The degree of crystallinity of authigenic illite was determined, by x-ray diffraction, for samples from the Lockatong Formation (Triassic) of the Newark basin of southeastern Pennsylvania and adjacent New Jersey. Sixteen samples from approximately 30 m of section in the lowermost Lockatong near Trooper, Pennsylvania, were analyzed.

Preliminary results ranging from 22° to 41° ^Dgr2^Thgr, indicate a range of metamorphism extending through the anchizone to the boundary with the greenschist facies, and indicate that the rocks were subjected to temperatures of ~ 300°C for a period of time.

An episode of elevated temperature might have resulted from proximity to an intrusive rock body, from burial a short distance beneath a lava flow, or from burial beneath ~ 8-10 km of overlying rock. None of these proposed mechanisms is consistent with the geologic evidence; another explanation for such a high temperature must be sought.

Four other samples from widely distributed localities within the Newark basin also fall within this range of illite crystallinity. Samples from the Gettysburg basin and the Connecticut basin are less well crystallized, suggesting that the thermal event hypothesized here was restricted to the Newark basin.

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