April 23-26, 2023
I attended the Institute of Lake Superior Geology meeting from April 23 to 26, 2023. I provide a brief description of the field trips in my Journal blog entry for May 1. These are notes I made during (and sometimes but not always about) the conference talks.
The conference papers were, for me, something of a mixed bag. Some were a bit too abstruse for me to follow, some ventured in to extreme detail on subjects I didn’t care so much about, but others introduced me to interesting comments, or provided provocative perspectives on what I had imagined were settled matters.
The star exhibit for the latter was a paper that argued that the mid continent rift was not a failed rift, but instead represented two separate events and were part of more global volcanism.. Just checking this with ChatGPT: “Recent studies have suggested that the Mid-Continent Rift [Laurentia] and the Siberian Traps [Rodinia] may have been part of a single, global-scale event known as the “Great Unconformity.” This event may have been caused by a superplume, a massive mantle plume that caused extensive volcanic activity and led to the breakup of Rodinia.“
ChatGPT is mistaken
-=> The mention of the “Great Unconformity” is erroneous, both in its timing and cause.
=> The Siberian Traps occurred at 250 Ma, not 1.1 Ga — ChatGPT now says that volcanic activity did occur in Russia around a billion years ago, and “resulted in the formation of a different type of igneous rock known as the Kola Alkaline Province, which is located on the Kola Peninsula in Russia.” — I am not sure about this and have not checked it yet.
Moral: Don’t trust ChatGPT on its own.
Rather than spacing out or multi-tasking when papers moved into territory I didn’t understand or didn’t care about, I used the time to understand new terms/concepts that had been presented, or to increase my understanding of things I knew something of. Here are some excerpts from my notes, favoring the more coherent and complete ones:
- Geological Formations in Minnesota
The Duluth and Beaver Bay complexes are intrusive rocks formed about 1.1 billion years ago during the Midcontinent Rift; these adjoin and are interspersed with the extrusive rocks of the North Shore Volcanic Group. The Animike Group is a geologic group composed of sedimentary and metasedimentary rock, having been originally deposited between 2.5 and 1.1 billion years ago during the Paleoproterozoic era, within the Animikie Basin. This group of formations is geographically divided into the Gunflint Range, the Mesabi and Vermilion ranges, and the Cuyuna Range. On the map, the Animikie Group is the dark gray northeast-trending belt which ranges from south-central Minnesota, U.S., up to Thunder Bay, Ontario, Canada. - Geological Formations and Features in Wisconsin
The Wisconsin Arch is a broad, positive area that separates the Michigan Basin on the east from the Forest City Basin on the west. The northern end of the arch is termed the Wisconsin Dome and is a region of Precambrian outcrops in northern Wisconsin. The rest of the arch is overlapped by Cambrian, Ordovician, and Silurian sedimentary rocks. The Keweenaw Fault is a reverse fault that bisects the Keweenaw Peninsula of the Upper Peninsula of Michigan. The fault thrusts lava flows of the Midcontinent Rift System onto sedimentary rocks of the Jacobsville Sandstone. The fault is part of the inversion of the Midcontinent Rift where a region that had previously undergone extension experienced significant contraction during the final stages of the Grenvillian orogeny. - MCR Rift and LIP
Rifts are segmented linear depressions that are filled with sedimentary and igneous rocks that form by extension and often evolve into plate boundaries. Flood basalts, a class of large igneous provinces (LIPs), are broad regions of extensive volcanism formed by sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift (MCR) is an unusual combination, because its 3000-km length formed during a continental breakup event 1.1 Ga ago, but it contains an enormous volume of igneous rocks that are mostly flood basalt. We show that MCR volcanic rocks are significantly thicker than other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR a rift’s geometry but a LIP’s magma volume. - Volcanogenic Massive Sulfide (VMS) deposits
VMS deposits are formed in association with submarine volcanic activity, commonly associated with ancient island arcs or back-arc basins, where the seafloor has been modified by subduction-related volcanism. Volcanogenic massive sulfide deposits re important sources of copper, zinc, lead, gold, and silver (Cu, Zn, Pb, Au, and Ag). VMS are divided into types bimodal, mafic or felsic, and also Sedimentary Exhalative (SEDEX)VMS deposits which are formed in sedimentary basins and are associated with the discharge of hydrothermal fluids through sedimentary rocks; other types are distinguished on the basis of what metals they tend to contain (CyprusOphiolites, KurokoZn, Pb, Cu, AlgomaCu) - Back Island Arcs
Back Island Arcsare long chains of active volcanoes found along convergent tectonic plate boundaries, and are the principal way by which continental growth is achieved. Back-arc basins are submarine basins that form behind an island arc, typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. They are formed by the process of back-arc spreading, which begins when one tectonic plate subducts under another, creating a trench between the two plates and melting the mantle in the overlying plate. - Forms of SiO2
(1) Chert is a microcrystalline or cryptocrystalline form of SiO2 with a granular microstructure. It is a sedimentary rock that forms either from the skeletons of diatoms and radiolarian, or via chemical precipitation of water-transported SiO2 in limestone or dolomite; in the latter case it often appears as nodules embedded in limestone or dolostone. In appearance it is dull with a waxy or porcelain like luster, and it is opaque.
(2) Chalcedony is a cryptocrystalline (too small to see) form of SiO2 made up of very fine fibrous intergrowths of quartz and moganite, a polymorph of quartz. It is similar in appearance to chert, but is translucent to transparent; it often occurs as a secondary mineral in cavities or veins, filling spaces created by the dissolution of other minerals or by hydrothermal activity. Agate is a term for banded chalcedony; Jasper is a term of opaque chalcedony. It can be sedimentary, metamorphic or igneous.
(3) Quartz is microcrystalline to crystalline, and transparent to translucent. It can be sedimentary, metamorphic or igneous. - Banded-iron formations
Banded-iron formations formed about 2Ga and were deposited for t200 Ma. They are characterized by bands of iron compounds and chert. Enough oxygen had accumulated in seawater so that dissolved iron was oxidized; iron reacts with oxygen to form compounds that precipitate out – including hematite, limonite and siderite. These iron formations are abundant in the Lake Superior region.
Microbial life played an important role in changing atmospheric conditions by releasing free oxygen as a waste product of photosynthesis. Free oxygen was taken up by elements with strong affinities for it – hydrogen, carbon and iron. Evidence for the change in oxygen levels is that the sediments of the earlier Archean are dark brown and black caused by unoxidized carbon, iron sulfide, and other elements and compounds. As oxygen levels increased in the atmosphere and oceans, the sediments changed. In the late Archean, sediments went through a transitional stage with the banded-iron formations; after this transition they demonstrate an oxygen-rich environment – shown by iron oxide-stained siltstones or mudstones called red beds. - The Sudbury Impact event
The Sudbury Impact event occurred 1,850 million years ago; it is theorized that this caused the end of the banded-iron deposits. In the impact a 16 km (10 mi) diameter meteorite collided with earth[12] in the current-day vicinity of Sudbury, Ontario, Canada.[13] The meteorite vaporized and created a 240 km (150 mi) wide crater.[12] Earthquakes shattered the ground hundreds of kilometers away and within seconds ejecta (cloud of ash, rock fragments, gases and droplets of molten rock) began to spread around the globe - The Penokean orogeny
The Penokean orogeny was a mountain-building episode that occurred in the early Proterozoic about 1.86 to 1.83 billion years ago, in the area of Lake Superior, North America. The core of this orogeny, the Churchill Craton, is composed of terranes derived from the 1.86–1.81 Ga collision between the Superior and North Atlantic cratons. The orogeny resulted in the formation of the Nena and Arctica continents, which later merged with other continents to form the Columbia supercontinent. The name was first proposed by Blackwelder 1914 in reference to what is - Skolithos burrows.
Skolithos are simple, vertical to slightly inclined tubes that have smooth walls with linings and structureless fill. Skolithos traces are straight, near-vertical and do not branch, and are usually found in rocks that represent non-marine to shallow-marine facies. - Palaeophycus burrows
A horizontal, passive fill, walled burrow tube probably made by a suspension feeder, detritus eater or carnivore. The burrow tubes are curvilinear to straight, and do not cross. - Fault mirrors (FMs)
Fault mirrors are naturally polished and glossy fault slip surfaces that can record seismic deformation at shallow depths. They are important for investigating the processes controlling dynamic fault slip. Slickensides, which are also naturally polished rock surfaces that occur when the rocks along a fault rub against each other, make their surfaces smoothed, lineated, and grooved. - .