Evidence of active equatorial upwelling during Late Jurassic and Early Cretaceous in the Zagros folded-belt, implication on development of rich hydrocarbon source rocks

Late Jurassic and Early Cretaceous are time of upsurge in breaking of the Pangaea supercontinent. During this time Arabian plate and Zagros folded-belt were located in vicinity of the equator. Volcanism related to splitting apart of the Pangaea enriched silica in the Tethys Ocean. During this time radiolarians are frequently observed in large quantities. Intermittent occurrences of radiolarians known as Radiolaria Flood Zone (RFZ) have been recorded in many rock units of Late Jurassic and Early Cretaceous in the Zagros such as Garau, Sargelu, Daryian, Fahlyian, Kazhdumi, and Gadvan formations. In this study intervals of RFZ are investigated via petrographic and rock-eval pyrolysis analysis. Obtained results mainly reveal radiation of radiolarians associated with high values of Total Organic Matter (TOC). Traditionally developed rock units in the Lurestan geological province such as Garau and Sargelu formation were regarded as offshore deposits. Characterized by bloom of radiolarians and high values of TOC. Though, occurrence of RFZ intervals in the Khuzestan geological province rock units were always controversial and mostly quoted to be tongues of the Garau Formation. Intermittent appearance of the RFZ intervals in the Khuzestan geological province are mainly sandwiched by shallow deposits and were supposed to be as a result of interfingering between relatively deep and shallow rock units such as Garau/Fahlyian, Garau/Kazhdumi or Garau/Daryian. Obtained results in this study suggest that radiation of radiolarian and TOC rich intervals are not necessarily deep sea deposits and are more related to permanent and monsoonal equatorial upwelling belts developed in Neotethys. Nowadays equatorial upwelling currents are quite upwelling currents commons in areas close to the equator. Consequently, we supposed activity of equatorial upwelling currents during Late Jurassic and Early Cretaceous. These oceanic currents brought the necessary nutrient and silica already produced at sea bottom by breaking of the Pangaea towards sea surface. The fertile sea surface was an ideal place for bloom of radiolarians. Radiolarian abundance in sediments increases particularly beneath high productivity areas such as water-mass fronts (equatorial zones, polar belts) and more generally under all areas of active upwelling (De Wever and Baudin, 1996). Radiolarians are one of the primary producers of organic matter in marine environments and this group have been common constituents of petroleum source rocks. This upwelling model explains occurrence of RFZ intervals sandwiched by shallow deposits. Moreover, record of this interval within a vast geographical area (up to 200 km) that could not be elucidated via interfingering models is reasonable. Incidentally, semi-restricted geometry of the Neotethys and influence of oceanic anoxic events during Late Jurassic and Early Cretaceous enhanced preservation of organic matters in these intervals. High productivity coupled with good state of preservation resulted in development of high potential intervals of source rocks within different rock units of Late Jurassic and Early Cretaceous. These rock units had an immense contribution in development of giant hydrocarbon systems of the region.