The effects of strong sediment-supply variability on the sequence stratigraphic architecture: Insights from early Toarcian carbonate factory collapses

Standard sequence stratigraphy interpretations are commonly built upon the premise of fairly constant sediment supply, leaving vast uncertainties about sequence architecture and sea-level change recorded during major paleoenvironmental disturbances, usually associated with carbonate productivity collapse and increased siliciclastic sediments supply. During relative sea-level rise, this process is referred to as drowning event. However, drastic collapse of carbonate productivity during relative sea-level drop remains poorly documented. The Pliensbachian/Toarcian boundary in Morocco provides an outstanding example of such a case, referred here to as poisoning event. Using bio-, chemo-and high-resolution sequence stratigraphic approaches on horizontal seismic-scale outcrop data, we characterize the architecture of vertically and laterally mixed siliciclastic/carbonate systems. During poisoning event, the standard sequence Lowstand-Transgressive-Highstand system tracts is altered into Highstand-Transgressive-Falling Stage system tracts with little or no evidence for Highstand deposits. Noteworthy, it is important to note that poisoning and drowning events can be confused together in sedimentary records where the shoreline trajectory cannot be clearly tracked, as it has often been the case in previous Pliensbachian/Toarcian studies. The outcome of this study calls thus for a closer re-examination of other carbonate factory collapse event attributed to drowning events without an unambiguous knowledge of their coeval shoreline trajectory.