Redbeds glow in the evening sun at Alcova Reservoir.
The broad Red Valley surrounds the high Black Hills (distant right skyline).
This Red Butte, one of many, is in the Red Valley south of Sundance, Wyoming. A hard cap of gypsum protects the soft red rocks from erosion.
The Redwall of the southern Bighorn Mountains, the old stomping grounds of legendary bandits like the Sundance Kid. Photo courtesy Casper BLM.
Redbeds in Wyoming date from the late Permian and Triassic periods, roughly 250 to 200 million years ago. The continents had again gathered to form a single supercontinent (Pangaea), as they had done a billion years earlier (Rodinia). [If you’re unfamiliar with how continents shift, grow, join, split and bump up against each other, see this friendly site about plate tectonics from the US Geological Survey.] Today’s Wyoming was on the west coast of Pangaea, not far from the equator. The western part of the state was underwater; the rest was a low muddy coastal plain with extensive tidal flats (Picard 1993).
Paleo-reconstruction of Pangaea during early Triassic time, with modern-day Wyoming added. From the EarthViewer App, free from the Howard Hughes Medical Institute.
[At this point, those of us who are interested shall digress for a brief overview of stratigraphy and nomenclature. Others can skip ahead to the next redbeds photo.]
Several redbed formations are exposed in Wyoming. The oldest is the Permo-Triassic Goose Egg Formation. The next in age and best-known is the Triassic Chugwater Group. The Chugwater Formation was described in 1904 by Nelson Horatio Darton, one of the great field geologists of the day. In 1967, the Chugwater was elevated to group status by High and Picard, who recognized three formations within (Picard 1993). However, following others (e.g. Knittel et al. 2004), I will continue to use “Chugwater Formation” or simply “the Chugwater”. Redbeds also occur in northeast Wyoming, made of the Permo-Triassic Spearfish Formation. The Spearfish is equivalent to both the Goose Egg and the lower Chugwater (no surprise here -- nature is never simple).
Redbeds of the Spearfish Formation, exposed by the Belle Fourche River below Devils Tower.
Chugwater redbeds are frequently capped by a thin resistant layer of Alcova limestone, a shallow marine limestone. For a short time, the seas advanced eastward, submerging the Wyoming coastal plain and depositing limey sediments on it. The Alcova is as extensive as the Chugwater and almost as easy to recognize. It’s “one of the most widespread marker beds in North America, covering at least 50,000 square miles” (Picard 1993).
The distinctive Chugwater capped by the equally-distinctive Alcova limestone, also in foreground.
Fast-forward now to the Jurassic and Cretaceous periods, roughly 200 million to 75 million years ago. During this time, Wyoming was periodically covered by an inland sea and thousands of feet of sediments were deposited. The Goose Egg, Chugwater and Alcova were buried under younger strata. With the overlying pressure, they were lithified -- turned to rock. And there they lay, hidden from view, unappreciated.
A rather dull trip across south central Wyoming 75 million years ago; strata forming modern-day redbeds labeled in red. Cross-sections modified from Knittel et al. 2004.
Then, fortunately, the Laramide Orogeny created the Rocky Mountains. Crust was uplifted, folded and fractured, and erosion uncovered the old rocks. Now the remains of Triassic tidal flats are nicely exposed as picturesque redbeds.
Traveling across south central Wyoming in 2014; locations of redbeds indicated with bold arrows.
On my recent trip through the Heartland of Laramide Tectonics, redbeds dominated the landscape around Alcova Reservoir. The scenery they create is striking -- almost surreal at times. As a bonus for geo-geeks, geologic structures dating to the Laramide Orogeny are visible too.
Alcova Reservoir sits among redbeds tilted during the Laramide Orogeny.
The Alcova anticline (uplift) is the ridge on the nearer skyline. Note redbeds, and gap cut by North Platte River.
The photo above was taken from a dip slope on Alcova limestone, looking northeast across Alcova Reservoir. The Chugwater is not far below my feet, under the limestone, yet it stands noticeably higher at the other end of the lake. That’s because there’s an uplift, a fault and a basin in-between.
Cross-section in area of Alcova Reservoir; dashed line is reservoir surface. Laramide structures are indicated with red arrows. Modified from Knittel et al. 2004.
Below is a view northeast toward Black Beach from Okie Campground. Nearly-vertical sandstone outcrops (right, below redbeds) are on the steep southwest flank of the Alcova anticline. The redbeds and other rocks behind make up the gently-dipping (away from viewer) northeast flank.
Here’s a closer look at the Black Beach area. A classic Laramide high-angle reverse fault runs behind the nearly-vertical sandstone outcrops on the right, bringing Triassic redbeds face-to-face with younger sandstones and conglomerates. This is the Alcova fault shown in the cross-section above.
The next photo was taken in the core of the Alcova anticline, with steeply-tilted limestone-capped redbeds on the left, and gently-dipping strata of the northeast flank on the right. These are the lithified remains of old tidal flats, back when Wyoming was coastal property near the equator. How cool to be able to go back in time 200 million years!
Another view of the gently-dipping (away from viewer) northeast side of the Alcova anticline.
Several redbed formations are exposed in the Alcova area. The photo below shows the older Permo-Triassic Goose Egg Formation on the left. It contains more gypsum than the Chugwater, and tends to be variegated. The younger Chugwater forms the hogback to the right; note the typical thin limestone cap. The dipping Goose Egg and Chugwater are overlain by younger horizontal strata, which must have been deposited after the Laramide Orogeny because they’re not folded. When horizontal rocks lie atop tilted ones, we have an angular unconformity. Field geologists are especially fond of them, maybe because they’re relatively easy to see.
|The variegated Permo-Triassic Goose Egg Formation.|
Why are redbeds red? That’s a really common question. As a seasonal ranger at Devils Tower National Monument, I would answer that the redbeds are made of iron-rich sediments that rusted (oxidized) because they were deposited in shallow water (I don’t remember where I learned this). Picard (1993) concluded that Wyoming redbeds turned red during diagensis -- while they were being converted to rock. The question is not settled, however, and they may well have rusted in place on the old tidal flats, as we used to tell the visitors. For more discussion, see this summary of redbed types and their genesis.
|Redbeds near Devils Tower. Google Earth doesn't do them justice color-wise.|
This is the third in a series of posts about the “Heartland of Laramide Tectonics” in south central Wyoming. Others feature an overview of the Folded Land and a look at the Great Unconformity. More highlights will follow.
Cavaroc, VV and Flores, RM. 1991. Red beds of the Triassic Chugwater Group, southwestern Powder River Basin, Wyoming: US Geological Survey Bulletin 1917:E1-E17.
Knittel, P, Van Burgh, Jr., DP, Logue, TJ, Strube, BE, and Jones, RW. 2004. Field guide for the Alcova area, Natrona County, Wyoming.
Picard, MD. 1993. The early Mesozoic history of Wyomng, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds. Geology of Wyoming. Wyoming State Geological Survey Memoir 5:210-248.
Snoke, AW 1993. Geologic history of Wyoming within the tectonic framework of the North American Cordillera, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds. Geology of Wyoming. Wyoming State Geological Survey Memoir 5:2-56.