In Memoriam: Burkhard Dressler (1939–2024)

Burkhard Bruno Otto Dressler passed away peacefully this April 2024 in Nanaimo (British Columbia, Canada), at the age of 84. His wife Bärbel Dressler was by his side, and his two sons, Wolfram and Andreas, sent him love from afar. Burkhard had been suffering for a decade from progressive medical problems that suddenly, in 2023, had accelerated.

Burkhard was the second of three siblings born to Hildegard and Gotthard Dressler of Schweidnitz, a town in Lower Silesia, formerly within Eastern Germany, now in Poland. He received his PhD in petrology, with a specialization in impact cratering studies, from the University of Munich/Technical University of Munich in 1970. He carried out extensive fieldwork, followed by detailed petrographic studies, for his PhD dissertation that was entitled “Petrology and shock attenuation, Manicouagan impact structure, Quebec, Canada.” In the following decades, Burkhard would emerge as a prominent figure in the study of impact structures in Ontario, Labrador, Quebec, the High Arctic of Canada, South Africa, and then in Mexico. How many impact workers can boast to have worked on all three of the largest impact structures on Earth? During his long and successful career, Burkhard Dressler published 56 refereed articles and chapters in monographs, with 26 of those having been dedicated to his impact studies.

Looking back in the early 2000s onto some 40 years of service to the Precambrian geology and impact cratering community, Burkhard described himself “as an experienced Precambrian field geologist and impact researcher.” His career-long interest in impact cratering and shock processes undoubtedly dates all the way back to his years as a graduate and postgraduate student in Munich, at a university located only 2 h from the Nördlinger Ries impact structure.

His career began around 1965 with shock petrographic studies of the igneous and metamorphic rocks of the Ries, and he proceeded to log the Wörnitzostheim drill core. From 1970 to 1975, Burkhard Dressler was employed as a staff geologist by the Geological Survey of the Quebec Ministère des Richesses Naturelles, working on the Proterozoic Labrador Fold Belt. By 1975, he, Bärbel, and two young children had relocated to southern Ontario, where he continued his career as a field geologist for the Ontario Geological Survey. He pursued mapping in the Southern, Superior, and Grenville provinces of Ontario. Between 1979 and 1981, he carried out his first investigations of the Sudbury structure and environs. This region includes two impact structures, the huge Sudbury structure and the only 7.5-km-diameter Wanapitei structure just a bit to the northeast. No in situ impactites were found exposed in the Wanapitei structure, but Burkhard studied the impact deformation in the country rocks around the lake and the impact glasses in suevitic breccia found in glacial deposits south of the lake. Faults and joints around the lake were identified to have a concentric pattern that could be clearly attributed to the impact event. The suevitic glasses showed petrographic and chemical features that allowed the recognition of the precursor rocks in the Wanapitei target. Burkhard's family would often accompany him on these trips, enjoying summers in the Canadian north.

Whilst his duties at Sudbury were governed by the priorities of his employer, he soon took up studies of the impact breccias and shock metamorphism, shock distribution and attenuation, and into the origin of the Sudbury Igneous Complex, as a whole. In particular, he recognized a more or less concentric distribution of large breccia zones in the country rocks around the Sudbury Igneous Complex, which also contained many small breccia bodies that did not show a distinct orientation. He also recognized a sequence of emplacement events of phases of the Sudbury Igneous Complex, and made the discovery of various phases of the ore-bearing Sublayer of the complex. A 1996 comparison of recrystallized glass fragments from Onaping impact breccias and the matrices of its “Basal Member” compositions with those from the Igneous Complex, however, remained inconclusive as to the origin of the Igneous Complex. The Basal Member matrix and the so-called Melt Bodies, on average, are geochemically similar to the upper unit of the Igneous Complex, the so-called Granophyre.

From 1981 to 1995, Burkhard's career progressed to the levels of Supervising Geologist, Section Chief, and then Chief Geologist of the Precambrian Geoscience Section in the Ontario Geological Survey. Throughout he continued his research on the Sudbury impact structure, which culminated in a series of fundamentally important and, still today, frequently referred chapters in the seminal Special Volume 1 of the Ontario Geological Survey, entitled “The Geology and Ore Deposits of the Sudbury Structure” of 1984. During this time, he also shared his Sudbury expertise with numerous graduate and postgraduate students, and researchers from many countries (Figure 1).

Burkhard's ongoing interest to stay abreast of progress in the field of impact cratering research led him to attend the 1987 “Cryptoexplosions and Catastrophes Workshop” held in July of that year in the town of Parys, in the heart of the Vredefort Impact Structure in South Africa. He also participated in an excursion to Namibia organized as part of that event, exploring the Gross Brukkaros volcano and, for contrast, the Roter Kamm impact crater. It was on this tour that Burkhard had the idea for the concept of the quintennial Large Meteorite Impacts and Planetary Evolution conferences. Back home in Canada, it was easy to convince some other prominent impact workers of the likely benefits of such a regular benchmark event. The first two of these conferences were held at Sudbury in 1992 and 1997, under Burkhard's chairmanship. He was also instrumental in editing the subsequent conference proceedings volumes, which were published as Geological Society of America Special Papers–a tradition that was to continue over the following decades. The LMI conferences then moved on to Nördlingen (2003–again under Burkhard's chairmanship), back to Vredefort (2008), and once again to Sudbury in 2013. The latest LMI conference was conducted in 2019 in Brasília (Brazil). This series of milestone impact gatherings, the brainchild of Burkhard Dressler, always in conjunction with important field excursions, has undoubtedly had an enormous influence on the entire impact cratering community as well as sections of the planetary community. Furthermore, some of these events brought together strong contingents of mining geologists with academic impact researchers, especially in the world-famous mining provinces at Sudbury and in the Witwatersrand Basin.

As early as 1983, while he still served as Section Chief of the Precambrian Geoscience Section of the Ontario Geological Survey, Burkhard met Buck Sharpton for the first time, who visited the Sudbury structure and was hugely impressed by Burkhard's field and laboratory skills. In 1995, Sharpton invited him to work at the Lunar and Planetary Institute (LPI) in Houston on a 3-year visiting scientist position, and Bärbel joined him for this sojourn. During this time, he and Sharpton secured research grants from NASA and Canada's Polar Continental Shelf Project that allowed the study of several large terrestrial impact structures, most notably the Haughton structure on Devon Island, the Slate Islands structure in the Superior Province, Manicouagan's central melt sheet, Marquez Dome in Texas, and the Chicxulub structure in the Yucatán of Mexico. The work on the Slate Islands impact structure in Lake Superior, together with Buck, entailed comprehensive sampling of impact breccias (Figure 2), mapping of the distribution of shatter cones, and a detailed petrographic and scanning electron microscopic study. The latter indicated a clear shock attenuation plan for the structure and a sequence of shock brecciation events. Furthermore, ⁴⁰Ar-³⁹Ar dating provided the first reliable estimate of the impact age for this structure, at approximately 436 Ma.

Toward the end of his time at the LPI, Burkhard, Buck, and a number of international colleagues initiated a deep drilling project at Yaxcopoil, in the central part of the Chicxulub structure. When this project came to fruition under the auspices of the International Continental Scientific Drilling Program (ICDP), Burkhard served, for 3 months in 2001/2002, as on-site chief scientist at the Yaxcopoil-1 drilling site. This job not only involved the technical supervision of daily drilling operations, but Burkhard also spent countless hours documenting the nearly 900 m of intact core that were recovered in this international research effort (Figures 3 and 4). Between October 2002 and March 2003, he continued this work in Mexico and at the LPI and NASA Johnson Space Center in Houston. He carried out electron microprobe and SEM studies on a large number of YAX-1 impact breccia core samples and concluded that “his macroscopic interpretations of the breccia cores during the on-site core logging process were overall substantiated by these detailed investigations and led to a plausible interpretation of the deposition mechanism of the breccia deposits (resulting from collapse of an ejecta curtain, and these materials having been overlain by suevitic fall-back breccia)”. Burkhard's meticulous descriptions of these drill cores provided an essential context for a large number of research papers that were subsequently published about this drilling, including several papers authored by himself.

In 2000 and 2002, Burkhard worked repeatedly as a visiting researcher with Uwe Reimold and Roger Gibson and their team at the University of the Witwatersrand, Johannesburg (South Africa). Over some 4 months, Burkhard conducted highly productive structural investigations, which included a field study of the distribution and orientation of pseudotachylitic breccias in the Witwatersrand region and, especially, in the Archean granitoid complex of the core of the Vredefort impact structure. Burkhard concluded that “he could not identify any specific orientation pattern for pseudotachylitic veins and dikes.” Burkhard and Uwe eventually published two seminal works entitled “Terrestrial Impact Melts and Glasses” and “Order or Chaos? Origin and Mode of Emplacement of Breccias in Floors of Large Impact Structures.”

After having completed his review of pseudotachylitic breccias in impact structures in 2004, Burkhard retired from active duty–after some 40 years, with much of that time spent on field mapping. He and Bärbel moved into their retirement home in Nanaimo on Vancouver Island (BC) and focused on enjoying their private passions, including gardening and their beloved nature hikes.

In 1989, the German Academic Exchange Service invited Burkhard to present a 4-week-long lecture tour with talks on Canadian impact craters at several German universities and the Geological Survey of Germany. For his outstanding work in impact cratering research, Burkhard Dressler was honored by the International Astronomical Union, who named a 7-km-wide main belt asteroid after him [15,359 Dressler (1995 GV2)]. The impact cratering community followed suit and honored Burkhard with a nomination in 2009 for the Barringer Medal and Award of the Meteoritical Society.

Burkhard Dressler is remembered by his friends and colleagues for his many excellent contributions to impact cratering science, including fundamental field and petrographic investigations related to the geology of the Sudbury structure and many other impact structures in Canada and beyond. He carried out important efforts in synthesizing widely disseminated results, and made a major contribution to the Yaxcopoil-1 Chicxulub drilling project. Burkhard taught his field skills to a host of postgraduate students and never shied away from sharing his experience and knowledge about impact cratering. He was a rigorous, enthusiastic geologist, whose many collaborations and research outputs have left an indelible impact within the field of impact science.

Above all, however, Burkhard is remembered for his dedication to his beloved wife Bärbel. His sons Wolfram and Andreas remember him as a kind and affectionate father. Wherever Burkhard went, he formed lasting friendships. He will be terribly missed by his family, his friends, and all who had the honor of knowing him.