Here’s yet another picture of the Great Unconformity –this time in southwestern Montana. Once again, Cambrian sandstone overlies Precambrian gneiss. You can see a thin intrusive body, called a dike, cutting through the gneiss on the right side. You can also see that the bottom of the sandstone is actually a conglomerate –made of quartzite cobbles derived from some nearby outcrops during the Cambrian.
And that’s me in the photo. My left hand is on the sandstone –some 520 million years or so old; my right hand is on the gneiss, some 1.7 BILLION years old. There’s more than a billion years of missing rock record between my two hands. Considering that the entire Paleozoic section from the top of the Inner Gorge in the Grand Canyon to the top of the rim represents about 300 million years and is some 3500′ thick… yikes!
And… just like in the Grand Canyon and elsewhere, there is Cambrian age shale and limestone above the sandstone. This rock sequence reflects rising sea levels during the Cambrian. It’s called the “Cambrian Transgression”, when the sea moved up onto the continent, eventually inundating almost everywhere. If you look at the diagram below, you can see how this sequence formed.
If you look at time 1, you can see a coastline in cross-section, with sand being deposited closest to shore, mud a little farther out, and eventually carbonate material even farther out. As sea levels rise (time 2), the sites of deposition for these materials migrates landward, putting mud deposition on top the earlier sand deposition and so on. At time 3, the sequence moves even farther landward, resulting in carbonate over mud over sand. If these materials become preserved and turned into rock, they form the sequence sandstone overlain by shale overlain by limestone –just what we see on top the Great Unconformity.