Abstract

From eastern Gaspe southwest to central Chihuahua, the Lower Devonian marine sedimentary sequence carries brachiopod faunas of singular provincial affinity. Four zones, based on rensselaerid evolution, are recognized: 1) Nanothyris Zone, comprising the Manlius-Coeymans and Kalkberg-New Scotland intervals (Gedinnian); 2) Rensselaeria Zone, comprising the Becraft-Port Ewen of the upper Helderbergian plus the Oriskany (Siegenian); 3) Etymothyris Zone; and 4) Amphigenia Zone. The two latter zones comprise the Esopus and Schoharie-Bois Blanc (Emsian).

Manlius-Coeymans equivalents are not widespread and are surely recognized, beyond the New York-Pennsylvania-Maryland region, only in Gaspe (St. Albans Formation) and as far southwest as western Tennessee (Rockhouse Formation). The belt of marine rocks appears to be narrow and represented predominantly by limestone and calcareous shale.

In the upper part of the Helderbergian, including Kalkberg-New Scotland as well as Becraft-Port Ewen equivalents, brachiopod-rich limestones, terrigenous rocks, and volcanics are far more widespread. Limestone is the principal rock type from New York southwest through Pennsylvania, Maryland, Virginia, and West Virginia and in a broad area embracing southeastern Missouri, Tennessee, Oklahoma, Texas, and Chihuahua. Northeast of New York the limestone belt gave way to one rich in terrigenous rocks with a central core of volcanics from northern New Brunswick to north-central Maine. A land area in southern New Brunswick, Maine, and northern Nova Scotia separated marine terrigenous deposits with brachiopods belonging to the Old World Province (Rhenish Community) in southern Nova Scotia.

Oriskany equivalents are equally widespread but in many places lie unconformably upon upper Helderbergian rocks. In the southern United States and in Chihuahua, limestone and chert predominate, but the Oriskany Sandstone and its equivalents span a broad area in New York, Pennsylvania, and southwest to westernmost Virginia. A limestone belt can be traced from eastern New York, through Montreal, and thence northeast to eastern Gaspe. The argillaceous belt is widespread from southern New England to Gaspe. Nova Scotia was still isolated.

Esopus time saw a restriction of marine faunas to terrigenous facies occurrences in Gaspe and in the Matapedia Valley of Quebec as well as in eastern New York, Pennsylvania, West Virginia, and to limestone in central Texas. Most of the rest of eastern and southern North America apparently was emergent during Esopus time.

Schoharie time was again represented by widespread marine sediments. Terrigenous rocks occur in a belt west of a land area extending from Gaspe to the southwest, traceable through New York, Pennsylvania, and as far south as northern Alabama. A broad inner limestone belt, containing much chert, occupied the area around the Great Lakes and to the west and south of central Tennessee and stretching into west Texas and northern Mexico.