A search for shocked quartz at the Triassic-Jurassic boundary in the Fundy and Newark basins of the Newark Supergroup

Abstract

Triassic-Jurassic rift basins associated with the breakup of Pangea preserve key geological elements of a defining interval in Earth history. In eastern North America, the Fundy basin is the largest and deepest of nine major early Mesozoic basins of the Newark Supergroup. Overall, the stratigraphic succession is that of continental sediments, primarily fluvial, eolian, and playa lacustrine facies. In the Fundy basin, the Triassic-Jurassic boundary, as palynologically defined, is located within a few metres of the base of the North Mountain Basalt. In the Jacksonwald Syncline of the Newark basin, the boundary is precisely correlated to a "fern spike" coincident with geologically rapid Late Triassic extinctions. If meteorite impact is held to account for extinctions at the Triassic-Jurassic boundary, then shocked quartz, the most important and unequivocal evidence of terrestrial impact, should be preserved in strata of the appropriate time interval. Quartz-bearing rocks below the North Mountain Basalt display in thin section up to five different sets of subparallel, subplanar features, which contain numerous tiny voids causing the optical contrast. Within error limits, universal-stage measurements reveal that the features seem to be oriented parallel to rhombohedral planes, which would favour their shock origin. Transmitted electron microscopic observations of the same grains show, however, that the optically visible features are subgrain boundaries. Many perfect dislocations present in the quartz grains clearly indicate tectonic origin. This microstructure is distinctly different from that of shocked quartz, which is free of dislocations and contains amorphous silica lamellae, the so-called planar deformation features. A catastrophic impact event at the Triassic-Jurassic boundary, however, cannot be excluded, because this boundary has not yet been comprehensively investigated.