A piece ganked from the USGS may shed some light on the most excellent observations by you and your Junior Geologists backup team:
Death Valley National Park through time Quiet to Chaos: Cenozoic Time
After 150 million years of volcanism, plutonism, metamorphism, and thrust-faulting had run their course, the early part of the Cenozoic Era (early Tertiary, 65-30 million years ago) was a time of repose. Neither igneous nor sedimentary rocks of this age are known here. No great events were recorded here, simply the weathering away of the region to a rolling landscape of low relief.
Beginning in
Miocene time, the geologic tranquility was shattered. Volcanism and faulting started up again, but this time caused by extension in the crust rather than compression. The birth of the Death Valley landscape familiar to us today was beginning.
Usually younger geologic events are easier for the geologists to interpret, however here in Death Valley, the faulting and movements of rocks and indeed entire mountain ranges has been so extensive that scores of geologists are still marching up and down canyons, mapping rocks layers, testing theories, forming new ones, and enthusiastically debating these theories among themselves. In this section, as you will read about this very active chapter in Death Valley's history, do not forget that the same tectonic forces are going on today, and Death Valley will continue to evolve into the future.
Forces Driving Mountain Building in Death Valley
Big mountain building projects require big forces, and that usually means
plate tectonics. In early
Tertiary time, the North American plate was riding up over the Pacific plate, however, starting about 30 million years ago, the spreading center beween the Pacific and Farallon plates intersected the subduction zone, dividing the Farallon plate into two pieces. The northern one became the Juan de Fuca plate, and the San Andreas strike-slip fault system grew between the two surviving fragments of the subduction zone. For reasons still not fully understood, this change in plate motion began stretching the continental crust between the Sierra Nevada Mountains on the west and the Colorado Plateau on the east. In this region, known as the
Basin and Range province, mountain blocks were uplifted and valleys formed as the floors between ranges dropped along
normal faults .
In response to the shifting tectonic plates, strike-slip faults developed in Death Valley. Between two strike slip faults, tension gashes opened up, forming the modern basins of Death Valley. The rocks that would become the Panamint Range were stacked on top of the rocks that would become the Black Mountains. If that's not crazy enough, the Cottonwood Mountains, now north of the Panamints, were also piggy-backed on top of the entire stack!
In the next several million years, The Black Mountains began to rise, and the Panamint/Cottonwood Mountains slid westward off the Black Mountains along low-angle normal faults. Imagine a tall stack of magazines tilting and sliding sideways, except our geological magazines are several miles thick and 50 miles wide! Starting about 6 million years ago, the Cottonwood Mountains slid northwest off the top of the Panamint Range. And there's some evidence that the Grapevine Mountains may have slid off the Funeral Mountains! Some geologists aren't satisfied that we have enough evidence to believe that the mountains were stacked on top of each other, but were rather stacked adjacent to each other. Major research efforts are still underway in Death Valley.
In either case, as these mountains slid apart, the valley floors dropped and began receiving sediments washed off the newly formed mountains. During this entire time, volcanic eruptions spewed basalt flows and blanketed the area with volcanic ash, mixing with eroded sediments and forming spectacular rock layers such as the Funeral Formation and Furnace Creek Formation visible from
Zabriskie Point and
Golden Canyon. Geologists may disagree on the details of how far these mountains moved, but a reasonable estimate is that they've moved 95 to 130 km (60-80 mi) to the northwest. That's quite a distance to be moving entire mountain ranges! The mountains are still moving too - regional estimates suggest the mountains are moving on average about a half inch per year (although no motion has occurred in the last few decades). Thus while the rocks of Death valley's mountains may be millions or billions of years old, the mountain topography is very young, and still growing.
While these mountain blocks were shifting about, the floor of Death Valley was also dropping. Nature doesn't like deep holes surrounded by mountains, so naturally the hole began to get filled up. There is something like eight thousand of feet of gravel, sand, and mud overlying the bedrock of the valley floor. The fact that the valley floor is still below sea level tells us that it is still dropping, even as it receives more sediments.
Recent Geologic Changes
By about 2 million years ago,
Pleistocene time in Death Valley, the major topographic features of Death Valley had formed. However, there were still big changes ahead. Earth's climate began to oscillate between warm conditions (like today's climate) and colder conditions (ice ages). During these colder conditions, continental ice sheets expanded from the polar regions of the globe to lower latitudes, and the nearby Sierra Nevada Mountains sported alpine glaciers. There were no glaciers in Death Valley, but with the cooler and wetter climate, rivers flowed into the valley year round.
Since the valleys in the Basin and Range region formed by faulting, not by river erosion, many of the basins have no outlets, meaning they will fill up with water like a bathtub until they overflow into the next valley. During the cooler and wetter climates, much of eastern California, all of Nevada, and western Utah was covered by large lakes. Death Valley was the last of chain of lakes fed by the Amargosa and Mojave Rivers, and possibly also the Owens River.
Move forward in time | |
Quiet to chaos (Cenozoic time)
As a postscript, I'll add that it appears your base geologic map is the California state map. It's a magnificent piece of work, but in areas of highly complex geology, close-in detail must be sacrificed. As a result, an outcrop area labeled as "Tertiary Volcanics" includes rocks deposited or extruded from 65 million years ago all the way up to 5 million years ago. Most likely there are a goodly number of other larger scale geologic maps available for that area, quite possibly including 1:24,000 scale quadrangle maps, as detailed mapping of specific USGS quad sheets is a commonplace exercise for those earning Masters or PhDs in geology, and by both State and Federal Geological Survey field geologists. As the USGS piece above indicates, large scale tectonic activity persisted throughout the Tertiary Period, including the repositioning of entire ranges such as the Panamints, the Cottonwoods, the Blacks, the Grapevines, and the Funerals. That being the case, and with the fact that the valley floors are still dropping relative to the upthrown sides of the block faults, you may most certainly expect to see sedimentary and volcanic rocks deposited within the Tertiary tilted to a material degree. Another brief observation is that tuffs themselves can be deposited over land (subaerial deposition) or over water (subaqueous deposition). Thus the term "water-laid tuff" and the appearance of thinly-bedded and even laminated tuffs deposited in shallow water in quiet low-energy environments. Subaerial deposition over an alluvial fan or braided stream deposit in a valley floor can give you the "sediment-tuff-sediment" sequence you observed. Nice work, Ski! I particularly enjoyed your description of putting your mind in 3-D mode in order to gauge where, in other parts of the deeply incised canyon topography, your distinctive bed of tuff may crop out again. It's the 3-D aspect of mapping and studying field relations which made field geology/structural geology most appealing to me as a student and as a mineral explorationist.
Foy
