Tuesday, January 27, 2009
Groups, Formations, Beds, Horizons and Zones
A little snippet (edited) from an email to Dan:
Lucas et al 2007 (on the Timpoweap) raised the Thaynes and Moenkopi Formations to Group status. If we dont accept Lucas et al, do we extend the Sinbad Member into the Confusion Range or extend the Thaynes Formation into the Pahvant Range. To me the rocks in the Pahvant Range, a few thin limestone beds in a fairly thick shale sequence, are more like the rocks in the Confusion Range than they are to the rocks in the San Rafael Swell (type Sinbad) where you get a fairly thin set of limestone beds with very little shale. Having hiked up to the Virgin in Dog Valley with Arnaud and Dawn, we noticed alot more thin limestone beds above the anasibirites beds, I didnt notice a clear contact to separate the Sinbad from the "Middle Red Formation" unless you just use the first set of red beds, the Virgin could probably be mapped. Ideally a new name for the rocks in Dog Valley and the Confusion range would make me happy, but that is beyond the scope of our abstract. So I was thinking of just calling them both the lower part of the Thaynes Group. Arnaud put Sinbad Formation on his strat sections for Dog Valley, and Thaynes Group for the DH sections. I think he used Sinbad Formation because the geologic map of the Richfield Quad and The Geology of Millard County both call it Sinbad* (or we discussed it in the field?). I really dont think it should be called Sinbad.
A formation is a mappable unit of rocks, a group is a set of similar formations, what separates one formation from another?
Jim asked me yesterday if I had heard of the Claraia Zone. I remembered seeing it somewhere like in Silberling & Tozer 1968, but Jim corrected me and read it as the Claraia Beds. It seems that Hintze used it in the Geologic History of Utah and/or The Geology of Millard County* (I will have to check on these). Is there a Claraia Zone? I also remember seeing Xxxxxx Horizon on a few papers. I think "Zone" is a formally defined unit, whereas Beds and Horizon are informal, can they be used interchangeably? Is an Horizon just a little thicker than a set of Beds?
*after reading the Geology of Millard County and the Richfield Map, It seems Hintze didn't refer to the Sinbad, he only calls it Moenkopi.