Igneous rocks are rebels. Sedimentary rocks follow straight-forward rules—they are deposited in horizontal layers, with the oldest sediments on the bottom. Igneous rocks can do what they want. Molten rock can eat away at other rocks below ground, opening up a cozy space to cool and solidify. It can also come flying—or oozing—out of a volcano, quickly crystallizing on the surface. Or it may squirt through crevices like fractures or boundaries between sedimentary layers, inserting itself as a sheet in any number of orientations. Where these walls of igneous rock cut across rock layers, they are called “dikes.”

Every now and then, when conditions are just right, sediments get to play this game, too. When they’re over-pressurized, water-soaked sands can sometimes get injected into fractures to form “clastic dikes”. Most often, these clastic dikes invade sediments or sedimentary rocks. Only very, very rarely, does sand get to turn the tables on those igneous hooligans, forming dikes of sandstone within igneous rocks.

In Colorado’s Front Range, near Colorado Springs, you can find that strange inversion. Along the Ute Pass Fault, the Tava sandstone forms dikes and similar formations within the billion-year-old Pikes Peak Granite, as well as some even older crystalline rocks to the south. Sheets of sandstone up to six meters thick cut through the rocks, which would confuse the heck out of any young geology students an instructor was mean enough to bring out there.

The sandstone “injectite” probably formed because of old earthquakes on the Ute Pass Fault. During the earthquakes, rapid changes in fluid pressure drove the sand into open spaces like fractures, including the ground-up rock along the plane of the fault. In some places, new fractures were actually opened up by the hydraulic pressure and subsequently filled with sand. (Sound familiar? Natural gas fracking involves fracturing shale with hydraulic pressure and propping the fractures open with grains of sand.)

The actual age of the Tava sandstone has long eluded geologists. It must be younger than the billion-year-old granite it’s inside, while other evidence indicates that it’s older than 300 million years old. That span of plausible ages is pretty broad, but sedimentary rocks aren’t easy to date—and bizarre sedimentary dikes doubly so. At least an igneous dike would contain crystals suitable for radiometric dating.

A new study from Colorado College’s Christine Siddoway and the University of Arizona’s George Gehrels turns to a clever bit of detective work, examining bits of zircon within the sandstone. Zircon, an igneous mineral, is a standard time capsule because its crystal lattice forms a tight cage that holds onto both radioactive uranium atoms and the lead they decay into. Zircon crystals, while usually tiny, are also fantastically tough, outlasting other igneous minerals that crumble to dust. Measure the age of wee zircons plucked from a sandstone and you can determine the age of the igneous rocks that broke down to generate that sand.

If other sedimentary rocks in the region formed from the erosion of those same igneous rocks, the same collection of zircons will be there. So by comparing the ages of the zircons, you might be able to find a rock that was deposited at the same time as the Tava sandstone—a rock with an age that is less mysterious.

The researchers dated zircons in six samples of the Tava sandstone and four other sandstones in the area, which ranged from around 500 million to 300 million years old. The Tava sandstone contained lots of zircons that were between 970 million and 1.3 billion years old, and three of the six samples also contained zircons around 1.4 and 1.7 billion years old. Two of the local sandstones had zircons aged 1.4 and 1.7 billion years, but none had much in that 1-1.3 billion year age range.

However, the researchers did find a good match with other rocks that have gotten zircon fingerprints. Surprisingly, they were from Utah, Arizona, and California. The matching rocks were older, coming in between 680 million and 800 million years old. That predates the Cambrian period, during which multicellular life took off and diversified.

These sedimentary rocks come from a time of continental congregation before Pangaea—a supercontinent called Rodinia. A massive mountain range rose up along a similar line as today’s Appalachians (which formed later, in a separate event). Erosion of those mountains produced sediment carried by rivers all the way to what is now the American West.

The Tava sandstone appears to be the first example of sediment from this time found on the eastern side of the Rockies. That means that studying it may teach geologists something about what that part of Rodinia was like—even if the sand was jumbled up and jammed into rock fractures.

Because some of those zircons are probably from local rocks, the Tava may also hold information about the widespread erosion that swept the decks of most continental rocks—a period of missing history called the Great Unconformity that ended at the start of the Cambrian period. The Tava is a scrap of a missing page from that history. Now we have a good idea which page it came from.

Lithosphere, 2014. DOI: 10.1130/L390.1 (About DOIs).