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The Ashgillian (Upper Ordovician) Montgomery Limestone occurs as slide blocks in melange of the Shoo Fly Complex, northern Sierra Nevada, northern California. Brachiopods and sphinctozoan sponges from the Montgomery Limestone have closest biogeographic ties to coeval faunas of the eastern Klamath Mountains (Yreka terrane), and in the case of the brachiopods, to east-central Alaska (Jones Ridge). The latter was part of North America in the Ordovician. A small collection of Montgomery rugose corals yielded one species that is known elsewhere only in the Yreka terrane and in northern Maine. Montgomery tabulate corals have affinities with contemporaneous faunas of the Yreka terrane, northern Europe/Asia, Australia, and eastern North America. The apparent absence of similar tabulate taxa in western North America may be an artifact of incomplete collecting.

As a whole, the biogeographic data indicate that the Montgomery Limestone was deposited close enough to Ordovician North America for faunal interchange to occur, and during its deposition was probably relatively near that continent.

A comparison of lithologic units of the Shoo Fly Complex with those of the Yreka terrane indicates that some units in each area have no counterparts in the other (e.g., schist of Skookum Gulch in the Yreka terrane), and other units have general similarities in age (where known) and lithology, but differ in detail. The Yreka terrane has been interpreted as the remnants of an Early Cambrian arc and Ordovician-Devonian arc–fore-arc–accretionary prism, and the Shoo Fly Complex as a fragment of a Devonian or older accretionary wedge. Available biogeographic and stratigraphic data can be reasonably explained, as has been done by earlier authors, by a paleogeography in which the Yreka terrane and Shoo Fly Complex were parts of the same arc-trench system but were situated at different points along the strike of the arc. Lateral changes along strike in tectonic conditions and source areas could account for the observed disparities.

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