Thermal maturity assessment study of the late Pliensbachian-early Toarcian organic-rich sediments in southern France: Grands Causses, Quercy and Pyrenean basins

Highlights

• Thermal maturity of late Pliensbachian-early Toarcian organic-rich sediments.

• First reflectance data for Jurassic hydroids.

• Similar thermal evolution for the Grands Causses and Quercy basins.

Abstract

Thermal maturity of late Pliensbachian-early Toarcian organic-rich sediments in the Grands Causses (Suèges section), Quercy (Caylus section) and Pyrenean (Pont de Suert section) basins was determined through multiple parameters, including Spore Coloration Index (SCI), hydroid random reflectance (HR_r) and spectral fluorescence of Tasmanites algae (λ_max). The main objective of this study is to test the effectiveness and make comparisons of organic matter thermal maturity in these three sections by different techniques and particularly hydroid reflectance.

For the Suèges section SCI presented a value of 3.5–4.0. HR_r ranges between 0.36% and 0.47%, which corresponds to 0.45%–0.52% VR_eq, and shows a good correlation with SCI values. Spectral fluorescence analysis presents a λ_max of 560 nm for most samples. The fluorescence spectral maximum seems to be redshifted in comparison to other thermal maturity parameters.

The Caylus section exhibited SCI of 3.5–4.0. HR_r ranges between 0.32% and 0.50% corresponding to 0.42%–0.54% VR_eq. Spectral fluorescence analysis points out a λ_max of 560 nm, displaying a good correlation with SCI. When R_eq values determined through SCI are compared with equivalent vitrinite reflectance values determined through λ_max, a redshift of the fluorescence spectrum maximum is detected.

For the Pont de Suert section SCI ranges between 8.0-8.5 and 8.5–9.0. HR_r varies between 0.85% and 1.18% which corresponds to 0.76%–0.98% VR_eq. Even though there are lower values than those obtained by SCI, VR_eq is reliable and acceptable to determine the thermal evolution stage. Furthermore, HR_r values also present a good correlation with SCI.

These data indicate that the majority of the samples from the Suèges and Caylus sections are in the immature to early mature evolution stages and Pont de Suert section samples are in late mature to early overmature evolution stages for liquid hydrocarbons generation, supporting a very similar thermal evolution for the Grands Causses and Quercy basins and an entirely different thermal history for the Pyrenean Basin. This is in accordance with the similar tectonic and sedimentary context of the Quercy and Grands Causses basins. The Pont de Suert section, located near the collision front of the Pyrenees mountains, records a more complex sedimentary and tectonic history. In addition, this study presents the first reflectance data for Jurassic hydroids, which show a good correlation with other rank parameters.

Introduction

During the geological history of sedimentary basins, the organic matter (OM) goes through a physiochemical transformation that is controlled firstly by biological activity followed by thermodynamic factors (Tissot and Welte, 1984). These progressive and almost continuous changes in the chemical and physical properties of the sedimentary OM reflect and are an indicator of the thermal and burial history of the basins (Tissot and Welte, 1984). To quantify thermal maturity of sedimentary rocks, several optical parameters were developed during the years, being vitrinite and huminite reflectance the most commonly used. However, when absent, other organic particles of different origins (e.g. solid bitumen, zooclasts) can be used as alternative to assess thermal maturity (Hartkopf-Fröder et al., 2015).

Animal-derived organic particles are often reported as minor components of the sedimentary OM. These components can be classified as belonging to: (i) the Zoomorph Subgroup, when composed by discrete unitary animal-derived particles, whether whole or fragmented, including foraminiferal test-linings, chitinozoans, and scolecodonts; and, (ii) the Zooclast Group, which comprises unknown organic, structured, fragmentary particles (clasts), with an angular outline (the most common varieties include graptolites, conchostraceans and other crustacean fragments, arthropod exoskeletal debris, organic linings from some bivalve shells, tintinnids, and insect cuticle fragments) (Mendonça Filho et al., 2014c, Mendonça Filho et al., 2012, Tyson, 1995).

Mendonça Filho et al. (2016) described for the first time the occurrence, in dispersed OM, of Cnidarians (Phylum Cnidaria), Class Hydrozoa (Hydrozoans), and Hydroida Order (Hydroids); namely its free-swimming medusoid forms. Recently, Mendonça Filho and Gonçalves (2017) included these animal components in the classification system of the individual palynological components from the Zoomorph Subgroup, together with cuticles of ostracods, within the Zooclast Group, and Fonseca et al. (2017) presented the first paleoenvironmental application of Hydroids in a palynofacies based study of the Toarcian Oceanic Anoxic Event organic record of the Grands Causses and Quercy basins (southern France). In literature, the fossil record of these organisms is mostly restricted to forms with a carbonate skeleton. Nonetheless, Eisenack, 1935, Eisenack, 1934, Eisenack, 1932) defined 4 genera of Hydrozoa polypoid forms with chitin skeletons (2 from the Ordovician, 1 Silurian and 1 Jurassic). Kozlowski (1959) described 15 genera and 22 species of polypoid forms of Ordovician Hydroids with chitin skeletons and doubted Eisenack, 1935, Eisenack, 1934, Eisenack, 1932) conclusions due to sample fragmentation. Bertrand (1987), based on Kozlowski's (1959) descriptions, presented the first reflectance measurements on Ordovician Hydroids.

Hydroid remains were described as being consistently observed in polished blocks of Ordovician to Devonian marine sedimentary rocks (Bertrand and Malo, 2012, Héroux et al., 2000). Nevertheless, these remains are usually less often recorded compared to graptolites, chitinozoans or scolecodonts (e.g. Bertrand, 1987, Héroux et al., 2000), possibly due to identification issues. Bertrand, 1991, Bertrand, 1990 and Bertrand and Malo (2012) suggested that hydroids can be a major constituent of the zooclast/zoomorph fraction in proximal marine sedimentary settings, although hydroid reflectance has been used only as a supplementary maturity parameter, compared to reflectance of graptolites, chitinozoans and scolecodonts, which are more often employed to determine the equivalent vitrinite reflectance (e.g. Bertrand, 1990, Bertrand, 1987, Bertrand and Malo, 2012, Bertrand and Malo, 2005, Bertrand et al., 2003, Héroux et al., 2000, Héroux et al., 1996). Hartkopf-Fröder et al. (2015) suggested that this may be due to lower abundance of hydroids compared to other zooclasts and zoomorphs. Mendonça Filho et al. (2016) suggested that hydroid fragments have probably been classified as undifferentiated zooclasts due to lack of knowledge and experience in their identification. Nevertheless, hydroid reflectance has been used in a few previous maturation studies of Ordovician and Silurian sedimentary rocks (Bertrand, 1993, Bertrand, 1990, Bertrand, 1987, Héroux and Chagnon, 1994, Héroux et al., 2000, Héroux et al., 1996, Bertrand et al., 2003, Bertrand and Malo, 2012, Bertrand and Malo, 2005).

The description of the first occurrence of components from the Phylum Cnidaria, Class Hydrozoa, Order Hydroida in the record of particulate OM of the late Pliensbachian – earliest middle Toarcian sedimentary successions of the Grands Causses and Quercy basins in southern France was made by Fonseca et al. (2017). This opens the opportunity to pursue a thermal maturity study of the dispersed OM of this time interval in different basins of southern France, especially focused and based on the hydroid reflectance.

The Lower Jurassic sequence in southern France has been the main focus of stratigraphic, sedimentological and geochemical studies for decades (Cubaynes, 1986, Cubaynes et al., 1989, Emmanuel et al., 2006, Harazim et al., 2013, Trümpy, 1983, and references therein). However, no emphasis has been given to the study of the thermal maturity of the OM present in the organic-rich carbonate Toarcian sedimentary rocks of the basins in this region. Therefore, this study aims: (i) to estimate and compare the OM thermal maturity, through organic petrology techniques, of the upper Pliensbachian-lowermost middle Toarcian sedimentary successions of the GCB, QB and PB; and, (ii) to assess the consistency of values obtained using different techniques (Spore Coloration Index, spectral fluorescence and hydroid reflectance). So far, no published data on reflectance of Jurassic hydroids are available, and their use as a major index for the assessment of sedimentary OM maturity has not been attempted. This paper, then, is the first approach to the use of hydroids as a key thermal maturity parameter for OM of this age.