| Résumé | (disponible en anglais seulement)
The external Humber Zone of the Appalachian Orogen of western Newfoundland is characterized by a Cambro-Ordovician carbonate-dominated platformal succession
that was structurally overridden and deformed in Middle Ordovician through Devonian times (Taconian, Salinian, and Acadian orogenies) and locally disrupted during the Carboniferous (e.g., Waldron et al., 1998). Contractional structures are largely
thin-skinned in style (low-angle, largedisplacement thrust faults), and are crosscut locally by late thick-skinned (basement-involved) thrusts. The Early to Middle Ordovician Taconian Orogeny involved initial obduction of ophiolitic complexes as well
as structural slices of deep-water slope facies rocks, some high in organic content. Post-Taconian deformation, which included significant displacement on low-angle thrusts, resulted in structural juxtaposition and folding (potential petroleum traps)
of platform carbonates (potential petroleum reservoirs) above organic-rich rocks (potential petroleum sources). The largest traps involved the late-stage inversion of thickskinned normal faults, which were initiated in the Taconian foreland prior to
arrival of the thin-skinned Humber Arm Allochthon and the development of a structural triangle zone (tectonic wedge) along the leading edge of the orogen in Devonian time (e.g., Stockmal et al., 1998, 2004; Cooper et al., 2001).
The timing of
thermal maturation of the potential source rocks relative to the timing of formation of the major structural traps has obvious and critical implications for the petroleum prospectivity of this area. Hydrocarbon seeps, first recognized nearly 200
years ago, have spurred sporadic exploration for over a century. The most recent round of serious exploration occurred in the 1990s, with muchreduced activity continuing to the present. The first modern well, Port au Port #1, was drilled on Port au
Port Peninsula in 1994-1995 and discovered the Garden Hill South field, which produced light sweet crude oil (51 °API). Follow-up wells, including one in the near offshore in the Gulf of St. Lawrence, have been disappointments. The principal
challenge is development of a reliable and predictive reservoir model. The leading model involves karstification associated with a flexural bulge unconformity, coupled with hydrothermal dolomitization associated with the thrustinverted thick-skinned
normal faults (Cooper et al., 2001).
Samples for apatite fission track (AFT) analyses were gathered mainly from outcrop but also from two of the recent exploration wells on the Port au Port Peninsula. These samples span the entire
stratigraphic section from the crystalline Grenville-age (ca. 1 Ga) basement, through the clastic-dominated Lower Cambrian platformal and slope successions, to the Middle Ordovician to Lower Devonian clastic foreland succession, and finally to the
remnants of a regional Carboniferous to Permian successor basin (the Maritimes Basin). Structurally, samples are from the fartransported Humber Arm Allochthon, the parautochthonous basement and platformal succession, the essentially autochthonous
foreland succession (structurally uplifted above the leading-edge triangle zone), and the post-orogenic overlap succession. All samples show significant annealing with mean track lengths ranging from 11.5 to 12.9 microns. Of the 20 samples we have
modeled in detail, ten have a full suite of probe analyses and three have partial analyses (F, Cl, OH, O). All but two of the 20 pass the ?2 test for uniformity, and all have been examined for possible multiple age populations using the binomial
peak-fitting program, Binomfit (Brandon, 2002). The two samples that fail the ?2 test are cuttings samples from the Port au Port #1 well, and Binomfit clearly indicates discordant grains (three grains out of a total of 34 in one sample, and one grain
out of 36 in the other) that are likely caved from higher up the borehole. Removal of the contaminant grains results in solid ?2 passes. Those samples for which we have full probe analyses have been examined for possible multiple compositional
populations, using measured ages and track lengths, and the calculated rmro parameter of Carlson et al. (1999). The majority of grains are fluorapatites, with some modest substitution of Ca by Na, Mn, and Fe. Calculated rmro values across the suite
of samples range from 0.5088 to 0.8621, with mean values of interpreted populations ranging from 0.6513 to 0.8488. The low-temperature thermochronology of this structurally complicated region is assessed using an updated version of the inversion
routine described by Issler et al. (2005) that allows for simultaneous inversion of samples with multiple compositional populations. The multi-kinetic inversion program (AFTINV) yields an objective set of thermal histories that fit observed AFT ages
and track length distributions. AFTINV allows for incorporation of various geological constraints (e.g., depositional age, overlying stratigraphy, unconformities, limits to erosion and deposition rates, etc.), as well as thermal maturation
indicators, if available (e.g., %R0, CAI, Tmax, etc.). The ability of AFTINV to handle multi-kinetic populations in a single sample is essential to interpreting some of the western Newfoundland AFT samples. AFTINV generates a set of forward-modeled
time-temperature histories (nominally 300) that satisfy all input constraints. For some samples run as single populations, AFTINV will not produce more than a few (if any) acceptable time-temperature histories even after tens of millions of forward
models. However, these same samples, run simultaneously as separate populations (as many as four), will result in a full set of 300 solutions in a few million forward models or less. Assessment of this suite of 20 samples indicates that following
post-depositional burial and post-orogenic erosion, all AFT samples were reheated into the AFT partial
annealing window (~60-120 °C; ~oil window) in post-Visean times, due to burial beneath the Carboniferous to Permian Maritimes Basin. The
magnitude and timing of peak temperatures, however, are dependent in part upon structural position with respect to the inverted thick-skinned thrusts. On the Port au Port Peninsula, the principal thrust-inverted thickskinned fault is the Round Head
Thrust (RHT), in the footwall of which lies the Garden Hill South field referred to above. At the deepest stratigraphic levels within the Cambro-Ordovician platform, in the immediate hanging wall of the RHT, peak temperature occurred prior to fault
inversion (during Devonian Acadian orogenesis). At equivalent stratigraphic levels in the RHT footwall, and at shallower levels in both footwall and hanging wall, peak temperatures were not achieved until post-Visean burial beneath the Maritimes
Basin.
Peak thermal maturity therefore post-dated the late Acadian development of large, thick-skinned structural traps. Thus, these structures have high potential prospectivity, as evidenced by the initial discovery well. Modeled AFT samples
from western Newfoundland, Anticosti Island, and the Gulf of St. Lawrence all indicate that the Maritimes Basin was substantially thicker and of greater extent than
commonly believed, with concomitant implications for petroleum exploration
throughout the region. |