Sunday, August 1, 2010

Why Looking for a Hidden River is like Searching for the Right Asylum

Back in my college days, I was lucky enough to intern at the Cleveland Museum of Natural History for one summer. One aspect of my project involved studying the quarrying history of a local building stone. In the course of my research, I noticed that on many 1800s-era geologic maps, asylums were often located very near to quarries. As it turns out, this was not a coincidence – quarries often hired asylums resident as cheap labor, and in turn, the residents got a chance to leave the grounds and get some stimulation. Hence, a somewhat disturbing lesson: if older geologic maps don’t show quarries, look for asylums. Investigating the geologic record often works in the same vein.

As it turns out, the majority of geologic events in the earth’s history were never preserved in the rock record. Even a geologic event that is sufficiently significant, long-lived, and plainly lucky enough to be preserved may never be discovered – it has to be exposed, accessible, and properly observed and studied. Often, geologic events are not interpreted based on their direct effects, but by side effects and related events. Metaphorically speaking, the quarries are gone, but the asylums remain. In a prime example, in looking for evidence of an ancient transcontinental river in the U.S., geologists had to look in….desert deposits.

How to find evidence of a river nobody can see

Many of the archetypical rock formations of the American southwest – the towering red cliffs, psychedelically wavy hillsides, etc (pictures at left from here) – are the deposits of ancient Jurassic ergs* (sand seas). These ergs were particularly enormous – perhaps rivaling in size the Empty Quarter in the Arabian Peninsula (the largest modern erg) (Dickinson and Gehrels, 2009).

With any erg, the obvious question is “where did all the sand come from”? (Granitic crust is only 1/3rd quartz at most, so a lot of rock has to be eroded to get a sand sea). Two easy answers presented themselves – the (Ancestral**) Rocky Mountains are practically next door, and there were plenty of voluminous sandstones across North America that could have been reworked. But there was also a third possibility, first proposed by Marzolf (1988) – an enormous transcontinental river originating in the Appalachians on the east coast. Unfortunately, Jurassic rocks aren’t exposed anywhere in the U.S. between the Rockies and the Appalachians, so direct evidence of this river was out of the question. But, there are the ergs…

Aspen, Colorado, circa 170,000,000 years ago (from here)

* Deserts and ergs aren’t synonymous, but the two coincided in this instance.

** Not the same as the modern Rocky Mountains, but let’s not get into that right now…

Marzolf’s hypothesis was essentially ignored until University of Arizona geologists William Dickinson (emeritus) and George Gehrels decided to take a look at some of the deposits in the Jurassic ergs in the 1990s. Specifically, they looked at the zircons in the sandstones – unlike the commercials, zircons, not diamonds, are forever in the geologic record, lasting hundreds of millions of years (the oldest known mineral on earth is actually a zircon (Wilde et al., 2001)). Over their long lifespan, zircons can be transported and reworked over thousands of kilometers, but always carry an age signature from their original host rock. When sandstone were sampled from various localities in the ergs, the ages of the zircons fell into three categories: 1/4th matched ages with the basement rocks of the Ancestral Rockies, 1/4th matched ages with reworked ancient sandstones, and the remaining half were divided into four ages too young for the previous two categories (Dickinson and Gehrels, 2003). When the younger zircons were investigated further, it was discovered that the four age ranges fit nicely with….granite bodies that compose the Appalachians (Dickinson and Gehrels, 2009). Furthermore, paleowind measurements from the sandstones showed consistent southern winds (Dickinson and Gehrels, 2009). Combined, this suggested there was some major source of transportation that carried sands from the Appalachians to the western U.S., where they were deposited and carried by winds to build ergs to the south (see cartoon below, heavily inspired by Fig. 1 in Dickinson et al., 2010). And a prime candidate would be….a major transcontinental river!

The long history of Canadian immigration

Recently, Dickinson et al. (2010) tested the "transcontinental river" hypothesis from another angle. I mentioned before that Jurassic rocks aren’t exposed between the Rockies and the Appalachians, but they do exist – under a whole bunch of other rocks. In this case, Dickinson et al. (2010) compared sandstones from the Jurassic ergs to the west to subsurface (fluvial) Jurassic sandstones in the Michigan Basin (star in figure above), an area that would have been right in the middle of some of the northern tributaries of the transcontinental river. This time, the zircon ages were more nuanced: the ergs and the fluvial sandstones both contained zircons with ages matching “Grenvillian” source rocks, but the zircons from the Michigan sandstones lacked ages matching “peri-Gondwanan” source rocks (Dickinson et al., 2010). As it turns out, the “Grenvillian” source rocks are found in northeast Canada, and the “peri-Gondwanan” rocks are found in the southern Appalachians. So it would make sense that tributaries from northeast Canada (upper arrow in figure above) would carry zircons that ended up in Michigan and eventually the western ergs, but tributaries from the southeast U.S. (lower arrow in figure above) would carry zircons that ended up in the western ergs only (Dickinson et al., 2010).



Dickinson, W.R., and Gehrels, G.E. 2003. U-Pb ages of detrital zircons from Permian and Jurassic eolian sandstones of the Colorado Plateau, USA: Paleogeographic implications: Sedimentary Geology, v. 163, p. 29–66. (pdf here)

Dickinson, W.R., and Gehrels, G.E., 2009. U-Pb ages of detrital zircons in Jurassic eolian and associated sandstone of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment: Geological Society of America Bulletin, v. 121, p. 408–433.

Dickinson, W.R., Gehrels, G.E., and Marzolf, J.E. 2010. Detrital zircons from fluvial Jurassic strata of the Michigan basin: Implications for the transcontinental Jurassic paleoriver hypothesis: Geology, v. 38, no. 6, p. 499–502.

Marzolf, J.E., 1988. Controls on late Paleozoic and early Mesozoic eolian deposition of the western United States: Sedimentary Geology, v. 56, p. 167–191.

Wilde, S.A., Valley, J.W., Peck, W.H., and Graham C.M. 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175-178. (pdf here)

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