Jacobsville Sandstone is well known around Lake Superior as a common architectural buildingstone of many towns and cities around the lake's perimeter. The red massive to cross-bedded sandstones underly the border cities of Sault Ste. Marie, Ontario and Michigan and is reflected in the construction of many churches and heritage buildings.

PRECAMBRIAN SHORES    What may not be well know is that this sandstone and its related cousins, the Bayfield and Freda sandstone (found on the southwestern end of the lake) represent the expression of a body of water, not that disimilar to the present day Lake Superior, but which existed here about 1 billion years ago.

ANCIENT BASINS  To understand the nature and extent of this ancient basin, we have to look for clues using a variety of tools that earth scientists have at their disposal. The most important of these tools relies on observation. For example, we know from the exposure of Jacobsville sandstone rock outcrops around the lake that this rock formation is geographically restricted to areas near Lake Superior. Seismic reflection studies indicate that the formation underlies most of the current extent of Lake Superior and reaches thicknesses of up to 8 kilometres.

ANCIENT SURFACES Even more intriguing is the environment just prior to the deposition of the sandstone. At various locations around the lake, the contact between the sandstones and the underlying crust represents the ancient surface or unconformity which existed just prior to deposition. Rocks below this surface display deep weathering similar to that found in current equatorial latitudes suggesting that just prior to the deposition of the Jacobsville sandstones, the land was located closer to the equator.

FOSSIL SOILS The presence of deep weathering at this unconformity surface is also accompanied by the development of a 'fossil' soil or paleosol. The paleo weathering profile represents a transition from relatively unweathered rock (1 and 3 - see figure below) to the development of a leached and weathered horizon and 'lag' deposits which may be transitional into a basal conglomerate.

(same area as Photo Location 1)

EQUATORIAL LATITUDES Further evidence for an equatorial latitude for the Jacobsville formation can be found in paleomagnetic determinations in samples from the Jacobsville formation and related cousins such as the Freda and Nonesuch formations in Mishigan. Remnant magnetism in minerals provides a snapshot of the original positions of the poles. Since the magnetic poles of the earth are generally found close to the poles of the earth's rotation axis, we can infer the location of the rock sample by restoring the paleopole position.

Using paleomagnetic techniques, the 'travel path' of rocks that form the Lake Superior basin have been reasonably well established and is referred to as the 'Logan Loop'. The Jacobsville formation is positioned at the end of the loop with each point on the loop representing a slightly younger age for the rocks.

Based on the paleomagnetic data, the Jacobsville was deposited during a time when it was just south of the equator and represents  the youngest member of the rock formations comprising the Lake Superior basin.

LAKE SUPERIOR PAST AND PRESENT   
The location of the sub-Jacobsville unconformity relative to the current and past water levels of Lake Superior is also quite interesting. In many exposures the unconformity surface is relatively shallow dipping and occurs near the present day elevation of the lake. This superposition is quite dramatic along the south side of Goulais Bay (Photo Location 3) where it is possible to walk along the current pebbly shoreline of Lake Superior and walk adjacent to the exposed Jacobsville unconformity which outcrops along the shore. In places, it is difficult to differentiate between the current pebble beach and the basal conglomerate which comprises the lower part of the Jacobsville.

The close spatial association between these two 'surfaces' which are widely separated by time is also mimicked in part by the drainage systems which feed into the lake. In the Jacobsville, paleo current directions determined from cross-bedding and ripple marks formed by ancient river systems, indicate that rivers and streams which deposited the sandstone and conglomerates drained into the Lake Superior basin (see location map above). The dip of the sandstone beds is  generally moderate to the north and north east from 1 to 10 degrees along the north shore of the upper peninsula of Michigan. On the Ontario side, the Jacobsville sandstone beds have a more variable dip ranging from 5 to 20 degrees to the south, southwest and west.This is quite similar to the current drainage system on the eastern end of Lake Superior.

In the upper peninsula of Michigan, the Jacobsville is unconformably overlain by younger rocks of Paleozoic age (less than 550 my old) which form the Michigan Basin. The Jacobsville unconformity which outcrops along the lake shore in places (see Photo 2), sub-crops beneath the Paleozoic sedimentary rocks in a pattern which mimicks the north shoreline of the upper peninsula but located about 0 -20 kilometres south. This would suggest that at least during early Paleozoic times (550 my ago) the Jacobsville shoreline was not far from the current shore.

Credits:
Photographs for locations 3 and 4 were provided by Mike Hailstone, District Geologist, Sault Ste. Marie Resident Geologist District, Ministry of Northern Development and Mines, Ontario.

References:

Hamblin, W., 1958, Cambrian Sandstones of northern Michigan, Michigan Department of Conservation, Geological Survey Division, Publication 51, 146 p.

Kalliokoski, J., 1982, Jacobsville Sandstone, in Geology and Tectonics of the Lake Superior Basin, edited by R.J. Wold and W.J. Hinze, Geological Society of America, Memoir 156, pp 147-155.

Ojakangas R. W. and Morey, G.B., 1982, Keweenawan sedimentary rocks of the Lake Superior Region: a summary, in Geology and Tectonics of the Lake Superior Basin, edited by R.J. Wold and W.J. Hinze, Geological Society of America, Memoir 156, pp 157-164.

Van Schmus, W.R., Green, J.C., Halls, H.C., 1982, Geochronology of Keweenawan rocks of the Lake Superior region: a summary, in Geology and Tectonics of the Lake Superior Basin, edited by R.J. Wold and W.J. Hinze, Geological Society of America, Memoir 156, pp 165-171.

Halls, H.C. and Pesonen, L.J., 1982, Paleomagnetism of Keweenawan rocks, Geology and Tectonics of the Lake Superior Basin, edited by R.J. Wold and W.J. Hinze, Geological Society of America, Memoir 156, pp 173-201.