Despite their discontinuous occurrence and poor preservation, knowledge about Triassic carbonates from North America has increased considerably during recent years. Their characterization represents a uniqueway to better assess evolution and recovery of the biosphere after the major Permo-Triassic biological crisis in the Panthalassa Ocean. The Eastern Klamath terrane, located in Northern California, is a key terrane due to its geographic position. It is placed halfway between the terranes of the Canadian Cordillera and the Northern Mexico counterparts, both extensively studied and characterized in recent decades, leaving a gap in knowledge along the Pacific coast of the United States. A few kilometers north-east of Redding, Shasta County, California, Upper Triassic carbonates (i.e., the Hosselkus limestone) crop out as a narrow north–south belt about 20 km long, near the artificial reservoir of Lake Shasta. All the accessible localities in this region have been extensively sampled for microfacies and micropaleontological analysis, leading to new insights about the depositional condition and age of the Hosselkus limestone. A depositional model has been proposed for the first time, corresponding to a steep slope system subjected to platform progradation and collapse, recording shallow water facies and associated fauna in the form of calcareous breccia. Numerous conodont specimens have dated the whole succession as Upper Carnian. Identification of shallow water organisms, associated to a reliable stratigraphic interval, allowed comparison of the Hosselkus limestone with other Upper Triassic carbonates from the Panthalassan domain. Despite the faunal affinities, especially with buildups developed at middle-paleolatitudes, the Hosselkus limestone is among the oldest of the terrane-based carbonates in Eastern Panthalassa. Thanks to peculiar geodynamical and bathymetrical conditions, allowing carbonate deposition slightly earlier than in other terranes, the Hosselkus limestone probably acted like a pioneer reef and may have had a great influence in the further expansion of carbonate buildups in the eastern part of the Panthalassa Ocean.