Deep Marine Systems

Processes, Deposits, Environments, Tectonics and Sedimentation

Kevin T. Pickering, Richard N. Hiscott

• Publisher: American Geophysical Union
• Year: 2015
• ISBN:
  9781118865422
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Deep-water (below wave base) processes, although generally hidden from view, shape the sedimentary record of more than 65% of the Earth's surface, including large parts of ancient mountain belts. This book aims to inform advanced-level undergraduate and postgraduate students, and professional Earth scientists with interests in physical oceanography and hydrocarbon exploration and production, about many of the important physical aspects of deep-water (mainly deep-marine) systems. The authors consider transport and deposition in the deep sea, trace-fossil assemblages, and facies stacking patterns as an archive of the underlying controls on deposit architecture (e.g., seismicity, climate change, autocyclicity). Topics include modern and ancient deep-water sedimentary environments, tectonic settings, and how basinal and extra-basinal processes generate the typical characteristics of basin slopes, submarine canyons, contourite mounds and drifts, submarine fans, basin floors and abyssal plains.

Part 1
Process and product

Chapter One
Physical and biological processes

(a) Experimentally produced turbidity current. Courtesy Jeff Peakall. (b) Upper part of sediment slide deposits (Facies F2.1) draped by siltstone turbidites (Facies D2.2 and D2.3) in deep-marine volcaniclastics, Miocene Misaki Formation, Miura Peninsula, southeast Japan.

1.1 Introduction

This chapter has two main functions. First, there is an introduction to the main processes responsible for the physical transport and deposition of sediments derived from land areas and carried into the deep sea. Second, the origin of pelagic sediments (oozes, chalks, cherts) and organic-rich muds (e.g., black shales and sapropels with >2% organic matter) is explained. For these sediments, transport of material from an adjacent land mass is either not required, for example in the case of accumulation of biogenic skeletons, or is far less important than the chemistry of the seawater at the site of deposition. The biogenic process of bioturbation is considered in Chapter 3.

The three main processes responsible for transporting and depositing particulate sediments seaward of the edges of the world's continental shelves are (i) bottom-hugging sediment gravity flows (e.g., turbidity currents and debris flows), (ii) thermohaline bottom-currents that form the deep circulation in the oceans and (iii) surface wind-driven currents or river plumes that carry suspended sediment off continental shelves. Tidal currents, sea-surface waves and internal waves at density interfaces in the oceans appear to be only locally important as transport agents on the upper parts of slopes and in the heads of some submarine canyons.

In order to appreciate how sands and gravels encountered in deep-marine petroleum reservoirs are deposited, it is essential to understand the dynamics of sediment gravity flows. A sediment gravity flow (SGF; Middleton & Hampton 1973) is a bottom-hugging density underflow carrying suspended mineral and rock particles, mixed together with ambient fluid (most commonly seawater). A SGF is a special type of particulate gravity current (McCaffrey et al. 2001) – in other flows belonging to this broad category the particles can be snow and ice (e.g., in a powder snow avalanche), or the fluid phase can be hot volcanic gases (e.g., in a pyroclastic surge). In engineering practice, such mobile solid and fluid mixtures are called granular flows, slurry flows or powder flows, depending on the size of the particles, whether the fluid phase is a liquid or a gas and whether cohesive forces are significant. In this book, we will use the more geologically relevant term ‘sediment gravity flow’, but anyone undertaking a literature search needs to be aware of the alternative terminology used in other disciplines.

The particles in SGFs spend most of their time in suspension rather than in contact with the seafloor. In the more dilute SGFs, particles in suspension eventually settle to the seafloor where they accumulate, either with or without a phase of traction transport. This is called selective deposition (Ricci Lucchi 1995; incremental deposition of Talling et al. 2012), because particles are deposited one by one according to size, shape, density or some other intrinsic property. In concentrated dispersions or cohesive debris flows, the particles are not fully free to move independently of one another and therefore accumulate by massive deposition (Ricci Lucchi 1995; en masse deposition of Talling et al. 2012). The distinction between selective deposition and massive deposition (or en masse deposition) is a useful one, because the former deposits are commonly laminated and the latter commonly structureless, poorly organised, plastically deformed, or contain evidence of intense particle interaction and/or pore-fluid escape. The ultimate end-member example of en masse deposition is coherent sliding in which masses of semi-consolidated material move downslope while retaining some of the organisation and stratigraphy of the original failed successions. Sediment slides come to rest as deformed, folded and/or sheared units.

Let us start by considering a typical event responsible for basinward sediment transport along a continental margin. The transport can be divided into four phases: (i) a phase of flow initiation; (ii) a period during the early history of the flow when characteristics of the transporting current change rather quickly to a quasi-stable equilibrium state; (iii) a phase of long-distance transport to the base of the continental slope or beyond and (iv) a final depositional phase. In many cases, the concentration of solid particles changes systematically along the flow path. Particle concentration is an important variable because mixtures of sediment and water can only become fully turbulent if the concentration is low. Without turbulence, it is difficult to suspend and transport mineral-density particles for long distances, and tractional sedimentary structures like current-ripple cross-lamination cannot form. Figure 1.1 shows how a range of deep-marine transport processes can be assigned to one or more of these four stages of flow evolution, and shows how the flow concentration might change through time. For example, sediment suspended by a storm on a continental shelf or by tidal currents in the head of a submarine canyon forms low-concentration suspensions that might continue to move downslope as turbidity currents, transferring particulate sediment tens to hundreds of kilometres farther seaward. Other initiation processes, like the disintegration of sediment slides on steep slopes, can generate more concentrated SGFs such as submarine debris flows. These debris flows may never become more dilute or develop turbulence, and therefore are less likely to transport their sediment load far into the deep-marine basin.

Figure 1.1 Simplified conceptual overview of the evolution of sediment gravity flows and other deep-marine transport processes as a function of concentration. The horizontal axis is time and/or space, but no units are implied because the evolution of some flows is much longer than for others. For example, turbidity currents might flow for only hours to days, whereas contour-following geostrophic bottom-currents (i.e., thermohaline currents) have velocity fluctuations lasting thousands of years. Notice that non-turbulent flows tend to deposit en masse, so that the deposit is simply the original flow, arrested in place when driving forces are no longer adequate to keep the material moving. In contrast, turbulent flows lose their sediment load by settling, and therefore become increasing...

Table of contents

• Cover
• Title Page
• Copyright
• Table of Contents
• Preface
• About the companion website
• Part 1: Process and product
• Part 2: Systems
• Part 3: Plate tectonics and sedimentation
• References
• Index
• End User License Agreement