Glen S. Stockmal

Structural and tectonic evolution of the Humber Zone, western Newfoundland, 1. Implications of balanced cross sections through the Appalachian structural front, Port Au Port Peninsula

The Appalachian structural front in western Newfoundland, which is principally a submarine feature that comes ashore on Port au Port Peninsula, is interpreted to be a structural triangle zone, or tectonic wedge. Rocks within the triangle zone, which are therefore transported, include the Taconian (Middle Ordovician) Humber Arm Allochthon, Taconian foreland clastic sediments, and the Cambro-Ordovician platform succession. The transported clastic sediments and platformal rocks, as well as structurally involved Grenville crystalline basement exposed east of the peninsula, compose the Acadian (Siluro-Devonian) Port au Port Allochthon, which carried the Humber Arm Allochthon as a high structural slice; Acadian transport of tens of kilometers westward is suggested. This interpretation contrasts markedly with the traditional and widely accepted interpretation of the Taconian and Acadian orogenies in western Newfoundland. Here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced, balanced cross sections across the Port au Port Peninsula. These sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone, as well as the envisioned structural linkage between exposures on Port au Port Peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast. The southeast vergent Tea Cove thrust, which formed an early upper detachment to the triangle zone, is offset and overturned by the north-northwest vergent Round Head thrust. The Round Head thrust is interpreted to flatten into a second southeast vergent thrust, the Red Brook detachment, which formed a late upper detachment to the triangle zone. Some map-scale structures are interpreted as fault-bend folds above oblique ramps on the Red Brook detachment. Allochthonous crystalline basement within the triangle zone, beneath its attached platformal cover, is interpreted as a result of thrust reactivation of a preexisting basement-cutting normal fault, which may have formed during rifting of the early Paleozoic Iapetus Ocean. This interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on Port au Port Peninsula, and is supported by the restored cross sections.

Structural and tectonic evolution of the Humber Zone, western Newfoundland 2. A regional model for Acadian thrust tectonics

The Humber Zone of the western Newfoundland Appalachians represents a Cambrian-Ordovician passive continental margin which was deformed in Taconian (mid-Ordovician) and Acadian (Silurian-Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment-filled Acadian foreland basin. Previous models suggest that the on-land shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show sub-horizontal reflections between 2 and 5 s two-way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal decollement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.

Basement-Involved Inversion at the Appalachian Structural Front, Western Newfoundland: An Interpretation of Seismic Reflection Data with Implications for Petroleum Prospectivity

Abstract Recent hydrocarbon exploration in western Newfoundland has resulted in six new wells in the Port au Port Peninsula area. Port au Port No. 1, drilled in 1994/95, penetrated the Cambro-Ordovician platform and underlying Grenville basement in the hanging wall of the southeast-dipping Round Head Thrust, terminated in the platform succession in the footwall of this basement-involved inversion structure, and discovered the Garden Hill petroleum pool. The most recent well, Shoal Point K-39, was drilled in 1999 to test a model in which the Round Head Thrust loses reverse displacement to the northeast, eventually becoming a normal fault. This model hinged on an interpretation of a seismic reflection survey acquired in 1996 in Port au Port Bay. This survey is now in the public domain. In our interpretation of these data, the Round Head Thrust is associated with another basement-involved feature, the northwest-dipping Piccadilly Bay Fault, which is mapped on Port au Port Peninsula. Active as normal faults in the Taconian foreland, both these faults were later inverted during Acadian orogenesis. The present reverse offset on the Piccadilly Bay Fault was previously interpreted as normal offset on the southeast-dipping Round Head Thrust. Our new interpretation is consistent with mapping on Port au Port Peninsula and north of Stephenville, where all basement-involved faults are inverted and display reverse senses of motion. It also explains spatially restricted, enigmatic reflections adjacent to the faults as carbonate conglomerates of the Cape Cormorant Formation or Daniels Harbour Member, units associated with inverted thick-skinned faults. The K-39 well, which targeted the footwall of the Round Head Thrust, actually penetrated the hanging wall of the Piccadilly Bay Fault. This distinction is important because the reservoir model invoked for this play involved preferential karstification and subsequent dolomitization in the footwalls of inverted thick-skinned faults. The apparent magnitude of structural inversion across the Piccadilly Bay Fault suggests other possible structural plays to the northeast of K-39.

REPLY TO DISCUSSION: A field guide to portions of the Jurassic Fernie-Kootenay Trans-Canada Highway section, Banff, Alberta: new sedimentological and structural observations and interpretations

We thank Hamblin and Walker (2000) for their interest in our recent observations and interpretations of the Jurassic Fernie-Kootenay section near Banff town site (Cant and Stockmal, 1999). As we stated in our paper, our intention was “to provoke discussion” and “to reassess some uncertainties and ambiguities regarding the sedimentological interpretation”. We have clearly succeeded in these objectives. However, Hamblin and Walker (2000) have misread our intentions, and have cast our contribution not as presenting a viable alternative interpretation, but rather as a denunciation of their conclusions. Although we offered compelling evidence that some of their conclusions are likely in serious error due to extensive deformation in the lower part of the section, they are mistaken that we have completely dismissed their sedimentological interpretations in favour of our own. Throughout our paper we emphasized that our new sedimentological interpretations are viable alternatives, never once stating or implying that the interpretations of Hamblin (1978,1979,1983) and Hamblin and Walker (1979) must be incorrect, or that we must “reject” them. For example, on page 12 of Cant and Stockmal (1999), with reference to the lower section, we state “It is therefore possible that these beds could be true turbidites..., but could also be interpreted as storm return-flow deposits, or turbid underflows discharged directly from fluvial or deltaic distributaries”, and on page 15 we state “Rather, these units could equally as well be interpreted as tempestites...”. We emphasize our use throughout our paper of “could”. Nowhere do we emphatically state that these particular rocks are tempestites, and not turbidites. Similarly, with reference to the upper section, nowhere in the text do we state that it is absolutely non-marine — in fact, a lagoonal setting was specifically suggested. Rather, we offer alternative views, which Hamblin and Walker (2000) have chosen …