Early diagenesis of Late Cretaceous chalk-chert-phosphorite hardgrounds in Jordan: Implications for sedimentation on a Coniacian-Campanian pelagic ramp

Abstract

Hardgrounds and omission surfaces are rare in the predominantly pelagic and hemi-pelagic chalk, chert and phosphorite lithofacies association that forms the Upper Cretaceous (Coniacian to Maastrichtian) Belqa Group succession in central Jordan. However, newly-described hardgrounds of regional extent at the base of the Dhiban Chalk Member (Campanian) in central and south Jordan reveal a complex history of sedimentation and early diagenesis. Following drowning of the Turonian carbonate platform during the Coniacian, the chalk-chert-phosphorite association was deposited on a pelagic ramp in fluctuating water depths. The Mujib Chalk and Dhiban Chalk members represent highstand sea levels, separated by a regressive, lowstand chert-rich unit (Tafilah Member). Hardground successions can be traced over 100 km, and show an early diagenetic history of phosphatisation and biogenic silica lithification from opal-A to opal-CT and quartz that resulted in penecontemporaneous chert deformation, followed by submarine bioerosion and colonisation by corals and/or bivalves. Subsequent deposition of detrital, remanié phosphatic chalk passing up into pelagic coccolith-rich ooze reflects a transgressive third-order sea-level rise during the Early Campanian. These events provide a time-frame for early silica diagenesis and subsequent hardground development. Regional variations in the hardground successions and their early diagenesis are attributed to their precursor host sediment and relative palaeogeographic position on a homoclinal ramp at the southern margin of the Neo-Tethys Ocean.