

528 E. BLACKWELDER GEOLOGY OF WASATCH MOUNTAINS, UTAH 



of 1,000 to 1,500 feet. In the lowest beds there are corals such as 

 Halysites and Favosites. At a slightly higher horizon there are abundant 

 shells, which Kindle thinks are the same as his Pentamerns fauna of the 

 Bear Eiver range. All the specimens, however, are poorly preserved, and 

 show best upon the weathered surfaces. Between the Silurian horizon 

 and the fossiliferous Mississippian there is a thick, dark limestone which 

 ^ corresponds satisfactorily with Kindle's Devonian Jefferson formation 

 east of Cache Valley. Further search may be expected to reveal the 

 Jefferson fauna in these beds. 



Farther south the formations have not been identified by fossils, but 

 beds lithologically similar, and lying conformably between the Ordovician 

 and the Mississippian limestones, have been found near the summit of 

 Mount Morgan. These are probably of Silurian and Devonian age. In 

 the main range, near Ogden, the absence of this portion of the Paleozoic 

 group is apparently due to the overthrusts which cut out the Middle 

 Paleozoic portion of the succession. 



A NON-MARINE MEMBER IN THE MISSISSIPPIAN LIMESTONE 



One of the best sections of the limestones which represent the Missis- 

 sippian (Lower Carboniferous) period in Utah has been overlooked until 

 now. It is exposed around the sources of the south fork of Ogden Eiver. 

 The system can there be divided into three formations : An upper and 

 lower limestone, with a middle shale. The upper formation is a succes- 

 sion of dark limestones rich in fossils, which have been determined by 

 Dr. G. H. Girty as middle to late Mississippian in age. The lower lime- 

 stones are chiefly dolomitic and devoid of fossils. The formation between 

 these comprises a series of reddish or pinkish beds in which shales and 

 thin-bedded limestones predominate. This middle formation has special 

 characteristics which may be noted here. 



The shaly formation is separated from the limestone beneath by an 

 obscure unconformity, marked by sandy breccia containing pieces of the 

 subjacent limestone. Above, however, it intergrades with the purely 

 marine limestone of the Upper Mississippian formation. The total thick- 

 ness is about 250 feet. 



The origin of these strata appears to have been continental rather than 

 marine. This conclusion is supported by a variety of evidence. A sug- 

 gestive fact is that no fossils were found even in the calcareous layers 

 and in sections which are well exposed. N'ear the base of the formation 

 there are many layers of lavender and maroon shale, with abundant sun- 

 cracks. The individual cracks are filled with a mixture of mud and sand. 

 Sparsely dotted over the surfaces of the shales and also upon thin layers 



