Pierre Jutras

Alleghanian deformation in the eastern Gaspé Peninsula of Quebec, Canada

Juxtaposition of the Mississippian Risti- gouche and Cannes-de-Roches Basins, which are subbasins of the composite late Paleozoic Maritimes Basin, occurred through dextral movement along the north- west-striking PerceFault system in the east- ern GaspePeninsula of Quebec. The north- west-striking faults are truncated by small north-northwest-striking dextral strike-slip faults, which probably developed as region- al stress gradually rotated clockwise from north-northwest-south-southeast to north- east-southwest. This study provides the first evidence in eastern Quebec for significant post-Acadian block displacement other than normal faulting and indicates that Al- leghanian deformation extended much far- ther north than previously thought. Iden- tification of these structures formed during the Alleghanian orogeny but more than 1000 km away from areas of peak Allegh- anian metamorphism in the southeastern United States underlines the far-reaching effects of continental collisions. It also casts doubt on the age attribution of brittle strike-slip faults elsewhere in the Gaspe ´ Peninsula, away from Mississippian expo- sures. Such brittle faults were previously associated with late stages of the Acadian orogeny but could in fact be considerably younger.

Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation

Atmospheric carbon is estimated to have been ~500 times more abundant in Hadean time than at present, and its concentration has been gradually decreasing since then due to its storage in sedimentary rocks. Consequently, rain pH has been gradually increasing through geologic time, leading to the common assumption that groundwaters are less acidic today than they were in the distant past. However, this assumption overlooks the fact that root-forming land plants increase the carbonic acid concentration in soils by one or two orders of magnitude. In the absence of rooted land plants, reactions between minerals and rainwater are known to promote alkalinity. It is hypothesized that groundwater pH must have been, on average, highest shortly before the Late Ordovician to Silurian proliferation of root-forming land plants. To verify this hypothesis, we studied the mineralogy and geochemistry of the youngest known pre-Silurian paleosols to have developed on primary rocks, and provide evidence that they have evolved in predominantly alkaline groundwaters despite warm and humid paleoenvironmental conditions. Today, a lush vegetation cover would thrive in such a climate, and the system would be necessarily acidic due to large inputs of organic acids. Paired with previous observations indicating that early Paleozoic sedimentary rocks are especially rich in detrital illite and K-feldspar, there is now enough evidence to believe that there was a greater tendency for alkalinity during this time period than during previous and subsequent geologic periods.

Significance of paleosurfaces in interpreting the paleogeographic and stratigraphic evolution of the late Paleozoic Paspébiac graben, a recently identified basin in the southern Gaspé Peninsula of Quebec, Canada

The remote-sensing study of an exhumed upper Paleozoic surface in the southern Gaspe Peninsula of Quebec has provided indirect evidence for the presence of a previously unidentified post-Acadian (post–Middle Devonian orogeny) graben-fill unit. Subsequent work on stratigraphic sections in that area confirms that the graben-fill unit, the new Saint-Jules Formation, is indeed petrographically distinct and older than previously documented upper Paleozoic strata in that area, which were collectively assigned to the Visean (Upper Mississippian) Bonaventure Formation. Paleogeo-graphic reconstruction from sedimentary facies, paleocurrent, and provenance studies concurs with the geometric detail that was provided by the preliminary geomorphic study. The present paper therefore presents the case study of an upper Paleozoic stratigraphic unit that was first inferred by geomorphic constraints prior to being recognized in the field through the study of stratigraphic sections. The study underlines the stratigraphic relevance of analyzing exhumed paleosurfaces at the margins of ancient sedimentary basins.

Basin inversion at the Mississippian–Pennsylvanian boundary in northern New Brunswick, Canada

Abstract Upper Paleozoic successions in the area of Bathurst, New Brunswick, were studied in an attempt to solve longstanding stratigraphic debates and to correlate the Carboniferous geology of this area with that of the adjacent Gaspe Peninsula of Quebec. Based on new spore dates and petrographic analyses, the abandoned Bathurst Formation is reintroduced. Paleogeographic reconstructions from facies analysis and provenance studies indicate that the Mississippian Ristigouche Basin formed the source area of the Pennsylvanian Central Basin due to a fault inversion event that occurred near the Mississippian–Pennsylvanian boundary. As a result, the Mississippian Bonaventure Formation, which was sourced from the south, is separated by the east-west striking Rocky Brook-Millstream Fault from the lower Pennsylvanian Bathurst Formation, which was sourced from the north with reworked detritus of the former unit. A possible correlation is made between Pennsylvanian sedimentation in northern New Brunswick and Pennsylvanian faulting in the adjacent Gaspe Peninsula of Quebec.

Reinterpretation of James Hutton's historic discovery on the Isle of Arran as a double unconformity masked by a phreatic calcrete hardpan

Because it is partly masked by a phreatic calcrete hardpan (PCH), a rare and poorly known type of rock that can transgress stratigraphic boundaries, there has been ongoing controversy concerning the exact position of James Hutton9s first discovered unconformity on the Isle of Arran in southwest Scotland. The unconformity separates folded Neoproterozoic to lower Paleozoic (Dalradian) metasedimentary rocks from upper Paleozoic red beds. The massive PCH developed in Late Devonian red conglomerate above the unconformity, but it also assimilated some of the underlying basement rocks, thus giving the false impression that the unconformity is at a lower position, as both host materials are almost entirely replaced by calcrete. At Hutton9s discovery site, only a small remnant of the deeply calcretized Late Devonian conglomerate was preserved from erosion prior to being disconformably overlain by lower Carboniferous red conglomerate and sandstone. Thus, there are two unconformities at Hutton9s historical site, but the younger has previously gone unnoticed, and the two red bed successions on each side of the disconformity were previously thought to belong to the same unit.

Intra-basaltic soil formation, sedimentary reworking and eodiagenetic K-enrichment in the Middle to Upper Ordovician Dunn Point Formation of eastern Canada: a rare window into early Palaeozoic surface and near-surface conditions

Mafic flows of the Middle to Upper Ordovician Dunn Point Formation of eastern Canada were deeply weathered under warm and relatively humid conditions before being buried by subsequent flows. In the absence of superior plants, and in the context of relatively low atmospheric carbon levels, the soils developed alkaline groundwater conditions through mineral–water interactions, which resulted in an enhanced mobility of Al relative to Si in most palaeosols of that formation. Although the vegetation cover was volumetrically insignificant compared with that of subsequent geological times, it was apparently producing very efficient chelates, which, for most palaeosols of the succession, generated a well-defined cheluviation pattern for not only Al and Fe, but also and mainly Ti, which is typically immobile in modern soils. The resulting soils developed an Al–Fe–Ti-depleted upper horizon that was enriched in Si, probably through periodic ground saturation. Long-term climatic variations related to orbital cycles are inferred to have accounted for a second type of soil in the succession, which contrasts with the former by showing a Si-depleted and less Al–Fe–Ti-depleted upper horizon. Some soil material was substantially reworked by surface runoff, but such occurrences can be easily differentiated from in situ soil material in terms of texture, structure and composition. A thick overlying rhyolite flow is thought to be responsible for providing abundant K in solution, which was incorporated in the underlying basalt palaeosols as exchangeable cations within a probably montmorillonitic clay precursor to the Fe–Mg-rich phengite that later developed during deep burial and orogenic compression.

Record of climatic fluctuations and high pH weathering conditions in a thick Ordovician palaeosol developed in rhyolite of the Dunn Point Formation, Arisaig, Nova Scotia, Canada

A thick Ordovician intra-rhyolitic palaeosol was studied to investigate the peculiarities of early Palaeozoic continental environments, shortly before the development of vascular plants, and to compare its pedogenic patterns with those of previously studied intra-basaltic profiles from the same succession. Well-defined K-eluviation from the top of the palaeosol is proportionally met by K-illuviation at the base of the profile, a pedogenic behaviour that was masked in the underlying intra- basaltic profiles due to subsequent eodiagenetic K-enrichment associated with rhyolite emplacement. Greater stability of K (and eventually Mg) than Si in the intra-rhyolitic profile suggests high pH soil water at the time of pedogenesis despite the low base contents of the acidic host rock, and despite evidence for humid weathering conditions. We also observe that, at the base and at the top of the profile, Al2O3/Zr ratios are substantially lower than those of the host rock, suggesting Al-leaching and therefore extreme weathering conditions, whereas the middle portion of the profile shows Al2O3/Zr ratios that are similar to those of the host rock, suggesting Al stability and therefore less extreme conditions. We interpret these variations in Al2O3/Zr ratios as signatures of a cyclic change from sub-humid periods during which Al was stable throughout the profile, to more humid periods with a well-defined seasonality, during which only the juvenile zone of weathering at the base of the profile couldhavedevelopedsufficientlyhighpHduringdryseasonsforAltobeleached,leavingtheAl2O3/Zr signatures from previous stages untouched higher in the profile.

Climate fluctuations recorded in phreatic and vadose calcretes of the Lower Carboniferous Clyde Sandstone Formation of Machrihanish, Kintyre Peninsula, SW Scotland

The Galdrings cliffs of Machrihanish, in the Kintyre Peninsula of Scotland, expose a Lower Carboniferous clastic succession that hosts a wide variety of calcretes, including thick and massive host-replacing phreatic calcrete hardpans (HRPCHs), which are geologically rare, and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters. Isotopic values of 53 samples from various calcretes distributed in a c. 45 m thick succession provided well-defined trends of covariance between carbon and oxygen isotopes, thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates. Although values from HRPCHs are more constrained, all other forms of calcrete in the succession (pedogenic nodules, pillar calcrete, laminar calcrete and invasive phreatic calcrete cement in sandstone) follow similar stable isotopic trends. An aridification trend preceded the formation of each HRPCH occurrence, corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation. The better constrained isotopic values of HRPCHs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements, which may in part explain their rarity in the geological record.

The Role of Salt Tectonics, Glacioeustatic Variations, and High pH Evaporitic Groundwater in the Development of Synsedimentary Paleokarst within Carboniferous Polymictic Fanglomerate at Hopewell Cape, Atlantic Canada

A succession of Lower Carboniferous polymictic conglomerate in eastern Canada is truncated by synsedimentary endokarst conduits, which are clogged by lithified karst infill. Although such rocks do not usually host karst because of their polymineralic and poorly soluble contents, there are contextual and geochemical lines of evidence suggesting that groundwater conditions may have been highly alkaline at the time of karst formation, thus substantially increasing the solubility of common silicate minerals. The succession was deposited in a hyperarid climate and in close proximity to an evaporitic marine body. Because of a combination of high subsidence rates along a basin-bounding normal fault and entrapment within a salt expulsion minibasin, the evaporitic body of water was forced to prograde toward the source area of the sedimentary basin. This restricted sea was also responding to ongoing glacioeustatic variations at the time, which are interpreted to have generated a cyclic progradation of evaporitic groundwater into basin margin fanglomerates, thus favoring their early cementation by calcite. Karstification of the siliciclastic material is interpreted to have occurred during cyclic retreats toward lowstands, when groundwater alkalinity may have been highest.