Stratigraphic Emmental: High-resolution allostratigraphy reveals multiple intra-formational unconformities in shallow ramp mudstone: Upper Cretaceous, Western Canada Foreland Basin

Abstract

Marine mudstone of Coniacian age (c. 89.51–86.49 Ma) was deposited on a storm-dominated ramp spanning the foredeep of the Cretaceous Western Canada Foreland Basin. Marine flooding surfaces define 18 allomembers that thin over 300 km, from c. 140 m in the proximal foredeep to c. 20 m close to the forebulge crest. The broadly conformable succession of allomembers is partitioned into five ‘tectono-stratigraphic units’ by low-angle unconformities that bevel off c. 10 to 20 m of strata over ‘arches’ that have a length scale of c. 50–100 km and are bounded by relatively linear zones of flexure. Depositional history involved two alternate modes: ‘Background’ deposition of subtly-tapered allomembers took place on a planar sea floor, subject to regional flexural subsidence, with sea-level modulated by Milankovitch-scale (c. 125 kyr) eustatic cycles. ‘Flexural’ events deformed the strata into troughs and arches across narrow zones of flexure. Arch crests were bevelled off, probably by submarine wave erosion. Eroded sediment did not accumulate in troughs but was advected beyond the study area by storm-driven processes. Cycles of deposition, warping and erosion were repeated five times on an average timescale of 600 kyr. Arches and troughs do not coincide with Precambrian basement structures, and their origin remains enigmatic. Changes in in-plane stress may have effected the localized vertical motion.