Genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models: a review

Abstract

A re-appraisal of the factors controlling the deposition of carbonate megabreccias and their sequence stratigraphic significance is presented based on an examination of the mechanisms for instigating carbonate seafloor slope instability and a review of the depositional settings of these distinctive rock deposits. Limestone megabreccias have classically been interpreted as formed by the catastrophic collapse of high-angle metastable ‘oversteepened’ carbonate platform margins. However, a review of megabreccia occurrence shows that these deposits formed on a broad range of carbonate slope angles and that metastable oversteepened slopes are not necessary for their genesis. A re-evaluation constructed from first principles of mechanisms promoting submarine gravitational instability indicates that pore-water overpressure of confined aquifer horizons beneath the seafloor, rather than slope oversteepening, is the critical control on megabreccia deposition. Catastrophic release of overpressure may initiate gravitational instability of previously kinematically stable slopes. The relative importance of endogenic processes intrinsically linked to the depositional system and exogenic processes operating independently in generating megabreccias is assessed. Two particularly important mechanisms in generating megabreccias are endogenic processes causing overpressure at discrete hydrologically confined horizons beneath the seafloor during relative sea-level falls, and increases in stress as pore-fluid drains from the sediment when the platform-top becomes subaerially exposed during relative lowstands of sea-level. The increased likelihood of megabreccia genesis during relative sea-level falls is supported by an empirical trend identified from a review of the stratigraphic distribution of documented megabreccias. Exogenic causes of gravitational instability with inherently random periodicity, especially seismicity, may account for the small number of documented megabreccias that do not conform to this underlying trend. The proposed endogenic pattern favouring megabreccia deposition during relative sea-level falls is important in a sequence stratigraphic context. Megabreccia deposits may build volumetrically significant toe-of-slope wedges and aprons during relative lowstands of sea-level, partially compensating for the decrease/loss in the supply of fresh granular sediment as the platform-top carbonate factory contracts. Lowstand wedges and aprons in carbonate systems therefore generally differ in composition from equivalent highstand deposits in being composed not of calciturbidite deposits but of megabreccias cannibalized from the upper slope.