Origin, sequence stratigraphy and depositional environment of an upper Ordovician (Hirnantian) deglacial black shale, Jordan

The upper Ordovician succession of Jordan was located 60°S, less than 100 km from the Hirnantian ice sheet margin. New graptolite dates indicate glaciation ended in Jordan in the late Hirnantian (persculptus Biozone). The succession records two glacial advances within the Ammar Formation and the subsequent deglaciations. Organic-rich black shales (Batra Formation) form part of the final deglacial transgressive succession that in-filled an existing low stand glacial continental shelf topography. The base of the black shale is coincident with the maximum flooding surface. During transgression, interfluves and sub-basin margins were breached and black shale deposition expanded rapidly across the region. The top of the black shales coincides with peak highstand. The “expanding puddle model” (sensu Wignall) for black shale deposition, adapted for the peri-glacial setting, provides the best explanation for this sequence of events.

We propose a hypothesis in which anoxic conditions were initiated beneath the halocline in a salinity stratified water column; a fresher surface layer resulted from ice meltwater generated during early deglaciation. During the initial stages of marine incursion, nutrients in the monimolimnion were isolated from the euphotic zone by the halocline. Increasing total organic carbon (TOC) and δ¹³C_org up section indicates the organic carbon content of the shales was controlled mainly by increasing bioproductivity in the mixolimnion (the Strakhov model). Mixolimnion nutrient levels were sustained by a continual and increasing supply of meltwater-derived nutrients, modulated by obliquity changes in high latitude insolation. Anoxia was sustained over tens to hundreds of thousands of years. The formation of black shales on the north Gondwana shelf was little different to those observed in modern black shale environments, suggesting that it was the nature of the Ordovician seas that pre-disposed them to anoxia.