Rift and salt-related multi-phase dolomitization: example from the northwestern Pyrenees

Abstract

The Meillon (Callovo-Oxfordian) and Mano (Tithonian) Formations are dolomitized carbonate reservoirs that actively produce oil and gas (Aquitaine Basin, France). In this study, the dolomitization conditions of their counterparts exhumed in the northwestern Pyrenees are detailed using a combination of field observations, petrography, fluid inclusion microthermometry, elemental and isotopic geochemistry, and carbonate U-Pb geochronology. Dolomitization occurred in several stages spanning from the Neocomian (pre-rift) to the Albian (syn-rift, associated with mantle exhumation and active salt tectonics). Both formations were first massively dolomitized in near-surface to shallow burial conditions during the Berriasian-Valanginian, likely triggered by the influx of marine-derived waters. Between the Barremian and the Albian, the Early Cretaceous rifting caused the upward influx of hot fluids associated with the partial to complete recrystallization of the initial dolomites. During the Albian, subsequent dolomites precipitated in both formations as high-temperature (T > 160 • C) veinand pore-filling cement. Distinct fluid inclusion chlorinities and rare earth element patterns between the Meillon and Mano Formations point to fluid compartmentalization during this stage. Whereas dolomite cements indicate the involvement of evaporite-derived brines in the Meillon Formation, precipitation was likely related to clayderived water in the Mano Formation. Lastly, a final episode of dolomite cementation occurred only in the vicinity of faults and volcanic intrusions during the Albian when the highest temperatures were recorded in both formations (T > 250 • C). These saddle dolomites precipitated from hydrothermal water with a mixture of mantle-, crustal-, and evaporite-derived waters channeled by faults and active diapirs. Subsequent quartz and calcite cement precipitation reveals a temperature decrease in a post-rift to inversion setting (post-Cenomanian) and indicates fluid compartmentalization between both formations. This study highlights the major control exerted by rifting, combined with the presence of diapiric salt, on dolomitization, making carbonate platforms of modern salt-rich passive margins potential targets for exploration.

Triassic evaporites: a vast reservoir of brines mobilized successively during rifting and thrusting in the Pyrenees

“…% eq. NaCl) are observed in dolomites (DC2 to DC3 in Mano formation, Motte et al, 2021) related to early diagenesis. Syn-rift fluids have Th ranges from 130 to 280°C, with a few data between 300 and 340°C (dolomite rim, Salardon et al, 2016, syn-rift Dolomite DC4 -Mano, Motte et al, 2021.…”

The North Pyrenean Frontal Thrust: structure, timing and late fluid circulation inferred from seismic and thermal-geochemical analyses of well data

“…A single study shows fluid circulations related to the extensive stage using structural evidence (Salardon et al, 2017). A few studies have used absolute dating to determine the exact timing of fluid circulation (Incerpi et al, 2020;Motte et al, 2021). So far, they have only obtained ages showing fluid circulation during the extension phase.…”

“…This is consistent with the late fluid circulations observed in the Chaînons Béarnais (NPZ) showing a high salinity around 18 wt % eq. NaCl (Motte et al, 2021). In the Aquitaine Basin, compression fluids have very low salinities around or below seawater (Renard et al, 2019;Elias-Bahnan et al, 2020), while the late fluid circulations observed in the South Pyrenean Zone have low salinity around 7 wt% eq.…”

“…Source, nature and timing of the fluid circulations in the central part of the South Pyrenean Zone are well documented for Cenozoic synorogenic deformation phases (e.g., Rye and Bradbury, 1988;Travé et al, 2000;Lacroix et al, 2014;Beaudoin et al, 2015;Crognier et al, 2018;Cruset et al, 2018). In the North Pyrenean Zone and Aquitaine Basin most studies dealing with fluid circulations have focused on the Mesozoic syn-rift history (Salardon et al, 2017;Corre et al, 2018;Quesnel et al, 2019;Renard et al, 2019;Elias-Bahnan et al, 2020;Motte et al, 2021;Cathelineau et al, 2021), the syn-orogenic imprint remaining poorly constrained. One of the main syn-orogenic faults carrying fluids in the north of Pyrenees is the North Pyrenean Frontal Thrust (NPFT; Connan and Lacrampe-Couloume, 1993) which separate the Aquitaine Foreland Basin from the North Pyrenean Zone (NPZ; Fig.…”

Multiple fluid flow events from salt‐related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees)

“…Finally, Rd3 dolomite exhibits saddle textures, indicating temperatures of precipitation between 60 and >250°C (Gregg & Sibley, 1984;Motte et al, 2021;Sibley & Gregg, 1987;Sijing et al, 2014). This dolomite replaces carbonate breccias made of fragments of Sinemurian limestones and cement Cc11 with U-Pb ages from 37.1 ± 0.7 Ma to 34.4 ± 0.9 Ma at the vicinity of the Coll del Verdet, which has been interpreted as a secondary weld zone by Saura et al (2016).…”