Extensive drilling on the south shore of Great Slave Lake has yielded new information on the structure and lithology of the Pine Point, Presqu'ile and Slave Point formations of Middle Devonian age. In most places along a 5 mile-wide belt, traced 30 miles in a N. 70° E. direction from a point 11 miles south of the mouth of the Buffalo River, the Slave Point formation varies from an upper phase of thin-bedded fossiliferous limestones to a lower phase which is mainly sub-lithographic limestone. These beds overlie coarsely recrystallized, vuggy dolomite of the upper part of the Presqu'ile which rests upon a considerable thickness of fine, sugary-textured and compact crystalline dolomites. The Presqu'ile dolomites overlie the shaly limestones of the Pine Point formation.

North of this belt, the coarse-grained dolomite lenses out and, in the western part of the area, the stratigraphic interval is occupied by green shales. The lower dolomites interfinger toward the north with shaly limestones lithologically similar to the Pine Point formation. South of the belt, the coarse dolomite grades into finely crystalline dolomite.

Although the regional dip is westerly, the structure is complicated by local folding and initial dips. North of the belt, northerly dips up to 125 feet per mile are found. South of the belt, the beds commonly dip more gently south.

Sphalerite, galena, and marcasite occur in greatest abundance at three horizons within the Presqu'ile formation. Sulphur, gypsum, pitch-like bitumen and small traces of oil are found in increasing abundance toward the west in the Slave Point and Presqu'ile formation.

The Presqu'ile formation shows many of the characteristics of a biostromal deposit with off-shelf facies extending into the basin of Great Slave Lake. The structure and development of the Paleozoic rocks may be related initially to the structure and topography of the underlying pre-Cambrian rocks.

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