EASTERN TRIASSIC RASINS 



1 ,, 



surface is marked by the Chicopee shale; this is bordered on both sides 

 and the north by the Longmeadow sandstone, and this in turn by the 

 Sugarloaf Arkose. Along the east side is a coarse border aggregate called 

 the Mount Toby conglomerate. These formations are clearly facies and 

 grade into each other or are interdigitated. The Mount Toby conglomer- 

 ate is a fanglomerate in large part and an actual talus in others. There 

 can be little doubt about its relation to a great border fault; but in places 

 bedrock crops out surounded by conglomerate, and the position of the 

 fault is obscure. 



Intercalated in the elastics and grouped close together in their cen- 

 tral part are three lava sheets of diabase. The middle one, the Holyoke 

 diabase, is the thickest and in places reaches 400 feet. Between it and the 

 upper are sandstones that contain large and small reptile tracks which are 

 very well known. Shortly after the third lava outpouring, an explosive 

 eruption took place; and fragments and dust of diabase were spread over 

 a large area to form the Granby tuff. Over the tuff was spread rusty sand 

 in which most of the tracks have been preserved. In the southern part of 

 the basin "dolerite" sheets have been intruded. Dikes are few. 



Here as in the other Triassic basins, normal faults cut and displace the 

 beds and volcanic sheets. See sections, Fig. 9.4. 



The red color and salt crystal impressions have led a number of writers 

 to envision a semiarid climate; but Krynine, on the other hand, contends 

 that the flora and swamps suggests a precipitation of about 50 inches a 

 year and a temperature of 70° to 80° F. Fresh arkoses and fanglomerates 

 can easily form under tropical humid climate in regions of steep topogra- 

 phy. Desiccation marks indicate alternating dry and wet seasons. 



STRUCTURE OF BASINS 



All the Triassic basins in the eastern United States are bordered on one 

 side or the other by major normal faults. A great fault, although irregular 

 and with branches and perhaps steps borders the Newark basin on the 

 west. The Deep River basin has a major fault on each side. The Con- 

 necticut Valley Triassic is bordered on the east by a major fault, also of 

 a complex nature. The long and very narrow basin that stretches from 



North Carolina into Virginia is bordered on the west by a fault. The sev- 

 eral other small and detached basins are shown with faults on either the 

 east or west sides on the Geologic Map of the United States. 



Associated with all the great border faults and perhaps due to them is 

 a general dip of the beds and sills toward them. See cross sections of Fi'_is. 

 9.2 and 9.3. The dips range from 5 to 50 degrees and are more generally 

 10 to 20 degrees. The Triassic beds are not folded as the underlying 

 Paleozoics and metamorphics, upon whose beveled edges they rest un- 

 conformably. 



Strike faults within the sediments are known, somewhat parallel with 

 the border faults, and many transverse faults cut and offset the beds and 

 sills. In places the transverse faults terminate against normal strike faults 

 and produce a rhombic pattern. Some of the transverse normal faults have 

 been traced out into the folded and thrust-faulted Paleozoic rocks which 

 they also offset. 



The normal faults within the basin cut the Triassic sediments and sills, 

 yet some of the dikes associated with the sills follow cross faults. It is gen- 

 erally concluded that the faulting is later than most of the beds, but before 

 the end of the period of volcanic activity, so that most of the sills are cut 

 by the faults, yet some dikes were injected immediately into the fractures 

 when they formed. 



ORIGIN OF BASINS 



The Triassic basins of the Piedmont province and of the Connecticut 

 Valley have a similar history. The troughs in which the sediments were 

 deposited are due mainly to downfaulting with a major fault or chain of 

 faults on either the outer or inner side. The trough block rotated by set- 

 tling most adjacent to the border fault. The border faulting is conceived 

 as a fairly continuous process during which the sediments accumulated in 

 the basins as they were progressively deepened. Stose and Bascom ( 1929) 

 represent sedimentation in the Newark basin to have started considerably 

 before the border faulting began (see Fig. 9.2); then with the onset of 

 faulting the previously deposited beds which came from the southeast 

 were tilted, and the site of later sedimentation, with continued faulting. 



