Simple clam shell helps explain when mountains formed

allen holder | Kansas city star

The makeup of the clam shell, below, gives clues to when the Rocky Mountains formed. The view at left is of Aspen Mountain in Colorado.

The mighty Rocky Mountains extend nearly 4,000 miles between British Columbia and New Mexico.

Geologists divide them into ranges that include the Bighorn, Beartooth, Black Hills (famous for Mount Rushmore), Sierra Madre and Uinta.

The exact cause of their uplift is still somewhat of a geologic mystery, but one key to the puzzle would be better information on the timing of the uplifts. But how do you determine when a mountain started to rise?

Some of the most interesting research articles are those that answer questions by combining data from different disciplines. One recent paper in the Geological Society of America Bulletin used paleontology, tectonics and geochemistry to determine when different Rocky Mountain ranges formed.

But first you need to know a little about clam shells.

Clam shells are made largely of the mineral aragonite, or calcium carbonate. And they also contain strontium, a silvery metal used in flares and fireworks to produce a deep red color. Its atomic size and chemical properties make it a good substitute for calcium. It has two common isotopes, strontium-86 and strontium-87.

The ratio of those two isotopes in a particular river or stream is determined by the kind of bedrock exposed in the area — erosion of igneous and metamorphic rocks such as granite and gneiss produces high ratios of strontium-86/strontium-87, while erosion of carbonate sedimentary rocks such as limestone produces low ratios.

Clams in those streams make their shells using available materials. That means the strontium isotopes of their shells can indicate the ratios in the water in which they lived.

Because those ratios are preserved in fossil clam shells, scientists can determine the kind of bedrock that was exposed to erosion when those clams were alive.

As mountains are uplifted, younger rocks (those more recently deposited at the surface) erode first.

In the Rockies, the younger rocks include large amounts of limestone that was deposited during the Paleozoic Era and subsequent Mesozoic, between 550 million and 250 million years ago. The mountain cores are composed mainly of gneiss and granite, which formed much earlier during the Precambrian Era, more than 2.5 billion years ago.

The recent research measured the strontium isotope ratios in fossil clam shells from six basins — low areas that received sediment as the surrounding mountains rose and were eroded.

Among the results, sediments in the Bighorn Basin came mainly from carbonates, so exposure of the granite core in the Bighorn Mountains (which is prominent today) occurred sometime after the clams were alive.

The Black Hills might have reached heights of 3 miles by the late Paleocene, and the Green River Formation (famous for containing huge amounts of oil shale) did not receive sediments from a Precambrian source before the Early Eocene, but then it came on strong.

Amazing what information you can get from a lowly clam.

gnidovec.1@osu.edu

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.