Lower and middle Paleozoic rocks in the southeastern Great Basin include thin sequences of gray, generally unfossiliferous dolostones that were deposited in cratonic margin paleoenvironments. Age-diagnostic conodonts recovered from a 295-m-thick section of Mountain Springs Formation indicate that Lower and lower Middle Ordovician, Upper Ordovician, and upper(?) Lower Devonian rocks occur in the section. Distinct disconformities provide evidence for a mid-Ordovician hiatus and a Silurian through Early Devonian hiatus. These cratonic margin rocks thicken, become lithologically more diverse, and form a more temporally continuous section of miogeosynclinal rocks 15 to 20 km to the west. This narrow, northeast-trending facies change is a linear geologic element that is offset ∼40 km by the Las Vegas Valley shear zone.

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