From the Hall of Disjointed Memories, Part 9: Some Lovely Basin-Fringe Stratigraphy | North Park, Colorado, USA

Taken somewhere in North Park, on either the western or eastern fringe of this intermontane basin, in one of my field-geology class mapping-exercise areas. More than that I cannot say.

Incidentally, in common with all other images posted in this series, this is a square-format Instamatic photo. I've had to crop off the left side of the frame because it had a swath of darkness across it. I suspect I let the camera's lanyard swing in front of the lens at the wrong instant. Fortunately, what remains is, geologically speaking, more than sufficient.

How good to be out of that consarned campground, and out in the field!

Despite the lack of precise geolocation, I'm pretty sure that this shot of a hillside offers us a succession of three formations I and my fellow students encountered again and again that summer. But if you're familiar with the stratigraphy in these parts and catch me making a mistake, please let me know.

My fifty-years-later interpretation, from bottom to top:

- Sporting that gorgeous Colorado-red color, the Chugwater Formation. Now, apparently, it's known to be just Triassic in age, but back in the 1970s I was told it was Upper Permian to Triassic. That sounded especially nifty because it meant that it straddled the Paleozoic-Mesozoic-era divide, and our world's worst mass extinction. In any event, it ranges in composition from shale and siltstone to sandstone and was deposited in shallow marine conditions.

- Lying unconformably (with an erosional break in the depositional record) above the Chugwater, the Sundance Formation. Its lower member is pale-gray sandstone, and its upper is mostly glauconitic (hence greenish) shale. Also marine in origin, it dates to the next period, the Jurassic. From this vantagepoint I'm not exactly sure how far upslope it extends, though William J. Hail Jr., in his 1979 USGS report on the geology of the northwestern part of North Park (Bulletin 1188) says that it has a maximum thickness of 125 ft / 38 m.

- And still farther upslope, the Jurassic Morrison Formation, famous for its dinosaur remains. To quote Hail directly, "the Morrison has a maximum thickness of 430 feet [131 m]; it consists of nonmarine varicolored claystone and siltstone and brown calcareous sandstone in the lower part and predominantly of greenish-gray claystone and siltstone in the upper part."

So I'll let you judge whether the greenish-gray bed visible at upper left is upper Sundance or upper Morrison. But keep in mind that maximum cited thicknesses for formations are not necessarily their true size here.

There may also be some Lower Cretaceous Dakota Sandstone up on top. It makes a good, resistant capstone and hogback-former. But, as usual in this series, I really can't remember if I saw it there.

You'll find the other photos and descriptions of this series in my Natural History: Colorado album.

Taken in: United States / Colorado / Cowdrey (show map)

Taken on: July 1, 1974

Tags:

geology

landscape

rock

rocks

stone

Colorado

Chugwater Formation

Sundance Formation

Morrison Formation

stratigraphy

sandstone

siltstone

shale

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Camera: MAKER NAME 96660, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
ISO:	50
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
Software:	Inpaint
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