The Upper Cretaceous (Santonian-lower Campanian) Haslam Formation (Nanaimo Group) of southern Vancouver Island and the Gulf Islands of British Columbia and the San Juan Islands of Washington includes: 1) inner submarine-fan facies consisting of channel-axis conglomerate and sandstone intercalated with channel-margin and interchannel turbidites, and 2) outer submarine-fan and basin-plain facies consisting of fan-lobe and lobe-fringe turbidites, thin-bedded turbidites, and shales. Outer submarine-fan facies are more common in the lower part of the formation and in the westernmost exposures on Vancouver Island, whereas inner submarine-fan facies characterize the upper part of the Haslam in the San Juan Islands and are more common throughout the formation to the east and southeast. These submarine-fan facies were deposited in a forearc basin on rocks of the Wrangellia exotic terrane seaward of the Cretaceous magmatic arc in the Coastal Plutonic Belt. The Haslam submarine-fan complex prograded to the northwest or west into the forearc basin. The sediment supplied to the fan was derived from terrane to the east and south where metasedimentary, sedimentary, metavolcanic, and volcanic rocks were the dominant source areas during the late Santonian and early Campanian. Haslam sandstone and conglomerate records the first appearance of sediment from the east in the Nanaimo Basin. Older Upper Cretaceous (Santonian) rocks of the Nanaimo Group, the Comox Formation, were derived in large part from Upper Paleozoic to Lower Mesozoic volcanic and metavolcanic rocks and Jurassic plutonic rocks of the Insular Belt. Later Nanaimo formations (Campanian-Maestrichtian) that overlie the Haslam were derived from low-grade metamorphic and plutonic sources with only minor input of volcanic and sedimentary material. Thus, three petrologic intervals can be recognized in the Nanaimo Group on southern Vancouver Island, the Gulf Islands, and the San Juan Islands: the Santonian interval (Comox Formation) characterized by sediment derived from the Insular Belt; the late Santonian-early Campanian interval (Haslam Formation) characterized by sediment derived from a sedimentary and subordinate volcanic source to the east; and the Campanian-Maestrichtian interval Extension to Gabriola Formations derived from a low-grade metamorphic and crystalline source to the east. Paleocurrent dispersal patterns are similar for the Haslam and Extension-Gabriola intervals, which suggests that sediment supplied to the forearc basin was derived from uplifted subduction-magmatic arc complexes that were probably related to the sourthern margin of the Insular Belt and were deformed when the magmatic arc shifted westward in the mid-Cretaceous. The petrologic intervals are interpreted to reflect unroofing of this source terrane, although other interpretations are possible if transcurrent faulting was active in this region in the late Cretaceous.