST parasequences G to J are thinner than SI HST cycles (15-30 m), and display peritidal caps across the entire transect. Molar-tooth cracks are oriented NW-SE as they are in SI, and depositional strike appean to have remained the same in S2. The transgressive surface marking the TST is the base of a 0.5 m thick layer of domal stromatolites which correlates to a 50-m-thick limestone stromatolite reef (Parasequence K). Like its larger S1 predecessor, this bioherm was also nucleated on debrites. The reef marks the top of the section, but the upper surface of the outcrop appears to be the original domal surface of the buildup, and may have been exposed through weathering of a less resistant overlying unit. The HST is thin, essentially contained within Parasequence L at Section 3. Chapter 3-CycIicaIl~packoged carbonate romp

Seo-bel history during S3.-

The TST of this sequence is very thick in the study area, and represents progressive drowning of the Victor Bay ramp, compared to abrupt fiooding of the entire Society Cliffs

Formation prior to the deposition of the lower Victor Bay member. During early TST, which is only preserved in Section 3 as the incomplete Parasequence M, a thin layer of Conophyton stromatolites was deposited coevally with ribbon limestone. Inboard of the northern extremity of the transect, in the vicinity of Section 4 (Fig. 3.2B), shallow-wzter sedimentation persisted longer, but was terminated by deposition of deep-water carbonaceou limestones of the Athole Point Formation. Thick calciturbidites within the middle member of the Athole Point might be the distat representation of highstand shedding. The portion of

S3 preserved at Pingo Valley and elsewhere is a deepening-upward succession, and shallow- water sedimentation does not resurne until the deposition of Strathcona Sound Formation siliciclastic rocks nearly 1 km higher in the section.

DISCUSSION: MECWANISM FOR THE CREATION OF CYCLES AND SEQUENCES

Characteristics of Victor Bay cycles.

The cyclic nature of Bylot Supergroup strata has been recognized at many scales

(Jackson and Iannelli, 1981). Geldsetzer (1973) interpreted kilometre-scale cyclicity as tectonically controlled second- to third-order "megacycles". At the other end of the spectrum, Knight and Jackson (1994) interpreted metre- to decarnetre-scale cyclicity within the Elwin Subgroup (Fig. 3.2) as the product of nsing sea level (caused by eustasy, tectonism or sedirnent loading) followed by progradation of siliciclastic depositional envùonrnents.

Jackson and Iannelli (1981) desded cycles in the upper Victor Bay Formation up to 30 m thick, and proposed a tectonic origin for thi~ershale-carbonate cycles in the lower Victor

Bay member. Chapter 3-CycIicaII)~-packagedcmbonate rarnp

The 2&50 m Pingo Valley carbonate cycles required the creation of several tens of metres of accommodation space, and a broad facies transition fiom intermediate outer ramp to supratidal inner ramp. This situation suggests a rapid rate of sea-level rise that exceeded sediment accumulation potential. Flooding surfaces juxtapose strongly contrasting environments, as would be expected across a sequence boundary-not within a sequence.

Victor Bay cycles are virtually identical to Mesoproterozoic cycles of the Wallace and Helena-Siyeh formations of the Belt-Purcell Supergroup. Whether the Belt Basin was marine or lacustrine has been debated (Grotzinger, 1986b; Winston and Lyons, 1993); however recent 6"~values fiom Helena rocks suggest deposition under marine conditions

(Frank et al., 1997). The 1.4-1.3 Ga Wallace and Helena-Siyeh cycles exhibit the sarne spectrum of muddy facies as Ca. 1.2 Ga Victor Bay peritidal cycles, fiom nbbon limestone to cryptomicrobial laminite, and are characterized by abundant molar-tooth mudstone

(O'Connor, 1972; Grotzinger, l986b; Winston and Lyons, 1993).

The Wallace and Helena-Siyeh formations appear to record a distinct

Mesoproterozoic style of carbonate sedimentation that recurred -100 Ma later in the Borden

Basin. The Belt cycles, however, are only 1-10 m thick (-20% of Victor Bay cycles). The

interpreted distances of progradation (200-300 km) of peritidal-like facies within individual

cycles (Gmtzinger, 1986b) indicate a low gradient on the ramp or shelf. Therefore, even

small sea-level fluctuations would have had a far more profound effect on the lateral

migration of the carbonate factory than on the steepened profile of the Victor Bay ramp.

A utogenic us. allogenic influence.-

Packaging of carbonates can be generated by either autogenic or allogenic

mechanisms, and the continuity and character of cycles permit a distinction to be made

between these forces. Autogenic, sediment-driven processes tend to result in thimer, metre-

scale cycles with limited lateral extent perpendicular to depositional strike (Pratt et al.,

1992). On shelves, progradhg tidal flats cause the carbonate factory to shrink and thus Chapter 3-CyclicaIly-packaged carbonate ramp choke itself off (Ginsburg, 1971, 1975; James, 1984) whereas on rarnps carbonate production simply shifts basinward as sediment accumulates in shallow water (Burchette and Wright,

1992). Autogenic controls can produce laterally discontinuous facies successions (Pratt and

James, 1986; Satterley and Brandner, 1995), and tend to dominate where sea-level change is

minor, resulting in parasequence boundaries of limited lateral extent (Adams and Grotzinger,

1996). Cycle thickness (>>IO m), a wide range of facies bathymetry, and lateral continuity

both along (>20 km) and across 910 km) depositional strike strongly imply allogenic forcing

for the Pingo Valley cycles.

Tectonics.-The effects of tectonism have been recognized at and above sequence

scale in the Borden Basin (Geldsetzer, 1973; Jackson and Iannelli, 1981; Narbonne and James,

1996). CIear evidence of a tectonic control on the formation of parasequences, however, has

not emerged. Certainly, given the history of extensional tectonics in the Borden Basin, local

vertical movement of rift blocks should be taken under consideration when assessing local

controls on subsidence (Hardie, 1986b). Shallowing-upward cycles can hypothetically result

fkom stick-slip normal faulting (Cime, 1986), but these should remain in the range of 1- 10

m, almost an order of magnitude thinner than Victor Bay cycles. Comparable cycle

thicknesses (e.g., Sami and James, 1994) and sirnilar spectra of shallow- to deep-water facies (e.g., Haywick et al., 1992) are nported fiom rapidly subsiding foreland basin settings. Bath

compressional and extensional tectonic subsidence, however, usually result in deepening-

upward successions, and rapid episodic downdropping associated with tectonism tends to result

in intempted cycles and variable cycle thicknesses. In contmst, facies stacking at Pingo

Valley is consistent from one cycle to the next, and the shoaling-upward cycles are complete.

The upward-shallowing trend is not intempted and predictably culminates in shallow-water or

supratidai facies.

Eus~us~.- Tectono-eustasy tends to influence ocean volumes at the long-term, first-

order scale (Vail et al., 1977). Continental reconstructions (Dalziel, 1991, 1997; Karlstrom

et al., 1999) imply that the Mesoproterozoic assembly of Rodinia was already underway Chapter 3-C'yciicaii''p~ckaged carbonate ramp during the deposition of the Bylot Supergroup (Kah et al., 1999). Sea level in the global ocean would therefore have been falling over the tens of millions of yean of supercontinent formation (Woaley et al., 1984), and would not have significantly influenced sea level at the scale or fmluency observed at Pingo Valley. The thickness of the cycles, their completeness, and lateral continuity is better reconciled with glacio-eustasy.

In a greenhouse world, fifth-order precessional (19-23 ka) forcing dominates

(Koerschner and Read 1989; Goldhammer et al., 1990; Wright, 1992). resulting in parasequences a few rnetres thick. Sequences are generated by third-order sea-level variations

(Fischer, 1980; Veevers, 1990). This overall pattern is comparable to the Belt-Purcell cycles described above, which Grotzinger (1986b) suggested were formed during eustatic sea-level oscillations of a few metres. In contrast, variations in ice-cap volume during glacial periods are driven by fourth-order eccentricity (100400 ka) and obliquity (40 ka) that produce hi&- magnitude fluctuations in sea-level. These result in thicker decametre-scale cycles (Read,

1998) similar to those of the Victor Bay Formation.

Placing the birth of an ice age between the Belt Supergroup (1.4-1.3 Ga) and Bylot

Supergroup (1.2 Ga), however, poses some problems. The importance of glacio-eustatic effects on sea-level change in the Mesoproterozoic is still unlaiown. Furthemore, the global carbon isotopic curve does not exhibit the ciramatic excursions that characterize the

Paleoproterozoic and Neoproterozoic (Kauhan and Knoll, 1995). Nonetheless, during the deposition of the Bylot Supergroup, the othem*sefiat Mesoproterozoic 613c cwebegins to exhibit an increased variabiiity, with strong negative shifks, ca. - 1.1 Ga @ah et al., 1999). In addition, whereas Victor Bay cycles share some attributes of high-fiequency parasequences defined by flooding surfaces, they are more reminiscent, in their thickness and bathymetric spectrum, of fourth-order icehouse sequences defined by unconfomities (cf.

Tucker et al., 1993; Koerschner and Read, 1989; Read, 1998). Unconformities are more

difficuit to identiQ on ramps than on shelves because facies belts (and thus the carbonate

factory) will shift offshore during lowstand, and ths supply of sediment is maintained Chapter 3-CjxIicaIIy-packaged carbonate romp

(Burchette and Wright, 1992). During sea-level fall the exposed inner ramp is prone to karsting in humid climates, whereas arid conditions tend to generate evaporites. Arid conditions are interpreted for the supratidal Victor Bay ramp (Sherman et al., 2000), and could explain the absence of karst on exposure surfaces. These postulated inboard unconformities could be correlative to tirne-equivalent peritidal caps (cf. Van Wagoner et al.,

1988). Therefore the parasequences (A-M) identified in the study area could be traceabie inboard into sequences, and the sequences (S IS3), into sequence sets.

Another aspect must be considered when interpreting the cycles as glacio-eustatic.

Tidal-flat facies are rare on icehouse Paleozoic ramps because the rate of intertidal sediment accumulation is usually too low ta match the high rate of sea-level change (Read, 1998).

Such facies thus only occur during stillstand when these two rates are matched, and the

resulting deposits are of minor thickness and lateral extent. Tidal-Bat dololaminite, however,

is an important component in the thick Victor Bay cycles, and implies significant supratidal

accumulation during highstand and early lowstand. This high supply of sediment does not

necessarily negate the icehouse hypothesis, given the vigorous mud production suggested by

the facies themselves. The primary source of sediment on the Victor Bay ramp was lime mud

interpreted as inorganically precipitated fiom CaCOg-supenaturated waters of the inner- and

mid-ramp (Sherman et al., 2000). The volume of mud created by whitings in modem tropical

waters (Robbins et al., 1997) is thought to be far inferior to that produced during highstand in

the Mesoproterozoic water-column (Grotzinger, 1989). It is therefore likely that the rate of

accumulation on the imer ramp was greater than in Phanerozoic icehouse ramps, and could

explain a thick stillstand record.

Scale of Mesoproterozoic cycles.-

If the Victor Bay Formation was indeed deposited under icehouse conditions, it would

follow that the scale of cycles in adjacent strata rnight also be unusually thick, perhaps

defining an upward-thickening trend in younger strata. Cycle thicknesses of Bylot Chapter 3-CyciicaiIy-packaged carbonate rantp

Supergroup strata compiled fkom Jackson and Iannelli (1981) and Kah (1997) suggest that, generally, fluctuations in base-level occurred on the scale of a few metres during the early bistory of the Borden Basin but increased dramatically in amplitude during deposition of late

Eqalulik and üluksan Groups. Maximum cycle thickness increased to several tens of metres in the uppennost calcareous member Arctic Bay Formation (Fig. 3.3). Then, decametre- scale shale-to-sandstone and conglomeratic cycles were deposited in Fabricius Fiord rocks coeval to basal Society Cliffs Formation. Packaging within Society Cliffs carbonates varies regionally (Kah, 1997): oolitic-stromatolitic dolostone cycles 1-10 rn thick occur near

Milne Inlet (Fig. 3.2A) whereas 1O-50 m microbial dolostone cycles occur to the east.

Bundles of up to 15 oolitic cycles are correlated tu 1-3 microbial cycles, suggesting that the oolitic facies were more sensitive to minor fluctuations in accommodation space, and therefore recorded higher-lrequency cyclicity not evident in the inboard restricted lagoon

(Kah, 1997). Evidence of exposure in the oolitic cycles points to a mie eustatic sea-level

&op on the scale of a few metres rather than autogenic response. Finally, the thickest cycles

are present in the Victor Bay Formation.

Are Victor Bay facies packaged in decametre-scale cycles because they occupy a wide

range of water depths and are therefore less sensitive to small-scale sea-level change, much

Iike the Society ClifEs lagoonal microbialites? It would appear that this is not the case, as

demonstrated by the metre-scale packaging of identical facies in the Belt cycles. Decametre-

scale cycles in the Victor Bay Formation may indeed be one of the fint indicaton of a

Mesoproterozoic glacial episode. The period of the global icehouse-greenhow supercycle

has been estimated at 0.4 Ga. based on the Phanerozoic and late Neoproterozoic record

(Veevers, 1990). When this penodicity is extrapolated Merinto the Proterozoic, the 1.2

Ga Bylot Supergroup falls into the same portion of the supercycle as present-day Earth, a

short-tenn icehouse within a greenhouse phase (Veevers, 1990). Altematively, if the

frequency of global icehouse-greenhouse cycles has changed over tirne, it is also possible that Chapter 3-CycIicaIly-packaged carbonate ramp late Mesoproterozoic Victor Bay rocks record the onset of a global icehouse that persisted into the late Neoproterozoic.

CONCLUSiONS

I) Three types of decametre-scale shallowing-upward cycle typify the Victor Bay ramp succession: peritidal, shallow-subtidal and deep-subtidal. Cycles are upward-shallowing and distinguished on the basis of capping facies: (a) peritidal cycles indicating shoaling to imer-ramp conditions; (b) shallow-subtidal cycles representing sedimentation close to fair- weather wave base, therefore straddling the boundary between the imer ramp and the mid- camp; and (c) deep-subtidal cycles accumulating on the outer rarnp, entirely below storm- wave base. Al1 cycles are characterized by thin transgressive bases and thicker aggradational to progradational highstand caps, especially in the peritidal and shallow-subtidal cycles.

Individual cycles can be followed laterally from shallow to deeper water equivalents, forming regionally distinct parasequences with continuity over several kilometres perpendicular to paleoslope.

2) Parasequences are assigned to systems tracts that define three hectometre-scale

sequences. The steep mid-rarnp was a locus of mass-wasting and a nucleation site for

stromatolite reef growth during deposition of the fint sequence. Development of thick

biostromal buildups in this portion of the ramp was coeval with deposition of pentidal cycles

on the gently sloping inner-rarnp. Sea-Ievei fa11 early during the second sequence led to the

development of a lowstand wedge outboard of the mid-rarnp flexure. The third sequence

records backstepping and drowning of the ramp in a thick ûansgressive systems tract.

3) Lateral continuity of cycles is consistent with an allogenic rnechanism of sea-level

change. Decarnetre-scale packaging and broad range of depositional environrnents within

cycles suggest high-magnitude / high-fiequency fluctuations which are distinctive of glacio-

eustasy. Victor Bay ramp carbonates therefore provide Werevidence of a late

Mesoproterozoic icehouse world. Chapter ,tycIicaIiy-puckaged carbonate ramp

ACKNOWLEDGEMENTS

This project was supported fuiancially through Natural Science and Engineering

Research Comcil of Canada (NSERC) gants to GM. Narbonne and N.P. James, a GSA

Research Grant and a Queen's Doctoral Travel Grant to A.G. Sherman, as well as Northem

Studies Training Program grants to A.G. Sherman and J.E.M.Harrington. Logistical support fiom the Polar Continental Shelf Project was essential to the success of this research (PCSP contribution No. 02200), as was assistance fiorn Breakwater Resources Ltd. / Nanisivik

Mines. J.F. Read, M.E. Tucker, and J.J. Lukasik are gratefblly acknowledged for their helphil and constructive reviews of the manuscript. J.E.M.Harrington is thanked for his assistance in the field during the summers of 1994 and 1995. CHAPTER 4:

TECTONIC INVERSION AND CARBONATE SEDIMENTATION:

MESOPROTEROZOIC BORDEN BASIN, BAFFIN ISLAND, ARCTIC CANADA'

ABSTRACT

Distribution of facies in the lower half of the Bylot Supergroup of Borden Basin suggests ovemll deepening to the West of this Mesoproterozoic aulacogen. In marked contnist, the upper half of the succession records a reversa1 in the overall deepening trend, such that the eastem portion undenvent a relative deepening as the West experienced a

relative shallowing. Strata deposited during the reversa1 belong to the Victor Bay Formation,

a rarnp composed predominantly of limestone. Kanting of the carbonates and development

of an angular unconfonnity in the west contrast with backstepping and drowning of the ramp

in the east, followed by rnantiing by deep-water limestone, carbonaceous carbonate, and

turbidites. Increased accommodation space during biis time, through both giacio-eustatic sea-

level rise and tectonic subsidence, ied to a profusion of stromatolite pinnacle reefs and large

biostromes. This bathymetric inversion is best reconciled with development of a foreIand

basin superimposed on the aulacogen. The inverted sequence is consistent with cnistal

rethickening and uplift dong reactivated basement faults. A compressional event at this time

in the Bylot Supergroup would be roughly coincident and consistent vergence with two phases

of post-Racklan defonnation in the northwestem Arctic.

' From: Sherman et d. (in review). Chapter GTectonic inversion and carbonate sedimentution

INTRODUCTION

The tectonic regime during the mid- to late Proterozoic along the northem margin of

Laurentia is poorly understood, in large part because key structural and stratigraphic evidence has been removed or is hidden under Paleozoic basins. The large-scale extensional history of the area is evident in the 1.27 Ga Mackenzie igneous event (LeCheminant and Heaman,

1989, 199 l), but late Mesoproterozoic sedimentary basins that were created during this rifting event record a subsequent history not entirely explained by a fuither mstal extension.

This is evident in the most continuous sedimentary record of the region, the Bylot

Supergroup, which records initial nfting, a passive margin phase, and a second tectonic stage

(Jackson and Iannelli, 1981, 1989) when the rift-to-drift pattern was disrupted and the basin morphology fundamentally reanruiged. Deposition of a carbonate ramp, the Victor Bay

Formation, spa~edthis cntical time period. During this time, the ramp underwent local exposure and coeval drowning, and hosted the acme of reef development in the ByIot

Supergroup.

The purpose of this paper is to offer an explanation for the regional facies trends in the Victor Bay Formation consistent with the sedimentological and tectonic evidence. Strata correlated across the Borden Basin record ramp progradation into the basin, followed by relative sea-level rise and growth of stromatolite pinnacle reefs, and backstepping of the carbonate facies. A divergence of sea-level histories is tagged to a tectonic event that drowned the eastem part of the basin and uplifted the western potion. Western strata expenenced a relative sea-level fall, whereas sea level rose in the east. The existing aulacogen mode1 (Olson, 1977, 1984; Jackson and Iannelli, 1981) is not compatible with ali observations, including the Iocalization and timing of uplift, which are better reconciled with alternative scenarios, such as basement reactivation within a foreland basin. In either case, proper interpretation of the Victor Bay Formation is crucial to identifLing the transition between passive and active tectonic phases and establishing its timing, which we propose began during Iate UIuksan time, earlier than suggested by others (Jackson and Iannelli, 1989), Chapter GTectonic inversion adcarbonate sedimentation and associated with a late Mesoproterozoic orogenic event in northwestem Laurentia. We interpret a basin history that integrates new and previous observations, with the intention of clarifying the tectonostratigraphic evolution and its paleocontinental implications along the northern margin of Laurentia during this tirne.

Geological Background

Regional tectonic setting.-

Rifting of Rae Province crystalline rocks at the northem margin of Laurentia in the late Mesoproterozoic created a series of basins in the eastern Arctic Archipelago of Canada and northwestem Greenland collectively referred to as the Bylot basins (Fahrig et al., 198 1).

Correlated on the basis of lithostratigraphy (Jackson and Iannelli, 1981) and

chemostratigraphy (Kah et al., 1999), the Borden, Fury and Hecla, Aston-Hunting, Thule

and Independence Fiord basins (Fig. 4.1) are interpreted as either separate contemporaneous

aulacogens (Olson, 1977, 1984; Jackson and lannelli, 198 1: Fahrig, 1987) or the now-

isolated remnants of a larger basin (Jackson and lannelli, 1981). The Borden Basin contains

the thickest stratigraphie succession, the 4 km-thick Bylot Supergroup which occupies moa

of Borden Peninsula of northern Baffin Island and is preserved in three grabens separated by

horsts of crystalline basement. They are the North Bylot Trough, Eclipse Trough, and, the

largest of the three, the Milne InIet Trough, where most of the Victor Bay Formation crops

out (Fig. 4.2).

Stratigraphy, age, and timing.-

The siliciclastic, carbonate, and volcanic rocks of the Bylot Supergroup

nonconformably overlie basement gneisses of Archean to Paleoproterozoic age and are

unconformably overlain by lower Paleomic siliciclastic and carbonate rocks of the Admiralty

Group (Fig. 4.3). Mesoproterozoic syn-sedimentary extensional faulting was recognized by

Jackson and IanneIIi (Jackson et al., 1978, 1980). Some post-depositional folding occurred

on northem Bylot Island, but the rocks remain largely unmetamorphosed. Chapter ATectonic inversion and carbonate sedimentarion

Fig. 4.1 - Distribution of the Bylot bmim in the Arctic Archipelago and Greenland. Modifiedfiom Trenui (1989). The Borden Barin (outlined area) ir enfarged in Figure 4.2. Chupter GTectonic inversion cvui carbonate sedirnentation

LEGEND A Fautt FAULT ZONES CHFL CAPE HAY BP BROONR PENINSULA 0lce Fautt zone Downthrown hFZ AKTINEQ O WIN INW Phanerozofc % bhk HMFL MO UN TAIN 88 BAIUGEBAY a STRATHCONASOUND Vf~t0rBay Formation TFZ TIKERAKLUUAK AS ADAMS =UND Reef locality: Mesoproterozoic UlTLiTIKEWKDJUAK FF FABRICIUS F~ORD seâimentary rocks @ Stathcona River KOLU-W TS (undMded) TREMBLAY SOUND @ mngo ~alley AIRMAGDA (Crystalline basement @ Bay CBn CEJaRAlBORDEN

Fig. 4.2 - Geological map of the Borden Peninsuia und Bylot Island, showing disiribution of the Victor Bay Formation und major siructurai elements of the Borden Basin, including fdt zones, grabens (troughr). and horsts (highs). Extent of outcrop of the in Borden Basin. Most of the outcrop of the Victor Bay Formation occurs in the Milne Inler Trough, the largest of the three major îroughs. Three mojor reef localities are indicated. The boxed area tk enlarged in Figure 4.5. A* Jackson and Iannelli (1981) and Jackson and Sangster (198 7). Chupter 4-Tectonic inversion and carbonate sedimentation

The Bylot Supergroup is subdivided into a lower volcanic and siliciclastic Eqalulik

Group, a middle, predominantly carbonate Uluksan Group, and an upper, largely siliciclastic

Nunatsiaq Group (Jackson and Iannelli, 198 1). The Eqalulik Group consists of plateau basalts and interlayered marginal marine sandstones of the basal Nauyat Formation, overlain by alluvial to marginal marine sandstones and siltstones of the Adams Sound Formation, and topped by marine shales and minor carbonates of the Arctic Bay Formation (Jackson and

Iannelli, 1981). The üiuksan Group consists of largely peritidal dolostone (Society Cliffs

Formation) and laterally equivalent continental alluvial conglomerate (Fabricius Fiord

Formation) that are overlain by marine shale, limestone and dolostone of the Victor Bay

Formation. The lower Nunatsiaq Group consists of dark lirny shale and calciturbidites of the deep-water marine Athole Point Formation that are overlain by red and green basinal shale, submarine fan sandstone and siltstone, minor marine carbonate, and alluvial fan conglomerate of the variable and complex Strathcona Sound Formation. The upper Nunatsiaq Group consists of the Aqigilik and Sinasiuvik formations that together make up the Elwin Subgroup.

These units are fluvial to marginal marine sandstone and siltstone. Al1 formations are presumed to have been deposited regionally, with three exceptions: (a) Nauyat basalts do not occur in southeastern Milne Inlet Trough, where the Adams Sound Formation overlies basement; (b) the arkosic congiomerates of the Fabricius Fiord Formation occur only in southem Milne Inlet Trough and pass to the north into dolostones of the Society Cliffs

Formation; and (c) shales and carbonates of the Athole Point Formation were deposited only

in the eastern portion of Milne Inlet Trough. Elsewhere, the siliciclastic Strathcona Sound

Formation directly overlies the Victor Bay Formation.

Geochronological studies of cross-cutting Franklin dykes provide a minimum age of

0.72 Ga (U-Pb on baddeleyite; Pehrsson and Buchan, 1999). The maximum age of 1.267 Ga

(U-Pb on baddeleyite; LeCheminant and Heaman, 1989) was obtained fiom Mackenzie

igneous rocks correIated geochemically and paleomagnetically to Nauyat basalts. Chapter ATectonic inversion und carbonate sedimentation

Stratigraphie and paleomagnetic data imply that the entire succession is close to the maximum age (see discussion in Knight and Jackson, 1994). Pb-Pb isotopic analysis of the underiying Society ClifYs Formation provides a late Mesoproterozoic mode1 age of 1198 *24

Ma (Kah in Samuekson et al., 1999).

Most formations were defined during the fint regional mapping program by Blackadar

(1956, 1970) and Lemon and Blackadar (1963) and placed within the Eqalulik and Uluksan

Groups. Jackson and Iannelli (1 98 1) established the Bylot Supergroup and subdivided the

Uluksan Group by placing the upper siliciclastic succession into the new Nunatsiaq Group.

They dehed the Fabricius Fiord Formation (Jackson et al., 1978) and partitioned al1 formations into members (Jackson et al., 1978; Iannelli, 1979; Jackson et al., 1980; Jackson

-m - K

Fig. 4.3 - Stratigraphy of Bylot Supergroup in the Milne Inlet Trough, modified frum Hofmann und Jackson (1991) and Iunneili (1992). me enkarged portion of the stratigraphic column is a schematic representation of the upper (nuhan and lower Nunatsiaq groups dong the NW-SE mrLs of the trough. Chapter Q-Tectunic inversion and carbonate sedimentation and Iannelli, 1981). In an unpublished thesis, Iannelli (1 992) detailed the sedimentology and stratigraphy of the Bylot Supergroup, proposed new reference sections for al1 major units, and formulated a detailed stnictwal interpretation for the evolution of the Borden Basin.

Knight and Jackson (1994) elevated the ELwin Formation to subgroup statu, raised its two members to formation grade, and identified several new membea.

The tectonostratigraphic evolution of the basin was fint addressed by Geldsetzer

(1973) who recognized the influence of uplift and subsidence on Borden sedimentation and discussed it in terms of epeirogenic movements. Jackson and Iannelli (1 98 1) linked the pattern of sedimentation to synsedimentary extensional tectonics, and described cyclic packaging of units. They also proposed the current mode1 of four tectonostratigraphic stages

(Jackson and Iannelli, 1989).

(1) Continental tholeiites and siliciclastic rocks derived from basement blocks were

deposited during regional extension, themal uplift, and cmstal thiming, which was followed

by extensive block faulting, basin-wide foundering, and deposition of thick basinal shales of

the Arctic Bay Formation.

(2) The tectonic quiescence that followed led to SE to NW progradation of a

carbonate platfonn-passive margin sequence of Society Cliffs Formation carbonates with

coarse siliciclastic alluvial-fan deposition at the margin of Bat-order grabens as the Fabricius

Fiord Formation. A smaller-scale subsidence event was localized to the Milne Inlet Trough

and is recorded as lower Victor Bay member shales. A remto passive margin conditions

resulted in deposition of the upper carbonate member of the Victor Bay Formation, albeit

under overall deeper-water conditions.

(3) Throughout stages 1 and 2, the basin deepened to the NW, but this changed during

deposition of the basal Nunatsiaq Group as it underwent extensive block faulting. During this

time, drowning of eastm Milne Inlet Trough coincided with uplift of its western half with

deposition of siliciclastic conglomerate and development of an angular unconformity. Chopter 4-Tectonic inversion and carbonate sedimentation

(4) Post-rift siliciclastic fluvio-marine deposition followed, resulting in deposition of the upper Nunatsiaq Elwin Subgroup. The Mesoproterozoic sedimentation record ends where the Elwin Subgroup is truncated at the sub-Paleozoic unconformity.

Evidence of Stages 1 and 2 rift-to-drift history includes the pattern of narrow, parallel principal grabens bounded by listric faults zones, synsedirnentazy faulting throughout deposition of the Bylot Supergroup, and development of Fabricius Fiord alluvial fans adjacent to major fault zones (Iannelli, 1979; Jackson et al., 1978, 1980; Jackson and Iannelli, 198 1).

The sequence of coarse siliciclastics, shales, and carbonates is typical of the transition from uplift and highland erosion to subsidence and marine incursion.

In contrast, the Uluksan-lower Nunatsiaq transition in Milne Inlet Trough (Stages

34) is problematic in a strict rift-reactivation regime. Jackson and Iannelli (1 989) and

Iannelli (1992)propose a second rifting phase that caused foundering of eastem Milne Inlet

Trough. Active instead of passive rifting (Turcotte, 1983; Turcotte and Emermann, 1983)

was interpreted as the mechanism for uplift in the West and simultaneous subsidence in the

est. Active rifting does not explain, however, why crustal movements were not

accommodated along the structural grain defined by NW-SE-trending normal faults created

during Stage 1.

The alternative hypothesis herein explored is that of differential subsidence and uplift

caused by compressive stresses acting on a complex rifted margin. Sedirnentological and

stratigraphie evidence fiom the Victor Bay, Athole Point, and Strathcona Sound formations

indicates that the fundamental structural changes affecting the Borden Basin were already

being recorded in the Uluksan carbonate succession.

METHODS

Lithostratigraphic interpretation and regional correlation of the Victor Bay

Formation and adjacent strata are based on 22 measured sections at eight localities. These

data were supplemented by aerial sunreys that allowed interpolation of strata beh~een

sections. Shatigraphic data and trends from Geldsetzer (1973). Jackson and Iannelli (1981), Chapter 4-Tectonic inversion and carbonate sedimentation

Knight and Jackson (1994), and Narbo~eand James (1996) are also utilized to arrive at a comprehensive interpretation.

STRATIGRAPKY AND CORRELATIONS OF VICTOR BAY STRATA

Distribution and Regional Trends

Depositional trends in the Uluksan Group and lower Nunatsiaq Group reflect a radical shift in provenance, direction of sedirnent transport, and depositional environments in the

Borden Basin over time. Victor Bay strata are thickest in the Milne Inlet Trough where they occur on both limbs of a broad syncline plunging gently to the W.In Eclipse and North

Bylot tmughs it is more difficult to distinguish Victor Bay strata from underlying Society

Cliffs dolostones as (1) the Victor Bay Formation is dolomitized and thus resembles the

Society Cliffs Formation and (2) the shaly lower Victor Bay member, which elsewhere facilitates recognition of the contact, is absent. Distribution of Society Cliffs and Victor Bay lithofacies suggests a zone of deep water in the northwest and centre of Milne Inlet Trough, with shallowing southeast and toward the northeast margin of the trough (Jackson and

Iannelli, 198 1; Iannelli, 1992; Kah, 1997). The overall geometry of the upper Victor Bay carbonate platfonn is a ramp @kirbonne and lames, 1996; Sherman et al., 2000) deepening both to the northwest along the axis of the trough and southwest toward the centre of the trough (Jackson and Iannelli, 1981; Iannelli, 1992; Narbonne and James, 1996; Sherman et al., 2000).

Victor Bay Facies and Puckuging

Facies represent outer ramp to inner ramp environments (Sherman et al., 2000).

Outer-ramp facies are ribbon limestone, nodular Lirnestone, and carbonaceous carbonate, al1 exhibithg slumps and resedimentation features. Mid-ramp tempestites are lime mudstone with molar-tooth structure and interbeds of graded calcarenite cornposed of molar-tooth microspar fragments. Imer-ramp facies are rnolar-tooth calcarenite, larninated rnicrobial dolomudstone, and evaporite-bearing red shaie. Carbonate mud was the dominant Chapter GTectonic inversion and carbonate sedimentation sedimentary component on this low-energy rarnp, where organically mediated water-column precipitation was the principal source of sediment. Facies on this storm-dominated, microtidal, distally steepened ramp are packaged in 20-50 m parasequences, interpreted as glacio-eustatic fourth-order depositional units formed during high-magnitude, high-fiequency sea-level fluctuations (Sherman et al., 2000, 2001). Third-order packaging delimited by erosion and karst surfaces is likely tectonic in origin (Narbonne and James, 1996; Sherman et al., 2001).

Victor Bay Reefs

SrnaIl biohems of columnar stromatolites occur in most formations of the Bylot

Supergroup (Jackson and Iannelli, 1989). The Society Cliffs Formation in particular is nch in microbial textures in tidal-flat and shallow-subtidal facies (Kah and KnofI, 1996; Kah, 1997).

Extensive development of large stromatolitic reefs, however, is limited to the Victor Bay

Formation and specifically to strata preserved in the Milne Inlet Trough. The most spectacular exposures occur at opposite ends of the troua: (1) in the NW at Strathcona

River where exhumed reefs dominate the landscape (Fig. 4.4A, B), and (2) at White Bay in the SE, where reefs are stacked to a combined thickness of 270 m (Jackson and Iannelli,

1989) and exposed on a near-vertical cliff (Fig. 4.4C). The intemal structure of Victor Bay reefs is one of altemating layes of cm-width columnar forms and 1-10 m wide domal forms.

Laminae in columnar forms are either convex or conical. "Cryptodome" fabric-poorly

Iaminated microbial sheets-formed during the final reef stage at Strathcona River (Narbonne

and lames, 1996). Influence of reefs on the hydrodynamic regime of the inner ramp appears

limite& For instance, at Pingo Valley, peritidal cycles coeval to the lower reef do not show

evidence of decreased wave energy compared to cycles older than the phase of reef

development. Reefs also form a semi-continuous tract of large biostromes for at lest 40 km

along the northem margin of Milne Inlet Trough (Fig. 4.5A). The fint phase of biostromal

reefs defines the tract, and developed during the fint transgressive sequence. The second

phase of pinnacle-shaped reefs roughly coincides with outcrop of the lower reef tract (Fig. Gutpter 4-Tectonic invem'on and carbonate sedimentation

Fig. 4.4 - Reef development within the Mine Inlet Trough. A) First reefol phase at Strathcona River overlyng calearenites. A second reefal phase equivalent to reefS in (B) lies direct& on top of the lower reefi. Field of view approximately 300 m. B) Second reefl phase ut Stmthcona Riwr. Deep-water off-reef carbonates ococpy the foreground Field of vîew opproximatel) 500 m. C) Re@ at Whire Bay overlain by linry shales of the Athole Point Forntation. Field of view approx~~muteiyI km. Chapter GTectonic inversion and carbonate sedirnentation

Fig. 4.5 - A) Map of eastem Milne Inlet Trough, showing reef distribution and infirred ment of the reef tract. B) Enlargement of baxed mea in (A) showing the overlap of the two generations of reefs in the Pingc+Muia area. Chapter GTectonic Ulversion and carbonate sedimentation

4.5B). Bot. reef phases are limestone at Strathcona River in the west (Narbonne and James,

1996). only the upper phase is limestone at Pingo Valley in the centre of Milne Inlet

Trough, and both are dolostone at White Bay in the est. Reefs are overlain by Athole Point

Formation in the SE and by Strathcona Sound Formation in the NW (Fig. 4.3).

Correlations

Correlation at the scale of 20-50 m cycles takes into account regional change in facies composition within cycles. This has been facilitated by excellent exposure at critical localities, such as the inner-to-mid-nunp transition at Mala River-Pingo Valley (Fig. 4.6). where cliff-face exposures permit the observation of peritidal facies thinning and disappearing basinward (Sherman et al., 2001), and the mid-to-outer ramp transition along the west shore of Tremblay Sound (Fig. 4.7). In addition to sections measured for this study

(Fig. 4.8), regional correlations are facilitated by stratigraphie, structural, and chemostratigraphic data fiom previous workea (Lemon and Blackadar, 1963; Jackson and

Iannelli, 1981; Iannelli, 1992; Knight and Jackson, 1994; Narbonne and James, 1996; Kah,

1997; Kah et al., 1999).

An axial section through the northern margin of the Milne Inlet Trough (Fig. 4.9) highlights the northwestward thinning of both upper and lower Victor Bay membea. The line of section veers slightly oblique to depositional süike, accentuating this effect in the westernmost portion. Similarly, both members thin to the southwest (Fig. 4.10, 4.1 1).

Where observe& the transition from mid-ramp to outer ramp is everywhere abrupt, occming outboard of large reefs, and associated with slope failure and massive debntes. It is a classic distally steepened ramp (cf. Read, 1982, 1985; Burchette and Wright, 1992).

Outer-ramp facies also occur within the upper member of the Victor Bay Formation, where they form the base of inner and mid-ramp shallowing-upward cycles. Deep-water ribbon and nodular facies mark the contact with the Athole Point Formation, Growth of pinnacle reefs and backstepping of shallow-water facies toward the northeastern margin of the Milne Idet Trough also occurs in this part of the succession. This transition occurs only VICTOR BAY FORMA1'ION

sub-reefal olistostrome

VICTOR BAY FORMAT 'ION

SOClEPI CLlFFS FM.

Fig. 4.6 - Overfapping oblique air photographs of the MalePingo area outlined in Figure 4SB, showing outline of the Yictor Bay Formation and the location of reefs at the contact with the overlying Athole Point Formation. Location of meawred sections are indicoted by arrows. A) kwto the southwest, with Pingo Valley in the foreground. Cycles underiying the reef me characterized by grey-weuthering molor-tooth Iimestone and calcurenite. Foreground approximateb 3.5 km wide. NAPL photograph T31 9R-28 reproduced with pemission of the Nationul Air Photo Library of Canada. B) Yiew to the southwest, along the Mafa River valley. Be pale-weathering strata on the right side of the photograph ore tidal-Put dololminites that are lunhuard equivalents to the subtidal sub-reef sîrata in (A). Foreground approximately 3.5 km wide. NAPL photograph T319R-26 reproduced with permission of the Nation01 Air Photo Librmy of Canada. Chapter 4-Tectonic inversion und carbonate sedimen tation

VlCrOR BAY FORMATION

Fig. 4.7 - Oblique air photogroph of Victor Bay and a@acent sshta on the shores of Tremblay Sound, showing shallow-wafer carbonates und reefS at the Victor Bay-Athole Point contact on the north side of the syncline, passing into exclusively deep-water facies where it reappears on the southern limb of the fold. TS = Tremblay Sound section; AP = A fjied Point section. NAPL photograph T344L-IBO reproduced with permission of the National Air Photo Library of Canada. in the eastern basin, as the Athole Point peters out to the West and passes into the lower Strathcona Sound Formation. The conformable contact just east of Strathcona River is replaced by an angular unconfomity between the Victor Bay and Strathcona Sound formations.

HISTORY AND PALEOGEOGRAPHY DURING ULUKSAN-NUNATSIAQ TIME

Previous work by Jackson and Imelli (198 1) provided paleogeographic interpretations of the Borden Basin encompassing deposition of the entire Bylot Supergroup.

Here, given new observations and facies analysis of upper Victor Bay strata, a finer-scale picture of the evolving basin paleogeography at the Uluksan-Nunatsiaq transition has ernerged, aided in large part by the clues imparted by the location and degree of reef development in the upper member (Table 4.1). Signs of incipient drowning are expressed in

Victor Bay facies prior to Nunatsiaq deposition, indicating that tectonic activity had already increased tiom early Uluksan levels.

Tl) Late Sociep Cltifs Formation and early lower Victor Bay rnember deposition

The Society Cliffs Formation ranges from 260 m thick in the western portion of the basin and thickens to 856 m in the SE (Jackson and IanneIli, 1981). In the westernmost parts of the basin, the Society Cliffs Formation consists of parallel-laminated dolostone, Chapter 4-Temnic inversion and carbonate sedimentation

Table 4.1 - Smmarired history of the Victor Bay and Athole Point formations in the Milne Inlet Trough, drawn hmthe present stuc& and previous work (Jackson and lannelli, 1981; Jackson and lannelli, 1989; lannelli. 1992; Narbonne and James, 1996). Abbreviations: SC = Socieîy Cltfls Fm., u VB = upper Victor Bay member, /YB = lower Victor Bay member, AP = Athole Point Fm,, SS = Strathcona Sound Fm.

STRATIGRAPHIC INTERPRETATION UNITS

Upper SC and basai 1VB Upper SC shallow-water carbonate platform abruptiy drowned and covered by transgressive basinai shales of the Iower VB member. ûrowning of SC ramp areally restricted owing to tectonic subsidence limited to to the Milne Inlet Trough. Distibution of facies in SC and VB indicates that ramp sloped W-NW.

Uppenost LW: Upward increase in carbonate content within the lower VB member Sequence 1 (S la interpreted as progradation of upper member carbonates to the TST) NWand toward the axis of the basin. Shaliow-water carbonate deposition continued unintempted hmSC to VB the in Eclipse and North Bylot troughs.

Basal WB: Sequence 1 Progradation of shailow-water ramp carbonates into central Miine (S 1a earl y HST) Met Tmugh onto lower mernber basinal shales originates from N and E. Mas-wasting interpreted as a combination of high carbonate production and steep antecedent topography, possibly aggravated by existing fault scarp. Regionai seismicity also suspecte& as distribution of Smthcona River reefs are adjacent to major fault zones.

Middle uVB: Sequence Mid-ramp molar-tooth mudstones and grainstones at Pingo Valley 1 (S 1b late HST) slump, creating accommodation space for two pulses of stromatolite reef growth. Uppennost uVB: Reef deposition terminateci during significant relative sea-Ievel fdl. Sequence 2 (S2) Karsting and deposition of quartzose sand above the first phase of Strathcona River reefs coincides with non-depositionlerosion (inboard) and development of a lowstand wedge (outboard) at Pingo Valley. Progradation of this lowstand wedge is reflected in an increased number of slumped hemipelagic units on the outer ramp at Adams River.

Lower AP: Sequence 3 Thick debrites and development of pinnacle mfs at UV&-APcontact (S3) suggest slope instability and rapid increase in accommodation space. Deepening indicated by draping by deep-water AP carbonate facies and SW to NE backstepping dong PingwMala traosect and at Strathcona River.

T7 West: AP Eastern portion of the basin subsides. Uplift occurs in the west, and East: SS the Strathcona River ceefs are exposed. Bypass results in deposition of quartzose caIciturtiidites in the West Erosional unconformity at uppermost VB covered by marine to fluvial sihstones of the SS Fm. Siliciclastic system rocks progrades east and overiies AP turbidites. Chapter GTectonic inversian and carbonate sedirnentation

Fig. 4.8 -Mop of section locations. Correlation along the ais of Milne Inlet Tmugh (A-A 3 i&represented in Figure 4.9. Two inner- tu outer-ramp correlations perpendicular tu depositional strike are illustrated in Figure 4. IO WB3 and Figure 4.11 (CC3. A) Yiew along mis of Milne Inlet Trough, roughly parollel to depositional striùe of the Victor Bay Formation. B) Tramerse view. whereas dolomitized stmmatolites, oolites, mudnacked laminated dolomudstones, redbeds, and gypsum-bearing shdes, dominate in the SE. The upper daceis undulose and is overlain by the lower Victor Bay member which is composed of pyritiferous and dolornitic shales, dolosiltite, and fine-grained turbiditdevent beds. The lower member is -200 m thick in the

NW, attains 250+ m in central MIT, and thins to <30 m in the SE. The upper Society Cliffs Formation is interpreted as a shallow-water carbonate platfonn which was abruptiy drowned and covered by transgressive basinal shales of the lower r Q pdvmicliccongiomerate doloiaminWcryptomimbial laminites (T) molar-tooth limestone Q molar-tooth cairnnite nbbon and nodular linestone debrite f="b""sw"b"te @A mlumnar stiwnatolbs 0Dbomatoüücreef quarime dolosbne p" @ dobsiioas i Q shaIy-s~mdoiostone STRATHCONA SOUND FORMATION @ lirneslone, undifferentiated dolosbne, undifiecsntiated shale O sandstone siitstone - @QIu~FJ 5 km

/ UPPER 'ACTOP 5.i.Y MEMBER

LOWER VICTOR BAY , , MEMBER

SOCIE3Y CUEFS FORMATION ARCnC BAY

Fig. 4.9 - SE-NW cross-section along the mis of Milne Inlet Tmugh. Sections at Arrtic Bay and White Bay from Jackson and Ianneili (1981). and a? Strathcona River fiom Narbonne and Jmes (1996). ATHOLE POlNT FORMATION

i r.

MALA RIVER CAMP PROZAC - . ALFRED POINT . - WHKE BAY

Chapter GTeaonic inversion and carbanute sedkntatton

STRATHCONA SOUND FORMATlON

.' /

ATHOLE POINT FORMATION

'~unpfdds

UPPER MCTOR BAY MEMBEU

LOWVlCTOR BAY MEMBER

/DAMS SOCIETY CURS FORMATION PINGO PINGO hM.4 RIVER VALLEY VU RlVER

F& 4.10 - NE-SW cross-section along the Mala-Pingo-Adams trmect, showing southwestwtmd transition fiom inner- rmnp pen'tidai struta to mid-romp bioherms and ultimute& into outer-ramp hemipeIagic rock. See iegend in Figure 4.9. ATHOLE POINT FORMAT1ON

BAY MEMBER &l

LOWER VICTOR BAY MEMBER 5 km

SOCIETY CLIFFS FORWON

TREMBLAY SOUND ALFRED POINT C c'

Fig. 4.11 - 2VE-SW cross-section along the AljTe6Trem6[ay hamect, iIlwtrating ccevul inner rmnp and outer ranp facies. See legend in Figure 4.9. Victor Bay member. Absence of the shale mmber in Eciipse and North Bylot Troughs to the north suggests that drownhg of the Society ClBs ramp was restricted to the Milne Inlet

Trou*. Jackson and Iannelli (1981) and lannelli (1992) interpreted this as reflecthg tectonic subsidence limited to the principal graben. Inmeased abundance of shallow-water facies, thickening of the Society Cliffs Formation, and thinning of basinal Iower VB member imply that the rarnp sloped to the W-W.

T2) Late Iower Victor Bay member deposition: Sequence I (Na, TSlJ Fine-graiaed sediments become more calcitic upward in the lower member of the Victor Bay Formation. Abundance, thickness, and grain size of carbonate beds inmeases, and nbbon and noddar limestones appea.. At Nanisivik, thick (> 15 m) polymictic debrites mark the base of the upper member. The upward increase in detrita1 carbonate content within the lower VB member is interpreted as the progradation of upper member carbonates to the NW and toward the axis of the basin. Shallow-water carbonate deposition continued unintempted fiom Society Cliffs to Victor Bay time in Eclipse and North Bylot Troughs. Depositional styles were similar in both troughs, although the Victor Bay Formation contains fewer stromatolites and more molar-tooth mudstone in the Milne Inlet Trough.

T3) Eariy upper Victor Bay member deposition: Sequence I (SIa, early HSV

Transition into the upper member of the Victor Bay Formation is recognized by the appearance of shailower-water facies. in the deeper parts of the basin, basinal shales and thin dolosiltite beds pass into thicker-bedded carbonate event beds, and nodular or nibon carbonates, In the north and east of the trou&, in the shallower sections of the ramp, these indude molar-tooth limestone, calcarenite, and mal1 bioherms of branching and non- branching columnar stromatolites. The upper member thickens fiom 130 rn in the northwest to 702 m in the southeast (Jackson and lannelli, 1981), and in central Milne Inlet Trough hm200 m to -600 m toward the northeastem margin of the trough (Fig. 4.10). Mass Chapter GTeeîonic invernon mtd carbonate seduneriliütanon wastïng of shallow-water strata occurs at the mid-rampiinner ramp transition at the Pingo

Valley locality and coevally at the mid- to outer-ramp boundary in slurnped deep-water ribbon

limestones at Strathcona River.

Based on thickening of upper member carbonates and increase in shallow-water facies

to the NE, progradation of shallow-water ramp carbonates into central Milne Inlet Trough

ont0 lower member basinal shales appears to have originated fiom north and east. Mass-

wasting may have been brought on by a combination of high carbonate production and steep

antecedent topography. Additionally or altematively, regional seismicity may have triggered

slope Eiilure at Pingo Valley. The relationship of faults to slurnped Strathcona River rocks is

not clear; nonetheless the reef complex is immediately adjacent to the major Baillarge Bay

normal fault zone and is located just north of the Strathcona River Fault zone (Iannelli,

1992).

T4) Middle Victor Bay upper member deposition: Sequence 2 (Rb. lote HST)

Deposition of mid-rarnp grainstones and molar-tooth mudstones at Pingo Valley was

followed by development of slurnps and olistostromes, imrnediately followed by two pulses of

stromatolite accumulation at the mid-ramp: a 166 m dolomitic biostrome with a 9 m

calcareous dolostone to limestone cap. nie cap is conelated with the first reefal phase at

Strathcona River (Fig. 4.12), which at 25 rn is a much thicker accumulation (Narbonne and

James, 1996). Further east at Alfred Point, on the proximal portion of the ramp, a single

phase of reef growth resulted in a thin dolomitic biostmne.

The interpretation of a western-sloping ramp at this time (Fig. 4.13A) is mpported

by (a) thickening of the fim reefal pulse to the West, presurnably as a response to greater accommodation space downslope, and (b) transition nom dolomitic to calcareous mineralogy

of the nefs westward, toward the head of the Milne Inlet graben. Increased salinity through

evaporation in the shallow waters would more likely cause reflux dolomitization (cf. Soreghan

et al., 2000) than in deeper parts of the ramp. Chapter 4-Tectonic UNem'on and carbonate sedimentarion

T5) Late Victor Bay upper member deposition: Sequence 2 (S2)

Reef deposition was terminated during a relative sea-level fall. A karst surface nsulting nom exposure and deposition of quartzose sand above the oblate Strathcona River reefs coincided with non-deposition/erosion (inboard) and developrnent of a lowstand wedge

(outboard) at Pingo Valley. Progradation of this lowstand wedge is reflected in an increased number of slumped hemipelagic mata on the outer ramp at Adams River.

T6) Lower Arhole Point Formation deposition: Sequence 3 (S3)

Subsequent transgression at Pingo Valley resulted in deposition of a second reef phase characterized by pinnacle-shaped bioherrns growing on thick polymictic debntes. lnboard of

Fig. 4.12 - Correlation of sequence stratigraphie ewnts that shaped reef deveiopment at Snothcona River and Pingo ValleyeyNote that the cafcureous Athole Point Formation overlies the Victor Bay Formarion at Pingo Valley wherear the siliciclastic Strathcona Sound Formation lies above the same mit at Strathcona River. Chapter GTectonic Ùwem'on ami carbonate sedintentatron the reefs (e.g. Section 5, Camp Prozac), lenticular debrite layen accumulated to a thichess of 60 m. A high rate of reef accretion is interpreted, based on massive reef blocks of Conophyton stromatolites spalled hmthe nef at Alfred Point and overly deep-water carbonaceous shale, caicareous shale, and calcsiltite. Backstepping fkom southwest to

A Myto mid-Victor Bay time

Late Victor Bay to Athole Point time

Fig. 4.13 - Cartoons depicting the proposed tectonic evohtion of Milne Inlet Trough during deposition of the Victor Bay (YB) and Athole Point (AP) formations. The cross-section alung the Milne Inlet Trough, roughly corresponds to A-A' in Figure 4.8. Refrence localities me Strathcona River (SR), Mala Rber (Mt), and Whire Bay 0.A) The ktor Bay ramp dopes tu the West as it did throughuut most of the deposition of the upper member. B) Onset of cornpression directed easîwurd along the mis of the busin creates upl@ in the west and leab to bathymetric inversion in Milne Inlet Trough. northeast is observed dong the PingeMala transect and at Strathcona River, where reef development was greatest.

T7) Athole Point Formation deposition (B?l and Strathcona Sound Formation deposition (E) At this time, there is a major discontinuity in central Borden Basin east of the Strathcona River area. The eastem portion of the basin began to subside, and the depression was filled with deep-water Athole Point carbonaceous and calcareous limestones. Simultaneously, on the other side of the discontinuity, the large pinnacle reefs were exposed and karsted (Fig. 4.138). Poorly laminated microbialites accumulated in cestricted lagoons between the exposed reef tops. Tongues of reef-denved conglomerate and syndepositional microbialites were shed downslope to fom west-directed debrites (Narbonne and James,

1996). Quartzose calciturbidites in the eastern basinal succession are interpreted to be the product of erosion and sediment bypass as the Strathcona River area was uplifted.

Eventually, the erosional unconformity overlying the reefs was covered by marine to fluvial siltstones of the Strathcona Sound Formation. These siliciclastic rocks prograded into the eastern basin and overlie the Athole Point Formation.

DISCUSSION

Changing Nature of Basin During Victor Bay Time

A mode1 for basin evolution mut explain concomitant uplift and subsidence along the axis of the Milne Met graben, that led to simultaneous karsting of deep-water reefs (and adjacent strata) and drowning of an apparentiy "healthy" shallow-water carbonate ramp that up to that point showed repeated evidence of fuIL recovery fiom high-amplitude glacio- eustatic changes. It mut also address the possibility that chemical or bathymetric causes were responsible for a downhun in carbonate production pior to tectonic subsidence, thus making the Victor Bay ramp susceptible to drowning. Initial development of the Borden Basin follows the well-known pattern of continental rift-to-drift until late Uluksan-early Nunatsiaq time when basin evolution Chapter ATectonic inversion and carbonate sedimentm*on diverges from the expected path. The west-deepening trend that characterized the Mihe

Met Trough fiom early Eqalulik reversed in late Victor Bay the, when deeper-water environments were established in the east and the Athole Point Formation was deposited.

The SE then became the depocentre as the upper Victor Bay carbonate rarnp drowned.

Meanwhile uplift, karsting, and erosion of the ramp in the NW coincided with a transition from carbonate to siliciclastic sedirnentation. Backstepping of carbonates toward the northem trough rnargin occurred both at Strathcona River (Narbonne and lames, 1996) and at Pingo Valley-Mala River, suggesting that the basin axis remained deep throughout.

Reversai of the NW-SE bathymetric trend is interpreted as a tectonic event. Development of western-oriented and western-defiected cryptodome tongues at Strathcona River indicate that immediately West the reef complex the dope retained its original west-deepening geometry. Thus, the reef complex is situated on a portion of the ramp that was elevated after deposition of the S2 TST. On the east side of the complex, however, an erosional unconforrnity peters out immediately east of Strathcona River. The correlative surface is a confonnable contact between the Strathcona Sound Formation and the Athole Point

Formation.

Possible Basin Types and Their Diagnostic Feotures

Without the larger-scale structural context, relying on extensional indicators is

unreliable as extension can be generated within both zones of collision and zones of rift

(Bally and Snelson, 1980). Structural and stratigraphie complexities are added when tectonic

reactivation affects a previously rifted depocentre, creating a polyhistory basin (Kingston et

al., 1983). Clues to decipheMg basin history lie in the structural style of rifting and the

sediments themselves. There is no doubt that the Uluksan-Nunatsiaq transition represents a

tectonic event that restnictured the topopphy of the basin, and was not due solely to

eustatic sea-level change. Reconciling sequences identified at Pingo Valley with those

described fiom Stsathcona River (Narbonne and James, 1996) requires a change in tectonic

regime in late Victor Bay time. Initiaily, S La and S 1b recognized at Pingo Valley can be Chpter ATectonic imemon ami carbonate sedùnentatton respectively equated to the Subreefd and Reefal Sequence 1 at Strathcona River (Fig. 4.12).

Like the oblate biostrome at Strathcona River, the Slb reef grew to near sea level during early highstand and was therefore susceptible to karsting and dolomitization ddng late highstand.

Correlation between the two locations becomes more tenuous above this highstand phase.

The magnitude of relative sea-level rise appears to have been greater during transgression at the base of the Reefal Sequence 2 at Strathcona River (carbonaceous carbonate, backstepping reefs, pinnacle-shaped bioherms, etc.) (Fig. 4.12). In addition, the LST at the top of Reefal

Sequence 2 has no equivalent at Pingo Valley. A tectonically induced reversa1 of basin polarity during late Victor Bay time is proposed to account for this discrepancy, and this is supported by the expanded stratigraphie sequence in eastem Milne Inlet Trough where the

Athole Point Formation was deposited in deep water.

The early stages of Borden Basin development, as indicated by submarine to subaerial tholeiitic volcanic rocks at the base of the Eqalulik Group, are typical of a rift succession

(Bally and Snelson, 1980). The relatively minor separation (-90 km) between the northem and southem margins of the basin, however, suggests that, as in the other Bylot basins, little

(if any) oceanic crut was produced dong the axes of the major grabens (Veevers, 1981).

Continental neng cm be generated within several types of tectonic regimes, but the Bylot basins do not exhibit deformation amibuted to basins in convergence zones. Significant strike-slip/transform faulting has not been observed, thus they are not considered to be wrench basins. Nor is there any evidence that the Bylot basins developed over an older rift basin, in the way that the Paleozoic Michigan Basin developed through reactivation of the

Roterozoic Mid-continent rift of North Amerka (Klein and Hsui, 1987). The Michigan

Basin and other shallow intracratonic basins, however, subsided due to tectonic loading of the

lithosphere during migration of the Tacoaic foreland basin (Quinlan and Beaumont, 1984).

The transition âom shallow environments to deep-water is similar to the drowning of the

relatively shallow Victor Bay Formation and subsequnit deposition of deep-water Athole

Point facies. Jackson and Iannelli (1981) proposed creation and reactivaîion of an aulacogen on the basis of paleocurrents, faults, and relationship of the sediments with adjacent the basement (Table 4.2), and this concept works well for the Eqalulik and Uluksan groups.

Problems arise, however, when rift reactivation is invoked to explain sedimentation during

Nunatsiaq time. Lithospheric flexure, distance fiom the orogen, and orientation of the stress field in relationship to depositional strike are not fully compatible with the reactivated hotspot-active rifthg scenario. This interpretation (1) relies on extensional tectonics dunng

Athole Pointlearly Strathcona Sound time-for which evidence is tenuous, and (2) it nquires that the same active rifling style illicit a different response in the Eqalulik and early

Nunatsiaq phases. Updoming and crusta1 thinning precede foundering during the Eqalulik rift phase, but succeed it in the Nunatsiaq rifi phase (Iannelli, 1992). Reversing the sequence of crustal movements to apply a rifting scenario is unnecessary when the rock record is better reconciled with an alternate interpretation.

Geldsetzer (1973) first brought attention to the similarity between the Nunatsiaq Group and the late orogenic clastic wedge of the Central Appalachians. Hofian (1989, p. 488) suggested that the Bylot Supergroup represents a passive-margin-to-foredeep transition, based on the shift from northwestward paleocurrents in Eqalulik sands to southeastward transport for Nunatsiaq sands (Jackson et al., 1985). The upper Uluksan bathymetric

reversal, interpreted as an extensional response to themal uplifl to the NW associated with

rifi reactivation (Iannelli, 1992), could also have been generated through remobilization of existing basement structures during convergent tectonics. This compressional mechanism could create a complex paleogeography through movement of preexisting inter-block faults hundreds of kilometres nom the proximal foredeep (e.g. Alpine foredeep, Ziegler, 1987;

Antler foredeep, Dorobek et al., 1991 ; Marathon-ûuacbita foreland, Yang and Dorobek, 1995). Crustal thinning and dyke emplacement during the Mackenzie event (Galfey et al.,

1983; Jackson and Iannelli, 1981; Fahrig, 1987; LeCheminant and Heaman, 1991) has been Chapter GTectonic inversion and carbonate sedimt~rlrion

Tuble 4.2 - Interpretations of the stmcturui context of the Borden Bain.

Olson (1977,1984) Auiacogen reactivated nmw, elongate basin surrounded by crystalline during reaewed basement and defined by normal faults Jackson and launelLi rifîing (1981) severai similar basins at high angle to proposed Poseidon Ocean

paieocmnts dong axis of troughs

fault zones creating parallel but essentially separate troughs

continental tholeiites at base of succession

drarnatic contrast of deepwater Arctic Bay shales onlapping onto basement

Epicratonic or Iittle change in depositional facies near stntctural pericratonic basin boundarîes of the basin

low depositional gradients (cg. Society Cliffs Fm. 4-9 crn/km)

change in paleocturent direction

shift from carbonate to siIiciclastic deposition

This study Aulacogen to foreland basin inversion recorded in carbonate ramp basin

related to rifting and opening of the Poseidon Ocean and creation of the intracratonic Bylot basins (Jackson and Iannelli, 1981; Fahrig, 1987). Subsequent closure of Poseidon Ocean is postulated (Knight and Jackson, 1994, p.38) before emplacement of Franklin dykes, because of small-scale thn~st-faultingin the Strathcona Sound Formation, the Elwin Subgroup, and

Narssârssuk Formation of the ThuIe Basin. Evidence for minor compressionltranspression prior to intrusion of Franklin diabase is present on northern Bylot Island where evaporite- Chapter dTectonic Unterson anà carbonate sedünenîmion bearing mata of the Society Cliffs Formation are folded (Jackson and Cumming, 1981).

Faulting is most abundant, however, in the Strathcona Sound Formation (Jackson et al.,

1985), suggesting penecontemporaneous faulting during deposition that had ceased by Elwin tirne (Knight and Jackson, 1994). Compressive stresses would have been south to south- easterly, a similar orientation to thnisting in the Coppermine River Group (Hildebrand and

Baragar, 1991), and a subsurface thrust belt in Beaufort Sea that records compression of the

Poseidon margin (Fahrig, 1987; LeCheminant and Heaman, 1989).

High-magnitude subsidence and uplift in late Victor Bay time suggest that compression began as early as late Uluksan time, lasting into late Nunatsiaq time, and that this compression, not rift reactivation, was the mechanism r"ar differential vertical movement within the Milne Inlet Trough. Northwest uplift and southwest subsidence could be accommodated by compressional tectonics leading to lithospheric flexure (loading and isostatic compensation). The hinge point, where the Athole Point Formation thins and interfingers with submarine fan, slope, and deep marine red siltstone of the Strathcona Sound

Formation (lannelli, 1992, p. 232), occurs just West of a major fault developed transverse to the trend of the Milne Inlet Trough (Jackson and Sangster, 1987), and within a central "hinge zonet' across which transition from proximal to distal facies are noted in strata as old as the

Adams Sound Formation (Iannelli, 1992, p.27).

Drowning the Victor Bay Romp

Drowning of a carbonate platform generally occua rapidly, over a period not exceeding -1 Ma (Schlager, 1981). It can occur through pulses of relative sea ievel rise, either through local tectonic subsidence, glacio-eustasy, or tectono-eustasy. Reduction of benthic carbonate gmwth potential, due to continental drift or eutrophication can reduce the rate of accumulation and cause the platform to "give up" (James and Macintyre, 1985;

Neumann and Macintyre, 1985). An increase in slope height, whether by tectonics or through differential aggradation at the shelf break, can also affect sedimentation rates by reducing the and extent of zones favourable to benthic productivity. Finally, the Chapter ATectonic invemfonanà carbonate sedimentmion superposition of two or more aforementioned effects can push the rate of productivity of a carbonate factory below a cntical threshold.

The dominance of photozoans makes high accumulation rates on Holocene carbonate platfonns possible, and the growth of these platforms will outpace rapid sea-level rise unless the carbonate factory has already been compromised in some way (Schlager, 1981).

Environmental factors that can negatively affect photozoans, such as salinity changes, nutrient levels, and predation, were of minimal importance within Precambrian systems where the dominant factors was carbonate saturation of ocean water (Grotzinger, 1990).

Thus, despite lower accumulation rates (e.g. 20-80 cm,Grotzinger, 1986a), drowned

Precambrian carbonate platforms are even more paradoxical than drowned Phanerozoic platforms addressed by Schlager (1981).

Sedimentation on the Victor Bay ramp recovered despite repeated high-magnitude transgressions, because it was not predicated on the persistence of shallow-water conditions on an areally restncted shelf. Save benthic microbialites and ooids, lime mud was ultimately

the medium in which a11 other carbonate components were created via early diagenesis:

intraclasts, molar-tooth crack-fil], and noduIes (Sherman et al., 2000). It was located in the

water-column, and would presumably, as on most ramps, shift landward with rising sea-Ievel

(Burchette and Wright, 1992, others) instead of shrinking as it would on a shelf. To

effectively shut dom this type of carbonate production, relative sea-level rise would have

been accompanied by encroachment of waters inimical to the development of whitings:

likely cold, carbonate-undersaturated, and possibly bearing a high concentration of inhibitors

(e.g. ~e~+,Sumner and Grotzinger, 1996).

Nonetheless, the Uluksan carbonate system recovered from the drowning event that

led to deposition of the lower Victor Bay shale, and repeatedly survived hi@-magnitude

glacio-eustatic sea-level nses, even prograding into the basin during accumulation of

Sequences 1 and 2. Perhaps the robustness of the water-column carbonate factory led to

senescence or prolongation of carbonate sedmientation in the Borden Basin under conditions Chpter 4-Tectonic invem'on and carbollote sedimentmon that a benthic carbonate factory (e.g. a Phanerozoic one) could not have withstood. Eventually the Uluksarr-Nunatsiaq tectonic event led to backstepping of reefs and deposition of deep-water carbonates, and changes in temperature or water chemistry coincident with transition to a siliciclastic regime were Iikely a principal factor. It is also possible that a eustatic sea-level rise coincided with the tectonic subsidence, and this additive effect drowned the platform in the east. Regardless, shallow-water carbonate accumulation penisted in the

Eclipse Trough throughout this time.

Implications for Models of Northern Laurentia in lare Mesoproterozoic

Relationship of Borden Basin to other Bylot basim- By strict definition, an aulacogen is an elongate basin at high angle to an orogen, interpreted as the failed arm of a marine basin (Shatski, 1946; Burke, 1977). Thus, in a simple system, the aulacogen would be roughly normal to the orogen, which would in tum mimic the orientation of the former continental margin. Extension, however, is often accommodated along ancient Iines of wealcness in the craton (Brewer et al., 1983) which do not necessarily reflect the continental-scale extensional regime. It is therefore questionable to assume the morphology of an oceanic basin based on the orientation of a single continental rift. The bounding faults that define the Milne Inlet Trough extend beyond the Borden Basin and into late Archean rocks of the Committee Orogen (Jackson, 2000) where they pass into the westemmost limit of the NW-SE Paleoproterozoic (ca. 1.8 Ga) Northeast

Baffin Thwt Zone. This pre-existing suture in the high-grade crystalline basement may have controlled development of Mesoproterozoic normal faults when the area came under tension (Jackson, 2000), without necessarily saiking at high angle to the spreading axis of the Poseidon Ocean. Perpendicdar development of transforms and ridges during rifting can create an angdar margin of alternathg headlands and embayments destined to become structurai promontories and re-entrants during ocean closure (cf. Thomas, 1977). The sediientary -ter 4-Tectonic inversion tmà carbonate sedunentatron record of a retro-foreland basin migrating across this complex paleogeography of arches and troughs would be mercomplicated where rifts extended further into the continent. Faults reactivated during compression/ transpressionl transtension can involve cratonic basement, and cause jostling of existing horsts and grabens far Born the deformation fiont (Cross, 1993;

Dombek, 1995). With these caveats in mind, perhaps the preserved western edge Milne Inlet

Trough was sufficiently distant from the principal deformation fiont, that only foreland subsidence occurred. In these instances, from the perspective of the hinterland, the foreland basin is preceded by a forebulge where uplift and erosion create an unconformity on a continental margin succession. In cases of basement involvement and reactivation, the foredeep succession can precede the development of the unconformity through the process of crusta1 rethickening (Cohen, 1982). Movement along pre-convergence basement faults leads to uplift of half-grabens in the proximal foreland, essentially elevating a portion of the margin that had been placed in deep water by migration of the foredeep. An isolated carbonate platform tens of kilometres wide can develop on this culmination with slopes into deeper foredeep waters flanking the platfom on either side (cf. Permo-Carboniferous Dimple

Limestone platform; Wuellner et al., 1986). A similar situation arising in the Borden Basin would account for the uplift in the Strathcona River area relative to eastem Milne Inlet

Trough, and the occurrence of deep-water and slopes both east and West of this locaIity (Fig.

4.13). Deep-water Athole Point deposition would occur to the West in the distal foredeep, and conglomerates and glacis would prograde westward fiom the reef complex into the proximal foreland.

Faleucontinental reconstructions.-

The relationship between the Bylot basins is obscured by poor chronostratigraphic contml and the Iack of continuous outcrop between the depressions. Nonetheless, chemostratipphy and lithostratigraphy niggest correlation of the Uiuksan carbonates with dolostones of the Narssârssuk Formation of the Thule Supergroup and limestones of the uppemost Fury and Hecla Basin (Jackson and lannelli, 1981; Kah et al., 1999). This Chapter Q-Tectonic inversion mtd carbonate sedimenration suggests a change nom restncted rift sedimentation to a margin that overfiowed the confines of the grabens during üiuksan thne (Jackson and Iannelli, 1981 : Iannelli, 1992). A similar evolution is noted in the Thule Supergroup (Dawes, 1997). Furthemore, deep-water strata in

the uppermost Narssârssuk Formation (Kah, 1997) implies that the entire region was affected

by a major transgressive event during Athole Point time. If this is the case, then the

subsidence event was widespread, likely due to the tectonic subsidence observed in eastern

Milne Inlet Trough. Configuration of the northem Laurentian margin during development of

the Borden Basin and life of the Poseidon Ocean are still unclear. Orientation is difficult to

assess if we cannot confidently use the aulacogen orientation as a guide. It is improbable that

the compressive event was a result of the late Mesoproterozoic Grenville Orogeny. The

Grenville Front is >2000 km distant, and it is generally accepted that peak orogenesis

occurred 100 Ma later, CU. 1.l Ga (cf. Moore, 1986; Rivers et al., 1989). Most

interpretations place Sibena adjacent to the northem Laurentian margin during mid-to-late

Proterozoic (Rainbird et al., 1998; Pelechaty, 1996; Torsvik et al., 1996; Weil et al., 1998)

while others place it adjacent to western North America (Sears and Price, 2000). The

deformational event bracketed between the 1.27 Ga Mackenzie igneous event and deposition

of the Neoproterozoic (cl .O Ga) Shaler Supergroup (Hildebrand and Baragar, 199 1) may have

been due to the convergence of these continents west of the Bylot bains, and related to the

milder for compressive effects observed in the Borden Peninsula (Knight and Jackson, 1994).

Their subsequent separation occurred either in the late Neoproterozoic (Rainbird et al., 1998)

or early Cambnan (Pelechaty, 1996), pnor to deposition of the Paleozoic Arctic Platfonn

and Franklinian shelf.

CONCLUSIONS

1) Comlation across the Miine Inlet Trough illustrates the changing nature of the

Borden Basin during deposition of Victor Bay -ta. An early phase of progressive

shallowing and camp progradation into the basin is folîowed by an abrupt relative sea level

rise, growth of pimacle-shaped stromatolite reefs, and backstepping of the carbonate facies. Chqter 4-Tectonic irivemon and carbonate sedimentath

Whereas the drowning continues in the east, the western portion of the ramp experiences a relative sea-level falt.

2) Reversal of bathymetric trends along the axis of the graben occurred prior to deposition of the siliciclastic Nunatsiaq Group, and thus earlier than previously thought. The matest amount of reef growth in basin history occurred during this time, aided by rapid creation of accommodation space through both glacio-eustatic sea-level rise and increased tectonic subsidence.

3) Crustal rethickening in a foreland basin setting is proposed as the mechanism for basin inversion. Reactivation of fault blocks in the proximal foreland results in uplift and erosion of the previously drowned outer ramp, as observed at the Strathcona River reef cornplex. This is the opposite sequence of events expected when a peripheral bulge migrates over a carbonate pladorm, whereby shallow-water facies are uplified and exposed prior to drowning.

4) The nature of the basin inversion suggests east-directed compression. Orogenesis following the Mesoproterozoic Racklan Orogeny of northwestem Laurentia is a possible candidate as the source of east-directed tectonic disturbance. The fold and thrust belt would have been sufficiently distant fiom the Borden Basin that deformation would have been accommodated through lithospheric flexure and reactivation of basement blocks along existing normal faults.

ACKNOWEDGEMENTS

Herb Helmstaedt is thanked for refining the tectonic ideas and thus greatly improving this manuscript. Funding provided by NSERC grants to GMN and NPJ. Permission to publish

TriMet air photographs was granted by the National Air Photo Library of Canada. Logistical support from Polar Continental Shelf Project in Resolute Bay and Nanisivüc Mine made this project possible. Field assistance by Jay Harrington is much appreciated. GENERAL CONCLUSIONS AND SYNTHESIS OF RESEARCH

INTRODUCTION

The Victor Bay Formation is a -300-600 m thick carbonate unit within the -1.2 Ga

Borden Basin of northern Baffin Island and Bylot Island. These rocks represent paleoenvironments fiom shoreline to basin on a carbonate ramp with an overall muddy composition. They also provide a sedimentological record of a critical interval in the tectonic evolution of the Borden Basin, with implications for the paleogeography of northem Laurentia in the late Mesoproterozoic.

The nature of Precambrian carbonates and the conditions under which they developed are still poorly undentood because their geological context (continental configuration, paleolatitude, etc.) is often obscured or disrupted. Contributions hmchemostratigraphy, geochronology, and Precambrian biostratigraphy are steadily increasing, however, and are instrumental in piecing together an otherwise hgmentary geological record. Successions such as the Victor Bay Formation, with excellent preservation, extensive exposure, and broad range of depositional environments, are of great utility because they provide crucial information on the sediment dynarnics, sea-level history, and paleoclimate that complement our understanding of Earth's evolution.

The Victor Bay Formation and other Mesoproterozoic carbonate platforms are important links in the evolutionary chain of carbonate systems fkom the Archean to the Present. Characteristics of the hydmsphere, atmosphere, and biosphere are "fossilized in these rocks, and interpreting these clues is auciai to our understanding of fundamental changes ocdgwithin these realms over a billion years ago. To do so, we must take into account both (1) the sedimentary responses to the hydrodynamic regime and sea-level change which are universa1 to carbonate systems, and which recur throughout the rock record, and Chapter S-Concltrsions

(2) the facies unique to Mesoproterozoic carbonates. This means that despite the age of the rocks and the restriction of sorne facies to the Proterozoic, we can nonetheless address problems of physical sedimentology, sequence stratigraphy, and tectonostratigraphy.

CONCLUSIONS FROM THE RESEARCH

The Victor Bay Formation ramp was built by the accumulation of lime mud and grains derived fiom early cementation of this mud, but where microbialites, usually a signature of

Proterozoic rocks, developed only intemittently, during times of decreased mud production.

Sea-level ranged widely during the development of the ramp, resulting in unusually thick shallowing-upward cycles. Such cyclicity implies that the late Mesoproterozoic was a glacial penod when sea-level was strongly affected by the volume of fluctuating polar ice caps. The ramp was eventually terminated by a tectonic event that expressed itself through vertical movement on existing faults, which is here interpreted as a response to convergence of northern Laurentia and another land mas.

The Victor Bay Depositional Environment

Muddy Iow-energy environments, tempestites at the mid-ramp, and resedirnented outer-ramp facies collectively Iead to the interpretation of a low-energy, microtidal, storrn- dominated, distally steepened ramp architecture (Chapter 2). Lime mud constitutes 50-90% by volume of upper Victor Bay lithofacies, and is thus the primary sediment component across the ramp. It accumulated on the i~erramp as peritidal dololaminite facies, on the mid-ramp as molar-tooth mudstone facies, and on the outer ramp as nodular, ribbon, and parted limestone facies. Storm-reworked muddy sediments were the principal source of grains in the inner ramp and mid-ramp, such as dololaminite intraclasts and molar-tooth microspar grains. Early cementation of mud in nodular and riibon Iimestones created coarse particles that were incorporated into dope debntes on the outer ramp. This is the £ktdescription and interpretation of the full spectnun of facies across a Mesoproterozoic ramp. It allows Chapter 'i-Conclusiom expansion of the carbonate ramp model to include molar-tooth limestone and microspar calcarenite facies in the sballow-water depositional environment.

The Role of Ercstnsy

Three types of decarnetre-scale shallowing-upward cycles are distinguished on the basis of capping facies: (a) peritidal cycles indicating shoaling to inner-ramp conditions; (b) shallow-subtidal cycles representing sedimentation close to fair-weather wave base; and (c) deep-subtidal cycles accumulating on the outer rarnp, entirely below storm-wave base

(Chapter 3). Al1 cycles have thin transgressive bases and thicker aggradational to progradational highstand caps, especially in the peritidal and shallow-subtidal cycles.

Individual cycles can be followed laterally fiom shallow to deeper water equivalents, forming

parasequences with continuity over several kilometres perpendicular to palaeoslope.

Sequence stratigraphie analysis of the parasequences indicates transgressive phases

limited to few parasequences, passing into thick aggradational highstand conditions.

Development of a lowstand wedge, uncornmon in homoclinal rarnp settings, lends Mer support to the interpretation of the edifice as a distally-steepened ramp. The scale of cycles is compatible with high-amplitude allogenic control of accommodation space, likely high-

frequency glacio-eustasy. The global climate cycle predicts icehouse conditions CU. 1.2 Ga when extrapolated into the Mesoproterozoic (Veevers, I990), The Victor Bay cycles

provide some of the first tangible evidence to support this theory.

The Influence of Syntectonics

The Uluksan carbonate phase ended during Victor Bay tirne. Drowning of the ramp

in the east and coeval karsting and erosion in the West indicate that tectonic effects were the principal cause of its demise. The new interpretation proposed here is that uplift was caused

by convergent tectonisrn, and that the Borden Basin underwent a transformation from an

aulacogen to a foreland basin (Chapter 4). Compression was accornmodated dong pre-

existing normal faults. Mild and attenuated tectonic effects such as these imply an orogenic fiont at some distance 6om the hidy area, evidence of which is preserved in the northwestern Arctic (Hildebrand and Baragar, 1991). This event adds fùrther support to the theory that Laurentia came into contact with another continent in the late Mesoproterozoic.

CARBONATE SEDLMENTATION LN LATE MESOPROTEROZOIC TIME

Field investigation of the Victor Bay Formation has led to a better understanding of a carbonate system that is intermediate between stromatolite-dominated Paleoproterozoic platforms and muddy bioclast-dorninated Phanerozoic shelves. The Victor Bay ramp is an example of a carbonate factory where the generation of carbonate mud via whitings was the primary source of vimially al1 sediment, either directly by settling of lime mud from the water-column, or by seafloor cementation and reworking of the mud. Stoms were the principal agent of resedimentation in shallow water by ripping up the seafloor, creating intraclasts, and provided lime mud to the outer rarnp by exporting suspended fines (Fig. 5.1).

Yet stromatolitic reefs grew to impressive sizes when conditions for nucleation were favourable, and show evidence of tenacity and penistence once given an initial advantage.

Facies analysis indicates that carbonate sedimentation had entend a phase unique in

Earth history, during which pathways of carbonate production began to shift in favour of water-column precipitation at the expense of benthic microbialites (Grotzinger, 1990). The high proportion of lime mud and extensive development of molar-tooth limestone in the

Victor Bay Formation is similar to Mesoproterozoic and Neoproterozoic ramps (e.g. 1.3-1.4

Ga Belt Supergroup, Horodyski (1970) Grotzinger, (1986b); ~0.85Ga Xinmincun Formation

Fairchild et al. (1997)). Yet the Society Cliffs Formation that underlies the Victor Bay

Formation is rich in microbialites and ooids, indicating that the spectnun of carbonate pladorm types was still broad in the late Mesoproterozoic.

Calcification and preservation of microbial mat textures in the Victor Bay rocks was largely restricted to the tidal flat where carbonate saturation was presumably higher. This trend is dramatically illustmted in the pentidai zone of the Society Cliffs Formation where precipitation of carbonate and silica outpaced organic degradation, tbus recreating conditions OUTER RAMP INNER RAMP ,

Fig. 5.1 - Sediment dynomics on the Victor Bay ramp, an emple of mud and grain formation in a Iate Precambrian mud-dominated carbonate factor-y. Lime mud formed in the water column sentes on the mid-romp where degussing md cementafion create rnolar-tooth structure. Microbiul laminites fonn in the intertidal to supratidal zone. Grains are generated in shallow-water settings principal&y by storms, which me also the means of export of rnud to the outer rmp. Ine steepened profie of the distal ramp leads to n grawavrtationaIiyunstable dope prone to resedimentation. that existed throughout Proterozoic carbonate platfonns (Kah and Knoll, 1996). On the

Victor Bay ramp, Proterozoic-style sedimentation, i.e. mirobial accumulation, occurred during transgressions, as a result of cartionate-undersaturated waters that "smbbed" the water column, thus increasing light penetration to the seafloor. Stromatolites and ooids accumulated during these transgressions, and were consistently nucleated on hard or grainy substrates, suggesting that so4 muddy substrates (expected during muddy highstand periods) were less conducive to microbialite growth and other fonns of microbially-mediated benthic cementation. Correlation of stromatolites to periods of decreased lime mud production and rapid creation of accommodation space establishes an antithetical relationship between deposition of benthic microbial carbonate and precipitation of lime mud in the water column during the late Mesoproterozoic.

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Stratigraphic Logs

Map of Section Locations

VktOreayFm~ AR Adams Rfver

@ NanlrMkX MR Mala Rfver N NanMk PWN Pingo htley North

n Tmmblay Sound apoiymicb'c conglomerate dololaminitelayptornicrobial laminites

ribbon and nodular iimestone debrite carbonamus carbonate colurnnar shrnatolites 0stiprnaldibc reef a wMc grainStone quaibose dolostone doIosilo$ @ shalyiilty dolosbne limaslone, undifferentiated adoiostone, undiirenb'ated shale 0sandstone a siitstone 4 slurnp a chen ADAMS RIVER

VICTOR SAY UPPER MEMBER

VICTOR BAY LOWER - lai MEMBER

- Om ALFRED POINT ATHOLE

FORMATION py--f

UPPER VICTOR BAY MEMBER CAMP PROZAC

VICTOR BAY ATHOLE ' -* -1 = POINT UPPER T- FORMATION MEMBER a CP-2 *,

VICTOR BAY CP-2 UPPER . MEMBER 5011

VICTOR BAY LOWER MEMBER

SOCIEIY CLIFFS FORMATION ELWICE CAP

VICTOR BAY UPPER MEMBER

CD SOCIETY CLIFFS STRATHCONA SOUND FORMATION

tom

ATHOLE POINT FORMATION

VICTOR BAY UPPER MEMBER

VICTOR BAY 3WER MEMBER Appendix A

W VICTOR BAY \ UPPER MEMBER

VICTOR BAY LOWER MEMBER

SOClETYCLlFFS - FORMATION 1 1 PINGO VALLEY NORTH AMOLE POINT FORMATION I

VICTOR BAY MEMBERUPPER -Mt VICTOR BAY UPPER MEMBER

VICTOR BAY LOWER MEMBER

SOCIETY CLlFFS FORMATION PINGO VALLEY SOUTH

VICTOR BAY UPPER MEMBER VICTOR BAY UPPER lm MEMBER

VICTOR BAY PV-2 b LOWER MEMBER Appendix A

TREMBLAY SOUND

HCS

VICTOR BAY UPPER MEMBER

VICTOR BAY LOWER MEMBER APPENDIX I3

ûriented Features at Pingo Valley

Molar-tooth cracks

Azimuth Location-Section-Unit Metets 275 PV-2b-3 11.2 145 PV-2a-10 219.7 140 PV-2a-10 222.2 142 PV-2a-1O 224.2 135 PV-2a-13 236.6 135 PV-2a-13 238.1 141 PV-2a-19 273.7 142 PV-3-1 0.5 040 PV-3-4 56.5 120 PV-3-16 202.9 130 PV-3-16 202.9 132 PV-3-16 202.9 155 PV-3-16 202.9 155 PV-3-18 207.9 135 PV-3-19 229.9 030 PV-3-19 229.9 1O0 PV-3-23 282.8 140 PV-3-23 283.3 145 PV-3-27 319.3

Stromatolite Elongationt

Slump Fold Axis in Nodular Limestone

Azimuth Location-Section-Unit Meters 145 PV-2a-23 305.4