Marine influence during deposition of the Kiliranjao Brown Shale, Central Sumatra, from palynology point of view

Abstract

The Eocene to Oligocene succession of the Kiliranjao Brown Shale was deposited in a lacustrine environment. This study combines palynological analysis and XRD analysis technique to investigate the possibility of marine influence during deposition of Kiliranjao lacustrine shale. Two biostratigraphic zonations of Karbindo section are Middle to Late Eocene Proxapertites operculatus Zone and Oligocene Meyeripollis naharkotensis Zone. The marine flooding surface, as shown by the peak of the abundance of mangrove pollen, is used to delineate the section into five parasequences. The tripartite stratigraphic architecture of Karbindo section is consists of early rift deposit (TS1), syn-rift deposit (TS2), and syn-rift deposit (TS3). Middle to Late Eocene coal and limestone facies (parasequence I) represents the TS1. Late Eocene to Oligocene parasequence II to IV (below MFS) shows the TS2 (deepening-upward lacustrine succession), and the shallowing-upward lacustrine succession of Oligocene parasequence V (above MFS) indicate the TS3. The abundance of mangrove and back mangrove palynomorph assemblages and the high amount of carbonate mineral from XRD analysis are the evidence of marine influence to the deposition of Karbindo section Brown Shale. The marine incursion to the lacustrine environment occurred from the adjacent nearby open sea, that separated by shoal area, which can submerge below sea level during a marine transgressive event. Marine incursion events did contribute to the deposition of Kiliranjao Brown Shale though further work is still needed.