Application of mineral liberation analysis (MLA) for investigations of grain size distribution in submarine density flow deposits

Abstract

Textural and structural features of submarine mass density flow sediments are a record of depositional flow evolution and hydrodynamics. Record of changes in such flow movement are difficult or impossible to detect, especially in fine sediments, without quantitative grain size analysis. This study shows an automated measurement system of mineral liberation analysis MLA applied for the first time to grain size distribution. The analysis concerns heavy minerals extracted from very fine- and fine-grained sandstones of the Cergowa Beds (Lower Oligocene) in the Polish and Slovak Outer Carpathians, which macroscopically show unclear textural trends and represent deposits of steady and uniform turbidity currents, possibly initiated by hyperpycnal effluents. The MLA analysis showed that the tested material represents sediments deposited by flows with developing turbulence. Simultaneously, the study verified that a turbulence damping contributes to size-density sorting during transport and demonstrated that maximum rather than average grain size is controlled by flow concentration, therefore reflects flow hydrodynamics. Besides, MLA measurements recognised variations in flow motion recorded by three types of grain-size breaks: (i) in high-density flow when capacity-controlled conditions are changing so that high energy flow erodes the previously deposited material; (ii) at a sharp interface between high-concentration basal layer and the overriding flow with lower concentration, and (iii) at a transition between low- and high-density flows during deposition of the Bouma interval Tb or between Tb and Tc. The MLA test study appears to be promising because this method increases objectivity of large amount of data collection, therefore provides significant statistical representation. Besides, this automated system reduces human errors, bias and tedious manual analyses, thus opening new perspectives in analysis of detrital sediments and interpretation of depositional processes.