CRISIS FOR A GENERAL TERM REFERRING TO ALL TYPES OF SEDIMENT GRAVITY FLOW DEPOSITS: GRAVITE

Recent debates on deepwater sediment gravity flows, defined as bodies of sediment-water mixture that flow down slope due to the action of gravity, centered on the definition and depositional characteristics of the two main subcategories, namely turbidity currents and debris flows. There is no agreement among sedimentologists on what should be the main basis of classification of these flows. Probably, the only consensus is that turbidity currents are true Newtonian fluid whereas debris flows are not. Issues like, whether these flows can be both laminar and turbulent, can evolve from one type into another, and can have multiple sediment-support mechanisms are still in debate. As a result, one school’s ‘turbidites’ (deposits of turbidity currents) become another school’s ‘debrites’ (deposits of debris flows), and vice versa. In sedimentology, most of the time, we infer depositional mechanism of rocks based on their observable criteria. Because of some recent critiques on earlier concepts of deepwater deposits, it is felt that further research combining laboratory experiment and field observation is necessary to draw a fine division line between turbidites and debrites, as well as between any of their subtypes. However, as natural processes are continuum, there may be some transitional deposits sharing both the characteristics of turbidites and debrites. The absence of a suitable and concise general term for all types of sediment gravity flow deposits resulted in over- and miss-use of the term ‘turbidite’. For example, although submarine fans consist of different types of sediment gravity flow deposits, the term ‘turbidite systems’ has been used interchangeably with submarine fans. The term ‘gravite’ is proposed here for deposits of any kind of sediment gravity flows (including slides and slumps), irrespective of their depositional environment. There are many advantages in using this umbrella term. Especially, when the assignment of sediment gravity flow deposits to any particular subtypes like turbidite or debrite is either problematic or undesirable, the term gravite can be used conveniently without creating any debate of the recent kind. At the same time, the usage of this new term leaves room to proceed cautiously with rigorous investigation to subdivide these event deposits into any of their subtypes.