USGS scientists and colleagues have recently identified a unique rock layer in ancient rocks around Lake Superior. The rock pictured here, located near Marquette, Michigan, is composed of debris ejected from a massive impact crater that formed 1,850 million years ago. Here, layers of accretionary lapilli (the oblong, but originally spherical features), formed by an accretion of dust particles in a fast moving cloud of impact debris. Because this layer formed in a geological instant, it provides a precise time-line wherever it can be found. Its recognition has allowed, for the first time, a precise time correlation of the major iron ore deposits of the Lake Superior region and established that most of these deposits formed during a single episode of iron deposition.
Photo by William F. Cannon, USGS Science Emeritus, Reston.