Placerias Quarry Panorama

Here is a panorama of the area where the Late Triassic Placerias Quarry is located in the Chinle Formation near St. Johns, Arizona. Sharp eyes can pick out a red car at the quarry just left of center of the photo above the juniper trees. Historically this quarry is one of the most productive sites in the Chinle Formation, and has produced numerous skeletons of aetosaurs, phytosaurs, "rauisuchians", crocodylomorphs, dinosauriforms, and of course more than 40 individuals of the dicynodont Placerias hesternus. The limited exposures have made stratigraphic correlation of this site to other Chinle localities problematic; however, it is a beautiful field area.

The Basal Chinle Formation in Canyon DeChelly National Monument

The base of the Chinle Formation is well exposed in Canyon DeChelly National Monument just outside of Chinle, Arizona.  Here the Chinle fills paleovalleys incised into Permian-age rocks, in this case the aeolian DeChelly Sandstone.  The only trail accessible in the monument is the White House Trail, which leads down to abandoned ancient puebloan indian sites on the valley floor and in the canyon walls.  Thus, this trail is very popular with visitors to the monument; however, I'll bet that the majority of them miss this feature, a 'u'-shaped valey carved in the DeChelly Sandstone and filled with gravels of the Shinarump Member of the Chinle Formation.

The capping darker sandstone is the Triassic Shinarump Member of the Chinle Formation, and the buff sandstone is the Permian DeChelly. In the direct center of the photo is a dark patch in the buff.  This is a 50 foot deep gravel filled paleovalley cut into the DeChelly.  You can also trace it to the left and right in the photo.  Note the figure on the trail just to the left of the valley, and a figure under the tree just to the right.

Something looks very wrong with the photo above. This is the contact between the Triassic Shinarump (on the left and the Permian DeChelly (on the right). The contact between the two is vertical along the sides of the paleovalley. This is relatively flat lying strata. Pretty rare to see this.

One very cool aspect of a vertical contact (and unconformity) like this is that it is pretty obvious that the older unit (in this case the cross bedded sandstone) must have been fully lithified before the younger unit was deposited.  In this case the was a slot canyon in the DeChelly that was filled in in the Late Triassic.  Actually Triassic topography is visible here.  You can stand on the end and look inot where the canyon would have been about 220 million years ago.

Cool Places to Visit - Dinosaur State Park

There are actually quite a few places out there where you can experience the Triassic. I've already briefly covered Ghost Ranch, New Mexico and of course the Petrified Forest National Park in Arizona. Another great place is Dinosaur State Park (DSP) in Rocky Hill Connecticut. Now before somebody shouts it out I am aware that the main feature, the track bed, is Early Jurassic in age; however, when I visited as a kid in the 1970s they still thought it was Triassic (I've just dated myself I think). Anyhow, DSP also has a Triassic mural and obviously the Triassic-Jurassic transition is an important topic for them.

As I stated above the main feature is a sandstone surface covered (and I mean covered) with dinosaur footprints. The site was discovered in the mid-1960s by a highway construction project and the park was opened in 1968. A large dome protects the trackways. Over 500 tracks are exposed and at least three times that many are buried for preservation.

This place is a great resource and I know that it made a lasting impression on me as a child (I am originally from Connecticut).

You can read more about Dinosaur State Park here and here.