Petrography and internal structures of MDAC tubes
(Extracted from Blouet, PhD thesis)


Annular structures of infill sediments that represent different generations of depositional event can generally be observed on latitudinal and longitudinal sections of MDAC tubes (cf.  Krause et al., 2009; Haas et al., 2009; Agirrezabala, 2009). Optic and electron microscopic observations indicate that the internal  infills of MDAC tubes are  most of time composed by micrite, as well sparry, blocky and/or fibrous calcite, aragonite  (cf. Campbell et al., 2002; Agirrezabala, 2009). In some cases solid bitumen is presented in the cement (click here) (Agirrezabala et al., 2008).  

The example below is a MDAC tube in our study which exhibits a complex sequence of sedimentary infills and indicates a series of diagenetic events (fig. 1).

Fig. 1. A thin section of a latitudinal section of MDAC tube. Right) See in nature light. Left) Annotations indicate different kinds of infills.


Sedimentary infills can be very localized at specific parts inside MDAC tubes. A longitudinal section of the tube demonstrate that a  same paragenetic sequence is not necessary present all along the tube and can be absents at certain locations.

A longitudinal section of a studied MDAC tube's segment (Fig. 2) shows asymmetric depositional layers inside the central conduit (Fig. 3).

Calcite-filled cracks are observed cutting cross the central conduit or extended along the latter's side wall (Fig. 3).
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Fig. 2. A segment of a MDAC tube has been chose to study its internal structure. Upper) Zoom into the segment which is studied and is highlighted by a yellow square. Right) Zoom out of the tube.
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Fig. 3. Internal structure of the MDAC tube shown in Fig. 2. The location of this segment is highlighted by a yellow square in Fig. 2 above. The longitudinal section demonstrates that some areas inside the tube may have more or less infill deposition and a heterogeneous distribution of micritic cements.
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Fig. 4. Interpretation for infill deposition inside the MDAC tube of Fig. 3. The chronology of paragenetic sequence is noted by numbers.


 Results from several geochemical analysis (not shown in here) demonstrated that infill events of the MDAC tube were complex, and the superimposed order of infill layers were not necessary indicated the chronological order of the infill depositions (Fig. 4).

 A reconstruction of order of infill deposition is based on the geochemical results (not shown in here) as well referred to the infill geometries (Fig. 5).



On the latitudinal section of the MDAC tube in Fig. 1, a  more detailed structure of the internal infill is revealed under microscopy. Fine lamination of sand layers are paralleling to the micritic layers (fig. 6).

Fig. 6. A thin section of a latitudinal section of MDAC tube under microscopy. Left) The thin section shown in fig. 1 in earlier. Middle) A zoom in view of the white square area in picture in the left. Laying structure is visible. Right) Laminating sand layers are highlighted by black dot line 



Hypothesis on the formation of MDAC tube and the infill process
(Presented in 11th Conference of Gas in Marine Sediment by Blouet, J. P., Imbert, P. & Ho, S., 2012)

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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
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Planar view of the tube's latitudinal section
Key words: Jean-Philippe Blouet fault Sutieng Ho methane authigenic carbonate