Sequence Stratigraphy of Deepwater Fans & Mass Transport Debris

Introduction
Until the recent advances in high frequency seismic acquisition and deepwater exploration the methodology of sequence stratigraphy has more commonly been applied to sediments that accumulated in coastal, shelfal and other shallow water settings rather than deepwater. Interpretation of shallow water systems has been favoured because evidence of changes in sea level tends to be preserved in sediments that accumulated close to the shoreline and this evidence is thought of as unlikely to be associated with a deepwater setting. Never the less the Exxon model for sequence stratigraphy has been applied to deepwater though it predicts higher rates of sediment accumulation than occur in reality.

As with those from shallow water, deepwater sediments exhibit relatively conformable successions of genetically related strata that are subdivided by hierarchies of surfaces. Similarly the origins of these surfaces range from erosional to those that are depositional. The higher frequency surfaces are more commonly autocyclic and tied to changes in point source migration rather than base level driven by eustasy. Eustatic events tend to be of lower frequency and have chronostratigraphic significance. Chronostratigraphic events associated with deep water include those driven by eustasy, signals of climatic change, storms and ash falls.

In deepwater evidence of sea level change is expressed by changes in the accommodation that result in gradational change in bathymetry and are reflected in repeated and cyclic changes in the character of the lithology and the occurrence of condensed sequences (eg; graptolites, goniatites, ammonites and foraminifera) and organic and/or radioactive shales. However most cyclic changes are tied to changes in the character of the sediment transportation and the source terrain. Autocyclic sediments are distinguished from allocyclic ones by the localization of the cycles or their absence. This can be seen in the Congo Fan in the illustration below. Not also the condensed sections that drape the autocyclic fan lobes. These condensed layers are believed to be driven by high positions of the sea.

Correlations between Zaı¨Ango seismic data, Site 1077 of ODP Leg 175 (Shipboard Scientific Party, 1998b; Wefer et al., 1998, 2002) and seismic stratigraphy proposed by Uenzelmann-Neben et al. (1997). The Axial and Southern Fans are in homogeneous gray tones, while the three main channel/levee complexes of the Northern Fan are shown individually; the thick black sinuous line represents the present-day active Zaire Canyon/ Channel. (b) Conceptual schematic cross section (approximate location on a) illustrating the general architecture of the fan and the relationships between the Northern, Southern, and Axial Fans. White areas refer to hemipelagic deposits. Not to scale. (c) Composite seismic section (upper) and line drawing (lower) from Site 1077 (ODP Leg 175; Shipboard Scientific Party, 1998b; Wefer et al., 1998, 2002) at the base of the Congo continental slope to the first channel/levee complex of the Northern Fan (location on a). Bn, Bs, and Ba = basal surface of the Northern, Southern, and Axial Fans, respectively; Tsu = top surface of the ‘‘slope unit’’. The inset shows the proposed correlations with the seismic stratigraphy established by Uenzelmann-Neben et al. (1997).

Seismic
Depending on the rates of sedimentation evidence of a eustatic signal viewed on seismic tends to be restricted to condensed sections and associated with third order events. For instance this is true of the study by Posamentier and Kolla (2003) when they used seismic data to describe how many, but not all, deep-water basin-floor successions characteristically have a base formed by mass-transport deposits that are overlain by turbidite frontal-splay deposits. These in turn are overlain by leveed-channel deposits and may be capped by another mass-transport unit covered by a drape of condensed-section deposits. They explain that this succession as related cycles of relative sea-level change on the associated shelf edge. They argue that commonly deposition of a deep-water sequence starts with a relative sea-level fall and terminates when this is followed by is a rapid relative sea-level rise. The drape of condensed-section deposits reflcts the sea level high.

Schematic depiction of the relationship between relative sea level and type of dominant mass-flow process. The succession comprises debris-flow deposits at the base (corresponding to the initial period of relative sea-level fall), overlain by frontal-splaydominated and then leveed-channel-dominated sections (corresponding to the subsequent period of early and late relative sea-level lowstand respectively). The succession is capped by deposition of debris-flow and condensed-section deposits (corresponding to periods of rapid sealevel rise and highstand, respectively) (Posementier & Kolla 2003).

A) Offshore Indonesia seismic reflection profile illustrating the stratigraphic succession of a deep-water sequence. Debris-flow deposits (1) overlain by frontal-splay deposits (2), channel–levee deposits (3), and again debris-flow deposits (4). The entire succession is inferred to be mantled by a thin veneer of condensed-section deposits (5). B) Schematic depiction of an idealized deep-water depositional sequence, with two hypothetical log profiles shown (Posementier & Kolla 2003).

Well Logs
Deep water sequence stratigraphy relies heavily on the use of Gamma logs, in which the high values in Gamma signal are equated with the reduced sedimentation associated with high positions of the sea. Well logs with high Gamma signals are traditionally taken to to be equated with maximum flooding surfaces and are often associated with condensed sections. For example Cornell et al (2001) found that they were able to use Gamma ray analysis of the upper Ordovician in the northern Appalachians to tie the surface and subsurface stratigraphy. They equated these with condensed intervals, sequence boundaries, and organic rich shales. Similarly working in Pliocene-Pleistocene sediments of the northern Gulf of Mexico, Crews et al (2000) identified condensed sections in wire-line logs that were from wells that penetrated the sediments of intraslope basins. They used this information to create a sequence stratigraphic framework that integrated high-resolution biostratigraphic data from wells with wire-line logs, mud logs, and seismic data. They recognized two major types of condensed sections: carbonate-rich condensed sections (CRCS) and shale-rich condensed sections (SRCS). They found that paleontologically, both CRCS and SRCS are characterized by a high relative abundance of calcareous nannofossil and foraminifera and an increase in diversity. The CRCS, in contrast, have a low Gamma-ray (siliciclastic sand) signature and high spontaneous potential (SP) (at or near shale baseline) wire-line log response. The SRCS are characterized by Gamma-ray and SP responses that are approximately at the shale baseline or slightly higher on wire-line logs. These two types of condensed sections can occur within the same depositional sequence, suggesting multiple factors controlling the kinds of sediments deposited within a condensed section.

Samson et al (2006) indicate that organic-rich shales with elevated Gamma-ray, or “hot streaks”, tie with stratigraphic condensed sections; representing relatively long periods of continuous geological time amalgamated in and represented by relatively thin slices of stratigraphic section. They argue that they indicate starved sedimentation conditions that are associated with deep-water settings. They record how Upper Carboniferous cyclothemic black shales from the mid-continent of North American provide geographic proxies to constrain paleoceanographic conditions, which can be correlated over large distances in the subsurface.

Sercombe and Radford (2007) take a contrary position that high Gamma ray ‘hot' Eocene shales in southern deepwater Gulf of Mexico wells suggest very high global temperature excursions created extensive algal blooms and anoxic oceanic conditions in water depths of greater than 20,000'. They argue against these being maximum flooding surfaces since they would have been beyond the influence of eustatic sea level changes in very deep water would be minimal. The deep-water ‘hot' shales alternatively suggest very high global temperature excursions that created extensive algal blooms. However these periods of high temperatures punctuated by brief episodes of extreme heat would explain the association of marine deep water ‘hot' organic shales and terrestrial death assemblages and use them for time markers.

Outcrop
In outcrop the most prevalent sequence stratigraphic marker for deepwater sediments is the existence of condensed sequences of pelagic and occaisional benthic fauna. In the case of the Ross Formation, and the associated Clare Group condensed sections of goniatites (Hodson. & Lewarne, 1961) are equated with maiximum flooding. However as Ten Veen and Postma (1998) record how one can measure amplitude variations of the Gamma-ray in outcrops. They record of late Miocene hemipelagic successions on Crete with high Gamma-ray signals. They tie these with the amplitude variations of a published theoretical insolation curve. The studied sections, which are well constrained paleomagnetically and biostratigraphically and cyclostratigraphically, are more than 100 km apart. They were correlated on basis of gamma-ray count rates. Ten Veen and Postma (1998) associated these stratigraphic events with warm periods and so times of potentially high sea level.

References Cited
Cornell, Sean R., Brett, Carlton E., and Mclaughlan Patrick I., 2001, "Sequence Stratigraphy and Spectral Gamma Ray Analysis of Upper Ordovician Carbonates of the Northern Appalachian Basin: Linking Surface and Subsurface Stratigraphy", GSA Annual Meeting, November 5-8,
Crews, Jennifer R., Paul Weimer, Andrew J. Pulham and Arthur S. Waterman, 2000, "Integrated Approach to Condensed Section Identification in Intraslope Basins, Pliocene-Pleistocene, Northern Gulf of Mexico", AAPG Bulletin; v. 84; no. 10; p. 1519-1536
Hodson, F. & Lewarne, G.C. (1961), "A mid-Carboniferous (Namurian) basin in parts of the counties of Limerick and Clare, Ireland". Quart. Geol. Soc. Lond., 117, 307-333.
Nilsen, Tor H., Gary S. Steffens, and Joseph J. R. Studlick, 2006, "Mass Transport Deposits in Deepwater Outcrops: Depositional Setting(s), Types, and Recognition", SEPM Research Symposium: The Significance of Mass Transport Deposits in Deepwater Environments II, AAPG Annual Convention, April 9-12, 2006 Technical Program
Posamentier, Henry W. & Venkatarathnan Kolla, 2003, "Seismic Geomorphology and Stratigraphy of Depositional Elements in Deep-Water Settings", Journal Sedimentary Research, Vol. 73, No. 3, P. 367–388
Reading, H. G., & Richards, M. (1994). "Turbidite systems in deep-water basin margins classified by grain size and feeder system". Bull. Am. Ass. Petrol. Geol., 78, 792-822.
Samson, Timothy M., Cruse, Anna M., and Paxton, Stanley T., 2006, "Spectral Gamma Ray Logs as Paleoenvironmental Indicators In Carboniferous Black Shales", Geological Society of America Abstracts with Programs, Vol. 38, No. 4, p. 23
Sercombe, William J. and Radford, Thomas W. 2007, "Deep Water Gulf of Mexico High Gamma Ray Shales and their Implications for Flooding Surfaces Source Rocks and Extinctions", AAPG & AAPG European Region Energy Conference and Exhibition, Technical Program
Stow, D.A.V., 1994. "Deep-sea processes of sediment transport and deposition. In: Sediment Transport and Depositional Processes", ed. by K. Pye, Blackwell Sci. Publ. pp 257-293.
Stow, D.A.V. and Mayall, M., editors, 2000. Deep-water Sedimentary Systems: Thematic Set, Marine and Petroleum Geology, Volume 17, No. 2.
ten Veen, Johan H. and Postma, George, 1996, "Astronomically forced variations in gamma-ray intensity; late Miocene hemipelagic successions in the eastern Mediterranean Basin as a test case", Geology; January v. 24; no. 1; p. 15-18