Trinidad Lake State Park
The K-Pg Boundary and much more!
Trinidad Lake State Park contains one of the best exposures of the famed K-Pg (Cretaceous-Paleogene — formerly known as the K/T, Cretaceous-Tertiary) boundary, where Cretaceous- and Paleogene-aged rocks are separated by an iridium-rich layer that represents an asteroid impact and possibly the extinction of many of the Earth’s dinosaur species. There is an interpretive display at the visitor center. The actual site may be visited by taking a short hike down into Long Canyon where an interpretive sign and a self-guiding brochure are available. Summer ranger-led walks are available that occasionally focus on geology.
Visiting Trinidad Lake State Park by Vince Matthews
Fishers Peak underlain by volcanic rocks. The four massive cliffs represent four outpourings of lava.
Heading back to I-25 for our easy trip to Denver, I wondered why more people don’t come to Trinidad Lake State Park and enjoy the fascinating geology. How amazing to find evidences for an ancient, Louisiana-type swampy environment; a long-ago fiery past of voluminous volcanic eruptions; abundant faulting and modern-day earthquakes; rich energy resources; and of course, the world-famous K-Pg boundary! As we neared the interstate, the sinking sun was highlighting the volcanic rocks capping Fishers Peak. Below, in quaint, historic Trinidad, was the wonderful B&B we used as a base for the weekend to explore Trinidad and the state park.
After a marvelous Saturday morning breakfast in Trinidad we drove out to the south entrance to the park. Approaching the Entrance Station at Trinidad Lake State Park, we passed over a bridge and I couldn’t believe my eyes. A deep, narrow-walled railroad cut revealed a beautiful display of the rocks underlying this area. I had to check this out because exposures of Earth’s third dimension this good are rare.
Overlapping channels in railroad cut with black, igneous dike cutting across in the lower left.
I wheeled around and drove back 0.2 miles and turned right onto Frisco Canyon Road. Driving along parallel to the railroad I hoped to get an unobstructed view down into this deep cut.Shortly, the view exceeded my expectations. About 0.5 miles after turning onto the road, was a fantastic array of overlapping, discontinuous sandstones recording the shifting channels of ancient rivers. Cutting across those layers was one of the most beautiful examples of an igneous dike that I have ever seen. The black, tabular rocks with sharp borders slicing across the white sandstones dramatically illustrate how the once-molten material worked its way upward toward the surface. It is hard to imagine that at the time the hot liquid (~1800 °F) was working its way upward, the surface was more than a half-mile higher than it is where we are now standing. Indeed, it was at the elevation of Fishers Peak (3,400 feet higher). Fishers Peak is capped with volcanic rock that cooled from molten lava flowing out on that higher surface. Dikes of molten material were feeding those flows. The hot liquid that didn’t make it to the surface solidified underground forming dikes such as this one. Since that time millions of years ago, the Purgatoire River has removed huge volumes of material.
If the dike fed the volcanic flows on Fishers Mesa, then why is the dike over here on the other side of the interstate? The answer is that the flows were much more widespread and where we are standing would have been a half mile deep in the earth underneath the lava flowing out on the surface.
A little farther west was a fault cutting and displacing the sedimentary layer. In fact there were two opposing faults that form a little graben. How amazing. I have only seen three grabens in nature during my decades-long career in geology. What a treat this railroad cut turned out to be.
Igneous dike cutting across horizontal sedimentary layers. Here we are getting a look about a half mile into the earth where molten material rose up to the surface, which was 3,400 feet higher when this was happening.
After soaking in all of these wonders, we returned to the Entrance Station to get our pass. The ranger told us that we should be sure to visit the K-Pg boundary layer in Long Canyon. Little did he know ― that was the main reason we were here. So, we crossed the dam getting a wonderful view in all directions. At the stop sign to Highway 12, I noticed some discontinuous layers in the high road-cut directly ahead of us. So, I pulled onto the shoulder to study it a little more. Slowly the realization came that the layers were actually chopped off by a fault. Because a similar fault farther west at Segundo had generated a swarm of earthquakes in 2002, I wondered if this fault was capable of generating earthquakes also.
Abandoned coke ovens at Cokedale.
Heading west on 12 toward the Long Canyon turnoff, we soon passed Cokedale, a nearly abandoned center of the once-bustling coal mining industry. The scenic skeletons of coke ovens in the valley to the south explained the name. Huge black waste piles give mute testimony to the time when this was the most productive coal basin in Colorado. More than 180 million tons of coal were produced from 150 mines in the Trinidad Coal Field since the late 1800’s. The coal was formed at a far-distant time when this area looked like the vast swamps of Louisiana. Decaying organic matter and fine-grained muck would have created a Cajun’s paradise (had there been any Cajuns around back then). Ancient rivers also meandered back and forth across the coal swamp depositing sand channels.
Now coal mining is over, at least for the time being. However, lots of energy is still being taken from the coal in the form of natural gas. This area is riddled with more than 1,300 coalbed methane wells. Since you probably wouldn’t realize that without my mentioning it, it shows what a minimal environmental impact this industry has on the landscape.
Sandstone channel along Highway 12 on the road to Long Canyon turnoff.
Several miles down the road I spy an outcrop of rock that I can’t believe. Here is a sandstone channel deposit that I had only seen previously in drawings in textbooks. These drawings illustrated a sandstone body completely encased in shale with”feathered” margins. I thought these drawings were idealized representations, not something that actually existed. Yet, here it is for real! Never have I seen an example this marvelous. It is completely encased in shale and it has all of those little feathered edges that I never really believed could be seen in nature.
Spanish Peaks – view north from Long Canyon Road.
Turning south onto the Long Canyon road, our excitement mounts, as we will soon be seeing the important K-Pg boundary. Climbing out of the Purgatoire River Valley, we soon get an unusual view of the Spanish Peaks. Most people only get to see them from the north, so this is an unexpected treat to get a clear southern view.
K/Pg boundary layer is just above white layer under the overhang shadow.
Finally, we reach the trailhead parking lot for the Long Canyon Trail. Walking down the trail our anticipation grows. In a short time, we reach the decorative, explanatory sign. There above the sign, underneath a block of sandstone, is the K-Pg boundary layer — that thin layer composed of the fallout from the asteroid collision that wiped out the dinosaurs along with 70% of all species on Earth.
Southern Colorado was an extremely important area for providing geologic data that helped the asteroid collision hypothesis develop into a theory. Indeed, because this area is so important, the Smithsonian Institution quarried a 2 ½ ton block of rock containing the K-Pg layer from near here, to put on display for all the world to see in Washington, D.C.
In 1980 scientists proposed that the Cretaceous period ended on Earth with the impact of an asteroid or comet (about the size of Denver) traveling at 100,000 miles per hour. Southern Colorado sites and geologists played key roles in proving the impact hypothesis, locating the crater at Chicxulub (CHEEK-shə-loob) on the Yucatan Peninsula of Mexico, and proving that Chicxulub was the source of the impact debris. The existence of an anomalously high level of a rare element, iridium, at the K/Pg boundary in coal-swamp deposits in Colorado proved that the iridium anomaly was not just some phenomenon created by seawater, but was indeed fallout from an extraterrestrial event. Indeed, Colorado has the highest iridium content ever measured in terrestrial rocks. Colorado sites also showed the presence of shocked quartz grains that could only be caused by impact. The size of the shocked quartz fragments and the existence of two layers of fallout debris in Colorado sites also indicated to crater searchers that the impact must have been located on, or near, North America.
A finger on the world-famous K-Pg boundary in southern Colorado.
Once Chicxulub was proposed as the site of impact, Colorado once again provided key evidence confirming it. The nail was put in the coffin by comparing the chemistry of minerals from Chicxulub’s rocks with the chemistry of minerals in the K-Pg boundary layer from a number of sites. Most of these samples were from Colorado. Colorado also provided the world with its first known, impact-shocked grains of the mineral zircon.
What a treat to finally touch that tiny layer that provides so many answers to major questions that puzzled scientists for decades. Our visit is complete, and we can return home with a rich geological experience tucked into our memories. Everything we have seen today is available to us because of the slow persistent wearing away by the Purgatoire River system. The river removed many cubic miles of material, one pebble at a time and carried it out to the plains and eventually on to the Mississippi Delta south of New Orleans.