AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Mass transport complexes (MTCs), their internal geometry and control on overlying turbidite deposits: Barreirinhas Basin (Equatorial margin off north-east Brazil) as a case study

Abstract

Mass transport complexes (MTCs), also known as mass transport deposits (MTDs), are forms of mass movement that are primarily gravity-driven (Armitage et al., 2009). These type of deposits can cover extensive areas (>104 km2), have thicknesses in excess of 102 m (in some cases, as can be seen by an example in the Barreirinhas Basin, the thickness of an MTC can reach 103 m) and contain sediment volumes greater than 103 km3 (Kneller, 2016). It is important to understand these deposits as they constitute as much as 50% of deep-water basin fill and can have major implications for hydrocarbon exploration targets (Weimar and Slatt, 2004). Wireline-log responses (and core data) from within MTCs confirm a high percentage of shale (Piper et al., 1997); this makes MTC deposits viable top and base seals, and, in stratigraphic trap settings, MTCs too can provide important up-dip traps if they truncate sand-filled channel or fan deposits. From a detailed outcrop study, Armitage et al. (2009) were able to categorize the MTC surface topographies into three tiers, each separated by an order of magnitude. Tier 1 is too small to be observable on seismic; however, tiers 2 and 3 are visible on a seismic scale and represent ten to several tens of meters, and hundred to several hundreds of meters in magnitude (respectively) and are able to significantly compartmentalize sandstone deposits. Deposition of turbidites on top of MTCs can be affected in the following ways: (1) MTC failure may create accommodation space by sediment evacuation and thus create conduits favorable for focusing turbidity currents, (2) obstacles created by MTCs can re-direct turbidity currents, and (3) irregular top surfaces of MTCs create a favorable local topography for sediment ponding (Armitage et al., 2009). In the Barreirinhas case study, one can observe, on high quality 3D seismic, primarily type tier 3 features (as described by Armitage et al., 2009). It can be seen that sand deposition above the MTC (these sheet sands are formed by turbiditic flows that transport proximal sands into deeper parts of the basin) occurs preferably along the edge of the MTC deposit. Along its longitudinal axis, where the deposit is thickest, the MTC has more chaotic internal reflections and a more irregular top surface. A possible explanation for the more chaotic internal reflectors within the center of the deposit could be that the internal velocity within the MTC is expected to be higher within the interior in comparison to its edges. It appears that the fan preferentially deposits on top of the more flattened edge of the MTC deposit. Imbricate thrusting found in MTC deposits reflect the compaction that can be observed within an MTC. These can be around 1 km in width and create baffles for sediment deposition. This too creates accommodation space and results in localized ponding of sandy deposits. If enough sand is brought in by the turbiditic flow these ponded deposits will fill to spill and create tortuously interconnected bodies, as is described by Smith (2004). At seismic scale, it is not always possible to determine whether adjacent ponded sandstone deposits overlying MTCs are connected or not. As can be seen in the Barreirinhas Basin, MTCs have a variety of different aspect ratios (width-to-height ratio), with higher aspect ratios being more conducive for larger turbidite deposition above its surface.