Fossiliferous Middle Cambrian boulders from a previously unknown boulder unit in the Haymond Formation of Pennsylvanian age in the southeastern part of the Marathon basin, Texas, contain trilobites, brachiopods, and mollusks indicative of the seaward margin of a late Middle Cambrian carbonate platform. The source area was to the southeast of present outcrops. Paleogeographic relocation of the source, about 100 to 200 km southeast of the present Marathon basin, provides the first clear evidence for the position of the seaward margin of the earliest Paleozoic carbonate platform along the southern sector of the United States. Metamorphic rocks of the “interior zone” of the late Paleozoic Ouachita orogen now lie well inboard of the seaward edge of the carbonate platform and are most likely allochthonous on a scale of tens of kilometres.

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