Late Miocene to Pliocene stratigraphy of the Kura Basin, a subbasin of the South Caspian Basin: implications for the diachroneity of stage boundaries

Abstract

Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regional stages that are defined by changes in microfauna and associated extreme (>1000 m) variations in Caspian base level. However, the absolute ages of these stage boundaries may be significantly diachronous because many are based on the first occurrence of either transgressive or regressive facies, the temporal occurrence of which should depend on position within a basin. Here, we estimate the degree of diachroneity along the Akchagyl regional stage boundary within the Caspian basin system by presenting two late Miocene‐Pliocene aged measured sections, Sarica and Vashlovani, separated by 50 km and exposed within the Kura fold‐thrust belt in the interior of the Kura Basin. The Kura Basin is a western subbasin of the South Caspian Basin and the sections presented here are located >250 km from the modern Caspian coast. New U‐Pb detrital zircon ages from the Sarica section constrain the maximum depositional age for Productive Series strata, a lithostratigraphic package considered correlative with the 2–3 Myr‐long regional Eoakchagylian or Kimmerian stage that corresponds to a period of extremely low (>500 m below the modern level) Caspian base level. This new maximum depositional age from the Productive Series at Sarica of 2.5 ± 0.2 Ma indicates that the regionally extensive Akchagyl transgression, which ended the deposition of the Productive Series near the Caspian coast at 3.2 Ma, may have appeared a minimum of 0.5 Myr later in the northern interior of the Kura Basin than at the modern Caspian Sea coast. The results of this work have important implications for the tectonic and stratigraphic history of the region, suggesting that the initiation of the Plio‐Pleistocene Kura fold‐thrust belt may have not been as diachronous along strike as previously hypothesized. More generally, these results also provide a measure of the magnitude of diachroneity possible along sequence boundaries, particularly in isolated basins. Comparison of accumulation rates between units in the interior of the Kura subbasin and the South Caspian main basin suggest that extremely large variations in these rates within low‐stand deposits may be important in identifying the presence of subbasins in older stratigraphic packages.

Drainage network reveals patterns and history of active deformation in the eastern Greater Caucasus

“…In Sarica and Vashlovani, the deposits immediately overlying this alluvial fan complex are interpreted to have been deposited in a lacustrine setting that correlates with the Akchagyl regional stage (Forte et al, 2014b), which represents an extreme sea-level highstand in the Caspian that submerged much of the Kura Basin (e.g., Popov et al, 2006).…”

“…Foreland-basin deposits now exposed in the Kura fold-thrust belt suggest Pliocene-Pleistocene alluvial fans extended 2-3 times farther from the Greater Caucasus into the Kura Basin than today (Forte et al, 2014b). In Forte et al (2014b) exposures of Pliocene alluvial fan deposits in the Sarica and Vashlovani sections were described (Fig.…”

“…In Forte et al (2014b) exposures of Pliocene alluvial fan deposits in the Sarica and Vashlovani sections were described (Fig. 7A), and dated to ca.…”

3D geometry of a shale-cored anticline in the western South Caspian Basin (offshore Azerbaijan)

“…Extrapolating linearly the age of the PS-top and PS-surfaces; i.e., 5.9 and ~3.1 Ma, respectively (e.g., Inan et al, 1997;Abreu and Nummedal, 2007;Forte et al, 2015), we estimate that the H7-H8 interval has an age of 3.5-3.4 Ma. This is the estimated time for the onset of folding and the syn-growth folding extends henceforth to Present (Fig.…”

“…The top of the PS, referred in this work as the PS-top surface, corresponds to a regional unconformity, which shows significant evidence of erosion in the basin. Recent studies suggest that the PS-top is diachronous in the western margin of the SCB, becoming slightly younger from the offshore domain (3.2 Ma) to the Kura basin (up to 2.7 Ma; Van Baack et al, 2013;Forte et al, 2015).…”