On Tuesday, October 3, 2023, at 7:00 p.m., we’ll be having our monthly meeting featuring…
Boston Area Geology: What Lies Beneath Us
with Scot Krueger, BMC President
About the presenter…
Our own president, Scot Krueger, is a retired geologist with a long family history connected to the Boston Mineral Club. Having returned from a career away in California and Texas, Scot has taken the time to read and explore to better understand the geologic history of the area. Based on what he has learned and synthesized, he has written over a dozen articles about the topic for the BMC newsletter.
This presentation will cover the Precambrian geology of eastern Massachusetts, including the metamorphic and igneous rocks of the basement and the meta-sedimentary rocks of the Boston Basin. The talk will allow the story to be told in more detail than the articles, with more pictures, and give club members a chance to ask questions.
The basement rocks of the Avalon terrane, which underlie eastern Massachusetts, are what geologists term an “exotic” terrane. It has a geologic history that is different from that of the nearby continental rocks of the same age, suggesting it has been transported a large distance over geologic time. Avalon is a series of volcanic arc rocks that originally formed along the margins of the ancient continent we call Gondwana. The Boston Basin represents a series of sedimentary rocks that were deposited in a rift that formed as the Avalon terrane rafted away from Gondwana more than half a billion years ago. The wandering Avalon terrane was later crushed when Gondwana collided with Laurentia (now North America) in the collision that formed the Appalachians about a third of a billion years ago. When Africa subsequently split away from North America in the Jurassic, creating the Atlantic Ocean, the Avalon terrane was left stranded on our side.
The Boston Basin fill consists of two main formations: Roxbury Conglomerate “puddingstone” and the younger Cambridge Slate. The Roxbury Conglomerate can be subdivided into multiple units, revealing an interesting slice of geologic time. One of the more interesting subunits is the Squantum Tillite, which is believed to have formed during a time we refer to as the Snowball Earth, when most of the landmass of the planet was covered in glacial ice.
The presentation will walk through the geologic history that has been teased out of the Avalon terrane and the Boston Basin, despite these rocks having mostly formed before the evolution of animals with hard parts that leave fossils. It will end with the arrival, just at the end of the filling of the Boston Basin, of the odd Cambrian trilobites, which allowed Boston to play a small role in the development of the theory of plate tectonics in the last century. Many hand samples of the various units discussed will be available to examine.
The presentation will be preceded by socializing and a brief business meeting and followed by our traditional mineral specimen raffle.
We hope to see you there!
See the Monthly Meetings page for directions and other information.