Thursday, April 7, 2016

Death Valley National Park geology, Part I: Proterozoic to Cambrian

As I mentioned a few months ago, one of the main components of my day job over the past few years has been preparing paleontological resource summaries for National Park Service Inventory and Monitoring networks. Between new and updated summaries, I've completed ten (Greater Yellowstone, Mediterranean Coast, Northern Colorado Plateau, Northern Great Plains, Northeast Coastal and Barrier, Northeast Temperate, Sonoran Desert, Southeast Coast, Southern Colorado Plateau, and Southern Plains). Each network has its own foibles and presents different challenges. Some are much more easily tackled than others; smaller parks generally have less to worry about than larger parks, with small urban cultural or historical units usually being the simplest. After finishing the Southern Plains Network last spring, I knew that the Mojave Desert Network (MOJN) would be one of the obvious candidates for the next project. It was completed under a superseded format, so it was already in the hopper, and we were already working with Death Valley National Park and Lake Mead National Recreation Area, so the iron was hot. The attraction for me was the challenge. The MOJN parks have superb geological records exposed by the combination of Basin and Range faulting and restricted plant cover, and they cover vast areas.

The geologic crown jewel of the MOJN parks is Death Valley National Park, or DEVA for short. In the National Park Service, the complexity of the geology at DEVA is surpassed only by the large parks of Alaska, which have not yet been explored as thoroughly. DEVA covers an area approximately the size of Connecticut (somewhat smaller by total area, somewhat larger by land area alone, which is more fair of a comparison given one of these entities has a coast and the other doesn't, at least not since some time in the Mesozoic). Its geologic record is one of the most complete that can be found in the NPS, a particular highlight being a stretch from the Neoproterozoic into the Permian that covers approximately 500 million years (approximately 790 to 290 million years ago) with only minor hiatuses. Obviously the park does not include everything; it didn't have much in the way of terrestrial Mesozoic deposition that has persevered to this day, so no dinosaurs, and it's not really a place for temperate well-watered terrestrial deposition. It's got just about everything else, though. Volcanic activity? Local and secondhand, Precambrian to the present. Igneous intrusions? Ditto. Faulting? Wouldn't exist without it. Marine deposition? Basically the entire Paleozoic column. Lakes? Anything from freshwater to hypersaline. Rivers? At various times, yes. Glacial? Well, if you don't mind turning back the clock, there are Proterozoic rocks from "Snowball Earth" episodes. Metamorphism? Yep, that's there too.

January term (J-term), 2001, University of St. Thomas field camp, at Ubehebe Crater in DEVA. The author is in the back row with a brown hat and mustache.

Furthermore, DEVA was situated at or near the continental margin from the late Precambrian into the Mesozoic, so it often straddled multiple depositional environments. Later, during the Cenozoic, faulting created numerous distinct depositional basins, and frequent volcanic eruptions both in the area and farther abroad laced the park with flows and ash beds. Finally, large-scale faulting has juxtaposed unrelated rocks in many areas. The combination of these factors makes a simple geologic column for the park at best an abstraction, because there are different stratigraphic sequences in different sections of the park. (It could have been worse; at least there hasn't been much damage done from crossing state lines and contending with different naming systems for the same darn rocks.) I ended up with a list of about 60 sedimentary units to cover and more than 950 references I wanted to find, not counting some internal unpublished stuff. Eventually, after much wailing and gnashing of teeth, over a couple of months I boiled it all down to about a hundred pages of text.

Two things occur to me at this juncture: first, perhaps I should ask for more money; second, I really ought to write a book.

With all that in mind, discussing the geology of DEVA is not something you do in one blog post. Fortunately, there are some natural places to break it up stratigraphically, with the first chunk being the Proterozoic into the Cambrian. Below is a simplified stratigraphic chart for this interval of DEVA (yeah, you'll need to expand it):

And now, the world's longest caption: Unconformities are not depicted except as a consequence of a missing time unit. The correlations are more general than exact; certain subtleties are omitted. Unit descriptions are shortened (Dm=Dolomite, Fm=Formation, Grp=Group, Ls=Limestone, Mt=Mountain, Qz=Quartzite). Contemporaneous units are depicted with the more northerly formation on the left side of the table. The depicted lateral divisions are not proportional to DEVA land area (e.g., the northern and southern Precambrian–Cambrian sequences do not split the park in half; the true division is in northwestern DEVA). Ga refers to billions of years, Ma to millions of years. The Cambrian is not divided into formally accepted Early, Middle, and Late, so those words are not capitalized.

The sources for this chart are, of course, gone over in the hundred pages of text, so for the moment you may just have to trust the guy in the back row with the brown hat and mustache. (Remember, we're talking bigger than Connecticut. Even stripped down to just geologic highlights and paleontology, omitting the delights of metamorphism, economic geology, volcanic activity, faulting, etc., it needs every one of those pages.) I can provide references for specific points, should somebody want them.

The basement rocks of Death Valley include igneous and metamorphic rocks that go back as far as the Paleoproterozoic, with some metamorphic rocks dated to 1,780 to 1,660 Ma (million years). These ancient rocks belong to the Mojave crustal province, one of the chunks that makes up the core of North America. We get into the first sedimentary formations with the Pahrump Group, which appears to have been deposited over several episodes. The oldest part of the oldest formation, the Crystal Spring Formation, may be more than 1,400 million years old, followed by the rest of it sometime between 1,320 and 1,080 Ma (older formations tend to be difficult to date neatly), and then the rest of the Pahrump Group between approximately 790 and 635 Ma. The early part of the sequence may correlate to the assembly of a supercontinent (Rodinia), and the later part may correlate to the break-up. The 1,080–790 Ma hiatus may be a time of continental stability. In terms of depositional setting, the Pahrump Group units include terrestrial and shallow marine beds, so we're in the vicinity of a coast. The uppermost unit, the Kingston Peak Formation, appears to include beds related to two "Snowball Earth" episodes, the Sturtian glaciation (715 to 670 Ma) and the Marinoan glaciation (650 to 635 Ma). The Kingston Peak Formation is also a basket case of complexity, and is liable to be split up. In terms of life, we're looking at stromatolites and other microbial structures, and microfossils. Stromatolites make up the bulk of certain beds in the Crystal Spring, Horse Thief Spring, and Beck Spring formations. They also show up extensively in the overlying Noonday Dolomite, otherwise more famous for "Noonday tubes" which have sometimes been described as burrows but which are more likely features that formed during the development of the stromatolites.

Rifting west of Death Valley continued to perhaps as recent as 580 million years ago ("west" being modern west; at the time, the continent would have been rotated in another direction). Much of what we now know as California did not exist at the time, and would only be supplied to the continent much later. With the end of rifting, the coast changed from an active tectonic margin to a passive margin: think the modern Atlantic coast of the United States, as opposed to, say, the Andes, with a seafloor trench and volcanoes. The Death Valley region became a shallow marine setting that hosted deposition of carbonates (e.g., limestone, which often became dolomite) and siliciclastics (e.g., sandstone, siltstone, shale; rocks made up of tiny bits of other rocks). What you got in a given place depended on the water depth and the availability of sediment from land. What is now northwestern Death Valley got more carbonates; these became the rocks of the White–Inyo facies, on the left side of the table. Most of what is now in DEVA got less carbonates; these rocks are known as the Death Valley facies. South of Death Valley there is another equivalent sequence, which can be found at Mojave National Preserve and the vicinity (the famous Marble Mountains trilobites).

The White–Inyo facies is not exposed as extensively in the park as the Death Valley facies, but one of the classic White–Inyo localities, the Waucoba Spring section described by Walcott as the type section of his Waucoban series, is located in the northwest part of the park. The Waucoban used to be considered the early Cambrian because it had the earliest body fossils in North America. The base of the Cambrian was later backed up and is now considered to be marked by the first appearance of the trace fossil Treptichnus pedum, which coincidentally shows up in both the lower Wood Canyon Formation (Death Valley facies) and the upper Deep Spring Formation (White–Inyo facies) in DEVA. The lower Cambrian rocks of both facies are noted for the presence of the archaeocyath sponges, one of the great "one-hit wonders" of life on Earth. Fairly typical Cambrian assemblages of brachiopods, hyoliths, various snail-like mollusks, trilobites, and extinct things that defy classification are present above the Treptichnus marker in both facies.

When we continue: a fairly straightforward first half of the Paleozoic gives way to collapsing margins, and by the Permian everyone gets their own formation.

[Edit: if this was all a little dry for you, maybe you'd rather read about sloth-dung-filled Rampart Cave?]

Selected references and chart references:

Bahde, J., C. Barretta, L. Cederstrand, M. Flaugher, R. Heller, M. Irwin, C. Swartz, S. Traub, J. D. Cooper, and C. Fedo. 1997. Neoproterozoic-Lower Cambrian sequence stratigraphy, eastern Mojave Desert, California: implications for base of the Sauk Sequence, craton-margin hinge zone, and evolution of the Cordilleran continental margin. Pages 1–19 in G. H. Girty, R. E. Hanson, and J. D. Cooper, editors. Geology of the Western Cordillera: perspectives from undergraduate research. Pacific Section, Society of Economic Paleontologists and Mineralogists, Los Angeles, California. Book 82.

Cornwall, H. R., and F. J. Kleinhampl. 1964. Geology of Bullfrog quadrangle and ore deposits related to Bullfrog Hills Caldera, Nye County, Nevada, and Inyo County, California. U.S. Geological Survey, Washington, D.C. Professional Paper 454-J.

Corsetti, F. A., and J. W. Hagadorn. 2000. Precambrian-Cambrian transition: Death Valley, United States. Geology 28(4):299–302.

Hagadorn, J. W., C. M. Fedo, and B. M. Waggoner. 2000. Early Cambrian Ediacaran-type fossils from California. Journal of Paleontology 74(4):731–740.

Hunt, C. B., and D. R. Mabey. 1966. Stratigraphy and structure, Death Valley, California. U.S. Geological Survey, Washington, D.C. Professional Paper 494-A.

Johnson, B. K. 1957. Geology of a part of the Manly Peak Quadrangle, southern Panamint Range, California. University of California Publications in Geological Sciences 30(5):353–423.

Keller, M., J. D. Cooper, and O. Lehnert. 2012. Sauk Megasequence supersequences, southern Great Basin: second-order accommodation events on the southwestern Cordilleran margin platform. Pages 873–896 in J. R. Derby, R. D. Fritz, S. Longacre, W. A. Morgan, and C. A. Sternbach, editors. The great American carbonate bank: the geology and economic resources of the Cambrian-Ordovician Sauk Megasequence of Laurentia. American Association of Petroleum Geologists, Tulsa, Oklahoma. Memoir 98.

Macdonald, F. A., A. R. Prave, R. Petterson, E. F. Smith, S. B. Pruss, K. Oates, F. Waechter, D. Trotzuk, and A. E. Fallick. 2013. The Laurentian record of Neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death Valley, California. Geological Society of America Bulletin 125(7–8):1203–1223.

Mahon, R. C., C. M. Dehler, P. K. Link, K. E. Karlstrom, and G. E. Gehrels. 2014. Detrital zircon provenance and paleogeography of the Pahrump Group and overlying strata, Death Valley, California. Precambrian Research 251:102–117.

Mahon, R. C., C. M. Dehler, P. K. Link, K. E. Karlstrom, and G. E. Gehrels. 2014. Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California; a record of the assembly, stability, and breakup of Rodinia. Geological Society of America Bulletin 126(5–6):652–664.

Palmer, A. R., and R. B. Halley. 1979. Physical stratigraphy and trilobite biostratigraphy of the Carrara Formation (Lower and Middle Cambrian) in the southern Great Basin. U.S. Geological Survey, Washington, D.C. Professional Paper 1047.

Petterson, R., A. R. Prave, B. P. Wernicke, and A. E. Fallick. 2011. The Neoproterozoic Noonday Formation, Death Valley region, California. Geological Society of America Bulletin 123(7–8):1317–1336.

Richards, C. A. 1957. Geology of a part of the Funeral Mountains, Death Valley National Monument, California. Thesis. University of Southern California, Los Angeles, California.

Smith, E. F., F. A. Macdonald, J. L. Crowley, E. B. Hodgin, and D. P. Schrag. 2015. Tectonostratigraphic evolution of the c. 780-730 Ma Beck Spring Dolomite; basin formation in the core of Rodinia. Special Publication - Geological Society of London 424.

Stewart, J. H. 1965. Precambrian and Lower Cambrian formations in the Last Chance Range area, Inyo County, California. U.S. Geological Survey, Washington, D.C. Bulletin 1224-A:A60–A70.

Stewart, J. H. 1966. Correlation of Lower Cambrian and some Precambrian strata in the southern Great Basin, California and Nevada. U.S. Geological Survey, Washington, D.C. Professional Paper 550-C:C66-C72.

Stewart, J. H. 1970. Upper Precambrian and Lower Cambrian strata in the southern Great Basin, California and Nevada. U.S. Geological Survey, Washington, D.C. Professional Paper 620.

Walcott, C. D. 1908. Cambrian sections of the Cordilleran area. Smithsonian Miscellaneous Collection 53:167–230.

Walcott, C. D. 1912. Cambrian geology and paleontology II, no. 10. Group terms for the Lower and Upper Cambrian series of formations. Smithsonian Miscellaneous Collections 57(10).

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