Sedimentology, depositional processes and sedimentary architecture of mudstones in the Cretaceous strata of the Western Interior Seaway of Utah

Abstract

Mudstones are the most abundant sedimentary rock type in the rock record and are therefore important archives of ancient depositional environments. However, they are arguably the most poorly understood sedimentary rock type as detailed sedimentological studies are uncommon compared to coarser-grained, sandstone deposits. In this PhD, a combined field, petrographic, sedimentological and sequence stratigraphic approach was applied to over 500 m of stratigraphy across 30 vertical sections through the Upper Cretaceous Mancos Shale within the well-constrained stratigraphic framework of the Book Cliffs, Utah, USA. This shallow-marine mudstone succession is dominated by facies that are interpreted to represent deposition in two major depositional environments: (i) offshore transition, and (ii) offshore. Based on differing sedimentological and ichnological characteristics, the offshore environment is further divided into three sub-environments: (i) distal offshore, (ii) middle offshore, and (iii) proximal offshore. Petrographic analysis of samples from each facies indicates this mudstone succession is dominated by the products of a range of depositional processes, including; (i) low-density turbidity currents, (ii) low-strength cohesive debris flows, (iii) transitional flows, (iv) wave-enhanced sediment-gravity flows, (v) hyperpycnal flows, and (vi) grains transported as bedload. There is limited evidence for deposition via suspension fallout in this setting, which challenges the traditional interpretation that mudstones are dominated by the product of suspension fallout. Moreover, if this study is representative of shallow-marine depositional systems, calculations indicate that suspension fallout accounts for somewhere between 12% and 19% of mud accumulation. Whilst the vertical stacking of facies and parasequences is well-ordered and predictable, the most noticeable feature is their spatial distribution. The distribution of facies is highly complex resulting in a high degree of spatial heterogeneity over hectometre- to kilometre-scales and parasequences generally lack continuity over similar scales. Moreover, there are differences between stacking patterns in shoreface and offshore environments as multiple mud-dominated parasequences in the distal environment are considered to be coeval with a single proximal sandy shoreface parasequence. These findings provide important information on (i) the processes responsible for the deposition of mud in shallow marine environments, (ii) local-scale mudstone heterogeneity, and (iii) the down-dip issue of stratigraphic hierarchy.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor), Stephen Flint (Supervisor) & Rhodri Jerrett (Supervisor)