Paleofluid-flow during syntectonic flexural extension and Taconian overthrusting, Québec external zone, Northern Appalachians.
Donna Kirkwood, Mohamed Ayt-Ougougdal, Thomas Gayot, Georges Beaudoin (Département
de Géologie et de Génie Géologique, Université
Laval, Sainte-Foy, G1K 7P4, Canada) and Jacques Pironon (CREGU, BP 23, F-54501,
Vandoeuvre-les-Nancy, France)
1. Introduction
Recent field work in the Québec Promontory Nappe has produced results
that place constraints on the tectonic evolution and paleofluid-flow in the
Québec Taconian foreland basin. New stratigraphic and structural data
combined with fluid inclusion petrography, microthermometric measurements
and stable isotope analyses demonstrate that rocks of the Ordovician outer
shelf underwent syntectonic flexural extension during the lowermost Caradocian
and were then thrusted further northwest over the autochthonous margin during
the Taconian Orogeny. Two different fluids migrated through fracture networks
developed in the Ville de Québec Formation that we tentatively relate
to chronologically distinct tectonic events.
2. Geological setting
In the Québec City area, rocks of the Humber zone lie in tectonic contact
with rocks of the undeformed continental margin. The geology in the Québec
City area has been subdivided into three tectonic zones, namely the autochthonous,
parautochthonous and allochthonous domains (St-Julien, 1979). The autochthonous
Cambro-Ordovician shelf sediments were deposited on the Precambrian North-American
basement and are overlain by rocks of the parautochthonous domain which consists
of moderately deformed and imbricated strata originally deposited on the shelf.
To the southeast, the allochthonous domain, also known as the external part
of the Humber zone, comprises tectonically transported strata originally deposited
on the continental shelf edge, slope or rise. The tectonic boundary between
the parautochthonous and overlying allochthonous domain, termed Logan’s
Line, is a northwest verging, southeast dipping thrust fault.
In the study area, the allochthonous domain (external Humber zone) is divided
into five nappes each displaying distinct sedimentary units, internal stratigraphy,
and characteristic structural style (St-Julien, 1995). The studied rocks belong
to the Québec Promontory Nappe, the lowest structural unit of the allochthonous
Humber zone. According to St-Julien (1995), the emplacement of the allochthonous
thrust nappes occurred through progressive accretion and foreward stacking
over the Cambro-Ordovician sedimentary prism during the Middle Ordovician
Taconian orogeny. Consequently, the uppermost Chaudière Nappe comprises
the oldest and farthest-travelled rocks, whereas the lowest and most northwestward
structural unit, the Québec Promontory Nappe, is made up of the most
proximal and least-transported strata.
3. Stratigraphy
The Québec Promontory Nappe comprises rocks of the Ville de Québec
and Citadelle Formations. The Ville de Québec Formation is a chronostratigraphic
correlative of the Chazy and Black River Groups and is interpreted as representing
a shallow marine platformal sequence (St-Julien, 1995). It is composed mainly
of thin- to thick-bedded argillaceous to sandy limestone, interbedded with
dark shales, and rare lenses of pebbly limestone. The Citadelle Formation
is a chaotic unit characterized by slumps and debris flows. It is composed
of a matrix of either black shale or dark, sandy argillites that contain blocks
of argillaceous or sandy limestone, and calcareous shale, very similar to
the Ville de Québec, as well as “exotic” blocks of calcarenites,
chert, dolomitic limestone and limestone conglomerates (St-Julien, 1995).
St-Julien (1995) interpreted the Citadelle Formation as a sedimentary olistostrome
derived by slumps and debris flows shed from the uplifted Ordovician carbonate
platform and from approaching allochthonous units. He also interpreted the
repetitious nature of the Citadelle within the Ville de Québec Formation
as thrust slices in incorporated within the Québec Promontory Nappe.
Both shelly fossils and graptolites have been collected in exposures of Ville
de Québec and Citadelle strata (see Riva, 1985). Graptolites have been
recovered from exposures in the Ville de Québec, ranging from the Nemagraptus
gracilis Zone in the lower part of the formation, to the Diplograptus multidens
Zone in the upper part (Riva, 1985). Suggestions of younger fossils have come
from two collections in the uppermost part of the Ville de Québec Formation
and have been assigned to the Climocograptus americanus Zone (Riva, 1985).
Fossils identified within blocks of the Citadelle Formation range in age from
Chazyan to Black Riverian.
Stratigraphic and structural relationships between the Ville de Québec
and Citadelle formations have been the subject of debate for many years (Riva
1985, St-Julien 1995). Our work has established that the debris beds and mélanges
of the Citadelle Formation occur as ten meter-thick intervals within the top
part of the Ville de Québec Formation. Numerous soft-sediment deformation
features, and syn-sedimentary faults occurring exclusively within the Ville
de Québec limestones, highlight the contact between the two formations.
Bedding near some faults shows normal drag on the downdropped block. A set
of orthogonal extension fractures also occur perpendicular to bedding within
the more competent lithologies of the Ville de Québec Formation. Small
normal offsets can be observed along most of the fractures at the base of
the beds. In most cases, the debris beds of the Citadelle occur in depositional
contact directly above the highly fractured and faulted argillaceous limestones
of the Ville de Québec Formation. Such features strongly suggest a
tectonically active environnement during sedimentation of the upper Ville
de Québec and Citadelle Formations.
4. Structural Geology
Rocks of the Québec Promontory Nappe were folded and thrust over the
autochthonous platform during the Taconian orogeny (St-Julien, 1995). Structurally,
the Québec Promontry Nappe is characterized by a regional NNE-SSW trend.
A well-developed, steeply-dipping first-phase cleavage (S1) and local folds
(F1) are observed throughout the nappe. Fold axes strike parallel to the regional
NNE-trending folds and plunge steeply towards the SSW. Low-angle thrust faults
cut through the steeply-dipping, overturned limbs of the regional anticlines
and synclines.
Fracture sets and veins were studied within the Québec Promontory Nappe
in order to characterize the nature of fluids and timing of their migration
within the nappe. Distinct fracture sets were recognized. Their geometry,
orientation, and kinematics are compatible with syn-tectonic normal faulting
within the basin and Taconian folding and thrusting. The paragenetic sequence
of vein and fracture fill consists of fibrous calcite (I), idiomorphic calcite
(II), and/or quartz crystals and/or bitumen. Hydrocarbon fluid inclusions
and precipitation of bitumen and impsonite (Levine et al. 1991) in open fractures
attest to petroleum migration within rocks of the Québec Promontory
Nappe.
4. Fluid-inclusions and stable isotopes
Both calcite and quartz crystals within the veins and open fractures contain
two-phase aqueous and hydrocarbon inclusions. Only fluid inclusions in calcite
II crystals and quartz were studied. In calcite II, they occur sporadically
in planar clusters with sizes ranging form 10 to 20 µm. In quartz, hydrocarbon
inclusions are very abundant, range from a few µm to a few mm in size
and are generally related to latest stages of crystal growth.
Fluid inclusion homogenisation temperatures and estimated salinities indicate
mixing between warm meteoric water, (<60ƒC, d18Owater = 0 ‰) and a
low salinity basinal fluid at higher temperature (> 160ƒC, 4 wt. % eq.
NaCl, d18Owater = 11 ‰). Progressive mixing is documented by dominance
of water during early calcite II and influx of basinal brine during quartz
precipitation. Within a quartz crystal, FI in core to rim growth zones also
document progressively warmer and more saline fluids. The basinal brine is
associated with hydrocarbon gases and fluids that are found in inclusions
in calcite but more commonly in quartz. Raman spectroscopy and fluorescence
and FT-IR microspectroscopy define 4 types of HC gases and fluids with increasing
homogenisation temperatures: 1) dry gas (methane); 2) wet gas with methane
CO2, ethane, propane, butane; 3) light condensate; 4) heavier condensate which
is depleted in methane but enriched in heavier alkanes compared to the light
condensate.
Calcite carbon and oxygen isotope compositions plot along a trend of decreasing
d18O and increasing d13C values that is consistent with methanogenesis during
burial or heating by the infiltrating basinal brine. Calcite precipitation
could also be a product of temperature increase.
6. Discussion and conclusion
We interpret the stratigraphic and structural relationships in the Québec
Promontory Nappe as reflecting the development of an extensional basin on
the outer shelf in the immediate foreland of advancing nappes. Thus, the outer
shelf was dissected by extensional faults defining local topographic highs
from which were shed debris flows into fault-bounded basins. Rocks of the
Ville de Québec Formation record the earliest tectonic activity of
the outer shelf in the Québec part of the Northern Appalachians, i.e.
lowermost Caradocian. Tectonic instability and subsidence of the outer shelf
is attributed to flexural extension of the North-American margin during the
Taconian orogeny (Bradley and Kidd, 1991).
Structural relationship of fracture sets combined with fluid-inclusion data
and isotopic composition of vein infill help constrain the P-T history of
the basin. Results suggets that a warm (60ƒC), low salinity fluid of meteoric
origin migrated through fracture networks and normal faults in the Ville de
Québec rocks during the extensional collapse of the outer shelf in
the lowermost Caradocian. This fluid was progressively mixed with a warmer
(150-160ƒC) high salinity, basinal fluid associated with hydrocarbon gases
and fluids. P-T modelling of inclusions related to the second pulse suggests
a 3 to 4 km maximum burial depth for rocks of the Québec Promontory
Nappe that we equate with thrusting of the nappe over the autochthonous margin
and/or overthrusting by the Chaudière Nappe.
7. References
Bradley and Kidd, 1991. Flexural extension of the upper continental crust
in collisional foredeeps. GSA Bull, 103: 1416-1438.
Levine, J.R., Samson, I.M. and Hesse, R., 1991. Occurrence of Fracture-hosted
Impsonite and petroleum fluid inclusions, Québec City region, Canada.
AAPG Bull., 75: 139-155.
Riva, J., 1985. The Citadel Formation: Its age on the basis of Trilobites,
Graptolites and Brachiopods. In: Field trips guidebook, (ed.) J. Riva, Canadian
Paleontology and Biostratigraphiy Seminar, Ste-Foy, Québec, 7p.
St-Julien, P., 1995. Géologie de la région de la ville de Québec.
Ministère des Ressources Naturelles du Québec, MB 94-40, 62
p.