Mudrock Microstructure: A Technique for Distinguishing between Deep-Water Fine-Grained Sediments

Shereef Bankole,Jim Buckman,Zeinab Smillie,Dorrik Stow,Helen Lever

doi:10.3390/min11060653

Shereef Bankole, Jim Buckman + Show 3 more

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https://doi.org/10.3390/min11060653

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Journal: Minerals	Publication Date: Jun 20, 2021
Citations: 2	License type: CC BY 4.0

Affiliation: Heriot-Watt University

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.

Highlights

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
It is somehow difficult to differentiate between contourites and hemipelagites based on their contents in some samples but in others, hemipelagites calcareous content is higher, predominantly coccoliths, and roughly up to 50% or more (Figure 11)
The actual mean size of deep-water sediments is controlled by three principal factors: (a) sediment supply, i.e., the grain size distribution of the original source material; (b) distance from the source, i.e., the distance travelled in the turbidity current or bottom current, or the distance from shore for hemipelagites; and (c) current velocity—in that this affects the current’s capacity to transport and/or winnow sediment

Summary

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. 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 tendency 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]

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