Paleofluid-flow in a foreland basin, Northern Appalachians: from syntectonic flexural extension to Taconian overthrusting.

Donna Kirkwood*, Mohamed Ayt-Ougougdal*, Thomas Gayot*, Georges Beaudoin* and Jacques Pironon***
*Département de Géologie et de Génie Géologique, Université Laval, Québec, Canada, email: dkirkwoo@ggl.ulaval.ca
*** CREGU, BP 23, F-54501, Vandoeuvre-les-Nancy, France

Abstract
Structural relationship of fracture sets combined with fluid-inclusion data and isotopic composition of vein infill help constrain the P-T history of the Québec foreland basin. Results suggest 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.
Keywords: Foreland basin, paleofluid-flow, extensional collapse, petroleum migration.

1. Introduction
Recent 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 rocks that we relate to the chronologically distinct tectonic events.

2. Geological setting
Cambro-Ordovician rocks of the northern Appalachians are divided into five tectonostratigraphic zones (Williams, 1979). The northwestern most zone, the Humber zone, consists of deformed and transported sedimentary strata and crystalline basement rocks that represent the ancient continental margin of eastern North America.
In the Québec City area, rocks of the Humber zone lie in tectonic contact with rocks of the undeformed continental margin (fig. 1). The geology in this area has been subdivided into three tectonic zones, namely the autochthonous, parau-tochthonous 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. It is divided into five nappes each displaying distinct sedimentary units, internal stratigraphy, and characteristic structural style (St-Julien, 1995). These are, from northwest to southeast, the Québec Promontory, Pointe-de-Levy, Bacchus, Rivière Boyer and Chaudière nappes. 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 structural unit, the Québec Promontory Nappe, is made up of the most proximal and least-transported strata.

3. Shelf and foreland basin stratigraphy
The Middle Ordovician Taconian orogeny was brought about by collision of the North-American passive margin with a subduction complex at the leading edge of series of magmatic arcs (St-Julien and Hubert, 1975, Williams 1979, Stanley and Radcliffe, 1985). The first sign of tectonic activity along the margin is a period of uplift and erosion of the platform as recorded by unconformities in the carbonate succession throughout the northern Appalachians and has been attributed by many to the passage of a peripheral forebuldge (Bradley and Kidd, 1991, Knight et al., 1991, Lavoie, 1994).
In the Québec part of the northern Appalachians, the evolution of the autochtonous shelf to foreland basin sequence is well documented (see Lavoie, 1994). The stratigraphic succession of the platformal sequence comprises initial, siliciclastic sediments (Potsdam Group), deposited on the unstable passive margin of Laurentia during the opening of the Ordovician Iapetus ocean. The Potsdam is followed by shallow-water carbonate strata (Beekmantown, Chazy, Black River and Trenton groups), that record establishment of an Ordovician carbonate platform and the progressive drowning and foundering of the tectonically active shelf.
The Québec Promontory Nappe comprises the Ville de Québec and Citadelle Formations. The Ville de Québec Formation is a chronostratigraphic correlative of the Chazy and Black River Groups (see below) and is interpreted as representing a shallow marine platformal sequence (St-Julien, 1995). The Citadelle Formation is a chaotic unit characterized by slumps and debris flows that contain blocks of argillaceous or sandy limestone, and calcareous shale, very similar to the Ville de Québec, as well as more “exotic” blocks. St-Julien (1995) interpreted the Citadelle Formation as a sedimentary olistostrome derived by slumps and debris flows form the uplifted Ordovician carbonate platform and from approaching allochthonous units. He also proposed a faulted contact between the Ville de Québec and Citadelle Formations to explain the repeated interlamination of melange-type rocks as well as the abrupt transition between the two units.
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 occur in depositional contact directly above the highly fractured and faulted argillaceous limestones of the Ville de Québec Formation.

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. Four distinct fracture sets were recognized. The first two sets are almost always completely filled with calcite and/or quartz. They are oriented 1- N-S, with shallow to moderate dips and 2- E-W, with moderate to steep dips. Their geometry, orientation, and kinematics are compatible with syn-tectonic normal faulting within the basin. The last two sets are oriented either NE or NW and are compatible with 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.

5. 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. Two-phase or multi-phase aqueous inclusions occur more sporadically in quartz and contain two or more solid phases.
Fluid inclusion homogenisation temperatures (fig. 2) and estimated salinities indicate mixing between warm meteoric water, (<60 C, d18Owater = 0 ‰) and a low salinity basinal fluid at higher temperature (> 160 C, 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. In some quartz crystals, the core is characterised by inclusions with heavy condensate followed in the growth (time) direction by light condensate and wet and dry HC gases. This distribution of HC compositions is consistent with thermal cracking of HC. 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 are 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 (fig. 3) 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
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