Megabreccia shedding from modern, low-relief carbonate platforms, Nicaraguan Rise
Single-channel seismic reflection data from the margins of lowrelief (150-250 m, measured from edge of bank to basin) carbonate platforms on the northern Nicaraguan Rise reveal complex seismic intervals consisting of mounded, chaotic seismic facies interspersed with discontinuous, parallel/laminated seismic facies. We interpret that these intervals contain megabreccias (chaotic facies) and sandy turbidites (parallel/laminated facies). One megabreccia is exposed on the sea floor displaying an overall fan shape having individual blocks measuring nearly 300 m across and >110 m high. The source area consists of a scalloped embayment with a headwall scarp 180 m high. Reflections within the platform are sharply truncated by this escarpment. This single megabreccia is ∼120 m thick and extends ∼27 km along slope and ∼16 km out into the basin. Other megabreccias within the basin have individual blocks measuring >400 m across. Rocks from dredge hauls are a mixture of shallow- and deep-water facies. Shallow-water facies consist of mixed, skeletal grain-stones and Halimeda packstones. Deep-water facies are massive chalks, chalks with shallow-water skeletal grains, and chalk-block breccias. This indicates that the megabreccias formed as a result of bank-margin collapse, during which the ensuing debris flow eroded into slope and basin facies, mixing rock types together. We speculate that bank-margin-collapse events, resulting in megabreccia formation, may have been seismically triggered, and we emphasize that these large-scale, mass-wasting events occurred along margins of low-relief carbonate platforms.