MIDDLE AND LATE DEVONIAN LAURENTIAN FAUNAS FROM NE WASHINGTON: NOT A DISCRETE TERRANE

Middle and Late Devonian (Givetian and Frasnian) brachiopod and coral faunas from NE Washington resemble coeval taxa on the Laurentian miogeocline and craton. However, remarkably few outcrops of Devonian carbonates in the region resemble disjointed blocks often associated with smaller terranes. At Limestone Hill, Pend Oreille County, Washington, are approximately 50m of recrystallized carbonates whose stratigraphic relationships are unclear. The locality is in a structurally complex region, with an apparent Mesozoic eastward movement on thrust faults. Sorauf (1972) suggested that the succession may be a carbonate mound established on shelf muds. Owing to the presence of hermatypic corals (e.g. Peneckiella) the succession reached the photic zone, and conglomerates reported by Greenman et al. (1977) could possibly be buildup front breccia, although the breccias appear to be older that the larger carbonate bodies. However, there is little evidence of the succession reaching wavebase. Givetian faunas (Sorauf, 1972) include the corals Peneckiella, Hexagonaria, Trapezophyllum, Phillipsastrea, and Aulacophyllum. Frasnian fossils are represented by Emanuella russelli, Allanella sp., Thomasaria sp., Cyrtina sp., and an athyridid. Brachiopods are very small, are mostly articulated, and Emanuella displays shell thickening suggestive of muddy substrate habitat. This fauna is remarkably similar to coeval Frasnian strata in central Idaho and Nevada. The closest Frasnian material includes the Jefferson Formation of Idaho, in which Thomasaria occurs below a well-described buildup containing Peneckiella, Thamnopora, Syringopora, “Stachyodes” and “Amphipora” (Isaacson and Dorobek, 1989). The region falls within the “Eastern assemblage” of Coney et al. (1980), consisting of metamorphic terranes “... of possible continental affinity.” Whitehouse et al. (1992) demonstrated that there is probably Archean crust in NE Washington, suggesting that the continental margin of Laurentia was west of Pend Oreille County. Therefore, this and faunal information contained herein demonstrate that Limestone Hill is not part of an allochthonous terrane and contains a succession deposited outboard of the craton.