Date of Award
Doctor of Philosophy (PhD)
Professor G. V. Middleton
Coarse sandstones of the allochthonous Lower Ordovician Tourelle Formation occur in spectacular exposures along the south shore of the St. Lawrence River. The formation is characterized by thick-bedded, coarse, massive sandstones. Individual graded beds may be tens of metres thick, and may contain slabs of shale, siltstone, or bedded chert up to 16 m in length. Cross-stratification and unusual near-horizontal stratification are the only internal depositional structures. Fluid-escape pillars indicate post-depositional liquefaction. Associated facies are thick shale units, rippled siltstones, slurry sandstones, classical turbidites, slump sheets, and clastic injections.
The Tourelle Formation was deposited on the channelized mid-fan of a submarine-fan complex. Several major channels (to 13 m deep) contain sandstone fills which thin upward. Most asymmetric megasequences thin upward, and are also interpreted as channel fills. These cycles average 20 m thick. A model for the filling of submarine fan channels is presented. Well-defined thinning or thickening upward trends are not common, although thinning upward cycles dominate sequence (Markov) analysis.
At some outcrop localities, packets of coarse sandstone alternate with thick shale horizons, interpreted as mud blankets over abandoned mid-fan lobes. Rippled siltstones are interpreted as overbank deposits adjacent to distributary channels.
Thick sandstones can be divided into stratified or massive layers, which may be capped by normal tractional structures. Inverse-graded stratification bands (to 10 cm thick) are based by a near-horizontal scour surface. Several such "bands" form a division of stratification. Stratified layers are well graded (distribution grading), and have a strong and consistent a-axis fabric, with long axis parallel to flow. Upcurrent imbrication exceeds 20°. These layers were deposited beneath supercritical turbidity currents. Stratification bands formed by progressive "freezing" of a flowing grain layer at the base of the current.
Massive layers may be well graded, poorly graded, inverse-to-normally graded, or ungraded. a-axis grain orientation is most strongly developed at the base and top of layers. The middle parts of individual units may have bimodal or isotropic fabrics, with both upcurrent and downcurrent imbrications. Most massive layers, or divisions, were deposited rapidly from dense, supercritical turbulent suspensions as a result of decreasing current capacity at velocities on the order of 10 m sˉ¹. Unusually thick layers containing enormous blocks were probably transported by laminar debris flows on slopes of 0.02 to 0.05.
The average composition of Tourelle sandstones is 45.6% quartz, 8.2% feldspar, 13.4% rock fragments, 16.1% chloritic matrix, and 15.3% calcite "cement". These sands contained 10 to 15% primary detrital matrix. The most distinctive rock fragments are radiolarian chert, and mafic to felsic volcanic detritus. The heavy mineral suite contains rounded ultrastable zircon and tourmaline, abundant epidote group minerals and garnet, unstable prismatic hornblende and pyroxene, and 18% transluscent cherry-red chromite.
Tourelle detritus was derived by erosion of rising cordillera and obducted ophiolites in the interior of the nascent Taconic orogenic belt, and represents the first influx of detritus from a southeastern source. Previous sediments were eroded directly from the Grenville shield and its fringing carbonate bank. These sediments were recycled to form Tourelle sands.
A plate tectonic model is proposed which involves closing of a marginal ocean basin along a southeastward-dipping subduction zone, collision of the ancient continental margin of North America with a volcanic are situated on a micro-continent of crystalline Grenville basement, and obduction of a large sheet of ophiolite. Previously published models do not provide a source area for the Tourelle Formation, and must be considered inadequate.
The Tourelle dispersal system consisted of a series of small, closely-spaced and coalescing submarine fans which were consistently deflected to the west along a sloping, elongate depositional trough.
Hiscott, Richard Nicholas, "Sedimentology and Regional Implications of Deep-water Sandstones of the Tourelle Formation, Ordovician, Quebec" (1977). Open Access Dissertations and Theses. Paper 767.