|Sedimentary rocks of the Virgin River Gorge|
One recent afternoon I was moseying about, trying to eyeball some extensive gypsum layers in the distant cliffs. This gypsum is part of the Woods Ranch Member of the Toroweap Formation and would be located beneath some younger Kaibab Formation sediments at the top of the cliffs. These rocks are from the middle Permian, somewhere around 280–270 million years old.
I was attempting this eyeball exercise from the parking lot of the picnic area.
Did I mention the veneer of snow that covered the scene?
A recent series of arctic storms had blasted their way down from Canada and across the western states and southern Utah. For at least a week temperatures hovered in the teens and single digits (rare occurrences in these parts). These temperatures pretty much bled away any enthusiasm I might have had for going outside for any length of time longer than ten minutes.
But I digress.
There is a large gypsum mine just south of St George, Utah. I have always thought the mine was in the younger Triassic Moenkopi Formation since there is a load of gypsum in those particular sediments.
Upon further consideration I thought it might be part of the older Toroweap. But honestly, I had barely a clue what was what.
Astute WatchingForRocks follower, friend, and geo–prof Jerry Harris (who can also be found here visiting me in Yellowstone) emailed me the other day. He had been doing some pondering of his own:
I always thought that the gypsum mines were in the gypsum-rich Harrisburg Member of the Kaibab Limestone--the unit on top of the Fossil Mountain Member. But I don't know for sure!
Here is an always handy stratigraphic column of the Beaver Dam Mountains and the Virgin River Gorge with our Permian rocks of interest highlighted in red:
Let’s zoom in and take a closer look:
There is thick bedded gypsum at the bottom of the Harrisburg and also in the top and bottom of the Toroweap! It’s no wonder I get so confused. There is a lot of gypsum around here.
Jerry had an excellent idea:
If you like, I can take you to some neat outcrops of the Harrisburg member littered with gypsum boulders!
Oh, yeah. Field trip! We’ll go out as soon as the snow melts.
Stratigraphic columns courtesy of:
Thrusting and extensional structures and mineralization in the Beaver Dam Mountains, Southwestern Utah, Utah Geological Association, Publication 15.