Neoproterozoic rocks of the Newfoundland Avalon Zone

Pretnmbriun Resenrth ELSEVIER

Precambrian Research 73 ( 1995 ) 123-136

Neoproterozoic rocks of the Newfoundland Avalon Zone P a u l M. M y r o w Department of Geology, The Colorado College, Colorado Springs, CO 80903, USA Received 8 March 1993; revised version accepted 3 February 1994

Abstract

A ca. 15 km thick succession of volcanic and sedimentary Neoproterozoic rocks are exposed within the Avalon Terrane of Newfoundland. Petrologic and geochemical study of volcanic suites and sedimentological analyses indicate that most deposition took place in ensialic rift basins and associated continental arc settings. On the Avalon Peninsula, over 7500 m of volcaniclastic strata include initial submarine fan and slope deposits, which pass upward into a prograded wedge of deltaic and fluvial deposits. This flysch-to-molasse transition, and associated folding and volcanism, mark the Neoproterozoic Avalonian Orogeny. Post-orogenic erosion was followed by deposition of extensive Cambrian siliciclastic deposits. Recent U-Pb zircon dating of many of the Neoproterozoic volcanic units provides some constraints on both intra-Avalon and global correlations. More precise dates are needed to improve correlation and better constrain the geologic history of this region. Glaciogenic diamictites of the Gaskiers Formation exposed on the Avalon Peninsula may eventually also prove useful for global correlation. Biostratigraphic study of the latest NeoproterozoicLower Cambrian deposits has resulted in ratification of the Precambrian-Cambrian Boundary Stratotype in an exposure on the Burin Peninsula. Older Neoproterozoic rocks contain well-preserved Ediacara-type fossils and a low-diversity suite of microfossils including acritarchs and carbonaceous filaments, the latter of which have little biostratigraphic utility. Aside from work on the Precambrian-Cambrian boundary section, little or no chemostratigraphic, magnetostratigraphic or sequence stratigraphic work has been completed on the Neoproterozoic of the Newfoundland Avalon Terrane.

1. Introduction T h e Avalon Terrane o f eastern N e w f o u n d l a n d (Williams, 1979; Williams and Hatcher, 1983 ) consists o f Neoproterozoic volcanic, plutonic and sedimentary rocks and C a m b r o - O r d o v i c i a n sedimentary cover units. Tectono-stratigraphic equivalents extend through Atlantic Canada, the eastern coast o f the U n i t e d States, the British Caledonides, and the northwest coast o f Africa. Eastern Newfoundland (Fig. 1 ) is the largest and best exposed area o f Avalonian strata in the App a l a c h i a n - C a l e d o n i d e Orogen, thereby making

it the type area for these rocks (O'Brien et al., 1988b). This paper aims to summarize the present state o f knowledge o f the N e o p r o t e r o z o i c stratigraphy o f the N e w f o u n d l a n d Avalon Terrane, and provide a framework with which to evaluate the potential for definition o f a global stratotype section for the terminal Proterozoic.

2. Regional setting The N e o p r o t e r o z o i c o f eastern N e w f o u n d l a n d can be divided into three major assemblages

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P.M. Myrow / Precambrian Research 73 (1995) 123- I36

124

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West of Paradise Sound Fault including Burin Peninsula and Fortune Bay

Fig. 1. Newfoundland Avalon Terrane. Divisions into Eastern (E), Central (C) and Western (I4") Zones. Stratigraphy of these zones illustrated in Fig. 2. (O'Brien et al., 1988a; Fig. 2), the oldest being the Burin G r o u p ophiolite, which has been recently dated at 763+12 ~ M a (Krogh et al., 1988 ).

These rocks are significantly older than the remaining units in the Avalon, and therefore represent a distinctly separate tectonic event (O'Brien et al., 1988a, b) that is recorded in

P.M. Myrow I Precambrian Research 73 (1995) 123-136

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Fig. 2. Correlation chart of stratigraphic sections from the southern Burin Peninsula, Bonavista Bay-west Placentia Bay, and Avalon Peninsula regions. The Gaskiers Formation (GF) is a late Precambrian glacial diamictite unit. The Precambrian-Cambrian boundary, shown in a heavy line, is found within the Chapel Island Formation (CIF), below the Random Formation (RF). The occurrences of Ediacaran fauna are shown by the letter E. The first occurrence of trilobites shown by letter T. Figure modified from Smith and Hiscott ( 1984, fig. 1 ); correlations are modified from those of O'Brien and Taylor (1983). No age relationships are implied for the older units in this diagram. Lower, middle and upper stratigraphic assemblages referred to in the text are shown as 1, 2, and 3, respectively. Sources of age dates given in text.

many areas across the Pan-African belt (e.g., Stern and Hedge, 1985). A separate group of poorly understood 6 7 0 - 6 8 0 Ma volcanic and plutonic complexes that has recently been described in abstracts by O'Brien et al. ( 1992a, b ), intervene between this lower Burin Group assemblage and the two younger assemblages

(middle and upper) described below. These have not yet been described in detail, but their presence further complicates the tectonic history of this region. The middle assemblage (Fig. 2) is made up of volcanic and comagmatic plutonic rocks with U Pb ages dominantly between 635 and 630 Ma,

126

P.M. Myrow / Precambrian Research 73 (1995) 123-136

and time-equivalent and slightly younger volcaniclastic rocks (Krogh et al., 1988; O'Brien et al., 1992a, b). Most of the volcanic rocks--all or parts of the Harbour Main, Marystown, Musgravetown and Love Cove groups --are bimodal volcanics of terrestrial and marine affinity (Papezik, 1970; Strong et al., 1978; King et al., 1988). Based on geophysical and petrologic criteria, these units show evidence for extension of ensialic crust and local formation of oceanic crust, thus drawing comparisons with the Cenozoic Basin and Range (Strong et al., 1978; Nixon and Papezik, 1979; O'Brien et al., 1983; Miller, 1987; Krogh et al., 1988). However, coeval volcaniclastic flysch units, deposited in submarine fan and slope environments (Gardner and Hiscott, 1988; King et al., 1988; Knight and O'Brien, 1988), contain trace and major compositional constituents that reflect sedimentation adjacent to more evolved calc-alkaline volcanic arcs (Dec et al., 1989, 1992 ). The tectonic setting is therefore considered to be a back-arc region characterized by alternating zones of volcanic islands and deep-marine basins that were filled by aprons of volcaniclastic sediment (Knight and O'Brien, 1988; Dec et al., 1989, 1992). The upper assemblage of the Newfoundland Avalon Terrane consists of latest Neoproterozoic to Ordovician platformal sedimentary units. Deposition was initially centered in tectonically active, small linear basins in the Fortune Bay region. During the Early Cambrian these units onlapped the older flysch-to-molasse belts to the east (Hutchinson, 1962; Anderson, 1981; Hiscott, 1982; Smith and Hiscott, 1984; Landing et al., 1989). Initial basin development was due to transtensional tectonics (Smith and Hiscott, 1984). Younger Cambrian and Ordovician deposits accumulated in linear basins affected by repeated epeirogenic movements (Hutchinson, 1962; Landing, 1992 ). 3. Stratigraphic overview Neoproterozoic to Lower Cambrian rocks of the Avalon Terrane of Newfoundland comprise three geologic regions (Figs. 1, 2). An eastern

zone--Avalon Peninsula--consists of (in ascending order): ( 1 ) rift-related (?) bimodal volcanics (Harbour Main Group), (2) volcaniclastic flysch deposits of submarine fan and slope origin (Conception and St. John's Groups; 4-7 km thick), and (3) coarsening-upward shallowmarine to proximal alluvial fan molasse deposits (Signal Hill Group; 5 km thick) (King, 1980, 1990; King et al., 1988). A central zone of rocks is exposed between the Avalon Peninsula and the Paradise Sound Fault (Fig. 1 ). Neoproterozoic rocks of this region include interstratified volcanic and sedimentary rocks of the Love Cove Group which are conformably overlain by the deep-marine arc-related deposits of the Connecting Point Group, and then the volcanic and sedimentary strata of the Musgravetown Group (O'Brien et al., 1988b; O'Brien and Knight, 1988; Dec et al., 1989, 1992). The deep-marine deposits of the Connecting Point Group are sedimentologically similar to, and have been generally correlated with, strata of the Conception Group (McCartney, 1967; Anderson, 1972). The Musgravetown Group contains a thick basal volcanic unit (Bull Arm Formation) and sedimentary units, which are correlative with the Signal Hill Group of the eastern zone, and similarly records deposition in terrestrial environments (King, 1980; O'Brien et al., 1988b). A western zone, west of the Paradise Sound Fault, contains a thick section of bimodal volcanics and associated minor sedimentary deposits that accumulated contemporaneously with the sedimentary deposits to the east. The stratigraphic and age relationships between these dominantly volcanic units (Love Cove, Marystown, Connaigre Bay, Long Harbour, and Rock Harbour groups) have not been established, but they are considered to be generally lithostratigraphic equivalents (O'Brien et al., 1988b). Detailed correlations between rocks of this western zone and those further east, across the Paradise Sound Fault, are not well established. Rocks exposed in and around Fortune Bay include a thick conformable section of uppermost Precambrian-Lower Cambrian sedimentary units (Green and Williams, 1974; Anderson, 1981;

P.M. Myrow / Precambrian Research 73 (1995) 123-136 Interpretation

Member N

<

Paleobathymetry Low ~ h

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1

J

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<[ "1O-

MFS

m

-3

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<7 i

FS

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er

3

Shelf : Shelf sws

Fig. 3. Generalizedstratigraphic section for the Chapel Island Formation showinginformal members,inferredpaleobathymetry, and sequence-stratigraphic interpretation. SB = sequence boundary, FS= flooding surface, MFS=maximum floodingsurface, TST=transgressive systems tract, HST=highstand systems tract. PrecambrianCambrian boundarystratotypeshownby letterS. Figurefrom Myrowand Hiscott ( 1993). Hiscott, 1982; Smith and Hiscott, 1984; Myrow et al., 1988; Myrow and Hiscott, 1993 ). The Precambrian-Cambrian Boundary Stratotype is located in this section on the southwestern tip of the Burin Peninsula, within member 2 of the Chapel Island Formation (Narbonne et al., 1987; Fig. 3 ). 4. Avalonian Orogeny Numerous pulses of late Precambrian orogenic activity, evidenced by plutonism, defor-

127

mation, and mild metamorphism, have been referred to as the Avalonian Orogeny (Lilly, 1966; Rodgers, 1967, 1972; Hughes, 1970). The first pulse is represented by the angular unconformity between the Harbour Main Group and the overlying Conception Group on an ancient horst (McCartney, 1967; Poole, 1967; Rodgers, 1967, 1968). An earlier ~680 Ma compressive tectonic event has recently been reported by O'Brien et al. (1992a, b), the details of which have not been published. A younger event is locally represented by the contact between continental molasse deposits of the Signal Hill and Musgravetown groups, and underlying folded flysch strata of the Conception and Connecting Point groups: the "H.D. Lilly Unconformity" (Rodgers, 1968; Hughes, 1970; Anderson et al., 1975; King et al., 1988, Stop 2-10 ). The final phase of the Avalonian Orogeny is recorded by a sub-Cambrian unconformity (below the upper assemblage, Fig. 2). Onlap of Cambrian strata occurred from west to east so that progressively younger strata rest on a variety of older units ranging from folded molasse deposits in the west, to deeper level granites, flysch and volcanic units (e.g., Harbour Main Group) towards the east (Rodgers, 1967; McCartney, 1967; King, 1980; Anderson, 1981; O'Brien et al., 1988b). This is a profound unconformity that records as much as 10 km of erosional downcutting (see summary in Landing et al., 1988).

5. Geochronology A number of U - P b zircon dates have been recently published for plutonic, volcanic, and associated volcaniclastic rocks from the Newfoundland Avalon Zone (e.g., Dallmeyer et al., 1981; Krogh et al., 1988) (Fig. 2). Rocks of the 7632=~.8 Ma Burin Group are the oldest rocks in the eastern Avalon region (Krogh et al., 1988). Volcanic and plutonic units from the southeast Avalon Zone have recently yielded U - P b zircon dates of 673+3 and 681 ___3Ma (O'Brien et al., 1992a, b), and therefore represent a separate, poorly understood, tectonostratigraphic assem-

128

P.M. ~ v r o w / Precambrian Research 73 (1995) 123-136

blage of rocks for this region. U - P b zircon ages of 606_2.9,+37622.6_+23. and 631 _+2 Ma have been reported from different stratigraphic positions and outcrop areas of the Harbour Main Group (Krogh et al., 1988), a lower unit in the middle assemblage (Fig. 2). The Love Cove Group, exposed in the central outcrop belt (Figs. 1, 2), is roughly age-equivalent with the Harbour Main Group, based on a recent U - P b zircon date of ~620+_ 1 Ma by G. Dunning (see Dec et al., 1992 ). These groups are, in turn, also age-equivalents to rocks that immediately overlie the Burin Group and the Rock Harbour Group. Rhyolite clasts from this unit have yielded a U - P b zircon date of 62 aJ - -+l . 719 Ma (Krogh et al., 1988 ) On the eastern Avalon Peninsula, zircons from ash beds in the Ediacaran fauna-bearing Mistaken Point Formation have yielded a U - P b date of 565 + 3 Ma (G. Dunning, pers. commun, to A. Benus in Benus, 1988). A U - P b date of ~ 6 1 0 Ma on a thin ash bed in the middle part of the deep-marine deposits of the Connecting Point Group (O'Brien et al., 1992b; Dec et al., 1992) indicates age-equivalence with plutonic rocks (Holyrood Granite ) (O'Brien et al., 1992b ), the younger parts of the Harbour Main Group (Krogh et al., 1988 ), or the older portions of the Conception Group. Ash beds are found throughout the Neoproterozoic section from the lower Conception Group to the basal Signal Hill Group providing the potential for tight age constraint on a variety of physical and biogenic features. The Long Harbour Group, a thick succession of subaerial bimodal volcanic rocks and subordinate volcaniclastic units, exposed in Fortune Bay, has been U - P b dated at 552+ 3 Ma (R. Tucker, S. O'Brien, C.F. O'Driscoll, pers. commun., 1994). This date, derived from rocks directly below the Rencontre Formation in an exposure in northern Fortune Bay, provides a maximum age for the unconformably overlying sedimentary units that span the PrecambrianCambrian boundary. An older age of 608_+72° Ma has been reported from volcanic rocks that underlie the Precambrian-Cambrian stratotype section on the Burin Peninsula, the Marystown Group (Krogh et al., 1988). No ash beds have been found in the sedimentary Precambrian-

Cambrian boundary strata in Newfoundland. However, a U - P b zircon date of 530_+0.9 Ma is reported from an ash bed in the second Cambrian trace fossil zone, the Rusophycus avalonensis Zone, in rocks roughly equivalent to the Chapel Island Formation exposed in New Brunswick (Isachsen et al., 1994).

6. Glaciogenic diamictite The Gaskiers Formation of the Conception Group is a 300 m thick glaciogenic deposit exposed primarily on shoreline outcrops in and around St. Mary's Bay. The formation is sedimentologically complex and consists of massive to poorly stratified diamictite with minor units of thin-bedded, fine-grained, turbidites and mudstone. The presence of chattermarked garnets (Gravenor, 1980), "flat-iron" clasts (Briickner and Anderson, 1971), dropstones, and abundant striated and faceted clasts, all attest to a glacial origin (Williams and King, 1979; Anderson and King, 1981; Anderson, 1987; Eyles and Eyles, 1989). Gravenor (1980) interpreted these diamictites as resedimented ice-marginal deposits. A lithofacies analysis by Eyles and Eyles ( 1989 ) indicates that, taken in conjunction with sedimentological data from surrounding units, the Gaskiers was deposited as sediment gravity flows in a mid-slope environment. Deposition was contemporaneous with volcanism, as indicated by the presence of volcanic bombs, ash fall tufts, and agglomerate layers (Williams and King, 1979; Eyles and Eyles, 1989 ). Within the Avalon Zone of Newfoundland, a number of units containing mixtite deposits have been considered to be potential time-equivalents to the Gaskiers Formation, including units in the Musgravetown Group (Briickner, 1977), Rock Harbour Group (Hiscott, 1981 ), and Connecting Point Group (Dec et al., 1992). However, none of these mixtite units contain any evidence for direct glacial influence (e.g., striated or faceted clasts). Numerous outcrops along the shores of Conception Bay, which contain large dropstones (Nixon, 1974) and mixtite units, have

P.M. Myrow / Precambrian Research 73 (I 995) 123-136

been described as the Gaskiers Formation (Anderson and King, 1981 ). Recent work by the author (Myrow, unpublished data) has revealed striated and faceted clasts, neither of which had been previously found (Anderson and King, 1981 ). Additionally, a limestone bed found at two localities within Conception Bay at the top of these mixtite-bearing units may represent a cap carbonate. Samples collected by the author will be isotopically analyzed to determine if they display anomalous values characteristic of cap carbonate beds (i.e., Jenkins et al., 1993). Study of submarine exposures of the Virgin Rocks Shoal (Lilly, 1965, 1966), 200 km southeast of St. John's, have revealed mixtite layers that have also been interpreted as glaciogenic diamictites (Briickner, 1969 ) and tentatively correlated with the Gaskiers Formation (Briickner and Anderson, 1971 ). Assuming correlation of these various outcrops, the distribution of the Gaskiers Formation would extend 125 km north and south, and 275 km east and west (Anderson and King, 1981 ). Late Precambrian glacial deposits have been identified in areas around the globe including the British Isles, Africa, Australia, North America, China, South America, and elsewhere (Hambrey and Harland, 1981 ). The most extensive glacial deposits are the Varangian ( ~ 610-590 Ma) tillites, which are generally found below Ediacaran fauna (Knoll and Walter, 1992). In the Mackenzie Mountains, glacial deposits occur below a typically diverse Ediacaran-type fossil assemblage but above a simple assemblage of diskshaped fossils (Twitya disks) of possible metazoan affinity (Hofmann et al., 1990; Narbonne et al., 1994). Poor age constraints, and the presence in many areas of several stratigraphically separate glacial diamictite units (i.e., Varangian, Marinoan, Sturtian; see Hambrey, 1983) have hindered direct correlation of these units. Lithostratigraphic correlation of widely spaced regions based, in part, on tillite units, has been attempted (Aitken, 1991; Young, 1992a, b), despite poor geochronologic control. Volcanic units within the Gaskiers Formation and ash beds from underlying and overlying strata could provide U - P b zir-

129

con dates for this episode of Neoproterozoic glaciation (work in progress: P. Myrow, E. Landing, S. Bowring). Such age constraint would only be a first step towards global correlation, for greater geochronologic control is needed on other sequences that contain Neoproterozoic glacials.

7. Chemostratigraphy There have been no stable isotope studies on the thick succession of Neoproterozoic rocks of the Avalon of Newfoundland. Stable isotope analyses of the Cambrian part of the section have been recently published. Strauss et al. (1992) give carbon and sulfur isotope data from the five members of the Chapel Island Formation (uppermost Vendian to Lower Cambrian). Brasier et al. (1992) present carbon- and oxygen-isotope data for members 3 and 4 of the Chapel Island Formation and for younger Cambrian strata (Tommotian and Atdabanian equivalents). Thermal effects on the stable isotopic signature of these rocks are suggested by illite crystallinity and high thermal alteration indices for kerogen and organic-walled microfossils (Strauss et al., 1992), both of which indicate temperatures between 140 and 200°C. Depleted ~180 values ( - 10%o PDB or lighter) support a similar conclusion (Brasier et al., 1992). The paucity of carbonate units and the low values of organic carbon for the boundary stratotype on the Burin Peninsula (Strauss et al., 1992 ) have discouraged attempts at using stable isotope stratigraphy for global correlation with other Precambrian-Cambrian boundary strata. The great thickness of Neoproterozoic strata in Newfoundland has not been subjected to isotopic analysis, but the paucity of carbonate units and a similar or higher grade of metamorphism may limit the utility of such studies. It should be noted that low levels of total organic carbon have not precluded successful stable isotope analyses of other Neoproterozoic sections (e.g., Narbonne et al., 1994).

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P.M. Myrow / Precambrian Research 73 (1995) 123-136

8. Biostratigraphy The oldest macrofossils from the Newfoundland Avalon Terrane are a diverse assemblage (at least 30 species; Anderson and Conway Morris, 1982 ) of well-preserved imprints of soft-bodied organisms from the upper Conception Group (Drook, Briscal and Mistaken Point formations) and lower St. John's Group (Trepassey and Fermeuse formations). This Ediacaran fossil assemblage includes: ( 1 ) discoidal, "medusoid"-like, lobed and annulated forms; (2) abundant "spindle-shaped forms" with no apparent polarity; (3) Charnia-like frondose forms with stalks and attachment disks; (4) "starshaped forms" which Jenkins (1992) considers a possible trace fossil; and (5) "bush-like forms" and other branched forms (Anderson and Misra, 1968; Misra, 1969; Williams and King, 1979; Anderson and Conway Morris, 1982). Taphonomic and sedimentological data indicate that the fauna represent in-situ life assemblages (Landing et al., 1988, Stop 1B; Jenkins, 1992; Seilacher, 1992) that were rapidly buried by volcanic ash. Unfortunately, a full taxonomic description of this assemblage has never been presented. A few faunal elements are similar to specimens recovered from Charnwood Forest, England (Jenkins, 1992), particularly Charnia masoni, which has a global distribution. However, in general the composition of the Newfoundland assemblage is highly unusual and probably endemic, reflecting either biogeographic isolation or a deep-water origin that is almost unique relative to other Ediacaran deposits (Conway Morris, 1989 ). The latter seems less likely, given the complete non-overlap of fauna between the Newfoundland assemblage and deep-water Ediacaran assemblages from western Canada (e.g., Narbonne and Aitken, 1990). The rich and enigmatic fauna from Newfoundland is proving important in the ongoing debate about the nature and affinities of Ediacaran fossils (see Seilacher, 1989, 1992; Fedonkin, 1992; Jenkins, 1992). Problematic discoidal structures referable to Aspidella terranovica occur within the Fermeuse Formation of the St. John's Group. These prob-

lematica, which have a central sandy plug and both annulations and rare radial structures, have been interpreted as both inorganic features (Hofmann, 1971 ; Conway Morris, 1989 ) and as substrate-attached medusoid body fossils (King, 1980; Jenkins, 1992 ). There have been few studies of microfossils from the late Precambrian of Newfoundland. Hofmann et al. (1979) analyzed approximately 300 samples from nearly all Neoproterozoic units in the Avalon Terrane. They describe a generally poorly preserved suite of organic-walled microfossils that include nonseptate filamentous forms (Taeniatum sp.) and a few acritarchs. The microfossil assemblage is not abundant or diverse, but it should be noted that globally Vendian acritarch assemblages are simple and of low diversity (Sokolov and Iwanowski, 1990; Knoll and Walter, 1992; Vidal and Moczydlowska, 1992). The stratigraphic ranges of the microfossils described by Hofmann et al. (1979) are quite long and are therefore of relatively little stratigraphic utility. Specimens of Badinella, an acritarch common in, but not restricted to, Vendian rocks, were reported from the upper two thirds of the St. John's Group (Timofeyev et al., 1980; Anderson et al., 1982); unfortunately, neither descriptions or illustrations of Bavlinella or other reported microfossils are provided by these authors to verify their identifications. Specimens of Bavlinella were also reported from the Gaskiers Formation (A. Knoll in Hofmann et al., 1979; Timofeyev et al., 1980), but the Riphean age inferred by Timofeyev et al. (1980) is clearly contradicted by recent work including U - P b age dates. A remarkably thick and expanded section of latest Vendian to Cambrian rocks exposed along the southwestern tip of the Burin Peninsula contains the recently ratified Precambrian-Cambrian boundary stratotype. The stratotype point is located 2.4 m above the base of member 2 of the Chapel Island Formation (Narbonne et al., 1987). The formation has a remarkably abundant and diverse assemblage of trace fossils (Crimes and Anderson, 1985; Narbonne et al., 1987; Narbonne and Myrow, 1988) that define three ichnozones: the uppermost Vendian Har-

P.M. Myrow /Precambrian Research 73 (1995) 123-136

laniella podolica Zone, and the Lower Cambrian Phycodes pedum and Rusophycus avalonensis zones. Carbonaceous fossils in the Vendian Harlaniella podolica Zone include Sabellidites cambriensis and impressions of vendotaenid algae including Tyrasotaenia gnilovskaya (Narbonne et al., 1987; Landing et al., 1989). The Cambrian portion of the Chapel Island Formation also contains a suite of shelly fossils (Bengtson and Fletcher, 1983; Landing et al., 1989; Myrow and Landing, 1992 ), calcified microbial remains (Myrow and Coniglio, 1991 ), and impressions of the soft-bodied chondrophore megafossil Kullingia delicata (Fedonkin) (Narbonne et al., 1991 ). Microfossils from this section include cyanobacterial sheaths referred to as Eomycetopsis (H. J. Hofmann, written commun., 1985) and acritarchs (Narbonne et al., 1987; Strauss et al., 1992 ). A lowermost Cambrian-aspect acritarch, Granomarginata squamacea, recovered from a sample in member 5 of the Chapel Island Formation (Strauss et al., 1992), is one of many acritarchs associated with a major Early Cambrian acritarch radiation documented on the East European Platform (Moczydlowska, 1991 ). Other filamentous and spheroidal microfossils have been described from the Lower Cambrian Random and Brigus formations (Nautiyal, 1976; Martin and Dean, 1983 ).

9. Lithostratigraphy and sequence stratigraphy Lithostratigraphic correlation of units across the Newfoundland Avalon Terrane has proved difficult due to lateral facies changes that reflect, in part, the time-equivalence of volcanic and sedimentary units. These facies changes reflect periods of contemporary arc volcanism and deepsea sedimentation (lower Assemblage 2; Fig. 2 ) in alternating NNE-trending belts (Knight and O'Brien, 1988). Structural complexity is also a minor limiting factor for correlation, primarily in the western region of the Avalon. Recent stratigraphic and geochronologic work (e.g., Krogh et al., 1988 ) has resulted in considerable advances in regional correlation. Sedimentological and stratigraphic studies of flysch

131

units have led to identification of regionally correlatable mixtite units, rapid changes in sediment provenance, and patterns of sediment dispersal (Gardner and Hiscott, 1988; Knight and O'Brien, 1988; O'Brien and Knight, 1988; O'Brien et al., 1988b; Dec et al., 1989, 1992). These advances hold potential for sequence-stratigraphic analysis despite lateral continuity of exposure that is less than that found in the bestexposed Neoproterozoic sections (e.g., Australia, Namibia, Mackenzie Mountains). Correlation of succeeding molasse units has also improved greatly with recent mapping and stratigraphic work (King et al., 1988; O'Brien et al., 1988b; King, 1990). A sequence-stratigraphy interpretation of the Precambrian-Cambrian boundary interval, the Chapel Island Formation, is provided by Myrow and Hiscott (1993) (Fig. 3). They document a sequence boundary at the top of the fine-grained, shelly fossil-bearing member 4, that is overlain by a thick shoaling unit of micaceous siltstone and sandstone (member 5), and quartz arenite and sandstone units of the Random Formation (see also Myrow, 1987; Myrow et al., 1988). These features are evidence for a previously unrecognized orogenic episode that involved rapid subsidence and subsequent progradation of a thick clastic sedimentary wedge. Uplift and erosion of a metamorphic/plutonic source area is indicated by the presence of abundant detrital mica, but none of the potential source rocks, the older volcanic and volcaniclastic rocks exposed within the present boundaries of the Avalon Zone, contain more than trace values of mica or recycled sedimentary detritus (Papezik, 1973; Poole, 1973; Dec et al., 1992 ). Consequently, the source area of these deposits ( 1 ) is covered by younger strata, (2) underlies the present sea floor in offshore localities, or (3) has been tectonically removed by movement along the Dover and Hermitage Bay faults (Fig. 1 ). A post-Random unconformity records uplift and local erosion of up to 750 m of strata (Fletcher, 1972; Landing, 1992). Overlying Cambrian deposits of fine-grained siliciclastic mudstone and thin carbonate units contain evidence for repeated episodes of epeirogenic

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P.M. Myrow / Precambrian Research 73 (1995) 123-136

movement and basin reorganization. The evidence includes major shifts in position of depocenters, reversals in direction of marine onlap, and formation of fractures related to possible crustal extension (Hutchinson, 1962; Fletcher, 1972; Landing and Benus, 1988; Landing, 1992 ).

10. Magnetostratigraphy

A detailed magnetostratigraphic study of the Neoproterozoic rocks of the Avalon of Newfoundland has never been completed. Paleomagnetic analyses of the Marystown Group by Irving and Strong (1985) suggest that these volcanic rocks retain a Precambrian signature. The declinations are scattered, but the inclination data indicate a paleolatitude of 35 °. The potential for magnetostratigraphy of the Precambrian-Cambrian boundary section on the Burin Peninsula was explored by Kirschvink (1979) who analyzed green siltstone and mudstone units from the Chapel Island Formation on the Burin Peninsula. His results indicated that the sediments were remagnetized, possibly during the Acadian Orogeny, and that the section was of no use for paleomagnetic polarity stratigraphy. Recent analysis of the redbeds of the Rencontre Formation by J. Hodych (written commun., 1994) has revealed a stable remanence that varies considerably stratigraphically. Work in progress will indicate whether the variation is a primary temporal signature or if it is due to spatial variations in a secondary magnetization. A paleomagnetic study of exposures of the Gaskiers Formation from the east side of St. Mary's Bay by Morgan (1980) indicates that the rocks failed a fold test and suffered considerable magnetic overprinting during the Devonian Acadian Orogeny. Morgan (1980) and D. Symons (unpublished data; pers. commun., 1994) both suggest, however, that a somewhat scattered but stable magnetization from the red mudstone unit at the top of the Gaskiers is potentially of primary origin.

I I. GSSP for terminal Proterozoic

The most suitable locality for a terminal Proterozoic boundary stratotype in the Newfoundland Avalon Zone would be near (?), or at the top of, the Gaskiers Formation. Exposures of this unit are found on the south coast of the Avalon Peninsula along the eastern margin of St. Mary's Bay and on Great Colinet and Little Colinet Islands. Descriptions of these localities, particularly the excellent exposure at Double Road Point, are given by Anderson (1987) and Eyles and Eyles (1989). Exposure of the younger, wellpreserved deep-water assemblage of Ediacaran fossils at Mistaken Point is excellent, and a field guide to this locality is included in Landing et al. (1988). The positive attributes of the Neoproterozoic sections of eastern Newfoundland include the presence of (1) potentially datable glaclogenic diamictites, (2) well-preserved and abundant Ediacaran fauna, (3) abundant volcanic rocks and sedimentary units with intercalated volcanic ash beds that are amenable to U - P b dating, and (4) presence of the Precambrian-Cambrian Boundary Stratotype. If precise U - P b dates can be derived for volcanic units in the Gaskiers Formation then this will be the only locality in which Varangian tillites have been directly dated, a major advantage for a stratotype. (Similar age tillites in Russia and Namibia also hold potential for radiometric dating; A. Knoll and J Grotzinger, pers. commun., 1993. ) Problems with the Newfoundland section include (1) a poor microfossil record, (2) a megafossil assemblage dominated by endemic forms, ( 3 ) an unknown chemostratigraphic and magnetostratigraphic record, and (4) some uncertainty in correlation of units across sections of the Avalon Zone due to contemporaneity of volcanism and deposition, and structural complexities.

Acknowledgements

The author gratefully acknowledges Scan O'Brien and Arthur King for a thorough review

P.M. Myrow / Precambrian Research 73 (1995) 123-136

of an early draft of this paper. Thanks are also extended to Guy Narbonne and John Grotzinger for critical reviews. Discussions with J. Hodych, D. Symons, E. Landing, S. Bowring, R. Tucker and J. Kirschvink were also helpful.

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