Recurring Themes in Foreland Basin Stratigraphy: Sheets, Wedges, Trampolines, and the Possible Role of Climate

Abstract

Much of the Cretaceous fill of the Western Canada Foreland Basin was deposited on low-lying delta plains and low-relief shallow muddy ramps. Isopach maps of allostratigraphic units provide primarily a representation of subsidence; water depth can largely be ignored as a first-order control on thickness. Shallow marine strata ranging from Mid Albian to Early Campanian show that marine mudstones, and some alluvial successions, form wedges thinning towards the forebulge; progradational sandy shoreface successions are more sheet-shaped. This observation corroborates the predictions of numerical models in which sand & gravel are trapped in the proximal foredeep during active tectonic subsidence, escaping only during tectonic quiescence or isostatic erosional uplift. Isopach maps of individual allostratigraphic units show, however, that ‘sheets’ and ‘wedges’ are actually subtly arcuate in plan, a few hundred km in width, and represent ∼ 500 kyr. Successive mudstone wedges are offset laterally, by several hundred km, along the strike of the foredeep. Each offset, lenticular unit indicates a lateral shift in the locus of maximum subsidence, each depocentre being inferred to have lain adjacent to the locus of active thickening in the deformed belt. A trampoline deformed by an energetic jumper provides a visual metaphor for the arcuate foredeep to forebulge geometry! The temporally & spatially discrete depocentres imply activity in correspondingly discrete sectors of the tectonic wedge, bounded by transfer zones. Averaged over several million years, stacked depocentres combine to produce an apparently simple prismatic wedge that fits simple models of foreland basin subsidence. Despite the mechanistically logical relationship between tectonism, stratal geometry and facies, some wedge-shaped foredeep units comprise aggradational, fine-grained alluvial successions that are repeatedly punctuated by paleovalleys 20-35 m deep, filled with multi-storey fluvial sandstone. Paleovalleys are not traceable outside the proximal foredeep. Cycles of incision and aggradation have a frequency of ≪1 myr. There is no geometric evidence for repeated tectonic uplift and subsidence on this timescale, and no evidence that eustatic changes propagated inland sufficiently far to trigger fluvial incision. Cyclic changes in river discharge, linked to wetter and drier climate cycles in the Milankovitch band, may have been responsible for alternate aggradation and incision on the coastal plain.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016