GENESIS OF LIMESTONE MEGABRECCIAS AND THEIR SIGNIFICANCE IN CARBONATESEQUENCE STRATIGRAPHIC MODELS - A REVIEW

A re-appraisal of the factors controlling the deposition of carbonate megabreccias and their sequence stratigraphic significance is presente d based on an examination of the mechanisms for instigating carbonate seafloor slope instability and a review of the depositional settings o f these distinctive rock deposits. Limestone megabreccias have classic ally been interpreted as formed by the catastrophic collapse of high-a ngle 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 oversteepe ned slopes are not necessary for their genesis. A re-evaluation constr ucted from first principles of mechanisms promoting submarine gravitat ional instability indicates that pore-water overpressure of confined a quifer horizons beneath the seafloor rather than slope oversteepening, is the critical control on megabreccia deposition. Catastrophic relea se of overpressure may initiate gravitational instability of previousl y kinematically stable slopes. The relative importance of endogenic pr ocesses intrinsically linked to the depositional system and exogenic p rocesses operating independently in generating megabreccias is assesse d. Two particularly important mechanisms in generating megabreccias ar e 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 megabre ccias. Exogenic causes of gravitational instability with inherently ra ndom periodicity, especially seismicity, may account for the small num ber 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 stratigrap hic 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. Low stand wedges and aprons in carbonate systems therefore generally diffe r in composition from equivalent highstand deposits in being composed not of calciturbidite deposits but of megabreccias cannibalized from t he upper slope.