Natural geomorphic variability recorded in a high-accommodation setting: Fluvial architecture of the Pennsylvanian Joggins Formation of Atlantic Canada

The varied form and architecture of suites of channel bodies is a product of the intrinsic geomorphic variability of the drainage network as well as extrinsic factors such as climate, eustasy, and tectonics. We investigated the balance between intrinsic and extrinsic controls on the basis of eighty two fluvial-channel bodies in the Pennsylvanian Joggins Formation, which is superbly exposed along the Bay of Fundy in Nova Scotia, Atlantic Canada. Based on their internal architecture and three-dimensional form, these channel bodies were divided into fixed, meandering, and multistory types, and each type is present in both coastal wetland and inland floodplain facies associations. Although the facies associations are organized into cyclic packages that probably represent relative changes in base level, the channel-body types are not positioned systematically within the cycles. Apart from a small group of unusually deep meandering channels within the wetland deposits, comparison of paleochannel depth and width/depth shows that active channels throughout the formation were similar in size and shape. The multistory bodies are small channel belts or valley fills that do not appear to record the basinward advance of facies belts but rather are genetically linked with the associated floodplain deposits. These observations suggest that the varied form of the Joggins channel bodies largely reflects the geomorphic variability of the original drainage network, with the largest, meandering channels in the coastal zone. Preservation of a representative drainage network was probably facilitated by rapid subsidence on bounding faults, enhanced by high-frequency, low-magnitude subsidence events caused by salt withdrawal. The effect of extrinsic factors on channel type and geometry appears to have been modest in this high-subsidence setting.