Introduction to Sequence Stratigraphy of Deepwater Clastic Systems

Preamble
The long history of interest in gravity-driven deep-water sedimentation is founded on early key work by Kuenen, Shepard, Bouma and others (link to Historical Review). Research in this field of sedimentary geology has recently accelerated. This is largely driven by the interest of the oil industry as their exploration on continental margins around the world has extended into increasingly deep water. Also parallel developments in seismic and seafloor imaging have provided fundamental new insights into the character and three-dimensional structure of deepwater deposits. Increasingly sophisticated laboratory and numerical simulations can now be used to replicate natural processes.

Concurrently the development of sequence stratigraphy has extended our understanding of deepwater deposits. Recognition of the concept of system tracts has stressed the connections within the depositional profile between shoreline, shelf and deep water settings. Deep-water systems are no longer viewed in isolation, but as an integral part of the margin response to up-dip accommodation and supply variations. These externally-forced changes provide a high-level framework in which to assess the behavior of successive gravity currents and the depositional architecture they build.

The new understanding of the processes and stratigraphy of deepwater systems is important to both the better prediction of sand occurrence during hydrocarbon exploration, and the assessment of the three-dimensional geometry and reservoir properties of the sandstones and interbedded non-reservoir facies during field appraisal and production.

Deepwater Clastic Systems Section Organization

This set of pages has two purposes and sections:

The first section provides a summary of key generic characteristics of clastic deepwater sedimentation, focusing on gravity currents and their deposits from the bed-scale up. The hierarchical organization of beds into elements, element stacks and systems is reviewed, and the relationship between system character and its wider context (sediment source and type, climate, tectonics, sea-level changes) is stressed. These relationships determine how deep-water systems respond to external forcing, and underpin the sequence stratigraphic surface-based approach beyond the shelf break.

The topics covered are listed below and can be accessed by the pull down menu above or by clicking on the appropriate thumbnail below.

Generic Deepwater Topics	Link from Icon
Deepwater Processes: Outline of the key gravity flow types and their deposits at bed-scale. Flow classification schemes. Introduction to concept of deep-water hierarchy.
Sedimentary structures: A pictorial inventory of typical sedimentary structures found in clastic deep water deposits
Deepwater depositional elements: Review of the key elements (channels, sheets, mounds etc.) found on slopes and deep-water basin floors.
Source and supply variations and their impact on system style: Background on the controls on deepwater sedimentation and the classification of fan and other turbidite systems
Deepwater sequence stratigraphic concepts: Outline of key stratigraphic surfaces and guide to stratigraphic subdivision of deepwater basinfloor and slope successions.

The second section examines a case-study of Carboniferous basin floor and slope deposits in large cliff and foreshore exposures of the Ross Sandstone and Gull Island Formations along the Atlantic coast of western Ireland. These rocks of the Clare Basin provide one of the best exposed and most thoroughly studied deepwater systems in the world. Deposition took place under icehouse conditions and evidence points to high-frequency fluctuations of sea-level and supply during basin filling. Details of the basin setting and wider stratigraphic framework are provided first, and then the depositional architecture of the Ross and Gull Island systems is illustrated, focusing on bed types, elements, element stacks and key surfaces. A number of alternative models for the distribution of channel and sheet elements in the Ross are presented, and the implications for larger-scale architecture assessed. The expression of sequence stratigraphic surfaces in the slope and basin-floor and the response to external forcing are also considered. The sedimentary sections that envelope Ross Sandstone and Gull Island are then described; including the deepwater basin Clare Shale below, and the shallow-water delta deposits of the Central Clare Group that overlie the deepwater basin fill. Walther's Law suggests that the vertical evolution of the Namurian Clare Basin depositional settings can simply and elegantly interpreted as a single progressive shoaling cycle, with the now vertically superimposed sediments from each of these depositional settings originally lieing laterally equivalent to each other. This section ends with a gallery of photographs, maps and diagrams of the Clare Basin and its sediments; exercises provided to understand deepwater sedimentary fill; and an extensive list of references to the literature describing deepwater sediments.

Clare Basin Case Study	Link from Icon
Clare Basin, Co. Clare: introduction to character of Upper Carboniferous Clare Basin fill and biostratigraphic and depositional framework.
Ross Sandstone Formation: exposures, depositional setting and general character of a high net-to-gross sandy-basin floor fan system. Application of sequence stratigraphy.
Gull Island Formation: character of a low net-to-gross, base-of-slope/slope turbidite system
Clare Shale Formation: character of the deepwater basinal shale at base of section
Central Clare Group: character of shallow-water delta deposits capping section
Deepwater Galleries: collection of the different image galleries associated with this section. Many photographs are of Co Clare rocks
Exercises: use of sequence stratigraphy to interpret depositional setting and predict reservoir quality in deepwater successions
Literature references:
Student: class summaries from classes on the sedimentary section

This set of pages on deepwater systems was compiled by Peter Haughton (field observations and insight) and Christopher Kendall (some field observations, much armchair geology and web magic). Their intent has been to introduce students and professional geologists to the interpretation and prediction of the laterally continuity and quality of deepwater sediments using an understanding of depositional processes, sedimentary architecture and their tie to the principles of sequence stratigraphy and allostratigraphy.
The site is illustrated with a mix of personal field observations and photographs of the Ross Sandstone and Gull Island formations, the published observations of others and their measured sections. While outcrops of any deepwater succession have "unique" characteristics imposed by the specifics of the depositional setting for that location and time, they also carry many common attributes.