O’Brien, Brian H.

English

University of New Brunswick: Centre for Digital Scholarship Journals

GAC–MAC 2012:FIELD GUIDE SUMMARYPROLOGUEGeoscience Canada intends to publish summaries of  forthcoming field trip guides that are national or international in scope.Authors may wish to submit summaries of  these guides in order to ‘advertise’ their trip to the broad geoscience community.They are strongly advised to do so well in advance of  the field trip. In particular, we welcome summaries from field trips thataccompany forthcoming GAC–MAC annual meetings, or divisional meetings. Here, we publish summaries from three of  thefield trips to be held in conjunction with the GAC–MAC 2012 annual meeting to be held in St. John’s May 27th-29th.Peri-Gondwanan Arc-back-arc Complex andBadger Retroarc ForelandBasin: Development of theExploits Orocline of CentralNewfoundlandGAC–MAC 2012, St. John’s post-meeting field tripBrian H. O’BrienDepartment of  Natural ResourcesGeological SurveySt. John’s, NL  Canada  A1B 4J6E-mail: brianobrien@gov.nl.caFIELD TRIP OBJECTIVESThe main objective of  this field trip tonorth-central Notre Dame Bay is toinvestigate the Ordovician evolution ofthe Iapetan island arc and back-arcrocks located within the type area ofthe Exploits Subzone of  the DunnageZone. The approach will be to high-light the main depositional and mag-matic events recorded in the Early andMiddle Ordovician parts of  theExploits Group, the eastern Wild BightGroup and contemporaneous igneouscomplexes. The field stops arearranged in an effort to track the pre-tectonic geological history of  the EarlyOrdovician volcanic and plutonic rocksthat comprise the oldest peri-Gond-wanan ensimatic arc, and to demon-strate their tectonostratigraphic linkageto a younger Middle Ordovician arcand a Late Ordovician overstepsequence.Participants will also have anopportunity to study some of  the Mid-dle Ordovician strata outcroppingwithin the Red Indian Line mélangebelt at the margin of  the peri-Laurent-ian Notre Dame Subzone and to com-pare or contrast them with coevalrocks in the peri-Gondwanan exten-sional arc and back arc sequence. TheModel to explain the structural arcuation of  the Newfoundland Dunnage Zone inthe Mid Paleozoic. During the orogenic accretion and regional metamorphism ofthe Late Ordovician–Early Silurian Badger Group, composite Laurentia was under-plated by a peri-Gondwanan microplate. Subsequent tectonic wedging was antiver-gent and resulted in the overthrusting of  the peri-Gondwanan rocks along withtheir terrestrial cover. This was facilitated by a switch from sinistral to dextralmovement during orogen-parallel convergence.4GEOSCIENCE CANADA Volume 39 2012 5various stratigraphical and structuralrelationships that Late Ordovician stra-ta display with the above-mentionedrocks will be examined immediatelysouth of  the Red Indian Line near themargins of  a mudstone-rich basin inthe olistostrome-bearing BadgerGroup. In the field trip area, theNotre Dame Bay oroclinal flexure isoutlined by greenschist facies stratifiedrocks that illustrate evidence of  inho-mogeneous regional ductile deforma-tion. As the oroflex evolved, the earlyformed tectonites were locally over-printed by secondary transpressionalstructures as regional metamorphismcontinued. The presumed Silurianstructural history will be addressed, inpart, by examining the development offolds, foliations and oblique faults in(a) the southern margin of  the RedIndian Line mylonite zone and (b) anarcuate slate belt that controls disposi-tion of  the rocks of  the Exploits Sub-zone and its overstep sequence in thispart of  central Notre Dame Bay. SUMMARY OF FIELD TRIP STOPSFor all stops on this field trip, theguide book systematically lists 1) geo-graphic access, 2) large-scale strati-graphic and structural position, 3) per-tinent features in the exposure exam-ined, and 4) regional tectonic signifi-cance. LIST OF STOPS FOR THE AFTERNOON OF DAY ONE:Stop 1-1: Early Tremadocian peri-Gondwanan primitive oceanic-arcsequence disposed in a tectonic horse(sericitic alteration zones in metabasiteschist of  the Glovers Harbour Forma-tion of  the Wild Bight Group)Stop 1-2: Early Tremadocian tonalitewithin the infrastructure of  the oldestperi-Gondwanan magmatic arc (syntec-tonic intrusion in ductile shear zonesconfined to the South Lake IgneousComplex arc ophiolite)Stop 1-3: Tonalite-bearing pebblywacke sourced from Exploits Subzonerocks near the southwest margin of  thePoint Leamington basin (BadgerGroup channel-fill deposits above ascoured earliest Katian – latest Sand-bian substrate) Stop 1-4: Concretionary shaley flyschin the distal part of  the Point Leaming-ton basin (graptolite-bearing abyssalmudstone and siltstone turbidite ofuppermost Katian age in the PointLeamington Formation of  the BadgerGroup)  Stop 1-5: Multiple mudstone-rich olis-tostromes within an unbroken forma-tion of  siltstone turbidite (intraclastsand extraclasts within slump sheets oflatest Katian age in the centre of  PointLeamington basin)LIST OF STOPS FOR DAY 2:Stop 2-1: The Tea Arm–SaundersCove transition from hematizedboninitic andesite to jasperite andreplacement chert in the lowerExploits Group (compacted, silicifiedand slumped blocks of  cupriferous pil-low lava in a size-graded carbonate-bearing breccia)Stop 2-2: Chert–mudstone–turbiditetransition in a condensed overstepsequence of  the graptolite-bearingUpper Ordovician Shoal Arm Forma-tion (southwest-directed structures inpart of  a triangle zone below over-thrust sheets of  the Exploits and WildBight groups)Stop 2-3: Pillow breccia of  island arctholeiite composition within thePhillips Head volcanoplutonic complex(thrust-bounded inlier of  the WildBight Group extensional arc sequencewithin the New Bay Formation of  theExploits Group?)Stop 2-4: Mudstone-rich olistostromalsequence within the early MiddleOrdovician Saltwater Pond Member ofthe upper New Bay Formation of  theExploits Group (detrital megaraft ofarc-derived pillow lavas associated withre-sedimented beds of  plutonic boul-der conglomerate)Stop 2-5: Volcaniclastic turbidites,pyroclastic flows and gabbro sills ofthe Charles Brook Member of  thelower New Bay Formation (northeast-ern subbasin of  the Lower–MiddleOrdovician Exploits Group near therefolded slates of  the Sunday IslandSyncline)Stop 2-6: Strong Island radiolarianchert and Lawrence Head within-platebasalt of  the uppermost ExploitsGroup (late Middle Ordovician infill ofthe Exploits back arc basin in thenorth-central Exploits Subzone)LIST OF STOPS FOR THE MORNINGOF DAY 3:Stop 3-1: Primary depositional fea-tures and block-within-block hierarchyin the Boones Point Complex (giantolistolith of  slump-folded sandstoneturbidite and re-amalgamated granite-bearing conglomerate set in the blackshale matrix of  the mélange) Stop 3-2: Altered tectonic sliver ofNotre Dame Subzone mid-ocean ridgebasalt within folded turbidites of  theMiddle Ordovician Moores Cove For-mation (bivergent faulting and lithos-tratigraphic separation in the CottrellsCove Group within the Red IndianLine imbricate thrust belt) Stop 3-3: Polydeformed block-in-matrix mélange and scaly-foliated peb-bly mudstone near the Red Indian Line(strongly cleaved turbidites from theNotre Dame Subzone thrust south-westward above a Boones Pointmelange tract marked by large exoticblocks) Stop 3-4: Boundary between theMoores Cove and Fortune Harbourformations on the southwesterly-over-turned limb of  the Cottrells Cove Syn-cline (large displacement or non-depo-sition at the contact between Tremado-cian island arc lavas and Darriwilianabyssal plain turbidites?)

Peri-Gondwanan Arcback-arc Complex and Badger Retroarc Foreland Basin: Development of the Exploits Orocline of Central Newfoundland

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Peri-Gondwanan Arcback-arc Complex and Badger Retroarc Foreland Basin: Development of the Exploits Orocline of Central Newfoundland

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