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Henry Halls - 1987 Goldich Medal Recipient

Henry is currently a Professor at the University of Toronto at Mississauga where he continues his research on the Lake Superior Region
   

I was born in 1941, attending High School in Tiverton, Devon, UK. I then obtained my B.Sc. in Geology at the University of Sheffield, followed by an M.Sc. in Geophysics at the University of Durham. It was during my attendance at a landmark Royal Society symposium in 1964 on “Continental Drift” that I first met Tuzo Wilson, who played a major role in getting me accepted at the University of Toronto in Canada for a Ph.D. Graduating in 1970, I then moved to Erindale College (subsequently the University of Toronto at Mississauga) as a faculty member where I have remained ever since, attaining the rank of full Professor in 1979.

The subject of my Ph.D. thesis was “the Geology and Geophysics of the Lake Superior Region”. Although registered in the Geology Department, my primary thesis supervisor was Professor Gordon West in the Department of Physics. My subsequent Departmental thesis defence was before 12 professors representing members of both departments, and I was the first student to graduate from the Geology Department while being a de facto graduate student in the Physics Department!

Professor West played a major role in shaping what was to become for the next 20 years my major scientific pre-occupation – understanding the origin of Lake Superior, what rocks lay beneath it, and how they correlated with observed outcrops around the margin. I owe Gordon my deep gratitude for accepting me into his very large and active geophysical program which at that time was engaged in ship-borne seismic refraction and magnetometer surveys to understand the faulted, volcano-sedimentary Keweenawan basin, formed of stratigraphic units each with a distinctive geophysical response – a veritable geophysical Utopia! Unravelling the unexpectedly complex structure beneath the lake would indeed prove tractable using geophysical methods.

One enigma in 1970 was a shallow high seismic velocity layer “The Upper Refractor” that was only 10 km beneath the lake. This was a puzzle because the velocities seemed to be too high for any rock unit that was exposed. My first project was to collect rock samples from all Keweenawan sequences around the lake and then to subject them to pressures of up to 2 kb in order to see if any had the high velocities. The only ones that did were volcanics that were amphibolitised right at the base of the Powder Mill group in Wisconsin, and certain units in the Mellen and Duluth complexes. It was concluded that the upper refractor represented either the subsurface continuation of these intrusives or that the deeper parts of the Keweenawan volcanics were subject to a deep burial metamorphism, long suspected by geologists such as Walter White to be the source of the Michigan native copper deposits.

A continuing puzzle about Lake Superior at that time was its origin. Our seismic refraction work (Halls and West 1971, Can. J. Earth Sci.) revealed for the first time that Lake Superior was almost entirely underlain by flat lying low velocity units that could be identified, on the basis of surrounding geology and the high pressure velocity measurements, with the Jacobsville and Bayfield sandstones. It was further shown that various islands and peninsulas in the lake, like Isle Royale, Michipicoten Island, and the Keweenaw Peninsula, owed their existence to faults that had upthrust the older units of the Keweenawan sequence (volcanics and more indurated clastics) through the Bayfield-Jacobsville sandstone cover to form surface outcrops that were more resistant to erosion by scouring ice in the last ice age. This ice however easily eroded out the more friable sandstones leaving the lake depression. In common with most of the large lakes in Canada, Lake Superior is thus a glint lake… a lake formed at the erosionally susceptible region where hard basement rocks emerge beneath a softer, sedimentary cover.

After finishing my Ph.D. I continued my Lake Superior studies, mainly in paleomagnetism and aeromagnetic interpretation. My main focus of interest was to try and trace a prominent magnetic polarity reversal around the lake. It was found in several Keweenawan volcanic sequences in Michigan and Minnesota and eastern Ontario. I discovered that it was also present along the Ontario north shore where it occurred across an unconformity with a conglomerate within the Osler Volcanic group. Endeavouring to trace this boundary eastwards brought the Slate islands into prominence because A.P. Coleman had reported breccias on the islands. When I first went there in 1973, I asked the lighthouse keeper, Jack Bryson, if there were any agates on the island. When he said that, yes there were some found on the beaches in the southwest part of the island, I was immediately hopeful that unmapped Keweenawan volcanics, possibly preserving the reversal were to be found. A short trip immediately verified that a thin sequence of about 20 lakeward dipping basaltic flows were present and that they rested conformably upon a red bed sequence that passed within a few metres down into an iron formation capped by a grey argillite. The following year in company with Jim Franklin we came to the conclusion that we had a condensed sequence of Animikie, possibly Sibley and Osler volcanics all within a thickness of about 10 metres! The only strange thing about the sequence was the presence of thin veins of a fine grained red breccia. My subsequent paleomagnetic results were also strange. The sequence had not captured the reversal, but instead there was another magnetization component, absolutely alien to Keweenawan magnetostratigraphy, which was also carried by the breccia veins. I concluded that the mystery component was very rapidly acquired and in 1975 published a paper in Nature speculating that the magnetization was caused by intense shock, such as could be produced by meteorite impact. The other curious aspect of the volcanics was that they were sheared in a way that initially was a puzzle to me because I had not seen such a phenomenon before. They turned out to be shatter cones! It is not often in life that you address an audience of 200 people for the last talk at a lake Superior Institute meeting and be aware of absolute silence and anticipation of the audience, when I revealed that I thought the Slate islands were caused by a meteorite impact! Subsequently I showed (Halls 1979, Geophysical Journal R. Ast. Soc) that the islands carry the world’s only well-documented example of shock remanent magnetization, and in collaboration with Richard Grieve and Bob Stesky, that the Slate Islands are the central uplift of a complex crater about 30 km across, with the remaining outer parts buried under Lake Superior and probably covered by Bayfield-Jacobsville sandstone. Today the crater is thought to be about 400 million years old, and is an accepted crater joining the other 160 or so that have been found world-wide.

Today I believe that my study of the Slate Islands shatter cones with Bob Stesky remains the only detailed study that shows how shatter cones are modified both in shape and orientation by foliation, bedding and seismic anisotropy, and in the completeness of the cones by pre-shock joints. Later I led a field trip with Ron Sage to the Islands to show Buck Sharpton and Burkard Dressler the beautiful impact features which inspired their later work on the islands.

One of Lake Superior’s remaining enigmas is Superior Shoal. It is a shoal about 6 m deep at its shallowest point and sits in the centre of lake Superior the farthest from land of any point in the lake, and once a hazard to shipping. It is joined to lake Superior by a ridge. In 1986 I convinced the Harbour Branch Oceanographic Institute in Florida to visit the Shoal by submersible in order to see if it could be another impact site, a possible companion to the Slate Islands impact (cf. Clearwater Lakes impacts in Quebec). The trip down with my Graduate Student of the time (Matt Manson) was a thrilling experience. On one dive we dropped into a chasm at about 150 to 200 metres depth, whereupon the pilot informed us that he was getting returns from all directions on the echo sounder! We were in a chimney or canyon of some sorts. Inching forward we encountered a vertical to overhanging wall about 100 metres high composed of Kewenawan flows. The curious aspect was that the walls were glacially grooved… we were observing a canyon which had been preserved more or less in pristine condition by subsequent drowning beneath the waters of glacial Lake Superior!

After 1987, my involvement with Kewenawan geology began to wane as I became more interested in the paleomagnetism of Proterozoic dyke swarms of the world and what they could tell us about continental reconstructions. It turned out that much of this research took place in the general lake Superior region because many of these swarms and the interesting story they tell about crustal uplift and rotation across the Kapuskasing Zone (Halls and Zhang 2003, Tectonophysics) are a feature of the Superior Province that borders Lake Superior on its eastern and northern sides.
In summary I would like to thank all of my Lake Superior colleagues of ILSG, Lake Superior light house keepers, graduate students and above all Gordon West who helped in the various phases of my Lake Superior research that was ultimately thought deserving of the Sam Goldich medal.

Henry Halls. February 2006.