L’Anse Unconformity


L’Anse Unconformity

At L’Anse there is an unconformity which we will visit. Unconformities are places where the geological record has a demonstrable gap, where time is unrecorded. There are several kinds of unconformities, but angular unconformities pose some special problems. Geologists start with Steno’s Laws--proposed by Nicolas Steno in the 1600s. To devise claims that illuminate unconformities, geologists pointed to evidence for deep time. The most famous unconformity in geology is at Siccar Point, in Scotland where James Hutton explored deep time implications of earth science in a profound way.

mottled red-brown, fine grained well sorted large scale trough crossbedding, scattered mudstone intraclasts.

red, very fine grained; well sorted; thinly planar bedded; some small scale crossbedding; somewhat lenticular; assymetric ripples common; interval drapes concave-up bedding surface of underlying interval.

mottled red-brown; fine grained, well sorted; large scale trough crossbedding; minor planar bedding;  assymetric ripples, both cap and arc contained within interval; mudstone intraclasts; channelized lower contact.

very fine sandstone, red, thickly laminated to thinly planar bedded; some small scale crossbedding and cut and fill; asymetric ripples, becomes shaly in upper 10 cm.

Stratigraphic section of Jacobsville Sandstone, S of  US 41. from Abel, 1985

Between Baraga and L’Anse at the head of Keweenaw Bay, the unconformity that exposes the Lower Proterozoic Michigamme Formation, here a slate, in exposures below US 41 level, North of the road and along the shoreline. The surface of the slate is striated, and has been interpreted as either a glacial surface (Murray, 1955) or as resulting from lakeward sliding of the sandstone on top of a sloping slate surface (Kalliokoski, 1982).

Here one can visit both the slate and sandstone exposures.


Jacobsville SS
US 41
Michigamme Slate
Father Baraga