494 H. W. SHIMER PEEMO-TPvIASSlC OF NORTHWESTERN ARIZONA 



ness in terrestrial deposits naturally make such work less certain than it 

 would be in the case of a marine limestone. The Moenkopi throughout 

 the area as thus extended, consists of thin-bedded red shales and sand- 

 stoneSj separated at usually rare intervals by limestone lenses. These 

 limestone beds become more frequent from the type locality westward. 

 These limestone beds are usually thin, 5 to 10 feet in thickness; some of 

 them are fossiliferous, containing a marine fauna. 



After the deposition of the great thickness of Permian strata — the 

 upper Supai, Coconino, and Kaibab — came a period of erosion. The 

 Moenkopi seems everywhere throughout this portion of Arizona to be 

 separated from the imderlying Kaibab by a disconformity.-^ Eesting 

 upon the Moenkopi are the Shinarump, and upon this the Chinle, both 

 of Upper Triassic age. Upon purely stratigraphic grounds, therefore, 

 this would tend to place the Moenkopi within the Lower Triassic. The 

 marine invertebrate fossils, however, which at restricted horizons are 

 abundant in the Moenkopi red beds, considerably complicate this strati- 

 graphic simplicity. The majority of the fossils have Permian affinities. 

 With these are associated, however, a number of Triassic forms and* 

 others whose nearest relatives have always been considered as belonging 

 typically to the Mesozoic. Abetting this last consideration is the absence 

 from these strata of some of the most typical Permian groups.' To take 

 these differences up briefly : 



The Moenkopi is characterized by a conspicuous reduction in the 

 brachiopod fauna (see tabular list of species with distribution). Instead 

 of the usually strongly dominant position held by this class in the upper 

 Paleozoic, it here occupies a very insignificant position indeed. The 

 pelecypods are now the most important faunal element, both in number 

 of species and of individuals, with gastropods a close second. In this 

 respect the Moenkopi resembles the English-German lagoon type of 

 Triassic, with brachiopods rare and pelecypods exceedingly abundant. 



The Moenkopi fauna is further characterized by an absence of some of 

 the typical Permian genera. There are here, for example, no representa- 

 tives of Productus, Spirifer, or Chonetes, which, through the great pro- 

 lificness of their many species, everywhere assert themselves as the most 

 important elements in the seas during the closing years of the Paleozoic. 

 There are likewise absent from the Moenkopi such other characteristic 

 uppermost Paleozoic genera as Fusulina, Lopliopliyllum, Axopliyllum, the 

 Ortlddce, Stropliolosia, and Phillipsia. In the absence of these genera, 

 which, through their many species and innumerable individuals, give to 



23 See also Gregory : U. S. Geol. Survey, Prof. Paper 93, 1917, p. 30. 



