Introduction

In 1979 I made a preliminary attempt to correlate three important sections of Permian and Triassic rock graphically (Sweet, 1979). In the sections considered in that study, data on the ranges of conodonts in strata thought to be Lower Triassic were adequate to suggest that further work might result in a high-resolution biostratigraphic scheme of worldwide significance. Consequently, considerable time was invested in assembling data on the ranges of conodonts in widely distributed sections of Lower Triassic strata and, in 1988 (Sweet, 1988b) a high-resolution biostratigraphic framework was established for the Lower Triassic. That scheme resulted from graphic correlation of sections in Kashmir, Pakistan, Japan, western United States, northern Italy, and far-eastern USSR. Correlation based on the measured ranges in those sections of 29 conodont species resulted in a framework that is divisible at the 95% confidence level into 21 chronozones (or Standard Time Units), each equivalent to a unit 5 m thick in the standard reference section at Guryul Ravine, Kashmir, and each representing approximately the same interval of time.

In 1979 data on the scaled ranges of stratigraphically significant invertebrate fossils were sparse for Permian rocks in the three sections I considered. Consequently, I used crude information on the ranges of scaphopods, bellerophontids, productacean brachiopods, and a few species of cephalopods and conodonts. Since 1979, however, there has been a flood of information on the measured ranges of conodonts and other invertebrates in rocks of the Permo-Triassic boundary interval, which it is now possible to use to expand the high-resolution framework begun in my 1988 report.