Mass Transport Complex: Musing on Past Uses and Suggestions for Future Directions
Authors: Weimer Paul , Shipp Craig 
Published: October 2008

Mass-transport complexes (MTCs) are significant deposits in deepwater settings. The term MTC is a seismic stratigraphic term and can only be applied to features at a scale that can only be completely imaged on volumetrically large seismic surveys. MTCs vary in size and shape, from filling one intraslope basin to several 1000's of square km in unconfined settings. MTCs can vary in thickness from 5 m to 100's of m. Their upper surface is usually irregular, and it commonly eroded by overlying channel and related deposits. Basal surfaces vary from planar, to erosional, to stair step. Internal facies consists of (a) rotated and translated blocks, (b) thrusted blocks, and (c) chaotic facies. Where MTCs have been cored, they consist primarily of clay-rich sediments. In the future, we hope that non-proprietary information on MTCs can be shared in a public forum to improve the geoscience and geotechnical community's understanding of these features.

Introduction,

Mass-transport complexes (hereafter MTCs) constitute large volumes of sediments in deepwater settings. During the past decade, the extensive interpretation of 3-D seismic data by many petroleum companies has indicated that these deposits are quite common along most deepwater margins. In some basins, individual sequences in the upper Pleistocene may consist of more than 50 percent slides/deformed sediments- for example, deepwater Brunei, 50%, (McGilvery and Cook, 2003); offshore Nile- average of 50%, in some areas up to 90% (Newton et al., 2004); and offshore Trinidad- 50%. MTCs are rarely primary exploration targets in siliciclastic settings. However, these deposits important to study because (1) they constitute important aspects of deepwater sediment fill, (2) they can be important regional seals, and, most critically, (3) understanding their distribution in the shallow subsurface is important both for drilling hazard assessment and field development planning (Shipp et al., 2004). Specifically, the transportation and deformation of MTCs causes the expulsion of water (Piper et al., 1997). As a consequence, these features are commonly overcompacted in the shallow subsurface, so that drilling through these features can significant decrease in drilling time. With rig costs in deepwater averaging $0.25 to 0.4 million/day, shorter drilling times are imperative; accomplishing this requires the detailed study of these features. In addition, understanding the distribution of the upper 10's of m of sediments in the slope is important for deployment of subsea infrastructure.

Original Definitions

Weimer (1989, 1990) originally defined the term mass transport complex " as sediments that occur at the base of sequences and are overlain and/or onlapped by channel and levee sediments. They commonly overlie an erosional base upfan becoming mounded downfan, are externally mounded in shape, and pinch out laterally"seismic facies (are) hummocky and mounded reflections with poor to fair continuity and variable amplitude" (Figure 1). In its original usage, MTC had a sequence stratigraphic connotation and was used to distinguish it from the generic term "slide."

Jackson (1997) defined slides as "a mass movement or descent from failure of earth...or rock under shear stress along one or several surfaces"the moving mass may or may not be greatly deformed, and movement may be rotational or planar."