Alluvial Terracing in a Falling-stage Braidplain: the Lower Cretaceous Cadomin Formation, West-central Alberta, Canada

The Lower Cretaceous Cadomin Formation in the Western Canada Sedimentary Basin is a thin, regionally extensive, conglomeratic fluvial deposit that accumulated over many million years on the sub-Cretaceous unconformity. Based on a dataset of approximately 50 cores and 750 wireline well logs from west-central Alberta, detailed isopach mapping of the overlying deposits reveals the presence of a complex, terraced paleo-topography on the top of the Cadomin Formation, consisting of six terrace levels in the study area. These terraces flank a series of north–south valleys that feed into a larger east–west valley to the north of the study area. This larger valley is also bordered by terraces that step downward to the north. The gradients of the north–south valley thalwegs are steeper than the flanking terraces, indicating that each terrace is diachronous and was most likely formed by the headward migration of knickpoints generated by episodic incision of the trunk valley. This paleo-topography formed during a prolonged period of falling base level caused by unroofing of the adjacent orogen. Thus, the Cadomin Formation represents a falling-stage systems tract. The deposits underlying each terrace consist mainly of channel-thalweg and braid-bar deposits. Preservation of full channel-bar successions in many terraces is consistent with terrace abandonment as incision resumed following a period of mild aggradation. Terrace abandonment is also indicated by the presence of a capping layer of wind-blown silt. Pedogenic alteration of this loessite is greatest on the highest terraces and extends to considerable depths, indicating the existence of a significant hiatus at the top of the Cadomin Formation. This surface, which lies above falling-stage deposits, should be used as the sequence boundary, if the sequence boundary is thought to coincide with the time of lowest base level. This surface, although its formation was diachronous, represents a real landscape surface, unlike the composite erosion surface beneath the Cadomin Formation (i.e., the sub-Cretaceous unconformity). The alternation of incision and aggradation that generated the terraces was probably the result of allogenic fluctuations in sediment supply caused by climate cycles, as was the case for analogous Quaternary terrace staircases. Downstepping alluvial terraces are a viable mechanism for the progradation of alluvial gravels long distances from a mountain belt during periods of basin uplift, and may explain the relatively thin, but areally extensive, alluvial sandstone and conglomerate sheets that are common at major unconformities in the stratigraphic record. We suggest that signs of subtle terracing may have been overlooked in similar sheet-like alluvial deposits elsewhere, although they can be removed by erosion during shoreline transgression or by later fluvial-channel migration. In the case of the Cadomin Formation, the exceptional preservation of the terraces is likely due to a combination of the difficulty of eroding the conglomerate and indurated loessite cap, and of the low-energy nature of floodplain sedimentation in the overlying Gething Formation.