CHANGING IDEAS ABOUT THE BOSTON BASIN, 1964-2007

In 1964 Father Jim Skehan and the Department of Geology and Geophysics, Boston College, hosted the NEIGC. One of the field trips, led by the author, reviewed the geology of the Boston Basin. The Boston Basin rocks were then divided into a lower Roxbury conglomerate, part of which was believed to be glacial in origin, and an upper Cambridge argillite. At that time the rocks of the Boston Basin were thought to be late Paleozoic in age, partly because they resembled Carboniferous rocks of the Narragansett and Norfolk basins, and also because there were known late Paleozoic glacial deposits in the Southern Hemisphere (Dott, 1961). A Proterozoic age for the volcanic rocks that underlie the Boston Basin was determined by Kaye and Zartman (1980) and Proterozic microfossils in the Cambridge unit were identified by Lenk and others (1982). Father Jim (Skehan,1983), (Skehan and others 1985) then placed all the rocks in Boston area in the Avalonian terrane. Walcott (1889) showed there were two groups of early Paleozoic fossils, the Atlantic fauna, consisting partly of the trilobites Paradoxides harlani and P. davidi, found in the Avalonian rocks, and the North American fauna, including Cryptolithus and Ollenellus, found in rocks along the St. Lawrence and Hudson River valleys. Walcott believed the Appalachian Mountains kept the two groups from interbreeding, although most of the rocks in the Appalachians are now known to be younger than the Atlantic and North American fauna. Dalziel (1997) placed the Avalonian rocks and fossils in the chilly waters near the South Pole in the late Proterzoic and early Paleozoic. The Atlanic fauna were fossilized in shale, while the North American fauna lived in balmy waters near the equator and were fossilized in limestone. Then moving as a funeral ship, the Avalonian terrane brought the entombed Atlanic fauna into collision with North America, forming some of the Appalachian Mountains. So Walcott was right and the Appalachians do separate the two fauna, but while they lived only great distances and differences in sea-water temperatures prevented their interbreeding. All of this suggests that a glacial origin for some of the Boston Basin rocks is not only possible, but that it is likely.

Thanks Father Jim, for a great time!