This small quartzite boulder, found in morainal deposits between Stella Lake and Teresa Lake below Wheeler Peak in Great Basin National Park, shows cross-bedding defined by dark purplish to reddish laminations. Up appears to be to the left. Without a stratigraphic context it's hard to say whether this quartzite is part of the late Precambrian McCoy Creek Group (pCMc) or the possibly late Precambrian to mostly early Cambrian Prospect Mountain Quartzite (Cpm), but most slopes uphill from this spot are in the Prospect Mountain Quartzite. Depositional environment was likely nearshore to offshore marine.
Friday, September 2, 2011
Friday Field Photo: Cross Bedding in Quartzite
This small quartzite boulder, found in morainal deposits between Stella Lake and Teresa Lake below Wheeler Peak in Great Basin National Park, shows cross-bedding defined by dark purplish to reddish laminations. Up appears to be to the left. Without a stratigraphic context it's hard to say whether this quartzite is part of the late Precambrian McCoy Creek Group (pCMc) or the possibly late Precambrian to mostly early Cambrian Prospect Mountain Quartzite (Cpm), but most slopes uphill from this spot are in the Prospect Mountain Quartzite. Depositional environment was likely nearshore to offshore marine.
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6 comments:
*sniff* those poor sediments, all beat-up...breaks my heart
What about opposing facing directions - the dark layer being folded, then to the right the truncations look like tops to the right and to the left of the synform tops to the left?
Rockswhisperer, you may have something there. I thought the truncations to the right were harder to see, so just went with the clearer (to me) ones on the left. Next time I'm up there, I'll probably find an entirely different rock!
Eric, haha! I think they'll be okay, slowly making their way down to the valley floor where they can have a nice long rest...
That quartzite bears an uncanny resemblance to the Gog Group quartzites of the Canadian Rockies (e.g. around Lake Louise), which are coincidentally (or not?) also lowermost Cambrian. Ours has the same thin purple stripes, but the stripes include both real crossbed laminations and liesegang banding. The two sets of stripes are sometimes at an angle to each other, so you can occasionally see what appears to be intersecting crossbeds--that sure makes you do a double-take.
--Howard
I'll have to look at the laminations more carefully next time!
I know that there are Precambrian quartzites that are widespread across the west, including a middle Precambrian unit; I'm not so sure about the upper Precambrian quartzite-argillite formation or the early Cambrian quartzite.
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