Paleozoic Carbonate Buildup ("Reef") Inventory, Central and Southeastern Idaho

P. E. Isaacson

Knowledge of central and southeastern Idaho's Paleozoic rocks to date suggests that three styles of buildup ("reef") complexes occur in Late Devonian, Mississippian, and Pennsylvanian-Permian time. The Late Devonian Jefferson Formation has stromatoporoid and coral (both rugosan and tabulate) organisms effecting a buildup in the Grandview Canyon vicinity; Early Mississippian Waulsortian-type mud mounds occur in the Lodgepole formation of southeastern Idaho; there are Late Mississippian Waulsortian-type mounds in the Surrett Canyon Formation of the Lost River Range; and cyclic Pennsylvanian-Permian algal and hydrozoan buildups occur in the Juniper Gulch Member of the Snaky Canyon Formation in the Arco Hills and Lemhi Range. Late Devonian (Frasnian) carbonates of the Jeffers n Formation show buildup development on deep ramp sediments. Four distinct biofacies are recognized. Overlying a hardground is a nodular and encrusting stromatopoid wackestone with impressive Fasciculophyllum colonies. Next is a favositid biofacies, forming a bafflestone. The third biofacies consists of varied stromatoporoids, Amphipora, and metazoans. The final biofacies, apparently occurring above the buildup, consists of Syringopora in wackestone-mudstone. Overlying this is an erosional disconformity, followed by incipient Antler highland-derived sedimentation. Early Mississippian mud mound buildups occur in the Lodgepole formation of southeastern Idaho. Chesterian (Late Mississippian) complexes near Willow Creek summit (central Idaho) show typical Waulsortian mound development. Both complexes consist of fine-grained fenestrate bryozoan bafflestones, which are both bedded and unbedded. Occasional reef breccia is found with these buildups. Pennsylvanian-Permian complexes in the Arco Hills and Lemhi Range show eustatic cyclic development of coral/algal and hydrozoan buildups. Following coral bafflestones are Palaeoaplysina and phylloid algae (including Archaeolithophyllum and Eugonophyllum) dominated lithologies. The algae and hydrozoans apparently excluded shelly metazoans. The buildups did restrict circulation, evidenced by dolomites and shoreward shallow-water mudstones, in the Beaverhead Range. Each complex described above represents development on an unstable shelf and foreland (depending on timing), into which clastic sedimentation events, upward growth into wave case, and occasional subaerial exposure play a role in terminating the complexes. There may be buildups in other Paleozoic systems in Idaho and northeastern Washington. In the Early Cambrian, near Colville, Washington, are archaeocyathid-dominated lithologies indicative of a bafflestone. Within the Silurian Roberts Mountains Formation equivalents in the Lost River Range are contributions by organisms considered to be "reef" dwellers (i.e., Halysites and pentamerid brachiopods) within allodapic, deep-water carbonates deposited in a possible marginal basin adjacent to a productive margin.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.