Geological characteristics

The Escuminac sedimentary formation is distinguished by several characteristic rock types that have been found in all the outcrops studied so far. The formation shows little variation at the regional scale, which supports the idea that areas where the Escuminac Formation is exposed today were all part of a much larger sedimentary system.

(92 kb)The Escuminac Formation consists mainly of sand and clay rock beds. Each of these layers lined the bottom of the Miguasha estuary at one time or another. Within the cliff protected by the park, sandy and clayey beds alternate, forming a sequence about 120 metres thick.

In a general way, the sedimentary features in the sandy layers reflect the times of highest current energy within the ancient sedimentary environment. The Escuminac primary sedimentary structures – geological features that formed while the sediments were being deposited – include current ripples, horizontal laminations, tool marks and flute casts. Tool marks are seen on the lower bed surface and represent the traces left behind as objects were tugged along by the current. Flute casts, also present on the lower bed surface, form when flowing water scours localized depressions in the sediment. If tool marks and flute casts are found in the same bed, sedimentologists deduce that a turbidity current was responsible. A turbidity current is the sudden arrival of sediments in a turbulent current that moves swiftly down an underwater slope. Rocks formed by the sediments deposited during such events are called turbidites. Common in the Escuminac Formation, turbidites often contain entombed animals that were caught in the path of the turbulent flow.

(40 kb)Conversely, clay beds denote a calmer environment, with barely any water movement. A very finely layered clay-rich rock known as laminite is particularly abundant in the lower third of the Escuminac formation. The laminites are characterized by alternating layers fine and coarse sand, no layer being thicker than 1 mm. The laminite beds easily split into thinner, flat layers, and are often fossil-rich. They reflect small cyclical variations in sedimentation.

(48 kb)Among other sedimentary structures, desiccation cracks are worth mentioning. At the base of the Escuminac Formation, these desiccation structures are evidence of sporadic exposure to air. Rounded concretions, often containing fossils, are also noteworthy; these form by bacterial action around the remains of decomposing organic matter, and are particularly common in the middle of the Escuminac Formation.

The main Escuminac Formation outcrop is exposed on the southwest part of Miguasha Point. It is possible to follow the gently inclined layers for more than three kilometres along the cliff, at which point they plunge under the Carboniferous conglomerates and red sandstones of the Bonaventure Formation to the northeast. They resurface at the eastern border of the Municipality of Nouvelle, where the Ristigouche Syncline terminates. They are exposed again about forty kilometres farther east, at the end of the valley of the Grande Cascapédia River near New Richmond.