Abstract

The Newark-Gettysburg basin, one of several Triassic basins in eastern North America which have long been described as fault-troughs or grabens, is herein interpreted to have developed by simple downwarp, filled with sediment from all sides. Igneous intrusion occurred late in basin filling. Following deposition, the basin was tilted north and northwestward with simultaneous faulting and folding.

The strongest evidence against a graben origin is the absence of a continuous system of faults along the north and northwest margin, and the lack of syndepositional faulting either within the basin or at the basin margins. Aeromagnetic and gravity data do not support the presence of buried faults under overlapping sediment. The coarse, poorly sorted fanglomerates at the basin margins are not sufficient evidence of syndepositional faulting. Erosional escarpments at the basin margins were of sufficient relief to produce these fanglomerates during the arid climate extant during deposition.

The absence of large syndepositional faults along the basin margins, the limited range of igneous compositions (tholeiitic basalt), and the mimetic arrangement of the Triassic basins along the Appalachian Mountain system contrasts markedly with the well-substantiated margin faults, the wide range of igneous compositions (generally alkaline), and the transection of pre-existing regional structures in other continental graben systems, such as the East African rift system and the Rhine graben. The Newark-Gettysburg basin is most similar to the early stage of the Baikal rift, which initiated as a downwarp with crustal thinning and basaltic effusions.

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