Abstract

Devonian monograptids are present in the United States in both the Cordilleran and Appalachian geosynclinal rocks. In the Cordilleran region, monograptids of the M. hercynicus group occur in Nevada in the Windmill Limestone at Coal Canyon, Simpson Park Mountains, the Roberts Mountains Formation in the Roberts Mountains, the Rabbit Hill Limestone in the Monitor Range, and Western Assemblage rocks near Carlin. Monograptus hercynicus nevadensis occurs interbedded with Siegen-age brachiopods at Coal Canyon and with probable Late Siegen-age conodonts at Rabbit Hill in the Monitor Range, and at Carlin. Abiesgraptus and M. cf. M. praehercynicus also occur at Rabbit Hill. Other monograptids of the M. hercynicus group are interbedded with Gedinne-age brachiopods in the Roberts Mountains, and they occur below Siegen-age brachiopods at Coal Canyon.

Monograptus aequabilis occurs stratigraphically above beds bearing an Early Devonian (Gedinne-Siegen age) brachiopod fauna on the Forillon Peninsula, Gaspe. Monograptus microdon and M. microdon cf. var. silesicus occur in northern Maine. Beds with similar stratigraphic position in the succession there have yielded possible Gedinne-age brachiopods and Devonian plants. These graptolites are known from Gedinne-age beds in Europe.

Following a use by Murchison in 1839, the Silurian-Devonian boundary may be considered to be at the top of the Downton Castle Sandstone. That unit must be, at least in part, correlative with the Skala Stage of Boucot and Pankiwskyj (1962) as it stratigraphically overlies beds included in the Ludlow Series. It stratigraphically underlies beds that may be correlated with the Gedinne and Lochkov. Widespread occurrence of a member of the the M. hercynicus group (M. uniformis) and the first (oldest) appearance of M. aequabilis correspond closely with the base of the Gedinne and Lochkov. Inasmuch as the M. uniformis Zone may be correlated with the lower part of the Gedinne, and because the Skala-Gedinne (Pridoli-Lochkov) boundary may be relatively close to the upper limit of Murchison’s (1839) Silurian, the base of the M. uniformis Zone may be an appropriate position for the Silurian-Devonian boundary in graptolitic successions.