Long-term shoreline shifts on continental blocks during the Bajocian: an updated interpretation Dmitry A. Ruban, Emad S. Sallam Geologos 25, 1 (2019): 43–50 DOI: 10.2478/logos-2019-0004 Long-term shoreline shifts on continental blocks during the Bajocian: an updated interpretation based on synthetic stratigraphical and palaeogeographical developments on regional scales Dmitry A. Ruban1,2*, Emad S. Sallam3 1Cherepovets State University, Sovetskiy Avenue 10, Cherepovets, Vologda Region, 162600 Russia 2P.O. Box 7333, Rostov-na-Donu, 344056 Russia 3Department of Geology, Faculty of Science, Benha University, Farid Nada Street 15, Benha 13518, Egypt *corresponding author, e-mail: [email protected] Abstract Available reconstructions of Jurassic global sea level changes differ and are in need of an update. New stratigraphical charts and palaeogeographical developments for a number of large continental blocks or their portions of these (e.g., Germany, India, northeast Africa, northwest Australia, the Russian Platform and western Siberia) reveal regional long- term shoreline shifts (i.e., transgressions and regressions) during the Bajocian Stage (168.3–170.3 Ma). A comparison of these allows to document only a single coherent feature, namely the earliest Bajocian transgression, in the majority of the continental blocks considered. Undoubtedly, this event was triggered by a eustatic rise. However, long-term shoreline shifts were either weak to absent or differed between the blocks during almost the entire Bajocian, thus pro- viding evidence of the apparent stability of global sea level and the importance of regional tectonic activity as a control mechanism on particular transgressions and regressions. Interestingly, it appears that the earliest Bajocian eustatic rise was a constituent of a long-term eustatic pattern; the nature of this event has yet to be fully understood. Generally, the findings presented here are in better agreement with Anthony Hallam’s view of Jurassic eustasy and question some other global sea level reconstructions. Key words: Bajocian, eustasy, Middle Jurassic, platform, transgression 1. Introduction also for much younger intervals, such as the Juras- sic Period that has been studied in great detail. Two Reconstructions of global sea level (eustatic) chang- main alternative reconstructions of Jurassic global es in the geological past are of great importance for sea level changes were proposed by Hallam (1978, understanding the evolution of continental blocks, 1988) and Haq et al. (1987), respectively. Both of basin development, palaeoclimate dynamics and these have been updated subsequently, by Hallam biotic transitions (and vice versa). In spite of signifi- (2001) and Haq & Al-Qahtani (2005), respectively, cant progress since the late 1970s, our knowledge of with further reinterpretation by Ruban (2015). Fi- eustatic fluctuations remains incomplete and some- nally, Haq (2018) presented another revised version what uncertain (Ruban, 2016). Not only is this the of the Jurassic global sea level curve. These recon- case for Precambrian or Palaeozoic time spans, but structions (either in original or modified form) are 44 Dmitry A. Ruban, Emad S. Sallam actively being used in far-reaching interpretations activity, even if this is minor. The second approach (e.g., Boulila et al., 2018). Moreover, van der Meer employs the single factor of eustasy, the importance et al. (2017) have recently proposed yet another of which changed through geological time, while reconstruction, which is of low resolution, but suf- the third appears to be most reliable, although the ficiently accurate to allow judgements on Jurassic outcome of its application depends strongly on the eustasy to be made. Ruban & Sallam (2016, 2018) proper choice of regional reference records. As a presented new evidence that, generally, was in sup- result, our views of Phanerozoic eustasy remains port of Hallam’s views, although it became clear strongly biased and ‘chaotic’ (Ruban, 2016). The that further consideration of as many ‘fresh’ data as present study focuses on transgressions and regres- possible is called for to understand Jurassic global sions on continental blocks as appropriate reflectors sea level changes. of eustatic changes. Although shoreline shifts can be In a previous paper, Ruban & Sallam (2018) drew influenced by some regional tectonic activity, their conclusions on the absence of a long-term eustatic coherence for several large and tectonically distinct lowstand during the first stage of the Middle Juras- domains can be taken as evidence of a planet-scale sic, i.e., the Aalenian. They also presented some pre- eustatic event. liminary evidence of a widespread transgression For the purposes of the present study, regional during the Bajocian. If the latter prove to be correct, stratigraphical records of several large continental this is really significant when the short duration blocks have been examined (Fig. 1). Laurasia (sen- (2 myr) of this second stage of the Middle Jurassic su Seton et al., 2012) is represented by the territory (Gradstein et al., 2012; Ogg et al., 2016; Wiggan et of present-day Germany, the Russian Platform and al., 2018) is taken into account. Hypothetically, such western Siberia; West Gondwana (sensu Seton et a transgression could be related to a global sea lev- al., 2012) covers present-day northeast Africa and el rise. Ruban’s (2015) interpretation raises doubts Arabia, while East Gondwana (sensu Seton et al., over a eustatic rise during the Bajocian, and this 2012) corresponds to India and northwest Austral- concern is supported by data supplied by Ruban & ia. In view of the fact that the present paper aims to Sallam (2016) for northeast Africa, which also im- update current knowledge, only domains for which plies the doubtful nature of such a rise. Below, we more or less ‘fresh’ data are available are here cho- attempt to update current knowledge on long-term sen for analysis. The regions analysed are fairly shoreline shifts on major continental blocks during representative considering their location in differ- the Bajocian. The long-term (of stage length) pattern ent parts of the Middle Jurassic world (Fig. 1). The is emphasised because this is the sole one that can main data sources consulted for the present study be correlated on a near-global scale. Continental are: Germany; Menning & Hendrich (2016); the blocks are chosen so to minimise the outstanding Russian Platform; Mitta (2012) and Ruban & Sallam influence of tectonic factors on active margins and (2018); western Siberia; Kontorovitch et al. (2013); to deal with ‘flat’ domains where shoreline shifts northeast Africa; Guiraud et al. (2005) and Ruban were sufficiently broad to be well documented. & Sallam (2016); Arabia; Sharland et al. (2001) and Simmons et al. (2007); India; Raju (2007) and Pan- dey et al. (2009) and northwest Australia; Riding 2. Materials and method Global sea level changes can be documented using three principal approaches, namely in-depth exam- ination of any regional record of global representa- tiveness (e.g., Miller et al., 2005), modelling based on tectonic and/or geochemical proxies (e.g., Müller et al., 2008) and near-global comparisons (constrained by accurate stratigraphical correlations) of multiple regional records. The importance and efficacy of the last-named approach was demonstrated, in par- ticular, by Hallam (2001), Miall (2010) and Ruban & Middle–Late Jurassic plate tectonic setting (sim- Sallam (2018). In fact, all three approaches have the Fig. 1. plified from Seton et al., 2012) and location of con- same objective in common, namely a correct recon- tinental domains considered in the present paper. struction of global sea level changes, but all have Abbreviations: A – Arabia, G – Germany, I – India, certain limitations. The first approach cannot fully NEA – northeast Africa, NWA – northwest Australia, steer clear from the influence of regional tectonic RP – Russian Platform, WS – western Siberia Long-term shoreline shifts on continental blocks during the Bajocian: an updated interpretation 45 et al. (2010) and Tao et al. (2013). All of these are of coherence, it is possible to draw conclusions on synthetic, region-scale stratigraphical and occasion- common, eustatically driven shoreline shifts. These ally palaeogeographical updates of previously pub- can be justified to the standard ammonite-based lished information. However, such of vital impor- zones of the Bajocian (Ogg et al., 2012), taking into tance in our understanding of the global extent of account the average duration of the relevant trans- transgressions and regressions, as was document- gressions or regressions in the regions compared. If ed by Hallam (2001). Even if these developments no coherence is found, this implies either that the comprise both novel and previous data, the devel- influence of regional tectonic activity predominat- opments themselves are ‘fresh’ sources of updated ed or that global sea level changes were too weak information. to overcome the tectonic factor. Indeed, not for all New and accurate composite regional strati- continental masses are novel stratigraphical and graphical charts that are provided in the sources palaeogeographical developments available; terres- mentioned above and, whenever available, detailed trial deposition dominated on some Gondwanan regional palaeogeographical reconstructions have blocks and some continents (e.g., North and South been analysed in order to draw conclusions on di- America) were strongly