DEVONIAN VERTEBRATE CORRELATION IN THE WESTERN UNITED STATES

The Early and Middle Devonian sequence of the west-central United States represents transgressive and regressive events that took place across a broad carbonate platform. The sequence consists of plant- and fish-bearing sedimentary rocks, often representing channel-fills, and with scattered distribution. As these formations lack age-diagnostic marine invertebrates and are frequently separated from the under- and overlying sediments by unconformities they have posed difficult problems in their dating and correlation despite their often rich vertebrate assemblages. However, recent work has started to show that it is possible to relate the vertebrates, particularly the heterostracans, to the standard conodont scheme and to use them in local correlation.

Sediments of this type have previously been referred to as Beartooth Butte Formation and an equivalency of age has been assumed. However, it is now clear that such channel-fill deposits represent a range of ages even in the classic localities. The type locality at Beartooth Butte, Wyoming, appears to be late Emsian and a correlative of the lower part of the Grassy Flat member of the Water Canyon Formation in northern Utah, the Sandy member of the Sevy Dolomite in eastern Nevada, and the lowermost part of the Lost Burro Formation in Death Valley, California. The other classic locality at Cottonwood Canyon, Wyoming, is late Lochkovian/early Pragian and may be correlative with the vertebrate-bearing Holland Quarry Shale in north-western Ohio and localities in the Lost River Range in Idaho. Although vertebrate-bearing Beartooth Butte-type channel-fills are also present in the Middle Devonian a combination of lack of precision in identification and apparent long temporal ranges currently restricts their value in dating and correlation.

Macrovertebrate correlation cannot currently be extended beyond the immediate area of the western United States due to the endemic nature of the vertebrates, however, within that area the heterostracans, and particularly the cyathaspidids, are proving to be most useful. It is expected that work on microvertebrates in the future will allow correlation with the developing international microvertebrate zonal scheme.