Bathymetric Controls on Mass Transport Flow Pathways and Sedimentation

Abstract

Structural deformation in tectonically active margins can be accompanied by mass wasting and the emplacement of mass transport complexes (MTCs). MTCs may both erode and deposit along their flow pathways, generating topography that affects later flows and their deposit characteristics. Here, we investigate the role that bathymetrically complex substrates may play in modifying debris flow erosion and sedimentation patterns. The study is based on detailed near-surface mapping of a 1900 km² 3D seismic volume from the southern Magdalena Fan, offshore Colombia. The study area is characterized by two NE-trending anticlines that form the northern tip of the Southern Sinú Fold Belt. In front of the structures, a 200 m thick MTC overlies an erosional surface. The horizons interpreted as the top and base of the deposit can be continuously mapped across the study area; however, grooves on the basal shear surface reveal three transport directions linked to separate headwalls along the frontal fringe of the fold belt, and thus indicate the coalescence of several flow events into a composite MTC. Here we focus on the central MTC, which can be traced updip across the northern plunge of the southern anticline to the steep frontal limb of another structure at the edge of the data set. The emplacing flow initially trended NW, but was deflected to the SW upon encountering the back limb of the northern anticline. It changed trend again where it crossed the southern anticlinal structure through a narrow notch. Updip of this structure, the erosional relief is 500 m, and the deposit is 6 km wide, 100 m thick, and is composed of debrites and isolated blocks. As it traversed the structure, basal relief reduced to 250 m and the deposit, which is mainly composed of blocks, is only 40 m thick and 3 km wide. Downdip of the structures, the erosional surface widens to 16 km and is almost entirely filled by a 200 m thick unit composed of chaotic and folded reflectors and blocks. Here, the crest of an underlying levee directed the flow to the SW and is overlain by blocks aligned parallel to the direction of flow. The response of MTCs to underlying bathymetry can be reflected in their flow pathways, erosional relief and in the characteristics of the deposits. The presence of blocks, internal deformation, changes in thickness and matrix content can all be influenced by the interaction between MTCs and irregular substrates.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015