THE ORISKANY — PIC D'AURORE EPISODE 

 OF THE APPALACHIAN DEVONIC 



BY JOHN M. CLARKE 



Theme. In the northeastern Appalachians the Oriskany episode 

 zvas represented by a heavy deposit of shoal-zvater sand with the 

 same peculiar fauna that characterizes the sands of this period in 

 the central Appalachian region. This development appears to he 

 restricted wholly to one of the northern channels which follow close 

 upon the line of the River St Lawrence downthrow. 



The Oriskany sedimentation in New York is now understood as 

 the record of a transgressing shelf sea in which the obvious move- 

 ment of the water was progressively west-northwest. Both in 

 petrology and biology this movement is clear. The shallow waters 

 bounded a continent which lay not far to the north, about the in- 

 terior sea of Appalachia, and were receiving a heavy landwash 

 from subsiding shores; tides and the undertow washed the fine 

 muds far out leaving behind in the shoal waters a clean quartz sand. 

 In New York these sands carry what is historically the typical 

 fauna of the formation, for it was the first to become known and is 

 still to be regarded as the characteristic biota of the '' Oriskany." 

 This assembly was largely made up of heavy sheUs, both of brachio- 

 pods and gastropods, and it has been a frequently expressed con- 

 ception that such ponderous sheUs as these were a direct response 

 to the demand for more secure protection in the play and pounding 

 of the waves along the strand. There is, however, a well-known 

 and so-called " calcareous Oriskany " in southeastern New York, a 

 region where a larger and much more variant fauna existed and 

 where the sediments are indicative of deeper water because of the 

 presence of lime. These deeper sediments carry a very large siHca 

 and clay content, and the condition of the silica is not that of 

 quartz sand nor can it yet be safely regarded as the fine silicious 

 outwash from the shore. The petrology of this silica content has 

 yet to be fully deciphered. Its chert masses and the irregular dis- 

 tribution of the silica matrix indicate secondary change and quite 

 possibly an obscure benthonic fauna not yet known. The very 

 clear distinction between the fauna of the sands and of the hme- 

 stones is recognized, and it is evident that, in these New York oc- 

 currences at least, the heavy species which locally abound in the 



