Review ArticleFormation of microbial organic carbonates during the Late Jurassic from the Northern Tethys (Amu Darya Basin, Uzbekistan): Implications for Jurassic anoxic events
Introduction
The Late Jurassic is a period marked by multiple anoxic events (e.g., Trabucho-Alexandre et al., 2012; Martinez and Dera, 2015). More specifically, the Oxfordian and Kimmeridgian Stages are characterized by episodes of regional anoxia resulting in the accumulation and preservation of organic matter (OM) in several sedimentary formations throughout the world, e.g., the Staffin Shale Fm. (Middle Callovian–Lower Kimmeridgian, Isle of Skye, Scotland; Pearce et al., 2005), the Knollenkalk Fm. (Middle Oxfordian, Jura, Switzerland; Rais et al., 2007), La Manga Fm. (Middle–Upper Oxfordian, Neuquen Basin, Argentina; Palma et al., 2015), the Hanifa Fm. (Oxfordian–Lower Kimmeridgian, Central Arabian Basin, Saudi Arabia; Enay et al., 1987; Droste, 1990; Sharland et al., 2001; Vahrenkamp et al., 2015), the Smackover Fm. (Oxfordian–Kimmeridgian, Gulf Coast, USA; Oehler, 1984), the Spiti Shale Fm. (Oxfordian–Berriasian, Malla Johar, India; Jain et al., 1984; Jadoul et al., 1998), the Ringnes Fm. (Oxfordian–Kimmeridgian, Sverdrup basin, Canada; Mukhopadhyay et al., 1997), the Akkuyu Fm. (Lower Kimmeridgian–Tithonian, Taurus Mountains, Turkey; Baudin et al., 1999), the Kimmeridge Clay Fm. (Kimmeridgian-Tithonian, Dorset, UK; Tribovillard et al., 1994). A large number of these organic rocks show a fine-grained laminated fabric, sometimes called organic varves. This facies has been interpreted as mineralized microbial mats (or stromatolites) that formed in shallow stratified, hypersaline lagoons (Bernier and Enay, 1972; Tribovillard et al., 2000; Warren, 2011). Warren (2011) postulated that such laminated organic sediments could also reflect pronounced seasonal salinity and nutrient variations leading to blooms in primary productivity, which in turn would result in the formation of organic laminae.
At the global scale, the role of climate and tectonic on the multiplication of organic-rich formations during the Late Jurassic is still debated. On the Arabian platform, the Hanifa and Jubaila Fms. have accumulated in partially isolated and stratified intrashelf basins. In a recent review, Vahrenkamp et al. (2015) link the formation of such basins to a tectonic uplift at the margin of the Arabian Plate. Based on the presence of biosiliceous deposits, Eltom et al. (2017) have related the elevated organic productivity in these basins to regional upwellings developing at the shelf edge. Both models may explain the formation of organic sediments on large epeiric platforms (Warren, 2011). In addition, several authors proposed that the increase in organic productivity and the multiple anoxia recorded during the Late Jurassic could have been caused by elevated atmospheric pCO2 due to enhanced magmatic activity at oceanic ridges and in subduction zones (Jones and Jenkyns, 2001; Dera et al., 2011; Meer et al., 2014; Georgiev et al., 2017).
In the Western Tethyan domain, the Middle-Late Oxfordian interval is characterized by elevated carbon isotope values from bulk carbonate (δ13Ccarb) as reported by several authors (Bartolini et al., 1999; Louis-Schmid et al., 2007; Rais et al., 2007; O'Dogherty et al., 2018), interpreted as the result of palaeoenvironmental events (e.g., major currents reorganization, climate warming, sea-level rise, higher carbonate accumulation rates). Within this overall positive δ13Ccarb interval however, during the Transversarium Zone, a marked δ13Ccarb negative excursion was reported in the Jura and the Subalpine Basin by Padden et al. (2001). A less well-dated negative excursion was also documented in the whole Western Tethyan domain at the Middle-Late Oxfordian transition both in shallow water fossils (e.g., bivalves) and in deeper benthic organisms (e.g., belemnites; Martinez and Dera, 2015). By contrast, no well-dated δ13C record is available for this period in the Northern Tethys.
The Amu Darya Basin (ADB) is a vast sedimentary basin (ca. 400,000 km2) of the Northern Tethys. This basin hosts the organic-rich Khodjaipak Formation, which is a major source rock for hydrocarbons (Klett et al., 2006, Klett et al., 2012). This formation is dated from Late Oxfordian to Kimmeridgian in age (Abdullaev, 2004 or Mordvintsev, 2015). These 10–30 m thick deposits have been identified in more than two hundreds boreholes on the northeastern margin of the ADB (Besnosov and Mitta, 1995; Abdullaev et al., 2010), but outcrop only at two locations in the south-west Gissar mountain range (SW Gissar). In this study, field sedimentological investigations were carried out in one of these localities (Tubiegatan, Uzbekistan). To date, no detailed sedimentological, mineralogical or geochemical study has been published on these deposits. The Khodjaipak Fm. is located near the top of the Upper Jurassic carbonates of the Kugitang Series, and could have accumulated in depressions surrounding reefal buildups formed on the contemporaneous carbonate platforms (Mirkamalov et al., 2005). However, the absence of outcrops between the organic deposits and their potential stratigraphic counterparts in the SW Gissar implies a re-evaluation of the platform to basin correlations. The presence of ammonites in the Tubiegatan section however allows herein to specify the timing of the main paleoenvironmental events occurring at this period on the northern margin of the ADB.
In this study, the Khodjaipak Fm. organic deposits are characterized through a transdisciplinary approach including sedimentology, biostratigraphy, petrography, clay mineralogy, and organic and inorganic geochemistry. Our main objectives are to (i) determine the composition and the mode of formation of such organic deposits, (ii) propose a high resolution biostratigraphic and carbon isotope stratigraphic framework for the Tubiegatan section and (iii) establish the stratigraphic relationships with the contemporaneous carbonate platform in order to specify the depositional conditions of the Khodjaipak Fm. The resulting model will be compared to similar Upper Jurassic deposits worldwide in order to determine potential common patterns in their formation and the potential impact on carbon cycle of multiple organic carbon accumulations.
Section snippets
Geological setting
The Kugitang Series is a thick carbonate platform succession (~100–600 m) deposited on the NE margins of the ADB (Fig. 1A) from the Early Callovian up to the Kimmeridgian (Mirkamalov et al., 2005; Nugmanov, 2010; Carmeille et al., 2018). This series crops out in the southwestern Gissar mountain range (SW Gissar), in the northeastern part of the ADB. The Kugitang carbonate platform is subdivided into two major depositional sequences: (i) the Callovian Sequence (CS) equivalent to the Lower
Ammonite biostratigraphy and sequence stratigraphy
A total of 14 ammonites (entire or fragments) were collected along the measured section. Six of them were determined to the species level by one of us (RE) using the standard biozonation of the NW Tethyan domain (Cariou and Hantzpergue, 1997). A sequence stratigraphic scheme was built based on Maximum Flooding Surfaces (MFSs) and Maximum Regressive Surfaces or Sequence Boundaries (SBs), following the terminology of Catuneanu et al. (2011).
Petrography
A total of 90 samples were collected. To obtain a
Results and interpretations
Based on the vertical facies succession, we defined three lithostratigraphic intervals along the Tubiegatan section: (1) a lower carbonate interval; (2) an organic-rich marl and limestone unit corresponding to the Khodjaipak Fm. and (3) a second carbonate interval at the top. In terms of sequence stratigraphy, the whole succession can be interpreted as a transgressive-regressive third-order sequence with a proposed MFS located ca. 20 m from the basis of the section. This sequence can be
Organic matter
The Tmax from Rock-Eval analyses indicate the influence of a burial diagenesis consistent with the oil window (Fig. 7). The HI and OI values plotted in the Van Krevelen diagram (Fig. S2) suggest the presence of altered OM (Type IV) and/or continental (Type III) OM. Nevertheless, in the case of thermal diagenesis associated with marked burial of the sediments, the hydrogen bonds of organic molecules are particularly affected, leading to a decrease of the original HI values (Espitalié et al., 1985
Conclusion
The Khodjaipak Fm. is composed of organic-rich carbonates, marls and siltstones which were deposited on a vast part of the Amu Darya Basin. These deposits were newly dated to the Middle Oxfordian, Transversarium Zone (Luciaeformis and Schilli Subzones). The predominant facies are organic laminated siltstones, nodular mudstones reaching ~6% of total organic carbon, and thinly laminated microbial boundstones. These organic-rich deposits are overlain by shallow water lagoon and sabkha deposits.
Declaration of competing interest
None of the authors have any conflicts of interest.
Acknowledgements
This study was funded by Total S.A. The scientists of the Institute of Geology and Geophysics of Uzbek Academy of Sciences of Tashkent (Uzbekistan), especially Irina Sidorova and Dmitriy Mordvintsev, are acknowledged for their logistic support in the field. The authors express their gratitude to Grégoire Bex for its assistance in the field, Léa Pigot and Philippe Blanc for the preparation of the thin sections, and Anne-Lise Santoni, Théophile Coquerez and Ludovic Bruneau for their assistance
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