Wyoming Day 9 – Red Gulch

As predicted, it was raining when we got up this morning. So, after breakfast at Dirty Annie’s we drove to the Red Gulch Dinosaur Tracksite, about halfway between Shell and Greybull, for a quick paleontology and geology lesson.

As its name implies, Red Gulch is best known for its hundreds of dinosaur tracks. These are probably theropod dinosaurs, and there are several different sizes and morphologies preserved:

The tracks occur in several crossing paths on a single bedding surface, and while they almost certainly weren’t made simultaneously they probably were made during a relatively brief period of time. There aren’t a huge number of reported tracksites in Wyoming, so the mere existence of this site is pretty important. However, the geologic setting is what really sets this place apart.

Much of the trackway bedding surface is covered with ripple marks, which of course suggests flowing water (although wind can also produce ripples):

A closeup shows that the tracks cut across the ripples, so the dinosaurs were walking across a surface that already had ripples:

But some of the surface isn’t rippled, but instead is lumpy:

Up close, these lumps show thin, concentric laminae…they’re stromatolites (algal mats)!

Stromatolites, of course, are often associated with intertidal zones, where you also tend to get ripples. Intertidal zones also tend to have high evaporation rates. A curious feature found in a few places on the surface are square to near-square shapes, such as the one to the left of the lens cap below:

These appear to be salt pseudomorphs, essentially natural casts of salt crystals, which suggests high evaporation rates. The sediment itself is also interesting:

Notice that the circular grain in the sediment has a concentric structure. This is an ooid, a chemically precipitated grain of calcium carbonate. Like salt crystals, ooids are typically associated with high evaporation rates.

All of this data suggests that the dinosaurs were walking around on a coastal tidal flat, perhaps walking along the beach at low tide looking for fish, squid, or other stranded marine animals that could provide an easy meal. For Wyoming, that’s a little surprising, because most of the dinosaur-rich units such as the Morrison and Cloverly are fluvial (river-system) deposits, not marine. So what’s going on? There’s clearly more to the story.

The bedding surface that includes the tracks is also covered with a large number of curious, U-shaped depressions:

These are the bottoms of invertebrate burrows, most likely made by shrimp-like crustaceans. These burrows are typically pretty deep; they can go down several feet below the seafloor. This suggests that there was a younger seafloor above the track bed. Sure enough, if we examine the overlying sediment, we find this:

These are shells from marine mollusks; the trackways were flooded by the ocean, not a river. Moreover, these shells are a bivalve called Gryphaea nebrascensis, which is typical of the Sundance Formation.

That means, first, that some revisions had to be made to the interpretation of the Sundance Sea. It was generally thought that, during Sundance times, Wyoming was under fairly deep water. But Red Gulch shows that there were regressions and transgressions during this time. During the regressions, the Sundance beach was located in Wyoming, allowing dinosaurs and other terrestrial organisms move into the area.

Second, the age of this part of the Sundance is Middle Jurassic. When we talk about all the famous Jurassic dinosaurs in North America, we’re usually talking about the Late Jurassic Morrison Formation, or occasionally the Early Jurassic Kayenta Formation from the southwestern US or the northeastern part of the Newark Supergroup, which is also Early Jurassic. In fact, in North America, there are no Middle Jurassic dinosaur fossils at all! That makes Red Gulch pretty much the only record of dinosaurs in North America during this time period.

Red Gulch is located on federal BLM land, and can be visited by the public. BLM has built a nice boardwalk that includes interpretative panels and leads visitors to the site, and it is well worth a visit. Make sure to leave a donation in the box at the beginning of the boardwalk, so they can continue to maintain the site!

The BLM’s webpage on Red Gulch is here, and this paper details some of the work that was involved in studying the site.

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