KEWEENAW FAULT 97 



Relation of Keweenawan Formation to Cambrian and Precambrian 



Before the deposition of the Keweenawan the Huronian was uplifted and 

 suffered deep erosion, so deep that, according to Allen, 6 over a large part of 

 the Gogebic Range the Neo-Huronian, which he calls the Copps formation, is 

 removed. Consequently apparently Van Hise mistook the Animikie, or Middle 

 Huronian, for the Upper Huronian. However great the pre-Keweenawan ero- 

 sion may be, however, all agree that it existed. It seems to have gone so far 

 and so nearly was a peneplain produced, in the later stages of which conglom- 

 erates must be finer and often absent, that coarse basement conglomerates of 

 the Keweenawan are not often found. This is important, as indicating the 

 importance of the gap separating the Keweenawan from the Huronian. The 

 black shales of the Neo-Huronian also might indicate the fine-grained deposits 

 of advanced baseleveling. The Keweenawan formation seems, however, to 

 have been formed when the Huronian was raised above water in a desert 

 something like our Great Basin. The dolomitic marls, the micaceous shales, 

 tbe arkose sandstones, the slight chemical assorting or leaching of the sedi- 

 ments, as well as the abundance of salt, as I believe, largely connate waters, 

 all favor this conclusion to which, I think, Leith came before myself. I have 

 been especially interested lately in studying angular clay pebbles that I find 

 in the Keweenawan drill cores, and take to be broken up, dried crusts on the 

 top of desert clays. This desert basin formation, with accompanying lava 

 flows, like those of the Snake River Valley of Oregon, lasted until late into 

 the Cambrian ; for the Upper Cambrian, Jacobsville, or Eastern sandstone, 

 still is arkose, with as much as three-eighths of the sand grains feldspar, still 

 micaceous, and still has saline, though not so saline waters. They become dis- 

 tinctly fresher in the upper part. These Upper Cambrian sandstones also 

 contain pebbles of the Keweenawan, showing that the escarpment of the. 

 Keweenawan fault was neither peneplained nor covered at the time of the 

 Upper Cambrian. So I think it is pardonable if I am more confirmed in the 

 belief that the Keweenawan is more largely Cambrian than Precambrian — 

 more parallel to the Triassic, with which it was once confused — than to the 

 Permian. I emphasize the fact that the Keweenawan occurred after rather 

 than before the great Precambrian peneplanation, because Lawrence Martin 

 has acutely suggested this as a test whether the Keweenawan is really Cam- 

 brian. The line of contact of the base of the Keweenawan on the various 

 Huronian formations is, as Allen's map shows, fairly smooth, except where 

 disturbed by faulting that also affects the Keweenawan. 



The Paleozoic Invasions 



By the time the sea invaded the valley d, however, the fault-scarp was 

 pretty well worn down, as figure 3 indicates, and though I have indicated the 

 Eastern sandstone in figure 4 as not entirely covering the range, it may finally 

 have clone so. though even near Limestone Mountain (,/) there are some small 

 pebbles which may be Keweenawan. Such pebbles certainly come near the 

 Keweenawan fault in strata which one would think are not much below. I 



Loc. cit., chapter Hi, pp. 58-59. 



