MONTMORENCY FALLS
Montmorency Falls is located
northeast of Quebec City in southern Quebec Province, se Canada. The
waterfalls occurs at a major fault. Structurally, the cliff behind the
falls has been uplifted, and the rocks in the foreground of the pic below have
been downdropped and tilted. The water is falling over Precambrian
gneisses (basement rocks). Thin intervals of steeply tilted Trenton
Group limestones and Utica Shale occur downstream of the falls (not visible in
the photo below). The dark, tilted rocks in the right foreground are
basinal black shales of the Lotbinière Formation (Upper
Ordovician), which are graptolitic at this locality.
Montmorency Falls (white
area near center), cliff of Utica Shale (gray area to the southeast of falls),
and Montmorency Promontory exposure (1st area with brown rocks upstream of
bridge).
Satellite
photo provided by DigitalGlobe & Google Earth.
Upstream of Montmorency
Falls is a spectacular exposure of an ancient rocky shore. This is the
best place to observe the Montmorency Promontory, a paleotopographic
high. It consists of Precambrian gneisses with onlapping Ordovician
sedimentary rocks. The Ordovician succession here is mostly shallow-water
limestones. Some of the basal onlapping sediments are fills of Ordovician
tidal pools.
Above & below: Precambrian gneiss of the
Montmorency Promontory, a little upstream of Montmorency Falls, NE of Quebec
City, Canada.
The gray limestone shown in
the photo below is filling an Ordovician tidal pool developed in Precambrian
gneiss. The limestone is a fossiliferous unit informally called the Solenopora
gravels (Trenton Group, upper Middle Ordovician). Its name refers to the
abundance of Solenopora fossils. Solenopora is an extinct
genus of red alga (Rhodophyta, Solenoporaceae). Some of the Solenopora
material at this locality is still red.
Note: “Solenopora” is not
the correct generic assignment for fossil red algae at the Montmorency
Promontory. The type species of Solenopora is from Estonia, and
has been demonstrated to be an Ordovician chaetetid sponge. Some
Paleozoic fossil red algae formerly assigned to Solenopora are now
referred to as Graticula. Mesozoic fossil red algae formerly
assigned to Solenopora need a proper generic assignment as well (see,
for example, "Solenopora" jurassica from the Mesozoic of
Britain).
Above: exposed stratigraphy
upstream of Montmorency Falls. The Montmorency Promontory is Precambrian
gneiss forming a rocky rapids in the river. Onlapping the Montmorency
Promontory is a thin, discontinuous arkosic sandstone. Above that is
upper Middle Ordovician Trenton Group fossiliferous limestones, the basal unit
of which is the Solenopora gravels.
Above & below: “Solenopora” fossil
red algae directly encrusting Precambrian gneiss. This was a rocky shore
environment during the late Middle Ordovician.
