Clastic Sediment Partitioning in a Cretaceous Delta System, Western Canada: Responses to Tectonic and Sea-Level Controls

Abstract

The early–mid Cenomanian Dunvegan Formation represents a largedelta complex that prograded at least 400 km from NW to SE. Aregional stratigraphy based on marine transgressive surfaces andequivalent subaerial interfluves allows the formation to be subdividedinto ten transgressive–regressive allomembers, labelled J to A inascending order, each with an average duration of < 200 ky. Analysisof stacking patterns and facies distributions of parasequenceswithin allomembers allows transgressive, highstand, falling stageand lowstand systems tracts to be identified. Extensive valley systemsthat average 1–2 km wide and 21 m deep can be traced for up to320 km across the top surfaces of allomembers H to E. In their lower20–40 km, valleys are filled with muddy heterolithic tidal facies butthis changes to fluvial-dominated multi-storey channel-fills furtherup-valley. Interfluve surfaces are marked by palaeosols, the characterof which indicate a protracted hiatus with extensive physical, chemicaland biological modification of the parent material.Changes in flexural subsidence rate are indicated by isopach patterns.Allomembers J–F have a sigmoidal prismatic geometry, successivelyofflapping to the SE. There is no evidence of thickening towardthe orogen. In contrast, overlying allomembers E–A show progressivedevelopment of a depocentre along the western margin of the basin.The increasing accommodation rate on the updip coastal plain causedmarine deltas to be starved of sediment, leading to progressive backstepof shorelines. Simultaneously, alluvial deposits within the depocentreshow an upward increase in the proportion of subaqueous tosubaerial facies, culminating in the incursion of brackish and finallymarine waters. Thus tectonic subsidence rate had a first-order affecton both the volume of sediment available to build marine deltas andalso on the local character of facies that accumulated on the coastalplain. The onset of flexural subsidence in allomember E appears tohave resulted in subtle uplift of a forebulge, resulting in dramaticdeflection of river systems.Despite the clear tectonic signature, successive transgressions andregressions involved similar horizontal displacements of the shoreline,regardless of subsidence rate. This suggests that modest eustaticchanges also influenced the accommodation available. Based on themeasured horizontal excursions of the shoreline, the vertical thicknessof alluvial strata, and realistic alluvial gradients, an average eustaticexcursion of about 24 m is calculated. The incision of valley systemsis attributed in part to periods of eustatic fall. However, valleys seemtoo long to be explained by eustasy alone, and hence secular changesin discharge are postulated as an additional forcing factor. Climaticcycles in the Milankovitch band may have been responsible for botheustatic and discharge variations.