Geology of the Great Basin

12. The Cenozoic

12 the cenozoic twenty-five million years of erosion After uplift comes erosion. Just as we begin to age from the earliest moments of birth, so the Laramide Highlands began to erode. Erosive forces were scouring the rocks from the earliest time of uplift. They soon revealed signs of the abrasiveness of age; the rocks were deeply beveled. Erosion is the opposite of deposition. When rocks are being destroyed, no deposition is occurring at the site. Miles away, the accumulated detritus of erosion may pile up into new deposits. But at the site of destruction there is no record. The thief leaves no record except the empty shelf. Consequently , our knowledge of the early Cenozoic in the Great Basin is severely limited. The pages of the rock record have been torn out, and history is sadly incomplete. The picture is still not complete, and portions may have been forever erased. There are some scraps of information, like a few igneous rocks and a few outcrops of a clastic sedimentary unit in the central Great Basin. This sandy layer seems to have been deposited in a streambed. Associated with it are some lake sediments. Perhaps this stream and the lakes were the result of internal drainage. There may have been some post-Laramide rejuvenation along faults—lithologic Gerovital. The upthrust blocks may have obstructed streams draining the Laramide Highlands, creating a smaller version of the Great Basin long before the actual Great Basin was formed. There were large volcanoes to the east of the Great Basin during this early Cenozoic time. Our area, however, was so devoid of volcanism that geologists 138 The Cenozoic refer to the Laramide igneous gap when they refer to the first twenty-five million years of the Cenozoic in the Great Basin. This gap was soon to be violently and abruptly filled. the enigmatic ignimbrites One of the most puzzling time periods in Cordilleran history is the middle Cenozoic. This was the late Eocene–early Miocene time of about forty to twenty million years ago. The slow change of history was again punctuated by paroxysm. Violent volcanic eruptions burst out in the northeastern part of the Great Basin a little more than forty million years ago. A great wave of volcanism engulfed most of the Great Basin, spreading from the northeast to the southwest . Huge eruptions of white-hot ash, lava flows, and ash-flow tuffs buried the landscape. The chemistry of the parent magmas is similar to those that had earlier created the granites of the Sierra Nevada. Here in the Great Basin, however, the molten material blasted out of volcanic vents. Fiery-hot clouds Carson Sink, northern Nevada. The high ground surrounding the sink is marked by the horizontal shorelines of receding Ice Age lakes. Tom Brownold. The Cenozoic 139 of ash and gases seared the land surface. Instead of becoming the intrusive interior of another Sierra Nevada, the magmas belched out onto the surface. The Great Basin was blanketed by the guts of what would have been a lofty mountain range. The chemical nature of these eruptives created ash-flow tuffs composed primarily of rhyolite, the extrusive counterpart of granite. Incandescent, white-hot ash falls from fiery volcanic clouds are termed ignimbrites. These rocks fused by their great heat into a welded tuff. The Defense Department tests some of its nuclear weapons on the Nevada Test Site in these hard, resistant rocks. They are the dominant rock type in most of the ranges of the central and southern Great Basin. The volume of ash expelled is incredible—an individual ash fall might cover an area of four to five thousand square miles. One ash layer might be six hundred feet thick. In some cases, an original volume of just one ash-fall eruption might be from one hundred to five hundred Clover Creek, Nevada. Ignimbrites stand as steep cliffs, reflecting their dense inner structure. Volcanics constitute many of the ranges of the central and western Great Basin. Tom Brownold. 140 The Cenozoic cubic miles. The ash flowed across the land in scorching white-hot clouds, pouring down valleys and filling basins. Any declivity in the land surface was soon filled by ignimbrites. The land became so featureless that ash-flow sheets covered thousands of square miles with virtually no change in thickness or in the sequence of the flows. So much magma was expelled to the surface that the magma chambers...