3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

Abstract

The resolution of data acquired over modern seafloors does not allow imaging of the inner features of a fluid seep structure, particularly in the shallow subsurface. In the SE Basin of France (Drôme), fossil cold seep structures comprising fossil-rich carbonate lenses were identified about 30 years ago within the Oxfordian (Late Jurassic) Terres Noires Formation. These structures were first interpreted as pseudo-bioherms related to hydrothermal activity, but comparison with active seep sites on modern margins, together with isotopic analyses, led to a reinterpretation involving cold fluids instead. To date, all seep sites have generally been studied individually without considering any link to neighbouring or more distant sites. Based on 23 high-resolution stratigraphic logs within the structure coupled to mosaicked aerial photographs from a drone survey, 19 fluid seep events were correlated in the area, including two new sites exposed as a result of weathering. We have shown that each identified sub-site is composed of subvertically stacked fossil-rich carbonate lenses interbedded with marls, which developed in smooth, 4–6 m deep depressions beneath the local seabed. The nodules present within the depressions are of primary importance as they mark the area of active seeping. This general organization is very similar to the modern Regab giant pockmark in the Lower Congo Basin where only a few sub-sites are active at the same time. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized into clusters comprising up to seven sub-sites grouped together in the same stratigraphic interval and the same geographical zone. A sandbox experiment where gas is injected at constant flow rate at the base of a box filled with a matrix of water-saturated grains displays a pattern consisting of disturbed sediments inside a parabolic-shaped area. This parabolic shape was also identified on a seismic profile across the Regab giant pockmark, suggesting that the processes are similar for the Beauvoisin and Regab seep areas. The laboratory experiments also show that the seeping conduit is stable during a given period of time and suddenly shifts laterally. This is mainly as a result of the collapse of the conduit, the lateral migration and the reopening at a new position. The general log obtained in the Beauvoisin seep area suggests a similar pattern with periods of seeping alternating with periods of quiescence, each of which is c . 200 kyr. Even if a pockmark seems to have been inactive for a long period of time, this could be due to the lateral shift of the feeder conduit, meaning that the sub-seafloor is still charged with gas. This is of primary importance for risk assessment, hydrocarbon exploration and general understanding of geobiology at seafloor seeps.

The abyssal giant pockmarks of the Black Bahama Escarpment: Relations between structures, fluids and carbonate physiography

“…This is consistent with the model of Gay et al (2019) showing sporadic decametric seeps within a giant pockmark depression that act at different times as a function of available pathways in the sedimentary column and imply fluids lateral migration.…”

A record of seafloor methane seepage across the last 150 million years

“…reflection seismic) allow recognition of pockmarks and mud volcanoes on the modern seafloor and in the subsurface, which have been associated with discrete events of past methane release 27,28 . However, their occurrence in the fossil record is rare in the literature [29][30][31] , demonstrating a low preservation potential. The formation of gas hydrates has been investigated in various regions of the world 32 and is often recognised in seismic data though a bottom-simulating reflector (BSR) that marks the contact between possible gas hydrates (above) and free gas (below).…”

Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

“…The carbonate, lucinid and gastropod-bearing, constructions of Beauvoisin developed under synsedimentary conditions, during the Jurassic in the South-Eastern Basin of France (Artru, 1972;Bourseau, 1977;Rolin, 1987;Gaillard et al, 1996;Peckmann et al, 1999;Kiel, 2013;Tribovillard et al, 2013;Gay et al, 2019). Prior to the development of the pseudo-bioherms, from the Lias to the Dogger the shallowwater carbonate platform underwent frequent subaerial exposure or erosion indicating a stable margin with a slow rate of subsidence (Lemoine, 1985).…”

“…The Late Oxfordian is characterized by the so-called Argovian sequence composed of thick amber-colored dolomudstone layers alternating with light brown marls in a general thickening upward sequence. Within the giant pockmark of Beauvoisin the thickness of the Argovian sequence varies by a factor of 3 to 4 which could be due to syn-to-post depositional erosion or slump processes on slopes of the depression (Gay et al, 2019). Subsidence was mostly due to basement faults with kinematics controlled by salt withdrawal in the extensional domain of the margin (Mascle, 1988).…”

“…It was first interpreted as a hydrothermal vent (Rolin, 1987;Gaillard et al, 1996) and then reinterpreted as a cold seep site (Peckmann et al, 1999;Kiel, 2013). It is now considered as a long-lasting (3.44 Ma) pockmark in the vicinity of Triassic salt diapirs, active during the Oxfordian (Tribovillard et al, 2013;Gay et al, 2019). The carbonate constructions have been also termed "pseudo-bioherms" by the pioneering studies of authors from the University of Lyon (France; e.g., Artru, 1972;Bourseau, 1977;Gaillard et al, 1985).…”