Deep-water facies, processes and models: a review and classification scheme for modern and ancient sediments

Abstract

A review of previous work on modern and ancient deep-water facies, processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups. These facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes, through semi-permanent bottom-currents, to pelagic settling. In essence, the scheme is descriptive of the sedimentary attributes of sediments, although it is designed to aid interpretation of possible sediment transport/deposition processes. The classification scheme is three-tier with facies classes, groups and constituent facies, and is hierarchical to allow flexibility in its use.There are seven facies classes, with Classes A–E defined largely on the basis of grain-size differences, Class F on the basis of internal organization, and Class G on composition. The facies classes are: Class A, gravels, muddy gravels, gravelly muds, and pebbly sands, with ⩾ 5% gravel grade; Class B, sands, with ⩾ 80% sand grade and < 5% pebble grade; Class C, sand-mud couplets and muddy sands, with 20–80% sand grade and < 80% mud grade (mostly silt); Class D, silts, silty muds and silt-mud couplets, with > 80% mud, ⩾ 40% silt and 0–20% sand; Class E, muds and clays, with ⩾ 95% mud grade, < 40% silt grade and < 5% sand and coarser grade; Class F, chaotic deposits, with variable grain or clast sizes, and Class G, biogenic oozes, hemipelagites and chemogenic sediments, with < 5% terrigenous sand and gravel. The second-order classification into facies groups (A1, A2 etc.) is based mainly on organized versus disorganized sediments, the latter lacking marked stratification or grading and the former having clearly-defined primary sedimentary structures. At the level of facies, the criteria used to distinguish different types is more complex and more flexible.For the purpose of large-scale mapping or reconnaissance fieldwork, either the level of facies classes or groups may be appropriate, whereas for more detailed sedimentology the more detailed facies level will be necessary.