November 1, 2012
As it is subjected to weathering, certain gravels break down and drift away, thus rejuvenating the deposit.
By Bill Langer
The Curious Case of Benjamin Button is a 2008 American film very loosely based on the 1922 short story of the same name by F. Scott Fitzgerald. (I enjoyed the movie much more than the short story.) On the evening of Nov. 11, 1918, Benjamin was born with the appearance and physical maladies of a very elderly man. As Benjamin said, he “was born under unusual circumstances. While, everyone else was agin’, I was gettin’ younger…all alone.”
Let’s get on to the geologic part of this article. Rocks or gravels that are exposed at the land surface are subject to weathering through contact with the Earth’s atmosphere, water, plants, and animals. Physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, freeze/thaw cycles, water, and ice. Chemical weathering is caused by atmospheric or biologically produced chemicals. Consequently, earth materials near the land surface commonly are more weathered than those at depth. (For example, see “W is for Weathering,” Aggregates Manager, November 2011).
But like Benjamin Button, the gravels along the Agua Fria River (located west of Phoenix, Ariz.) suffer from a reversal of normal processes. While other gravels get stronger with depth, the gravels of the Agua Fria River keep gettin’ weaker.
Curious. What can possibly explain this “Benjamin Button” reverse-weathering phenomenon?
The gravels were laid down as three separate deposits called the upper, middle, and lower alluvial deposits. The lower alluvium was deposited about 100,000 years ago (Figure 1) and originally filled the entire Agua Fria valley. The gravels were subjected to intensive physical and chemical weathering for many tens of thousands of years, and that weathering took its toll on the gravels.
About 10,000 years ago (Figure 2), the Agua Fria River became rejuvenated and eroded and transported the older gravels (geologists call this reworking). As the gravels were being jostled around by the river, some of the weathered clasts were broken down into small particles and were washed away. This improved the overall quality of the middle alluvial deposit.
About 1,000 years ago the Agua Fria River once again became rejuvenated and eroded into the middle alluvial deposit (Figure 3). In doing so, it removed most of the remaining weathered gravels.
All of this is borne out by the results of a geological study of the state of weathering of the gravels. As it turns out, the gravels made of one particular rock type — tuffaceous (volcanic ash fall) rocks — are most susceptible to weathering (Figure 4). In the lower alluvial deposit, 14 percent of the tuffaceous clasts are decomposed. Seven percent of the tuffaceous clasts are decomposed in the middle alluvial deposit. The upper alluvial deposit was least weathered where less than 1 percent of the tuffaceous clasts are decomposed.
Maybe the reverse-weathered Agua Fria River gravels are not so curious after all.
Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey.
He can be reached at Bill_Langer@hotmail.com.