Seafloor and Shallow Subsurface Examples of Mass Transport Complexes, Offshore Brunei

Abstract

Abstract Two distinct depositional elements attirubuted to gravity mass wasting events along the continental slope Offshore Brunei are defined as local cohesive slump complexes and large-scale mass transport complexes. Cohesive slump complexes are relatively small (< 15km across) in comparison to the mass transport complexes. They record local slump or down slope creep that typically occurs along the basinward margins of syndepositional structures. The internal seismic character of these elements is organized and commonly exhibits extensional features along their updip margins and compressional features in their down dip ends. Mass Transport Complexes can be extemely large (> 100km long, 50km wide, and 250m thick). Seismic facies are characterized by a "matrix" of dim and chaotic amplitudes punctuated by "blocks" of organized, high amplitude facies The organized units are interpreted as rafted cohesive slump blocks up to 1.5km across and 100m thick. Basal surfaces of mass transport complexes are characterized by high-amplitude continuous reflections that commonly exhibit elongate grooves in map view attributed to gouging of the substrate by the rafted blocks during transport. The upper surfaces of these complexes at the seafloor are very hummocky, with local relief exceeding 100m. This relief corresponds to cohesive blocks that have been rafted to the top of the flow. The largest complexes are divided into subunits by locally mappable surfaces defined by continuous trough reflections. This suggests that they are composite features preserving multiple mass wasting events. There has been some debate regarding the distance of transport of the individual flows within composite complexes. Shallow subsurface examples document features that indicate short transport distances and others suggesting long transport distances. Short distance transport of 100's of meters to a few kilometers is indicated by:lobate external geometry,arcuate glide planes or pressure ridges perpendicular to flow within the features, and lack of elongate and linear erosional grooves. Long distance transport over distances of 10's of kilometers is indicated by:elongate external geometry,divergent grooves along the margins and at the terminus of the feature, andpresence of elongate linear grooves parallel to flow. Introduction Several outstanding examples of depositional features attributed to matrix supported flows that are the product of gravity mass wasting events have been documented on the modern seafloor and shallow subsurface offshore Brunei. A 10,000km2 survey of full offset, 16bit, 3D seismic data across the continental slope was used in the study. A series of seismic profiles, interval and horizon slice amplitude extractions, isochron maps, and 3D perspective views are presented to illustrate the observed mass wasting features. A shaded relief map of the waterbottom with a max-positive amplitude overlay provides a regional overview of the middle to lower ontinental slope (Fig. 1). This map documents the array of depositional elements developed on the sea floor and illustrates the relationship between sediment dispersal pathways and structurally controlled bathymetry (McGilvery and Cook, 2003). A series of strike elongate structures parallel to the shelf edge divide the continental slope into a stepped slope characterized by elongate, mini-basins 2-10 km wide and 20-60 km long.