Preliminary Investigations Into the Structural Geology of the Bonavista Peninsula, Northeast Newfoundland

Current Research (2016) Newfoundland and Labrador Department of Natural Resources Geological Survey, Report 16-1, pages 133-152 PRELIMINARY INVESTIGATIONS INTO THE STRUCTURAL GEOLOGY OF THE BONAVISTA PENINSULA, NORTHEAST NEWFOUNDLAND A. Mills, T. Calon1 and C. Peddle1 Regional Geology Section 1Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL, A1B 3X5 ABSTRACT The Bonavista Peninsula is divided into three Neoproterozoic siliciclastic tectonostratigraphic domains including, from west to east: the West domain comprising the 620–600 Ma arc-derived, turbiditic Connecting Point Group; the Central domain comprising mainly rocks of the terrestrial to marine, volcano-sedimentary Musgravetown Group (<600–545 Ma); and an East domain correlative to rocks of the East Avalon Basin (Conception, St. John’s and Signal Hill groups; <585–545 Ma). These domains are separated by north- to north-northeast-trending faults zones: the Indian Arm Fault and the Spillars Cove–English Harbour fault zone. Each domain preserves evidence of at least two regional deformation events. These have been traditionally recognized across the Avalon Zone of Newfoundland as the Neoproterozoic Avalonian Orogeny (D1) and the mid-Paleozoic Acadian Orogeny (D2). The main deformational elements that characterize the more easily recognizable D2 event include: north-northeast- and south-southwest-plunging, open to tight, upright folds with a regionally developed, axial- planar, north- to north-northeast-trending, subvertical penetrative cleavage (S2) that is nearly ubiquitous across all three domains but best expressed in Cambrian cover sequences. D1 is less evident, as it lacks a regionally developed penetrative cleavage. Steeply, but variably east- and west-plunging folds that are clearly overprinted by S2 cleavage provide clear evidence of Neoproterozoic D1 deformation. The F1 fold axes vary from a generally east–west trend in areas where bedding dips shallowly, to more variable trends close to north- and northeast-trending faults, where Acadian transposition appears to have reoriented the steep limbs, in particular, of Avalonian folds. North-directed Avalonian thrusting, with asymmetric, thrust-style folding has been recognized in the west and central domains. Avalonian deformation is also recognized in the East domain where it is clearly overprinted by D2 deformation. These data substantiate the evidence for Avalonian deformation documented elsewhere in the Avalonian Terrane of Newfoundland and characterize Avalonian deformation style so that its elements can be distinguished from later, Acadian deformation elements. INTRODUCTION mation and related metamorphism across the Avalon Zone are attributed to the Devonian Acadian event at ca. 390–370 Two regional deformation events are recognized in the Ma (Dallmeyer et al., 1983) or possibly at 420–400 Ma (van Avalon Zone of Newfoundland: the Neoproterozoic Avalon- Staal, 2007; van Staal et al., 2009). Paleozoic deformation is ian Orogeny and the mid-Paleozoic Acadian Orogeny (e.g., best expressed in the Cambrian cover successions, preserved Rodgers, 1967; Hughes, 1970; Dallmeyer et al., 1981, 1983; at Keels and near Ocean Pond on the Bonavista Peninsula, O’Brien et al., 1983). Previously cited evidence of Precam- where the subvertical, north-northeast-trending S2 fabric is brian orogenesis includes Precambrian magmatism (e.g., strongly developed, and F2 fold axes plunge shallowly to the Krogh et al., 1988), coarse clastic rocks derived from local north-northeast or south-southwest (see O’Brien, 1994; volcanic rocks (Lilly, 1966), and Precambrian angular Mills, 2014). unconformities (e.g., McCartney, 1967; Anderson et al., 1975; Mills et al., this volume). However, Precambrian This study has led to further recognition of Avalonian thrust tectonics have, thus far, been documented solely in deformation and the effects of Acadian overprinting. This the vicinity of Flatrock on the Avalon Peninsula (King, paper documents the deformation style in select areas across 1990; Rice, 1996; Calon, 2001). Regional Paleozoic defor- the Bonavista Peninsula. 133 CURRENT RESEARCH, REPORT 16-1 STRUCTURAL DOMAINS ON THE position of Acadian and pre-existing Avalonian fold struc- BONAVISTA PENINSULA tures (e.g., Plate 1D) and imply folding of steeply tilted beds of the CPG, overprinted by north-northeast-striking steep For this study, the Bonavista Peninsula is divided into Acadian cleavage. three structural domains, including (from west to east): the West domain, comprising rocks of the 620–600 Ma Con- Folds are more common in this black shale to argillite necting Point Group; the Central domain comprising main- unit than in any other succession within the CPG, and are ly rocks of the Musgravetown Group (<600–545 Ma); and variable in style, scale and orientation. Some examples the East domain, consisting of rocks of the Avalon Basin include tight, recumbent folds (Plate 1B), refolded isoclinal (Conception, St. John’s and Signal Hill groups; >582–545 folds (Plate 1C) and asymmetric folds that locally show Ma; Figure 1). Cambrian inliers are present in the West and apparent northeast vergence (Plate 1D), compatible with the Central domains (Figure 1). The domains are separated by inferred northwest–southeast Avalonian fold trend. north-northeast-trending faults or fault zones, namely (from Although these rocks have sustained both Avalonian and west to east): the Indian Arm Fault, and the Spillars Acadian deformation, it is locally difficult to distinguish tec- Cove–English Harbour fault zone (SCF). tonic structural elements from synsedimentary deformation. Some of the structures observed in the black argillite appear WEST DOMAIN to be incompatible with both Avalonian and Acadian structures observed elsewhere in the West domain and may rep- The Connecting Point Group (CPG) comprises the old- resent syndepositional structures. Moreover, the rheological est rocks on the Bonavista Peninsula (ca. 620 to >600 Ma; contrast between the black shale and the more competent see Mills et al., this volume). The >3500-m-thick pile of siliceous turbidites above and below this unit, may have Neoproterozoic epiclastic, marine turbiditic sandstone, silt- caused strain to partition into the incompetent shale that may stone, and shale, and minor conglomerate and mixtite occurs form zones of regional detachment during thrust tectonics. stratigraphically above the volcanic Love Cove Group, the remnant of a Neoproterozoic volcanic island-arc that sup- Interbedded Sandstone and Mudstone of the plied volcaniclastic sediment to the marine basin, now rep- Upper CPG resented by the CPG (Knight and O’Brien, 1988; Dec et al., 1992). Geochronological constraints include a 620 ± 2 Ma Thin- to medium-bedded, siliceous sandstone and mud- greenschist-facies volcanic rock from the top of the Love stone of the CPG were examined, in detail, at a roadside out- Cove Group (O’Brien et al., 1989) and a 610 ± 2 Ma tuff bed crop along Highway 230 (Figures 1 and 3A). At Station from the middle of the CPG succession (Dec et al., 1992; 15AM131, poles to bedding planes define a π girdle with an Mills et al., this volume). The CPG is overlain conformably F1 fold axis that plunges very gently to the southwest (Fig- to unconformably by the terrestrial to marine, latest Neo- ure 3B). Cleavage dips steeply to the east-southeast, again proterozoic, volcanic and sedimentary Musgravetown consistent with S2 cleavage elsewhere on the peninsula. All Group. measured bedding–cleavage (S0–S2) intersection lineations (L2) plunge gently to the south-southwest, indicating gentle Black Shale Unit of the Middle CPG south-southwest-plunging F2 folds (Figure 3B). The fold axis calculated from the bedding data is offset (to the west) Deformation in the CPG rocks is most pronounced in from L2. The offset is inferred to result from superposed the black shale and argillite exposed on the southeast side of deformation, i.e., the rocks may have been part of an F1 the Northwest Arm of Sweet Bay, east of Winter Brook (Fig- (Avalonian) fold train that was overprinted and modified by ure 1). A north-trending cleavage is locally developed (Plate D2 (Acadian) deformation. 1A), consistent with the orientation of the S2 cleavage that is regionally and penetratively developed across the Bonavista Lower Connecting Point Group–The Kate Harbour Peninsula, with only minor refraction across sub-Cambrian North Section unconformities (e.g., O’Brien, 1994; see also Riveros, 1998). At Station 15AM087, the intersection between bed- North of Kate Harbour, in the Sweet Bay area, several ding and S2 cleavage plunges steeply to the north-northeast faults are preserved in rhythmic bedded, fine-grained, (Figure 2), and is steeper than, but consistent with, the inter- siliceous sandstone and shale (distal turbidites; Figure 4). section of the average S2 with the π girdle for all bedding These rocks are variably reddened near the Kate Harbour data along this stretch of shoreline. The fold axis calculated fault (O’Brien, 1994; Figure 4), and comprise an imbricate from the regional bedding data plunges gently toward the thrust stack (Figure 5) that structurally overlies medium- to southeast (Figure 2) and does not lie in the Acadian (S2) thick-bedded, locally rippled, grey sandstone of the Kate cleavage plane. The orientation patterns demonstrate super- Harbour formation (upper Connecting Point Group; see 134 A. MILLS, T. CALON AND C. PEDDLE Figure 1. Regional geological map of the Bonavista Peninsula showing

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