Carbonate megabreccias in a sequence stratigraphic context; evidence from the Cambrian of North Greenland

In carbonate sequence stratigraphy, carbonate megabreccias have acquired particular significance, being deemed characteristic of the lowstand systems tract (LST) or the forced regressive systems tract (FRST). Large-scale mass-wastage can, however, result from factors other than sea-level change and it is rare that the sequence stratigraphic significance of megabreccias can be rigorously tested. In the Cambrian of North Greenland, erection of a robust sequence stratigraphic framework is facilitated by extensive fjord-wall exposures of the platform to deep shelf transect and by a well-developed carbonate-siliciclastic reciprocal sedimentation pattern within off-platform strata. On the basis of this independent framework, megabreccias are represented locally within the LST and the highstand systems tract (HST), but occur systematically above the HST. These HST-capping megabreccias are composite sheets tens of metres thick that extend up to 50 km distally and flank the platform for up to 400 km along strike. They comprise debris derived from the highstand platform margin and slope and are directly overlain by mixed carbonate-siliciclastic sediments of the succeeding LST. The HST-capping megabreccias are assigned to the FRST; they record extensive failure of the platform margin and upper slope during relative fall of sea-level and prior to the onset of lowstand sedimentation. Although the LST megabreccias are compositionally distinctive, the sole example of an intra-HST megabreccia differs from those of the FRST only in terms of areal extent. In the absence of an independent framework, therefore, the sequence stratigraphic affinities of megabreccias may be ambiguous.