Roadside Geology of Idaho (Roadside Geology Series)
David D. Alt
Format: PDF / Kindle (mobi) / ePub
From the ancient sedimentary formations in the north through the overthrust belt in the southeast, Idaho's rocks are as interesting as rocks come. The authors know these rocks well through their years of research in Idaho, which led to their theory explaining the flood basalts of the Columbia Plateau and the hotspot track of the Snake River Plain as the results of a giant meteorite impact that happened about 17 million years ago.
in relatively pale shades of gray and pink. Almost all consist of crystals large enough that you can easily see them without a magnifier, and many are full of complexly swirling bands of light and dark. Most basement rocks really are quite beautiful. Geologists call that complex of schist, gneiss, and granite the basement because nothing you can see at the surface suggests how deep those rocks may go. They lie beneath all the younger sedimentary and volcanic rocks that cover most parts of the
split into thin slabs along fractures that cut across the original sedimentary layers. Few of the Belt rocks show such slaty cleavage. Some people call them argillites, a term often applied to very hard and somewhat recrystallized mudstones that are not slates. Slates develop from rocks that recrystallized while they were being crumpled into folds. Evidently, the Belt rocks simply recrystallized with no accompanying deformation. The ex- 56 tremely deep pile of sediments with all those sills of
continent first began to split sometime around 1 1 00 million years ago. If so, a new ocean basin opened then along the line of the rift as the two pieces of continent separated. An oceanic ridge similar to the one that runs down the middle of the 59 modern Atlantic Ocean would have existed off the west coast of Idaho. It could have been responsible for the original deposition of metals in the rocks. Research submarines exploring oceanic ridges have revealed enormous hot springs on the ocean
moved off the Kaniksu batholith. is the torn southern edge of the slab that moved eastward off the Kaniksu batholith west of the Purcell trench. It would help if the timing of the Hope fault were not so hopelessly obscure. All the rocks on both sides of the fault are Belt formations a billion or more years old. Nowhere along its considerable length does the fault offset younger rocks, and the youngest rocks that cover it without offset are glacial deposits. Evidence that merely brackets the age
guide to their age. Radioactive age dates might not answer the question because it is possible that heat from the nearby granite batholith could have reset the radioactive clocks in older rocks. Outcrops of granite appear west ofthe road in the area a few miles north of Bovill. This is Cretaceous granite, part of the western Idaho batholith that invaded the surrounding Precambrian sedimentary formations sometime between 70 and 90 million years ago. 1 99 Idaho 3/97 Lewiston-BovillSt.