Carbonate mounds and fluid-seeps on the sea-floor?
Carbonate mounds are relevant geological markers for localizing an underlying focused fluid-flow system connecting a reservoir at depth through a network of preferential pathway (Gay et al., 2020). We basically provide a relevant field example for:
– (i) helping seismic interpreters to « ask » themselves: What could be the seismic response of such feature?
– (ii) Helping geoscientists for linking such carbonate building to an underlying fluid-flow system which needs to be understood.
More details about this outcrop can be found in the detailed works of Gay et al. (2019, 2020). The studied carbonate mound of this webpage is around 15 m high.
What is a carbonate mound?
Carbonate mounds are commonly made of calcareous building organisms living in relatively deep water environments on the continental slope (200 – 1000 m), right above seafloor fluid seepages (Ercilla et al. 2021). Fluid circulation leading to carbonate mound growth can be related to either hydrothermal hot springs, petroleum, sulfur or methane cold seeps (Moore and Wade., 2013). Most of the carbonate mounds are commonly less than 500 m in elevation. Undiscovered 10m-scale carbonate mounds are probably omnipresent at the present-day seafloor as well as into the fossil record (buried carbonate mounds).
« Carbonate mounds » outcrop details (Gay et al. 2020)
« The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls (Gay et al., 2020).
(i) Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian.
(ii) The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels.
(iii) The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene.
The fluids migrated through faults induced by the emplacement of Triassic- salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time. »
A seismic response for « carbonate mound »?
This seismic profile (with a pseudo-relief seismic attribute) shows a probable buried carbonate mound. The structure is 100 m-wide and is interestingly located in the footwall of a normal fault (confidential location). This fault could have provided the fluids leading to carbonate mound building when this latter feature was at the seafloor.
Such as sketched above in the outcrop interpretation, could we expect a seismic diffraction? (e.g. Schwarz, 2019).
Modern carbonate-cemented nodules and shell fragments have been identified on the sea-floor at -2167 m (Paull et al., 1995) ; They were associated with current-day gas-rich plumes (methane) located in the water column up to 320 m above the seafloor.
A core description?
A fictive borehole called « well A » would show cores made of carbonate lenses, cm-scale shells, surrounding nodules, and tubes (enclosed picture). A detailed sedimentological log is provided in the publication of Gay et al. (2019).
For instance, pseudo similar modern carbonate-cemented nodules and shell fragments have been identified on the sea-floor at -2167 m (Paull et al., 1995) ; They were associated with current-day gas-rich plumes (methane) located thtat have been detected into the water column up to 320 m above the seafloor.
Clyde H. Moore, William J. Wade, Chapter 6 – Marine Diagenetic Environment,Editor(s): Clyde H. Moore, William J. Wade,Developments in Sedimentology,Elsevier,Volume 67,2013,Pages93-131, https://doi.org/10.1016/B978-0-444-53831-4.00006-9
Gay, H., Alexiane Favier, Jean-Luc Potdevin, Michel Lopez, Delphine Bosch, Nicolas Tribovillard, Sandra Ventalon, Thibault Cavailhes, Martin Neumaier, Sidonie Revillon, Anna Travé, Olivier Bruguier, Doriane Delmas, Christophe Nevado; Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France. Bulletin de la Société Géologique de France 2020;; 191 (1): 35. doi: https://doi.org/10.1051/bsgf/2020036
Ercilla, G., David Casas, Belén Alonso, Daniele Casalbore, Ferran Estrada, Javier Idárraga-García, Nieves López-González, Mayte Pedrosa, Manuel Teixeira, Olga Sánchez-Guillamón, María Azpiroz-Zabala, Patricia Bárcenas, Francesco L. Chiocci, Marga García, Jesús Galindo-Zaldívar, Adelina Geyer, María Gómez-Ballesteros, Carmen Juan, Eleonora Martorelli, M. Pilar Mata, José Nespereira, Desiree Palomino, José Rueda, Juan Tomás Vázquez, Mariano Yenes, Deep Sea Sedimentation, Reference Module in Earth Systems and Environmental Sciences, Elsevier,2021, https://doi.org/10.1016/B978-0-12-818234-5.00129-2.
Paull, C. K., Ussler III, W., Borowski, W. S., & Spiess, F. N. (1995). Methane-rich plumes on the Carolina continental rise: associations with gas hydrates. Geology, 23(1), 89-92.