Quantifying lateral and distal variability within hybrid beds, case studies from Central and Northern Italy

Brooks, Dr Hannah1, Steel,Dr Elisabeth2

1 School of Earth Sciences, 253-283 Elgin St, University of Melbourne, Carlton Victoria, Australia, 2 Department of geological sciences and geological engineering, Bruce Wing/Miller Hall, 36 Union Street, Queen’s University, Kingston, Ontario, Canada.

Hybrid beds or linked debrites are deposits that form under bi- or tri-partite flow conditions, involving both turbulent and laminar flow conditions. Often, hybrid beds occur with distal or lateral flow transformation following significant entrainment of a muddy substrate and/or declining turbulent energy. Hybrid beds have been noted to make up significant proportions of deposits within basin floor setting worldwide, most commonly within the distal fringes of lobe systems. The study examines dip and lateral variations in facies and architecture in hybrid beds using detailed facies analysis of selected sections within the Castagnola, Marnoso-Arenacea, and Gottero Formations, deposited within three different basins within central and northern Italy. Sections within these systems were selected where hybrid beds could be traced out laterally or down-dip for several metres to several kilometres. In total 407 samples were taken for laser-diffraction grain-size analysis. Samples were selected through beds at 20 cm intervals and across/ down-dip at 10’s of metre to several kilometre intervals. Layers within beds were classified into facies divisions which were ran through ‘EMMA’ End Member Modelling Analysis, this splits beds into an optimum number of end members that when combined create the trends found within the dataset. This method is utilized to establish patterns of changes within beds laterally and down-dip which are otherwise difficult or too complex to be quantified from field data alone.

This detailed study of facies, architectural and grain-size changes within the targeted deposits will help to establish how flow processes varied as flows spread laterally and moved downstream. Through quantifying the amount of mud within the matrix and clasts at any one time within the flow it is possible to interpret how and when turbidites and hybrid beds erode and incorporate sediment from underlying substrate. Initial results from EMMA indicate that using three end members for each formation is an optimum number for observing basinal trends within beds. These end members constitute a sand-rich unimodal end member, a silt-rich unimodal end member, and a bimodal poorly sorted end member, interpreted to represent a high-density turbidite, a low-density turbidite and a debrite respectively. Recognised trends within the data include a decrease laterally in the sand-rich unimodal end member within the Marnoso-Arenacea section, interpreted as an increased distance from the flow input or the presence of a lateral basin slope. Through application of this methodology in basins with well-established bed correlation it is possible to provide novel understanding that will significantly augment traditional field techniques. It is evident that laser diffraction grain-size analysis and EMMA are vital tools in furthering our understand of process sedimentology and should be applied more widely.


PhD in Sedimentology, University of Leeds, UK, studying Permian deepwater sediments in the Karoo Basin, South Africa. Undertook postdoc in Chiba University, Japan studying Pliocene deepwater outcrops, and Queens University studying hybrid beds in central and northern Italy. Now, postdoc at University of Melbourne studying Neoproterozoic rocks in Flinders Ranges.

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