Depositional processes and stratigraphic architecture of deep-water mudstones

Abstract

Mudstones are important archives of palaeoenvironments of Earth’s history. These fine-grained sedimentary rocks dominate the deep-water stratigraphic record, but their detailed sedimentological study has been neglected compared to sandstone-prone deposits. Here, a combination of macroscopic and microscopic descriptions of more than 1200 m of cores from the Permian deep-water successions of the well-constrained Tanqua and Laingsburg depocentres of the Karoo Basin (South Africa), have allowed the sedimentology and stratigraphy of deep-water mudstones to be investigated. The deep-water mudstone successions are dominated by the products of a wide range of sediment gravity flows, including low-density turbidity currents, low-strength cohesive debris flows, transitional flows between turbulent and laminar, slumps and slides. The limited evidence for hemipelagic fallout, and the episodic nature of mud accumulation at all relative sea-level stands, challenge the paradigm that deep-water mud only represents deposition by slow rainout from suspension in quiescent environments. The investigation of mudstones beyond the sandstone pinchouts of basin-floor fans has allowed refinement of depositional models for deep-water lobes. A range of sedimentological, stratigraphic and ichnological criteria are provided to distinguish between mud deposited in lobe distal fringe environments from background mud accumulated beyond a lobe. A new hierarchical scheme stratigraphically subdivides distal basin-floor fans using thickness ranges of background mudstones as bounding elements of lobe distal fringe deposits, and indicates that deep-water lobes are much larger than previously estimated. For the first time, a set of sedimentological and stratigraphic criteria is proposed to differentiate slope mudstones from basin-floor mudstones. Basin-floor mudstones consist of a repeated and predictable alternation of low-density turbidites and debrites. Slope mudstones show a higher proportion of low-density turbidites compared to debrites, a higher bioturbation intensity, and a less predictable facies stacking pattern indicating accumulation in a wide range of sub-environments. These criteria may provide an important tool to refine palaeogeographic reconstructions of other deep-water successions.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor) & Stephen Flint (Supervisor)