130 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



NW. 



SE. 



A 



Fig. 9.2. Origin of the Newark Triassic basin. Reproduced from Stose and Stose, 1944. The 

 sediments are postulated to have first been derived almost entirely from the east. After intrusion 

 of the diabase dikes and sills and renewed faulting, much fanglomerate was washed in from 

 the west. 



are 60 to 70 miles long, and the Safe Harbor dike extends an equal length 

 before it is covered by the Cretaceous of the Coastal Plain. 



The largest sill in the southern part of the Newark basin is the Gettys- 

 burg, which is 1800 feet thick. Farther northeast in New Jersey four 

 great sheets of trap rock occur and form the Watchung Mountains which 

 are more prominent that the ridges of the great Gettysburg sill. The low- 

 est of the four sheets is intrusive and in places reaches a thickness of 1000 

 feet. It forms the Palisades of the Hudson. See section 31 of Fig. 8.21. 

 Above the Palisades sill and separated from it and each other by several 

 hundred feet of intervening Triassic shales are three extensive (buried) 

 basalt flows which, from bottom to top, are 650, 850, and 350 feet thick. 



The dikes are believed to follow tension cracks which in places become 

 faults and offset the Triassic beds. According to Stose and Stose ( 1944) 

 the dikes and the normal faults that the dikes follow represent major lines 

 of Triassic fractures. They cut across older structural lines, which are 

 nearly at right angles to them. Many of the diabase dikes originate in or 

 join the diabase sills which are most abundant along the northwestern 

 part of the basin. See map, Fig. 9.1. 



The diabase sills, with which many of the dikes connect at their northwestern 

 ends, coalesce to form extensive intrusive bodies in the northwestern part of 

 the Triassic area of Pennsylvania. The larger sills are the Haycock, Ziegler, Saint 

 Peters, Yorkhaven, and Gettysburg. They parallel the strike of the sedimentary 

 rocks for long distances, and then the intrusive body cuts across the strike at 

 right angles. Most of these crosscutting bodies extend to the northwestern edge 

 of the Triassic basin where they terminate against the faults that form the 

 boundary of the basin. Each of these intrusive bodies, therefore, has the form 

 of a great tilted trough bounded on the southeast side by the west-dipping sills 

 and at the ends by the crosscutting bodies and open at the west. 



The fissures through which the diabase entered the Triassic rocks are be- 

 lieved to lie near the northwest edge of the basin where the greatest amount of 

 progressive sinking and faulting occurred during Triassic deposition. The 

 rising magma broke through the Triassic beds near the vents in the form of 

 crosscutting bodies, and injected the beds to the southeast in the form of sills. 

 The magma extended still farther southeastward as dikes that followed vertical 

 fractures in the Triassic sedimentary rocks and continued into the older under- 

 lying rocks southeast of the limits of the basin of Triassic sedimentation. Some 

 of these dikes in the area southeast of the Triassic outcrops may have been 

 feeders of large diabase bodies in Triassic sedimentary rocks that are now re- 



