EASTERN TRIASSIC RASINS 



129 



during deposition. The arkose, a sandstone containing more or less feld- 

 spar or kaolin derived from granite or gneiss, indicates proximity at the 

 time of deposition to a shore of Precambrian crystalline rocks. 



The Lockatong formation consists chiefly of dark-colored fine-grained 

 hard and compact argillaceous rocks. Some beds are massive, and others 

 are flaggy. They show mud cracks and other evidences of shallow-water 

 deposition, but their materials are clay and very fine sand, some of the 

 beds also contain carbonaceous material. 



The Rrunswick formation, in its typical development, consists mainly of 

 a great thickness of soft red shale with local and thin layers of sandstone. 

 Northward and westward the sandstone increases in amount and coarse- 

 ness. It overlaps irregularly older Traissic formations and Paleozoic and 

 pre-Paleozoic formations. 



The three formations are not sharply separated by abrupt changes of 

 material, but usually merge into one another through beds of passage 

 which appear to vary somewhat in thickness and possibly also in strati- 

 graphic position in different areas. 



The thickness of the Stockton is estimated to range from 1000 to 3000 

 feet, the Lockatong from 1500 to 3000 feet, and the Brunswick from 12,000 

 to 16,000 feet. The total thickness of the Newark group as generally men- 

 tioned is about 20,000 feet, but figures up to 35,000 feet have been pro- 

 posed. This great amount is computed by the dip angle and the distance 

 across dip of the homoclinal beds, but several writers have suggested the 

 possibility of duplication of certain beds by faulting, and hence that the 

 figure may be excessive. Stose and Stose ( 1944) suggest that the beds over- 

 lapped from east to west in somewhat the manner shown in Fig. 9.2 and 

 that therefore the combined thickness of all the beds will not be found in 

 any one place. It cannot be doubted, however, that the long, narrow 

 troughs containing the Triassic sediments are very deep, undoubtedly 

 over 10,000 feet, and probably 20,000 in places. 



The age of the Newark group is probably Upper Triassic, but the high- 

 est beds may be lowermost Jurassic. According to Bascom and Stose 

 (1938), 



A comparison of fossil plants, crustaceans, and vertebrates of the Newark 

 with simliar forms of the Jura and Trias of Europe establishes a correspondence 



» 2* milCS 



Fig. 9.1. Triassic basin in western New Jersey, Pennsylvania, and Maryland. Stippled area, 

 Triassic sedimentary rocks; solid black areas and heavy black lines, Triassic diabase sills and 

 dikes; light black lines, faults. Reproduced from Stose and Stose, 1944. 



within general limits, but a correlation of exact horizons is not practicable. 



The Newark strata did not share in the folding that occurred at the end of 

 Carboniferous time and therefore must be of later date; they are, however, 

 clearly older than the lowest Cretaceous formations, which overlap them un- 

 conformably. They are thus separated from earlier and later deposits by inter- 

 vals of upheaval and erosion of unknown duration, but their position in geo- 

 logic history cannot be determined more closely than by the general correlation 

 of fossils above indicated. 



Igneous Rocks. The map of Fig. 9.1 shows the distribution of outcrop- 

 ping sills, lava flows, and dikes in the Newark group and in adjacent 

 rocks of the Piedmont. The sills and flows are confined to the Triassic 

 basin, but some of the dikes cross out into the older rocks of the Piedmont 

 and persist for many miles. The Conshohocken and Downington dikes 



