Original scientific paper

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

A.G. Plint

Abstract

The early–mid Cenomanian Dunvegan Formation represents a large delta complex that prograded at least 400 km from NW to SE. A regional stratigraphy based on marine transgressive surfaces and equivalent subaerial interfluves allows the formation to be subdivided into ten transgressive–regressive allomembers, labelled J to A in ascending order, each with an average duration of <200 ky. Analysis of stacking patterns and facies distributions of parasequences within allomembers allows transgressive, highstand, falling stage and lowstand systems tracts to be identified. Extensive valley systems that average 1–2 km wide and 21 m deep can be traced for up to 320 km across the top surfaces of allomembers H to E. In their lower 20–40 km, valleys are filled with muddy heterolithic tidal facies but this changes to fluvial-dominated multi-storey channel-fills further up-valley. Interfluve surfaces are marked by palaeosols, the character of which indicate a protracted hiatus with extensive physical, chemical and biological modification of the parent material.

Changes in flexural subsidence rate are indicated by isopach patterns. Allomembers J–F have a sigmoidal prismatic geometry, successively offlapping to the SE. There is no evidence of thickening toward the orogen. In contrast, overlying allomembers E–A show progressive development of a depocentre along the western margin of the basin. The increasing accommodation rate on the updip coastal plain caused marine deltas to be starved of sediment, leading to progressive backstep of shorelines. Simultaneously, alluvial deposits within the depocentre show an upward increase in the proportion of subaqueous to subaerial facies, culminating in the incursion of brackish and finally marine waters. Thus tectonic subsidence rate had a first-order affect on both the volume of sediment available to build marine deltas and also on the local character of facies that accumulated on the coastal plain. The onset of flexural subsidence in allomember E appears to have resulted in subtle uplift of a forebulge, resulting in dramatic deflection of river systems.

Despite the clear tectonic signature, successive transgressions and regressions involved similar horizontal displacements of the shoreline, regardless of subsidence rate. This suggests that modest eustatic changes also influenced the accommodation available. Based on the measured horizontal excursions of the shoreline, the vertical thickness of alluvial strata, and realistic alluvial gradients, an average eustatic excursion of about 24 m is calculated. The incision of valley systems is attributed in part to periods of eustatic fall. However, valleys seem too long to be explained by eustasy alone, and hence secular changes in discharge are postulated as an additional forcing factor. Climatic cycles in the Milankovitch band may have been responsible for both eustatic and discharge variations.

Keywords

Cretaceous; Cenomanian; Western Canada; Foreland basin; Deltas; Sea-level change; Tectonics; Eustasy; Clastic sedimentation; Sequence stratigraphy

Hrčak ID:

3790

URI

https://hrcak.srce.hr/3790

Publication date:

27.6.2003.

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