September 6. Triassic Climate

You can imagine that whatever the causes of the great extinction at the end of the Permian, the effects continued at least a while into the Triassic. It looks like it took maybe 4 million years or so for the recovery to be clearly underway. Extinctions are double-edged swords – while they decimate many species and eliminate countless individuals, they also clear the slate, opening up ecological niches, so that when conditions allow, the survivors that can change and adapt to the new circumstances have great opportunities to expand into vacated ecological realms. Extinctions favor the opportunistic, to some degree at least. 

So what were the conditions during the Triassic that life had to adapt to? To a large extent, Pangaea was still assembled into one big continent. We heard about small blocks rifting off to form the Cimmerian continent, and there is evidence that Pangaea had started to rift apart between today’s Greenland and Scandinavia – the first hints of the North Atlantic Ocean. But on the whole, it was still one big continent, and its tropical interior, distant from the sea was in many places pretty hot and arid. The red beds that characterize much of the Triassic are clear evidence for this.

On average, carbon dioxide levels in the atmosphere were high. Depending on which geochemical model you use, the level could have been as much as 1500 parts per million, 4 times today’s value. And oxygen levels had plummeted during the Permian, to possibly as low as 15% of the total versus 21% today. That continued during the Triassic, increasing slowly through the period. The overall average temperature was higher than today, probably at least 3ºC higher, and perhaps more. All of the geochemical models and actual measurements indicate hot, dry, carbon-dioxide-rich settings during the Triassic, and that meshes well with the kinds of rocks – commonly, red beds and evaporites – and terrestrial life, adapted to such conditions, that we actually observe. 

Arid and dry does not necessarily mean conditions like the middle of a modern desert. There had to be some water, both for life and to give the alternating wet-dry conditions that produce red beds and evaporites. And even modern deserts, with some relatively small exceptions, support life. The Triassic seems to have been an intensely seasonal time – in part because of the world ocean, Panthalassa, which would have generated monsoons bringing seasonal rains to at least the coastal parts of Pangaea. Large woody trees, hit hard by the Permian extinction, appear to have recovered to a large extent within about 6 million years, about 245 million years ago during the early Triassic.

You can and should imagine that we would not expect the 50-million-year time span of the Triassic climate to be uniform and boring. There was always variety, depending on where you were on the globe, and there was plenty of variation in time as well. There’s even evidence that at times during the Triassic the climate became much wetter. We’ll talk about one such period in the late Triassic later this month.

—Richard I. Gibson

September 5. Black Hills Racetrack

Devils Tower and red Spearfish Formation. Photo by Doc Searles under Creative Commons license 

The Black Hills of South Dakota is a dome, an elliptical arch with the oldest rocks at the center. If you think of it as a set of inverted nesting bowls, with each bowl representing a rock layer, you’ll be pretty close to a view of the Black Hills – but you need to slice the bowls off, or erode the rocks, so that what you end up with in map view is concentric oval rings representing each rock layer.     

The various layers have variable resistance to erosion. Resistant rocks like sandstones and limestones make ridges, called hogback ridges because they tend to stand with a relatively high, linear, rugged crest above the sloping flanks. Rocks that are easily eroded tend to form valleys.

The Triassic red shales of the Spearfish Formation, which is at least partly equivalent to the Chugwater that we talked about yesterday, are among the least resistant rocks in the Black Hills, so they form a low, flat zone within the overall dome and concentric pattern of geologic contacts. The zone lies between a prominent limestone ridge, made of Mississippian and Pennsylvanian Limestone, partly equivalent to the Mississippian Madison that we talked about a few months ago, and a sandstone hogback made from Cretaceous strata. The intervening valley is called the Red Racetrack, a nearly continuous lowland that surrounds the Black Hills.

Some of the lower part of the Spearfish Formation is probably Permian in age, but much of it is Triassic. Like the Chugwater it contains gypsum, but unlike the Chugwater the gypsum in the Spearfish is probably original. The depositional environment of the Spearfish was probably a sabkha, a muddy salt flat such as those found around parts of the Persian Gulf today. Restricted and ephemeral lakes allowed gypsum to precipitate occasionally, and the alternating incursions of seawater with arid periods allowed for the oxidation of iron in the muds and silts that became the Spearfish Formation. 

Most of the exposures of red rocks in and around the Black Hills, including around Devils Tower in northeastern Wyoming, are the Spearfish Formation. Gypsum appears as white bands within the red rocks. After 225 million years, probably during the recent glacial period when the Black Hills was much rainier than it is today, some of the gypsum dissolved in the subsurface. This left empty spaces – caves – that sometimes collapsed if they were close to the surface, creating sinkholes. Early Native Americans used some of these sinkholes as buffalo jumps, driving bison to them, and some, such as the Vore site adjacent to Interstate 90, are important archaeological sites.  

—Richard I. Gibson

Photo by Doc Searles under Creative Commons license 
Line drawing after USGS

September 4. The Chugwater Formation

Wyoming is a colorful state, and some of the brightest color comes from the Triassic Chugwater Formation. It’s a brick-red sandstone and somewhat finer-grained siltstone, colored by iron oxide. You’ve heard repeatedly about red beds, and how they indicate episodic exposure to air of iron-bearing sediments, and that’s the case for the Chugwater, too. Ripple marks, crossbeds, and mudcracks all suggest the alternation between river settings and more arid times, or at least shifting river channels and variable overbank areas. The package includes lake deposits in places, but they would have been ephemeral lakes, not lasting very long, geologically speaking.   

Steamboat Rock near Laramie, Wyoming (Chugwater sandstone) - by N.H. Darton, 1905.

Parts of the Chugwater contain rocks that indicate deltaic environments and brackish conditions, perhaps along a marine margin, especially in central and western Wyoming. In places, the Chugwater formation is more than 1,000 feet thick. It’s common to find gypsum, calcium sulfate, within the Chugwater, a finding that might indicate really arid, evaporitic conditions, but the gypsum crystal growth usually breaks and deforms the sedimentary layers, indicating that the gypsum grew in the rock after it had lithified or become solid, at least pretty solid.

Partial skeletons, usually skulls, of amphibians, reptiles, and cynodonts have been found in Chugwater rocks, but generally speaking fossils are uncommon there.

Chugwater Formation near Thermopolis, Wyoming

These Triassic redbeds extend from Wyoming into Colorado, South Dakota, and Montana, but they go by different names in places.

The Chugwater was named by USGS geologist N.H. Darton in 1905, for exposures near the town of Chugwater, in southeastern Wyoming. A possible origin for the name of Chugwater Creek is a Mandan Native American reference to the “water at the place where the buffalo chug,” meaning a buffalo jump where the bison made a chug sound on impacting the ground.

—Richard I. Gibson

Reference:
Red beds of the Triassic Chugwater Group, southwestern Powder River basin, Wyoming, 1991, Cavaroc, V. V.; Flores, Romeo M., USGS Bulletin: 1917-E

Photo credits:
Steamboat Rock composed of massive sandstone in the Chugwater formation. Twenty-six miles southwest of Laramie, looking north. Laramie County, Wyoming. 1905. Figure 6 in U.S. Geological Survey Folio 173, Photo by N.H. Darton.

Color photo near Thermopolis, WY, (public domain) by Jstuby