On the architecture of intra-formational Mass-Transport Deposits: Insights from the carbonate slopes of Great Bahama Bank and the Apulian Carbonate Platform

摘要

Large sub-tropical carbonate platforms shed huge volumes of sediment onto their adjacent slopes. Resulting carbonate successions are comprised of gravity-flow beds (turbidites, debrites), which are occasionally reworked into mass-transport deposits (MTDs) by the failure of inconsistently lithified portions of slope strata. Despite their significance in the depositional record, the internal architecture of MTDs along relatively tectonically stable and sediment-producing carbonate platforms is poorly understood in comparison with counterparts in active compressional tectonic settings. This study compares carbonate-slope-derived MTDs imaged in seismic reflection data with those observed in outcrop. The internal architecture of MTDs from two case studies is presented: the seismically-imaged MTDs in Miocene-Recent carbonate-slope deposits from the Bahamas, and MTDs off the Cretaceous Apulian carbonate margin exposed in outcrops of the Ionian Basin (Albania). Understanding the large-scale (kilometers to tens of kilometers) internal architecture of MTDs in seismic is crucial for identification of these structures in outcrop. In turn, only outcrop studies allow the detailed analysis of the reworking style within these massive sediment bodies at a smaller scale (meters to kilometers). The internal architecture of the studied carbonate MTDs testifies to the existence of an along-slope continuum of deformation structures, characterized by vertical partitioning into distinct mass flows during single mass-transport events. Mass-wasting processes are inferred from strain-partitioning structures, recording along-slope and slope-parallel architectural heterogeneities. Three deformation domains are recognized: extensional headwall domain, translational body domain, and contractional toe domain. Confined MTDs are limited by steep and deep lateral margins and frontal ramps that mark the boundaries between failed deposits and undisturbed strata. Vertical strain-partitioning is pronounced in the toe domain. Upward intensification of deformation within MTDs is common and corresponds to a top-down reorganization of failed strata during downslope transport. Intra-formational MTDs along carbonate slopes that are lacking exotics, such as the ones studied here, differ from extra-formational MTDs occurring in compressional tectonic settings that form sedimentary melanges. Ductile-layer deformation structures are predominant in carbonate MTDs as opposed to block-in-matrix fabrics in sedimentary melanges. The development of shear surfaces, rather than ductile shear zones, in the body and toe domains presents predictive (in modern environments) and diagnostic criteria (in fossil examples) of mass-wasting events along carbonate slopes.