CHERT FORMATIONS OF NOTRE DAME BAY 587 



In the southwestern part of the Kenai Peninsula, Alaska/ 

 chert is found in three formations of Triassic age. The rocks are 

 exposed on the southeast shore of Kachemak Bay, and on portions 

 of the shore to the south. 



The oldest series is composed largely of pillow lavas. It 

 probably has a thickness of about 3,000 feet, and throughout this 

 thickness pillow structure is well developed. In nearly every 

 section of these rocks chert is found forming beds as much as 40 

 to 50 feet thick. There are in places a few thin beds of tuffs. 



Younger than these pillow lavas, which cannot be referred to the 

 Triassic with absolute certainty, is a series of rocks composed 

 largely of thin-bedded chert. The areas of this formation as mapped 

 "include numerous small undifferentiated masses of ellipsoidal 

 lavas, the two kinds of rock being so intimately and complexly 

 associated as not in all places to permit an accurate cartographic 

 separation of the smaller areas of lava." The relations of the lava 

 to the cherts are believed to be due in part to consecutive deposition, 

 and in part to contiguity due to faulting. 



The cherts are everywhere intensely deformed, minute crumphng being 

 universal and faulting being common, so that it is impossible to make even the 

 roughest estimate of the thickness of the formation, nor can any upper and 

 lower part be determined in any local section. .... 



The cherts are thin-bedded and even bedded rocks consisting of hard 

 siliceous layers from half an inch to 2 inches thick, separated by thin films of 

 softer material. The hard siliceous layers are of fairly uniform appearance 

 except as they vary in color. They are of very fine grain, being uniformly of 

 almost glassy texture and not containing any recognizable detrital fragments. 

 The character of the material between the hard siliceous beds was not definitely 

 determined, but is probably shaly. The color of the chert at most places is 

 greeri, grey, or black. Brown or red cherts were noted at a few places, and 

 are beheved to owe their color to the infiltration of iron minerals 



The cherts show in thin section a fine-grained siUceous groundmass contain- 

 ing no indication of detrital grain, but contain rounded forms which suggest 

 the -simpler Radiolaria.^ 



There is also a series of Limestones whose exact relation to these 

 other rocks is not known. They are found in the vicinity of Port 



^ G. C. Martin, B. L. Johnson, and U. S. Grant, "Geology and Mineral Resources 

 of the Kenai Peninsula, Alaska," Bull. U.S. Geol. Survey, No. 587, 1915. 

 ^ Ibid., pp. 60-61. 



