minerals Article Mudrock Microstructure: A Technique for Distinguishing between Deep-Water Fine-Grained Sediments Shereef Bankole * , Dorrik Stow , Zeinab Smillie , Jim Buckman and Helen Lever   Citation: Bankole, S.; Stow, D.; Smillie, Z.; Buckman, J.; Lever, H. Mudrock Microstructure: A Technique for Distinguishing between Deep-Water Fine-Grained Sediments. Minerals 2021, 11, 653. https://doi.org/10.3390/ min11060653 Academic Editors: Huifang Xu, Paul Sylvester, Hanumantha Rao Kota, Theodore J. Bornhorst, Anna H. Kaksonen, Nigel J. Cook, Alexander R Cruden and Sytle M. Antao Received: 31 May 2021 Accepted: 15 June 2021 Published: 20 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Institute of Geoenergy Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK; d.stow@hw.ac.uk (D.S.); z.smillie@hw.ac.uk (Z.S.); j.buckman@hw.ac.uk (J.B.); h.lever@hw.ac.uk (H.L.) * Correspondence: sb222@hw.ac.uk Abstract: Distinguishing among deep-water sedimentary facies has been a difficult task. This is possibly due to the process continuum in deep water, in which sediments occur in complex associations. The lack of definite sedimentological features among the different facies between hemipelagites and contourites presented a great challenge. In this study, we present detailed mudrock characteristics of the three main deep-water facies based on sedimentological characteristics, laser diffraction granulometry, high-resolution, large area scanning electron microscopy (SEM), and the synchrotron X-ray diffraction technique. Our results show that the deep-water microstructure is mainly process controlled, and that the controlling factor on their grain size is much more complex than previously envisaged. Retarding current velocity, as well as the lower carrying capacity of the current, has an impact on the mean size and sorting for the contourite and turbidite facies, whereas hemipelagite grain size is impacted by the natural heterogeneity of the system caused by bioturbation. Based on the microfabric analysis, there is a disparate pattern observed among the sedimentary facies; turbidites are generally bedding parallel due to strong currents resulting in shear flow, contourites are random to semi-random as they are impacted by a weak current, while hemipelagites are random to oblique since they are impacted by bioturbation. Keywords: deep-water fine-grained sediments; turbidites; contourites; hemipelagites 1. Introduction Microstructure is one of the most important aspects of rock properties that has a significant impact and a wide range of applications in several areas of interest, including geosciences. For example, the importance of microstructure on unconventional reservoir quality and storage properties [1,2], seismic anisotropy [3,4], geomechanical properties [5,6], depositional processes [7,8], as well as post-depositional processes [9] are the subject of ongoing research. In a strict sense, microstructure in this paper is used to describe the total characterizable features from micrometer to nanometer features, such as pore size, grain size, porosity, connectivity, permeability, microfabric, mineralogy, orientation of crystals and grains, and other features at this scale. Early work on mudrock microstructure was based on pedantic notions and on clay particles structure. The presumption is that clays are very small and have a very high ten- dency to stick together. This informed early descriptive terminologies such as honeycomb, cardhouse, and bookhouse fabrics, which are several descriptions of clay flocs and their arrangement [10]. The advent of scanning electron microscopy in the early 1970s led to the first direct observation of mudrock microstructure [11]. Interested readers of many of the earliest works, including hypotheses related to the formation of edge to edge (E-E), face to face (F-F), and edge to face (E-F) clay crystal interactions, are referred to a review by [12]. Despite the huge interest in mudrock microstructure, studies linking them to depositional processes are very rare [13]. There are very few works aimed at addressing microfabrics as a way of discriminating between deep-water facies, and the outcomes of the little research that has been done Minerals 2021, 11, 653. https://doi.org/10.3390/min11060653 https://www.mdpi.com/journal/minerals